Flight trim issues

[ChiefK]

Flight trimming has always been a bit of a mystery to me. I manage to get my planes to track straight and level, but it gets "iffy" after that. My current issue is with my Seagull Ultimate biplane. The airplane requires down trim to fly level, it tracks poorly in inside loops and better in outside loops, and it will roll out of any attempt to knife edge as soon as rudder is applied. Engine is a O.S. .75AX which provides adequate power and airspeed. Radio is Spektrum DX7 - dual aileron servos, pull-pull rudder, dual elevator servos, standard throttle. Any help will be appreciated.

ChiefK

[HerkS]

Suggest balance between rudder trim and aileron trim may need adjusted. If you have level flight roll trim established by crossed rudder and aileron settings some adjustment of both may help with the loop problem. I'm assuming that you are not using aileron - rudder coupling.

I expect that the knife edge problem is something specific to the design.

[ChiefK]

The knife edge issue really bugs me. I read in another thread that rolling out was symptomatic of too much dihedral. Clearly NOT the problem here with only slight dihedral. As for the design, Ultimates seem to have a great reputation for flyability.... so don't know about that.

ChiefK

[kevinelp]

The knife edge issue really bugs me. I read in another thread that rolling out was symptomatic of too much dihedral. Clearly NOT the problem here with only slight dihedral. As for the design, Ultimates seem to have a great reputation for flyability.... so don't know about that.

ChiefK

I'm in the process of building one of these. It wont be in a flying state for another few weeks though, so I can't comment on the flight characteristics. My suggestion would be to check the dual elevators are tracking together - maybe they need some tweaking.

Also, it seems no right-thrust is built into the fire-wall, so judicious use of some washers may help your tracking issue.

I have to admit, your making me a little anxious about the knife-edge characteristics - something I also am interested in the ultimate performing okay.

If you have an incidence meter, you could check the wing incidences to see if something is astray.

Keep us informed if you have any success.

cheers

- Kevin

[mnowell129]

If the airplane is nose heavy at all and you are carrying some up elevator, then when on knife edge the swept wing will behave as if it has dihedral and try to roll out of the knife edge. I've seen this quite often with ulimate's because of the amount of sweep.
Try moving the CG back until it doesn't take any down trim for inverted, this should help the knife edge. You may still have to do some electronic mixing to make it perfect.....

[ChiefK]

I did notice that the engine thrust line seems to be zero-zero. In order to stay aligned with the cowl, it needs to stay there. I specifically looked for changes in attitude with changes in engine speed. None seen.

I did fly the Ultimate today and tried knife edge several times. In addition to "rolling out", the plane swerves hard toward the landing gear.... possibly due to the down elevator trim I need to fly straight and level. I did notice some slight down deflection on all four ailerons, so I have adjusted that now.

One of my club members has an incidence meter which he'll let me borrow. Although nothing looks out of alignment, it won't hurt to check and either note an issue or rule out an issue there. Plane balances exactly on CG recommended.

Inverted flight requires very little down elevator.

Fellow club members express approval and appreciation for the "normal" flight characteristics. The airplane apparently is smooth and impressive looking at takeoff, in normal flight, and in landing sequence.

Not using any automatic aileron/rudder coupling, and did try to work with aileron/rudder trim variances but noted no particular improvement.

I'll keep working it. Too nice an airplane to not try to resolve these issues.

ChiefK

[dayhead]

In reading your post the down-deflected ailerons caught my attention.
In level flight at 1 G loading, these ailerons might have a slight "flap" effect, increasing lift of the wing and perhaps be requiring your down trim to compensate.
In knife-edge flight the wing should be producing no lift. At a zero-lift angle of attack, the down ailerons might be producing a negative pitching moment. Think "flying wing" type aircraft here. This would result in the plane pitching toward the landing gear.

I don't fly this type of aircraft, but it seems reasonable to me that for this kind of flying a neutral, or "zero-zero" stability profile would be desirable.
We used to call this the "goes where it's pointed" stability profile.

You say the CG is in the recommended spot, but it should be kept in mind that no two airplanes can be exactly the same. It really could be that your particular plane needs it's CG just a smidgeon fwd or aft of the recommended spot. Respecting your airplanes' individuality will require your willingness to "fool around" with incidence angles, CG, and even the thrust line.

You say the plane has "slight" dihedral. Even a small amount will produce a yaw-roll coupling, so I'm surprised that a plane designed for this type of flying would have any at all.

[lincoln]

Seems to me that there's enough going on here that it might be worth digging up a series of articles on systematic pattern trimming. If memory serves, Dean Pappas had such a column, and I think he may have done it more than once over the years. Possibly in Flying Models, but I'm not sure. And I wouldn't be surprised at all if someone else hadn't done the same thing. Perhaps there's a book. Having it all laid out systematically would probably save a lot of time.

[mnowell129]

Pattern trimming guide from NSRCA
http://www.nsrca.org/trimA.htm

[ChiefK]

As a followup, I raised the ailerons 1.5 turns on the clevises and reduced rudder throw to 40%. These changes resulted in significant improvement. The airplane still requires some down elevator trim to fly straight and level, but a little less than previously. The reduced rudder throw seems to have reduced the coupling effect without reducing the ability to spin and reverse spin the airplane. That said, I now suspect that there is an incidence problem somewhere. Changing the incidence of the top wing would not be too difficult, but I'm uncertain how that would affect the other control surfaces and thrust line. I'm not the original purchaser, and assembler, but I do have a copy of the owners manual. Seagull does not specify any incidence settings. I assume they intend zero/zero and no engine offset. All this said, the plane still has a way to go to fly and track cleanly. I will continue to experiment, but flight trim is not a particular strength. All thoughts or suggestions will be appreciated.

Thanks for all responses so far. I will continue updates until I solve this or until I lose the plane.

ChiefK

[Thomas B]

The very tall rudder and fin are part of the knife edge problem. If you made some part of the rudder at the top fixed instead of moveable, it will reduce the roll coupling in knife edge. You could then increase the throw again and gain more knife edge performance, most likely.

That very tall rudder is acting like another aileron and rolling the model.

I have always felt that the rudder and fin were a little too tall on some Ultimates.

Some of the other issues do sound like incidence. It would be worth putting an incidence meter on it and see what it really is.

[Thomas B]

............

[HerkS]

ChiefK -- Down elevator trim to fly level

Any conventional aircraft has to be balanced at least somewhat nose heavy in order to be stable in pitch. So when flying level there is a nose down moment that has to be offset by aerodynamic forces. When the aircraft is flying level, but very slowly, more up elevator trim is needed to offset this nose-heavy weight effect. As the plane flys faster, the weight moment stays the same, but the aerodynamic forces increase with speed squared. Thus when you fly faster you have to put in down elevator to fly level. This is a fact of life.

If you change wing incidence you will change the speed at which the elevator lines up with the stab, but the physics will still be the same.