Angle of attack and mathematical "proportionality"

[Steve Pomroy]

Hi Photofly.

The information you have is accurate. But it refers to zero-lift drag, which includes parasite and wave drag, but not induced drag.

The outboard spanwise component of the flow shifts the lift distribution toward the wingtips. This strengthens the wingtip vortices and increases induced drag.

This is one of the reasons forward sweep is of interest to aircraft designers. Forward sweep reduces induced drag while still increasing critical mach and reducing the drag rise. Forward sweep also eliminates the dutch roll problems associated with backsweep. The problem is that forward sweep creates structural problems (static divergence). Composite materials are showing some promise in solving this problem, but until they do, sweepback will be the rule for high-speed subsonic designs.

Cheers,
Steve
http://www.skywriters.aero
http://www.flightwriter.com

[photofly]

The outboard spanwise component of the flow shifts the lift distribution toward the wingtips. This strengthens the wingtip vortices and increases induced drag.

The lift distribution can be controlled by an appropriate choice of planform and washout. Presumably these are used to minimize the adverse effects of sweepback?

[Steve Pomroy]

Sure, all kinds of design features are used to optimize the wing. Sweepback is just one feature. But keep in mind that sweepback is a major element of planform. "Planform" encompasses wing area, aspect ratio, sweep, and taper ratio. High aspect ratio and some optimum taper ratio can be used to help offset the negative effect of the sweep. Ditto for both geometric (washout) and aerodynamic (airfoil variation) twist.

Cheers,
Steve
http://www.flightwriter.com
http://www.skywriters.aero

[dr.aero]

Steve...

Simple yes. Correct no. I think this is one of the points you've been trying to make: simpler is not better when accuracy suffers. So, no beer for you. You can still have a cookie because you're partly right.

Exactly my point!

Sweep increases the critical mach number and the drag-rise mach number. So in effect you do get less drag at high speeds. But one of the prices you pay is in the form of increased drag at low speeds. The spanwise component of flow increases induced drag.

I believe you mean "high speeds" as in speeds approaching Mach 1. I'm sure you know but as you go above Mach 1, a short stubby wing starts becoming more attractive as it has less drag than the swept wings - like you illustrated in your other comment, the X-15 and F-104.

http://imgur.com/a/EjEWj

"An immediate conclusion is that sweepback is a device of greatest application in the regime of transonic flight."

A couple images supporting your comment of keeping the wings inside the mach cone to reduce drag:

http://history.nasa.gov/SP-60/i-5-1.jpg
http://www.worldaffairsboard.com/attach ... ockum9.jpg

Modern fighters try to take the best of both worlds (sweep and straight) to achieve low drag going above Mach 1-1.5 and then low drag at higher mach numbers. A bit of sweepback with a short/straight wing (think delta wing - involves taper ratio): http://farm3.staticflickr.com/2340/1584 ... z.jpg?zz=1