9.3 Drag, L/D, and Ground Effect
Key Takeaways
- Parasite drag (form, skin friction, interference) rises with the square of airspeed. Induced drag falls as speed rises and is highest at high AOA.
- Total drag is a minimum at L/Dmax. That AOA is best glide and, for a propeller airplane, is typically near maximum range.
- In the region of reverse command (back side of the power curve), holding altitude at a lower speed requires *more* power.
- Ground effect is significant within about one wingspan of the surface: upwash, downwash, and wingtip vortices weaken, so induced drag drops.
- An airplane can become airborne below the published takeoff speed and can float on landing. Leaving ground effect raises induced drag, reduces stability, and produces a nose-up moment.
ACS PA.I.F.K3 and the slow-flight knowledge in PA.VII.A both assume you can read a drag curve. The same wing that makes lift extracts a tax. That tax has two families, they move in opposite directions with airspeed, and the runway invents a third plot twist called ground effect.
Parasite drag rises with V²
Parasite drag is everything that is not the price of making lift. PHAK splits it three ways.
| Type | What it is | How designers fight it |
|---|---|---|
| Form (profile) drag | The turbulent wake from the airplane’s shape — cowling, gear, antennas | Streamlining, wheel pants, retractable gear |
| Skin-friction drag | Viscous shear in the boundary layer (about a playing-card thick) | Smooth paint, flush rivets, a clean airplane |
| Interference drag | Two airstreams colliding, worst where surfaces meet at right angles | Fillets and fairings at wing roots, strut junctions, gear wells |
Parasite drag increases approximately as the square of airspeed. Double the speed, roughly four times the parasite drag. That is why VNE is a structural and a drag problem, and why a dirty airframe on a cross-country is a fuel problem.
Induced drag falls with speed
Induced drag is the tax on lift. High pressure under the wing spills around the tips toward the low pressure on top and rolls into wingtip vortices. The vortices drive a downwash that tilts the relative wind downward. Lift is always perpendicular to the relative wind, so that tilt points a component of lift aft. That aft component is induced drag.
Induced drag increases with AOA and therefore is highest when you are slow (you need a large AOA to keep L = W). PHAK: induced drag varies inversely with the square of airspeed. Double the speed in level flight, induced drag falls to about one-fourth. High aspect ratio (long, skinny wings) and winglets reduce the tip spill; a stubby wing pays more.
Total drag, L/Dmax, glide, and range
Plot parasite (rising) against induced (falling) and total drag is a U-shaped curve. The bottom of the U is minimum total drag, which occurs at one specific AOA: L/Dmax (maximum lift-to-drag ratio). At that AOA, CL/CD is greatest.
PHAK consequences you must not mix up:
- Best glide (maximum distance in still air, power off) is at the L/Dmax AOA. The AFM publishes a speed for a weight; if you are lighter, the same AOA occurs at a slightly lower speed. Pitch for that speed. Do not “stretch” a glide by raising the nose — that raises induced drag and shortens the distance.
- For a propeller airplane, maximum range (most miles per gallon in cruise) is typically at or near L/Dmax. Maximum endurance (most time airborne) is slower, near minimum power required, not minimum drag.
- Any AOA higher or lower than L/Dmax produces more total drag for the same lift.
Region of reverse command
PHAK Chapter 11 draws a line through that curve. Region of normal command (faster than the minimum-power / L/D neighborhood): to hold altitude, a higher airspeed needs a higher power setting. That is the cruise world.
Region of reverse command (the back side of the power curve, slow flight): to hold altitude, a lower airspeed needs a higher power setting, because induced drag is exploding. Pitch up without adding power and you do not “hold it”; you sink. This is why slow flight and a dragged-in final feel backward. It is also why the fix for being low and slow is power first, not more back-pressure.
Ground effect: the surface edits the vortices
When the wing flies within about one wingspan of the surface, the ground interferes with the tip-vortex / downwash pattern. Upwash, downwash, and wingtip vortices all weaken. Induced AOA and induced drag fall. Less thrust and a lower AOA will hold the same CL. PHAK’s numbers at constant CL:
| Wing height above the surface | Reduction in induced drag |
|---|---|
| Equal to the wingspan | 1.4 percent (almost nothing) |
| One-fourth span | 23.5 percent |
| One-tenth span | 47.6 percent |
So ground effect is defined out to a wingspan, but you only feel it in the last several feet — the flare, the soft-field liftoff, the low-wing trainer more than the high-wing. PHAK’s knowledge-test wording is often that the airplane is usually affected less than half a wingspan above the surface.
Takeoff. Reduced drag can make the airplane become airborne below the AFM recommended takeoff speed. Becoming airborne is not proof you can climb. As you leave ground effect you will need a higher AOA for the same CL, more thrust to cover the returning induced drag, and you will see a decrease in stability and a nose-up change in moment. Static-port pressure also changes: PHAK lists a reduction in static-source pressure and an increase in indicated airspeed leaving ground effect. If you rotated early on a hot, high, heavy day, the airplane can lift into ground effect, refuse to accelerate, and settle back onto the remaining runway or the obstacle.
Landing. The same reduction in induced drag and the effective increase in CL produce float. Excess speed in the flare becomes a long, floating trip down the pavement. You may need a power reduction to offset the extra lift. Do not shove the nose down in the flare to “put it on” — that is a bounce and a propeller strike, not a ground-effect technique.
Do not dump the nose to “gain speed”
PHAK’s takeoff warning is about speed deficiency, not about a pitch-down trick. It does not teach “once you hop into ground effect, dump the nose to accelerate.” Forcing the airplane off early and then pushing over can put the wheels back on the runway, or put the propeller closer to the surface, while you are still below the speed that will climb out of ground effect.
The published takeoff speed exists to give you climb performance once the vortices come back. Soft-field technique in the Airplane Flying Handbook is a different, deliberate maneuver: lift off at the lowest practical speed, then hold a level attitude in ground effect and accelerate to Vx or Vy before climbing. That is a slight pitch reduction to stay in the cushion, not a dive for airspeed. High power, high AOA, and low speed in that cushion also make left-turning tendencies (torque, slipstream, P-factor) obvious — expect right rudder, and expect the nose-up moment as you finally climb out.
Scenario: Jordan on a hot soft field
Jordan’s 172 feels light at 50 knots on a 95 °F grass strip and skips into the air a foot off the grass. Ground effect just cut a large fraction of his induced drag. If he hauls back to “climb,” he leaves the cushion with too little speed, induced drag slams back, the nose wants to pitch up, and the stall horn may be the next sound. If he shoves the nose down to “gain speed,” he can touch again. The PHAK-consistent action is the soft-field one: hold it in ground effect, accelerate to the book climb speed, then climb. The 50-knot hop was not a takeoff.
Which statement correctly separates parasite drag from induced drag?
For a typical light propeller airplane, the angle of attack that produces L/Dmax is also the AOA for which performance values?
An airplane becomes airborne well below the AFM recommended takeoff speed and is still within about a tenth of a wingspan of the runway. What should the pilot understand about ground effect?