Yaw stable and hard turns

[willfly]

Hello All,
I have a high wing R/E model with 10 oz AUW. The fuse is 2" in height, 36" in length and wing is 48" with polyhedral breaks midway. Wing tip is 4" higher then chord.

Here is the problem: The plane is yaw stablized. On a full rudder, the model yaw's as much as 20-30 deg but resumes flying in the same direction as before. This is making turning very difficult and I have to hold on to turn stick for 1-2 seconds to change flying direction. This happens on power as well as, on a glide.

I had seen pitch stability, but have discovered yaw stable for first time. Any explanations for this behaviour. I doubted not having enough dihedral as a cause, because then the plane will have a tendency to spin rather then turn.

[vintage1]

I am not entirely clear as to waht teh problem you are describing is, but whichever it is, its dihedral.

Planes with too little dihedral yaw, but take ages to bank into a turn, and once in, stay there and are the devil of a job to get out..I know: I knife edged a low wing RET model into the ground after a gust tipped it. No way could I get it back..

Planes with too much dihedral need constant rudder to stay in the turn but usually turn in very quickly and are very responsive to ruder. I build my RET's that way because they can handle turbulence better.

[willfly]

It looks like the problem was indeed having not-enough dihedral. I realized that the plane did not bank when rudder was applied. It just yaw'ed.

Anyway, this plane is destroyed (uncontrollable), but will make another one with lots of di.

[Sparky Paul]

I've added dihedral to planes which indicated they needed more, by sawing off the tips and reattaching..
This one had ailerons and no dihedral, as an SAE lifter, but these didn't handle the rolling well enough, when the wing was mounted on this fuselage and tail as an electric, with rudder control instead of ailerons.

[willfly]

I repaired the wing and the fuse and added a lot of dihedral (25 deg) and flew it again today. Now it is way better in the turns. Thanks for the help.

[vintage1]

25 degress is really too much. Rule of thumb is 7-10 for a low winger and 5-7 for a high wing.

You only need enough with an RC model to ensure that yoiu can get in and out of a bank.

Too much mans you have to hold rudder in te turn.

[markdrela]

25 degress is really too much. Rule of thumb is 7-10 for a low winger and 5-7 for a high wing. My guess is that he means a 25 degree center bend. So he really has 12.5 degrees of dihedral. That's about right for a R/E sailplane, and only slightly excessive for a R/E powerplane.

FWIW, dihedral should always be defined from the horizontal. That's what is aerodynamically significant, and it's the convention in the aero business.

[willfly]

Thanks for pointing this out, will keep it in mind.