On Wednesday, May 7, 2014 1:20:44 PM UTC+12, Bill D wrote:
So, you're saying the pilot will be safer if they don't learn to perform the return to runway maneuver when it's safe to do so?

I can assure you that the higher a glider's performance, the safer it is. It's the old, low L/D gliders that can run out of altitude before getting lined up with the runway.

I agree with you. I'm shaking my head every time I read this thread.

In a modern glass glider (such as the DG1000's I instruct in) with a 40 knot stall speed and being towed at 70 knots you should be able to execute a safe 180º turn with *zero* loss of height.

Just slowing down from 70 knots towing speed to 55 knots gains you 80 ft on top of whatever you already had.[1]

How much height do you lose in a 45º banked turn at 55 knots? Most modern gliders lose no more than 120 fpm at 45 knots in straight and level flight.. A 45º banked turn gives 1.41 Gs (1/cos(45)), which needs sqrt(1.41) = 1.19 times more speed for the same angle of attack and L/D. 45 knots times 1.19 is 53.6 knots. So 55 knots in a 45º turn has a little more margin above stall than 45 knots in straight and level. The sink rate will be 120 * 1.41 = 170 fpm.

Converting to SI and using a=v^2/r, a 45º banked turn at 55 knots (28.3 m/s) has 115.3m radius, or 725m circumference for a full turn. A 180º turn (362m) will take 12.8 seconds. In 12.8 seconds at 170 fpm you'll lose 36 feet.

So the height loss in the turn is only about half the height gained from slowing down from towing speed to circling speed!

It would actually be better to start turning immediately, but these calculations assume you delay (deliberately or not) and climb straight ahead (no pull-up required) for several seconds before starting the turn.

With 18m wingspan in a 45º bank your wingtip is 18m/2*sin(45) = 6.4m or 21 ft below you.

So you theoretically could do this from absolutely zero height, with nearly 20 ft to spare.

I wouldn't want to try it! But from 100ft? No problem at all. IF you start from normal towing speed and reasonably benign weather conditions.

Even if you're releasing from a sick tug that's slowed to 55 knots, you'll be fine from 200 ft.


Another post mentioned that glider pilots make mistakes when flying close to the ground because they are not trained to do so and don't do "ground reference" manoeuvres like power plane pilots do.

Obviously that person lives in very flat ground, because I can assure them that here we're flying close to ridges and peaks a LOT, from almost the first flight, doing 180º turns at the end of ridge lift runs, or circling low over a peak or head of a gully looking for a thermal. We'd very often be only 100-200 ft or so above the terrain while doing so.


[1] handy formula: X knots of kinetic energy is worth (X/5)^2 feet of gravitational potential energy. e.g. 70 knots = (70/5)^2 = 14^2 = 196 ft. 50 knots = (50/5)^2 = 10^2 = 100 ft. Less drag loses of course. You'll never turn speed into quite that much height, and you'll need more height than that to get speed. But the differences are large in a high performance glider at moderate speeds.