LESSONS · AERODYNAMICS · 12 MINUTES · AUSTRALIA AND NZ
Drag and best glide.
Two kinds of drag change in opposite ways as airspeed changes. Where they are equal, total drag is least and the lift-to-drag ratio is at its best. In still air, that is the speed that glides furthest.
PREDICT FIRST
Your engine stops and you want to glide as far as you can in still air. Which speed do you fly?
Two kinds of drag
Parasite drag is all the drag that has nothing to do with making lift. It comes in three types: form drag, interference drag and skin friction. It grows with the square of airspeed, so doubling your speed gives four times the parasite drag.
Parasite drag is comprised of all the forces that work to slow an aircraft’s movement. As the term parasite implies, it is the drag that is not associated with the production of lift.
Induced drag is the cost of making lift. Fly slower and the wing needs a larger angle of attack to hold the aircraft up, and induced drag grows. It falls with the square of airspeed, so doubling your speed cuts it to a quarter.
Induced drag is inherent whenever an airfoil is producing lift and, in fact, this type of drag is inseparable from the production of lift. Consequently, it is always present if lift is produced.
Conversely, parasite drag increases as the square of the airspeed. Thus, in steady state, as airspeed decreases to near the stalling speed, the total drag becomes greater, due mainly to the sharp rise in induced drag. Similarly, as the aircraft reaches its never-exceed speed (VNE), the total drag increases rapidly due to the sharp increase of parasite drag.
Parasite drag climbs as you speed up. Induced drag climbs as you slow down. The shaded bar stacks the two into total drag, and the glide below stretches or shrinks with it.
Heights are stretched about 3.7 times, so the path looks steeper than it is. At this L/D of 9.2 the real glide angle is about 6°. Still air.
L/D
9.2
Glide from 3,000 ft
4.5 NM
Best glide speed
65 kt
Parasite drag
Induced drag
Total drag
Weight
Faster than best glide by 15 kt. Parasite drag is the bigger part, total drag is higher than its least, and the glide is shorter.
Example aircraft, not a real type: heavier, clean, it glides best at 65 kt with an L/D of 10. Still air. The glide uses the level-flight drag curves, which holds for a shallow glide. Extra drag raises both kinds of drag by the same share, so best glide speed stays at its clean value; the handbooks give no best glide speed with drag-producing parts out. The model has no stall, so its slowest speeds may be below a real aircraft's stall speed.
The bottom of the curve
Add the two together and total drag makes a U. It is least at one speed. One part grows with the square of speed while the other shrinks the same way, so the lowest point comes where the two are equal.
As seen in Figure 5-6, at some given airspeed, total drag is at its minimum amount.
The lift-to-drag ratio, L/D, compares the lift a wing makes with its drag. In level flight lift equals weight, so the speed with the least drag is also the speed with the best L/D.
Note that the maximum lift/drag ratio (L/DMAX) occurs at one specific CL and AOA. If the aircraft is operated in steady flight at L/DMAX, the total drag is at a minimum.
The glide ratio is how far the aircraft travels for the height it loses.
The glide ratio of an airplane is the distance the airplane travels in relation to the altitude it loses. For example, if an airplane travels 10,000 feet forward while descending 1,000 feet, its glide ratio is 10 to 1.
With no thrust, only lift, drag and weight act, and the lift-to-drag ratio alone sets the glide angle. The example aircraft has an L/D of 10 at best glide, so from 3,000 ft it covers 30,000 ft, about 4.9 nautical miles, in still air.
This tells us a very simple and very important fact: the glide angle depends only on the lift-to-drag ratio.
Everything so far assumes still air. Wind changes the distance over the ground. AFH says a tailwind carries the aircraft further because of the higher groundspeed, and a headwind shortens the glide because of the slower groundspeed.
Wind affects the gliding distance. With a tailwind, the airplane glides farther because of the higher groundspeed. Conversely, with a headwind, the airplane does not glide as far because of the slower groundspeed.
Away from best glide speed, in either direction, drag rises and the glide shortens. Once you are at best glide speed, raising the nose to stretch the glide slows you below it, and the glide gets shorter.
The pilot should not attempt to “stretch” a glide by applying back-elevator pressure and reducing the airspeed below the airplane’s recommended best glide speed.
Drag-producing parts cost distance too. Turn on the extra drag in the simulation at the same airspeed and watch the L/D and the glide shrink.
Since the highest glide ratio occurs at maximum L/D, certain considerations should be given for drag-producing components of the airplane, such as flaps, landing gear, and cowl flaps. When drag increases, a corresponding decrease in pitch attitude is required to maintain airspeed. As the pitch is lowered, the glide path steepens and reduces the distance traveled.
The same aircraft glides from the same height in still air, once heavy and once light, each time at the best glide speed for its weight. How far does it glide when heavy?
For example, if two airplanes having the same L/D ratio but different weights start a glide from the same altitude, the heavier airplane gliding at a higher airspeed arrives at the same touchdown point in a shorter time.