What is the Keel Effect and how does it happen? Is it the same as Pendulum Effect or it doesn't exist?

The Pilot's Handbook of Aeronautical Knowledege by the FAA has an explanation that says:

An aircraft always has the tendency to turn the longitudonal axis of the aircraft into the relative wind. This "weather vane" tendency is similar to the keel of the ship and exerts a steadying influence on the aircraft laterally about the longitudonal axis. When an aircraft is disturbed and one wing dips, the fuselage weight acts as a pendulum returning the plane to its original attitude

I know I'm like answering my own question, but is this explanation quite right? Because in this question it is said that perhaps it doesn't exists or it means something different.

  • Yes, this is right. You can demonstrate this by kicking in some rudder in straight and level flight then release the pedals, the aircraft will straighten out on its own. – Ron Beyer Mar 31 '16 at 13:33
  • The only thing that I see that is wrong with the explanation is that it assumes a high wing aircraft. Low wing aircraft don't have as much "righting" tendency as a high wing, so the "pendulum" effect is lessened, but the "keel" or weather-vaning effect is still there. – Ron Beyer Mar 31 '16 at 13:47
  • @RonBeyer not entirely, the dihedral configuration (where present) adds to this effect in low wing aircraft. – falstro Mar 31 '16 at 13:49
  • @falstro Yes, depending on the degree of dihedral effects how much the effect is there, but I'd say its stronger in a high-wing having flown both myself the low-wing that I usually fly requires a little more opposite aileron to come back where the high-wings is more of a relaxing on the controls into the turn. – Ron Beyer Mar 31 '16 at 13:51
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    @RonBeyer Has it been decided whether the "pendulum effect" is a real thing or not? There seems to be some disagreement over it. I can't find it at the moment, but I'm certain Peter Kämpf has started before that there is no passive aerodynamic way to right a plane about the roll axis – TomMcW Mar 31 '16 at 19:28
up vote 5 down vote accepted

According to the keel effect page on Wikipedia it is the rolling moment created by the fuselage and vertical surfaces in a sideslip if the center of pressure is vertically offset from the center of gravity.

Functions of a keel

The keel of a ship (besides shifting the center of gravity below the metacenter) provides a stabilizing side force when the ship rolls. The same is achieved by a tail surface, but in aviation this is called roll damping. The much larger wing tends to dominate roll damping, so the effect of the vertical surface is small.

The next task of the keel is to balance the side force and rolling moment from the sails in crosswind conditions. It does this by letting the boat build up a sideslip condition until the side force from the keel and hull balances the side force from the sails, and the rolling moment from the side force and from heeling balances the rolling moment from the sails. The keel effect is this rolling moment due to a sideslip condition.

A few aircraft with very small wings and large vertical surfaces do indeed suffer from excessive moments created by side loads on those vertical surfaces. The best known of those is probably the Lockheed F-104 Starfighter. The massive tail on the upswept rear fuselage made 10° anhedral necessary to get the sideslip-induced rolling moment right.

Pendulum effect

The Wikipedia page also mentions the pendulum effect and calls both the fuselage contribution to the dihedral effect. Man, if there is ever a competition for the most misleading name of an effect, this would be the winning entry.

A pendulum is a mass mounted below the pivot or the metacenter, so it will stabilize in the down position. It seems the adherents of the pendulum effect belief believe that the plane is somehow hinged at the center of the wing. This is so wrong! A flying aircraft is not hinged, so all motion is around the center of gravity.

This is different with airships: As @JanHudec points out in the comments, the buoyancy of the lifting gas always works against gravity whereas the lift of a wing acts perpendicular to its span and the airspeed. This does produce a moment when the airship rolls, and the heavy gondola is pulled down by the same effect which pulls down a pendulum. Also, due to an airship's low density (by definition very close to the density of the air it displaces) the aerodynamic side forces on an airship are much larger compared to the inertial forces. This lets the airship effectively pivot around a point near the center of the envelope. Note that airships have no means of roll control - it is just gravity which ensures that the gondola stays on the lower side of the hull.

Now you can argue that the same is true for aircraft, only with much smaller aerodynamic forces. Yes, but those side forces are vanishingly small in comparison to the wing's aerodynamic forces which, again by definition, are close to the weight of the aircraft. A small change there will completely dominate the picture, and the small loads on the fuselage are just a rounding error. By moving the control surfaces, the pilot can shift the lift vector around almost as he/she likes, and the forces on the fuselage are insignificant.

Note that knife edge flight, a situation when the side force on the fuselage really is close to the aircraft's weight, needs lots of fuselage area and a powerful engine. Fuselage loads in normal flight are a fraction of those in knife edge flight.

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    It seems like I remember you saying one that there is no passive aerodynamic way to make an airplane return to wings-level after an upset. Is that basically what you're saying here, that the effects that are supposed to do that are just too insignificant to have any real effect? – TomMcW Apr 1 '16 at 0:41
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    @TomMcW: No, I certainly didn't mean to say that. If you have no sideslip, there is indeed no passive aerodynamic way to right the airplane up. Here we are talking about a sideslip, and that results normally in a rolling moment, but this need not be in the uprighting direction. If the aircraft banks left wing down and yaws nose left, the sideslip will make it roll further to the left. – Peter Kämpf Apr 1 '16 at 6:00
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    I believe the main difference with airship is, that buoyancy always acts upwards in the centre of lift, so if the centre of gravity is not directly below it, the lift and weight forces will shift laterally without changing direction, thus creating a restoring rolling moment similar to how a hinge works. But dynamic lift is always perpendicular to the airfoil, so no moment is created with banking alone. – Jan Hudec Apr 1 '16 at 12:27
  • @JanHudec: Yes, that is right, and I totally overlooked this detail. I will edit the answer. – Peter Kämpf Apr 1 '16 at 20:52

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