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General control line discussion => Open Forum => Topic started by: Tim Stagg on October 31, 2016, 07:32:03 AM
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I hesitate to post this questions because there are so many factors that you can not know without seeing the airplane or watching the flight. I will do my best to explain what the airplane is doing....my questions is, what are the most probable causes so I can start experimenting one by one till I fix the issue.
OK...looking down the lines as the airplane is flying... the inboard wing tends to move up and down on the roll axis of the airplane....maybe only a 1/2" up and down but it is not stable in roll. The plane fly's well other than this, turns well...locks in and does not hunt up or down...it just is constantly rocking. Apparently this is much less visible from the outside of the circle.
Plane specs: 60 Size super Chipmunk with a time machine 60 foam wing
Time Machine foam wing is pretty thick and a blunt leading edge.
Without going into very single detail about the plane......does something based on your past experience come to mind as to what it could be??
Any ideas??
1. Do I have a twist or warping feature on the inboard or outboard wing maybe?
Tim
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Let me start out by saying that I do not have a clue. However it would seem to me that some configuration of the air frame is dynamically unstable. IE some force on the air frame changes with the roll angle. I know not much help - But hay I am sure someone will have an answer.
D>K
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A long time ago, I had a model that did this. It rocked badly in windy conditions, and just noticeably in calm air. I never found the cause, but at the time I suspected the rather heavy block tips that I hadn't hollowed enough. It was a tapered I-beam wing, and although the model's long gone, I remember that the flaps were very narrow at the tips, with about three or four inches unsupported beyond the outermost hinge: could it have been that they were flexing, perhaps under the influence of the tip vortices?
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I hesitate to post this questions because there are so many factors that you can not know without seeing the airplane or watching the flight. I will do my best to explain what the airplane is doing....my questions is, what are the most probable causes so I can start experimenting one by one till I fix the issue.
OK...looking down the lines as the airplane is flying... the inboard wing tends to move up and down on the roll axis of the airplane....maybe only a 1/2" up and down but it is not stable in roll. The plane fly's well other than this, turns well...locks in and does not hunt up or down...it just is constantly rocking. Apparently this is much less visible from the outside of the circle.
Plane specs: 60 Size super Chipmunk with a time machine 60 foam wing
Time Machine foam wing is pretty thick and a blunt leading edge.
Without going into very single detail about the plane......does something based on your past experience come to mind as to what it could be??
Any ideas??
I would suggest adding tipweight until it is steady, then seeing what it does in the corners. Also. check that the leadouts are sufficiently aft of the CG, by at least 3/4".
I am not sure what is happening, and without seeing it, this is little better than guessing. If it start hinging in the maneuvers, maybe it needs the tip weight AND a tab.
Stunt planes are weakly stable in roll at the best of times, and the stability is due to a very slight (when trimmed correctly) tendency toward outboard roll contending with a very slight restoring torque from the line tension. *Usually*, if it is just wandering around in roll, it's due to either not enough tipweight or slight inboard yaw due to too-far-forward leadouts. The too-far-forward leadouts are usually the result of trying to correct for rudder offset and shoving the leadouts further forward to "compensate". The leadouts need to be where they have to be, which is from 3/4" to an inch and a quarter or so behind the CG. Then adjust the rudder offset to match it, thus resulting in essentially zero rudder offset.
I agree that the hinge lines should be properly sealed but that's very unlikely to be causing this particular problem
Brett
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What Brett says, after you verify that the flaps are firmly nailed down. I'm suspecting some sort of low-level flutter, if that's possible (normal flutter would involve the flaps falling off, possibly with bits of wing attached).
Are your hinge lines sealed?
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I'd be onboard with what's been said but might add: if there is much rudder offset or the leadouts are way off-likely too far back- or maybe a crooked fuse will cause the airplane to oscillate or fishtail some forcing alternate wings into the slipstream causing changing lift to each panel. Do as suggested above then creep the leadouts forward between flights to find the magic spot. After that it could be simply a design issue with the wing tips or too aggressive tip airfoil compared to the root making the tips sensitive to turbulence. From the front view rounded block shaped tips are a little less stable than simple 'V' shaped tips. If it's design you may just have to live with it.
Dave
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Thanks for the suggestions, based on your thoughts here is a little more info:
Brett, I will need to see where the CG falls, but I would guess that my lead-outs are forward of where you are suggesting they should be. I have an ounce of tip weight in the plane currently, I had 1.5 in it originally, but I have been slowly reducing the weight because I thought I was getting a little hinging. maybe I am going the wrong direction...I will have to try more weight . I have no offset in the rudder, however I am using a pusher prop setup. I tried the tractor rotation yesterday but did not really see any real difference in the wing. I do have a wing trim tab and have been adjusting a little down trim...not for an unlevel wing, but to see what if any effect it had on the wing....maybe I am going the wrong direction...I will try that also. Lastly, I do have a very minor building error in the that the wing stab alignment are not 100% parallel.....its not much but if you think it is the culprit...I may have to fix.
John,Tim, The wing tips are rounded so I guess that could be an issue, but they are light hollowed blocks so I dont think the weight is an issue. The hinge lines are well supported and close...but not totally sealed, I will try sealing them.
Also, the nice thing about electric is this plane is still in silkspan and primer so I can make changes. I will try to snap some pictures so you can at least see, what it looks like.
Thanks again everybody.
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Hey Tim:
Even if that stab misalignment isn't causing this problem, it'll bite you later -- so you probably want to fix it regardless.
The comment on yaw made me think of a plane that I've built here. It had much too little fin area, and would come out of a sharp maneuver rocking badly -- to the point where it'd be banging on the lines until it calmed down. Adding fin area helped. I did not try playing with tip weight or leadout location, so it could have been that I missed an easier fix. However, if it's a new design, and if it doesn't have much side area in back, and if you get desperate, you might want to try pinning on some temporary fin area and see how it goes.
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Fix the Stab. I can feel 1/64" misallignment.
Your leadouts are too far forward if they are in front of where Brett says.
My LO's are .........3" behind the CG.
Fiddle with your LO's first.
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My LO's are .........3" behind the CG.
Did you mean LE ??
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Brett, can you clarify the leadout measurement? Is it to the front lead, or the center of the adjuster? Thanks.
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Did you mean LE ??
( I think Paul might take a bit to check in so I being the nosey guy I am elect to jump in here)
I am betting he in fact meant behind the CG,, Mine are similarly located, though I think not quite as far back,, becuase leadout position is relative to CG, but typically measure from the flap hing line so as to be consistant...
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Hi Tim,
As I don't know you nor have I seen you fly I'd like to know how you categorize your current flying skills. Do you fly complete patterns including verticals, hourglasses and overheads? If so are there any maneuvers in the pattern where you feel their performance may be inhibited due to trim problems? In particular, significant line tension issues? Have you had significant experience in the "magic arts" of stunt; bench and flight trimming, power trains, aerodynamic fundamentals, etc.
Are there any obstacles around your flying site that may be causing even modest breezes to result in disturbed air; trees, bushes, parked cars, adoring crowds of observers, etc.
I, too, would be interested in the results of sealing the hingelines (in particular--given the roll issues--the flaps). If the gaps vary from one side to the others minor control inputs during level flight could cause an imbalance in lift change between the left and right wings resulting in small (as you suggest) rolling resultls. (This has been said a million times on these fora but...Nothing bad can happen by properly sealing hingelines--i.e. don't restrict control freedom when doing so--and a lot of very good stuff can result from doing so that might not have been blatantly obvious during construction and bench trimming.
The answers to questions such as these might give us more to work on.
Ted
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Did you mean LE ??
No, I said CG and that is what it is. Seems a bit far back, but every electric I have flown has responded well to this.
The Canadian contingent were also very skeptical but soon came around when they experimented with it. There is also someone else in WW who does what I do, and this person also flies with them back.
Good luck Tim.
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Ted:
Relative to how Tim flies: he flies Expert and is really quite good. And he keeps beating me at all of our local contests!
So, hmmmm, maybe you could help him get a little more roll on his maneuvers? Yeah, that would help a lot.... VD~
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How does this LO's 3" aft of the CG compare with the location of the LO's of a glow powered Impact, measuring both from the hingeline? I'm thinking that the unchanging CG makes the difference in LO position work? ??? Steve
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How does this LO's 3" aft of the CG compare with the location of the LO's of a glow powered Impact, measuring both from the hingeline? I'm thinking that the unchanging CG makes the difference in LO position work? ??? Steve
Nice try Steve, but most of my IC stuff the LO's were 0.75" (or so) aft of the CG. The CG change is not 2.25".
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No, but with an IC plane, the LO position has to be a compromise, due to the CG shift. More so with a thirsty setup like David's. Too far aft early and too far forward late in the tank? How much farther forward is the CG on an electric Impact vs. a glow one? Howard isn't saying, but we had to look at his clay wart all Summer. I think he said that was 1 oz. I doubt that would make 1/8" change.
Ooops, had to go to the door. A big kid in a Clown Suit driving a Preus (sic), wanted imported chocolates. Scram, kid! H^^ Steve
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Brett, can you clarify the leadout measurement? Is it to the front lead, or the center of the adjuster? Thanks.
Center, presuming they are reasonably close. Note that I am not saying this is optimal, just safe.
Brett
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No, but with an IC plane, the LO position has to be a compromise, due to the CG shift. More so with a thirsty setup like David's. Too far aft early and too far forward late in the tank? How much farther forward is the CG on an electric Impact vs. a glow one? Howard isn't saying, but we had to look at his clay wart all Summer. I think he said that was 1 oz. I doubt that would make 1/8" change.
Ooops, had to go to the door. A big kid in a Clown Suit driving a Preus (sic), wanted imported chocolates. Scram, kid! H^^ Steve
My electric CG's are about an inch forward from IC. The LO's moved BACK an inch to and inch and a half from the IC location.
Clearly not fuel load CG shift.
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Ted:
Relative to how Tim flies: he flies Expert and is really quite good. And he keeps beating me at all of our local contests!
So, hmmmm, maybe you could help him get a little more roll on his maneuvers? Yeah, that would help a lot.... VD~
Point taken, Scott! Thanks.
Ted
p.s. How much would a little more roll be worth to you????
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Ted:
I need about 20 points worth of roll on Tim's plane. On one of my good days (and one of Tim's average days) that will make me competitive.
So, how much is that dollar-wise per point? Do you use PayPal or should I send a check? ;D
By the way, we do want Tim to figure out this problem so he'll finish the plane and post the pictures here. He is a top-notch builder and we have had him over to my shop a bunch of times to give tutorials on building/finishing to our club members.
Scott
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Thanks for the nice words Scott...but in all honesty Scott and I battle neck and neck all of the time.
I do fly Expert and although I have not been to the NATS as of yet, I tend to be in the top 5 at Brodak. I dont think my wife would support two weeks of vacation devoted to model airplanes and I really love the Brodak event. But I will be attending the NATS when Brodak ceases to see how I measure up against the best. I look forward to meeting you all.
Paul, Ted, OK, I will seal the hinge lines....move the leads back..and possibly add some vertical stab area....this had been another thought of mine. One other thing I have noticed that when the power stops and I glide in for landing the rocking can get much worse with less forward motion... Hmmmmm
And yes the Stab miss alignment...not sure how it happened....it could not have been a cocktail I may or may not have had that night ::).....hate to cut in and fix it....but that is why I have not applied any paint yet
Ty....Alabama is still the plan...just working through aging parents issues :-\
The site we fly can cause turbulence on days when the wind is coming from the west...but I am comparing the turbulence of this airplane verses others that dont have this problem....and most of that affects overhead stuff more than level flight.
I cant wait to get back out an try some of these fixes to see what works. thanks all
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Ted.....
Are there any obstacles around your flying site that may be causing even modest breezes to result in disturbed air; trees, bushes, parked cars, adoring crowds of observers, etc.
I, too, would be interested in the results of sealing the hingelines (in particular--given the roll issues--the flaps). If the gaps vary from one side to the others minor control inputs during level flight could cause an imbalance in lift change between the left and right wings resulting in small (as you suggest) rolling results.......
Heck, if the hinge lines aren't sealed just the airflow changes going around the circle could be enough to force slight roll oscillation. There might even be some unfortunate coupling between the roll of the plane and the line length and line tension. All the possible interactions all get less significant when you get the close to right line length, CG, and leadout position though.
Phil C
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OK,
Nothing like pulling up your dress and exposing your undies n~ n~
Pictures now attached that may show something to someone including:
hinge gap shot
tail misalignment
CG and lead-out position
Side and Top views
If you need something else ask I will try to capture.
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Outboard end of the stabilizer is down? I wasn't sure if you meant that, or hingeline skew. I would question whether the wing tip tab pushrod/horn/hinges are stiff enough...any slop there? I would consider adding a second pushrod/horn and loading the inevitable slop in the opposite directions, to lock it up solidly...maybe it's flexing or fluttering a little?
This wobble does strike a resemblance to "Dutch Roll", tho without any dihederal, I'd wonder how that could be. LE sweep would act similar, but can't see that being enough to cause Dutch Roll with almost any amount of fin area. But I'd also think about adding fin area, at least temporarily.
Is the fin LE rather blunt, by chance? I think a fairly sharp fin LE might be a good thing. I have Don McClave's old Skylark...it's a copy of an experimental variant that Ed Southwick had on his wall, but didn't like. It has a lower A/R wing...and it sucks. Lots of top fliers flew it when Don had it, and they did all sorts of trim changes. All said that it sucked. The only thing I can see that I really don't like is that the fin LE is very blunt, particularly at the top. As the model slows during tricks, that might make the fin (which is a lifting airfoil, per Nobler) stall. It's horrible late in the pattern, during the verticals and overhead stuff. But I never noticed the wing wobbling. Maybe it does, and I just never noticed. I didn't fly it very much, because it sucks, and I was flying Intermediate then. Maybe I should get it down and do something to the fin. Or shift the LO's back an inch and a half... D~ Steve
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Steve,
Yes you are correct the outboard stabilizer is down about 1/8 at tip...that is the error I need to fix...but I am not sure that is causing this issue....but needs to be fixed none the less.
The fin leading edge is nicely rounded expect at the base of the dorsal fin...it is a little blunt there.....easy enough to try that if other things that have been suggested dont work...I am thinking I will tape on some more fin area first.
The trim tab is pretty rigid however I tried to keep it neat and compact and it may not have enough leverage where it needs it to keep it steady at speeds...it is possible it could be fluctuating a bit in flight. And as i stated earlier in this thread it is adjusted down a little because I thought that may be the problem...I now understand I need to adjust it up possibly.
Thanks for your thoughts
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"Yes you are correct the outboard stabilizer is down about 1/8 at tip...that is the error I need to fix...but I am not sure that is causing this issue....but needs to be fixed none the less."
Commonly caused when adding the top or bottom blocks/shells out of the fuselage jig, after installing the wing and stabalizer...twisted fuselage. If you look closely at Paul's Impact drawing, he sheeted the top and bottom of the fuselage with cross-grain (i.e., spanwise) balsa to make a box first, then added the blocks/shells after. Also, I figure three tubes should be stiffer in torsion than one bigger tube. Howard will want the ciphers, of course. :## Steve
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Tim;
Even tho I am just a beginner in PA, my Excalibur 109 had the same problem. In my case I started with the leadouts aprox 1/4" front of CG
and my plane exhibited the same roll properties you are describing with your plane. I had forgotten about the CG being "different" with Elec. as oppsoed to IC power. I started moving the LO back and wound up with them (the center between the 2) at 3/8" to the rear of the CG on my plane. The rolling tendency stopped.
I tried a bit further back but the battery drain went UP, so put them back and all is well. Nice and level wings. ~>
But then again, that is on my plane.
NOW, as for Scott, This coming season I will be working with him on a few things HE needs to NOT DO. Being a past F3A Judge and International Judge when ever we were stationed over seas, I am going to apply that to Scotts flying and have him chomping at your heels,,,,,,,,,,,,, VD~ VD~
Carl
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Ted:
I need about 20 points worth of roll on Tim's plane. On one of my good days (and one of Tim's average days) that will make me competitive.
So, how much is that dollar-wise per point? Do you use PayPal or should I send a check? ;D
'snip'
Scott
Cash dolla's only...in unma'ked bills...under the thoid rock sout'west of da second rose shrub from da nort' end of da vics...er, uh...target's juke joint flow'a ga'den. My brudda, the local Consigliere, after due leverage, has detoimined dat a rate of $100.00 per point or a deceased Palimino's head from y'er herd is da going rate for such sensitive perfessional services. Please advise so dat da "contract" can be fulfilled.
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OK,
Nothing like pulling up your dress and exposing your undies n~ n~
Pictures now attached that may show something to someone including:
hinge gap shot
tail misalignment
CG and lead-out position
Side and Top views
If you need something else ask I will try to capture.
Tim,
After carefully studying the close up the cowling the source of your problem became obvious. There is no cooling inlet for the cylinder head so the crankcase is shaking its head in an attempt to cool the top end and the rocking mass in motion thus generated has been transmitted to the roll axis. Cool that puppy and the problem goes away.
You're welcome.
Ted
p.s. You didn't ask us to solve your directional problem but I did notice your prop was backward which is probably the cause of the canard-esque appearance of the ship in flight. No extra charge. ;) ;)
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Sorry. Boredom set in.
Ted
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So, Ted, if you are bored, where would the leadouts go if the plane did not have flaps? Same place or would it work out otherwise?
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Tim I have seen this in CG little to far back but iif this that is the case the planes usually don't lock in well. I have also seen this if the lead out holes are a little to big and I think the plane rockes around in the big holes. Tip weight can help this. Good luck my friend.
Joe Gilbert
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Solve the whole problem and go back to IC power. LL~ LL~ LL~ LL~
I've flown planes minus the rudder with no notice of change in the handling of the plane. It was the Thunderbird II. The rudder was taken off on too low of an outside loop. Of course I don't know or didn't know it wasn't suppose to fly with out the fin/rudder.
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Solve the whole problem and go back to IC power. LL~ LL~ LL~ LL~
I've flown planes minus the rudder with no notice of change in the handling of the plane. It was the Thunderbird II. The rudder was taken off on too low of an outside loop. Of course I don't know or didn't know it wasn't suppose to fly with out the fin/rudder.
In many cases that improves the performance because it had offset to begin with, and once it is gone, it's not there to screw up the trim.
You want A LOT of effective fin area to help keep the airplane flying straight ahead and stable. It's particularly important if you want to handle a lot of power, because there's little else you have to control the yaw axis, once you realize you can't (or don't want to) use the leadouts to control it. THAT is why my airplane looks like "3 billboards in formation" as Ted so delicately states it. But you can't have a lot of offset, or you had better be willing to fly 500 flights a week to learn to fly around the problems. Even on my airplane, even 1/32" of offset can completely screw up the trim. As can slight products of inertia (but that's a different thread...).
Note that a Rabe rudder would hypothetically allow you to handle extra power/prop diameter (in terms of yaw), too, but almost always cause far more problems than they solve. The rudder effect on something like a Bearcat (almost no stability from the fuse and a pretty high aspect ratio fin/rudder) is extremely strong and it takes only *tiny* amounts of movement to offset any reasonable precession.
But they almost always wind up with large motion and cause the problem they are intended to resolve. The idea is probably close to right, but in practice almost no one seems to be able to adjust them properly, and the typical linkage is such that you can't adjust it in small enough increments. Keith Trostle's linear cam method is one of the few I have seen work well enough to be a net benefit, but when Windy, et. al. tried a conventional mechanism he wound up with about 20x too much movement, and the airplane fishtailed all over the place in the maneuvers. I took one look at Windy's attempt to emulate the Impact and Trivial Pursuit (Testarossa, or "Impactarossa" as some people {such as myself} called it) and saw 15 degrees of offset on a huge rudder and knew not to worry any more.
Brett
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Brett:
So, when people fly a Genesis or one of Casale's "guppy planes" at VSC do you see trim problems? or not? They don't have much side area, but neither do they have a fin/rudder.
Would you build a Nobler with a symmetrical vertical stab?
Scott
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So, when people fly a Genesis or one of Casale's "guppy planes" at VSC do you see trim problems? or not? They don't have much side area, but neither do they have a fin/rudder.
No, and that is because they don't have any rudder offset to begin with, or at least it is very ineffective. Precession is more-or-less negligible with modern props so that was never a real problem as soon as we got good engines. In any case, compute the center of lateral area (crude approximation of the X CP position) on a Genesis and you will find it *very far aft* of the CG - which is what is needed for passive yaw stability. Compare that to a Bearcat, where the X CP is very close to the CG, thus providing minimal yaw stability - unless you do something else, like add an *active compensating component", AKA a Rabe Rudder. Almost as if some had thought this problem through...
Very few current competitive airplanes have Rabe Rudders and those that work the best have *very minimal* movement, indicating that it's a minor effect at most.
BTW, I don't need to go to VSC to see a Genesis fly, I flew one for several years myself, until various people around the midwest got tired of seeing it. It was my first modern airplane.
Would you build a Nobler with a symmetrical vertical stab?
Maybe or maybe not. If I was going to run it with a Fox, the detrimental effects of the very large amounts of rudder offset on a stock Nobler might be overcome by the fact that you needed more line tension than you can get when trimmed correctly. Which is of course why he made it that way in the first place. If you are going to try to fly <5 second or so laps with a Fox, you had darn well better figure out a way to manufacture line tension.
If I had an Aero-Tiger I would be inclined to straighten it out, but for fear of incurring the wrath of the rulebook lawyers I would probably make it stock but with an adjustable rudder section, then adjust it to remove the effect of the offset by moving the section to make it come out net neutral.
Brett
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So, Ted, if you are bored, where would the leadouts go if the plane did not have flaps? Same place or would it work out otherwise?
Hi Russell,
Yup, still bored.
Relative to the CG there isn't much difference whether a ship has flaps or not. Remember, the only reason the lines shouldn't exit "on" the CG is because of their aerodynamic drag which draws them "backward" from the direction of flight. Thus, if the leadouts are located at or forward of the CG the drag on the lines would attempt to yaw the airplane "into" the circle toward the pilot lessening line tension. We make them adjustable so as to be able to fine tune their exit point in order to allow the plane to track tangent...or a tiny bit "out"ward from the pilot.
The proper location for the leadouts relative to the CG (I always speak in terms of the center of the two leadouts at the tip...which should be in the neighborhood of a half inch apart or so) is a function of the plane's weight, it's speed, the length of the inboard wing and the diameter and length of the lines, that results in the plane flying tangent to the circle or very slightly nose out in calm air. For nominally normal "stunt" planes that's going to work out somewhere between, say, 3/4" to 1-1/4" or so. This assumes, of course, that the aircraft is properly trimmed, especially in the roll and yaw axes.
The primary difference between a design with or without flaps, by the way, is that the CG location as a % of the Mean (average is OK) Aerodynamic Chord must be further forward on an unflapped ship to retain some control feel feedback or "resistance to input" when maneuvering (details if you're interested). Flapped ships have significantly greater input feedback due to the additional pilot effort required to deflect the flaps as well as the elevators and, thus, are more amenable to CGs back in the 25%MAC area that is commonly discussed nowadays for "top gun" stunt ships with big tails...i.e., the Marilyn Monroe configuration.
I'll let P.W. discuss the whole weirdness of much further aft CGs improving the performance of stunt ships powered by hair dryers. None of it makes sense to me...other than the fact that it appears to work. I'd love to know why, too.
Ted
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I'll let P.W. discuss the whole weirdness of much further aft CGs improving the performance of stunt ships powered by hair dryers.
It's worse than that. The CGs are farther forward; the leadouts are farther back.
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Very few current competitive airplanes have Rabe Rudders and those that work the best have *very minimal* movement, indicating that it's a minor effect at most.
There is of course one very noticeable exception to this. One guess whose model it is. ;)
Somehow he seems to make it work with both a lot of movement and a lot of offset.
Pat MacKenzie
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There is of course one very noticeable exception to this. One guess whose model it is. ;)
Somehow he seems to make it work with both a lot of movement and a lot of offset.
I am not going to play "gotcha" games. Do whatever you want.
Brett
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Not playing a gotcha game.
Exception to the rule perhaps?
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the thing that gets ignored in all of these conversations ( well not by those who know) is that these things are a system, bolting on the magic widget wont fix anything even if it fixed it on bellcrank joes airplane. all this stuff has to work together,,
every change or affect , affects something else...
just because your airplane works great with a 12x6 apc prop doesnt mean mine will,,,
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Not playing a gotcha game.
Exception to the rule perhaps?
I'd like to see how much the rudder moves on that one at 10 degrees up and 10 down. What it does at 45 degrees doesn't matter, because you can't use all that during a normal corner.
Graphs have been made from in flight data before. I'd like to see one, with a second curve for the rudder travel. I'd bet that Igor probably has that info. H^^ Steve
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It's worse than that. The CGs are farther forward; the leadouts are farther back.
Oooops. As usual the eagle eyed Howard espied my errantly embarrassing typos; for which I thank him as it allows me to amend my remarks!
I, of course, meant greater "aft of the CG" location of the 'leadouts'." The more forward volts and amps CG I more or less understand inasmuch as the Harley vice Hair dryer crowd adds anywhere from six to eight oz of high octane propellant forward of the CG prior to launch thus approximating to some degree the dry weight divergence from the anti-greasers. It's somewhat embarrassing to admit that I've never checked the CG/leadout relationship on my ships with a full tank (Where's the red-faced "embarrassed" emoji, Sparky!). I'll have to do that if I ever get to a field and fly again.
Thinking on that subject suggests that the changing CG with IC stuff is the reason I've often felt the response rates on my ships were at their best around the start of the figure eights through the hourglass; slightly sluggish prior and modestly subject to tension issues afterwards. That would approximate a noticeable increase in leadout sweep at the response peak but I wouldn't think any where remotely close to three inches. There must be some other magic in the anode/cathode recipe.
Thanks again for outing me, Howie!
Ted
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If I may offer small conjecture here. I do not fly electric but have an experience that could in some way explain the CG issue. My airplanes use either the RO Jett .61 or .76. The .76 weighs 1 ounce more than the .61 but I can sense absolutely no CG trim differences by changing engines- but certainly power differences. Seems the .76 adds so much more air volume over the wing as to overcome the added nose weight, ie more lift on the same prop and roughly the same RPM.
My thought is this; IC is pulsed power, a burst every revolution with a dead spot in between. Electric is solid-state tug- no real pulses. Just a steady power stream and airflow over the wings which in the same way might give a different sense to the CG.......
Dave
The leadouts? Danged if I know but I'll think on it. I do remember reading quite some time ago about somebody ( thinking it may have been Bill Werwage). Discovered a second 'sweet spot' for his lead outs considerably farther back from the norm. Might not be an electric thing at all. Since I can't move my leadouts that far back without balsa surgery I doubt I'll be finding out.
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I'd like to see how much the rudder moves on that one at 10 degrees up and 10 down. What it does at 45 degrees doesn't matter, because you can't use all that during a normal corner.
Graphs have been made from in flight data before. I'd like to see one, with a second curve for the rudder travel. I'd bet that Igor probably has that info. H^^ Steve
What I think would be great theater would be to sit in the press box while Brett and Igor fly each other's airplanes a few times and then interview them afterward as to their impressions...positives and negatives. I've never flown Igor's obviously very capable machine (although I've marveled at the large deflections of his rudder) but have flown Brett's several times and been enormously impressed with its state of trim and uniform response to inputs. I'd pay to hear them discuss their impressions of each others' ships.
Just a quick comment on Steve's "never 45 degrees" comment above. I think it is mostly true with today's big tailed/aft CG configured stunters but must be qualified by the recognition that the amount of control deflection necessary for a given radius of pitch change is dependent on a number of factors but most especially on the CG location and the size and aspect ratio of the stab/elevator.
Embarrassing story on myself which I've told before but a long time ago.
At one of the very windy Lincoln, NE nats (1980s or so) during the Walker fly off my flights had required all of the control throw I could put into whatever '...tation" I was flying at the time not just to fly a nice corner but hanging it on the required line to avoid crashing (Then as now I always made ~40-45 degrees obtainable for the simple fact that if you ever do need it it's there, if not....).
Some sorta' new guy with the initials PW flew a bit later in just as much wind by measurement but flew a flight that made it look like the wind wasn't blowing at all. Only standing ovation I can remember for a stunt flight. Being nosy I casually went to the pits and pretended I picked up his airplane by mistake but was actually checking his CG which (at this pre-flashlight-fuel era) was probably an inch further aft than mine at the mid (quasi MAC) span. I was, at the time, using the CG David F's dad and I had divined and utilized as gospel after measuring dozens of published winning/noted designer's stunters on magazine article plans) all of which showed CGs right around 15% of the chord at the halfspan/MAC.
A dim light bulb flickered in my head.
Being slow but not totally stupid I later went home and started not only moving my ship's CG aft gradually but also spending a lot of spare time reading Martin Simon's great little book "Model Aircraft Aerodynamics" and paying attention to things such as CG relationships to the Neutral Point of the aircraft, tail volumes, airplane response to CG vice Neutral Point, pitching moments of cambered (flapped #^ #^) wings, etc. and came to believe that PW hadn't just lucked into the configuration via accidentally building an oversized and heavy tail but deciding to go ahead and use it just to see what happened.
I never, designed/built (other than classic ships) another airplane that wasn't nominally configured so as to be plenty stable at CGs right around the "Center of lift of the average wing chord; roughly the ubiquitous 25% MAC bandied about on stunt forums nowadays.
IOW, if the CG is too far forward you couldn't pull the nose up for takeoff with 45 degrees of deflection let alone fly an hourglass!
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Oooops. As usual the eagle eyed Howard espied my errantly embarrassing typos; for which I thank him as it allows me to amend my remarks!
I, of course, meant greater "aft of the CG" location of the 'leadouts'." The more forward volts and amps CG I more or less understand inasmuch as the Harley vice Hair dryer crowd adds anywhere from six to eight oz of high octane propellant forward of the CG prior to launch thus approximating to some degree the dry weight divergence from the anti-greasers. It's somewhat embarrassing to admit that I've never checked the CG/leadout relationship on my ships with a full tank (Where's the red-faced "embarrassed" emoji, Sparky!). I'll have to do that if I ever get to a field and fly again.
Thinking on that subject suggests that the changing CG with IC stuff is the reason I've often felt the response rates on my ships were at their best around the start of the figure eights through the hourglass; slightly sluggish prior and modestly subject to tension issues afterwards. That would approximate a noticeable increase in leadout sweep at the response peak but I wouldn't think any where remotely close to three inches. There must be some other magic in the anode/cathode recipe.
Thanks again for outing me, Howie!
I wasn't picking on a typo. The CG is about 1" farther forward on my electric Impact than on my last IC Impact, and the leadouts are 7/8" farther aft (relative to the wing TE) on my electric Impact than on my last IC Impact.
These discussions always cause somebody to point out (never more than qualitatively) that fuel has an effect on IC-powered airplanes' CGs. Yes, we have taken that into consideration. On my plane, half a tank of fuel moved the CG forward .42" and up .01".
It's a mystery. I have a couple of hypotheses as to why the leadouts need to go back so far on electrics, but they are too bizarre to admit in public.
(Edited to split a poorly written paragraph in twain.)
Edited again to apologize to PW for not noticing that he said almost the same thing above
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Seems the .76 adds so much more air volume over the wing as to overcome the added nose weight, ie more lift on the same prop and roughly the same RPM.
How the heck does it seem that?
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Back to Tim's original issue (remember that?), I am hesitant to add anything to the advice you've gotten from the West Coast Varsity, but I will, of course.
Looking at the picture, I see what appears to be an adjustable tab outboard of a flap. If the moveable part of the flap aligns with the adjacent fixed part on one tip at a different flap deflection than it does on the other tip, I would expect a little roll or yaw moment increment whenever the moveable part moves past the fixed part on either wing. A quick way to tell if that's the culprit would be to put a fence between the fixed and moveable parts to remove the discontinuity. Lots of people have such a flap configuration without a problem, but I've seen trouble from it on a former local yokel's airplane.
Edited to add a link to show how out-of-date I am: http://grammarist.com/spelling/movable-moveable/
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...it could be simply a design issue with the wing tips or too aggressive tip airfoil compared to the root making the tips sensitive to turbulence.
I doubt it. Despite modeling myth, airfoils have the same lift curve slope at low angles of attack.
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I wasn't picking on a typo. The CG is about 1" farther forward on my electric Impact than on my last IC Impact, and the leadouts are 7/8" farther aft (relative to the wing TE) on my electric Impact than on my last IC Impact.
These discussions always cause somebody to point out (never more than qualitatively) that fuel has an effect on IC-powered airplanes' CGs. Yes, we have taken that into consideration. On my plane, half a tank of fuel moved the CG forward .42" and up .01".
It's a mystery. I have a couple of hypotheses as to why the leadouts need to go back so far on electrics, but they are too bizarre to admit in public.
(Edited to split a poorly written paragraph in twain.)
Edited again to apologize to PW for not noticing that he said almost the same thing above
Howard;
Just curious...........Are you running CW or CCW propeller rotation? If you've flown both, did it have any significant influence on the CG or LO locations?
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How the heck does it seem that?
Well I can take one of my airplanes in good trim with a .61 in it. I can add 1 ounce- or even 1/2 ounce and can feel the airplane getting nose heavy and sluggish to turn. I then can remove the .61 and the added weight, drop the .76 in and the airplane responds just like the .61 without added weight- just a noticeable addition of power in maneuvers. That's been pretty cut and dried for me.
The other issue I've convinced myself of the same way- trial and error with way too many design configurations over the years. If you want the most lift and tightest turn then you start shoving that tip high point forward and holding thickness toward the tips. The down side to that is the rough ride you get in anything but perfect air. However if you pull the high point back and taper your percentage the airplane is overall smoother and gets through rough air better though sacrificing pure lift and turn. Like most everything a compromise is likely better.
Dave
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Just curious...........Are you running CW or CCW propeller rotation? If you've flown both, did it have any significant influence on the CG or LO locations?
CW now (looking from the back). I regret that I forgot the effect of prop direction on CG and leadout position. I'll have to look at the notes. Whenever I'd switch, the whole thing had to be retrimmed, so it was probably awhile before I got to CG.
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I always find it interesting how the topic of the post changes as the chain of responses grows in number. Kind of the similar to the old test of whispering a short story to the person next to you in a circle of people and as the story comes back to you it is not even close to what was started.
Any whoo....all of this discussion is much appreciated, I have many ideas of things to look at the next time I can get to the field...if the weather and my job every cooperate.... something to be said about retiring y1 y1 ahhhh some day.
The most interesting thread out of this discussion seems to revolve around CG and leadout position...I cant wait to try out these theories. I guess the biggest thing that is puzzling and also probably what interests me the most about control line is how very different 2 models can be trimmed to find the best balance...there is definitely no one size fits all...... just when you think you have things figured out HB~> HB~>
I want to thank everyone for their comments. I will post results to some of these ideas as soon as I can fly the darn thing again....this weekend does not look good at the moment so it may take a couple of weeks to let you know what I find.
Once I get this issue tamed I have to reinstall the wheel pants which I took off thinking some misalignment with them...or aerodynamic force caused by them was my problem......
Super Chipmunks need to have wheel pants to look the part....but they are a pain ~^
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Not playing a gotcha game.
Exception to the rule perhaps?
What if THAT is the rule and not the exception? VD~
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I doubt it. Despite modeling myth, airfoils have the same lift curve slope at low angles of attack.
Only for wings of infinite span. Low aspect ratio wings have a smaller lift curve slope; I suspect that for any wing that doesn't have a perfect Prandle planform the lift curve slope at the tips is lower than at the root -- it pretty much has to be for the whole planform vs. aspect-ratio thing to work. It would follow, then, that a tip that effectively suppresses vortexes would be more effective, i.e. it would have a larger effective chord, which would mean it would have a lift curve slope that's closer to that of a wing of infinite span.
It may also be that a tip that has a sharp edge responds differently to air that's not perfectly normal to the span than a rounded tip, which would translate into a different response to gusts.
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I'd like to see how much the rudder moves on that one at 10 degrees up and 10 down. What it does at 45 degrees doesn't matter, because you can't use all that during a normal corner.
Graphs have been made from in flight data before. I'd like to see one, with a second curve for the rudder travel. I'd bet that Igor probably has that info. H^^ Steve
That rudder is at 20 degrees in neutral, and goes 20 degrees right and 15 degrees in (from neutral).
However those +/- numbers do not mean too much since you do not know size of rudder, prop gyro moment etc.
And controls travel is limited to 27 degreees. So it is measured at its max deflection.
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What I think would be great theater would be to sit in the press box while Brett and Igor fly each other's airplanes a few times and then interview them afterward as to their impressions...positives and negatives. I've never flown Igor's obviously very capable machine (although I've marveled at the large deflections of his rudder) but have flown Brett's several times and been enormously impressed with its state of trim and uniform response to inputs. I'd pay to hear them discuss their impressions of each others' ships.
I never flew Bretts model, but I think I can imagine, as I also had IC models with such configuration. Model can be very well trimmable with minimal LO offset as well with extreme offset (I know you will not agree and you will and you also did write theory against, but it simply so as we can see :- ))) ). So I belelieve his or your or any other top pilot model will fly well without troubles. The point of that LO vs rudder offset is that electric models ALLOW do tricks which you cannot do with IC because the fuel delivery will be so corrupted that you will say that it is simply wrong, however they work well on electric :- ))) ... they show advantage in extreme conditions, so pilot will probbaly not see it in first flight, may be he will see them later, and may not at all, but he will see it clearly when he goes back to old model :- ))
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What I think would be great theater would be to sit in the press box while Brett and Igor fly each other's airplanes a few times and then interview them afterward as to their impressions...positives and negatives. I've never flown Igor's obviously very capable machine (although I've marveled at the large deflections of his rudder) but have flown Brett's several times and been enormously impressed with its state of trim and uniform response to inputs. I'd pay to hear them discuss their impressions of each others' ships.
Yes, but having flown dozens of airplanes with similar trim, I can make a pretty good guess. It's very close to how the Impactarossa was set, which had some different parameters, but not different enough to matter to this question. The biggest difference is probably that the Max Bee has very little passive yaw stability for the same reasons I mentioned above, and, the fin/fuselage/rudder combination has an unusually low aspect ratio even by "59 Caddy Tailfin" standards, so it takes a lot of offset to get it to do anything. Unlike, say, my airplane, where 1/32" of rudder deflection has DRASTIC ill effects.
Igor is trimming his airplane to not fly tangent to the circle. If you want that, then do it his way. I would very strongly suggest that if you want to do it that way, you make some provision for rotating the principle inertial axes of the airplane to match the yaw angle. The fact that there are maybe 10 people reading this that know what that means, and maybe only 3 people have actually tried experimenting with it, indicates what I think the issue is going to be. My experience has been that virtually no one can manage to make it work Igor's way, and those who tried ended up in the 10-gallon-a-month club, but it's no skin off my nose.
If you want to fly tangent to the circle, then do it my way. You certainly DO NOT NEED to manufacture additional line tension with any modern power system and otherwise reasonable tail volume/CG/flap effectiveness. But it's not one of those "little from column a, a little from column b" issues, the concepts are utterly incompatible.
Brett
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The biggest difference is probably that the Max Bee has very little passive yaw stabilit.
Exactly, that is the point :- )) that is how was fuselage designed :- )) ... and why it needs Rabe rudder ... and also why its deflection needs to be almost symmetrical right and left.
However I do not think it is difficult to trim ... especially if you ignore (or better if you never learned) some rules which does not apply in this case. There are many people who got to very good trim also without me.
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That rudder is at 20 degrees in neutral, and goes 20 degrees right and 15 degrees in (from neutral).
However those +/- numbers do not mean too much since you do not know size of rudder, prop gyro moment etc.
And controls travel is limited to 27 degrees. So it is measured at its max deflection.
Just to be clear :), does that mean the deflection varies from 5 degree right at full up elevator, to 20 degrees right at neutral, to 40 degrees right at full down?
Thanks,
Pat MacKenzie
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It's a mystery. I have a couple of hypotheses as to why the leadouts need to go back so far on electrics, but they are too bizarre to admit in public.
I would love to ear your (or any ones) theories, no matter how bizarre.
There clearly must be something to it, just based on what is in this thread. It does seem like rather an extreme change. (Both the CG shift and the lead out movement)
Could it be that Igor's big rudder offset and this big change in leadout position are just two different ways of getting to the same point?
Pat MacKenzie
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I would love to ear your (or any ones) theories, no matter how bizarre.
There clearly must be something to it, just based on what is in this thread. It does seem like rather an extreme change. (Both the CG shift and the lead out movement)
It may be easier to understand from a "electric lets you get away with..." perspective rather than "electric requires you to..." but I certainly can't see the mechanism or why it might be different.
Brett
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Here is a picture of Igor's Max Bee which shows the deflection of the Rabe-rudder.
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Just to be clear :), does that mean the deflection varies from 5 degree right at full up elevator, to 20 degrees right at neutral, to 40 degrees right at full down?
Yes, 40 at full elevator down.
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I would love to ear your (or any ones) theories, no matter how bizarre.
I do not think it is too bizarre and I wrote it few times, firs time on Stuka, but last time I tried to find it I failed, so I am not going search again, I think it was deleted at some database accident or what. And I am also not so good in text visualizations like Ted ... but I will try simple question:
You have 2 identical models. One is trimmed yawed out and one is yawed in. You fly figures on downwind side of circle and strong wind is blowing from your back. Model has some fuselage area and that area acts like a turbine blade. What it does with first model, what second and which one you will fly rather? 8)
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Only for wings of infinite span. Low aspect ratio wings have a smaller lift curve slope; I suspect that for any wing that doesn't have a perfect Prandle planform the lift curve slope at the tips is lower than at the root -- it pretty much has to be for the whole planform vs. aspect-ratio thing to work.
That is a wing planform effect. Lift curve slopes of different symmetrical airfoils (2D wing cross sections) are the same near zero angle of attack (or zero lift), and I thought Tim S.'s problem was happening in level flight, in which case the wing would be at near zero lift, hence no airfoil would be more "aggressive" than another. I read Tim's original post again. He didn't specify level flight. Maybe you could pick a tip airfoil that operates on a part of the lift curve where lift is petering out while the root airfoil is still in the linear range. Maybe that's what Dave was writing about. I started writing this paragraph to say that Dave was full of beans, but now I think he might have a good idea. I still think he's full of beans about a prop turning the same RPM moving more air with one engine than with another.
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That is a wing planform effect. Lift curve slopes of different symmetrical airfoils (2D wing cross sections) are the same near zero angle of attack (or zero lift), and I thought Tim S.'s problem was happening in level flight ...
I thought you were commenting about Dave's comments about wind gusts. We're having too much fun with this thread.
I suspect that it's either yaw or your flap-weirdness thing. Our toy airplanes do have an under-damped roll response (I've got data captures to show it!), but it must be easy to keep from exciting it or every plane we flew would be rocking and rolling all over the circle.
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I do not think it is too bizarre and I wrote it few times, firs time on Stuka, but last time I tried to find it I failed, so I am not going search again, I think it was deleted at some database accident or what. And I am also not so good in text visualizations like Ted ... but I will try simple question:
You have 2 identical models. One is trimmed yawed out and one is yawed in. You fly figures on downwind side of circle and strong wind is blowing from your back. Model has some fuselage area and that area acts like a turbine blade. What it does with first model, what second and which one you will fly rather? 8)
Is this your info on L.O position/yaw that you were referring to?
Posted on SSW By Igor Berger. A rather interesting approach and runs contrary to some people's opinion.
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=130453&mesg_id=130453&listing_type=search
Yaw and leadouts on pictures after all"
Fri Dec-03-04 03:55 AM
I see my texts are not very clear, so I did some pictures. Hope this will show what I mean. I do not know if my ability to make self explaining pictures is better than my English, but I hope all together will be enough to understand what I mean after all.
So first of all usual understanding what is happening here.
I assume:
1/ Model is tangent to the circle
2/ fuselage & engine has no side aerodynamic forces
3/ lines are outside of wing
4/ bellcrank (BC) is at (CG)
Here is Ted’s visualization from "Bellcranks and CGs redux":
We have dragy lines and moving mass point CG at end of them. Centrifugal force of CG makes tension in lines, so they tend to be straight, but drag makes its usual curvature.
So let’s call the centripetal force Fc. The line drag of lines Fd. It is clear, that stable situation is, if the angle makes another force Fy which is in size equivalent to Fd but with opposite orientation. All is on pic1.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44601;image)
Static (flat) theory tells us where to put lines in leadouts to keep tangent position of fuselage.
So we know the shape and we can convert the CG point to real model and fix lines on place, which will match actual position of lines. Nothing happens. Line drag is counterbalanced by centrifugal force of CG to LO position and forces are balanced. Pic2.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44587;image)
Now assume that we have the same model and we fly overhead. The shape of lines curve is different, because the force FC is less gravity FG. The drag is the same. It means lines are more round. Pic3.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44589;image)
Therefore if LO is in wing fixed on the same place, the nose will yaw inwards. That is happening because CG position is not aligned with LO position and therefore any change in that force leads to yawing. Pic4.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44591;image)
Such a model is not flyable, so “flat” theory cannot work for aerobatic model. … at least at those conditions over.
Now another try.
Assume that we have little bit functioning rudder and it makes constant force Fr on tail. Tail is of the same length as wing. So that force is permanently yawing nose out and thus inboard tip forward. Just opposite than the line drag Fd is. Pic5.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44593;image)
It means that it is the force, which counterbalances line drag instead of CG position. If we want reach no friction in LO, we must put BC far forward, but we know that BC position has no effect to yaw and thus we can live it in CG.
Both line drag and also force on rudder are aerodynamic forces and every change in speed has proportional effect to both of them, so they are in balance at every speed. It means that CG can stay aligned with LO not making any yaw.
As the CG is aligned with LO and not making yaw, then also variation in line tension does not make any VARIATION in that nonexistent yaw. Aerodynamic forces are still in balance, thus also if curvature of lines is different, the resulting stable orientation of fuselage still tangent. Pic6.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44595;image)
We can fly overhead or strongly pull handle and model will still keep its angle.
It is not only CG or (exclusively) only rudder what can balance the line drag. They can work together. Assume the rudder is little smaller and its effect is too small for line drag. Its force is not enough, model tends to yaw in, but we can put lines little back and give CG chance to balance the rest. No problem, but it will make lower line tension overhead.
Opposite situation – if rudder is stronger than necessary, it will lead to opposite situation. We will move leadouts FRONT, CG will fall aft of LO thus not allow outboard yaw caused by excessive rudder force (Fr>Fd) and we are still at tangent position. But lack of line tension will point nose OUT … Dick, are you watching? No gismo, no screws, no tricks, just simply proper design/trim. It means LO moved forward will improve line tension overhead – sounds familiar?
I am not calling for any change. We are able trim models and they fly well. I am only explaining what is happening here. So if we use calculation in hope that “flat” theory is proper and works also for our stunt models, then we simply get situation on pic 2. But we fly on circular path and that makes forces permanently yawing out. The CG can in that case fall to in-flight level of LO, or front of or aft of LO. That situation is on Pic 7.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44597;image)
So the rudder, LO position can very effectively place CG on proper place making that proper response not allowing too much yaw, but also keeping good tension overhead. Pic 8 shows detailed configuration. Fvr is variation of line tension and it gives idea what is its effect on yaw.
(http://stunthanger.com/smf/index.php?action=dlattach;topic=13003.0;attach=44599;image)
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YES Brent!!! That is. I see it is still on stuka, but pictures are lost.
Well I see it is already 12 years :- ))
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Are you now going to show the difference between IC and electric?
Static solutions are nice, but don't explain the difference.
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Are you now going to show the difference between IC and electric?
Static solutions are nice, but don't explain the difference.
I have no idea why the CG needs to be (or can be) different. I also only have weak arguments for the aft leadout position (although I think most people are not actually running a static yaw offset). But I am concerned with the rest of the theory and I have seen nothing since to change my mind, far from it. The original was in 2004 on SSW, as I recall.
The point I made before and still see is that there are a few things missing from Igor's illustrations once you try to turn a corner. Certainly they are correct as far as they go, but the kinematic and dynamic effects are missing.
Take the last drawing, where rudder offset is balanced with the leadouts. I think we all generally agree that you have to have that, regardless of what yaw angle you want to run. If you have rudder offset, you have to move the leadouts back to compensate or you end up with wild line whip reactions when you start to maneuver.
To turn without incurring any sort of disturbance, the airplane has to rotate around the vector Fc (the radial vector, from the pilot to the CG). The problem is, unless you do something to force it, *it can't and won't rotate around that axis* because they aren't the principle axes. Barring other inputs, it wants to rotate around the Y axis of the airplane, and that isn't aligned with the radial vector. If the airplane rotates around its own Y axis by 90 degrees, it winds up vertical with a large roll angle, effectively appearing to have inadequate tip weight. The usual solution is to add tip weight to keep it from hinging. But the effect of tip weight varies depending on the pitch acceleration, so you can't have it balance at all rotation rates (including zero). This leads to what I previously called "Twister Disease". No one appears to have been able to resolve this satisfactorily that I have seen over *many* examples.
A possible solution would be to skew the principle axes with respect to the airplane, so the momental Y axis lines up with the radial vector at whatever yaw angle you want to run. You can skew the principle axes around the yaw axis by adding mass at the inboard tip TE and the outboard tip LE, and it has certainly been done. The problem is that to get any consequential shift, you have to use a lot of mass and effectively that means you can only make small adjustments rather than wholesale changes. And, in any case, if you were to successfully get the principle axes shifted around, now you get huge aerodynamic torques because, for example, the lift from the tail is centered in the wrong place, there's a huge dihedral effect, and a host of other issues.
The alternative is to create some torques that oppose all this. I can easily believe that the moving rudder can compensate for a torque that appears as a function of the pitch rate as was intended. Precession is certainly that way, but the kinematic effect is completely independent of the rate and cannot be corrected by moving the rudder in the direction of the elevator. You might be able to move the leadouts off the natural "trail" position and use the dihedral coupling roll and yaw to induce a yaw angle that then rolls the airplane as required, but that also requires that you put force on the lines that almost guarantees that the line whip will get excited.
This was all hashed out long before in various SSW posts. The method I recommend for adjusting the rudder and leadouts will align the geometric axes of the airplane to that Y lines up with the radial vector and remove most of the roll.yaw dihedral effects. Building the airplane with matched parts from side to side will get the inertial Y axis lined up pretty close to the geometric Y axis. I think it's close enough that small dynamic balance effects and precession are both in the negligible range most of the time, or at least close enough that other effects like details of workmanship will swamp the effect. If you have enough passive yaw stability and don't use excessively large props, and don't build too light (which increases the line tension stabilizing forces without exacerbating the other issues), it's below the threshold of caring about it.
I have seen next to no one able to trim with a significant static yaw angle successfully. The few exceptions are those case (like the Firecracker) where there is very little passive yaw stability so that slewing around in yaw doesn't encounter any significant impediment - not that it doesn't yaw, it just doesn't care. Otherwise you get wild fishtailing in the maneuvers. The worst example I have ever seen in an otherwise competently constructed airplane was a 54-ounce, PA65 airplane with a 3-blade Bolly 13-4 or so and a very large fin with lots of offset.
big fin = lots of passive yaw stability/authority
offset fin = lots of aerodynamic yaw torque
huge very heavy prop = lots of precession and other aerodynamic destabilizing force
abnormally light weight = reduced stability provided by the lines and in conjunction with very heavy .018 lines, huge influence of line whip effects
This airplane was in an almost continual pretty high frequency roll/yaw oscillation that only damped out in the two laps between maneuvers in perfect air. Anything else, and it got upset, and stayed upset with very light damping. It would make about 3 swings in the space of one leg of a square loop, for example.
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Take the last drawing, where rudder offset is balanced with the leadouts. I think we all generally agree that you have to have that, regardless of what yaw angle you want to run. If you have rudder offset, you have to move the leadouts back to compensate or you end up with wild line whip reactions when you start to maneuver.
To turn without incurring any sort of disturbance, the airplane has to rotate around the vector Fc (the radial vector, from the pilot to the CG). The problem is, unless you do something to force it, *it can't and won't rotate around that axis* because they aren't the principle axes. Barring other inputs, it wants to rotate around the Y axis of the airplane, and that isn't aligned with the radial vector. If the airplane rotates around its own Y axis by 90 degrees, it winds up vertical with a large roll angle, effectively appearing to have inadequate tip weight. The usual solution is to add tip weight to keep it from hinging. But the effect of tip weight varies depending on the pitch acceleration, so you can't have it balance at all rotation rates (including zero). This leads to what I previously called "Twister Disease". No one appears to have been able to resolve this satisfactorily that I have seen over *many* examples.
Brett, I would say you have too pessimistic approach to the point. I think it is better to go from opposite side, we can simply presume it DOES make clean turn (because I see it clean, no ill yaws, rolls etc). So now questions is how it is possible.
There 2 independent problems:
1/ Permanently yawed model has misaligned aerodynamic center (AC) of tail and AC of wing like you wrote. That configuration will clearly need tip weight to solve the rolling problem. And it is also clear that that tip weight will need to be different in tight turn and different in large radius and level flight. So far all my models needed LESS tip weight in tight turn and more in loops. It is very clearly visible in overdone (too tight) corners when outer wing rolls out. My solution for that problem is trim tab on right flap. Its size has smaller percentual effect in small deflection and stronger at large deflections - just because of flap to wing ratio. Well, it is not "mine" solution, it is known trick to use larger flap on righ wing, mine is only variable size for efective trimming. So far I always solved that problem to extend that even in tightest corner I did not see any roll and model has still plenty of line tension for example in eight overhead in wind when the wind tends to roll model inwards.
2/ There is also another, not so obviouse problem which you did not mention, it is fact that yawed model is not dynamically balanced. Every wing (left and right) has its CG, left wing has CG front of main GC and right wing aft of CG. If model rotates, centrifugal force will tend to force them out of rotating axle, means to yaw the model out. We have also fuselage, front of fuselage has CG right of the model CG and tail has CG left of the model CG. So it will tend it yaw in. But since fuselage has heavy battery balanced by long tail, it has likely stronger effect compared to wing, so model will tend yaw in in both positive and also negative turns. Looks like we need rudder which deflects right in every turn to keep constant yaw. We have also precession which is the same (in absolute value) in positive and negative turns, but with opposite direction - means we need rudder in in positive and the same amount out in negative turns. So if I sum both effects, I will need little more rudder out in negative turns and less in on positive. That probably answers why I need 20 deg out and only 15 deg in.
Yes I agree, it looks complicated, but we know and we use those 2 tricks for years and both work. May be not everyone can set it for 100%, but small roll or yaw is still better than if model yaws in during flight overhead - and thanks to that yaw still makes mentioned problems :- )))
And I do not think my model is trimmed different way than your, I bet your model also flies yawed, and I bet it is also trimmed the same way, just much less yawed then mine :- ))
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All right. I think everybody on Stunthangar should throw in a couple of C-notes to buy Igor a ticket to fly to San Francisco with his airplane. We can arrange accommodations at the David Fitz' Bed and Breakfast and then hold the Igor/Brett airplane swap showdown at David's OK Corral--oops--Napa flying site (we can all go wine tasting afterward...or maybe Igor and Brett should go before flying to impart a touch of daring-do to the event). We'll give each guy six flights on the other guy's ship and then hold a contest with a couple hundred of Sparky's closest stunt buddies all judging to determine...I'm not sure what... Never mind, they'll both fib and say they're more convinced than ever that their own is clearly superior and we'll have solved no secrets whatsoever.
Frankly, after reading all of their back and forth I'm afraid to fly my own stuff lest they prove themselves un-airworthy...I'm too old to build any replacements.
Ted
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Brett, I would say you have too pessimistic approach to the point. I think it is better to go from opposite side, we can simply presume it DOES make clean turn (because I see it clean, no ill yaws, rolls etc). So now questions is how it is possible.
<<snip>>
2/ There is also another, not so obviouse problem which you did not mention, it is fact that yawed model is not dynamically balanced. Every wing (left and right) has its CG, left wing has CG front of main GC and right wing aft of CG. If model rotates, centrifugal force will tend to force them out of rotating axle, means to yaw the model out. We have also fuselage, front of fuselage has CG right of the model CG and tail has CG left of the model CG. So it will tend it yaw in. But since fuselage has heavy battery balanced by long tail, it has likely stronger effect compared to wing, so model will tend yaw in in both positive and also negative turns. Looks like we need rudder which deflects right in every turn to keep constant yaw. We have also precession which is the same (in absolute value) in positive and negative turns, but with opposite direction - means we need rudder in in positive and the same amount out in negative turns. So if I sum both effects, I will need little more rudder out in negative turns and less in on positive. That probably answers why I need 20 deg out and only 15 deg in.
I did mention that, it's the same as the dynamic imbalance/product of inertia issue.
The reason it moves outboard more than inboard is that moving it outboard increases the line tension (over what it would have had otherwise) so the restoring force from the leadouts is greater. Move it inboard and it removes line tension, meaning you can tolerate less of it because the restoring force is reduced. Same with the original Rabe rudder (which was intended to solve a different problem).
And I do not think my model is trimmed different way than your, I bet your model also flies yawed, and I bet it is also trimmed the same way, just much less yawed then mine
Maybe. I can bound the maximum static yaw angle to less than 1/4 degree since the 1/8" inboard landing gear hides the outboard landing gear in dead air with no other disturbances.
I understand what you are trying to do, the explanation from 2004 was clear enough. If you fly with a static yaw angle it creates a bunch of axial coupling from a variety of effects (products of inertia, aerodynamic, etc), and you are using the rudder and tipweight adjustments, and a movable rudder to create torque to counteract them.
All I am saying is that of the hundreds and hundreds of people who have tried this, I haven't seen anyone one of them manage it particularly well, and they ended up flying hundreds of flights a month to try to work around it by trying harder.
I can see that now I am going to have to start carrying my Zona saw around to cut people's rudders to remove the offset, when everybody gets the great idea to try to fly sideways again.
I don't claim anything with regard to understanding the forward CG or aft leadouts for electric (or 4-stroke) and I am more-or-less taking Paul's word for it. Even though my engine can be set to run on either side of any electric system (more steady than a feed-back electric system and more punchy than a governor system). His airplane doesn't appear to fly yawed out, at least not that I can tell.
Brett
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All right. I think everybody on Stunthangar should throw in a couple of C-notes to buy Igor a ticket to fly to San Francisco with his airplane. We can arrange accommodations at the David Fitz' Bed and Breakfast and then hold the Igor/Brett airplane swap showdown at David's OK Corral--oops--Napa flying site (we can all go wine tasting afterward...or maybe Igor and Brett should go before flying to impart a touch of daring-do to the event). We'll give each guy six flights on the other guy's ship and then hold a contest with a couple hundred of Sparky's closest stunt buddies all judging to determine...I'm not sure what... Never mind, they'll both fib and say they're more convinced than ever that their own is clearly superior and we'll have solved no secrets whatsoever.
Frankly, after reading all of their back and forth I'm afraid to fly my own stuff lest they prove themselves un-airworthy...I'm too old to build any replacements.
Ted
AAwwwww, Ted....You can't bring common sense into a discussion like this! Especially since it's reasonable given past contest results, to assume, that both or either approach can work.
One thing that I do wonder about is that it seems that this entire discussion ignores the action of the flyer and his handle movement to follow, lead, or trail the aircraft. My opinion is that this is a very important part of what actually happens regarding line tension and yaw or lack thereof! Probably very difficult to factor that into the diagrams but I believe it is a very real input and likely the primary reason for different reaction to different setups!
No...somebody prove me wrong. It really seems to be the only thing that explains the "WHY". I don't think any of the top fliers do so with the handle in a static position as the diagrams or equations always seem to show. I believe it's actually a much more complicated dynamic situation, than described by anything so far!
Simple body, handle "English" to overcome forces that seemingly cannot be statically or dynamically balanced by straight forward trim methods because they are constantly changed by influencing dynamic and aerodynamic forces that do not balance.
Randy Cuberly
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All right. I think everybody on Stunthangar should throw in a couple of C-notes to buy Igor a ticket to fly to San Francisco with his airplane.
Curiously enough, Igor goes to Silicon Gulch to visit his kid. Last time he went, he hand-delivered some stuff to Jimby.
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One thing that I do wonder about is that it seems that this entire discussion ignores the action of the flyer and his handle movement to follow, lead, or trail the aircraft. My opinion is that this is a very important part of what actually happens regarding line tension and yaw or lack thereof! Probably very difficult to factor that into the diagrams but I believe it is a very real input and likely the primary reason for different reaction to different setups!
No...somebody prove me wrong. It really seems to be the only thing that explains the "WHY". I don't think any of the top fliers do so with the handle in a static position as the diagrams or equations always seem to show. I believe it's actually a much more complicated dynamic situation, than described by anything so far!
Simple body, handle "English" to overcome forces that seemingly cannot be statically or dynamically balanced by straight forward trim methods because they are constantly changed by influencing dynamic and aerodynamic forces that do not balance.
That's an interesting thought. I find myself doing peculiar hand jive, like on the last corner of triangles. When that happens, I figure I'm compensating for some mistrim, but I'll watch the Varsity more carefully.
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Curiously enough, Igor goes to Silicon Gulch to visit his kid. Last time he went, he hand-delivered some stuff to Jimby.
:- ))) ... I was twice here this year. Once in February (and that is one of reasons why I traveled to Perth with 2 weeks old model and I trimmed it there every evening :-P ) and second time in August, but I must say that there is lot of other fun than flying models :- ))
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The reason it moves outboard more than inboard is that moving it outboard increases the line tension (over what it would have had otherwise) so the restoring force from the leadouts is greater. Move it inboard and it removes line tension, meaning you can tolerate less of it because the restoring force is reduced. Same with the original Rabe rudder (which was intended to solve a different problem).
That is right only in case that rudder does not solve all movements, if you again try that "optimistic" approach, that rudder keeps in turns that orriginal static yaw and model does not hinge right left in turns, then gyro moment, which is on both sides the same in absolute value, will need also +/- the same moment of rudder and thus +/- the same deflection :- )) Yes yes, I know it brings another problems, but they are also solvable :- ))
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:- ))) ... I was twice here this year. Once in February (and that is one of reasons why I traveled to Perth with 2 weeks old model and I trimmed it there every evening :-P ) and second time in August, but I must say that there is lot of other fun than flying models :- ))
I hear you learned something important about Bay Area traffic around 5 in the afternoon, too! Specifically, don't plan on getting anywhere in any reasonable amount of time.
Brett
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All right. I think everybody on Stunthangar should throw in a couple of C-notes to buy Igor a ticket to fly to San Francisco with his airplane. We can arrange accommodations at the David Fitz' Bed and Breakfast and then hold the Igor/Brett airplane swap showdown at David's OK Corral--oops--Napa flying site (we can all go wine tasting afterward...or maybe Igor and Brett should go before flying to impart a touch of daring-do to the event). We'll give each guy six flights on the other guy's ship and then hold a contest with a couple hundred of Sparky's closest stunt buddies all judging to determine...I'm not sure what... Never mind, they'll both fib and say they're more convinced than ever that their own is clearly superior and we'll have solved no secrets whatsoever.
There are no secrets. The kinematic effect described above, for example, *is not a matter of opinion* and has been mathematically perfectly well-understood since the early 1800's. I think Igor and I understand what he is attempting pretty well, since it's a technical matter that us not subject to debate. The only opinion involved is how well you can resolve the well-understood static/dynamic/kinematic using the available trim features. Having seen many people try to do what he is doing, I am skeptical, but...
after all this time of both helping other people trim, evaluating their airplanes, and advising them, I think I am capable of making my points without fabricating data or failing to be objective about the results.
Frankly, after reading all of their back and forth I'm afraid to fly my own stuff lest they prove themselves un-airworthy...I'm too old to build any replacements.
I gave my opinion on how to proceed for the original posited problem about 30 responses ago. No one has to follow it. If the rest of it is of no interest or a pointless debate, no one is compelled to read it. People have done this by-guess-by-golly trial and error with some success for 60+ years, no one is required to understand it to brute-force their way to a solution.
Brett
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I wasn't picking on a typo. The CG is about 1" farther forward on my electric Impact than on my last IC Impact, and the leadouts are 7/8" farther aft (relative to the wing TE) on my electric Impact than on my last IC Impact.
These discussions always cause somebody to point out (never more than qualitatively) that fuel has an effect on IC-powered airplanes' CGs. Yes, we have taken that into consideration. On my plane, half a tank of fuel moved the CG forward .42" and up .01".
It's a mystery. I have a couple of hypotheses as to why the leadouts need to go back so far on electrics, but they are too bizarre to admit in public.
(Edited to split a poorly written paragraph in twain.)
Edited again to apologize to PW for not noticing that he said almost the same thing above
A big thanks goes out to Howard for actually giving some dimensions for both CG shift and LO changes between IC and e'ons. Unlike previous posts. I appreciate it. We all should. H^^ D>K Steve
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A big thanks goes out to Howard for actually giving some dimensions for both CG shift and LO changes between IC and e'ons. Unlike previous posts. I appreciate it. We all should. H^^ D>K Steve
Well...
Dimensions are pragmatically valuable but do nothing to help lay people understand the physics involved which is of more interest to me. I can measure the difference between the CG vice leadout rake trim on Brett's plane versus Igor's/Paul's/Howard's/ Orestes'/Jimby's, et all; I just don't understand why the difference is so great nor why it is so much different from otherwise nominally similar stunt ships????
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After 15 days I still do not see anywhere where it tells us if this is occurring during level flight with no control input or is this seen during a control input. If it is during control input then it is a easy problem to find.
If I missed this point somewhere forgive my oversight. HB~> HB~> HB~> HB~> HB~> HB~> HB~> HB~> HB~>
Ed
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... I just don't understand why the difference is so great nor why it is so much different from otherwise nominally similar stunt ships????
Me neither, but here are my bizarre hypotheses:
1. Dynamics: This I think generalizes what Brett said above. For a stunt plane to make a nice corner, it must start rotating about a given axis, then keep rotating about that axis, then stop rotating, all without any tendency to rotate about another axis. This requires the vector sum of moments on the airplane to be in the same direction as the rotation throughout the maneuver. One of the component moments is due to rotating the airplane about an axis that's not a principal axis, and that's a component we can vary by adjusting the leadout position. Dynamic differences between our electric and acoustic Impacts include:
a) the magnitude (which I haven't calculated) of the moment due to precessing the rotating machinery,
b) the moment due to deflecting air through a prop offset 2 degrees from the airplane body axis,
c) all moments due to the time histories of RPM,
and d) a difference in the direction of principal axes due to different mass distribution.
Moving the leadouts might be changing the axis about which the airplane rotates, thus changing the off-principal-axis moment component and also some aerodynamic moment components.
2. Statics: Maybe the line drag is way different between electric and acoustic airplanes.
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Well...
Dimensions are pragmatically valuable but do nothing to help lay people understand the physics involved which is of more interest to me. I can measure the difference between the CG vice leadout rake trim on Brett's plane versus Igor's/Paul's/Howard's/ Orestes'/Jimby's, et all; I just don't understand why the difference is so great nor why it is so much different from otherwise nominally similar stunt ships????
No, but it's a start, unlike those others. Can't measure what we don't have access to. Not that I ever measured the CG shift of any of my IC planes.
Maybe it's all about the vibration of IC's and induced circular motion of the flying lines changing the drag of the lines, vs. no vibration and a different drag factor. Increased, I 'spect. Increased drag would align with the LineIII etc., where bigger lines or higher speed shifts the LO's aft. No, I don't see why a flying line whipping around like a jump rope would cause less drag than one being nicely behaved. I think that's what Howard mentions in item 2.0 above, but I wouldn't swear to it. LL~ Steve
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Just for record:
This plane which I built for WCh 2002 had CG at 15% with empty tank. (however it was small tank aproximately 160ccm )
http://www.netax.sk/hexoft/stunt/the_max_ii.htm
The xtreme forward CG position was due to its new logarithmic unit on flaps.
It is the same plane which I flew 2008 reworker to electric in 2008 and I fly it as B model till last year and still hanging on wall, so I can measure CG position. It is at 20% now - with battery installed (charged to half >:D )
And I do not remember that I shifted CG dramatically, I think it was just slow progress in time. ... moments are very similar, light motor/engine and heavier tank, the same lines, 2 blade 12" props on both (now I fly 3 blade NOT hollowed)
So may be question for Paur and Howard - are you speaking about the same but reworked model, or you mean 2 different models? (sorry if I missed answer in text before) And if not, are controlls, area, etc... the same?
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I fly an IMpact that is electrified, it exhibits the same characteristics mentioned,, the CG is forward over an inch from the plans location of the glow impact. THe leadouts are WAY back, not quite as far as Paul has his, but then my model is 7 ounces heavier and flies a touch faster so that matters.
Steve, I have told you several times that the cg is farther forward than a glow model WITH a full tank.
I am not sure Igor's findings apply directly because his trim is different vis a vis the Rabe rudder and other trim. But I know Pauls setup, I know my airplane the closer I got it to Pauls setup the better it flew..
I would love to know the why but for now I am equally happy with just having the performance without the why.
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I am not sure Igor's findings apply directly because his trim is different vis a vis the Rabe rudder and other trim. But I know Pauls setup, I know my airplane the closer I got it to Pauls setup the better it flew..
I would love to know the why but for now I am equally happy with just having the performance without the why.
What will happen on the Max Bee with a fixed rudder?
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...or higher speed shifts the LO's aft.
THe leadouts are WAY back, not quite as far as Paul has his, but then my model is 7 ounces heavier and flies a touch faster so that matters.
Both the sideways acceleration on the airplane and the line drag increase with the square of speed, so speed doesn't matter much for leadout position, except for: a) part of the wind, and b) a puny Reynolds number effect.
Mark, I notice that you tend to be a little slow in releasing the shift key, which could cause it to burn out prematurely. Hope this helps.
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Both the sideways acceleration on the airplane and the line drag increase with the square of speed, so speed doesn't matter much for leadout position, except for: a) part of the wind, and b) a puny Reynolds number effect.
Mark, I notice that you tend to be a little slow in releasing the shift key, which could cause it to burn out prematurely. Hope this helps.
Howard, ok point taken about the speed effect, however would not the weight be an issue?
as to the shift key, mine gets neglected regularly so I try to give it a work out occasionaly to keep it in sync with the remainder of the keys,,,,,
However part of the problem is my recent replies to vairous political fear based posts which have caused me to be a bit more pronounced with my shift key usage, perhaps its a carry over?
out of curiousity, does your more robust Impact tend to have the leadouts in the same place as Pauls less robust versions?