Wing Pylon Drag Question

[DarrenW]

I've noticed that the speed of an aircraft is affected more and more by wing pylons as altitude Increases. With parasitic drag decreasing at higher altitudes how could the pylons provide more drag? Does a greater induced drag have something to do with this?

Here's an example:

P-51 Mustang Performance

I've noticed this phenomenon with the F6F Hellcat too. Any ideas?

 

[tomo pauk]

The new design of wing racks reduced the speed loss on the P-51D to just 4 mph.
IMO, the racks represent the 'drag increment', ie. addition to the parasitic drag.

 

[DarrenW]

Thanks Tomo I forgot that the early pylons were more draggy but how come they reduced aircraft speed more as altitude increased? I'd think that they would have less of an affect on speed as altitude increased because of decreasing parasitic drag.

 

[elbmc1969]

A few uninformed guesses:

1. While parasite drag decreases with altitude, that assumes constant speed. If we assume that the racks represent the same proportion of parasite drag, we should expect the speed loss to be greater.

2. Level flight at higher altitude requires a higher angle of attack at a given speed, and going faster may not make up for this. The turbulence caused by the racks may change considerably with angle of attack (almost certainly), producing more drag. This could even reduce the lift under part of the wing enough to require a still higher angle of attack.

 

[DarrenW]

You made some very logical observations. Both make perfect sense to me. Thank you!