Flap Reflex

[lpaaruule]

Has anyone experimented with flap reflex positions in cruise flight?

I've read that some gliders will allow their flaps to be moved up slightly to reduce drag, and effectively increase wing loading. I also saw that Aircraft Spruce sells a Reflex Flap positioning system that has been used in RV-10s. The flaps are usually reflexed 10 degrees up.

With manual flaps, it would be easy to return them to zero degree position quickly if needed, and I'm sure a stop could be made to prevent accidentally moving it to the reflexed position.

Seems like a simple way to either increase cruise speed, or reduce the effects of turbulence.

[John Monnett]

Seams like a good idea but the flap interface with the rear spar/ skin TE would prevent reflex. Although we have a lot of experience in reflex flaps from our Monerai and Moni days and both had airfoils and structure to permit reflex that works to some extent, I doubt it would be worth the effort on the Sonex.

[NWade]

Speaking as a glider pilot and owner of a flapped glider (ASG-29): While reflex can be effective for some situations, it’s worth noting that glider airfoils are markedly different in shape than the Sonex airfoil.
After all, a glider’s wing needs to provide a high amount of lift at lower speeds (while climbing in lift, such as a thermal) and yet also create laminar flow across as much of the airfoil as possible (chord-wise) without any laminar separation bubbles - to minimize drag and support a high L/D (glide ratio). Given the metal skin and round-head rivets, a Sonex wing just isn’t going to be able to achieve the same laminar flow and low-drag state at high speeds (which is exactly what glider wings are trying to do with flap reflex).
Furthermore, gliders tend to have larger-span flaps than the Sonex wing planform- and modern gliders also tend to have interconnected ailerons and flaps so that all of the wing control surfaces move as ratios of each other (essentially making the entire trailing edge of the wing a multi-segment flaperon). The traditional flap & aileron division on the Sonex wing would lead to a discontinuity in lift and drag between the inboard and outboard sections of the wing with the flaps reflexed. In other words, you’d only be reducing drag (and lift) over a portion of the wing, and with the rest of the wing still experiencing higher lift & drag the overall improvement would likely be pretty small. It might allow you to use a bit less elevator trim in high speed cruise; but that reduction in trim drag won’t net you more than a few knots of speed. And given that discontinuity I’d also want to make sure that the change in span-loading wouldn’t be a problem (although given how beefy the Sonex spar is, I wouldn’t be surprised if it’s no sweat).

To be clear: it’s not a *bad* idea; but modifying the flaps & rear spar & wing skin to support reflex would be a heck of a lot of work for a very small gain. If you want to pursue that angle and see noticeable gains in either top end speed or a larger overall speed envelope, then you’d want to design a new wing (including selecting a different airfoil) with full-span camber-changing in mind.

Take care,

—Noel
Sonex #1339

[lpaaruule]

Thanks for the replies.

I definitely don't want to do something that's going to involve a lot of work! :-) I'm enjoying flying my Sonex too much to have it grounded for any length of time.

I'm actually more interested in the wing loading than increasing speed. There have been times that I've flown well bellow normal cruise speed due to chop. It would be nice if I could soften them a little, and it would (maybe) even be easier on the airframe. Of course, an increase in speed is always welcome.

I have no plans to make any changes, but if someone had reported great results, I'd consider it.

[GraemeSmith]

To soften the blows - you want to fly heavy. It increases the inertia of the plane and reduces the apparent effect of the turbulence. But you also want to slow to Va or less so as not to over stress the plane. You don't want to fly faster in turbulence. Especially if light.

[radfordc]

To soften the blows - you want to fly heavy. It increases the inertia of the plane and reduces the apparent effect of the turbulence. But you also want to slow to Va or less so as not to over stress the plane. You don't want to fly faster in turbulence. Especially if light.

Because Va goes up as weight increases you can fly faster in turbulence at max gross weight.

[NWade]

Please keep in mind that “Va” and “maximum structural cruising speed” (VNO) are two different things!

Va correlates to max speed you can apply full control deflection (usually a positive G pull) and stall the aircraft or run out of control authority before putting enough stress on the airframe to damage it.

VNO designates the start of the yellow arc on the airspeed indicator - where the aircraft structure may not be able to take 50 FPS gust loads without being damaged.

—Noel

[GraemeSmith]

Please keep in mind that “Va” and “maximum structural cruising speed” (VNO) are two different things!

Va correlates to max speed you can apply full control deflection (usually a positive G pull) and stall the aircraft or run out of control authority before putting enough stress on the airframe to damage it.

VNO designates the start of the yellow arc on the airspeed indicator - where the aircraft structure may not be able to take 50 FPS gust loads without being damaged.

—Noel

Good important point. Just finished reading a load of WWII reports on DeHavilland Mosquito losses caused by structural failure. In an attempt to make the aircraft easier to handle with extreme CG's caused by war stores carried - the aileron balance horns and rigging tension was adjusted to make the aircraft easier to handle. After the bombs were dropped and with a lot of fuel burned off - the aircraft CG went to the other end of the envelope. The downside was that pilot induced full aileron deflection to keep the much lighter aircraft upright in turbulence - while flying at high evasion speeds - simply tore the wings off.

[509sx]

I just don't get it, but I see everywhere this notion that maneuvering speed is weight dependent. If the weak link in the airframe is say, the motor mount, bomb racks, or seat structure, I could get that a G limit makes sense. But if the limit is wing bending moment, that is driven strictly by dynamic pressure (and AoA), and weight doesn't matter. In this case the issue is the maximum load the wing can generate at the AoA for max lift, just before stall.
I have seen data that suggests that CL greater than the max available steady-state can be generated in a dynamic maneuver, momentarily (in very short time scales), before the flow has time to reach a new equilibrium around the airfoil. In this case being lighter should be good in that it would allow the airframe to react to gust loadings by accelerating in the appropriate direction, easing the load.
O.K. so what am I missing?
Art
Southern WA
Sonvair 2700

[GordonTurner]

At a given: Weight, speed. Maximum AOA will give a specific G (acceleration).

The structural limit is a specific G (acceleration), in our case 6.0.

At a lighter weights slower speed will give the 6.0 G (acceleration). At a higher weight a higher speed will be required to attain the same acceleration.