Big Canopies in Turbulence
I have spent much of my life studying parachute stability. It has become an obsession of sorts, spurred by a fairly sizable stint in a wheel chair- funny how that works. I have designed and built many, many canopies with the goal of creating collapse-proof canopy. I have failed. It is impossible.
This is the case because, despite the best efforts of the designer to increase internal pressurization and dynamic stability, the canopy can still be flown badly and become unstable. This will always be so. The job then, falls on the educators, and on the pilots themselves to learn and rehearse the essential survival skills that increase the chances that the correct action will be taken in the spur of the moment.
I stated in my original article on turbulence, entitled “Collapses and Turbulence”, that the key is to maintain lots and lots of airspeed and line tension. I still hold that this is generally the truth. However, upon re-examining the situation, I have realized that my perspective on the situation is based on my frame of reference. I fly sub-100 square foot cross-braced speed machine that falls out of the sky like a homesick bowling ball. I do not really represent the whole. The average-size parachute is 150-170 square feet in the civilian world, and much larger for students and military jumpers. In further exploring the issue from the perspective of lighter wing loading and larger parachutes, I have discovered that this is not necessarily best way to fly a larger canopy in chaotic air. Here is why this is so:
If the parachute has a great deal of drag, i.e. a light wing-loading, thick airfoil or is a large parachute in general, the rules change. Such canopies are less capable of maintaining high speeds unless flown very aggressively. Due to the high drag variable at the canopy end of the drag equation (“Rag Drag”, as I call it) the excess airspeed makes the canopy itself want to retreat behind the jumper far enough to reduce the airspeed far below the unadulterated full flight speed. This momentarily increases the likelihood of a collapse. The parachute levels off in mid air, slows down, and for a brief moment, becomes vulnerable to collapse.
Therefore, when flying a canopy with a short, powerful recover arc, aiming to increase the speed beyond full flight becomes a double-edged sword. If the timing is wrong, such as when leveling out high (prematurely), the situation can become very dangerous. The truth is, leveling off well above the ground is dangerous for any wing-loading, and can happen with any parachute due to an incomplete plan or an imperfect execution.
Parachutes flown below one G, at speeds less than full flight speed tend to be more susceptible to collapse. So, if the pilot is quick with their "Surge-Prevention Input", (what paraglider pilots call "flying actively", the risk of collapse is significantly reduced as the negative pitch oscillations will be minimized, thereby diminishing the likelihood that the wing will reach a low enough angle of attack to actually achieve negative lift and dive toward the jumper (i.e. collapse and scare the daylights out of you).
Given the fact that the only preventative or corrective response to a collapse is to stab the brakes as quickly as possible, the sooner the pilot responds to the forward surge, the less the input necessary to avoid or correct a collapse. Therefore, a canopy with a great deal of slack in the brake lines will require more motion on the part of the pilot to create any appreciable effect. This means that a canopy that is in full glide, with the toggles all the way up in the keepers and three inches of excess brake line trailing behind will take longer to see an increase in the angle of attack due to the control input than one with no slack in the brakes at all.
So then the question is posed: “Do we shorten the brake lines on larger canopies to help the pilot prevent collapses?” The answer to this is no, we cannot. This will result in serious bucking during front riser input. It will also mean that following a few hundred jumps, the canopy will be in significant brakes when they think they are in full flight, due to their “lazy arms” pulling the tail down when they should be flying arms up. This will result in lower average airspeeds that will reduce the parachute’s flare power, as well as it’s penetration into the wind. This will also result in more oscillation and distortion in turbulence.
The answer comes to us from our sisters and brothers in the paragliding world. They teach their students to hold a touch of tension on the brakes when flying through turbulence. The goal here is not to put on the brakes and deform the tail, but to simply take up the slack on the brake lines, in preparation for a 12-24 inch strike on the toggles to prevent a collapse. Some teach their students to hold about 5 lbs of pressure on the brakes, while others teach that we should hold no more than two inches below the “Feel Point”. Either way, taking the slack out of the brakes is like standing ready in the door, even when you can't see the count.
So, on larger canopies, it appears that a light touch on the brakes may help prevent collapses. However, it is not because the canopy is more stable in this configuration, but simply that the pilot is more prepared to prevent the wing from surging forward in the pitch window. Once the wing has passed through that parcel of turbulent air, however, the job remains to regain the full flight airspeed, while maintaining positive G's. Letting the wing surge back into full flight too quickly can send the wing out of the frying pan and into the fire. Get it back to speed gently, but get back there as quickly as possible. These are opposing goals, so the actions of the pilot once again become pivotal, calling upon trained skills and acute attention to sensation.
Ultimately, the best way to handle turbulence is to deny it battle. Despite what your ego is telling you, you already have enough jumps. I know you want more, but sometimes the best way to go is to sit on the ground and watch the inexperienced jumpers get experienced.
Live to fly another day.
Brian Germain Big Air Sportz www.bigairsportz.com
More articles in this category:
- Saved By The Beep - by Brian Germain (Posted: 2011-05-15)
- The Abort Zone - by Brian Germain (Posted: 2010-09-01)
- An Inconvenient Truth Regarding PLF's - by Jack Guthrie (Posted: 2010-03-02)
- The "D" Point by Brian S. Germain - by (Posted: 2009-11-01)
- Big Canopies in Turbulence - by Brian Germain (Posted: 2009-06-03)
- Canopy Control - A DVD Review - by Eduardo Guillen (Posted: 2009-04-06)
- Canopy Formation Part II - by Peter A. Pfalzgraf (Posted: 2009-02-16)
- Canopy Formation Parachuting - by (Posted: 2009-01-06)