V-tail and Adverse Yaw

[HELModels]

I'm still tweaking, crashing, repairing, test hopping, crashing some more, and on and on. What I built is a high wing pusher with v-tail. The motor is Gobrushless Cdrom and runs great! What is happening is with about 1/3 throttle, I give it a good toss and starts to slightly climb out - whoopie. I give a little rudder and turns o.k.(V-tail moves opposite of aileron for turns) when I goose it and try to turn, I get real bad pitch up and have to shut it down before it stalls. How is it that adverse yaw(or whatever it is called) only shows with high power?

Is this caused by thrust angle or function of this V-tail?
It is self righting - chop power, hands off and returns to level, which seems desirable.

suggestions?

[BMatthews]

It's not adverse yaw. Adverse yaw can only happen from using ailerons.

Can you post a picture of you model? I suspect there's other factors a'foot here.

[Andy W]

Prop wash across the surfaces, acting differently on one vs. the other.. ?
..a

[Sparky Paul]

Detail the "Vee-tail moves opposite the ailerons" thing.
If you have ailerons, why is the tail steering at all?
Is the rudder function seperate?
Can you fly it with the rudder, and not the ailerons?
A plane with no dihedral responds the way you describe, when turning with the rudder. The nose rises in the turn until the plane is sideslipping badly.

[BMatthews]

Also what you describe sounds a lot like a balance point too far forward with too much up elevator trim to compensate. Quite possibly combined with little or no downthrust. You add power and the model tries to climb. This is normal but too much makes it hard to control. Ways to dampen the effect are to move the balance back a bit but not to the point the model is no longer stable and the other is adding downthrust. Fine tune the two options until the model flies decently.

But I still think there's other factors at work here. A picture would help a lot. Top view and 3/4 view preferably.

[HELModels]

Detail the "Vee-tail moves opposite the ailerons" thing.
If you have ailerons, why is the tail steering at all?
Is the rudder function seperate?
Can you fly it with the rudder, and not the ailerons?
A plane with no dihedral responds the way you describe, when turning with the rudder. The nose rises in the turn until the plane is sideslipping badly.

Sorry, the point of "Vee-tail movesoppositethe ailerons" was that months ago I set the vtail up to move like ailerons and theresult was painful. travel direction is opposite of how ailerons move. There are no ailerons on it.

It has plenty of dihedral as wing came off an old freeflight. The CG is 23-25% but the old freeflight had CG almost 40-45%. That seems way too far back.

When I redid it, I kept the same tail length, changed tail to flat plate and increased the tail semispan 18% to make the Vee.

The prop is 7X4.

[HELModels]

Prop wash across the surfaces, acting differently on one vs. the other.. ?
..a

That's why I incorrectly called it adverse yaw, but apparently I've got it trimmed for slow slow flight. I guess I could give it more downthrust or increase tail aspect ratio And reduce tail area And move CG back a mm or 2 And reduce decalage. Or I could probably just trim it in flight. We shall see.

[HELModels]

None of those solutions might be necessary since I took a look at the throws on the V-tail. I'm using a GWS mixer and it puts more up than down, so in a turn the one tail goes up more than the other goes down. It is even on straight up and down. How do I correct that? If I switch the servos around and then reverse the transmitter channel?

[Sparky Paul]

It appears you may have the surface horns offset from the hinge line or the arms on the servos aren't set dead center at neutral. This creates the differential in up-down. Moving the surface horns or offsetting the output arms on the servos might adjust this difference, but changing the polarities or channels won't.

[HELModels]

For crying out loud, that isnt it either. The GWS mixer does not have differential. I had uptrim and that would make one go up more than other goes down. It still doesnt answer why uptrim is grossly amplified in turns with high power and brings it back to other possibilities which a picture isnt going to solve. I guess it could be prop wash. I called it adverse yaw because it was similar effect, different cause. Maybe it should be Yaw Amplified Pitch/YAP.

I'll try the thrust angle angle

[BMatthews]

If the wing came off an old free flight and you kept the same tail moment arm length and the tail is the same oversized one used on the old free flight then you can safely move it back to 35% to 40% anyway.

23 to 25% is much too far forward in any event. The uptrim required to balance your model for level gliding and low power flight is then reacting to the added power and speed and making the model pitch up as you're finding.

So move the balance back and retrim the elevator and you'll find that the pitching up isn't anywhere near as bad as you have now. The elevator control will become a bit more sensitive so you'll need to reduce the amount of elevator throw but the model will still be stable. Your problem now is that it's TOO stable as in it wants to fly at one speed and one speed only. So when you add power it noses up and slows down until it can fly at that same speed with the new power.

On top of that you've already identified a major control system problem. You need to solve that as well obviously. On top of your pitch up with added power it sounds like you also are getting unwanted up elevator when you input rudder. If the mixer you're using ends up being the problem then you may need to go back to a conventional tail with separate controls. Also study your output arm to pushrod geometry and the pushrod to control horn geometry. They should be the same for both sides. If a control horn is leanding forward or back more than the other or you tried to compensate for a long pushrod by offsetting the output arm you now have differential and the controls won't move in harmoney in a V tail setup. To set it up right the output arm much be at 90 degrees to the pushrod and the hole in the control horn of the controls much be at 90 degrees to the hinge line. If either of these is out and if one is different from the other by an easily visible amount (a degree or 3 won't matter) then you'll get significant difference in the control surface travels.

[HELModels]

Arghh! Those are all good suggestions. When I installed the horns, I was real careful. I'll double check horns and servo arms. Real stable at one speed is what I have now for sure. If moving the CG back helps, then it could lose 1 ounce and be 35% which would be great, maybe. We shall see. Stable within a speed range is difficult to accomplish, but I know it has been done before.

I remember freeflight was about tuning too. When the fuel ran out, the balance changed- they climbed straight up mostly. I also remember gliding OOS as the dethermalizer forgot to kick in. WitchDoctorX never came home.

The one this is based on had Left turn climb with power, gliding right turn.
All the tuning was in the motor and tail.

[Bill Glover]

What BM said.

Here's a diagram I did for somebody else showing how easy it is to put differential throw into a setup without realising you're doing it. If both sides of a V tail are done the same you can easily get much more 'up' throw than 'down' on each surface, or vice-versa. And a difference in geometry from one side to the other will make the plane turn when you apply elevator.

Also a couple of shots of an ARTF I bought where the horns on the all-flying V tail halves had been very badly installed. That would have given all sorts of problems.

[Sparky Paul]

I just installed one of the GWS vee-tail modules in a plane which was controlled by a computer radio featuring the wee-tail option, but changed to a radio without it.
Looking at the control throws for elevator and rudder, they are the same on both sides with the GWS part..
I expect the plane won't handle any differently.
Weather hasn't been cooperating.... :(

[HELModels]

I removed any trim and checked horns and arms and everything looked good.
I took BMathews advice about moving the CG back and that Worked. This meant I had to fly without the Cowl to move the CG. Wingloading went down by 20% and funny it now flies like the old freeflight did.

Time to build a lighter cowl.

Thanks for the help!
Harry

[HELModels]

Well, been test hopping in the yard with some success. All suggestions were considered and what has worked is CG roughly where it was on the old freeflight and thrust angle canted a little more skyward(mounted on top of wing TE). The 45% CG seriously improved the glide. The correct thrust angle improved turning behavior with no tip stall as slowed down in turn.

BMath, I'll beglad to post somepics when it is prettied up.

Thanks,
Harry

[BMatthews]

Excellent. Glad it helped out. Bring on the pics when you can.