Subsonic Flight Flashcards
What is subsonic flight ?
Flight at which the free airstream over the entire aircraft is subsonic
What are subsonic aircraft’s design constraints/ performance goals?
Short range , high payload , short takeoff and landing and a ceiling of up to 35000ft (but usually around 25000ft due to propeller effectiveness)
What are some common design features of subsonic aerofoils?
Straight rectangular wings, thick camber , uniform chord over the span, minimal high lift devices, engine mounted in front of wing to gain extra lift from prop wash.
What is the main design limitation of propeller driven subsonic aircraft ?
Airspeed at tip of propeller becoming /near supersonic , inducing drag and compressing the air.
Why is compressibility considered negligible when referring to subsonic flight?
The airframe is not accelerated fast enough to catch up with the pressure wave generated.
What is the main influencer of an aircrafts speed capability?
The propellor. An aircrafts speed is proportional to the efficiency of the propeller.
What is one of the main ways commercial aircraft achieve their operational efficiency?
By maximising payload. This can be done by making the fuselage as wide, high and long as required then matching a high lift wing design and respective engine power.
Aerodynamic performance is not a major factor as long as the aircraft meets a cost per seat/payload.
What are some of the disadvantages of subsonic aircraft?
- Engine power vs weight - increase power, increase weight
- propeller efficiency - propeller design has reached its limits in aerodynamic efficiency without shrouding
- Size vs performance - increase size requires more power/more engines, increases weight and cost
- Size vs runway performance - increased size means increased runway requirement/reduced number of smaller airfields available
- Wing efficiency - straight thick wings (no sweep) are approaching relative airflow velocity limitations without encroaching on buffet boundaries induced by flow separation
