Trimming question

[Randy Powell]

Warning-- long, rambling post ahead. Proceed with caution.

OK, I've outlined some problems with a classic model I'm flying (or at least trying to fly) elsewhere here on the board. Now I'm looking for input because I'm about out of ideas. It's a Simon's Shoestring. Model is about 600 square inches and weighs, at this point, about 49 oz. The engine run, from a PA 40UL, is flawless. I'm using an 11x4.5 N three blade Bolly and it gets good drive pretty much everywhere. I've tried other props, both two and three blades, and none seem to effect the problems much one way or the other.

Here are the problems. First it's, well, squirrelly. I doesn't really have a classic sort of "hunt" in level flight, but seems to take constant corrections to keep it going straight. A hunt tends to be a gentle "wandering around". This is much more jerky. You can settle it in a groove (eventually), but it seems any sort of turbulence or disturbance bounces it out and you've back to trying to get it to settle again. In maneuvers, it seems to have some strange bobbles that are unpredictable. Initially, on hard outside turns, the outboard wing tip yawed toward me pretty badly. The addition of a wiggly rudder seems to have fixed that, but I can still see it fighting the rudder on outside turns. It wants to come at me but the moving rudder prevents it. I've played with the CG endlessly. It's been moved 3/4" in each direction from the original CG marked on the plans (right over the spar, about 22% MAC). When moved forward, the squirreliness gets better and it seem more solid, but the turn suffers hugely. Moving it back brings the turn back, but it get jumpy. I've hit a spot where it turns alright, and is not too jumpy, but the plane clearly isn't very happy.

I'm convinced that something is out of alignment, but I've check the silly thing endlessly with incidence meters, laser levels, dial indicators and anything I could think of to measure accurate alignment and it seems pretty good. There may be a slight (and I mean SLIGHT), twist in the inboard stab. The leading edge is straight looking at it from the front of the plane but appears (by visual inspection) to be slightly high on the inboard side when viewed from the rear. I've checked it with an incident meter and if it is twisted, it's much less than a half a degree. Not enough to register on the incidence meter. The stab is dead square to the TE of the wing (plan view). The stab was built with a degree of positive incidence. More to insure that it wasn't negative that any true positive incidence. The engine has about a degree of down thrust and out thrust. Again, not so much set up to have that as much as to insure that it didn't have any in thrust of up thrust. When measured now, it's actually about a half degree of down thrust and about a degree or perhaps a bit less of out thrust. The wing isn't twisted, or if it is, I can't find a measuring instrument to tell me that it is. Using an incidence meter, the thing is dead square down the full length of both wings. Both flap TE's are straight as far as I can tell. In a effort to help with an apparent inboard roll on both inside and outside maneuvers I had early on in the trimming process, I added a tab to the inboard flap. That seem to fix that, but is another thing to look at.

The leadouts are currently set at about 5/8" back from the CG (measured to the center of the guide between the leadouts). They've been moved from as far forward as they'll go (1/4" behind the CG when the CG was 3/4" forward of the plan) to pretty far back (most of 2" behind the CG). While this effects line tension in various parts of the pattern, it has had zero effect on the problems.

Adjusting the pushrod length (dialing in a bit of down elevator) seems to have settled the hunting/jumpiness problem in level flight quiet a lot. It's still there, but much more manageable. It still has some odd bobbles in maneuvers that aren't predictable.

The last thing is much harder to describe. The thing just doesn't feel solid out there. It's hard to fly precisely, if that makes sense. I can fly a decent pattern on it, but it's a lot of work. Sometimes, particularly coming toward the ground, the thing just doesn't seem to want to turn well. Under turns and over turns are common. Opening the line spacing makes it worse, but closing it doesn't really seem to help.

Wish I could be more exact. Anyway, any thoughts would be appreciated.

[Charlie Pate]

 Whew!
Flap horn stiff enough?
flaps stiff enough
flap area equal? (good ) (bad) equal length wing panels a factor. 

same as above for elevator:; stab
In my humble opinion,it sounds as if something is changing(flexing)

I was fortunate to meet Bill and see his plane;It was  a nice ship!

[Randy Powell]

Charlie,

Flaps are equal length. Initially, the outboard flap was bigger, but I went a bit overboard. It was either trim the outboad flap or add a tab to the inboard. I did that. I believe they are stiff enough. The elevator is a solid balsa sheet of "C" grain, pretty hard wood. It's probably flexing a bit, but no more than you would expect.

[Bob Reeves]

No help.. Seems like when I have a trim problem it's one that isn't easily diagnosed or cured.. I hate spending months building an airplane only to have it do something that nothing seems to help with.. I can sure relate, have two that I'll get back to someday, one could be the twin to the one you are describing...

Hope you find a specific answer and let us know how it goes..

[tom hampshire]

Hi Randy...How many flights?  Any chance of free play in the flap linkage when the leadouts are pinched together and stretched taut?  Second, what happens if you dial in more down elevator?  Good luck, I have little faith in troubleshooting anything by internet.  Tom H.

[John Miller]

Randy, I feel your pain. We had a similar problem with one of the Pathfinder LE's in the beginning. It just wasn't a happy flier. Like you, time was spent measuring, and everything seemed OK.

It was sitting there, and from about ten feet away, you could see that there was a slight twist in the inboard stab. Not much, but when everything was lined up just right, you could just see it.

The stab, being covered with polyspan, was heated with the heat gun, and twisted back to level.

Problem gone.

Hope this information is helpful.

CYA

[Dennis Adamisin]

First an easy one - Hinge gap sealed?  All seals tight?

2nd easy one - I did not hear you mention tip weight.  Won't help the hunt but might help your wing wobble.  Frankly that sounds like the more serious problem anyway...

3rd easy one: Inspect the hinges for looseness or even breakage.  Had that happen - twice - with results like you describe.

I'd suggest extreme measures: MAKE it do SOMETHING
* Like Tom H said crank some down elevator into it.  Idea is to get the flaps and elevator fighting each other a litle to HOLD it in trim. Watch out for unequal turns.  You said you have positive int eh stab - terrific, but if overdone then force UP elevator into it.  Again goal is to force the flaps & elevator to balance out each other.  It will also force the wing to fly at a slightly different AOA - which should at least get you into a different regime to trim from.

* Might want to do this with the wiggly rudder disabled: add tip weight until it clearly hinges to the outside of the turn in both directions.  Does it hinge the same amount both ways?  If not you might get an indication that the flaps could stand a tweaking.  I would GUESS the outboard wing is low in level flight - maybe just a hair, and its hidden until you do an outside turn.  Does the outside half of the overhead feel different than the inside?  worse case, you might get to add a tab to o/b flap to counter the heavy tip weight.  Then reactivate the wiggly rudder and trim it to suit.

* You know this one's coming: You said forward CG made it slightly better.  How long are the flaps?  If full span then lop off about 4" I/B of the tip.  The remaining flap will behave like it is stiffer, and the reduced control load will give you a harder corner.

[Mark Scarborough]

Randy,
from my vast,, *cough cough cough** experiences,, I think it sounds like one of my planes that had issues with load sensitivity on the bellcrank . controls were dynomite free when on the ground giving it the old pull pull test on the leadouts. but when you got any line tension on them the bellcrank sort of seized up and got really stiff. Just a thought there. The other thing about the hunting, if all the alignments are within reason, I would look for soemthing more subtle,, big surprise there huh. HOw did you shape the horizantal stab? is it airfoiled? I have been reading some info about stabs with to generous of a radius on the leading edge creating a hunting problem like you describe. Not really hunting as much as just wandering, like an old ford with worn kingpins,, it will keep heading that way till you woah it back, then it heads that  other way. and if youhit a pothole, lord look out you dont know where your going,,again, just a thought.I hear the diagnostic for that one is to tape a peice of skinny piano wire on the leading edge to create a sharper seperation of airflow, to make it more pitch sensitive.  This could also change the yaw tendancy as well if on stab stalls first due to airflow or other stuff,,((pleas not my throwing around of significant scientific terms there for your benefit,, lol)
good luck bud,,shes a beaut,, hat seeing these problems..
hmm another thing, any flexing in the wing structure torsionally?

[Jim Pollock]

I agree with John, it sounds like a stab warp problem.
It doesn't take much.  For instance If Paul's Impact
had a 1/64" variation in the hinge line being straight.
The plane was much more difficult to fly and diagnose
the problem.

Jim Pollock   

[Randy Powell]

Eric V suggested the control problem. Weirdly, this thing has the smoothest controls I've ever put in a plane. I had thought that perhaps under load the leadout guide might be dragging, but it isn't. I had a friend hold the plane and I pulled pretty hard and worked the controls. they feel good.

Dennis,

Yep, all hingelines are sealed. the plane has about 40 flights on it all together. No sign of loose controls (though I keep having to shim up the movable rudder. I have to remind myself never to use a plywood horn again). No broken hinges.

I already cranked in some down elevator. That seem to help the difficulty with level flight. I suspect that John is right and it's the stab. I've measured it every way I can think of the objective measure says it straight. But looking at it, I just get the feeling of a warp. If this last pass doesn't do anything, I may cut into it and do a little twisting.

Mark. the stab is triangular (which is what the plans call for). And it's pretty sharp.

I did some work on the controls (fixing, hopefully permanently, the loose rudder issue (which could be causing the bobble in maneuvers). I also took what little down and out thrust I had put in. I also put some silicon in the leadout guide. We'll see what we get. If it still have the problems, I'll cut the stab and mess with it.

Thanks for the input. As Tom pointed out, when you get to this sort of un-obvious stuff, it gets really hard to diagnose. Especially on the internet. I'll keep messing with it and maybe Paul will show up at a flying session at the narrow and tell me what to do with it (probably to include hanging in on the wall).

[Steve Helmick]

1) I was thinkin' that Ted would suggest the sharper stab LE.
2) If the hinged surfaces don't come off (not sure in the case of the Shoe), I don't see any hope of accuracy with an incidence meter.
3) Heat and opposite twist applied to solid balsa will correct any warp. The question is...will it stay put, and will the paint survive? If it ain't straight, the finish don't matter...but the finish might be repairable.
4) I recall Bruce Perry struggling to get his "Jester" to groove prior to "Prairie Fire" a few years ago. The next AM, he was elated, because he re-read PW's Impact article, and the Trimming Flow Chart in particular. Moved the LO's back a couple of tads and it grooved wonderfully. Maybe not the solution for the Shoestring, but perhaps the same information source might be worth a read?
5) Well, you asked. Good luck...pretty plane...I want to see it win Classic at Regionals... Steve

[Howard Rush]

I'm wondering what Mark is wondering.  Assuming nothing is twisted or excessively flexible, put a lot of downrig in the elevator (lots of down when the flaps are neutral) and see if that fixes the squirreliness.  It might affect the difference between insides and outsides, but if it fixes the squirreliness, we can go from there to get the inside-outside thing fixed.  For the theory, see Igor's famous piece: http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=133980&mesg_id=133980&listing_type=&page=6 .

[Ted Fancher]

Hi Randy,

Almost too much information and too many variables in your opening post.  I'm vaguely familiar with Bills ship but don't have plans for it.  I'd much rather start out with some basic planform information and work from there.  I'd be interested in the wing area and aspect ratio including flaps; the stab/elevator area and aspect ratio; (tail moment is of some interest but nothing from that era was outlandish in any way); how large the flaps are both in percentage of the wing area and percent of the chord at the root and tip; whether the wing, stab and thrustline are in line; any built in variations from "O-O-O"; the overall weight (I guess you said 49, right); flap elevator ratio; basics of the control system geometry.  Also, what are you pulling it around with and on what length/diameter of line.

My belief is that an airplane is an airplane and with the right information you should be able to set it up prior to first flights within a small percentage of what ought to work based on the above parameters.

First thing that comes to mind is the wing loading sounds pretty low if it is the more or less ubiquitous 600 or so square inch ship of the era.  Lightly loaded ships don't generally feel solid with the aft CG locations common to our modern,  higher loaded ships. Second thing is the concept of a triangular stab sounds like a potential source of pitch issues if for no other reasons than the only "wedge" shaped surfaces I'm familiar with are the ones on the X-15! Even I never flew stunt that fast.

At any rate, I know you're a good and accurate builder.  I've got to think there is something haywire with the configuration that is giving you fits. Gimme some numbers.

Ted

p.s.  Shoot, let's cut to the chase.  Put the CG at 15% of the average chord, locate the midpoint of the leadouts one inch aft of the CG when you balance it on your finger tips at the wing tip ... a little further forward if you're flying on "short" lines a tiny bit aft if you've made the mistake (in my opinion) of flying on maximum length lines.

Oh, and before you set that up, add about six ounces of weight to the CG, split between the top and bottom of the wing next to the fuse on both sides (i.e. 1.5 oz on the top and bottom of each wing at the root -- centered fore and aft on the CG suggested above).  Put a piece of crystal clear tape on the finish where the Prather stick on weight will be put so you don't have to pull off the paint with the sticky foam backing if it doesn't work.  You can simply peel the tape off the wing and it'll take the stick on weights with it.

I can pretty much guarantee a lot of the imprecision of maneuvers will abate remarkably when you've done this.  Still might be some issues with the stab airfoil but this should settle down the squirreliness quite a bit.  If so and you're embarrassed to be flying a 55 oz 600 square inch stunter we can trim it differently so you can win the lightest wing loading trophy if that's important to you.

[tom hampshire]

Hi Randy - Me again...  Before you cut the stab, try a succession of turbulators, taped on the top and bottom (either or both) of both sides.  Just strips of 1/16 balsa held on with tape.  I have had this reveal a problem with a triangular stab which had too sharp a leading edge.  That airplane was also a hunter, not extreme, but enough to upset pilot concentration.  Snow today.  Rats! Tom H.

[Randy Powell]

Thanks guys.

Steve,

>>2) If the hinged surfaces don't come off (not sure in the case of the Shoe), I don't see any hope of accuracy with an incidence meter.<<

I made a little widget that allows me to slide the meter foot between the stab and elevator.

Ted,

The thing is pretty standard, though it's has a pretty small TVC and is pretty short coupled for the era. The wing is a Skylark airfoil and the planform is the same with a bit more sweep to the leading edge, I think. Balance is right behind the spar. I weighed it last night and with the tail weight now added, it comes in at 50oz. Hinge lines are all sealed (elevator and flaps).

BTW, unlike some, I don't give a rip about what a plane weighs. As long as the wing has the juice to carry the payload adequately, I'm happy. Remember me? The guy with the 75oz, 650 square in high aspect ration plane? I added most of 9oz to that plane to get it to fly consistently. I have no problem piling on the weight if that will help.   

I was concerned about the wedge shaped stab (as per plan) when I built the thing, And I was exceedingly careful with alignment. As noted above, the engine and wing are (now again) 0-0. The stab has right on 1° incidence. Initially, the thing had an absolutely wicked yaw problem on outside maneuvers (a problem that seems to periodically plague me for some reason). I ended up adding a Rabe rudder to the thing and that seems to mostly have go away. Oh, I'm flying it on 60' x .015 lines. Hard point handle.

I should note two things. This is the second Shoestring I've built. The first one was a great flying plane. I could never get the OS FP35 to run worth a darn and that ended up causing the plane's demise. But that plane was a terrific flier. So I know the basic design is sound. The only difference, designwise, between that plane and this one is, that plane was built exactly according to the Flying Models plan. This one, based on comments from Bob Hunt about the original plane, has a Skylark airfoil. The original one I built weighed about 55oz, so I could be that Ted's comment about it being too light for the wing loading might very well be the case. If I don't get anywhere with other things, adding weight might be the way to go.

The other thing is, I've use Paul's trim chart and have done all the normal sorts of trimming things we all try. Like a lot of things, I do something to the plane and the problem I'm working on gets better. But unlike what usually happens, it seems that while that problem gets better, new ones are introduced. To me, this is a classic "something's out of alignment" problem. Usually, trim changes are at least somewhat predictable. Add wing tip weight until the wing starts to drop in maneuvers then back off until it doesn't do that anymore. But if you add wing tip weight and the flatness of the turn improves but now the plane is yawing, then something's not right.

The turn is pretty equal. As Howard had suggested, I've dialed in some down elevator and it improved the squirreliness in level flight quite a lot. Still doesn't really track well, but it's not horrible anymore. I added tail weight and while the squirreliness got worse, the turn and predictability in maneuvers got better.

Last night, I went back to the drawing board on trim, re-bench trimming it and found a couple of things. The CG is about an 1/8" back from the plan location. Calculating it, it's about 16% MAC. Should be pretty much in there. I recalculated the line sweep and moved the leadouts back almost 5/8". This puts them about 1" back from the CG at the tip. I readjusted the Rabe Rudder according to Al's article, recently re-read. I also did a sort of weird thing that may make a difference. The first Shoestring I built didn't have an adjustable rudder. It was set up just like the plan except I didn't put in the rudder offset (it was set up 0-0 to the centerline). So I thought the hingeline gap in the rudder might be doing something. So I taped the rudder hingeline. Might make no difference, but it was something to try. I also checked the tailplane and with the down elevator dialed in, I have about 2° of positive incidence.

So, I'll fly it again this weekend and report back. If none of this works, I'll try both adding some ballast and perhaps tweaking the stab or first adding Tom's turbulators. Interesting idea.

Thanks again for comments. Even if it's none of the things suggested help, it gives me ideas to try.

[John Miller]

2 degrees of positive incidence at the stab seems like a lot to me. I can see with that much it would make sense to remove the downthrust. I don't see a problem with a little out thrust though.

I still, admitably from a distance, have suspicions with the stab.

First, I would really look at a possible stab warp. It doesn't take much to cause problems. My preferred stab leading edge shape has evolved to an ellipse, with the high point about 60% of the stab at the root. This makes for a relativly sharp entry point, which I think helps.

Adding weight, at the CG, may help, but I would try, or at least look closely at the stab first.

What happens at the end of the flight? What does the plane do, in level flight, when the engine quits? How does it track through the round manuevers?

[james dean]

HI Randy,

Since we're just rambling here. Heres some more rambling for ya.  It would be my observation looking at the design itself that maybe the problem is just that......the design of the plane.  Taking a race plane design of the proportions that the Shoestring presents seems to have some inherent flaws for the purpose of C/L stunt.

For example, the aerodynamics of the fuselage.  The shape of the fuse is designed to slip thru the air in a linear fashion. It is smooth and rounded in all respects from the airscoops forward  on back to the tail feathers. A beautiful airplane and very pleasing to the eye.  In an wind tunnel it would be very "dynamic" in terms of aerodynamic efficiency. But consider the aerodynamic constants of tethered flight.

It's just an intuitive thing but think about what George Aldrich did with the Nobler.  Examined carefully, you can see that the fuse has a shape that lends itself as a wing the greater the angle overhead it goes.  It presents itself as a "lifting Body" at overhead angles and the closer to 90 degree's overhead the more it has this effect.

The depth of the fuse' from nose to tail and especially the fuse depth and wing like shape of it after the canopy rearward has to lend itself to the stability weather at level flight or at higher angles overhead.  Maybe think of it in terms of the veins of the fletching of an arrow.

The gist of my thought being that the shape of the fuse of the Nobler plan form acts more like a wing at overhead angles and more like the fletching of an arrow at level flight.

Given the shape of the Shoestring's fuse, as beautiful as the lines of it are, it is lacking this directional stability that I'm observing in the Nobler plan form. (IMHO)  I always thought that the Nobler was kinda homely looking until I looked at the creative genius of George Aldrich! The "function"  of the "form" justifies the shape so to speak.

Well anyway, I hope that was at least entertaining if not thought provoking.

[Randy Powell]

Hi John,

Hmm, well, as far as landing, the plane glides fine. No tendency to drift of dive. Wish the gear was a bit farther back, but it's at the plan location. Has a slight tendency to bounce on landing. Oh well.

As far as tracking in round maneuvers, I have to really concentrate or the bottoms of both inside and outside loops come out really high. It seems to want to turn inside of where I am pointing (if that makes sense). It's as if, on both outside and inside round maneuvers, I have to consciously give less and less control input as the plane comes toward the ground to insure that it stays on track. As a comparison, with my PA plane, I just set the track and forget it. the plane stays on the track unless I change it. With this plane, it seems I have to change control input to keep it on track.

Not sure how clear that is, but hopefully, you get the idea.

James,

The only thing to contradict this is, the previous one I built was a great flier right off the board. Wish I had it back, I'd put the great engine in this version in it and have a great plane. Sigh... Such is life.

[EddyR]

Randy  Every thing others have said are true,BUT. I know you build very accurate planes and I do also and some times this happens. I have found over the years that hunting is almost always caused by leadout position and to much tip weight.I have moved the leadouts forward more than would ever seem plausible and it corrected the problem on that one plane. Have someone look up at the bottom of the plane in flight and be sure it is not aimed out. It needs to be straight. Most of my planes have not had this problem and they could be flown a long way out of adjustment but once in a while one comes along that has these problem. I agree with Ted about the weight if it has the airfoil to carry more weight try it.
Ed

[John Miller]

Hi John,

As far as tracking in round maneuvers, I have to really concentrate or the bottoms of both inside and outside loops come out really high. It seems to want to turn inside of where I am pointing (if that makes sense). It's as if, on both outside and inside round maneuvers, I have to consciously give less and less control input as the plane comes toward the ground to insure that it stays on track. As a comparison, with my PA plane, I just set the track and forget it. the plane stays on the track unless I change it. With this plane, it seems I have to change control input to keep it on track.

Not sure how clear that is, but hopefully, you get the idea.

Tightening up in the round manuevers tends to indicate a tail heavy condition. Opening up tends to indicate a nose heavy condition. So, try moving the CG forward until the plane tends to fly like your PA plane.

After that, try adjusting your leadouts and tip weight.

I'm glad to hear that the plane doesn't zoom up, or down immediatly after the engine quits. It indicates that your thrust line, wing, and stab incidences are likely OK.

[Randy Powell]

John,

Well, I just changed a ton of stuff so I want to fly it again before I do anything. But if it's better, I'll continue on the track I'm on. If not...

One of the things to look at with CG stuff is, how much does the turn quality change throughout the flight as the tank empties. For most designs, there is a window for most critical elements: CG, moment arms, aspect ratio, etc. As long as you're in the window, the plane does OK. Sometimes the window is short and other times it can be pretty big. Generally speaking, CG (depending on the design) can be a pretty wide window. As long as your in the window, the plane should fly OK (more or less). There is certainly an optimum point in the window that, when you hit, you know you it. But when you are approaching the edge of the window, it can get interesting. If, as the tank empties, the plane starts to be easier to fly in maneuvers; if the clover leaf is easier to fly well than the inside loops, then you are probably near the edge of the window and need to move the CG back.

With this plane, there isn't a lot of difference in how it does loops (as described) at the front of the pattern versus the end. It's just as hard to keep it on track in the clover leaf as it is in the inside loops. Weirdly, after moving the CG back a bit, it doesn't bounce out of every corner anymore (no matter how much I closed up the line spacing). It is, however, hard to turn 90° accurately. It seems I always underturn or overturn. I think the thing is fighting some misalignment that cause it to be inconsistent. But we'll see.

[RC Storick]

Randy just send it to me. Unless te bellcrank is in the wrong place, then there is noting I can do without a rebuild..

[Mark Scarborough]

Randy, I still feel like there is something there that is changing under duress in manuevers, loading situation. Your over turning, and under turning the 90 corners makes it seem that way , again, cant say "why" so much as just a feeling. it really sounds like what I felt with my Nobler under load you couldnt find center on anything, it was changing all the time. weird stuff for sure, I know your pretty logical with your approach to trimming so its not something normal I wouldn't think. ?

[Randy Powell]

Mark,

Could be. I don't think anything is particularly flexing, but it may be that some misalignment, that is overcome under load, comes into play when the load is reduced. As often happens when we have a plane that has a twist somewhere, we make some trim adjustment to overcome it. But that doesn't mean the problem went away. It's just masked.

Here's my best guess (provided the other stuff I've done hasn't fix it - we'll find out this weekend):

The stab has a twist on the inboard side. While it doesn't show up when measured, looking at it, I just have a feeling. It's said that the human eye can detect as little as a 1/64" of misalignment. This twist is causing the plane to very slightly roll right and turn left (anyone that has ever built a hand lauch glide knows what I'm talking about. You tilt the stab to get a left hand turn). The wiggly rudder is overcomming the tendency, but the initial trim misalignment remains. When the plane is under load, this washes out the tendency, but when the load suddenly changes (as in a sharp turn or changeable winds), the effect of the twist comes back in, thus making the thing unpredictable. It's OK, it's not OK. Gee the problem is fixed. Whoops, no it's not.

Now, this is supposition. It could be that the problem was caused by other stuff I've talked about above and I'm imagining the twist (if it's there it's subtle enough that I can't measure it). If that's the case, the thing should fly OK this weekend. If not, I'm getting out the X-Acto knife and me that the stab will have a session together.

[Howard Rush]

Look at Igor's SSW post on flow transition.  To  me, it explained why airplanes with smooth controls can "hunt", why positive incidence works with airfoiled tails, why zero incidence works with Impact tails, what trip strips do, what a twisted stab could do, and the subtlety of stunt aerodynamics. 

[Randy Powell]

Yea, but Howard, that's you.   

Thanks

[RC Storick]

Look at Igor's SSW post on flow transition. 

Or we could look at it here.. Quote Igor Burger

You remember David's (F) Article in S/N? Here on forum were several times guesses why that small coincidence. There were guesses that it is gyro moment, or thermal expansion of fuselage on top or downwash. I beleived that it will be the surface flow deflected by wing and still existing at tail which is little higher than wing (suction under the tail). Now playing with analyzer able to work with more segments at once I modelled that situation (on pic). The wing is at 0 AoA and having no lift. The tail is little over the centerline also at 0 AoA. And guess what. The tail has small negative lift (pitching up). I rotated the tail by 0.1 deg at once and results are in following table:


AoA 0.0:
alpha____Cl_____Cd________Cm 0.25
0.0_____-0.011__0.00881___0.002
0.1_____-0.002__0.00267___0.001
0.2______0.001__0.00266___0.000



so it hows that IN THIS CASE the neutral AoA on tail is somehwere between +0.1 and +0.2 deg.

I do not know how far to believe those numbers, but at least this theory is mathematically proofed

End Quote

Pie are square but in reality Pie are round.

[Howard Rush]

That's the start of the conversation.  Then Paul asked Igor a question, which Igor answered in posts 12 and 17. 

So, Randy, I think you can either shorten or lengthen the flap-to-elevator pushrod.  Then there will be some level-flight elevator deflection one way or the other.     That will keep the transition at the crack on one side of the tail and at the LE on the other.   If that stops the hunting, then you can do something else to make the transition stay put with the right pushrod length.     

[Ted Fancher]

Randy,

Just skimmed the Shoestring article.  Noted a couple of things.

Bill's original weighed 57 oz.  590 square inches on a 57 inch span,  a 5.5 aspect ratio.  The plans show one to one controls with the commonly used 1/2" control horns and a bellcrank with a much longer output arm (didn't try to measure it).  The result is a comparatively high aspect ratio airplane with a wing loading of just under 14 oz/sq ft. with very fast controls.  All powered by a Fox .35.  It was apparently a very competitive airplane among some pretty darn good flyers, Baby Hunt, Gene Schaffer, Silhavy, McFarland, and many more.  It also shows the CG pretty darn far forward (probably to deal with those fast controls).  If you continue the CG out to the wingtip it shows it balancing right at the leading edge where the tip glues to the basic wing.  Again, I couldn't measure it with any accuracy as far as % of the average chord but I think it is well forward of even 15%.

By the way, I totally agree with (I think it was) John Miller regarding the tendency to want to pull out of loops high.  That's a pretty classic symptom of being a bit tail heavy.  You could add a bit of nose weight or you could add a lot of weight at the CG.  I still think that's worthy a shot.

By the way, did you use the same horns shown in the plans or did you go a bit more modern with the dimensions?

Ted

p.s.  Denny mentioned disconnecting the wiggly rudder while you sort out the trim.  I agree with that.  To the extent the rudder will be of value it will be as a final refinement.  You shouldn't be using it as a primary trim device for basic trim.  All the things you have talked of doing in terms of CG and leadout changes etc. will ultimately require a different setting for the yaw control for which the rudder is intended.  Fix the thing straight ahead until you get the "funnies" worked out and then see if judicious use of the rudder can optimize any remaining yaw issues.

[john e. holliday]

Randy, old DOC Holliday here.  I have fread and reread all the expert advice you have been getting here.  I had a plane that I followed all the flow charts on trying to tame it down.  It had balance point right on the leading edge and was still a hand full to fly consistently.  I finally cut into the fuse to get to the bellcrank.  Moved the push rod toward the pivot point as much as I could without binding.  It was one sweet flying plane after I did that. 

If all the above advice from the experts doesn't help, send it to VSC with Leo.  I'll send you twenty bucks for it.      Have fun,  DOC Holliday

PS:Really I hope you get it figured out.  jeh

[Randy Powell]

Ted,

The controls are more modern. Tom Morris units. The bellcrank is at .7 inches and the horns are, I think, 1 3/8" and 1 1/8" at the flap and a slider at the elevator. Trust me, that fuse bottom block is paper thin under the elevator horn to get that length horn in. All adjustable.

As far as the rudder. I have both a Rabe setup and a static pivot. The thing had a wicked yaw initially, but the Rabe unit solved that. But it shouldn't have been there to begin with. The first one I build didn't have that problem. So I have to assume that something is out of alignment. I'm still voting for the twisted stab and may just cut into that. But first I want to try it with the stuff I've done. I may also try weighing it down at the CG. If it contines to be as squirrelly as it was, that and the pushrod lenght will continue to get attention.

Or I may just give up and fix the Novi.   

[Shultzie]

Ted,

The controls are more modern. Tom Morris units. The bellcrank is at .7 inches and the horns are, I think, 1 3/8" and 1 1/8" at the flap and a slider at the elevator. Trust me, that fuse bottom block is paper thin under the elevator horn to get that length horn in. All adjustable.

As far as the rudder. I have both a Rabe setup and a static pivot. The thing had a wicked yaw initially, but the Rabe unit solved that. But it shouldn't have been there to begin with. The first one I build didn't have that problem. So I have to assume that something is out of alignment. I'm still voting for the twisted stab and may just cut into that. But first I want to try it with the stuff I've done. I may also try weighing it down at the CG. If it contines to be as squirrelly as it was, that and the pushrod lenght will continue to get attention.

Or I may just give up and fix the Novi.   

Randy...
Don't give up on that beautiful Shoestring...yet!
Hummm?
Although I have never believed in the old stuntflyer's daddytales when the say PIGS CAN'T FLY
since I have built and flown waaaaaaaay too many of them to know that PIGS DO FLY and here is proof positive...THEY DO FLY!! (but how well do they fly is totally a different animal?

[Howard Rush]

Randy is known for building beautiful airplanes and not liking them.  I think that his airplanes may suffer from low self-esteem.  The rest of us praise his airplanes when we see them, but I'm afraid that they may not get the respect they need in the home.

[Shultzie]

Randy is known for building beautiful airplanes and not liking them.  I think that his airplanes may suffer from low self-esteem.  The rest of us praise his airplanes when we see them, but I'm afraid that they may not get the respect they need in the home.

AMEN!...
Randy's beautiful models...are truly masta'pieces of the CLPA KIND! I can't imagine that Shoestring can't be trimmed to the perfection that model truly deserves...and after watching Randy fly that model...
I would say that .HIS PATTERNS ARE LOOKIN PURTY PURTY HECKITITY DARNED COMPETITIVE  AND GOOD ENOUGH TO STEAL AWAY SOME CLASSICAL HARDWARE FROM THE TOP THREE BIGGIN-ZAAAAS' THIS YEAR.

[Mark Scarborough]

Perhaps we should seek the services of an in home counseler for Randy and his family of finely crafted "children" I too have to agree with Howard,, I think some measure of low self esteem may be causing this issue. I am now nurturing two of Randys orphaned children of stunt.. one is the Freelancer, the first time I had it out it suffered greatly not knowing how to react to the tender gentle hand I dealt it,, the second time it was much happier,, perhaps it was that Jessica wiped it down for me,, and that led it to feeling more loved and appreciated,,
or maybe it was all its brethern that left never to return to the hanger that scared it into compliance?

[Randy Powell]

Howard,

Hey, I like the Slider. I'd probably be happier with the Shoe if I had used Rhomboids.

[Randy Powell]

Well, flew at the Narrows today to mixed reviews. Definitely much better, the the thing still wanders around like a drunken sailor. The hunt has smoothed out. It's not jerky now. but it doesn't track worth spit. Next thing to try it to Whitely it (down thrust). If that doesn't work, I guess it's becomes a hanger queen till I think of something else. I also plan to take out material to try turbulators on the stab.

[Howard Rush]

Just what shape is the stab?  Did you take the twist out?

[Randy Powell]

Howard,

Yep. Seemed a little better, though it was hard to tell. I kept having to shorten the down line. Gets to a point of diminishing returns, though. The last crank really made it quick on inside maneuvers and terrible on outsides. I about ended up like Donnie and the Chip. I figure if I dial in some down thrust, I can re-trim for turn if it cures the wandering problem.

[Howard Rush]

The point of changing the pushrod length is to see if the hunting is being caused by the transition point wandering, causing a limit cycle.  So you'd change pushrod length a whole lot-- probably well into the range where insides and outsides don't match.  If the hunting doesn't go away, it's probably not being caused by the transition point moving, so a turbulator probably won't fix the hunting either.  If the pushrod length change fixed the hunting, then you can restore the length to where it matches insides and outsides or gives the same level-flight pitch angle upright or inverted.  Then you would change something about the stab.  You could try turbulators, maybe at 10% stab chord.  See Igor's post to see how big to make them.  You said the stab airfoil is a triangle? Pointy end in front and straight lines back to the hinge? 

[Perry Rose]

I built a plane with a triangle airfoil on the stab and it wouldn't stay level either. I read some place that the air separates at the high point and the elevator operates in reduced/turbulent airflow for a bit before it hits smooth airflow. That null zone changes with speed on top of it all.  I scrapped the thing.

[tom hampshire]

Hi Randy - My usual terse writings have failed to explain my thoughts, although Howard touched on the nub of it.  What both adding down elevator and experimenting with turbulators can do is to test the limiting case(s) of whether the hunting is caused by flow separation or a dead zone over the tail group.  Neither should be expected to fix the problem, only identify it.  If trying these changes does not change the hunting, then the problem is most likely the tip weight/line rake combination, and that  should be explored with cut and try changes.  For purposes of identifying the source of the problem, I'd suggest making the changes fairly large at first, so the effects are pronounced and easily evaluated.  My guess is that you are pretty close to a breakthrough, so don't hang it up until you have pursued both avenues fully.  Ugh, it's still snowing.  Tom H.

[Ted Fancher]

Howard,

Yep. Seemed a little better, though it was hard to tell. I kept having to shorten the down line. Gets to a point of diminishing returns, though. The last crank really made it quick on inside maneuvers and terrible on outsides. I about ended up like Donnie and the Chip. I figure if I dial in some down thrust, I can re-trim for turn if it cures the wandering problem.

Randy,

Is that what you really meant?  It's turning real fast inside and slow outside? You are shortening the down line (at the handle, right?) to try to regain even turning?

Sounds to me like you're going in the wrong direction with the pushrod.  This is an especially predictable evaluation since the hunting isn't getting significantly better.  Before doing the downthrust thing, shorten the push rod until the trailing edge of the elevators are down 1/4" or so with the flaps at neutral.    Unless I've misread something along the way you're going in the wrong direction!  You'll also want to reset your handle neutral if you've been shortening the down line based on flight to flight considerations with the (IMHO) inappropriately rigged flap/elevator relationship. Then try a flight or two.

Re the triangular stab.  I hesitate to suggest this because I have great respect for the designer but...

The triangular stab thing could well be nothing more than a simplified way to "draw" the stab for the plans.  Most of us old school designers used nothing more sophisticated than a variety of french curves for "designing" planes, especially airfoils.  The stab/elevator could be a bit challenging unless one had the right -- very long -- curves to make the section look like an airfoil.  A much easier way to draw them would be to draw a small circle at the leading edge and draw straight lines back to the hinge line ... whatever form the original section might have been following shaping.

Unless Bill mentioned something about a reason for the triangular section (and I can't think of any particularly reasonable aerodynamic theory for doing so), I would consider shaping the stab airfoil in a more conventional manner.

Ted

[Randy Powell]

Hi guys,

OK, two things. First, I built one of these before with the triangular stab and it flew great. So, while it could be a contributor to the problem, I don't thing it has to be since the first one flew really well with no sign of a hunt.

Second, all the changes that have been made have resulted in an at least flyable plane. It's not nearly as unpredictable now. But the hunt it has is new, really. It was sort of jerky in level flight before. That seems to have gone away an now it has a more traditional hunt. Could be that the line rake/wing tip weight situation is now fouled up. Shortening the pushrod seemed to be helping till the last time when the thing went kinda weird. The turn is biased now and I wasn't clear above (Sorry Ted), it turns much easier outsides than insides. I reversed it above. In re-reading it, I realized my error.

I can now turn it flat without bounce (except that induced by the pilot). I could be that I need to slow the elevator a bit and bring back the line spacing to sooth things out a little, but I'll worry about that later. Last night I checked alignment (again). Put the incident meter on the engine shaft, the wing and the stab after blocking the plane up. It appears that there is about 1 degree positive incidence in the stab and about the same in the engine (with the wing at 0-0). This is weird since when I built it, I was sure that the engine was about a half degree down thrust. So, I'll wedge the engine to at least 0-0 and try again.