Thursday 18 July 2013

Reflex glider - how does it work - part 3/4

Why reflex airfoil is 'safe'?

The problem with the certification of the reflex wings is that the whole weight of the pilot and gear is equalised by the lift force generated on the leading edge of the airfoil. In this case it is very difficult to pull the collapse deliberately because even when the test pilot is pulling the A risers with his full weight he is not able to add more force upon them then his own weight acting on the risers.

This is the reason why reflex wings cannot pass the certification tests - they cannot be tested. The tests define how quickly and in what configuration the wing is coming out the deformations, not how easy it is to cause the deformation.

For example test pilot of Dudek Synthesis (and the author of the original text in Polish) was not able to cause 50% collapse on the full open trimmers so the wing could not pass the test.

Another thing is that in the wings with reflex airfoil the surface which is generating the lift force is much smaller in comparison to the classic airfoil. From this we can conclude that to equalize the pilots weight the effective wing load will be higher in comparison to the classic airfoils. And this is causing reflex airfoils to fly faster then the classic ones.

In the above picture we can see the lift force distribution on the reflex wing without speedbar (angle of attack 6 degrees )on the left hand side and with full speedbar on the right hand side (angle of attack 2 degrees).
We can see how the lift force moves forward with speedbar loading only A lines and making the wing more resistant to deformations.

In summary reflex wings are more stable, more resistant to deformations, faster. The downside is worse gliding ratio, certification is impossible in some  configurations according to nowadays certification rules. Wing design is more complex as well.

The complexity in wing design are caused by the fact to control the glider we use  trailing edge. When the pilot pulls the brakes the wing is lowering the trailing edge and the airfoil is not reflex anymore. In reflex mode the whole lift force is acting on the leading edge. When we pull the brakes (even a little) then lift force is also generated on the trailing edge. While not increasing the drag force enough it can cause a turn in the opposite way that pilot wants. For example we pull right hand brake in the reflex mode, additional lift force is generated on the trailing edge of the right wing (total sum of lift forces on the right half of the wing is higher then on the left), right wing is going up according to the increased lift force and we turn left. Normally this would be overcompensated by drag force generated on the right half of the wing (in classic airfoils) but in reflex mode the drag force can be not enough. In this way we can have left turn caused by pulling the right brake.

The other problem is that pulling the brake we kill the reflex airfoil characteristics which is causing the center of the pressure to move backward. This can result in wing tip collapses when pulling the brake even a little in reflex mode.

This is the end of part 3.

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

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