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.

 

[alphy]

There are several factors that contribute to the decrease in aircraft efficiency during low level, high speed flight.

Firstly, at low altitudes, the air is denser which means there is more air molecules for the aircraft to push through. This increased air resistance, also known as drag, requires more engine power to maintain the same speed. This leads to a decrease in engine efficiency as more fuel is needed to produce the same amount of thrust.

Secondly, the airflow over the wings is affected by the proximity to the ground. At low altitudes, the air is disturbed by the terrain and obstacles, causing turbulence and disruption to the smooth flow of air over the wings. This turbulence creates additional drag, reducing the lift to drag ratio and decreasing overall aerodynamic efficiency.

Additionally, flying at high speeds also creates a phenomenon called wave drag. As the aircraft approaches the speed of sound, shock waves start to form around the aircraft, increasing drag and reducing efficiency. At low altitudes, these shock waves are more pronounced, further decreasing the efficiency of the aircraft.

In summary, low level, high speed flight decreases aircraft efficiency due to increased air resistance, disrupted airflow over the wings, and the formation of shock waves. These factors all contribute to a decrease in the lift to drag ratio and engine efficiency, ultimately reducing the overall performance of the aircraft.