L/D Max and Flaps Settings

[VDEE7]

I'm looking for good explanation to the effect of flaps settings on L/D MAX Airspeed, If L/D MAX airspeed with the flaps up is 74k for instance, Would that change if the Flaps were extended? since that would increase induced and parasite drag! Thanks.

[SteveC]

<moved to Tech Talk forum>

[VDEE7]

Ops!!.. Thanks

[meritflyer]

Originally Posted by VDEE7

I'm looking for good explanation to the effect of flaps settings on L/D MAX Airspeed, If L/D MAX airspeed with the flaps up is 74k for instance, Would that change if the Flaps were extended? since that would increase induced and parasite drag! Thanks.

Yes, L/Dmax would in fact change with the introduction of flaps. You instantly increase parasite drag of the wing.

To what extent, I am not sure. I am sure tgrayson has a formula for figuring it out.

[tgrayson]

Originally Posted by VDEE7

I'm looking for good explanation to the effect of flaps settings on L/D MAX Airspeed, If L/D MAX airspeed with the flaps up is 74k for instance, Would that change if the Flaps were extended? since that would increase induced and parasite drag! Thanks.

If you draw out the drag curves, you'll see increased induced drag tends to increase Vldmax, whereas increased parasite drag decreases Vldmax. So they counteract each other. The end result will depend on the airplane, type of flap, and flap setting.

If I had to hazard a guess, I'd say that small flap settings tended to increase induced drag more than parasite drag, with a resulting increase in Vldmax. Large flap settings would tend to increase parasite drag more than induced drag, so it'd likely slow down Vldmax.

The reasoning is that the reason that flaps affect affect induced drag is via a further distortion of the lift distribution from the ideal elliptical. Also, if the flaps increase the chordline, the effective aspect ratio is reduced. Since the greatest increments in lift coefficient take place at lower settings, it seems likelier that these outweigh the parasite drag changes.

As I recall, a C-172 publishes a best glide speed for the full flap setting about 5 knots below the clean speed, so that is consistent with the above.

[VDEE7]

Makes sense, thanks alot.