Gliders don't always go where they are pointing

You may have seen recently a K-13 plunging Stuka-like to the ground at 80 knots, rounding out at the last second. This was a try-out of the energy-dumping idea of the National Coach, intended to show that landing ahead after a cable break was possible more often than many pilots think. (The obvious questions of where will the first too-late round out and the first through the far hedge incidents occur was quickly answered in part by the Lasham pilot who performed the latter feat.) This reminded me of a 1987 Newsletter article which has a message some of you may not be too aware of, that gliders don't always go where they are pointing. At that time, our puny Ford diesel winch had a struggle getting gliders high enough to reach the hill into stiffish winds, and I showed that if you pointed a K-13 at the top of the hill after the winch launch you would probably arrive at the bottom, and if you pointed it at the bottom you might well arrive at the top.

Many pilots get an exaggerated idea of how much the flight path angle changes with speed, which they get partly from the typical plots of glider performance polars. These are always shown with different airspeed and sink scales as in the left hand figure below. The true polar shape is shown drawn to identical scales in the right hand figure. This polar is taken from a K-13 brochure, and it shows that over a sensible speed range the flight path angle changes by roughly 2 degrees, from about 2 to 4 degrees. This 2 to 1 ratio is quite typical for most gliders.

The other reason for the misconception is the changes in the glider angle of attack, or AoA, its own attitude relative to the flight path. The next figure shows that while the flight path angle changes by 2 degrees between 40 and 90 knots, the AoA changes by about 9 degrees, so the total change in glider attitude seen by the pilot is 11 degrees.

The glide angle is the same at both 40 and at 60 knots, despite the 6 degree change in attitude. With a stiff headwind, however, the glide angle over the ground is much better at 60 knots than at 40 knots, for reasons which of course you all know, hence the earlier article's "point" about getting to the hill. At that period I could often almost see a cloud of disbelief filling the front cockpit while trying to persuade pupils to go fast enough to reach the hill. I doubt if that is a problem now, but there is another condition which catches the pupil out. If orographic cloud starts to form ahead, a common reaction is to put the nose down to get underneath. The polar shows this is pretty useless, since you will hit the cloud at the same spot at 60 knots as if you kept plodding on at 40 knots. From a distance of 1000 feet, even 90 knots would make only a 35 foot difference to the height on reaching the cloud (though in both cases there would actually be a bit of extra height lost in diving to reach the higher speed). Obviously you simply open the airbrakes instead (the blue thingy on the left is not just a going-in-to-land lever).

Now return to the death-plunge approach. With no wind, the nose points roughly along the glide path at 80 knots with brakes open, rather than below it as the figure above shows for brakes closed. This is because the brakes cause a loss of lift, needing an increased angle of attack to maintain lift. You can obviously see the reference point in a sensible place in the canopy. A strong wind introduces a remarkable effect, however. Assume a top-end approach with a 35 knot wind at circuit height, a bit strong but quite possible even with only 20 knots on the ground. At this wind speed, with full brakes the glide angle over the ground is about the same at 60 and 80 knots, so assume a 60 knot approach for illustration even if more would be prudent in such a wind. Because the ground rises as you approach, the general airflow is downwards, so add another 2 knots to the sink. The figure below shows how the elements add up to an approach path very much steeper than the glider's attitude. Now the reference point is far beneath the nose, and if you think you are going to reach the ground about where you are pointing you are in for a huge surprise. It shows clearly why crossing the fence at 800 feet would not be too high and why going more than a short distance behind the fence is unwise in a strong wind. I once saw a bold instructor, insisting on flying the Pilatus when nobody else wanted to operate, show that 1000 feet was a good idea.

In case you are thinking of doing an 80 knot approach all the way to the round out, the polar for full brakes doesn't go that far but suggests a sink rate of well over 2000 feet per minute, which is possibly some 40 feet per second. Don't dither about starting the round-out!

John Gibson

12/1/96