Bill Crawford’s Flightlab Blog

Most aircraft are laterally stable, up to a point. If you put them in a shallow bank, then fold your arms in feigned indifference, they tend to return to level flight. But first they sideslip in the direction of the low wing. On aircraft with conventional dihedral angle—the wing tips higher than the wing roots—the sideslip leads to an increase in the angle of attack on the low wing, and to a decrease on the high. That difference produces a rolling moment that brings the wings back to level and simultaneously makes the sideslip disappear. The reaction is called dihedral effect. It gives an aircraft lateral stability. But remember, a sideslip has to happen first.

A swept wing exhibits dihedral effect as well, even if the swept wing has no dihedral angle. Highly swept wings sometimes have anhedral—wing tips lower than wing roots—because sweep can produce too much dihedral effect at low speeds. Anhedral knocks some out.

Ok, flight-test time. Wings are level; the aircraft is in trim. Note the position of the aileron control. Now enter a 5-degree bank and return the control to the original position. It’s important that any unintended aileron deflection, possibly from control system friction, not be allowed to mess things up. Keep the rudder neutral. The aircraft should slowly start to roll level after a few moments. Its velocity vector has a component of motion (sideslip) toward the low wing, which leads to a wings-level rolling moment from dihedral effect. The aircraft’s lateral stability provides positive spiral stability. Sideslip also produces a yawing tendency, but dihedral effect dominates at smaller bank angles. Note that we are not pulling back on the stick as we normally would in a regular turn.

That was boring. So do it again at 15 degrees and see what happens. Try it left and right to check for asymmetry, perhaps caused by propeller effects. Then try maybe 30 degrees and see what happens. You’ll probably find that the aircraft generates a slow, up-and-down phugoid motion, superimposed on the bank angle.

At some larger bank angle the aircraft will forget about leveling its wings and will depart into a spiral dive. Roll the wings level and recover. Note the strong nose-up pitch moment. Once the wings are level, the aircraft, as it tries to get back to trim speed, recovers on its own. That’s actually the phugoid you saw before, coming to the rescue!

Test pilots typically place an aircraft in a given bank angle, center the ailerons (or bank the aircraft with rudder while holding the ailerons fixed), and then time the interval required to reach half the bank angle for the spirally stable condition, or double the bank angle for the unstable. Again, it’s important that control surfaces are positively centered during these tests, because any residual deflection caused by friction or by our abysmal ineptitude can create an apparent difference in spiral characteristics. Control friction confuses things when you’re assessing an aircraft. Friction in the elevator system makes you think that longitudinal (pitch axis) stability is lower than in reality; friction in the ailerons that prevents them from returning to center when released leaves you with a roll rate that you didn’t ask for. Normally, unless the aircraft has severe rheumatism and virtually creaks, you’d accommodate to such things without really being aware of the control adjustment required.

An aircraft’s “spiral mode,” as it’s called, is essentially a contest between lateral stability caused by dihedral effect, and directional stability caused, mostly, by the aircraft’s tail. The tail makes the aircraft want to yaw, or weathercock, its nose into the relative wind—into the direction of a sideslip if that’s what happens to be going on.

As the aircraft yaws into the sideslip, the outside wing moves faster than the inside wing. The resulting difference in yaw rate across the span leads to a difference in lift, and the aircraft generates a rolling moment. It doesn’t amount to much, however, because at low bank angles the opposite moment due to dihedral effect wins out, and the wings return to level (positive spiral stability). As you add bank increments you’ll find a point—if the atmosphere’s not too turbulent—where bank angle remains constant (neutral spiral stability). The opposing rolling moments produced by dihedral effect and roll due to yaw rate cancel each other out.

At greater bank angles, roll due to yaw rate finally gets the upper hand. Now the aircraft will continue banking into a spiral dive (negative spiral stability). Nature has arranged things so that the coefficient of roll moment due to yaw rate goes up with wing coefficient of lift, so the roll moment that leads to spiral departure is more pronounced at low speeds, when the coefficient of lift is high. Therefore, you’ll increase your spiral bank angle more quickly at lower entry speeds. Nature also got it into her pretty little head that for a given bank angle, yaw rate varies inversely with airspeed. So, for this reason as well, roll due to yaw rate increases as you slow down.

Various other effects also participate in spiral departure, but I don’t have the energy to carry on about them. I’m pooped and distracted: I just shoveled out the Blizzard of 2006, mostly onto my neighbor’s property. Also, I’ve got one eye on the Winter Olympics—imagine the satisfaction it must give Michelle Kwan to know that 1.7 billion people now go to bed thinking about her groin muscle. And, of course, I’m cruising the Net to see how Dick Cheney’s hunting buddy is doing.

Spiral dives lead to impact craters. So we can’t demonstrate to completion. But poking carefully at the edges we can see that a spiral dive doesn’t really tighten into a vicious corkscrew, as some melodramatic illustrations might suggest. You don’t go round and round and round until you hit. That’s either because the ground is just too near or, if not, because the load factor gremlins gleefully pull the aircraft apart. Because of the g load and potential loss of consciousness, many who die in spiral dives probably don’t notice the crash.

By the way, an aircraft with an “over-banking” tendency that forces you to hold outside aileron during a normal turn is likely to be spirally unstable. At proper altitude, enter slow flight and begin a coordinated turn. Note aileron position in response to over-banking. Then let go of the ailerons and see what happens.