Plane nose pitches up with throttle

[kist2001]

It's a scratch built high wing aileron plane. Like an ugly stick.

It will run hands off stable on glide or with a smaller motor. I recently switched to an Axi for more speed... Now despite gobs of down angle on the motor the nose of the plane pitches up violently with throttle. I can compensate with down-elevator but would like to cure this problem.

Should I add some incidence to the wing (did I say it glides like a kite when the motor is off :rolleyes: ). Any help would be greatly appreciated. -Ken

[jfitter]

If the airplane is otherwise trimmed correctly, then the problem is not enough down thrust. This is common in high wing configurations with powerfull motors. Do not be surprised if you need 15 to 20 degrees of down thrust.

It's all about force couples. Draw the diagrams and see for yourself.

[pmackenzie]

The plane might also be nose heavy.
In normal cruising flight a nose heavy plane will need extra up trim to fly level.
If you reduce power the plane will drop the nose, and if you increase power the plane will want to climb.
If you have it trimmed out to glide "hands off", then any power input will cause it to go nose up.

There are a couple of in flight ways to test CG:

Assuming both you and the plane can fly inverted:
Trim for level flight at normal cruise. Roll inverted. Keep the same power setting to eliminate thrust line effects.
If it takes a lot of down to maintain altitude, move the CG back.
If it takes no down to fly inverted it is neutral.
If it wants to climb inverted, then it is tail heavy.

If neither you nor the plane can fly inverted:

Trim for normal cruise or glide. Without changing power put the plane in a shallow dive. Release the elevator stick.
If the plane pulls out of the dive by itself quickly, it is nose heavy.
If it continues in the dive it is neutral.
If the dive gets steeper ( "tucks" ) the plane is tail heavy.

Most sport planes should gently pull out of the dive. This means that the CG is slightly forward of the neutral point.
IMO a slightly nose heavy sport or scale plane will be a bit easier to fly than a neutral one.
Inverted is easier if you have to push on the stick a bit.
Landings are easier since the elevator position will effect the rate of descent and not just the attitude as it would in a neutrally balanced plane.
You can also judge airspeed in glide based on stick pressure. Very handy on final. With a neutral plane you don't have that feedback.

Only after the CG is set can you test for thrust line problems, since they will be masked by CG problems.
Hope that helps,
Pat MacKenzie

[kist2001]

Thanks, you are awesome! -Ken

[willfly]

If possible, try raising the moto-prop higher so that thrust is in line with wing drag.

[vintage1]

this sounds like a nose heavy model to me.

[jay flay tway]

I thot it was a fact of life that standard configuration (non-canard) planes pitch up under increased throttle unless you want to introduce a bunch of inefficiency with a lot of downthrust.
Just like increasing throttle will roll a low-wing no-dihedral plane and requires pilot compensation with aileron input.
Am I right? Not sure.

[phat23]

Climbing under increased throttle is normal, sure. But Ken says ".......nose of the plane pitches up violently with throttle." That's not normal.

[jay flay tway]

Right. I didn't read it carefully enough. Sounds like it is nose heavy, probably.

[vintage1]

Also with a lot of drag aboive the thrustline - common with high wingers - you get this.

Only solution is to angle thrustline so it passesd more or less through the wing.

My old 1965 'sharkface' flew with about 20 degrees of downthrust.. short nose shoulder wing and way to much power :D and no elevator...

[jay flay tway]

Yet another thing I hadn't considered yet, drag above and below the thrustline affecting pitch (or in gliders, above and below the drag line if there's such a term). There are so many facets to flight--baffling and fascinatingly complex.

[kist2001]

As a update. I moved the CG back and forward until I was afraid to lose control. I added incidence to the wing. I even had the thrustline angled down radically....with no success. I guess a high wing plane was just not meant for the abuse I want it to take. :-)

I'm going back to the PJS outrunner I had in it before. It was a sweet flyer with no bad tendencies. -ken

[jay flay tway]

Is it highly overpowered maybe?

[kist2001]

yes...

[SaMx]

just a thought, but wouldn't adding incidence increase the problem? I would think you would want to have less incedence on the wing to avoid having the nose pitch up. I know that on FF HLGs you want the wing to be parallel to the stab, so there is no effective incidence, to help prevent the nose from pitching up violently on the launch. Some even have negative incidence (actually positive incidence on the stab). I would remove some of the incidence on the wing, otherwise the stab acts like it is always giveing down elevator.

[Highflight]

While difficult to diagnose your specific problem without looking at it in person, I will add that I once had an aircraft that had the same tendencies of "zooming" upward when given high throttle just like yours.

While I had a really good idea about what needed to be done, I was feeling awful lazy so what I actually did was to very simply use the programming in my radio where I adjusted down elevator to begin at a certain point of throttle and give more and more down as higher throttle was applied.

The crazy thing is; it not only worked, it worked perfectly, and tamed that nasty model to where it felt just fine.
It helped to have a Multiplex Evo that allows you to make adjustments while actually flying (set it up perfectly on one flight), but any radio that allows even basic programming should allow you to "cheat" like that.

Highflight