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Static, Dynamic and Accelerated Stalls

By adminon - Read 15136 times

Before discussing static and dynamic stalls we should first review some terms involving stability. Static stability refers to how something behaves in a steady state. A parachutist suspended under a normally functioning canopy would usually be said to have positive static stability. That is to say that when not changing the position of the toggles or risers, things like airspeed and heading will not change very much from one moment to the next.

Dynamic stability refers to what happens after something has been disturbed from its static state. Once a parachutist changes the position of the toggles or risers, dynamic stability comes into play as the body swings under the canopy.

A static stall refers to what airplane pilots would call a slow deceleration, approach to landing or simply a normal stall. Basically, at an altitude from which it will be safe to recover from the stall, slowly increase the angle of attack and let airspeed bleed off. Keep increasing the angle of attack, gently, until the stall happens.

Under a parachute, this is fairly simple. Slowly pull the toggles down and wait. Do not pull down the toggle so quickly that you swing forward from your normal position under canopy. Hold the toggles all the way down and wait. You should notice the sound of the airspeed decreasing, perhaps a slight rocking in the saddle and then perhaps a noticeable increase in descent rate.

This is a basic stall. For all intents and purposes it is the most genteel stall your canopy will demonstrate.

Recover from the stall by decreasing the angle of attack -- let the toggles up and resume flight.

The dynamic stall is different because of the suspended weight swinging under the canopy.

Again at an altitude from which it will be safe to recover, begin the maneuver from full flight -- toggles all the way up. This time instead of slowly pulling the toggles down, pull them down as quickly and as far as you can and hold them there.

A few different things are happening this time around.

Because your airspeed initially hasn't changed all that much, but you've increased the angle of attack dramatically, the wing is now creating a lot more lift. A function of creating lift however is also the creation of drag. Your canopy as a result will not only go up but also slow down. Your body on the other hand is following Newton's first law of motion and will continue at its current speed and direction. Unfortunately this also asymmetrically loads the front and rear risers, which continues to further increase the wing's angle of attack.

It's a vicious little circle there for a moment or two as increasing the angle of attack slows the canopy more and more. As your body swings farther and farther forward, rapidly, the wing exceeds the critical angle of attack and it stalls.

Your body may have been thrown quite a bit forward of the leading edge of the wing and even slightly upwards of your normal place under the canopy. As a result, your body may feel a much more definite falling or even backward motion than it did during the static stall.

Stalls can happen at any airspeed and at any attitude. All that is required is for the wing to exceed the critical angle of attack.

Up to this point in this discussion, we've been looking at stalls in a fairly normal manner. If you didn't know better you may have thought that the stall had something to do with the speed of the wing or its attitude in relationship with the horizon.

That's normal. Many people make this mistake. After all, we've demonstrated the stalls from a slow deceleration and with a fairly normal relationship to the horizon, earth and sky. That's about where most discussions on the subject begin and end. So, some people might make the mistake of thinking that a stall can only happen if you're flying too slow or if the leading edge of the wing is pointed toward the sky.

Unfortunately, this is just dead wrong.

To make matters worse, some maneuvers that you may perform, turns for example, create G force. Basically, your body wants to continue in a straight line but is getting pulled in another direction by lift. As the bank angle increases, so does the G force. In an airplane, maintaining altitude during a turn, the G force increases at a rate equal to one divided by the cosine of the bank angle. So, at a bank angle of 60 degrees the G force will be 2.

You might not get exactly 2 Gs under a parachute turning with a 60 degree bank though since generally you're not maintaining the altitude and the equation becomes quite a bit more complicated taking into account your descent rate, but in most cases it will definitely be greater than 1.

So what does this have to do with stalls?

As the G force increases so does the amount of lift required to offset it. With the same angle of attack, the airspeed at which the stall occurs will be increased by the square root of the G force. Airplane pilots would call this an accelerated stall.

Let's say you're pulling a sustained 2 Gs pulling out of a steep swoop, your canopy will have an accelerated stall at 1.414 times its static stall speed.

The really insidious part of this is when a person is snapping the toggles down to pull out of a too steep and too low swoop, the dynamic stall comes into play. The body continues in a straight line, increasing the angle of attack and aggravating the stall with really bad results.

It's my opinion that this could be the primary factor in some botched hook turn landings.

Paul Quade is a Certified Flight Instructor and the camera flyer for the Open Class 4-way team, Perris Lightwave.





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