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Questions following RV-6 flight.

Yes, we were pretty surprised too! It really made that difference.
If I hadn’t experienced it I would not have believed it. I suppose it’s a combination of less drag from the wing and the tail so perhaps the difference had not been that great with just aft CG.

The Flightdesign CTSW that I fly on occasion jumps from 160 km/h to 220 km/h in a heartbeat when you bring it to negative flap from zero.

ESSB, Stockholm Bromma

The bit I find surprising is the 10 knot speed improvement. Presumably that comes from unloading the tail. I knew that being tail-heavy was meant to buy you some efficiency, but that’s more than I was expecting.

Last Edited by kwlf at 11 Jun 22:03

That’s what I was thinking too, CP moves forward due to an effectively shortened chord which in turn makes the aircraft tail heavy.

ESSB, Stockholm Bromma

Maybe there’s a John Roncz or Don Dykins among the crowd that can explain it all…

I’m not… but lots of flying wings use reflex in order to improve stability. At a stab, it’ll move the center of pressure forward, which will mean that the CoG is effectively further back hence the aircraft becomes less stable.

Last Edited by kwlf at 11 Jun 05:36

Thanks for the input LeSving.

I honestly cannot tell you exactly why the owner thought the flaps were incorrect from the beginning. He did, adjusted them and we had an interesting experience to say the least.. I’m glad it didn’t get any worse than it was or perhaps I wouldn’t be sharing this episode with the rest of you. Lessons learned.

ESSB, Stockholm Bromma

Well, to me the situation just seemed very odd (and still does). The correct position of the flaps and aileron is clearly stated in the building manual. They shall follow the foil, and to do that these wooden foil templates are included (they are printed on the wooden boxes that came with the kit). Due to the curvature of the foil, and because the flaps/ailerons are simple straight looking bits with no obvious curvature, it is not possible to get the correct position by simply by looking at them. The manual also say what to do if you for some reason have discarded the box. It is also important that the flaps and ailerons are aligned relative each other.

Without the wooden templates to start with, why would you believe it was originally wrong? and what lead you to believe it was correct after the modification?

I mean, if the correct position of the flaps leads to a funny flying machine, this is a sign of a much more fundamental error. Misaligned horizontal surface, misaligned incident angle of the wing, or misaligned engine or a bit of all these things.

There is nothing right or wrong in a certified sense about experimental aircraft, and there is no need to be a rocket scientist. It’s just that when building the aircraft yourself you think about all these little things, in every detail. Things are discussed with other builders and with the kit manufacturer. The original builder would never do such a thing as misaligne the flap. But if he did, he would most probably do it on purpose just to see the effect or some other reason. He would not simultaneously do lots of other changes that would mask the effect.

This is just the way it is with experimental aircraft. You can do lots of things you would not do on a certified aircraft. It is still a good idea to know what you are doing and test it properly, at least when altering the the configuration of the main lifting surface. Just my opinion, I don’t mean to criticize or anything. But think about what you do to the wing by lifting the flaps. You lower the incident angle on the inboard part of the wing. For the aircraft to fly, more lift is transferred to the outer part of the wing and this will increase the bending and twisting moment on the main spar depending on how much the flap is lifted. The faster you fly, more parts of the lift is transferred to the outer part until you reach a point where all lift is made there and the inboard section does nothing. I doubt Vans designed the wing for this to happen, which is why he made those templates and the procedure for aligning flaps and aileron.

The elephant is the circulation
ENVA ENOP ENMO, Norway

matti wrote:

In the gliders with flap and aileron adjustment I have never found similar behaviour

That was why I didn’t think the flap setting was the problem to begin with based on my own experience with negative flap.
Maybe there’s a John Roncz or Don Dykins among the crowd that can explain it all…

ESSB, Stockholm Bromma

Nice outcome. Could this behaviour have something to do with less downwash after wing with negative flap setting. In this situation the horizontal has smaller incident angle and less negative lift. In the gliders with flap and aileron adjustment I have never found similar behaviour although I adjust my gliding speed mainly with this.

Matti
EFHV

LeSving,

I didn’t build the thing so I don’t know the details of the RV series. Let’s just say the flaps were not correct (apparently) although it wasn’t something you would catch visually unless you knew exactly what to look for and had the manuals at hand – perhaps the drawings as well. When I say discussed I mean to say that we talked about the upgrade and the fact that the flaps had been readjusted to be correct (or so it was believed).

We did not know why it behaved strange. Remember though that besides the flap there were some additional changes including a lighter starter and tailwheel that we first believed had caused a shift in CG to where it was too tail heavy. The flap adjustment was believed to be correct. After weighing the aircraft and realizing that it was indeed well within the CG range attention was turned to the only remaining change – the flap.

I would not have guessed that the slight change in neutral flap would have such a dramatic effect, but then I’m not a rocket scientist or aerodynamics expert as you may be.

Last Edited by Krister_L at 10 Jun 08:29
ESSB, Stockholm Bromma

It sounded no one knew why the aircraft behaved strange, yet “someone” had purposely misaligned the flaps and discussed about it. The manual of my RV-4 includes a procedure for aligning flaps and aileron, and also includes a “foil” in wood for proper alignment.

The elephant is the circulation
ENVA ENOP ENMO, Norway
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