FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsEasy
A helicopter's performance chart shows a hovering ceiling in ground effect (HIGE) of 10,000 feet density altitude but a hovering ceiling out of ground effect (HOGE) of only 6,000 feet density altitude for the same gross weight. This difference exists because:
- AHIGE is always calculated at a lower gross weight than HOGE for safety margin
- BHOGE figures include a mandatory fuel reserve that HIGE figures do not
- Cground effect increases translational lift, which is unavailable out of ground effect
- Dground effect reduces induced drag and required power, allowing a hover at higher density altitudes than without ground effect
Show answer & explanationAnswer & explanation
Correct answer: D. ground effect reduces induced drag and required power, allowing a hover at higher density altitudes than without ground effect
Ground effect reduces induced drag and the power required to hover by restricting rotor downwash and reducing tip vortices, allowing a hover at higher density altitudes (or heavier weights) than would be possible out of ground effect.
Why the other options are wrong
- A. Both charts are typically presented at the same gross weight; the difference is aerodynamic, not a weight adjustment.
- B. Fuel reserves are unrelated to the aerodynamic difference between IGE and OGE hover performance.
- C. Translational lift results from forward airspeed, not from proximity to the ground during a hover.
Hovering Ceiling: IGE vs OGE
The maximum density altitude at which a helicopter can hover, which is higher in ground effect (HIGE) than out of ground effect (HOGE) because ground effect reduces induced drag and power required.
- Ground effect reduces rotor downwash angle and induced drag
- Lower power required means better performance at altitude/weight in IGE hover
- HOGE requires more power margin since ground effect benefit is lost
- Performance charts list both HIGE and HOGE ceilings for a given weight
Memory trick: Ground effect is a cushion that lets you hover higher with the same power.