How does pitch relate to glide angle?

Question

When flying a glider, you have a polar curve. As your speed increases beyond best glide, your glide angle gets steeper. Say you are flying at best glide, you are looking through a specific point in your canopy to the spot you would land if you maintained this glide angle until you touched down.

Now say you double your speed. Of course your glide angle would get steeper, the spot will have moved down relative to the canopy at best glide. At the new pitch, looking through a specific point in your canopy to where you would land, would this point be higher, lower or the same?

The reason I ask this question is because I’m learning to glide. When you pitch the nose down to gain speed, it feels as though you are pitching down a long way, but your glide angle doesn’t seem to be affected as much as you would expect it to be. I guess a more accurate question title might be: “How does pitch relate to the polar curve?” But I’m trying to gain intuition on how glide angle changes in relation to what you perceive in the cockpit/canopy.

Answer 1

When you pitch the nose down to gain speed, it feels as though you are pitching down a long way

This is because the change of the cockpit angle towards the horizon is the sum of both the angle of attack and the glide path angle change.

This means that at low speed your speed changes should move the target point up when you go faster (the point is only affected by the change in glide angle while your view is affected by the sum of angle of attack and glide angle change) while at high speed, when a speed change means only very small changes in angle of attack) the target point does not move much relative to the canopy any more (but it will still move a bit up when going even faster).

How does pitch relate to the polar curve?

All you need to do is to add the angle of attack at the respective speed to the polar curve and you get the pitch angle. Like this:

The blue line is the arcsin of the ratio of sink speed to flight speed. It should not be a coincidence that it looks like the sink speed polar. To that line I added the angle of attack (red line) which shows in which direction relative to the horizon the longitudinal axis of the plane points. Since the cockpit is a bit angled down, your viewing angle should be reduced by that amount. Now it is easy to see that the angle changes a lot at the low speed end but stays almost parallel to the sink speed line at high speed. Since I assumed a zero-lift angle of attack of -2°, the red line crosses the blue one at 52 m/s.

Answer 2

Pitch attitude in itself isn't the big factor, other than as a speed control tool (in gliders). It's mostly about whether or not the position of the spot on the ground you're approaching changes (moves up or down in your view) at whatever pitch attitude you're holding, when that pitch attitude is being held constant. This is key for judging glide path on final in both gliders and power planes.

You know you are overshooting a spot on the ground (your glide is flatter than the angle that the spot represents), when, at a constant pitch attitude, the spot moves down toward the glareshield, and that you are undershooting if the spot moves up (glide is steeper).

So if you are gliding at max L/D in still air, say 30:1, and are at 500 ft agl, the spot you are heading for on the ground is 2.8 statute miles out ahead (15000 feet), and at the pitch attitude that is giving you the max L/D speed, held constant, the spot 2.8 miles ahead will stay in the same spot vertically, relative to the instrument panel.

If you lower the nose to best L/D + 20 knots when you are at 500 ft, say the speed increase drops the L/D to 20 (maybe not a very efficient glider). You pitched down to speed up, so the spot 2.8 miles out will move up in your view because you pitched over.

The key is what happens while you are holding the new pitch attitude constant. If your glide is now steeper because you sped up, the spot 2.8 miles away represents a shallower glide (a 30:1 glide) than your new 20:1 glide angle. What will happen is the 2.8 mile spot moved initially up to the new position when you pitched over, then instead of stabilizing at that spot, it continues to drift up because your glide path is steeper than the one represented by the spot.

The practical application of this is that when flying on final, and the spot is the runway threshold, your indicator that you are on the proper glide path is whether or not the threshold moves up or down in your view over the nose while holding a constant speed/attitude. If you hold a constant pitch attitude and the threshold moves down, you need to add spoiler to steepen the path, and if it moves up, you need to remove spoiler to make it shallower. In power planes, it's the same thing, but with throttle.