Originally Posted by glekichi
barit>
Thanks for the input.
I know exactly what you mean about l/d and that... im just thinking conceptually here... high school science style.
What if we ignored parasite drag and consider purely lift and induced drag.
The faster you go, for some reason, the energy required to fly becomes less?
Why is that?
Pointless perhaps, but im just curious.
Well, let us have a look at a wing.
Assuming a symmetrical airfoil, you could fly it unloaded. Angle of attack zero. Or in a case of asymmetrical, cambered wing at some angle of incidence to the fuselage, fly slightly nose-low so that there is no lift. There would be relatively small drag, but the drag would be still there. Lift/drag would be zero.
Alternatively, you could fly an airfoil with angle of attack near 90 degrees. A flat foil at a right angle to the airflow creates huge drag, but very little lift. The "leading" and "trailing" edges normally have a somewhat different shape from each other, so the angle of attack where exactly zero lift is produced is slightly different from, but close to 90 degrees.
Now what happens if a wing flies at an angle of attack which is large, say 45 or 60 degrees? Then the wing has huge drag, but also creates a lot of lift. This happens because it deflects a lot of air downwards, rather than up.
And what about smaller angles of attack? Well, what happens is that the wing deflects air slightly, generating moderate lift - at the cost of drag much lower than the lift. L/D gets to the ranges of 10, 20, 50...
So what about speed?
The lift has to equal weight in sustained level flight.
At certain speed, suitable for cruise, the lift equals weight at the angle of attack giving best L/D.
If the aircraft slows down, then to continue supporting the weight, the craft must increase the coefficient of lift by means like adopting high AoA and extending flaps - which increase lift at the cost of a drag which is high relative to the lift.
If the aircraft tries to speed up above the cruise speed then the drag increases with the square of speed (assuming low Mach numbers) while the coefficient of drag cannot be decreased much - the minimum at zero AoA is not far below the drag coefficient in cruise, and cannot be attained because that would mean no lift.
So, this is why airframes have a certain optimum speed.