Triming Question....Wing Waggle

[Tim Stagg]

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

[John Rist]

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.
                       

[John Park]

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?

[Brett Buck]

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

[Tim Wescott]

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?

[Dave_Trible]

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

[Tim Stagg]

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.

[Tim Wescott]

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.

[Paul Walker]

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.

[Motorman]

My LO's are .........3" behind the CG.

Did you mean LE ??

[Russell Shaffer]

Brett, can you clarify the leadout measurement?  Is it to the front lead, or the center of the adjuster?  Thanks.

[Mark Scarborough]

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...

[Ted Fancher]

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

[Paul Walker]

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.

[Scott Richlen]

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....

[Steve Helmick]

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


 

[Paul Walker]

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".

[Steve Helmick]

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! Steve

[Brett Buck]

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

[Paul Walker]

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! 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.

[Ted Fancher]

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....

Point taken, Scott!  Thanks. 

Ted

p.s. How much would a little more roll be worth to you?

[Scott Richlen]

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?   

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

[Tim Stagg]

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

[phil c]

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

[Tim Stagg]

OK,

Nothing like pulling up your dress and exposing your undies

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.

[Steve Helmick]

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

[Tim Stagg]

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

[Steve Helmick]

"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

[Carl Cisneros]

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,,,,,,,,,,,,, 

Carl

[Ted Fancher]

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?   

'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.

[Ted Fancher]

OK,

Nothing like pulling up your dress and exposing your undies

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.

[Ted Fancher]

Sorry.  Boredom set in.

Ted

[Russell Shaffer]

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?

[Joe Gilbert]

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

[john e. holliday]

Solve the whole problem and go back to IC power.

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.

[Brett Buck]

Solve the whole problem and go back to IC power.

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

[Scott Richlen]

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

[Brett Buck]

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

[Ted Fancher]

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

[Howard Rush]

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.

[pmackenzie]

 

  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

[Brett Buck]

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
   

[pmackenzie]

Not playing a gotcha game.
Exception to the rule perhaps?

[Mark Scarborough]

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,,,

[Steve Helmick]

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.   Steve

[Ted Fancher]

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

[Dave_Trible]

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.

[Ted Fancher]

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.   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!

[Howard Rush]

 

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

[Howard Rush]

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?