RAST – Frequently Asked Questions
That’s a very good question. RAST was originally developed to get more pressure in the glider’s rear section. It is typical with paragliders that the entire canopy gets distorted if there is braking on one side. Part of the control energy is then lost and it takes some time for the wing tip to keep up. RAST reduces this effect almost completely. This creates a major advantage for active flying in thermals and turbulence: both sides of the glider are felt very exactly and precisely through the control lines, there is more control and there is no longer the unpleasant refusal to turn in very turbulent lift.
This also explains the feedback from ARCUS-RS test pilots who were amazed that a paraglider with this damping can have such precise and direct handling.
Yet another advantage is that, with gliders that have RAST, in level flight the constant wave motion in the rear section is less marked. That means that the more the air is moving, the greater the difference when gliding between the same glider with and without RAST.
Much better in fact! This is because with active braking on the beginnings of a collapse, it is not a case of pushing air from the rear to the front, but of altering the profile by pulling down the trailing edge in such a way that no lift – but in this case it is in fact downforce – can build up on the underneath in negative flow.
On braking, an air cushion is created between the leading edge and the trailing edge, which straightens the curvature on the underside of the profile and thus prevents downforce being created.
With RAST, much less control travel is needed to straighten the curvature in the bottom surface. What’s more, with impulsive braking, the profile alters in such a way that lift is still created despite a negative angle of attack on the top surface.
In paragliders with RAST, the inside of the canopy is divided crossways to the flight direction by one or more panels with valves. This can influence the airflows in the canopy.
Under real operating conditions, we have not yet encountered any large-scale collapses or uncontrollable flying situations with our RAST series gliders.
In the simulations we have carried out during development and in the certification tests, we have to attempt to force the collapses in the measurement fields specified using force or tricks.
Depending on the model and particularly with the accelerated manoeuvres, the reactions can initially be dynamic as the air remaining in the canopy produces a good deal of resistance.
In contrast to that, however, there is the quick and reliable reopening and the small loss in height.
No, generally not. It is only on launch under ideal conditions that a glider with RAST may need more time to fill completely.
However, this is not a disadvantage as long as the pilot simply allows the glider time to reach its zenith, enjoys the stress-free take-off run and does not accelerate until after that.
Compared to other paragliders, the pilot may perceive there to be greater control pressure when pulling impulsively on the brake lines.
No. The difference to earlier attempts to make paragliders more stable (rigid) using bars or tubes is the fact that a paraglider with RAST can and should continue to collapse in the area in front of the partition.
This area can be compared with a car’s crumple zone which absorbs energy.
However the size and positioning of the partition can be used to influence canopy deformation. Thus, RAST does not prevent a harmless situation of the “ears” flapping, for example, but it does effectively counter large-scale collapses with steep folding angles.
It is not as simple as that unfortunately. A paraglider’s collapseresistance does not have any bearing on its rating in the certification test. In the EN test, the collapses performed on gliders equipped with RAST must be across the same area as other paragliders and they behave in just the same way as traditional gliders if there is a collapse of the same size.
Having said that, with our RAST gliders, to some extent it is no longer even possible to collapse the canopy using the methods described by the EN/LTF standard within the measurement fields specified!
Thus the glider must be ‘pitched’ or auxiliary lines must be incorporated in order to produce a collapse within the measurement field.
This in turn can have negative implications for the test results, as the glider is often preaccelerated by tricks or ploys such as these.
Some further discussion on this with the type-testing bodies is still needed given that RAST is designed primarily to prevent collapses and not to improve reactions to large-scale collapses.
- launch behaviour is very straightforward,
particularly in difficult conditions (e.g. in strong winds,
a tailwind or on steep terrain)
- there is a high level of canopy stability
- large-scale collapses are less likely to occur
- the canopy does not empty completely if there are any disturbances meaning less loss of height, quicker reinflation, less tendency to cravat and better control through the brakes
- there is improved performance in disturbed air due to less canopy movement