The Stall popularBy Brian Germain on 2006-07-18
The stall is one of the least explored and most feared aspects of flying. Avoidance of this flight mode causes many canopy pilots to be uncomfortable with flying slowly, and unpracticed in this important art. This article will discuss the governing variables relating to the stall, in hopes that this knowledge will help parachute pilots to become less afraid of this essential aspect of the flying experience.
First we must explore what a stall is. The assumption made by most canopy pilots is that the stall is caused by slow speed flight. This is not true. It is correlated with low speed flight, but a wing can stall at high speed too. A stall is caused by an excessive angle of attack. When the relative wind flows over an airfoil, it is bent in the general direction of down. This causes an opposite force called "Lift". When the orientation of the airfoil is changed to a higher angle with respect to the relative wind, it is said to have an increased angle of attack.
Air is quite cooperative. It is willing to be redirected and still flow in a fairly organized manner…up to a point. At a specific angle, all airfoils fail to bend the air into submission. This discrete angle of attack is referred to as a stall. It is coupled with a sudden drop in lift, and thus a significant increase in decent rate. Whether you are flying an F-16 or a Lotus 190, recovery from a stall is always the same: the pilot must reduce the angle of attack. On an airplane this requires forward pressure on the yolk or stick. On a parachute, we are simply required to let off the downward pressure on the toggles or rear risers that has increased the angle of attack in the first place.
Each parachute stalls and recovers differently. Depending on several governing variables, some parachutes will recovery nicely from a stalled configuration no matter what the recovery technique, while others will require very careful execution. Let's take a look at these issues one by one.
The characteristics of a stall on any ram air canopy are based on two main variables, and several lower order variables. The most significant governing variable is the flight mode when the stall is reached. If the canopy is in a sink, rather than level flight (zero decent surf), it will tend to stall in a more forgiving and docile manner. The second primary variable is the attitude about the roll axis when the stall is reached. In other words, if there is any bank angle when the stall precipitates, it will cause the lower wing to stall first, resulting in significant yaw energy, which can result in line twists.
There are several other things to consider when testing the stall of a canopy, including: canopy design, density altitude, wing-loading, aggressiveness of the control input, and most importantly, recovery technique. This will be discussed next.
If the wing is allowed back into forward flight quickly, it will dive aggressively toward the ground, causing a drop in the angle of attack, as well as the lift and therefore the overall line tension. This may allow the wing to surge below the suspended weight (you), and possibly cause a jumper/canopy entanglement. Further, if the release of the brakes is asymmetrical, the lack of line tension can allow the wing to surge unevenly about the yaw axis, causing line-twists.
The key to stalling any wing is to enter the stalled configuration in a sink, with the wing level and static about the roll axis. As soon as the stall is reached, the toggles (or rear risers) should be released only a few inches to allow for only a slight drop in the angle of attack. As soon as the brakes are released, the jumper should be prepared for a sudden increase in toggle pressure, as the tail of the parachute is about to get hit with a pulse of relative wind. If the pilot is unprepared for this, the toggles will usually be pulled upward and possibly in an uneven manner, often resulting in an aggressive stall recovery that may result in line twists.
When the brakes are released quickly to the full flight position, the wing doesn't have much drag. This means that there is very little to prevent it from surging forward in the window. When the brakes are released slowly, and then held down just above the stall point, the wing has a great deal of drag. You have two big barn doors at the back of the wing helping to prevent and aggressive surge.
Further, as you become more familiar with the stall and recovery characteristics of your wing, you may begin to fly "actively" with respect to the recovery process. In other words, as soon as the wing begins to fly forward in the window, the pilot jerks on the brakes to dampen the forward surge. It is important to do this minimally enough to prevent re-stalling the wing. A well-timed reapplication of the brakes during the recovery process will significantly reduce the amount of altitude lost in the stall. This can be very useful in the event of a low altitude stall. This maneuver can be practiced in relative proximity to another canopy in deep brakes. Be sure to keep your distance when you do this. By definition, a stall is a loss of control of the wing.
Rear riser stalls tend to be sharper at the onset, but quicker on the recovery. Therefore it is advisable to stall the parachute on the rear risers first before attempting to stall it on the brakes. Further, such maneuvers should always be performed at an altitude that will allow for a safe cutaway.
Given all of these concerns, one must ask "Why should I stall my parachute in the first place". There are several valid reasons why each jumper should rehearse stalls at altitude.
- In high angle of attack approaches, such as may be necessary in a tight landing area, stalls can happen inadvertently while maneuvering. This is why it is also important to practice slow flight maneuvering by lifting the toggle on the outside of the turn, rather than depressing the one on the inside of the turn.
- In order to reach a (near) zero ground-speed on a no-wind day, the pilot must have full "Toggle Authority". In other words, if the toggles are set too long, the pilot will be unable to access the slowest possible airspeed, and therefore will be forced to land with more ground-speed without the advantage of a headwind. Being able to finish the flare completely and then let up after landing to prevent the stall from pulling you onto your heels in an essential part of any no wind landing.
When you decide to practice stalls, I suggest taking the process step by step. Simply honking your brakes down with your eyes squinting in negative expectation usually results in a wild ride, and sometimes a cutaway. Try hanging out in slow flight for a while. Bring your toggles down to a bit more than half brakes and leave them there. If you are above the stall point, it isn't going to just stall all by itself. Watching people fly in deep brakes is usually similar to watching them light a firecracker. Your parachute isn't going to explode…promise.
When you get your canopy into the deep brake mode, take a deep breath in and let it out slowly. Relax your muscles. Let your legs hang limp. I find that nervous pilots can't connect with their parachute because it isn't touching their bones. If you soften your muscles, your will allow the leg straps to sink into you so that you can feel what is happening with the newest addition to your body: your wing. By truly relaxing under canopy, we begin to sober up from the adrenalin that is blurring our vision and skewing our perspective toward the negative.
Stalls are an essential part of flight. If you are to be fully in control over the wing, you must explore all aspects of your parachute's performance envelope. Ultimately, flying slowly is the most important aspect of flight because we land in slow flight. The more comfortable you are with your slow flight skills, the better your touchdown will be. Remember, the definition of a good flight is one that ends well.BSG
|this article is great, but uses many unknown, explained terms for the novice jumper, like me. what exactly is the roll axis, yaw axis, yaw energy? i think this article will suddenly become much clearer if i had a definition for these terms:)|
|I agree, as novice it is difficult to understand without an explanation. Please clarify.|
|See Brian's Website Bigairsportz.com he has a book that really breaks it all down. "The canopy and it's pilot" I am an A license and started reading the free online download preview last night. He is an awesome teacher. Very good buy!|
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