Thursday 27 June 2013

Reflex glider - how does it work - part 1/4

Reflex airfoil in paragliders

One of the paraglider pilots from Poland has explained quite well with simple words how the reflex airfoil in our gliders works and what makes it 'safe'.

At the very beginning I have to apologise  to all aerodynamic experts for the simplifications which I have had to make to make this article  comprehensible to all paraglider pilots.

To explain what a reflex airfoil is we have to go back to the beginning of aviation. These were times when planes looked like flying scaffolding. Some designers decided to get rid of the plane's tail. The problem was that the airplane was then pitching down. Why was it pitching down? Apparently the  lift force which was created by the wing was fixed to the point behind center of airplane's gravity causing the rotation.

The point where lift force was 'attached' to the wing is called the center of pressure. Its value and position depends on the total sum of all the forces acting on the airfoil. The airfoil is simply a wing's cross-section.

To solve the pitching problem described above the only solution is to put the center of pressure at the same place as the center of gravity. Only then will  the weight of the wing (airplane) will be pulling the wing down and the lift force will be pulling the wing up without rotation in any direction (which means without changing the angle of attack).

Unfortunately the airplane has to pitch  the nose down during landing. When the angle of attack decreased, the center of pressure was moving backwards but the center of gravity stayed in the same place. This caused an increasing tendency to pitch the nose down.  

The same effect was obtained when the angle of attack was increased (the nose was pitching up). The center of pressure movies forward and causes the wing to pitch up more.

The breakthrough was when aerodynamic experts designed an airfoil (reflex airfoil) where the center of the pressure was working in the opposite way - causing the nose to pitch down when the angle of attack was increased and vice-versa. This happened as early as 1906.  (A lot of people who think that the reflex airfoil was invented by/for paraglider pilots).

These two pictures will be helpful to understand what is the difference between classic and reflex airfoils. 
Classic airfoil:

How paraglider classic airfoil works

The top picture shows the wing flying on a high angle of attack. The lower picture shows the airfoil on a small angle of attack. The arrows show the pressure distribution along the airfoil and the arrow 'inside' the airfoil shows the magnitude of the lift force and its attachment point (the center of pressure). You can see that when lowering the angle of attack the point of pressure is moving backwards.

Reflex airfoil:

How reflex airfoil works in paragliders

This picture shows the reflex airfoil flying on high (upper picture) and low (lower picture) angle of attack. The arrow inside the airfoil shows how the center of pressure moves forward when the angle of attack is decreased.

This is the end of part one. Next part will be published in one week.

Take your time to understand the above because in the next part we will look more into what is happening to the airfoil when it is changing angle of attack in the reflex mode.

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

If you have any questions please post a comment or send an email to lukaszpg (at)

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