Trevor Hewson asks, with regard to roll control with rudder only on planes with sweep, but no dihedral:

I can fly my Lazy Bee (with dihedral) upside down (just!) without having to worry about rudder reversal. I have always assumed this wouldn't be the case if the rudder was working through wing sweep rather than dihedral...Is this true?<

From : Don Stackhouse

Yes, that's correct, sort of. What matters is the direction of the lift, i.e.: is the lift coefficient ("Cl") positive or negative. If positive, such as right side up or during an inside loop, a left rudder input will result in a left roll. If lift coefficient is zero, there will be no roll response from sweep effects during a rudder input. If the Cl is negative, such as during level inverted flight, or during an outside loop, then roll due to rudder will be reversed; i.e.: left rudder will result in a RIGHT roll.

Rudder reversal effects are not limited to this case alone. A conventional airplane with or without dihedral sees a reversal in the rudder required to counteract adverse yaw during negative "G" flight. For example, in a slow roll to the left, you start with left aileron (which is held throughout the maneuver, although the amount required to keep a constant roll rate varies during various parts of the maneuver), left rudder to counteract the adverse yaw from the ailerons, and some up elevator to hold altitude. The rudder gradually decreases, reaching zero at about 45 degrees bank, and continuing into a right rudder deflection, reaching maximum at about 90-135 deg. of roll. At this point, that right rudder is acting primarily as an upward force to keep the nose from dropping. However, as the airplane rolls the rest of the way into inverted flight (and the now "down"-deflected elevator is holding the nose up), you continue to hold right rudder. This is because when you are inverted (negative "G") you need "right" rudder (as seen by the pilot in the cockpit) to counteract the adverse yaw from "left" aileron. The rudder does not go back through zero deflection and on into leftward deflection again until the airplane is at about 225 degres of roll, where rudder is again acting like elevator and you need left rudder to hold the nose up.

For a whole chapter discussing this and other bizarre and wonderful phenomena related to slow rolls, consult my favorite book on the subject, "Aerobatics" by the great (and sadly, late) Neil Williams, ISBN no. 0-9504543-0-3.

There are lots of other possible factors that also act like dihedral, and that could therefore confuse the issue. If you had some dihedral and some sweep, then the roll reversal would still occur at large negative lift coefficients, but the dihedral might dominate at small negative Cl's, resulting in normal behavior for inverted level flight.

The combination of the shape of the fuselage and its position relative to the wing can also act like dihedral in some cases. For example, Steve Wittman's "Tailwind" has a zero-dihedral, constant-chord wing cabin-mounted on top of a very boxy fuselage with square corners. The Tailwind rolls quite nicely with rudder alone, despite having no dihedral. When the fuselage is yawed, it acts like a very low aspect ratio wing, with higher air pressure on one side and lower air pressure against the other. These fields of pressure extend out along the undersides of the wing roots, causing an increase in the local wing lift lift on the high pressure side and a decrease on the low pressure side. The airplane rolls in response to these unequal lift forces. The same effect in reverse can happen on a low-wing airplane, which is why low wing airplanes often need a couple degrees more dihedral than an otherwise similar high-wing airplane. However, the strength of this effect is very dependent on the shape of the fuselage. High, slab-sided, square-cornered fuselages like the Tailwind's tend to maximize this effect, while on a slim, well-rounded fuselage it may be barely discernable, if at all.

There are a host of factors that interplay to determine the behavior of each individual airplane design in any given situation. The factors that you overlook (and there will INEVITABLY be at least a few) will be the ones that catch you by surprise. Even if you've thoroughly done your homework, be ready for some surprises.

Don Stackhouse
DJ Aerotech