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Old 18-05-2010, 14:53
Cabruna Cabruna is offline
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Join Date: May 2010
Posts: 1
Default Vimd and Drag

Hi people!
Could someone tell me why if we increase the drag profile the Vimd becomes lees and if we increase the induced drag the Vimd increases?If we consider that Vimd is the best ratio L/D at level flight why by increasing Drag it becomes less in a straight way?

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Old 18-05-2010, 15:07
John H's Avatar
John H John H is offline
Bristol Instructor
Join Date: Sep 2005
Posts: 1,057
Default Re: Vimd and Drag

Hi Cabruna

The aircraft is affected by two types of drag. Parasite drag which increases in proportion to velocity squared and induced drag which comes from lift which decreases whith speed inversely to velocity squared. Add then both together gives total drag See notes page 3,15 fig 11-3-15

Where the value of induced drag and parasite drag is equal is the lowest section on the total drag curve. This is point of velocity for minium drag on the aircraft Vmd (Vimd) . The graph assumes straight & level balanced flight.

Now you use the term profile drag ? this is part of parasite drag (form + skin friction ) If this is increased the slope of the parasite drag curve would be steeper? Look at fig 11-3-15. this is the green line? if you draw this steeper it would cut the induced drag curve at a lower speed reference to the x axis at the bottom of the graph.

Vmd is not fixed value. it changes with weight & profile, but when ever an aircraft fly's at its vmd it is producing the value of lift required for the minimuim amount of drag (L/D Max) and the wing will be at its optimium angle of attack.

Hope this helps

John H
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