Well i just finished and test flew my combat plane and it may have been the wind but it seemed really unstable and sometimes at slow speeds it would really pitch up. can anyone explain this??

A few checks:
These parameters influence the pitch-up at low speeds
1) rearward centre of gravity
2) Stabilizer surface too little
3) motor pulls upward too much.

What airplane is it?

It sounds like your CG is too far back.

On some planes, a tendancy to hunt or pitch up or down at random can be caused by the ailerons being drooped, so you may need to reflex the ailerons a little bit. Reflexing means raising both ailerons.

If it pitches up like that, especially at low speeds, the CG is too far aft for the geometry of the plane. As the nose pitches up a little, the wing's Center of Pressure moves forward, causing the nose to pitch up even more. Move the CG forward a little at at a time until it flies the way you want it to.

If it only happens when the prop is generating thrust, then look at the thrust angle, but I don't think that is the problem, based on your description.

I'm with Kirk on this one. Add some reflex in the rons and try it again. It's a lot easier to turn your clevises than trying to shuffle weight. The plane will act and fly tail heavy when it wants more reflex.

r

That may work, by changing the pitching moment of the airfoil, but only when positively loaded. When you fly it upside down, it might makes things worse yet. If you never fly it upside-down, it might be a good solution.

Oddly enough, it does work quite well when inverted.

But it depends a LOT on the airplane, and the airfoil, which is why the first thing I did was ask what the airplane is. If it has an airfoil with 0-camber or nearly-0, then reflex may not help, or it may hurt as you say. But if it's a design with a bunch of camber, some reflex can do wonders.

I still think the problem sounds most like a CG problem, but I also think there is not enough information provided to really know what is going on.

Other info that would be nice to know:

- Is it a gradual pitch change or very sudden.
- The post mentions slow speed, does that mean it only happens on final approach to landing?
- After the pitch change, what happens? Does the nose flop down, or does a wing drop, or does the plane just continue to climb?
- What happens if you are flying full throttle and trimmed level, then suddenly pull off the power?
- What happens if you climb way up, get level, then cut the power to idle or even kill the engine and then enter a 45degree dive?

The results of this kind of testing will tell you if the problem is related to airspeed, engine thrust, or something else.

For example, if the plane is flown around at full throttle all the time, and the only time the pitch occurs is when the power goes from full-on to full-off on final, and the change is sudden, but isn't followed by a stall, it might very well be the thrust line as Pe Reivers suggested.

As you say, more information is needed to be definitive. Taking the post at face value and making several assumtions based on what I think is likliest: I have seen similar behavior several times before. With an airplane that I want to fly inverted a lot (assumption, based on "combat plane")), reflex would not be a good solution, since I want to maintain a minimal static margin that is symmetrical between upright and inverted.

I suspect that in the assumed case of an airplane that is somwhat negatively stable when upright (at least at high AOA), if we stabilize it only by reflexing the ailerons, then when the airfloil is negatively loaded, the pitching moment may change (toward the negative relative to the flightpath) to the point that it goes unstable again, depending on the positively loaded static margin and the characteristics of the reflex.

A airplane with a reflexed airfoil with a sufficiently large static margin when positively loaded will still be positively stable when inverted. I didn't mean to imply that all reflexed airfoils are unstable inverted.

Another consideration is that airspeeds do not always reach the minimums inverted that they do upright (AOAs do not always reach equivalent maximums), depending on the kind of flying done.

Right, I don't disagree with what you are saying about what's going on with the reflex.

However, oddly enough most combat planes don't spend much time inverted. So while you need to be able to fly inverted, and be stable, you don't need the same well-tuned performance when inverted as you do when upright.

In my experience most pilots spend most of their time upright, and mostly pull positive G's. You'll get an occasional negative G push out of a turn or a push over in to a dive or at the top of a bump-style maneuver. But sustained inverted flight isn't seen very often. Which is why the best flying combat planes all have cambered airfoils, and why my planes are set up to turn inside much harder than outside. (not that I can't go outside, or fly inverted, I can, and do once in a while).

(When I say inverted, I really mean with negative AOA. The top of a normal inside loop, for example, doesn't count as "inverted", the AOA is still hard in the positive direction. Just to be clear about things here).

Sounds like we are on similar pages. Sometimes it takes a few posts to figure that out[8D].

alright i will do another flight without changes and tell you how it goes( that is if the wind ever dies) but it is a home built you know with the whiffle ball bat fuse and coroplast surfaces. its pretty much a club design and a friend there has a computerized foam cutter so i cet the wings from him. i think my best bet would be to adjust the CG first

Simply checking the airplane would be a very wise thing to do before flying it again.

Check the CG location and check the aileron reflex. Then you can make adjustments if either/both happen to be incorrect.

Good luck and please post your findings when you get it sorted out.[8D]

D

As a quick check I do for my own designs:
Airfoil set at zero AoA; Engine at zero , and stab at zero. CG not further back than 25% of mean chord. All my planes fly quite well first off, and fly just a little better after final flight trimming.
I do not fly inverted (negative G) in combat. Just like Spitfires, I have to roll and yank. Thus I can use highly cambered foils for very tight turns.
No clunk either, the fuel line goes to the tank bottom. Prolonged inverted would kill my engine.

i checked my cg and my alerons before anything but it must not be right, hopefully tomorrow the weather will be good

Ok, i did my test flight. it seems to pitch up only i go from no throttle to full and if i start an intentional climb it continues to climb and climb. the same thing happened when i changed the cg

Ok, that sounds like a thrust line issue. If it's pitching up when you apply throttle, and pitching down when you back off the power then you need a bit of downthrust or you need to shim the wing a little.

When you say it keeps climbing, do you mean that if you don't touch the thottle, and just point the nose up a little that the plane continues to pull nose up higher until it basically loops over on it's own? If that's the case, then a little reflex might help. You should be able to fly straight and level at full power, then pull sharply to a 45 degree climb, and the plane should just continue ona straight 45degree climb. If it pulls to vertical or beyond, reflex might help. Same thing downward, if you fly level at full power then push down, does the plane hold a steady down angle, or does it steapen the dive on it's own. If it keeps tucking in to a steaper dive, try a little reflex.

Since reflex is easy to adjust and put in and take out for most planes, I'd say give it a try and see if it helps. If you just raise the ailerons a couple of turns (don't over do it), and it doesn't help, you can just put it right back.

sweet reflexing the rons worked after all. thanks guys

imagine that glad you got it squared away

r

Glad that fixed it .