RV-10 Flap Reflex position is just more drag

[mulde35d]

I am sure i will stir a hornets nest, but thought it important to share my experience. While rigging my RV-10 flaps I decided to set zero degrees at the angle that extended the upper and lower wing surface as flat. In other words, the flaps are an extension of the wings with no change in angle while laying a straight edge along the surface. From this position they could go up 3.85 degrees, also called the reflex position (-3 degrees). I understand the reflex position to change the wing chord line with an intended result of a pitch up. This would then allow for less downward lift on the tail thereby reducing drag and increasing airspeed. But thats not what happens. When the flaps are moved to reflex the nose pitches down, thereby requiring more downward lift and an increase in drag on the tail. I realize this has a lot to do with my particular aircrafts CG and configuration, but it came out pretty standard for RV-10's.

During my latest cross-country while very stable I moved the flaps from zero degrees to -3 degrees (reflex) three time and let the aircraft stabilize. Each time the airspeed decreased 1-2 knots and returned to the higher airspeed when placed back at zero.

My 2 cents, when rigging your flaps, airlerons, and wingtips. Set your flaps such that they are at zero degrees to your wings surface, then set your ailerons flush with your flaps (also zero degrees). Then make any final adjustments to your wingtips to be flush with the ailerons. Assuming your control surfaces don't have a significant amount of twist, this should provide a stable starting point for final in flight rigging with the least amount of drag for cruise flight. Obviously still follow Vans Instructions, but Van's provides very little info on flap and aileron angles, so hopefully this helps

A picture of the reflex position is below. I set zero degrees at the point where the flaps and upper chamber are flush (no change in angle). Needless to say, not planning to ever use reflex on the RV-10.

 

[Richard Connell]

I’m pretty sure that standard rigging is 0 degrees (like you) is trail. -3degrees is reflex and +10/20 etc for deployment down below trail.
There’s been a fair bit of ink spilt over the years on trail versus reflex.
The theory is that trail is indeed supposed to be faster at high altitudes and or higher weight so I think you are seeing what is to be expected.
Fwiw I haven’t seen any demonstrable difference over the years of experimenting. But I don’t have extensive test points.
I think you’d need to do a series of tests at various weights and altitudes to quantify it across loading and altitudes.

What I do tend to notice is the plane gets noticeably faster as it gets lighter. I just did 2 back to back 5h flights this week and we were a couple of knots faster at the end both times.

Great airplanes.

 

[BobTurner]

IMHO the analysis is a bit off here. It's complicated. When you move the flaps from in trail to reflex, the wing loses some lift from the last 8" of the flaps. This reduction in lift causes the plane to drop vertically (and the huge drag of the wings in the vertical direction cause the nose to rotate down). The pilot "catches" the drop with elevator, momentarily increasing tail drag. But as the plane re-establishes equilibrium, the center of lift of the wings has moved slightly forward, because the last 8" of flap isn't producing as much lift as before. This reduces the nose down torque about the cg, which means the nose up torque of the elevator is also reduced. e.g., the tail drag goes down slightly. I think I see a few more knots down low (9,000') in reflex than I do in trail. But the test is flawed, as I did not adjust the ailerons or the wingtips for an "in-trail" configuration. But the difference is pretty small. I did some in-flight testing at 15,500' (17,000 DA) (and about 400 lbs below gross) and thought in-trail was very slightly faster there. Although Vans engineering says it's okay to run in-trail at any speed (in 2008 they said it was NOT okay!) I do note that that means the flap linkage is constantly carrying some load. In reflex the flaps are up against the aft spar, and that carries the rotational load.