FAA Private Pilot Airplane (PAR)Aerodynamics and Aircraft SystemsEasy
A Cessna 172 departs a short runway with tall trees at the far end. Just after liftoff, while still within one wingspan of the ground, the pilot notices the airplane seems to fly with unusually little effort and reduced drag. As the airplane climbs above approximately one wingspan altitude, this effect diminishes and the pilot must lower the nose slightly and add power to maintain the same climb performance. This phenomenon is best explained by:
- AAn increase in propeller efficiency near the ground
- BA temporary decrease in air density near the runway surface
- CGround effect reducing induced drag near the surface
- DReduced parasite drag caused by ground friction on the airflow
Show answer & explanationAnswer & explanation
Correct answer: C. Ground effect reducing induced drag near the surface
Ground effect occurs when an airplane flies within about one wingspan of the surface. The ground interferes with the formation of wingtip vortices and downwash, reducing induced drag and making the wing more efficient. As altitude increases beyond one wingspan, this benefit disappears.
Why the other options are wrong
- A. Propeller efficiency is unrelated to proximity to the ground.
- B. Air density near the surface does not change due to proximity to the runway.
- D. Ground effect reduces induced drag, not parasite drag from ground friction.
Ground Effect
A reduction in induced drag and downwash that occurs when an airplane flies within approximately one wingspan of the surface.
- Most noticeable within one wingspan of the ground
- Reduces induced drag, allowing lift-off at lower airspeed
- Can cause premature liftoff and difficulty climbing out of ground effect if airspeed is too low
Memory trick: Close to the ground, drag is drowned.