Simply put, it is an aerofoil which – in case of decreased angle of attack – automatically tends to have it increased again. In other words, it always stays in neutral position regarding its load. On drawings it's an aerofoil that has its tail turned upwards :)
One of the most distinct features of the paragliders equipped with reflex airfoil as opposed to classic paragliders is their substantially increased stability on high speeds.
As always in nature, nothing is for free. In case of longitudinal self-stability you pay for it with lesser lift coefficient. It means that in order to get good glide ratio you have to choose proper wing shape and aerofoil thickness. With paragliders there comes additional problem, namely big forces needed to steer it. These are main reasons why you don't find full reflex aerofoils among free-flying paragliders. In our designs we solved these problems through carefully adjusted rigging as well as introducing several new solutions like ALC and TST systems.
With PPG or PPC canopies worse glide ratio is not that problematic, because there is always an engine to help, while self-stability means that even in turbulent air you can feel completely safe (and fly with hands in your pockets :) Considering limited manoeuvrability of a glider burdened with paramotor this is an extremely desired feature. Last but not least, with such big margin of stability reflex canopies can be safely flown at much higher speeds than classic paragliders.
In reflex-airfoil paragliders releasing trimmers and activating speed system results in shifting pilot’s weight towards leading edge, so that A and B rows carry almost all of it while C and D rows get relieved.
The opposite happens in classic paragliders – with increasing airspeed load on the A and B rows decreases, while more is carried by C and D lines. In some extreme cases full acceleration can lead to total unloading of the A risers, with subsequent frontal collapse (frontstall).
The important quesiton is, how to discern a reflex paraglider from a classic one? Currently there is quite a number of paraglider available claiming to be partially or semi-reflex, and no one knows for sure what does it exactly mean.
Sometimes such names are a mere marketing tricks, aimed to use established position of fully reflexed designs. However, there is a way to prove if there is a reflex attitude present or not.
In 2006 an idea was born within Dudek Paragliders concerning effective measurement of variable loads occuring at individual risers.The idea was brought to life by a Swiss test center Air Turquoise (http://www.para-test.com), while detailed technical solutions were prepared by École Polytechnique Fédérale de Lausanne.
Reflex paragliders can be tested in flight as the classic paragliders are, by traditional, well-established European Norm procedures. However, tests are performed with only fully closed trimmers (slow settings). The reason is, with released trimmers (fast settings) some manoeuvres routinely done during trials are impossible to execute or unreliable when attempted on reflex paragliders. This does not mean that a reflex paraglider is unsafe when flown with released trimmers – quite the contrary: for example it is so much tuck-resistant, that it can’t be properly tested with traditional methods. Such a situation happens only when the paraglider is a truly reflex one, and that can be proven by measuring in-flight loads of individual risers (link).
Dudek Paragliders performed such tests in case of Action/Reaction models (see above) as well as Nucleon and Synthesis.
Reflex profiles have been known in the aviation from its early times, and they are to be found in many publicly available profile catalogues. They have been tried in hanggliding and paragliding too, initially with little success. Especially paragliding application proved to be a hard task – lack of rigid elements, low wing loading and high steering forces discouraged many of the designers.
After many attempts, the first serially produced paraglider with really working reflex profile was created: it was the Traper of Dudek Paragliders.
Traper aroused high interest and won a high acclaim among competitors when it was shown at European PPG Championship (Hungary, 2002). It was there that Michael Campbell-Jones saw it and he couldn’t believe his eyes, seeing a practical realisation of an idea he tried to introduce for a couple of years. That galvanized him to enter close cooperation with our company. Michael significantly modified steering scheme in order to make it easy and smooth (hard steering is an inborn feature of reflex wings), and doing that he became a coauthor of Traper’s follower – the Action. Regrettably our ways soon parted, as Michael started promoting the Action as his own development without endorsing Piotr Dudek’s part, to the degree of concealing the very fact of manufacturing the Action by Dudek Paragliders. To this day you can find texts praising Michael as a father of reflex profile – which is absurd, since it was known almost for a century now and was never patented by anyone. Similarly, it is not true that he designed the Action.
In our following reflex designs (the Reaction, Synthesis, Plasma and Nucleon) we perfected steering scheme of the profile with several novel ideas, arriving at a reflex paraglider which is easy to launch, amiably at steering and exceptionally safe.
Our designs remain popular with pilots all over the world, dominating both the quantity (on average half of the competitors are using Dudek wings, with rest of the cake divided among several companies) and quality statistics, confirmed by numerous top places and world records. Here are some of the results:
2010 British National league (1st leg): 1st, 2nd, 3rd
2009 World Paramotor Championships: 1st PF1, 1st PL1, 2nd PF2, 3rd PL2
2009 Polish Paramotor Nationals : 1st, 2nd, 3rd...
2009 British Paramotor Open: 1st, 3rd, 1st (women)
2009 Spanish Paramotor Championships : 1st
2009 France Paramotor Open: 2nd
2008 Euro Paramotor Championships : 1st PF1, 1st PL1
2008 Japan Paramotor Nationals : 1st, 2nd, 3rd...
2008 Polish Paramotor Nationals : 1st 2nd 3rd...
2008 French Paramotor Open: 1st (women), 2nd, 3rd, 4th, 5th
2008 German Cup: 2nd PF1, 2nd PL2, 3rd PL1
2008 UK Paramotor Nationals : 1st, 2nd, 3rd, 4th...
2007 World Paramotor Championships: 1st PF2, 2nd PF2, 2nd PL1, 3rd PF1
...and many more.
Speed over a closed circuit of 50 km without landing (RPF1Tm): 65.4 km/h
Speed over a closed circuit of 50 km without landing (RPF1Tf): 45.14 km/h
Speed over a closed circuit of 100 km without landing (RPF1Tm): 51.9 km/h
Distance with Limited Fuel (RPF1Tm): 110.161 km
Speed over a straight 15/25 km course (RPF2T): 37.4 km/h
Distance over a closed circuit without landing (RPF1Tm): 343.6 km
Reflex profile paragliders have been produced for many years now. There must be a dozen thousands of them flying in the skies, and the number of flights absolved cetainly is a seven-digit one. With growing confidency more and more pilots go flying in even more difficult conditions, yet there are no reports of accidents allegedly caused by reflex-profile characteristics.
How is it then that so many pilots still consider reflex wings to be evil and traitorous? Popular meaning says they are docile in easy weather, but can get really nasty when struck by some bad mood. As usual, fear is a child of ignorance. And when there is no solid knowledge around, myths come in handy.
That’s why we decided to give short shrift to several most persisting of them, which grew over years.
Myth #1: "It is hard to take off on a reflex wing"
A reflex-profile paraglider (RPP) takes off differently than a classic paraglider (CP). RPP needs to be launched with one smooth pull, quickly arriving over pilot’s head. You can’t pull up the wing too slow or have a break in-between, as many pilots do. According to their CP experience they think it is safer to pull it up reluctantly than to go with full zeal and then brake it when it wants to overtake the pilot. This is all wrong – with RPP there is no possibility of overshooting: once the canopy is filled up and stays in the airflow (be it due to the wind or pilot’s movement), RPP firmly stays overhead and simply waits for you to take-off. This is one of its fundamental advantages.
Is such a start a difficult procedure? Well, it depends. To be frank, green pilots who started their training on RPPs under guidance of an instructor conscious of reflex technology have a lot less problems than seasoned classic wings pilots, who just switched to reflex.
Fact: A reflex-profile paraglider takes off differently than a classic paraglider. Problems can be caused by handling a RPP with CP habits, or by misjudging trim settings (in relation to wind speed). Pilots who do it properly have no problems with taking off on a reflex wing.
Myth #2: "It is hard to steer a reflex wing"
Harder handling is not caused exclusively by the reflex profile peculiarity; there is also a simple fact that RPP’s fly faster than CP’s (steering a classic wing at 60 km/h would require a lot of strength too). Should an RPP be steered with technologies similar to CP, you would really need a strongman to handle it at high speed.
However, ongoing design improvements finally worked out solutions that practically eliminate this problem. In latest wings brakes act more like “pulling in” than “pulling down”, thus bringing comfort to steering RPP at low speeds. At high speeds (open trimmers + speedbar) there is additional set of steering handles connected directly to wingtips (TST - Tip Steering Toggles), allowing for easy and effective handling.
Fact: Modern RPP design features more effective classic steering techniques at lower speeds, and alternative handles set (TST) for higher speeds.
Myth #3: "Dynamic RPPs reactions to collapses induced by test-pilots proves that the paraglider can be very dangerous when flown in turbulence.”
Test pilots are forcing collapses (frontstall, 50% asymmetric, 75% asymmetric) by pulling A-row lines or risers. Typically it’s an easy job for the test pilot, since classic paraglider’s inherent vulnerability to collapses (especially when accelerated) requires but a slight pull on the lines to cause a massive frontstall.
Quite the contrary for the reflex paragliders. Due to considerable resistance to collapses of a reflex profile, test-pilot has to pull with all his might for several seconds. Meanwhile, already fast-flying RPP accelerates even more, as pulling A-row diminishes wings’ angle of attack. At last a large and sudden collapse occurs, then reopens spontaneously and dynamically by the paraglider on its own. While unassisted reinflation is a welcomed behaviour, the dynamic exit is regarded as a sign of instability of the wing and thus disapproved.
Such judgement is wrong for two reasons.
First: any wing (including those considered very safe) flying at 50 or 60 km/h will exit the collapse dynamically, therefore gaining considerably worse marks – even as it would be still the same, safe wing.
Second: in real flying a collapse is caused by a stream of sinking air (turbulence) entered by a paraglider. Even at hypothetic 35 km/h speed (and RPP’s fly much faster than that) and theoretic maximal chord of 3.5 m (in reality much less, especially at wingtips) we have only 1/3 of a second between the leading edge entry in turbulence and the moment when entire profile is covered with it. It means that in real life aerodynamical force is acting on leading edge for tenths of a second only, since later on it engages entire canopy and is rather subduing that collapsing it. Moreover, we have to remember that the reflex-profile is automatically adjusting its angle of attack all the time, further limiting the exposition.
Fact: RPP’s reaction to collapses induced by test-pilots during certification procedures or SIV courses does not reflect real behaviour of the paraglider. A wing of fully reflexed profile will acknowledge even strong turbulence by slight acceleration and upsurge (when faced head-on) or a swing and slight turn when it is hit distinctly on a side.
Attention: another situation occurs when a paraglider enters rising air. There is not much difference between reflex and classic profile behaviour – a violent turbulence can “stop” a paraglider in flight and bring it to parachutal stall. Testing such a case would require simulating it on highest possible AoA (slowest trim setting). Usually such stall is entered by a slow letting out of a full or B-stall, and then time is measured in which the paraglider returns to normal flight – the sooner the better of course.
All in all, certificating of modern RPPs is a broad topic which deserves an article of its own, if not a number of them.
Myth #4: "All RPPs behave exactly the same"
Each year the offer of RPPs is getting broader. Each year new designers come to try the reflex-wing concept, enriching it with their own ideas and experiences. “Old” designers are playing with new trends. In effect we have several types of reflex-profile paragliders:
fully reflexed wings,
“half-reflexed” paragliders (i.e. with profile showing limited reflex behaviour),
semi-stable paragliders (i.e. with profile displaying only some features of typical reflex-profile)
partly reflexed paragliders (for example with reflex profile in central part of the canopy only).
When additionally we’ll take into consideration changes brought to profile by trimmers and speed system, we will have:
reflex behaviour in full speed range,
reflex behaviour at middle and high speeds.
To sum it up, we arrive at quite a number of possible RPPs. All these wings can (and usually do) behave differently, depending on their design and current trim/speed system configuration. Fully reflexed profile is hardest to exploit, but it is the one most welcomed by pilots wishing to fly as fast and safe as possible. Compromises offering limited reflexivity bring some features typical for classic paragliders (lower speeds or easier handling), but it is invariably with a loss of several welcomed reflex-profile features.
Fact: All RPPs flying in full-reflex mode feature increasing stability with increasing speed. But not all of the RPPs available are fully reflexed at every trim/speed system setting, and that’s why they can behave differently.
Myth #5: "Reflex-profile paragliders are said to be immune to turbulences.”
RPP is not a flying broom nor does it possess any magic powers of casting spells on weather. Turbulences won’t disappear simply because the flight is done on a RPP. Pilot will experience them as sequence of jolts, felt harder with increasing speed.
The truth is, an RPP is very resistant to turbulence-induced collapses. That’s why their pilots have considerable less work to do (if any at all) when controlling stability of the wing flown through rough air areas. And as we already know the faster he flies, the safer it will be (this concerns fully-reflexed paragliders only).
Fact: RPP does not erase unpleasant feelings in turbulence, but it is very resistant to turbulence-induced collapses.
Myth #6: "Reflex-profile paragliders can be flown only by experienced pilots."
We have to discern two notions: experience and education. The first is gained through practice, the latter by training. A pilot with considerable airtime gathered exclusively on classic wings can experience difficulties when he starts flying RPPs, thus perceiving them as potentially dangerous. Flying with brakes slightly pulled in is by far not the only one, but decidedly most widespread error. As (hopefully ; ) known, this technique while perfectly suited to CP will cause an unpleasant feeling of instability when applied to RPP.
Speaking generally, RPP’s range looks pretty similar to CPs: there are competition wings with high aspect ratio and sophisticated trimming as well as simple recreational wings. Competition paragliders require much experience, while recreational ones can be flown much easier. Still, flying both of them it is absolutely necessary to know and understand peculiarities of reflex-profile wing operation. There are following points to be learned:
influence of brakes, trim and speed system on flight parameters and safety
take-off and landing techniques, applicable trim settings and landing approach calculation
handling of a paraglider in specific flight stages.
Fact: Flying a RPP absolutely requires knowledge and understanding of its specific demands (besides all general paragliding knowledge of course). An educated pilot even with negligible experience will be perfectly right on his recreational RPP.
As you can see, most of the myths concerning RPP has its roots in treating RPPs from classic paragliders point of view. It his highest time to underline that reflex profile paragliders are different than their classic counterparts! The difference is important enough to justify additional training before flying a RPP. Be it brief, but in its crucial points it is indispensable.
To simplify the matter: it is better to think of a reflex-profile paraglider as of a paraglider with several features of an aeroplane. And conscious RPP pilot must learn these “aeroplane” characteristics of his wing, for otherwise he will be restricted to… mythology.
Recently there was a famous case of two American car thieves, bagged when trying to steal a Toyota Avensis. The police got them before they were able to drive away from the spot, since they could not operate the manual gearbox. All their previous driving experience concerned cars equipped with automatic gearbox only.
Basing on these facts, journalist George Rubber of local newspaper came to conclusion that Toyota Avensisis is a car which is incredibly hard to drive.
After further contemplation on plight of the hapless thieves (a prison term), George Rubber arrived at another one priceless deduction:
Under certain circumstances Toyotas Avensis can be very dangerous cars.
Wojtek Domański, Piotr Dudek