Twister Trim Help

[Motorman]

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[rich gorrill]

Hi M.M. I know this is not your first build, so let me ask a question, why did you start out with no tip weight? every profile I ever built calls for an ounce of tip weight to start. I,m no expert by far but if you don't have tip weight and you see the bottom of the wing inverted which means the outboard panel is light. Put some clay on the outboard panel for trial and error.

Rich

[Tim Wescott]

... Best I could get it was an even amount of inboard tip down both upright and inverted. There's no way to fix it by bending the flap horn...

If by "tip down" you mean "toward the ground" and not "toward the wheels" then this sounds like you just need more tip weight.

... I see the bottom of the wing on insides and the top of the wing on insides ...

Bottom of the wing on insides and top of the wing on outsides?  I.e., it's rolling OUT in maneuvers, where in level flight it's rolling IN?

Does it have any rudder offset?  I tend to run mind with very little, so if that part is built to the kit then it's way too much.  Ditto engine offset.  The consensus seems to be that just a hair of thrust to the outside is a good thing.

I can't imagine a warp that "cares" if the wing is upright or inverted, unless you have asymmetrical vortex generators.  I find it much easier to believe that you have a yaw problem.

Check that the elevator hinge line is square with the wing as you're looking down at it (I assume you know how), and that the elevator isn't tilted with respect to the wing.  It doesn't take much tilt at all to screw up a plane.

You may also want to check that both sides of the elevator move equally, that the elevator coupler is nice and stiff, and that neither of the elevators are loose on the coupler.  If one side of the elevator is moving more than the other, it'll always tend to roll away from that side in turns.  Which side of the plane is your elevator horn?  I can't remember what's "stock" for a Twister.

I'd try it with the lines brought in as close together as you can get them, around the current center (i.e., move the front line back 1/4 inch, and the back line forward the same amount).

[Mike Haverly]

Paul's trim chart assumes a straight airplane.  You are wasting your time until you address any warp or misalignment issues.

[Brett Buck]

Paul's trim chart assumes a straight airplane.  You are wasting your time until you address any warp or misalignment issues.

  His wing, at least, is probably straight enough after the tweaking. The problem is with the yaw trim, and affecting roll because they are coupled.

   Add a tiny bit of rudder offset and move the leadouts back.

    Brett

[Brett Buck]

Yeah, is that crazy or what?

   It's not at all crazy, you are flying crabbed sideways (nose-in)  in level flight.

     Brett

[Brett Buck]

Thanks Brett, I will make the adjustments. Another thing I just remembered, The plane came out tail heavy from a thick rustolium finish and a an AAC Brodak 40 so I had to add a big brass Higley heavy hub acorn nut and plastic prop. Could some of this be gyroscopic precession?

    Probably not to any great degree. Anything that is near the spin axis has negligible effect on the moment of inertia of the prop. Even in the worst cases, the effects of precession and/or p-factor are tiny, particularly with an airplane with strong passive yaw stability.

     Brett

[Tim Wescott]

... so I had to add a big brass Higley heavy hub acorn nut and plastic prop. Could some of this be gyroscopic precession?

Nope.  Gyroscopic precession goes as mass times diameter squared.  So a one inch diameter, one ounce hunk of brass on the crankshaft makes practically no change.

I have just about the same setup on my Twister (including too much Rustoleum behind the CG), and it does not misbehave in the way you describe.

[Dan McEntee]

I have a Francherised Twister built from a kit. I see allot of good things in it but I'm having a few problems.

Coming off the bench it flys straight upright but inverted I see the bottom of the wing. Following Paul Walkers trim chart I took about 15 flights tweaking the flaps. Best I could get it was an even amount of inboard tip down both upright and inverted. There's no way to fix it by bending the flap horn. The only thing I can figure is it has a wing warp that is more or less effective depending if it's upright or inverted?

2nd thing. When I'm in the 4th corner of the outside square it really noses in at me. It also seems generally a little lighter on the lines in all outside maneuvers. The down line is forward 1" from the up line.

There's no tip weight in it because I see the bottom of the wing on insides and the top of the wing on insides like it has too much tip weight.

Thanks for any help,
Motorman

     Sounds like it was less than bench trimmed from the beginning, and after flap tweaking, it was classic lack of tip weight with the "inboard tip down both up right and inverted."  If it flies straight upright, it should fly straight inverted. Some of your other issues could be it is just too slow, or sticky controls. First thing I would do is recheck for warps and alignment. If the brass spinner nut bothers you, mount the equivalent weight solidly to the nose and take it out of the equation. If I remember correctly, your flying site is tree lined, so make sure you are trimming/testing in calm air and not reading bouncing on turbulence as a trim problem.
     Type at you later,
  Dan McEntee

[Tim Wescott]

... it did seem to lose some power when it broke into a 2 stroke ...

If it loses power when it breaks into a 2 stroke, you have it adjusted wrong.  Because you should never ever adjust your 46LA to break!  Use a 4-pitch prop, and adjust it for a wet 2.  If it's breaking, it's too rich.  If it's going too fast in a wet 2, then the prop pitch is too much.  I suggest an APC 11.5 x 4 or 12.25 x 3.75 for starters, unless you can get a TT 11 x "4.5" (pitch in quotes because that prop is famous for having less pitch than it says).  Figure that when you change the prop that drastically, you'll have to fiddle with the venturi size.

If you just can't imagine life without a 2-4 break, I have some 40FP engines that have been "improved"; I'll sell you one.

[Brett Buck]

It was flying too slow as I was breaking in the engine and creeping up on the needle setting but it never really got faster by the end of the session. I had an extra head gasket in there because, brodak 40 likes to run away when brand new but, it did seem to lose some power when it broke into a 2 stroke so, that shim's coming back out. I think it's over propped with the Bolly Clubman 11.5 x 5 (cut down to 11"). Going to order some Clubman 10.5 x 5, they will have less blade area at the tips.

I guess the plane will pull more going faster and straighten out the wing? I added 25 grams to the tip, put .025" washers under the front of the engine, moved the lead outs back 1/8", adding a chunk of lead to the engine back plate to replace the prop nut, making an adjustable rudder and going leaner next time out.

Rain today.
Motorman

   The roll angle should be the same in level flight, and maneuvers, or at least pretty close, and it should not ever be rolled in at you.  If it is not, running it faster will certainly give you more line tension, but there is something wrong with the trim of the airplane.

   If the engine "slows down" when it breaks into a 2-stroke, you probably do not have enough compression. I see no reason to add additional head gaskets to an already weak engine, even though it seems to be a reflex reaction to any engine problem. If it is overcompressed, you will know it, it will backfire, refuse to run, quit unexpectedly, etc. - it is not subtle.

     Brett

[Dan McEntee]

If it loses power when it breaks into a 2 stroke, you have it adjusted wrong.  Because you should never ever adjust your 46LA to break!  Use a 4-pitch prop, and adjust it for a wet 2.  If it's breaking, it's too rich.  If it's going too fast in a wet 2, then the prop pitch is too much.  I suggest an APC 11.5 x 4 or 12.25 x 3.75 for starters, unless you can get a TT 11 x "4.5" (pitch in quotes because that prop is famous for having less pitch than it says).  Figure that when you change the prop that drastically, you'll have to fiddle with the venturi size.

If you just can't imagine life without a 2-4 break, I have some 40FP engines that have been "improved"; I'll sell you one.

     Ah, Tim, he has a Brodak .40 in it! Just a minor detail, but thought I should point that out! It's been a while since I read the Brodak.40 instructions but I think they state that you should not run more that 4 inch pitch. I have had good results with the thundertiger 11.5X4 and APC 11.5X4 and 11X4 props on mine and don't remember ever having a runaway, until I tried a tongue muffler on it and then it was just a VERY hard break but would always come back. I think they need the extra back pressure or the stock muffler, and they would be easily over propped with anything larger than the 11.5 and more than 4" pitch. I thought I had asked about lap times in my last post, but I guess I didn't, but never the less, trimming doesn't start until you are in the realm of your desired lap times. With all the symptoms described, and the late information of the lap times being "slow", I think everything is out the window and time to start from the beginning now.
   Type at you later,
   Dan McEntee

[Ken Culbertson]

It was flying too slow as I was breaking in the engine and creeping up on the needle setting but it never really got faster by the end of the session. I had an extra head gasket in there because, brodak 40 likes to run away when brand new but, it did seem to lose some power when it broke into a 2 stroke so, that shim's coming back out. I think it's over propped with the Bolly Clubman 11.5 x 5 (cut down to 11"). Going to order some Clubman 10.5 x 5, they will have less blade area at the tips.

I guess the plane will pull more going faster and straighten out the wing? I added 25 grams to the tip, put .025" washers under the front of the engine, moved the lead outs back 1/8", adding a chunk of lead to the engine back plate to replace the prop nut, making an adjustable rudder and going leaner next time out.

Rain today.
Motorman

That should do it.  I left a rather extensive post earlier that I guess I forgot to save.  The Twister is a mid 70's profile and it flies like one.  If you remember how to trim with engine offset, tip weight and rudder offset you can get a mix that will make it into a sweet flying plane.  I fly mine at 5.5 on 60' lines and it flies wonderfully.  Getting there took an adjustable rudder, engine offset and some nose weight.  I did replace the horns with Morris horns.  I have had problems with flap mounted horns putting all of the pressure on one side and causing a roll in or out both directions depending on which side the controls are on. Solid horns don't doo that as much.

You appear to be on the right track.  Once you get it trimmed you are going to love it.

[Tim Wescott]

     Ah, Tim, he has a Brodak .40 in it!

Uh -- oops?

[Ken Culbertson]

This is the first plane where I made my own Tom Morris style control horns. I used 1/8" A2 drill rod but the Morris stuff I've bent seems a bit stiffer, don't know what he uses.

Know what you mean.  His horns are a real SOB to tweak - which is a good thing.

Ken

[Tim Wescott]

Tim, I can see where the LA 46 would work quite well in a Francherized Twister and guys have recommended one. With my current tail weight I'm going to do a little weight and balance test tonight.

Sorry about the misdirection -- nothing that I said really applies to the Brodak 40, other than perhaps the underlying statement of "use the motor the way the motor wants to be used".  For the Brodak (with which I have no experience) my understanding is you want that 2-4 break, which implies a higher pitch prop, etc., etc., etc.

This is the first plane where I made my own Tom Morris style control horns. I used 1/8" A2 drill rod but the Morris stuff I've bent seems a bit stiffer, don't know what he uses.

The big thing you're getting with out-of-the-box drill rod is really high quality (and shiny) steel -- it's supposed to be annealed to a nicely machinable state.  Harden it, and it should be, well, like a drill bit.

Which is why I've used plain old music wire up until now.

[Ken Culbertson]

Well I compared the plastic prop and brass heavy hub to a wood prop and 1/4-28 hex nut and the difference was 90 grams of rotating mass.

Then I compared weights of the B40 and LA46 and difference was about 90 grams. That's not the best part.

I measured and the bolt pattern of both engines was exactly the same. LOL never knew that. All I had to do was widen the motor pocket .025" on each side and the LA46 dropped right in.

Now I have 3oz less rotating mass, the same CG and more power, can't wait to fly this thing.

Motorman

I fly mine with a Cobra 2820/12 which is a .46-.51 size electric.  You are going to like this plane.

Ken

[Brett Buck]

Well I compared the plastic prop and brass heavy hub to a wood prop and 1/4-28 hex nut and the difference was 90 grams of rotating mass.

  With a radius of gyration of <1/4". This has negligible contribution compared the prop, with a radius of gyration of maybe 2.5". That means that a gram in the prop is equivalent to *100* grams of weight on the shaft. So, your added angular momentum is the same as if you lightly sanded one prop blade to balance it. Negligible.

  Precession is not your problem, this is a pretty straightforward trim issue.

    Brett