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There are some paramotoring wings presumably designed for higher speeds. The method is to allow a portion of the wing near trailing edge to flutter in high speed, rendering it unaccountable for development of lift, but still accountable for drag (in my opinion). I believe is called reflex because the trailing edge will flex up in high speed.

The speed gain is minimal though. These wings fly at around 40km/h with stall at 24km/h and the reflex promises 60km/h, while the same wing trimmed non-reflex achieves 55km/h.

If there is anyone with piloting experience on those wings, please explain the reflex from your perspective.

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I'm haven't flown them but am very familiar with them and the reflex wing controversy because I was interested in getting into the sport (I normally fly power and gliders).

A reflex wing when in "reflex mode" offloads the aft part of the wing and shifts the pressure distribution forward. Effectively, the rear part of the wing is being allowed to "trail", or partly trail, up and not do any (or much less) work. It's a bit similar to gliders with flaps that have a reflex setting, where the flap is angled trailing edge up a few degrees for high speed penetration flying, which off loads the trailing edge and moves the wing's pressure distribution forward and makes it behave as if it had a narrower chord.

It's not related to reflexing flying wing aircraft trailing edges for trimming forces, where the trailing edge provides downforce in place of the absent tail to trim the body to a given AOA; that simply can't work and isn't necessary in a paraglider.

The effect of reflex is to somewhat reduce the effective wing area, increase the effective aspect ratio, and reduce induced drag in the process. A 26 sqm wing becomes effectively a 20 sqm wing (I'm just picking numbers here) with a narrower chord and what amounts to some loose fabric (the unloaded trailing edge) trailing behind it. Because the TE is unloaded, you can't use the normal trailing edge drag brake steering and have to use "tip steering" controls.

The different pressure distribution increases the inflation pressure at the front of the wing and makes it more collapse resistant, and this is one of the reflex wing's selling features besides slightly higher cruising speed. The downside is that when they do collapse, they do so violently with way more altitude loss and if you are close to the ground your wing will pretty much fling you into the dirt like a rag doll if you get a collapse. I don't believe there are any current wings that can meet EN cert requirements while in reflex mode. This means if you value your life you don't use reflex below a recoverable altitude, say 500 ft.

Having researched it a lot myself and noted the same thing you did, only a 5 kmph benefit, I'm not keen on reflex myself, especially after watching videos of how they behave when they collapse.

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    $\begingroup$ +1 for explaining that "reflex" in flying wings is completely different. $\endgroup$
    – Camille Goudeseune
    Dec 29, 2019 at 21:18
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    $\begingroup$ Thanks. There is a... somewhat colourful character, well known in the paramotor business, by the name of Schanze, who rants on about how dangerous reflex wings are, and I tend to agree, but he goes on about how they can't work because you "can't push on a string", as if it was a reflexed flying wing. He misunderstands what is going on. $\endgroup$
    – John K
    Dec 29, 2019 at 23:31
  • $\begingroup$ You can't reduce induced drag if the mass per span stays constant, unless you improve the span wise lift distribution (something I don't expect a reflex wing to do). My guess is that a reflex wing at low lift coefficients shifts the stagnation point a bit down, allowing a more favorable pressure distribution on the bottom of the wing. $\endgroup$
    – Peter Kämpf
    Jan 29, 2020 at 18:24
  • $\begingroup$ That may explain the increased internal pressure and collapse resistance. $\endgroup$
    – John K
    Jan 29, 2020 at 18:31
  • $\begingroup$ @PeterKampf Actually, can't induced drag be reduced by lowering AoA and going slightly faster? A parasail is a thin undercambered wing. It is great that the dangers of lowering AoA are pointed out. Some strange things start to happen with circulation under the wing at lower AoA. Dynamic pressure is needed there to hold it's shape. At lower AoA, it is more prone to collapse. $\endgroup$
    – Robert DiGiovanni
    Jan 8 at 17:27
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Three definitions first (simply): CoW (Centre of Weight) and CoP (Centre of Pressure). These two govern the aircraft (any) while flying (lets forget about thrust and drag now - the other two). AoA (Angle of Attack) is the anlge between the leading edge and the relative airflow.

In case of airplanes CoW and CoP are close together. For a Paraglider, CoW is significantly below CoP (normal flying). So it is possible that when AoA changes, CoW "moves out" from below CoP. The natural behaviour of all constructs in the air that CoW "wants to be" below CoP.

The difference between normal and reflex profiles is what happens when AoA change.

Let's say you hit an uplift stream (turbulent air going upwards). This will increase AoA.

In case of "normal" profile, this will result CoP move FORWARD. But, hence CoP "wants to" be over CoW, the whole glider will "slide backwards" a bit. This will increase the AoA even further, which will move CoP even forwards etc etc. It would cause the glider to stall, but by the time it would stall the pilot's pendulum will stabilize the system. So in rough air, normal gliders "move a lot".

In case of reflex profile, CoP moves BACKWARDS instead of forwards, moving the glider itself forward above the pilot, which decreases AoA, so the two effects negate each other. That is why reflex gliders are more easy on rough air.

The fact that they are faster has nothing to do with profile. Reflex gliders have different line distribution and different sizing, but these are independent "changes". Okay maybe not completely independent - a reflex glider, when fully trimmed in, has "almost like" a normal profile - to easy the job with takeoff and landing. But this means that they will get their relfex profile, once they are trimmed out, so weight is distributed towards the front lines. But the more full-bloodied reflex glider you get, the less this is a compromise.

IF you overload a normal glider, it will be fast as any relfex glider.

Also worth to note that reflex is more difficult to open once closed up for any reason. But they fold much harder.

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    $\begingroup$ "So it is possible that when AoA changes, CoW "moves out" from below CoP. The natural behaviour of all constructs in the air that CoW "wants to be" below CoP." Oh look, it's the pendulum fallacy! Please see here. $\endgroup$
    – AEhere supports Monica
    Jan 29, 2020 at 15:58
  • $\begingroup$ @AEhere supports Monica - Well, you can object to the "wants to" construction, but the end result is the same as if it does "want to". Imagine a heavy, streamlined weight fixed in position a ridiculously long distance below a wing by a thin but rigid rod. That wing will tend to be roll-stable even with no dihedral, and pitch-stable even with no horizontal tail. The reason has to do with the roll torque created by sideforce during sideslip following an accidental bank, and the pitch torque created by increased drag due to increased airspeed following an accidental dive. $\endgroup$
    – quiet flyer
    Jan 30, 2020 at 6:37
  • $\begingroup$ @AEhere supports Monica - In both cases the aerodynamic forces act, very loosely speaking, near the center of surface area of the aircraft, far above the CG, thus creating a stabilizing roll or pitch torque. $\endgroup$
    – quiet flyer
    Jan 30, 2020 at 6:51
  • $\begingroup$ @AEhere supports Monica - Naturally, all this is important to understanding paraglider stability dynamics, where the pilot is fixed in place far below the wing. Positive roll stability results, despite the anhedral wing shape. Positive pitch stability results as well. Now, what if the paraglider wing were sprayed shellac to freeze the fabric in the familiar shape that we normally see in flight, but the lines were removed, and the pilot rode on a scaffold that fixed him in place high ABOVE the wing. Would that aircraft have any hands-off stability in pitch or roll? $\endgroup$
    – quiet flyer
    Jan 30, 2020 at 6:53
  • $\begingroup$ @AEhere supports Monica - consider also a toy weight-shift-controlled radio-controlled "trike" powered hang glider with Rogallo delta wing-- the aircraft gets a boost in pitch and roll stability due to the heavy "trike" unit with wheels, electric motor, battery, and servos held in a fixed in position via the servo linkage, well below the wing. It could never under any circumstances be kept in sustained inverted flight. Just as a paraglider could not, even if we replaced the lines with rigid struts, and "shellacked" the wing to fix it in its normal positively-loaded shape. $\endgroup$
    – quiet flyer
    Jan 30, 2020 at 7:03
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Fluttering is usually undesirable because it can lead to tearing and usually increases drag, which tends to lower airspeed. A raised trailing edge is similar in function to an elevator, increasing drag and limiting airspeed.

The concept may be similar to that employed by the "Suzanne" record breaking hand thrown paper airplane design, that had a slightly upraised trailing edge.

Thrown (for distance) free flight paper airplanes are deceptively challenging because of their extremely high speed range. Discus Launched Gliders are trimmed to be staticly unstable$^1$ when launched, but can be re-trimmed by radio control once they reach height. No such luck with free flight.

John Collins ingeniously designed a "reflex" or flexible trim mechanism that would lay flat under higher airspeed dynamic pressure, but "pop up" ay lower airspeed.

On the world record throw by quarterback Joe Ayoob, the plane did indeed fly out straight, lost airspeed, and stalled, but recovered in the dive and flew on to a world record.

Essentially, at higher airspeeds, reflexing reduces static stability and drag$^2$, allowing the paraglider to go faster without changing control input.

The reflex concept is not without some foundation.

$^1$ going faster lowers Angle of Attack
$^2$ light "luffing" may be noted

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