I'm building a 1/6 scale U-2, 13' wingspan. The first ones with a 80' span had just ailerons. When the TR-1/U-2R was built the increased w/s required the addition of spoilers for roll control.
To those with large gliders: do you supplement the roll control with spoilers?
My thought is to mix in some spoiler after a certain amount of aileron, similar to what some airliners do.
Of course they could also be used together as speedbrakes for the descent.

Any thoughts?

Jack

Hi, Jack I fly sailplanes with up to 4m (160", Multiplex Alpina, ASH26) wing span, and never had the need for augmenting roll control with the spoilers.
In this kind of sailplane normally no more than the ailerons is requiered, and if you need some more roll control, you mix some aileron input to the flaps.
However, a friend of mine has a 4.5 m (177") Graupner Discus 2b, and he felt that he needed more roll control, and mixed in some spoiler, like you suggested.
In your case, I would install the spoilers, if you need them for roll control, good, you have them. If you don't need them, use them only as speedbrake.

Michael

The U2 was on the verge of a stall when operating at near its service ceiling and approaching the speed of sound. The usable speed range under these conditions was only about 10MPH which made for very tricky piloting indeed. The turn spoilers may have been needed because of these unusual operating conditions.

Very high aspect ratio sailplanes (model or full scale) do not generally need or use turn spoilers. The high aspect ratio wing at slow speeds results in lots of adverse yaw which is overcome with aileron differential (more up than down travel) and rudder compensation for coordinated turns. That’s what works best at our altitudes and speeds.

Good info
An interesting point on the U-2: Because of the high camber wing, at altitude, higher pitching moments at high speeds resulted. Kelly Johnson developed the "gust control" setting, setting the ailerons up 10 degrees and the flaps up 4 degrees thereby moving the center of pressure forward and decreasing the tail loads.

I guess differential ailerons and mixed rudder should be enough, but I think I may still wonder till the first flight.

I had a knights of the air U-2 ducted fan which I could not roll out of about a 60 degree bank and I didn't think I was slow. Another turbine U-2 had a similar end recently. I'm not sure these were stall related.

Thanks
Jack

Mr Vortex may remember me as a fellow U-2'er who has a kit in the back room waiting to be built.

FWIW - I've seen film of the U-2R at liftoff using the spoilers for roll control. There are relatively small spoilers right in front of the outboard flap segment. Deploying spoilers in front of extended flaps can create significant roll authority. I think they were there to auto-coordinate turning flight at low speeds, and decrease the chance of a tip stall with aileron deflection. Likewise, they would be a benefit at altitude by reducing the need to deploy any down elevator which could have cause a tip-stall in those conditions as well.

The wing has such a severe taper ratio that the plane will be extremely vulnerable to tip stall with the scale model of the U-2 as with the full scale because of Reynolds' numbers. Remember, you are probably running Reynold's numbers at Sea Level takeoff just as touchy as the full scale would have at altitude - due to the size difference. I would program the ailerons with up-only differential, setting the down-motion to 0, at least while in the takeoff/landing mode. Down-going aileron would only make the tendency to tip-stall worse on such small-chord tips. You could mix in a bit a reflex aileron with flap extension as well, again to combat tip-stall tendencies while low & slow. Also, mixing in rudder wouldn't hurt.

I would also be worried about wing twist with scale ailerons. The deployment of the ailerons could cause the wing to twist the opposite way and negate, or even reverse the intended roll. The wing needs to be kept torsionally stiff in order for the ailerons to be effective.

I currently have a 100" Pilatus B-4 aerobatic glider. It also has a large taper ratio. I tip stalled it once during aerobatics, and used at least 75' of altitude to get it flying again. When I land this plane, I reflex the ailerons as much as 30 (non-scale) degrees to keep from tip-stalling when maneuvering for approach. Flaps could be used instead to supply the effective wing-twist that helps prevent tip stall - but this plane doesn't have them. Since the U-2 will have them - they should be used on both takeoff and landing, not so much for the added lift, but for that added wing twist that helps prevent tip-stall. I also mix more rudder in for landing than for thermal cruise. My ailerons have a 3:1 differential all the time.

One reason you may have lost control of the KOTA U-2 could have been the scale sized tail surfaces. When flown too slow, the tail surfaces just become ineffective( Reynold's number again) and the plane seeks its own way, usually steeply towards the ground. This even makes the ailerons ineffective - as there is no rudder effectiveness to counter adverse yaw. I have done this with my Pilatus a few times - when trying to make one too many turns before settling into final approach. Good thing that plane is tough.

Sorry if this was a bit repetitive and rambling. It's late and I need to stop writing now.

Hi John,
sorry for the late reply, I was out flying the large aluminum tubes around the country.
I do remember you and that you have the big U-2. Thanks for your input to the large wing roll problems. I'll look at each point you brought up. I was going to plan to not incorporate flaps initially since I'll have the scale speedbrakes. But the Reynolds number is significant. Roll spoilers look more promising though the early U-2 did not have them.
I have incorporated a more semi-sym airfoil as the wing progresses toward the tip giving aerodynamic washout.
Jack

>"Likewise, they would be a benefit at altitude by reducing the need to deploy any down elevator "

Um, I meant *down aileron*. It was late...