As I said, I am fed up of writing long diatribes on PPRuNe.
However, suffering from insomnia, I will do a little.
First, throw away any misconceptions you bring with you from power flying. I don’t care what you were taught there, or what you teach if you are a power instructor. This is a theory course on your conversion to gliding, assuming you have not flown a glider. The differences, if you find any, are important.
DFC, your point challenged what WW wrote about how to handle a low cable break or winch launch failure, so first come with me on a virtual winch launch. We are going to do a simulated power failure. (i.e. I am PIC/PF and I know its coming. By the way, I am always prepared for it even if I don’t know.) I will do this as a demonstration. This exercise is sufficiently dangerous that gliding instructors are told ONLY to do it as a demonstration, and not to let student pilots have a go. It is taught only on the last day of an instructor course – so that if the would-be instructor crashes the glider, it does not ruin the whole week!
The wind is nil, or very light and down the runway. As the launch starts, the wingtip runner lets go at about 10 knots of ground/air speed, and we keep the training glider balanced on its wheel by stick/elevator in about its mid position, adjusting very slightly if we need to, to keep the fuselage level. The tail skid is just off the ground.
WE DO NOT ROTATE IN ORDER TO LIFT OFF.
As the airspeed builds up, the airflow meets the wings, which are set with a positive angle of incidence. At say 37 knots (a little above normal stalling speed of say 32 knots), the glider lifts off the ground, still fuselage level. Can you tell me why, from your power flying experience?
[In a full stall, at altitude, the wings will reach about 15 degrees of alpha before stalling. That is achieved there, as a demonstration, by raising the nose above the horizon and letting speed bleed away at 1 knot per second, typically, until it drops to 32 knots. In that attitude, the tail skid is below the main wheel. On the ground, we cannot get the tail lower than the wheel. So from a lower alpha we have to have a higher airspeed to achieve sufficient lift to become airborne.]
Now we are airborne, just above the ground, fuselage still level, and accelerating. As Instructor/PIC/PF I jam the airbrake with my elbow, while holding the cable release with my left hand, while I keep the stick in the same mid-elevator position.
I jam the airbrakes because I must prevent you, the newbie in the front seat, opening them. If you did, you would do what WW cautioned against. You would slam the glider onto the ground and probably break it. (I don’t care if your power flying expertise leads you to think otherwise, you are not going to learn you are wrong in my glider by breaking it. We shall come back to that topic later, on the next flight.) If I were not an experienced instructor, and had not got through my course doing the demo on the last day and succeeded in not breaking the course glider, I might get it wrong myself.
With power from the winch, we accelerate further, stick still in the middle, and the glider without our further intervention starts to rotate gently into the climb. WE MUST NOT INCREASE THE RATE OF ROTATION BY STICK BACK/MORE UP ELEVATOR. This beautifully harmonised training glider can manage a winch launch very well on its own.
If the winch launch fails in, say, the first 50 feet of climb, it really is important to handle it very carefully. In this pre-arranged failure, the winch driver chops the power when we are about 10 feet above the ground. The fuselage is still only slightly nose high, and speed is around 40 knots. I release the (now slackened) cable, and adjust the attitude to slightly nose down before we lose flying speed. At the normal flying attitude, at 40 knots, we are on a 1 in 30 glide angle. As we approach the ground, I round out (power pilots may use the expression “flare”, but we call it rounding out). We fly parallel with the ground, main wheel about 6 inches off. As the speed bleeds off, we prevent the glider landing by easing back the stick to increase alpha and maintain lift equal to weight, The drag slows us, and eventually, the change in alpha means nose higher, tail lower, and - - the tailskid touches the ground. We cannot bring the nose up and increase alpha any more, so as the speed drops further, lift can no longer hold up our weight. The main wheel drops the last 6 inches, gently, not fast, and we have a perfect landing. Now we fully open the airbrakes to stop as soon as we can.
Second flight. We have an uneventful launch, get quite high, and I am going to demonstrate the use and effect of airbrakes. We fly straight into the (almost nil) wind. We fly at a typical approach speed, 50 knots. Check your ASI and vario as we call it (the latter is our VSI) and note the sink rate: 2.5 knots. Yes, it is in knots, not feet per minute in hundreds. Divide one by the other and we see our instantaneous glide angle – 1 in 20, about 3 degrees.
Look over the nose. In front of us is a field. It is sliding down the canopy, showing that we are overshooting it. Look in the distance. That town with the white warehouse is sliding up the canopy. We will never reach it. Now look at that water tower. As we get nearer, it is in the same position on the canopy. If we do nothing else, we will reach the ground just where it is. That is where our 1 in 20 glide slope will take us to.
When I open the airbrakes, what do you think will happen to out airspeed, our attitude, and our sink rate and glide slope, if I don’t also move the stick? Be ready for a surprise.
OK, half airbrake. (Actually, air“brake” is a bit of a misnomer at this speed, as you see.) Our attitude has not changed! Our speed is still 50 knots; the “brakes” do not slow us down perceptibly. What has changed is the sink rate – now about 5 knots. At 5/50, we now have a glide angle of about 1 in 10. Why is this so?
The “brakes” spoil some of the lift. To maintain a steady speed, and steady albeit greater sink rate, alpha has to increase on the unspoilt part of the wing.
Yes, we have more drag, but we are now going down a steeper glide path, ALTHOUGH THE ATTITUDE HAS NOT CHANGED. So the gravity component has a greater vector along our glide path, and that makes up for the increased drag so the forces are still balanced..
(There are some gliders where opening the brakes DOES cause a SMALL speed reduction at normal approach speed, and would need a very minor correction with elevator. There are others, particularly an old favourite trainer with spoilers, where they cause the nose to drop and speed to increase, and a bit of up elevator/stick back is needed. Do what you have to do to maintain approach speed, but in this trainer the stick movement required and attitude change is very slight.)
In that demo, half brake increased the sink rate, virtually instantaneously, to 5 knots. I will show you full brake. Now the sink rate is - - 12 knots.
You remember that I jammed the airbrake lever when we had a low cable break/power failure. If you had just opened the brakes, we would have experienced an immediate increase in sink rate. Hitting the ground with a vertical component of 10 or 12 knots would break the wings. Even 5 knots sink rate would bend the undercarriage and damage the fuselage.
If I ever have another virtual flight with you, I might be able to show why we always use elevator to control speed and the brakes to control sink rate to achieve a stable final approach, contrary to what a boofy power pilot said, but that will do for today. And I am fed up with instructing, so I probably won’t.
Chris N.
Last edited by chrisN; 12th June 2009 at 02:35.