Why Does Low Level, High Speed Flight Decrease Aircraft Efficiency?

[kevjcarvalho]

hey,
I was reading Aircraft Design: A Conceptual Approach by Raymer and i came across the following line.
' The aerodynamic efficiency of an aircraft, expressed as the lift to drag ratio is greatly reduced during low level, high speed flight as well is its engine efficiency. '
Can someone tell me why. Thanks.

 

[abhi88]

Search for the reason Airliners fly at 30,000ft during cruise.
and, as far as speed is related, you must be aware of the 'wave drag' and 'critical mach number'.

 

[kevjcarvalho]

Thanks.

 

[dtango]

Here's a bit more explanation: 1) drag increases with the square of airspeed 2) drag also increases with increasing air density. Air density is the highest near sea level and then decreases the higher you go. So at low altitudes and high speeds, that's the worst possible combination from a drag perspective because it maximizes drag in level flight. We could further expound on all this but that's a basic explanation to Raymer's statement.