Drag & the power curve

Two kinds of drag, and the sweet spot

Definition

Drag comes in two flavors. Parasite drag — skin friction, form, and interference — grows with the square of airspeed, so it dominates when you're fast. Induced drag is the drag the wing makes as a by-product of lift, and it shrinks as you speed up, so it dominates when you're slow. Add the two together and you get total drag: a U-shaped curve. The bottom of that U — where parasite and induced drag are equal — is the best lift-to-drag ratio, L/Dmax. That single airspeed is your best-glide speed.

Explanation

Because the two drags trade places as speed changes, there's exactly one speed where total drag is least. Fly faster and parasite drag runs the total up; fly slower and induced drag runs it up. That least-drag speed (L/Dmax) also gives a propeller airplane its best range.

Figure 5-6. Drag versus speed. — Figure 5-6 · FAA-H-8083-25C, Pilot's Handbook of Aeronautical Knowledge (p. 102)

Why it matters

When the engine quits, L/Dmax is the number that keeps you airborne longest and reaching farthest. Guess too fast or too slow and you land short of the field you could have made.

From the Right Seat

I have students memorize best-glide speed cold and practice trimming straight to it — hands off — so it's automatic the day the engine gets quiet.

A DPE might ask

What are the two kinds of drag, and how does each change with airspeed?
What is significant about the speed for L/Dmax?

PHAK ch. 5 — Drag & the drag curve

The back side of the power curve

Definition

Because induced drag climbs so fast at low speed, there's a speed below which flying slower actually takes more power, not less. That low-speed regime is the region of reversed command — the 'back side of the power curve.' It's where slow flight and short-field approaches live: you control the descent with power and the airspeed with pitch.

Explanation

On the front side of the curve, easing power off lets the airplane slow down. Cross to the back side and that reverses: to hold altitude at a slower speed you have to add power to overcome the fast-rising induced drag. Get slow and low on a dragged-in approach and you can run out of power to arrest the sink — the airplane keeps descending even at full throttle.

Why it matters

This is why 'low and slow, dragging it in with power' is dangerous. Behind the power curve you have little margin — if the speed decays you may not be able to climb or even stop the descent without lowering the nose.

Common error

Students reflexively pull the nose up to stop a descent. Behind the power curve that backfires: raising the nose without adding power increases the angle of attack and induced drag, so the airplane sinks faster and slows toward a stall. The fix is power for the sink, pitch for the speed.

In the region of reversed command, to fly slower in level flight you need to...

Safety

On a slow approach, keep some power in and fly the airspeed with pitch. If the airplane starts to sink, add power first — pulling the nose up alone will only deepen the sink and bleed speed toward a stall.

A DPE might ask

What is the region of reversed command, and where do you encounter it?
If you're behind the power curve and start to sink, what corrects it?

Airplane Flying Handbook — Slow flight & the region of reversed command