|
For more on wings, see
Chapter 26: Choosing A Wing | and
Overview of
Reflex Wings
Reflex technology was developed well before paragliders came along.
Sailplane pilots have been using it, in the form of negative flaps,
since the 1970's.
Michael Campbell Jones was the first to try it on paragliders with his
Reflex brand and found that it had some very desirable characteristics.
Like all things aviation, there are trade-offs. It was indeed fast and
hard to collapse but it was also difficult to launch in no wind and
had high brake pressures. Many improvements have been made handling and launch characteristics
better but at some expense in other areas.
I have experimented with nearly every model of these wings and find
that, for the most part, they meet their mission well. As to collapse
resistance, yes, they are more resistant in the accelerated mode but, if
a collapse does happen going fast, it will be quite sporty. Any extra
safety can be quickly sacrificed by taking on more lively air than a
pilot otherwise would.
Comparing Free Flight and Motor Pilot Needs
The following comes from longtime glider maker Ozone, who produces both
regular and reflex gliders. I agreed with reader Ami Abramson who finds
their perspective enlightening—they compare and contrast the needs of
paraglider and paramotor pilots. I have no connection with the company.
"Ease of launch has always been an important point for both. But it has
to be said that with 30kg of gear on your back in no wind and on a flat
field, you want to make sure your wing will launch easily the first
time.
The effect of the motor on the glider is very specific and can only be
experimented by flying with an engine. The motor affects the roll,
damaging the quality of the turn. For both sports, a glider that rolls a
lot with short brake range and high spin tendency would not be ideal.
The turn ability of a wing is quite important in free flying as it’s
nice to be able to stay in the core of the thermal, in a small area of
lift, or even to land in a tiny field. In the past it has not been seen
as important in PPG as most the flying is done in a straight line at
high speeds. However, in the future PPG pilots will realize that
precision is a very important feature in a wing. More and more pilots
will realize how fun it is to play near the ground and with the terrain,
and in order to do this precise and agile handling is the key.
A Specific airfoil for PPG:
Because of what I have explained above, performance in glide and sink
rate for free flying and speed for the motor, it is obvious that wings
need developing specially for each category.
Everybody in PPG today has heard about “auto-stable” or “reflex”
airfoils. Here is the simple explanation of the benefits of this type of
airfoil:
As I’ve said earlier, pilots want to fly fast to cover long distances,
so needed in the concept is everything to make a paraglider fly at low
angle of attack (to go fast) with maximum stability. The “reflex”
airfoil acts as an automatic system that helps the wing to handle
turbulence by delaying the point of break (collapse) by positioning the
lift forces quite far forward on the profile.
In effect, the more you accelerate, the more effective the reflex is and
the more solid your wing becomes: This means it’s more solid, but as
I’ll explain bellow, that doesn’t mean it’s safer!
Also, a reflex profile is usually associated with poor aerodynamic
performance. In order to achieve maximum speed, a competition reflex
wing my require 100% thrust in order to maintain level flight in
trimmers out / fully accelerated position. This is the case with the
Viper, for instance, which is designed with an emphasis on speed.
So to summarize, reflex profiles have been developed for speed in
paramotoring and is the best known solution for speed with a certain
amount of solidity and comfort in flight.
The negative sides of reflex airfoils are less often discussed, but are
just as important:
-
The reality of a soft canopy concept, rigged with lines, is that the
pilot is always vulnerable to the possibility of canopy collapse. Reflex
in the canopy profile delays the point where the collapse occurs, but
the risk is ever present.
-
Additionally, another rule of soft canopy aerodynamics is that the
faster the airspeed and the lower the angle of attack at the time of
collapse, the more dynamic will be the reaction of the glider to the
collapse. Because a reflex wing will not deform at the leading edge in
turbulence, collapses are generally larger in surface and volume than in
a free flight profile, and as a result of this tend to be more
aggressive in dive and surge during collapse.
The forces described earlier are - “in a simple way” of explanation –
naturally making the airfoil to increase its Angle Of Attack (AOA), even
when outside influences are pushing it down, and there are other direct
consequences from that:
-
The altered point of lift on a reflex profile (put simply) naturally
encourages the wing to increase its Angle of Attack (AOA), even when
outside forces are encouraging it to decrease. There are other direct
consequences of this, such as:
-
Slower inflation / poorer launching. The glider is reluctant to rise and
needs to be held with the A’s until overhead otherwise the glider tends
to drop back.
-
Poor flying characteristics at high AOA (slower speeds). Delayed
recovery from stall and a short brake range (spin tendency), and poor
handling are symptoms of a reflex profile.
-
Inefficiency: Fuel consumption is noticeably higher.
In pure theory, reflex is actually not the ideal solution for reaching
higher speeds. Ideally, a symmetrical airfoil with a Moment as close as
possible to zero would offer the highest possible airspeed. In
comparison to a symmetrical airfoil reflex is actually slower. Reflex is
the solution that fits the needs of our soft canopies when flying at low
AOA (high speeds).
Speed is the most popular measure or performance in PPG, and Reflex is
the technology currently ‘in fashion’.
There are other answers to increase speed, for example reducing the
surface area of the wing dramatically. But like everything else, this
must be paid for with higher take off and landing speed, and poor
passive safety, which makes it a less than ideal solution for most
pilots.
In the future, PPG designers will continue to focus on reduction of
energy consumption and performance will center more on glide and sink
(efficiency) as well as speed. This is where the limits of reflex become
painfully apparent and designer’s skills will be tested when trying to
achieve this delicate balance.
Wings can be broken down into 3 types for the PPG market:
1. The Beginner Wing.
At the start you want to be able to learn without having to think about
your wing.
This means the lightest and the easiest possible must be offered. A
quick take off so you don’t have to run far. This necessitates good
enough performance and a slow minimum speed that provides enough lift to
fly. You’ll also want forgiving brake travel with predictable response
and a long brake range.
Your aim at the start is to gain confidence and skill in your launches
and landings, and become accustomed to flying the motor and coordinating
the turn in the air: the basics of ppg flight.
All these requirements show that there is no need for reflex in a
beginner wing. However, all of the classic ingredients of a good school
wing will work perfectly. The crossover with the needs of free flight
paraglider training is high, and similar wings are needed. For PPG we
have developed the new Ozone Indy as the answer to the beginner PPG
wing.
2. The Intermediate wing, for the widest range of pilots.
This is the wing that is easy and safe enough to be used by any pilot
that practices occasionally. But it needs to be fast enough so the pilot
can enjoy going for distance flights without getting bored or frustrated
with a slow wing.
The take off must be very easy. There is nothing worse, when you have
30kg on your back, than having to struggle to get your wing into the air
so you can start to fly. Also, the general passive safety of the wing
should and must be quite high. Certainly, the wing must have enough
‘reflex’ to allow the pilot to fly fast with comfort, but the effect of
the ‘reflex’ should not be so high that it damages the wing’s general
ease of use.
Getting these characteristics right is difficult and that is why it has
taken us some time to develop the OZRP into our new Intermediate wing
for PPG - the Roadster. You cannot just add a bit of reflex to a
paraglider design to get the answer. The whole concept has to be
thoroughly developed and tested to get the right blend of safety and the
required performance. Getting a good EN certification has been a strong
aim for this glider and we are happy to have achieved this with a
‘reflex’ wing.
3. The Competition Wing.
When speed is one of the most important parameters, like when pilots fly
in competition or practice long distance flights, then it’s necessary to
have top speed and glide performance, and hopefully plenty of agility.
This is where the OZRP has proved its incredible versatility and why it
has won every major event in the last 2 years with Mathieu Rouanet.
We’ve been the first to introduce the idea of a compromise in reflex
profiling, where the design doesn’t push reflex to an extreme that gives
the wing only one advantage and loses the rest.
The agility, efficiency, ease of launch and passive safety that are
by-products of the OZRP are like 2 wings in 1: Speed and stability of
the reflex, plus pleasure in flight.
Certification
Currently there are [effectively] only 2 types of certification for
paragliders (DHV and EN), and one dedicated to PPG wings which is
recognized only by the German authorities (DULV).
The DHV and the EN certification are very similar and after years in use
have become a recognizable standard for paragliders, with the flying
public well acquainted with the categories of each system.
The DULV is inspired by the DHV. The DULV test pilot performs a
selection of DHV flight tests with and without motor. The categories
have been reduced to a simple pass / fail, so in the end the wing has
DULV certification or not.
As ‘auto-stable’ airfoils become more popular in paramotor wings, there
is an obvious problem with the certification of these wings with the
current tests.
The current tests only measure the effect of a collapse, but not the
ability of the wing to resist collapse. Paramotor wings with
‘auto-stable or reflex airfoils will fail all current flight tests. The
reasons are explained above: the high resistance of these airfoils at
low angle of attack cause very dynamic reactions to collapse.
Ozone is currently working in concurrence with several other paramotor
manufacturers and Air Turquoise (An EN testing) to set up a specific
certification for the sport of paramotoring.
The idea would be as follows:
First, the wing should pass the EN tests in a defined configuration
(let’s call it ‘neutral’ position), to be sure the flying
characteristics and the behavior during maneuvers fit into a category of
safety. This category, like the current EN tests, will be defined by
European Community Law.
Second, a measurement would be taken to give a figure representing the
amount of reflex, such as a percentage point, that would define the
amount of reflex at full speed and / or untrimmed flight.
With this measurement pilots would know how much reflex there is in each
wing. Today, apart from what the manufacturer says and the feed back
from the pilots that have flown the wings, it’s impossible to know what
a wing is like in terms of amount of reflex. The current definitions of
no reflex, semi reflex, and ‘reflex’ are inadequate.
This measurement won’t be a stamp of approval or a measure of safety for
flight at full speed, but it will be information for pilots and a
reference to compare between wings.
To return to what was explained earlier, a high amount of reflex will
describe a wing with higher resistance to collapse but stronger side
effects, while a lower figure will show a wing with less resistance to
collapse but less trouble in other flying characteristics.
Whether or not a wing has reflex doesn’t mean it’s safe, or unsafe. This
is why we think it is an important point that a PPG wing for most pilots
should be treated like a paraglider and be tested as such. The brake
range, the behaviour at high angle of attack (spin tendency, stall exit,
big ears, etc), the spiral, collapse behavior, etc… need to be tested to
show that it is suitable for that level of pilot.
We don’t simply launch and go into the full speed or untrimmed flight
mode to turn the reflex ‘ON’ and then hope we’re safe!
Flying requires the pilot to go through various stages and making sure
the wing will behave within the defined categories during all stages of
flight is a much greater warranty of safety for pilots.
Disregarding this and only measuring the amount of reflex in a wing
would be disregarding over 20 years of research and experience of
paraglider testing.
Accelerator / Trimmers
In paragliding, most certified wings don’t have trimmers. The reason is
simple: for the same result (achieving higher speeds) the ‘speed bar’
accelerator system is safer because it can be released at any time. In
case of collapse, by just releasing the speed system (taking your foot
off the accelerator), the wing will react more gently and will behave
more closely to its ‘neutral’ configuration.
With trimmers, because you’re stuck in the full speed setting without
the possibility to return to neutral flight immediately, the reaction to
collapse will lead to severe pitch and or rotation.
PPG pilots should be aware that by using trimmers with a large range,
the wing is not certified at all. It’s very difficult to achieve a safe
result after collapse and therefore to get a certification with trimmers
longer than 5-6cm is almost impossible. This is why most of the wings
with proper certification will only be certified without trimmers or
only with very small ones.
So it’s very important to know if your wing has been certified when
making your choice. You should know if your wing is certified, but more
importantly, in what configuration of trim. It is important to note that
some major manufacturers of reflex wings advertise their wings as
certified, but fail to mention that the wing is uncertified the moment
you release the trimmers.
The [Ozone] Roadster has passed EN certification at full speed on the
12cm accelerator range. This proves that there is another solution to
high speed with higher safety! We hope to see the culture of the
accelerator growing slowly in the PPG world.
In Summary
We encourage PPG pilots to educate themselves and to ask questions. Some
manufacturers have pushed the reflex concept too far, suggesting that it
is the main ingredient in the ideal PPG wing.
There is little doubt that the concept is useful but we cannot forget
some of the other fundamentals that have proven, particularly to us
through our years of work on paragliders, to be just as crucial in PPG
wing design.
Ease of take off, precise handling, stable speed, and for when it is
needed: collapse recovery, are just as important in PPG as they are in
paragliding. The motor allows us to push the concepts differently but at
the end of the day a recognized level of safety and ease of use will
ensure that pilots get the pleasure and the performance we are all
seeking!" |
Courtesy FlyOzone.com