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Induced Drag and Parasite Drag - Which one decreases as IAS increases?

  • towsibali@gmail.com
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towsibali@gmail.com created the topic: Induced Drag and Parasite Drag - Which one decreases as IAS increases?

This question confuses me at all times.

As IAS is progressively increased - then happens to the induced drag and parasite drag - which one progressively increases and which one progressively decreases?

Similarly, As IAS is progressively decreased - then happens to the induced drag and parasite drag - which one progressively increases and which one progressively decreases?
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  • John.Heddles
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John.Heddles replied the topic: Induced Drag and Parasite Drag - Which one decreases as IAS increases?

If we look at low altitude, low Mach drag a bit idealistically, the two main components are

(a) lift dependent drag = lift induced = induced = vortex drag (or any similar terms you might prefer) which relates to that component of total drag arising as a direct consequence of the generation of lift. This part of the drag is high at high alpha (low speed) and then reduces as alpha reduces (speed increases).

(b) lift independent drag is the sum of all the other sources of drag which don't have anything particular to do with the generation of lift: things like profile drag, skin friction, interference drag, parasitic drag and other such terms you may come across. This part of the drag is low at high alpha (low speed) and then increases as alpha reduces (speed increases).

The main thing is that (a) is tied up with the production of lift while (b) is all the other stuff not tied up with the production of lift.

The total drag, then, is the sum of (a) and (b) at any particular speed. Because of the shape of the curves for (a) and (b), the shape of the total drag curve is high at low speed, reduces to a minimum as speed increases, and then increases again as speed increases further.

The typical picture can be found in every aerodynamics text book, in many places on the net, in Bob's books and so on and so forth. I took this picture from a site on the net and it is quite typical. I've omitted the axis scales - moving left to right is increasing speed, moving bottom to top is increasing force (in this case, drag).




You can see the two component curves and the sum (addition) of these to give the total drag curve.

Be aware that this sort of picture is a typically generic story based on simple theory. In practice, specific aircraft will have variations, a common one on larger aircraft being a drag bucket to provide better cruise performance. I couldn't find a suitable picture, but imagine a lower drag U-shaped section of the total drag curve in the cruise range for alpha or speed.

If you can memorise this picture and get a handle on the general shape, that's about all you will need for the exams.

Engineering specialist in aircraft performance and weight control.
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