The Art of Gliding

By Craig Skinner

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As we have all heard by now, the four forces acting on an aircraft in flight are Lift, Weight, Drag and Thrust. These four forces cancel each other out and the plane flies along quite happily in a state of equilibrium. What happens if we remove Thrust? Besides the obvious high pitched yell of "Dead stick" followed by an increased heart rate and a short prayer to the All Mighty to stay out of the corn field, the plane will continue to fly quite happily.

Lets look at the forces acting on a plane during a glide. When thrust is removed, a new equilibrium is achieved through a combination of Lift and Drag equalling Weight.

L + D = W

As you can see, the combination of Lift and Drag equals Weight. The gliding angle "a" is calculated using Tan a = D/L. A lower valve of D/L is better or to put it differently (this is the important part)

So you ask, how is L/D determined? Good question. The L/D ratio is determined during the design of the airplane and is a measure of the planes efficiency at flying. At what speed / Angle of Attack (AOA) will we get the most Lift for the least Drag. First lets have a quick review of drag and then we will look at the graphs. Drag is divided into two parts with Induced Drag (low speed) produced by the wings at high AOA's and Profile Drag (high speed) produced by the shape of the airplane. A low drag airplane will have long thin wings and a small frontal area, wait a minute, I just described a glider. Lift on the other hand, increases steadily up to the stalling angle. Ok, it's graph time.

Ok, so what does this graph tell us? Well the most important bit of information is that the best L/D ratio occurs at a constant Angle of Attack. Now assuming the weight is constant (and we are not doing any silly flight manoeuvres), we can say that a given AOA will produce a given air speed. Therefore (big breath)

So what is this speed? Well it varies between designs but I would guess that it is not too slow or too fast and has an AOA similar to that in cruise.

What effect does speed have on gliding angle? As we can see from the graph, any change in speed away from the optimum will decrease the ratio between Lift and Drag thereby produce a steeper glide angle. It may be tempting to "pull up" is you are going to land short but it will only make things worse.

Finally, what effect does weight have on gliding angle? Weight has no effect on gliding angle but will change the gliding speed. Lets look at the vector diagram above. If we increase the Weight vector, in order to maintain equilibrium, we must increase the Lift vector (more speed) which will increase the Drag vector (remember that both Lift and Drag are proportional to the square of speed). The gliding angle was fixed in the design and will not change. An increase in Weight will increase the glide speed but the angle will stay the same.

There you have it. How to fall out of the sky while giving the impression that it's all part of the plan.

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