RCCOMBAT Forum

I've been making up a batch of SSC and Open B birds for the upcoming season. Essentially they are nothing but J.A.B.A.C.'s (just another Battle Axe clone). It has been my belief that the 495 sq in wing on the standard Battle Axe should be a little bit bigger, something closer to about 550-575 sq in or so. Thus, I cut a batch of wings with a 1 in chord increase, made new pylons for them and mounted them on the rails. They turn a little tighter, as I had hoped, and they don't seem to be any slower. The c.g. is in an acceptable position and they are stable. BUT, they have a strange and unacceptable characteristic that I can't account for. When flying along, and without any control input, they will occasionally "bunt" down slightly and start a descent of about 20 deg or so. They recover with normal control input. Because of this, I can't take my eyes off of them to look for streamers. That makes them not very usable for combat. The only thing that I can think of is that because the wing has a larger chord and the trailing edge of the wing is a little closer to the stab, and therefor a little more short coupled, that somehow there is a "blanking" effect taking place from the down wash of the wing interfering with the stab. I can't think of any cure except to increase the size of the stab in hope of getting it out of the down wash. Anybody else got any ideas?

More info: I went out and flew it some more today and it diverges equally upward and downward, not just downward. I'm going to try stiffening the stab with some more skewers because I found it to be a little flexible near the leading edge, and if that doesn't work, I'm going to try a larger stab and perhaps change the incidence angle (it's set to zero)

Gary
Try putting a little weight on the nose to change CG and see what that does.

Gary
Try putting a little weight on the nose to change CG and see what that does.

After you trim it for straight and level flight is the elevator neutral or up or down a bit? If it is up or down you may need to adjust both ailerons up or down to get the elevator to neutral. Sometimes the ailerons and elevator are fighting to control the pitch and can cause similar signs. Being tail heavy and or a combination of both could cause it also.

I slid the wing/pylon back 1" on the rails to make the airplane more nose heavy, and got the same results. I'll take a look at the stab/aileron trim when I get back from this 5-day trip, but I seem to recall that it's pretty darn close to neutral. thanks for the ideas so far.

Is the trailing edge of the ailerons square or tapered? I've had squirrly results from tapered stock.
I only use 3/16 flat stock.

Have you tried a subfin?

I echo what Hattrick says...adjusting the reflex on the ailerons does help like he said. Usually only a couple of turns of the clevis on both ailerons is all it takes. I had similar problems with my SSC planes being very pitch sensitive. I ended up lengthening the fuselages on all of them and they are now much more stable for me. Of course making sure the CG is right is the first step. Good luck!

If you use two channels for ailerons (which I do) you can use flap trim to adjust reflex in flight if you set the plane up as having "flaperons".
I leave the elevator neutral and trim out using the flap trim. This seems to avoid the issues you are describing.

That requires two servos for ailerons. Right Keith???

boiler wrote:That requires two servos for ailerons. Right Keith???

That's right Bob
I got too much flutter with single servo set-ups, especially in Open B.

Blue Note wrote:Another thing to consider is the possibility of having an elevator servo that is double centering and drifting erratically. I have encountered such problems with several older Hitec HS-81 servos, even when they were new. At the time it was hard to diagnose as the amount of movement was very small. Sometimes they would later self correct on their own, but if I had trimmed for one position, it would suddenly be wrong. On the ailerons it is harder to tell what is going on, but on the elevator it can be much more obvious you have a problem. It might not be it, but just wanted to throw that out there.

I also had issues with servos not acting properly. After 2 new servos doing the same thing, I tried a different receiver. Same thing. I figured the transmitter saw too much rain. Different transmitter and everything worked just fine.

I did have the same thing happen with the Hobby King Oranage Futaba 2.4 clones. Failsafe disabled like the instructions said and it did the same thing. Once, you gave it input, it went to where you wanted it to go. But try to fly straight, and it wouldn't hold center.

Well, the problem is solved. And the award for best guess goes to BlueNote. Chris was right, it was a bad servo. I went out to the garage last night and started testing the elevator servo. I could hear it "chattering" and noticed that it wasn't centering accurately. It was very "soft" around neutral and would change "neutral" position without any radio input.

I changed out the elevator servo and went out to do some test flying today. The plane flies great now. It's stable, it turns a little tighter, appears to be just as fast and is simpler to build with a lower parts count.

So, THANKS! to all of you guys who gave me suggestions on what to look for. Now.... check 6!