THE STALL / SPIN CONSPIRACY
© John Heiney 2007
Aviation danger has many faces, any one of which can lead to bodily injury. The worst mishaps occur when two or more conspire to kill you. There is much to learn from aviators who have gone before us.
When I attended ground school for my private pilot license nearly 30 years ago, my instructor, a gruff old pilot told us that one of the easiest ways to die in an airplane is to fail to understand the concept of the departure stall, and the approach-to-landing stall. If runways could talk, they would all have stories to tell about take-off and landing crashes. Most would involve a stall and a spin.
Lots of airplane pilots have gotten themselves killed by performing an inadvertent spin while approaching or departing a runway. It is much easier to do with an airplane, but it does also happen to hang gliders. I have seen many pilots enter a spin while landing their hang glider, and usually they have no idea what happened. But, the most dangerous place to stall and spin-in is at launch or scratching along a ridge.
|This is Gerry Charlebois launching at Gunter in the Owens Valley. Hit by turbulence well into his launch run, Gerry did not give up; but made full-scale correction to bring that tip back up. Believe it or not he recovered from this hairy situation and got into the air safely.
Notice how far over he moved his body and legs to correct.
As we should remember from our hang gliding training, when you slow below minimum-sink speed two things begin to happen. Your sink rate increases and your roll response becomes sluggish. Further slowing causes a serious reduction in roll control, especially at higher wing loading. This leads us to the two most important things to understand about stalls.
1. A stall causes loss of control. When is loss of control most hazardous? When there is something nearby to collide with like the ground. If you have lost your ability to control the glider, it is impossible to avoid collision with another aircraft or the ground.
2. A stall takes altitude for recovery. More severe stalls require much more altitude.
To understand the stall we should remember that a wing is essentially a vane that deflects air downward. We utilize the resultant upward force as lift. A sheet of plywood will do this, but the shape of a wing gives it efficiency and stability. A properly shaped wing can fly at a high angle-of-attack to generate lots of lift. But there is always a limit.
When the limit is reached the air flow over the top of the wing begins to separate from the surface and turns turbulent. This interferes with the smooth reconnection of the top air with the bottom air at the trailing edge; consequently, lift production drops.
Once the smooth flow of air over the wing is disrupted it takes time for it to be restored. Rotating air has a life of its own and tends to stick around for a while. Ever hit a tip vortex from another glider long after you had expected it to have dissipated?
We avoid inadvertent stalls at all cost where we spend much of our time: scratching in light lift close to terrain. Yet, if we want to get up in a marginal situation, we must utilize that narrow speed range just above stall to get our best sink performance. It is this catch 22 that has sent many pilots, at an early point in their career (myself included) into the hill.
When you are scratching close to the ridge and you sense that you are getting too slow, you must have the self-discipline to instantly switch from competitive mode to self-preservation mode, and speed up. Remember, once the air on top has started to separate it takes time to re-attach. A little pull-in might not be enough to prevent a problem. Performance and winning are important, but what good are they if you are dead.
If you are way too slow and/or turbulence has bumped you towards the hill, you should use Bill Heaner's three step method:
1. Look in the direction you want to go.
2. Pull in significantly.
3. Use full-scale roll input to turn away from the hill.
In a panic situation you should do all three at the same time quickly. This is something to practice and always be ready for while scratching on a ridge. Fly as if you are "spring-loaded" and ready to "snap" away from the ridge. The closer to the ridge, the more intense your readiness.
There are two ways in which we use controlled stalls in aerobatic hang gliding. One is to do a mild straight-ahead stall to cause the nose to drop allowing a steeper dive. You ease the bar out fairly quickly feeling for a moderate push-back at which time you haul the bar in all the way into dive position. Be sure to pull in as soon as the nose starts to drop. If your wing loading is heavy, the break will be quicker and more abrupt. Be ready. The other way we utilize a stall is in initiating an intentional spin.
THE AGGRAVATED STALL
An aggravated stall on a tailless wing is one of the worst things a pilot can do. This is when you push out to stall from a speed above min sink. You can “zoom” up to a much higher-than-normal pitch attitude leaving yourself at great risk of tumble.
I guess the worst example of an aggravated stall is in blowing a loop attempt and coming to a stop somewhere between straight-up and flat-upside-down. If you stall in that quadrant of a loop, you will tumble no matter what you try to do to get out of it. The exception is if you make it all the way to the flat-upside-down position. If you find yourself falling into the sail in this position you should hang on the bar and ball-up. That should cause you to nose down and dive out of it. I am guilty of both of these peccadilloes early in my career.
In the early days of hang gliding people flew at very low wing loading on big gliders. Pilots would often “parachute” nearly straight down from a wingspan of altitude in an attempt to hit the target, if they were overshooting during a spot-landing contest.
Today’s gliders are rather intolerant of such parachuting. In fact, if anyone had the courage (or foolishness) to do a full flare at any altitude other than very close to the ground, it would certainly result in a tumble. When we misjudge our landing flare, and the nose rotates downward and slams into the ground, this is the beginning of a tumble that would likely continue if the ground were not there to stop it.
We have sort of a love/hate relationship with the stall. If we perform it just right on landing, we feel great and impress our friends. In most other cases it can ruin our day.
|Circa 1985 I decided to make "tip" smoke-holders to verify that one wing actually goes backward during a spin. I made an internal system for triggering the smokes, which I used later in competition and demos. The smoke trails not only indicate that the right tip is moving rearward, but that the glider is losing altitude rapidly.
Notice the twisting-up of the trailing edge on the right wing as it is forced the wrong way through the air. Spins are hell on your sail.
The spin occurs when part or all of one wing (left or right) stalls while the other side continues to generate lift. The stalled wing slows and drops while the other wing basically flies around the stalled wing. A properly entered spin will settle into a stable maneuver that will continue as long as you hold the bar out. The glider will rotate fairly rapidly and lose altitude very rapidly.
After you have done several properly executed spins, it can begin to seem quite benign. But any time you are spinning you are riding the ragged edge of disaster. A nice, stable, comfortable spin can turn into a tumble in the wink-of-an-eye. Three things can make a good spin go bad:
1. Turbulence in the practice area. Turbulence can cause the manageable stall occurring on one wing as you spin, to propagate quickly to the other side. Now the entire glider is stalled and you are still pushed out all the way. Can you say tumble.
2. Improper entry technique. Some gliders do not like to spin. Trying to force a spin when a glider does not want it, is asking for a tumble. Some gliders will drop into a spin easily just by pushing out to stall while banked in a turn. Most gliders require some technique. The classic spin entry method is to stabilize in a 30-40 degree bank, then slowly push out as you high-side the bar(opposite roll input). There are many possible variations to these inputs which might need to be refined for a particular glider. If you have no experience, take this very seriously. Have plenty of altitude in smooth air, and make sure your chute is current.
3. Trying to hold a glider in a spin when the glider is telling you “I don’t want to spin anymore”. You can get an idea of what full-stall back pressure feels like by pushing out in level flight. You feel back pressure on the bar, and as the glider stalls the back pressure builds. (NEVER DO AN AGGRAVATED STALL ON A TAILLESS WING.) While spinning, you stall about half the glider so you feel half the back pressure. Any time you sense the back pressure increasing it means the stall is progressing across to the other wing. Time to pull in and abort the spin.
|Steve Burns towed me up late in the day for this photo-flight during one of my aerobatics seminars at Hearne TX. At first glance I might appear to be diving in these photos. A closer look shows the smoke on the right curving upward towards the trailing edge while the left smoke stays straight. Also, notice that I am pushed out all the way at a very steep nose-down attitude.|
When I started doing spins with a camera mounted, I found that the pitch attitude gets much lower than I would have expected. The glider looks to be in steep dive attitude, but the slow shutter speed reveals the rotation. Never the less, you must notice if the nose begins to drop more. This is another way the glider tells you “It is time to pull in”.
The two most important things to know about the spin:
1. A spin causes rapid loss of altitude and can require lots of altitude for recovery to normal flight. If you wish to practice spins you should do it with plenty of altitude, and away from other flyers.
2. The way to get out of a spin is to pull in. There is no such thing as pulling in too much, unless your glider is questionable in pitch stability. When I am spinning in a contest, I go from full arm-stretch-push-out to fully pulled-in, “balled-up” max-dive position in less than a second.
|Notice the smoke heading toward the bottom surface and then curving around the trailing edge. Spins can get wobbley at times. I have seen the smoke go breifly in front of the leading edge!|
Have you ever done a spin attempt on landing? Most of us have. You are a few feet off the ground, wings nice and level, and it is just about time to flare. Suddenly, one wing drops, so you correct for the new turn(high-side) as you flare. If you analyze this situation you find you are actually making spin entry inputs. You are slow, in a bank, and you push out to stall. We are surprised when, despite our best attempt to roll back level, the glider banks and yaws even more resulting in miserable humiliation.
Tip: The best way to deal with a dropped tip on landing is to expect it. Always be ready to make a quick, full-scale counter roll input to bring the tip up; then return to center and flare. It is more important to bring that tip up than to do a full-stop flare landing, so be ready to run it out.
First published in Cross Country Magazine March/April 2007
I wrote the preceding as a supplement to my article “The Ups and Downs of Freestyle Hang Gliding” first published in the July 1994 issue of Hang Gliding Magazine, and viewable at johnheiney.com.