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What to do in Turbulence?

1/23/2004

Necessary Action when Encountering Turbulence

 

The purpose of this article is to illuminate on the facts necessary for jumpers to make an informed decision, so that they may better plan their corrective/preventative action when encountering turbulence while flying their ram-air parachute.

We are aware that others have published information contrary to what will be presented here. It is for that very reason we write this article. We believe this is a major safety issue, and that there should be a hearing and finding by the USPA Safety and Training Committee.

Encountering turbulence under a ram-air canopy can be unnerving at best. It can and has led to canopy collapse and sudden impact with the ground. Turbulence can happen at any altitude, however, it is the most dangerous when it occurs close to the ground because of the limited recovery distance.

To fully understand the problems created by turbulence we must first understand some basic aerodynamics. The principal, which keeps a ram-air flying, is one of pressure differential. It’s the same principal that makes airplanes fly. The pressure on the top of the canopy aft of the thickest point is reduced to less than ambient. This pressure reduction is caused by the flow of the air over the top surface, and the differential of this flow from the flow over the bottom surface.  When the air mass is encountered by an airfoil it divides at what is known as the stagnation point. From here some of the air goes over the top of the canopy and some of it goes below the canopy. The air going over the top is accelerated, ‘cause it has to go further, to keep up with the air going under the bottom. The further the air goes the greater the pressure reduction. This pressure gradient is called the “Boundary Layer”. It is progressive in its flow characteristics from the surface of the skin to the “Free Stream Velocity” (the full uninterrupted flow of the air around the airfoil). At the surface (of the canopy) there is no flow, only a reduced pressure. This is why when you stick your fingertip out the door of an aircraft you don’t feel much air flow. As you progressively expose your finger, the airflow becomes greater. It is this boundary layer we are concerned with. If this boundary layer becomes detached from the skin of the canopy, a loss of lift is encountered and what looks like a collapse ensues. This boundary layer can be blown off the top of your canopy as a result of turbulence.

Considering the above, and understanding the axiom “The moment of greatest lift on an airfoil is just prior to an impending stall”, which is taught to every student pilot, tells us that the boundary layer gets stronger as we approach a stall. Why is this? It is because the air flowing over the top must go further and necessarily faster to meet with the air passing on the other side of the wing. Therefore the pressure differential is greater, and the boundary layer is stronger, making it harder to blow off.

Understand we are not advocating that you fly at just above a stall. This would be dangerous, as a sudden gust could change the airflow inducing said stall. We do, however, recommend that you fly near deployment brake configuration or depth. This is the flight mode you would want to go to if your canopy did lose lift. Remember, pilots are taught to “reduce to maneuvering speed” when encountering turbulence. One of the considerations for this procedure in powered aircraft is to reduce the structural load, a minor concern for ram-air parachutes. But recognize that at deployment brake depth we have greater lift and a stronger boundary layer, which would be more difficult to separate.

Think of it in these terms, if you should be so unfortunate to lose your boundary layer due to turbulent conditions, your canopy may fully or partially collapse. When we want to make our canopies open or re-open faster, what do we do? We set the deployment brakes. If you want your canopy to re-inflate quickly you’d best be in some stage of braking (toggles pulled at or near brake setting). This will cause your canopy to recover more quickly- no matter what kind of canopy we’re talking about.

The only time we need to be overly concerned about turbulence is as we get close to the ground. If your canopy bumps and breathes or even partially collapses at 1000’ or 500’ feet - so what! There is plenty of time for it to recover. The time that is takes to recover is about equal to your deployment time.

The notion of  “sacheting” or front “risering” out of the turbulence doesn’t make sense aerodynamically, nor will it help you avoid the problem in the first place. Turbulence travels around with the air mass, rising with the thermal activity, descending in the downdrafts. Understanding thermal activity will prevent surprises, and help you to avoid mishaps. Remember for example, not to land directly downwind of tall trees or buildings on a windy day. Land on the upwind side of the runway on a hot (thermally) day, rather than the downwind side. Try to land where the clouds cast great shadows on the ground – you’ll more likely encounter smoother air.

For a wonderful explanation of turbulence as it relates to aviation, read Peter Lester’s Weather Smart article at http://www.met.sjsu.edu/~lester/ws_aug03.html.

Written by:

John Sherman, USPA I/E, Master Rigger, Commercial, Instrument, Multi-engine Pilot, Medallist In 4 Way, 10 Way, Style & Accuracy, US National Championships, Designer of the Racer, AngelFire Reserve, & FireBolt Elliptical Canopy, Tandem, Student, Military and BASE Canopies.

Nancy LaRiviere, USPA I/E, Tandem Examiner, Senior Rigger, Commercial, Multi-Engine Pilot, Double Bronze Medallist 2003 World Championships, Multi-time U.S. Nationals Medallist and U.S. Team Member, Holder of 10 World Records, 5600 jumps, Canopy 102 Coach, President of Jump Shack

 

 


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