Trim question....how can this happen

[Doug Moon]

Hello,
I flew another fliers electric model today. I has outstanding tension everywhere. Not overly heavy but very solid. Very nice flying model. Wing is level, doesnt throw the wing in corners. I noticed when trying to fly 5' level it wanted to hunt up to about 8-10' then settled in level with little input. I figured it was the handle. Then inverted it did the SAME thing. Hunted up to 8-10' then settled in level. I thought it was just me. Flew it again it did the SAME THING. I asked the owner of said plane and he said it does that to him too....  What gives???

 

[Dave_Trible]

Hello,
I flew another fliers electric model today. I has outstanding tension everywhere. Not overly heavy but very solid. Very nice flying model. Wing is level, doesnt throw the wing in corners. I noticed when trying to fly 5' level it wanted to hunt up to about 8-10' then settled in level with little input. I figured it was the handle. Then inverted it did the SAME thing. Hunted up to 8-10' then settled in level. I thought it was just me. Flew it again it did the SAME THING. I asked the owner of said plane and he said it does that to him too....  What gives???

Doug I'd call it flap to elevator alignment at first guess.  If this is adjustable tweak it a little one way then the other and see if you can dial it in. Other than that I'd say the controls are too tight (stick-sion) or the elevator is mounted with some incidence, likely negative.

Dave

[Doug Moon]

Doug I'd call it flap to elevator alignment at first guess.  If this is adjustable tweak it a little one way then the other and see if you can dial it in. Other than that I'd say the controls are too tight (stick-sion) or the elevator is mounted with some incidence, likely negative.

Dave

Could be incidence or flap to elevator.

Lines are Spectra, very smooth controls didn't feel drag anywhere.

[Dennis Adamisin]

Pusher prop? Try 1 degree up thrust; that’s one #4 washer between motor & mount at that the 25mm bolt circle.

Tractor prop? Try 1 degree downthrust.

[Ken Culbertson]

I had the same issue with my Sandpiper III, the one painted up like Igor's MaxBee.  It was also electric.  In my case it wanted to hug the ground.  The problem with having the same issue upright as inverted is that most solutions simply don't work.  You want the plane to track both directions so the fix for level is the opposite inverted.  So, the one thing that is the same both directions is the CG vs the Center of Lift.  If you are holding the handle level as the plane floats up or down it will induce a slight amount of control till it reaches equilibrium.   In your case that happens at about 8'.  This tells me that the CG is behind the center of lift.  In my case it was the opposite and moving it aft helped.  Or you could add a canard...

I think I know the plane and it is impressive - Ken

[Doug Moon]

Ken, the cg can’t be behind the cl it wouldn’t take off or probably crash badly. But I see what you’re saying. At the same time the plane is very stable. If the cg was far enough off to cause what you’re talking about I wouldn’t be able to fly very good patterns. And in this case the plane flew really well in the maneuvers. But on exit and around 5’ it was difficult to keep it tracking well. But the turns were nice, not too snappy and not too sluggish.

Dennis it’s a tractor prop. The owner tried pusher before and it really had some issues. Went to tractor and it’s prettt smoothed out except this weird thing. Will advise about the washer trick. Could be an incidence issue.

[Paul Walker]

How are you assuring that the incidence of the stab is ZERO, or slightly positive??

My new plane this year sort of did that. After careful examination, I had a RCH of negative incidence.  Shimmed it to perfect ZERO, and the problem went away.

If your plane is a one piece configuration, not sure how you could fix that.

[Ken Culbertson]

Ken, the cg can’t be behind the cl it wouldn’t take off or probably crash badly. But I see what you’re saying. At the same time the plane is very stable. If the cg was far enough off to cause what you’re talking about I wouldn’t be able to fly very good patterns. And in this case the plane flew really well in the maneuvers. But on exit and around 5’ it was difficult to keep it tracking well. But the turns were nice, not too snappy and not too sluggish.

Dennis it’s a tractor prop. The owner tried pusher before and it really had some issues. Went to tractor and it’s pretty smoothed out except this weird thing. Will advise about the washer trick. Could be an incidence issue.

If I am right about this, it wouldn't take very much of a CG shift to correct the problem.  IMHO this doesn't even happen unless the CG is nearly perfect.  In my case a quarter ounce in the tail stopped it.  At least yours wants to go up.  Mine wanted to count ants.  You can fix the upright with any of the usual suspects and that may be the best approach.  The pattern only has two laps of judged inverted and most can handle a little floating for that short of time.  Upright is the setup for all of the other maneuvers plus the two laps judged.

Ken

[Doug Moon]

How are you assuring that the incidence of the stab is ZERO, or slightly positive??

My new plane this year sort of did that. After careful examination, I had a RCH of negative incidence.  Shimmed it to perfect ZERO, and the problem went away.

If your plane is a one piece configuration, not sure how you could fix that.

Hello Paul,
I am not sure it is 0. This is a friends plane but he is a good builder and uses jigs. I am not sure on the incidence. It is a one piece plane.

[Larry Fernandez]

Hi Doug.
Keep in mind, I’m not a top five flyer but I get advice from the best flyers on the planet.
A few years back, I built A gorgeous Thunderbird that showed the very same flight characteristics as you described Brett suggested a tad bit of down thrust. I put washers under the front lugs of the motor, and magically, the problem went away. That  Buck character seems to know what he’s talking about.
Since then, all the planes I build have about two degrees of down thrust built in.
Just food for thought.

Larry, Buttafucco Stunt Team

[Doug Moon]

If I am right about this, it wouldn't take very much of a CG shift to correct the problem.  IMHO this doesn't even happen unless the CG is nearly perfect.  In my case a quarter ounce in the tail stopped it.  At least yours wants to go up.  Mine wanted to count ants.  You can fix the upright with any of the usual suspects and that may be the best approach.  The pattern only has two laps of judged inverted and most can handle a little floating for that short of time.  Upright is the setup for all of the other maneuvers plus the two laps judged.

Ken

As is said before his plane has a very stable cg setting. The turn is smooth and predictable. I have flown many planes of my own with improperly set cg and this was never result. Plus you can’t fix this with the usual suspects because it’s not the usual problem.

[Doug Moon]

Hi Doug.
Keep in mind, I’m not a top five flyer but I get advice from the best flyers on the planet.
A few years back, I built A gorgeous Thunderbird that showed the very same flight characteristics as you described Brett suggested a tad bit of down thrust. I put washers under the front lugs of the motor, and magically, the problem went away. That  Buck character seems to know what he’s talking about.
Since then, all the planes I build have about two degrees of down thrust built in.
Just food for thought.

Larry, Buttafucco Stunt Team

Hello Larry! I hope all is well these days. I think down thrust is going to be one of the very first things to try since it has a fixed stab and that can’t be adjusted.

Have a good one!

[Brett Buck]

Hello Larry! I hope all is well these days. I think down thrust is going to be one of the very first things to try since it has a fixed stab and that can’t be adjusted.

   Anything can be adjusted.  It's just a matter of how hard it is.

      Brett

[Steve Helmick]

Maybe I missed it, but are the hingelines sealed? I could sure see that causing such a problem. I believe Doug isn't a believer in sealing hingelines, though.  Steve

[Ken Culbertson]

Plus you can’t fix this with the usual suspects because it’s not the usual problem.

You mistook what I was saying.  What I meant you can cure *upright* floating at the expense of inverted then live with the inverted if all else fails.  Downthrust may very well be the answer coupled with the handle adjustment you will probably have to make.  This may be a more widespread problem than we think. 

Ken

[Doug Moon]

Maybe I missed it, but are the hingelines sealed? I could sure see that causing such a problem. I believe Doug isn't a believer in sealing hingelines, though.  Steve

Yes the hinges are sealed.

[Dennis Toth]

Could this be related to the vertical CG of the plane? Maybe that's why the downthrust / upthrust works for some. Maybe shifting this with heavier/lighter wheels could correct the issue?

Best,    DennisT

[Ken Culbertson]

Could this be related to the vertical CG of the plane? Maybe that's why the downthrust / upthrust works for some. Maybe shifting this with heavier/lighter wheels could correct the issue?

Best,    DennisT

Heavier or lighter wheels will also shift the horizontal GG.  How would we know which was the real problem, if either one was.  Most of the solutions I have read so far are for an asymmetrical problem.  This one is symmetrical, or is it?  What if there are multiple issues in play.  That is why I think he should use whatever it takes to fix it upright then look for another solution for inverted if the problem is still there.

Ken

[Doug Moon]

UPDATE: So the owner of the plane in question added some down thrust. Not sure how much. I sneaked out of work and went to the flying field just now for a test flight! WOW!! This plane is tracking and I was comfortable right off the wing over for a full 5' pattern. It is a electric and damn that was EASY! It is turning a tad faster inside vs outside but that's an easy line adjustment.

Thank you all for your input. Love this forum!!

[Crist Rigotti]

I understand the owner added some down thrust and  the problem was solved, he then adjusted the length of the elevator push rod so the inside/outside turn rates were the same.

Can anybody explain, aerodynamically, how adding some down thrust cured the problem of floating too high on both upright and inverted.

I'm happy for the owner!

[Dan McEntee]

I understand the owner added some down thrust and  the problem was solved, he then adjusted the length of the elevator push rod so the inside/outside turn rates were the same.

Can anybody explain, aerodynamically, how adding some down thrust cured the problem of floating too high on both upright and inverted.

I'm happy for the owner!

  Here is what I have been living by ever since I read this in Stunt News when it came out. It's pretty much what I've done on models I've built, purchased, and recued/rehabbed.
 
    https://stunthanger.com/smf/cadclassics-plan-service/bob-whitely-s-artical-things-that-work/

  Type at you later,
  Dan McEntee

[Brett Buck]

I understand the owner added some down thrust and  the problem was solved, he then adjusted the length of the elevator push rod so the inside/outside turn rates were the same.

Can anybody explain, aerodynamically, how adding some down thrust cured the problem of floating too high on both upright and inverted.

I'm happy for the owner!

  Stability. My guess is that the airplane has some built-in "up'stabilizer, so putting a constant torque on it forces it to operate to keep the same loading for normal near=level flight control motion. It is greatly exacerbated with a "flat stab", adding a pointy leadins edge reduces the problem. 

   My guess would be, without having seen it, that the airplane has a tiny amount of "up" stabilizer, which causes it to be unstable around neutral, just like many prior examples, and pushing outside the dead zone around neutral avoids the issue. If it had a removable tail, I would suggest a maybe .010-.015 shim under the stab leading edge, instead of downthrust. As it is, you would have to cut a wedge underneath the stab, bend it down to tilt in some "down" stabilizer,  then glue it back It doesn't take much, typically. but does require refinishing.

   Brett