Sweepback = Dihedral Effect?
 

Back in the early 50's I read in some mag that about 5 deg.'s of <u>sweepback</u> is equivalent to 1 deg. of <u>dihedral</u>. This is why sweptback jets don't need dihedral, and why some with <u>extreme</u> sweepback actually have <u>NEGATIVE</u> dihedral (or "anhedral", with wings that slope downward).

What about this, Aeronautical Engineers, do you agree, and if so, could you provide the EXACT no. of degrees of sweepback in my stated "formula" above?

FOR NON-BELIEVERS: Imagine a sweptback jet "falling off" to the left - the airstream does little for the <u>right</u> wing panel as it is somewhat in the "shadow" of the fuselage - the <u>left</u> wing panel, however, is highly affected since the plane is "FALLING" somewhat in THAT DIRECTION - the result is that the <u>left</u> panel is generating MORE LIFT now than the other one, tending to raise the left side again ---- THE SAME EFFECT AS <u>DIHEDRAL</u>.

Everyone agree?

Sweepback = Dihedral Effect?
 

Sweepback does indeed have similar effect as dihedral on lateral stability. However there is not a simple relationship. It depends on other factors such as wing planform, thickness, section, vertical location of the wing (high-low)etc. That being said, the ratio of five degrees sweep equals one degree dihedral is probably as good a rule of thumb as any to start.

Sweepback = Dihedral Effect?
 

If a swept wing sideslips, the downgoing wing experiences a flow direction more nearly at right angle to the wing, hence the apparent chord/thickness ratio increases, (so does lift) whereas on the other side the apparent chord increases, reducing the chord/thickness ratio, resulting in less lift on that side. Total result, a rolling force to reduce the sideslip. The effect is the same for a Delta wing ; the important thing is that the leading edge is swept, whether the trailing edge is or not. The "blanketing" effect of the fuselage on the wing is unimportant, it would have to be a pretty severe side slip!.

Sweepback = Dihedral Effect?
 

The flow fences on the wing of the Mig15 should tell you something about the effects of swept wing design. If you have flown on a Boeing or other commercial jet you will probably have noticed the vortex generators all over the wing, especially just forward of the movable surfaces. True also of the vertical and horizontal stabilizers.
Sweeping usually has more to do with the supersonic movement of air over the surfaces. Dihedral for inherent stability is not usually necessary due to computer control during flight. Power boost has eased the burden of moving surfaces at high speeds and made it much more usable for computers which have redundant systems in place. What the effect of sweep has on model aircraft design usually reflects high Reynolds numbers which most everyday sport flyers usually don't mess with.

Sweepback = Dihedral Effect?
 

Ballgunner, you are right that sweep is used to delay the onset of compressibility effects in jets. However, vortex generators are seen on both straight and swept wings, they add energy to the boundary layer and delay flow separation. They generally are placed to correct or minimize some quirk discovered during flight test. Although some fighter aircraft are basically unstable without computer aid, all transport category aircraft must meet inherent stability requirements for certification.

Sweep does indeed affect low Reynolds number, and subsonic aircraft. Moderate sweepback behaves similarly to dihedral and is used in most pattern airplanes for that purpose.