induced drag

[Killtron2000]

My understanding of lift is that it is almost entirely due to the deflection of air. So basically an airplane that weighs say 3000 pounds would have to accelerate 3000 pounds of air at 1 g directly downward or some equivalent of that. What I don't understand is if you need the same amount of lift for level flight on the back side of the power curve as the front then why is there so much more induced drag on the back side? So say you are going 150 knots and a 2 degree angle of attack is all you need to maintain level according to the graph the induced drag would be negligible in a small aircraft. But when you are going 60 knots with a 12 degree angle of attack the induced drag is many times higher but you are still making the same amount of lift. This doesn't make sense to me because when you are deflecting more air with less acceleration it should still make the same amount of drag as accelerating less air faster.

[seagull]

Originally Posted by Killtron2000

My understanding of lift is that it is almost entirely due to the deflection of air. So basically an airplane that weighs say 3000 pounds would have to accelerate 3000 pounds of air at 1 g directly downward or some equivalent of that. What I don't understand is if you need the same amount of lift for level flight on the back side of the power curve as the front then why is there so much more induced drag on the back side? So say you are going 150 knots and a 2 degree angle of attack is all you need to maintain level according to the graph the induced drag would be negligible in a small aircraft. But when you are going 60 knots with a 12 degree angle of attack the induced drag is many times higher but you are still making the same amount of lift. This doesn't make sense to me because when you are deflecting more air with less acceleration it should still make the same amount of drag as accelerating less air faster.

Because your "understanding of lift" is flawed. Dump that inane "newton" theory that is so popular with the GA crowd. Do a search on previous discussions on the topic.

[tgrayson]

Originally Posted by Killtron2000

This doesn't make sense to me because when you are deflecting more air with less acceleration it should still make the same amount of drag as accelerating less air faster.

The source of induced drag doesn't really have anything to do whether or not you're accelerating more or less air faster or slower. The essence of it is that a high angles of attack, some of the aerodynamic force generated by the wing is wasted because it doesn't point *up*, it points backwards.

This is generally explained in the aerodynamics books as a change in pressure distribution around the airfoil at high lift coefficients that produces a resultant force that has a component acting opposite the flight path.

However, if you wish to think about lift as a momentum change of the air, you have to take into account its initial momentum as well as its final momentum. Wingtip vortices change the direction and velocity of the air as it approaches the wing and as it leaves it, so there is obviously some way to depict these changes that produces a net force in the induced drag direction. However, aerodynamics books generally don't approach this topic from this perspective, so I can't add much.

[Ian J]

Geesh you guys kill me sometimes... this is a CURRENT thread: http://forums.jetcareers.com/showthread.php?t=33344 (Induced Drag)