Learning hub · Principles of Flight · Lesson 2
Drag, stability and turning flight
Lift gets you into the air, but drag decides how far you glide, and the way you turn decides how hard the wings must work. This lesson covers drag, stability and the effect of bank.
About 13 minutes, plus the quiz. Part of Principles of Flight. It builds on Lift, angle of attack and the stall.
1. Types of drag
Drag is the force that opposes motion through the air. There are two main kinds.
- Parasite drag comes from the shape of the aeroplane and the friction of the air on it: form drag, skin friction and interference drag where parts meet. It grows with the square of the speed, so it is the main drag at high speed.
- Induced drag is the price of making lift. It is largest at low speed and high angle of attack, and falls as the speed rises.
Add the two and the total drag has a lowest point at one speed. At that speed the aeroplane has its best lift-to-drag ratio (L/D). Below that speed, drag rises again, which is why a slow aeroplane needs more power to hold its height.
2. The best glide
With the engine at idle, the glide ratio is about the same as the L/D ratio (a windmilling propeller adds some drag). The best glide speed is the one with the lowest total drag. It gives the greatest distance in still air. The flight manual lists the speed, and it is the number to remember for an engine failure.
A mass change shifts the best glide speed but not the best glide ratio. The speed is a little higher at a higher mass. Into a headwind use a slightly higher speed, and with a tailwind a slightly lower one, within the manual’s guidance.
Worked example (invented): an aeroplane with a glide ratio of 9:1 is 2,000 ft above the ground. It can cover 2,000 × 9 = 18,000 ft, which is about 3 NM in still air (1 NM is about 6,076 ft).
3. Stability and the CG
Static stability is the first reaction to a disturbance. If an aeroplane is pitched up and its nose then tends to return, it is statically stable. Dynamic stability is what happens over time: the oscillations can die out (stable), stay the same, or grow (unstable).
Light aeroplanes are made stable on purpose. The tailplane does much of the work. The position of the centre of gravity (CG) matters. A forward CG is more stable but needs more elevator force and has a higher stalling speed. An aft CG is less stable and lighter on the controls, and if it is too far aft, recovery from a stall can become difficult or impossible. See loading and the centre of gravity.
4. Forces in a level turn
In a turn the lift is tilted. The vertical part of the lift must still balance the mass to hold the height, and the horizontal part pulls the aeroplane into the turn. So the total lift must be bigger than in straight flight. You get it with more angle of attack (back pressure) and, because that adds drag, a little more power.
The load factor n is the lift divided by the mass. In a steady level turn: n = 1 ÷ cos(bank angle)
| Bank angle | Load factor | Stalling speed increase |
|---|---|---|
| 0° | 1.00 | 0% |
| 30° | 1.15 | about 7% |
| 45° | 1.41 | about 19% |
| 60° | 2.00 | about 41% |
| 70° | 2.92 | about 71% |
The stalling speed rises with the square root of n. Worked example (invented): a stalling speed of 50 kt in level flight becomes 50 × 1.19 = about 59.5 kt at 45° of bank. The stall itself is explained in the stall lesson.
5. Adverse yaw
When you roll into a turn, the aileron on the rising wing goes down and makes more lift and more induced drag. The other goes up with less drag. The drag pulls the nose towards the outside of the turn, the opposite way. This is adverse yaw. It is strongest at low speed with large aileron movements. Counter it with rudder in the direction of the turn, and with coordination. Designers help with differential ailerons (the up-going one moves more) or Frise ailerons.
Training material only. Bank angles, speeds and limits for your flying come from your instructor and the flight manual. Steep turns at low level are dangerous.
6. Try it yourself
Work each one out on paper first, then open the answer to compare. The numbers are invented.
- An invented aeroplane stalls at 55 kt in level flight. What is the load factor in a steady 60° level turn, and what is the new stalling speed?
Show the answer
- n = 1 ÷ cos 60° = 1 ÷ 0.5 = 2.0.
- The stalling speed rises by the square root of 2, which is 1.414.
- 55 × 1.414 = about 78 kt.
- An invented aeroplane with a glide ratio of 10:1 has an engine failure at 3,000 ft above the ground in still air. How far can it glide in nautical miles?
Show the answer
- Distance = 3,000 × 10 = 30,000 ft.
- 30,000 ÷ 6,076 = about 4.9 NM.
- In practice a headwind, turns and a poor speed reduce this, so keep some margin.
- In a 45° bank at constant height an aeroplane has a mass of 1,000 kg. What lift does the wing have to make, in kg-equivalent? What happens if you fail to add back pressure?
Show the answer
- n at 45° is about 1.41, so the lift must be 1.41 × 1,000 = about 1,410 kg-equivalent.
- Without back pressure the angle of attack does not increase, the lift stays near 1,000 kg-equivalent, and the nose drops and the aeroplane descends.
- With more back pressure the speed also falls unless you add power.
7. Watch
A short video on induced and parasite drag. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.
Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.
More to watch
An introduction to load factor and why it grows in a turn.
Open on YouTube if the player does not load. Not made by VectorEFB.
A simple explanation of adverse yaw.
Open on YouTube if the player does not load. Not made by VectorEFB.
8. Check yourself
Five original practice questions. Answer them, press Check answers and read each explanation.
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