Our Reflex technology and paramotoring

The biggest evolution in our sport, aside from improved engines and better understanding of Paramotor frame geometry, is the introduction of our reflex profiles, into wings developed specifically for powered flight. It is now generally recognized that it has helped turn our sport into a practical and relatively safe, fun form of flying.

Some History
History has a habit of repeating itself. In the early eighties, similar developments took place as pioneers of ultra light aircraft bolted engines onto their existing hangliders. Whilst these early ultralights flew ok, they were not ideal.
A wing designed for power had a completely different set of requirements. A new breed of wing with stronger construction, different handling and more stability, was born

I was privileged to have been part of these early pioneering days. My name is Mike Campbell-Jones and for some inexplicable reason I was imprinted with a passion for light weight flying machines. My experience spans through hangliding, ultra-lights, general aviation, gliding, ballooning and paragliding to present day paramotoring.
It proved particularly valuable when developing the original Reflex paramotor wing in 1994.
 
Understanding Reflex technology
Reflex profiles are not new; they were first used extensively as far back as the 1930’s in tailless aircraft, such as the Horten brothers flying wings or the Fauvel tail-less glider, It provided these wings with aerodynamic pitch stability, where there was none.
Hangliders also adopted reflex in profiles to give gliders a positive trim. It improved safety, as it helped prevent tumbling. The effect was simple enough, more reflex more pitch stability, though less speed and performance as the angle of attack was increased.

But used in a paraglider Reflex profiles have a different effect. Because unlike the hangliders or flying wings the angle of attack is maintained through the lines connected to the pendulum weight of the pilot suspended below, in the same way as a wing with a fuselage and a tail-plane, acts as a lever, controlling the wing.
Reflex profile enhances this pitch stability, by adding an effective elevator into the wing, whilst keeping the centre of lift/pressure close to the leading edge. The wing loading is higher, as less of the wings area is used for lift. So stability and speed are increased without the need to change the wings angle of attack.
There are many other factors but the end result is there is also more efficiency at speed and a greater speed range (a flatter polar curve). So a bigger distance between the stall and cruising speeds and generally less likelihood of being robbed of that precious air speed in turbulence.  
For paramotoring, this is an exciting development.  


About stability
There are three types of stability necessary for any aircraft. Pitch, Roll and Yaw.
They make up the 3 axis by which most aircraft are controllable in 3D space. For paragliders the first two matter most, especially for those developed for powered use, since the low slung mass of engine/pilot and propellor thrust with all it’s associated gyroscopic and torque effects. This means that enhanced stability in both pitch and roll are even more vital than with non-powered paraglider wings.

Pitch stability -.
Inherent pitch stability is easily recognized in normal flight whilst flying through thermals or areas of turbulence.

• If the aircraft pitches forward as it enters the thermal and backwards as it exits. Then it is pitch positive.

• If the aircraft pitches backwards as it enters and forward as it exits. Then it is pitch negative.

This movement is very noticeable on paragliders because the centre of gravity (CG) is so far below the wing, a long way from the centre of lift/pressure (CP) compared to most aircraft. Remember the traditional fuselage and tail plane are replaced by the pendulum effect of the pilot and engine unit (CG).

• Performance paragliders for maximum efficiency have a CP around 30% along the chord from the leading edge (fig 01), which makes them inherently pitch negative. The wing’s stability is dependent on the pendulum effect of the CG (pilot weight) to control the angle of attack. In turbulence the CG (pilot weight) changes value. Imagine flying an aircraft where the size of the elevator changes in flight! This is why paragliding pilots are taught to fly actively, to constantly control the pitch of the wing in turbulence with the brakes - otherwise large changes in angle of attack can and do cause collapses. Indeed, some wings are even designed with more sensitive pitch control to enhance feedback in thermals.
  
  
• Low performance paragliders for beginners generally have more pitch stability to reduce the amount of input a new pilot needs to keep their wing open, this is considered safer. Designers generally achieve this by using flatter wing sections, which results in the centre of pressure being further forward - around 20% of the chord (fig 02). This gives more stability, but the wing often remains pitch negative.

• With a reflex wing section the centre of pressure is even further forward, around 15-20% of the chord. (fig 03); with its built in elevator the wing is now definitely pitch positive, so safer in turbulence. This has proven ideal for Paramotor wings as it means the pilot can fly safely with their hands off, as they do not need to fly actively to maintain stability. In fact, when the brakes are pulled pitch stability is partially reduced, as the reflex is removed and the centre of pressure moves further back from the leading edge. As this is the opposite behaviour of a normal paraglider, seasoned paraglider pilots find this hard to accept.

Accelerated flight
Another major difference between a reflex wing and a traditional paraglider occurs when the wing is accelerated with either the speed bar, the trimmers, or both. Despite the change in angle of attack, stability actually increases as more reflex is introduced, by pushing the centre of pressure even further forward and creating more elevator effect with the reflex. Combined with the extra speed, the wing cuts through the turbulent air better.

Roll or spiral stability

A factor often over-looked when adding Paramotors to paragliders, is that quite apart from the aircraft being able to fly in a straight line, it is vital to make sure the craft is spirally positive.
Paramotors and trike units have many different attachment points and propeller effects; so generally they need flatter front profiles (more dihedral), for obvious safety reasons.
Most paragliders are designed and tested around a specific harness width and pilot position. As it is, they sit on the boarder line, partly because being close to spirally neutral is ideal for thermal flying.

Spiral stability
It’s vital, and a factor often over-looked when adding a motor to paragliders, to make sure the craft flies spirally positive (will recover on its own from a spiral dive). The spiral stability of a wing largely depends on whether the wing is anhedral or dihedral and its centre of gravity. (Fig 04 shows how anhedral and dihedral works relative to both an aeroplane and a paraglider wing and how it effects spiral stability.)
Aeroplanes with anhedral have a negative profile angle, so their wing tips are lower at the tips than the centre. They’re normally associated with aircraft like fighter planes where a very high level of manoeuvrability is important. However, these wings are so unstable in flight that they usually require onboard computers to make the precise adjustments needed to control them. The result is an aircraft that is spirally unstable and, if put into a spiral, will tighten into the dive and quickly become unrecoverable.
Aircraft that have completely flat wings have zero anhedral or dihedral and are also very manoeuvrable. They fly on the edge of stability and are excellent for aerobatics or situations where the pilots are highly skilled and trained to handle them. They are spirally neutral so will remain in a constant spiral that neither tightens up nor opens out.
Aircraft that have a positive profile angle have their wing tips higher than the centre of the wing, so dihral. Whilst being less manoeuvrable they are easy and safe to fly and exit spins and spirals without pilot input. 95% of aircraft are designed like this, for obvious reasons. If the centre of gravity of a wing is moved lower, like in the example of the Cessna 150 in Fig 04, a wing may then have zero anhedral and still remain spirally positive. However it is interesting to note that even with this classic design, when more powerful engines are fitted, so more dihedral added.
Anhedral and dihedral in paraglider design has the same effect and makes a wing spirally stable or unstable. It is largely controlled through the arc of the wing. If a designer pulls the wing tips closer they create a negative radial arc and the wing becomes anhedral and consequently spirally unstable. A constant radial arc produces a spirally neutral wing, which like its equivalent in aeroplane designs will create a wing that is excellent for acrobatics and thermal flying, but sits right on the edge of stability. In order for a paraglider to remain spirally stable the profile has to have a positive radial arc with dihedral. It’s possible to have less dihedral because, as with the Cessna 150 in fig 04, our centre of gravity is much lower than the wing, however if the arc changes from dihedral to anhedral the wing will become spirally unstable.

why PARAGLIDER certification isn’t working with reflex or power
I have three main concerns regarding the current paraglider certification tests: the lack of measurement of a wing’s actual pitch stability, the emphasis of frontal collapses as the main measure of safety and the limited level of spiral stability testing.
Firstly, there are no current tests for actual pitch stability.
Secondly, in order to meet the current frontal collapse recovery tests we would have to alter our designs in ways that make them more unstable in other areas. As in reality reflex wings are virtually impossible to frontal collapse, putting such a great emphasis on recovery, to the detriment of security in more commonly occurring situations such as spiral stability, is misguided.
It is beyond doubt that the new breed of reflex profile paragliders are more collapse resistant than classic paraglider wings, however, the current certification systems are designed around paraglider technology: they fail to take into account that, whilst it’s imperative to test a paraglider’s reaction to a frontal collapse, as they do collapse, a reflex wing doesn’t. A natural frontal collapse on a reflex is about as likely as a wing falling off an aeroplane – yes it can happen, and the consequences could be serious, but it is incredibly unlikely. However, when the certification authorities do manage to force a reflex wing to frontal collapse, it will naturally have a more dynamic reaction. This is considered to be a valid measure of a wing’s security, as it is when applied to a classic paraglider that collapses easily, however, with an inherently collapse resistant reflex wing, it simply isn’t.
Thirdly, whilst paragliders with a negative profile angle are spirally unstable, they behave less aggressively during the asymmetric deflation tests currently used in the paraglider certification system. Therefore, beginner’s paragliders, like DHV 1 or CEN A graded gliders, are now being designed with more anhedral or negative radial arc to pass these tests.
I believe this is a dangerous road for designers to follow as these wings sit right on the edge of spiral stability and minute changes in the harness and pilot position can cause them to become spirally unstable and lock into spiral dives.
Most paragliders are designed to be close to spirally neutral as it is ideal for thermal flying. They are also designed and tested with the hangpoints set at defined distance apart, normally 42 cm. However, if the pilot tightens their harness chest straps just a couple of centimeters narrower than the tested width they change the arc of their wing. The wing changes from dihedral to anhedral and becomes spirally unstable. Over the last few years there has been an increase in paragliding fatalities with modern beginner wings, where pilots have tightened their chest straps to feel more comfortable in rough air, then entered spirals to descend, from which they never recover from. We are starting to see the same thing in paramotoring. Knowing that hangpoint distances are so critical for paragliders designed on the edge of spiral stability how, with all the different paramotor hangpoint systems, propeller effects and wing loadings, how can we safely entertain the current test system?
Further to that, wings designed to be inherently safer for powered use, need to use flatter front profiles for more positive spiral stability and/or use wing sections that are more pitch stable and collapse resistant - this includes beginner paragliders and reflex wings. However during certification test these wings collapse differently from the way DHV, DULV or EN demand in their asymmetric tests (Fig 05). They end up having much less wing area left flying than higher performance wings, which makes them tend to turn more during the recovery process – a key measurement during paraglider certification. The solution many designers are forced to take is to build in more anhedral, which takes the wing closer to spiral instability. Or increase the distance between wing and pilot which increases G-forces in spirals and the risk of twists, especially with propellors.

Conclusion
The current paraglider wing certification is unable to truly assess the security of a reflex profile paraglider. There are some areas of relevance between normal paraglider tests and paramotor wing tests; however, other areas are totally inappropriate for the direction our sport is going. In fact, the efforts made to pass the current tests are leading to a position where we could soon have thousands of wings flying that have a serious security issue with spiral instability.
Paramania, over the last couple of years has made a variety of proposals to the main testing authorities in attempts to make the paragliding certification process more relevant; however, so far none of our proposals have produced any clear response and very few of our proposed tests have even been carried out.
We are naturally disappointed as we believe the purpose of any testing body should be to raise levels of safety and set standards for the general public. For this it must be capable of evolving quickly to meet the demands of the new technology. Otherwise what value does it have?
As other manufacturers join us in the use of reflex in wings and paramotoring continues to grow rapidly and we are concerned for future safety. And feel its time that wing test bodies woke up!

Article by our designer Mike Campbell-Jones ::Father of Reflex wing Technology::

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