Thursday 4 July 2013

Reflex glider - how does it work - part 2/4

Why reflex gliders are safe?

Welcome to part 2 of the reflex airfoil post series.

Despite the fact that a paraglider does not have a tail there was no need to use reflex airfoils in our gliders. The center of gravity is located a few meters below the canopy which makes the the whole system stable  and the movements of the center of pressure does not affect it that much.

The fact is that the reflex airfoil has a much worse gliding ratio than a classic one. The other disadvantages are the difficulties with controlling such a wing (deformations of the airfoil to make a turn etc.)

Piotr Dudek was one of the pioneers in applying reflex airfoils to paragliding. His idea was successfully popularized worldwide by Paramania. Most paramotor pilots are flying reflex wings because they are more resistant to collapses, and faster.

What makes the reflex airfoil so safe and popular?

Please have a look at this picture showing classic  pressures in an airfoil

The airflow is most important above the upper wing surface because this is the place where the most of the lift force is generated.
The lift force in our case is caused by the lower pressure on the upper part of the airfoil (blue area). The brighter the blue,  the lower the value of the pressure. It makes the pressure difference between the upper and lower part of the wing bigger.

From the above pictures we can conclude the following for the classic airfoil:
- the lower the angle of attack, the lower the lift force generated
- the lower the angle of attack, the more the center of pressure is moving backwards

Reflex airfoil:

In the case of the reflex airfoil we can see that even though the angle of attack decreased, the center of the pressure did not move backwards. This makes the wing resistant to deformations and collapses. The pressure to 'keep the wing open' is always there.

Another characteristic of the reflex is that the area where the pressure is acting is smaller then in the classic airfoil. Additionally it is located at the front of the airfoil so the turbulence which is necessary to 'blow' this pressure out has to be stronger.

Another myth is related to the speed bar. When pressing the speedbar we are decreasing the angle of attack. In reflex wings the center of pressure is not moving backwards so the wing's collapse resistance does not change.

What is important to keep in mind is that if the deformation occurs on the full speedbar and opened trimmers, the result will be very violent to the pilot due to higher speed.

This is the end of part two of this article. Part three will be posted soon.

The author of the original text is Zbigniew Gotkiewicz. It was presented on Leszek's blog

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