I just built a twin-boom pusher plane out of correx with an inverted v-tail and need some tips on setup/design mods. The plane was inspierd by Sparky Paul's camera plane (OK, I pretty much copied it ).

It looks like this in-flight:


It generally flys OK, but I have some problems with tail wagging. It seems like after every control input, it oscillates for some time until it settles back into a stable condition. Also the tail hangs low in both right and left turns which may or may not be related.

General dimensions are 1.2m wingspan (polyhedral), 18cm root chord. Tail flat dimensions are 34cm span by 13cm (mean) chord. The v-tail angle was supposed to be 110 degrees, but turned out more like 115. It has a geared (2.33:1) speed400 with an 8x6, total weight 640g and seems to have plenty of power.

I'm thinking the tail might be a bit small (~20% of wing area) especially the vertical projected area, or not far enough back. I might be able to fix the tail-hanging problem with v-tail differential or a more forward CofG. Any ideas?

/Splat

You can increase yaw and pitch damping by increasing the tail moment arm length. Damping is proportional to tail area and to the square of the tail moment arm length so lengthening the tail moment arm is the way to go. Keeping the extremities as light as possible will also help.

What a splendid design!
I bet what you're seeing is the tail boom bending!
The loads are high, and the booms must be stiff!
Area wise it appears to have more than enough horizontally.
Slide the booms aft some, and stiffen them up.
Boom type planes tuck easily! (Sob!)

Thanks for the tips guys. I have been reading a bit more on E-zone and it seems what I have is the symptoms of a classic "dutch roll". This would indicate a lack of vertical tail area as I understand it. It is easy for me to cut a new tail, but to slide the booms back I would need to make new control wires (servos are fixed to the wing, not the boom). I'll try a new tail first with an angle of about 95 degrees, and slide the booms back if that doesn't fix it.

Boom flex is also a possiblity as you mention. Although I think then I would also see instability in pitch and a lack of elevator response. I only seem to have the instability in yaw, and it will pull clean tight loops with authority the way it is now. Anyway I will also try to brace thew booms a bit if I can figure out how to do it without adding too much drag.

Cheers,
Splat

Dutch roll (yaw instability) arises without any control input. Still, your vertical tail volume coefficient may be marginal and increasing vertical tail area will improve damping of the oscillations as well as providing more yaw stability. Moving the CG forward a bit will improve both pitch and yaw stability at the expense of control response. When stability increases so does the strength of corrections after a gust or abrupt control movement. It is overshoot of the correction that starts the oscillation. As the CG is moved aft, stability decreases, the frequency of oscillation is reduced and the amplitude of correction is reduced. At neutral stability there is no correction and the plane goes where it is pointed. At this CG location the amplitude of oscillation is zero and the frequency is also zero. Most pilots find a plane with a little stability more relaxing and pleasant to fly than one with neutral stability.

As damping is increased the plane becomes critically damped or even over damped. When damping is sufficient, there is no overshoot and thus no oscillation to the correction. The plane can be as stable as desired and still no oscillation. Because damping goes up as the square of the tail moment arm length, increasing this length can cure oscillation while increasing stability.

Looks like yours Sparky Paul . CU

You can both stiffen the booms and increase the vertical tail ( but decrease the horizontal ) area by tieing the booms together at the rear with tension, i.e. flex the booms towards each other and connect with strapping tape or kevlar thread or spectra thread or whatever. This will make the v-tail more vertical stiffen up the booms. You'll have to remount the tail, probably, because it will change the angle of incidence of it.

My gut feeling looking at it is that the rear end is wobbly and can easily twist or flatten out a bit while flying, and it really wants some pre-tension to keep it rigid. Anyway, that's what I've done.

Looks like a bit more vertical area did the trick. I cut a new tail with 95 degrees and it flies much better now with no more tail wagging. I didn't need to add any more stiffness to the booms, but thanks for the tips.

Cheers,
Splat