Flap Reflex
Posted: Thu Jul 30, 2020 10:13 am
by 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.
Re: Flap Reflex
Posted: Thu Jul 30, 2020 10:58 am
by 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.
Re: Flap Reflex
Posted: Thu Jul 30, 2020 11:32 am
by 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
Re: Flap Reflex
Posted: Thu Jul 30, 2020 1:03 pm
by 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.
Re: Flap Reflex
Posted: Thu Jul 30, 2020 7:16 pm
by 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.
Re: Flap Reflex
Posted: Thu Jul 30, 2020 10:14 pm
by radfordc
GraemeSmith wrote: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.
Re: Flap Reflex
Posted: Fri Jul 31, 2020 5:53 am
by 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
Re: Flap Reflex
Posted: Fri Jul 31, 2020 8:13 am
by GraemeSmith
NWade wrote: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.
Re: Flap Reflex
Posted: Tue Nov 17, 2020 10:53 pm
by 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
Re: Flap Reflex
Posted: Wed Nov 18, 2020 11:45 am
by 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.
Re: Flap Reflex
Posted: Wed Nov 18, 2020 12:15 pm
by Bryan Cotton
At 1G and a higher weight, you are running at a higher AOA for a given speed. Less angle to go before stall.
Re: Flap Reflex
Posted: Wed Nov 18, 2020 11:18 pm
by Rynoth
509sx wrote: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.
It IS the G limit that primarily dictates maneuvering speed. In addition the the last couple posts, consider this (very rough and unrealistic) example:
Aircraft is rated to +10 G’s.
At 100kts, the wings can generate 10,000 lbs of lift before reaching critical angle of attack (stall).
At 150kts, the wings can generate 15,000 lbs of lift before reaching critical angle of attack (stall).
If the aircraft weighs 1000lbs, maneuvering speed is 100kts, because a higher airspeed might max the G-limit before the wing stalls (1000lbs x 10G = 10,000.)
If the aircraft weighs 1500lbs, maneuvering speed is 150kts, because a higher airspeed might max the G-limit before the wing stalls (1500lbs x 10G = 15,000.)
Maneuvering speed, in effect, imposes a maximum G-load airspeed on the aircraft. This speed varies depending on aircraft weight.
…
As is with any limitations, the engineering complexities need to be effectively dumbed-down for a pilot to have a a safety envelope to operate within with clear-cut standards that account for various safety aspects. I don’t actually know (and don’t want to find out) which part of my Waiex will break first if I perform a max-deflection maneuver above maneuvering speed, but I do know that weight changes my safety envelope for that airspeed, and if I operate within that envelope the margins should be reasonably safe.
I will say that the worst thing to do during a maximum G pull is try to induce a roll at the same time, as this imposes additional stress (via the aileron) at the outboard of the lifting wing that may exceed the envelope in an unexpected (to the pilot) manner.
Re: Flap Reflex
Posted: Thu Nov 19, 2020 1:52 am
by gammaxy
509sx wrote: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 like and agree with Art’s fundamental question and I think there’s a lot of conflicting information about it. I think it ultimately depends on the design of the airplane and reducing Va as weight decreases is a safe way to avoid higher load factors on other parts of the airplane.
In the example Rynoth provided, the lighter airplane’s wings were under much less stress, so the limit does not seem to be bending moment on the wing. Maybe the engine falls off or the seat collapses beyond 10Gs.
Of course, I don’t believe you can do the reverse and safely increase weight beyond what’s listed in the POH for Va and expect Va to also increase. In that direction, the limiting factor might actually be aerodynamic loads on the wings or other surfaces.
As evidence that there might be a little more to this, I’ve found two examples from POHs where a higher Va is listed at a lower weight. In both cases, the higher speed is listed under the Aerobatic category.
https://www.vansaircraft.com/wp-content … speeds.pdf
RV 14 @ 2050lbs: 150 mph
RV 14 @ 1900lbs: 169 mph
https://www.extraaircraft.com/docs/serv … -09-20.pdf
Extra @ 2095 lbs: 140 kts
Extra @ 1918 lbs: 158 kts
Re: Flap Reflex
Posted: Thu Nov 19, 2020 1:40 pm
by Rynoth
gammaxy wrote:As evidence that there might be a little more to this, I’ve found two examples from POHs where a higher Va is listed at a lower weight. In both cases, the higher speed is listed under the Aerobatic category.
https://www.vansaircraft.com/wp-content … speeds.pdf
RV 14 @ 2050lbs: 150 mph
RV 14 @ 1900lbs: 169 mphhttps://www.extraaircraft.com/docs/serv … -09-20.pdf
Extra @ 2095 lbs: 140 kts
Extra @ 1918 lbs: 158 kts
This makes sense if dropping the weight puts the plane in a new category with a higher G-loading maximum. Although the actual reason the speed changed probably has more to do with C.G. and regulations.
There is certainly more to overstressing parts of an airframe than simply G-loading, but Va itself is determined a certain way and that’s what we use, and we hope that it covers all aspects of overstressing the airframe at various speeds. I.e. it’s partially (mostly?) a bureaucratic safety standard.
Re: Flap Reflex
Posted: Thu Nov 19, 2020 10:28 pm
by 509sx
I read somewhere that the wing was sandbag-tested, and failed at 10.5 Gs, based on 1100lbf gross weight. This is in F-16 territory!
With the typical 50% structural margin to ultimate, we have a 7G wing. I like having a strong airplane; it gives me confidence in turbulence. But as I was saying, a G-limit only makes sense if the airplane’s structural integrity is limited by something other than the wing. Are there any known strength data for the motor mount? Here we have a fixed mass attached to the airframe, and how many multiples of the engine’s weight the motor mount structure can support could drive a G-spec. Classically, though, it’s the wing structure that’s considered the driver, so the maneuvering speed is one value, irrespective of weight. If I’m not mistaken, the top of the green arc at 125 mph is really maneuvering speed, not max structural cruise speed, as on a certified airplane. The latter is driven by a gust criterion. The former is more conservative. The 6 G number is a standard minimum for aerobatic airplanes. I’ve never seen a payload weight minimum for aerobatic airplanes.
Art 509SX 2700cc Sonvair TD Southern WA
Re: Flap Reflex
Posted: Fri Nov 20, 2020 2:56 am
by peter anson
The g meter in my Sonex once recorded 6.9g in turbulence. You really don’t want to go there, or at least not too often. That was a high enough load so that the spars, where they cross over at the center of the fuselage, punched dents in the fuselage floor. A Solidworks model calculated a 4mm bend in the spar between the two wing attachment points and the dents protrude about 3mm which is pretty good confirmation.
Peter