by Bill Gargano
My work with powered parachutes began in late 1982 with the first "Buckeye" powered parachute wing built by GQ Security for the vehicle designed by Jack McCornack and my work with powered parachutes has continued non-stop throughout the years. Over those years, we (Quantum Parachutes, Inc.) have tested powered parachute designs to their extremes. That testing has shown us many things.
STALL PROOF? NOT EXACTLY.
My knowledge and experience has shown me that there are two general statements that can be made about powered parachute stalls. (1) Given proper assembly, training, care, preflight, and use (including weather and terrain conditions being within the vehicle and pilots ability), it is almost impossible to stall a "Buckeye" powered parachute. (note that this statement is likely true for other powered parachutes, but I do not have design information on other powered parachute wings to be able to make that statement) (2) All powered parachute wings are capable of stalling given the correct conditions.
WHAT CONTROLS THE SHAPE OF A POWERED PARACHUTE?
To better understand what causes a stall, we must look at what a powered parachute wing is. It is obviously not rigid. These parachute based wings cannot retain their complete inflated shape when not pressurized and are therefore capable of changing shape while in-flight when aggravated to do so. The fact that there is nothing more than air maintaining the wings rigidity tells us that any change in air pressure, no matter how it is caused, affects the performance of a powered parachute wing. The internal air pressure must always be greater than the external pressure, or the wing will stop flying. In other words, stall.
Powered parachute wings have a multitude of cells. When pressurized, each cells three dimensional inflated shape changes based upon the total amount of weight that is being carried (wing loading). All powered parachute wings change shape with changes in wing loading, and therefore perform differently when flying solo or tandem. Increases in weight lower the effective, or flying, aspect ratio and increase leading edge drag due to changes in mouth opening shape.
Take a look at a picture of any powered parachute wing in flight. Notice that the wing arcs (some more than others) spanwise. This provides a large component of the systems excellent stability. Notice that the vehicle is well below the wing tips. This places the center-of-gravity far below the aerodynamic center of the wing, providing a "neutral", hands off, flight mode that makes the vehicle very easy to fly and similar to a flying a parachute. Look at the profile of the wing and you can see that it is permanently set at one angle. This angle defines the flight envelope of the wing. Notice that the steering system or "brakes" are attached to the trailing edge and when pulled, induce drag. This is the quickest way to perform a controlled turn or to cause dynamic changes to the system, such as a landing flare. If you pull in both sets of brake lines far enough, the wing will stall.
WHAT IS A POWERED PARACHUTE STALL?
By definition, a powered parachute wing has stalled when the wings internal air pressure is equal to or less than the external pressure, and the airflow around the wing has separated. The wing collapses, and the rate-of-descent increases rapidly, until the wing is able to re-pressurize. Control authority, while severely weakened in a full stall, is maintained via the steering system. This definition describes both steady-state and dynamic stalls.
There is one other type of stall, often called a "metastable" stall, that can occur with some powered parachute wing suspension line trim settings. A powered parachute wing is in a "metastable" stall when the wing has been dynamically pushed to a very high angle-of-attack relative to the center-of gravity, and all trailing edge control inputs have been locked out. This high angle-of-attack sets the wing slightly behind the vehicle instead of overhead. The wing is "stuck" in this position resulting in a high rate-of-descent with no steering control.
HOW TO MAKE A POWERED PARACHUTE STALL.
The easiest way to stall any powered parachute is to drop the engine to idle (lighter steering line pressure); push both steering controls as far as they will go; reach out for the lower steering lines and pull them in until the wing stalls. Done quickly, these actions will result in a dynamic stall, where the wing rapidly drops behind the vehicle, the upper surface of the wing collapses, the vehicle swings back under the wing, and the rate-of-descent rapidly increases. Pulling the steering lines in further will cause the lower surface to collapse as well. Pulling the steering lines in slowly will cause a steady-state stall. At the onset of a steady-state stall you can feel the wing rock slightly aft, then forward. If at this point you were to gently let out some of the steering line, the wing would not go into a stall. However, if you continue to hold in the steering, or pull in more, the wing will fall off, aft, and stall.
A powered parachute wing is affected by weather and terrain. For example, a wind shear, or severe turbulence, can cause anything from minor disjointed movement of the system, to a complete collapse of the wing. The severity of the disturbance is related to your wing loading and piloting. The key to avoiding unexpected weather and terrain induced stalls, is for the pilot-in-command to understand the vehicle, the wing, and micro-meteorology. If you always fly in "good" conditions, you are not likely to ever be pushed into an unexpected stall. If you choose to fly in "questionable" conditions, or areas, you are placing yourself (and your passenger) at risk.
Powered parachute wings want to inflate and stay inflated. When pushed into a stall, the wing doesnt want to stay there. It wants air to re-pressurize. To get it re-pressurized, you need to let out just enough steering line to allow it to re-inflate. For example, if you push both steering lines to full stroke and a stall occurs, you would then change your steering to three quarters to one half stroke. This will allow the wing some forward velocity to re-inflate, without giving it the dynamic ability to fly so far forward that you would momentarily be able to see over the trailing edge. This method also significantly reduces the altitude required for recovery and maximizes system stability during stall recovery.
All powered parachute wings are capable of stalling. The pilot-in-command must pay attention to wing loading, weather and terrain conditions to help avoid entering a stall. The pilot-in-command must understand the powered parachute system. The pilot-in-command must know and respect their own limitations and the limitations of the powered parachute.