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מאת: נייג'ל פייג, 2006

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מקור: http://50k-or-bust.com/Flying_Articles/Deep_Stall.htm

Deep Stall in Paragliders - Nigel Page

I first wrote an article entitled 'Deep Stall' a long time ago after plummetting out of the sky on several occasions on a very porus glider. Paragliders are now considerably safer but recent events have caused me to look at deep stall again. The aerodynamics of deep stall have not changed, but the most probable causes are different for modern gliders than for early types as are the preferred recovery procedures. In this article I am going to try and keep the technical stuff to a minimum and concentrate on the practical side. Is deep stall still an issue? Decide for yourself.

What is deep stall?

For our purposes a deep stall is one from which a paraglider will not automatically recover when the brakes are let up or, at least, takes a long time to. All forward airspeed is lost and the canopy descends rather like a parachute and usually remains fully inflated. The rate of descent may exceed six or seven metres per second, comaparable to a rather small emergency parachute. Very quick! Deep stall is sometimes, very appropriately, known as parachutal stall. "Deep stall" should not be confused with "full stall" which is initiated and maintained by the pilot by pulling the brakes fully down.

Why is deep stall a concern?

Stall occurs when the angle of attack of the wing is too high. Aircraft are usually designed so a stall makes the nose drop, reducing the angle of attack and unstalling the wing. A paraglider is stabilised mainly by 'pendulum' action provided by the weight of the blob hanging underneath. This tends to keep the wing at a constant angle relative to the horizon rather than relative to the airflow whatever direction the thing is travelling, even vertically. Thus a paraglider does not recover from stall as easily as a conventional aircraft.

Causes

Modern certified gliders should not be prone to deep stall. i.e. They should not be stable in the stalled condition and should quickly recover to normal flight without any pilot input other than releasing the brakes. However, there are several 'fault' conditions which may make a glider prone to deep stall, or at least slow its recovery from a stall.

1) Porosity. Paraglider wings are made of woven fabrics coated to stop the air flowing through the weave. If the coating is damaged the wing may become liable to deep stall. Ultraviolet, water and abrasion can all damage coatings.

2) Line shrinkage. Sometimes paraglider lines can shrink, but the A and B lines are subject to higher loads than the C, D and brake lines and may shrink less or be stretched back more after shrinkage. The net result is that the wing pitches up and makes the glider prone to deep stall.

3) Brake lines set too short. Brake lines set only a little too short may not manifest any symptoms until the glider is stalled, perhaps following a deflation, when it may stay stalled or take a long time to recover. i.e. It will be in deep stall.

Modern paraglider fabrics are much better than earlier types and a reasonably cared for glider should do over 400 hours without becoming seriously porus. Line shrinkage can be caused by dampness or even prolonged lack of use and is more likely than porosity to be the cause of a modern paraglider becoming prone to deep stall.

Recognition

One thing which makes deep stall especially hazardous is that it can be difficult to recognise. A paraglider in deep stall will descend at a speed of the same order as its forward airspeed in normal flight. The pilot may 'feel' the airspeed but not recognise that it is in the wrong direction, vertical! The givaway is that serious sinking feeling, the ground coming up and the scream of the vario, but it still may mistaken for badly sinking air.

Action In Deep Stall

Firstly make sure the brakes are fully off. The decision of the pilot of whether to attempt a recovery or not depends mainly on height, terrain and experience. In deep stall the descent rate could be around 6m/s, about 200 feet in ten seconds.

Low. Trying to recover too low is likely to result in the pilot flying or 'swinging' into the ground at high speed. If low over benign terrain the safest course of action may be just to PLF.

Intermediate height. Modern emergency parachutes can be very effective even when quite low. The currently generally accepted rule is "If in doubt get it out!" Don't leave it too late.

Very high - Recovery. The object of a recovery attempt is to re-establish forward airspeed (as opposed to vertical airspeed) by pitching the wing down or inducing some sort of dive. Essentially a modern paraglider which remains stalled with the brakes fully off is probably faulty. How difficult recovery is will depend on how faulty! The manual for your paraglider may state a preferred method.

1) Applying speed bar. This pitches the wing down. This is probably the most effective method and should work with all modern gliders. OK if you can find the speedbar quickly, otherwise use another method.

2) Pushing forwards on the A lines to depress the front of the wing and pitch it down.

3) Pulling on brake, possibly to full stall, and releasing quickly. The idea is that the canopy is pitched down as it surges forwards to produce the necessary dive. Be ready check the canopy with the brakes if it surges too far forwards. This is probably best used only if other methods do not work.

Practice Of Deep Stalls And Recovery

It is useful to investigate deep stall but only over water with appropriate equipment and support facilities (SIV).

Avoiding Deep Stall

Maintaining good airspeed and not stalling in the first place is a good idea. Try to avoid over braking to pump out tucks. Make sure your brakes really are fully released when you think they are and that you don't have to stretch to get them fully off. Frightened humans tend to curl up and hang on to anything they happen to have in their hands. Not good if this is the brake handles. In rough air pilots can sometimes progressively apply more and more brake without realising it.

Glider Maintenance

Recently pilots have begun to be encouraged to have their wings inspected annually. A proper inspection should include a porosity test, a check for line shrinkage and brake setting. A modern glider passing properly executed tests should not remain stalled if the brakes are fully released. If you look after your glider yourself it is down to you to ensure it is safe.

Copyright © Nigel Page - March 2006