by Tommy H. Thomason

Tuesday, August 15, 2017

Relying on Museum Pieces for Accuracy Part 3

Restored airplanes, either static or warbirds, can lead a kit manufacturer and/or modeler astray from an accuracy standpoint. Missing parts, ersatz replacement parts, flat oleo struts, one-off test program modifications, etc. have all resulted in kits and built models with errors. Sometimes, however, what's there is ignored or disbelieved. A case in point is the Douglas AD (A-1) Skyraider vertical stabilizer.

In addition to thrust, the propeller on a single-engine airplane creates other forces that must be taken into account. Consider the following for a propeller turning clockwise from the pilot's point of view. When the propeller is inclined nose up to the relative airflow, the down-going blade produces more thrust on that side than the other, resulting in a turning moment to the left (this effect is known as P-factor). The turning propeller also creates torque, causing the airplane to roll to the left; opposing this requires right stick, which increases lift on the left wing and therefore potentially drag and a turn to the left (some aileron-control designs compensate for this). When the airplane is on its takeoff roll, the torque also puts more pressure and therefore more drag on the tire on the left side of the airplane, causing a turn to the left. The swirl from the propeller, equivalent to downwash from a wing, impinges on the vertical fin, pushing it to the right and therefore the nose to the left.

In other words, a lot of right rudder (which results in a turn to the right) can be required to oppose these forces that cause a left turn. They change with the throttle setting and, in the case of P-factor, angle of attack. More powerful engines and bigger, heavier propellers result in higher forces. The flight-control forces to counteract them decreases with airspeed. As a result, the designer of a powerful single-engine, propeller-pulled airplane sometimes provides a built-in assist like a vertical fin with the leading edge angled left, providing a right rudder effect.

The Douglas AD (A-1) Skyraider incorporates such a feature, with the fin angled left at three degrees.

You'll note that the fin also appears to have a cambered airfoil, creating lift to the right as in the application of left rudder. My guess is that this isn't as effective at low speeds during a high-power wave-off as the angling of the fin to the left (right rudder) but is important in a dive (the AD was designed as a dive bomber) when the fin angle created too much "right rudder" at that low angle of attack, reduced throttle setting condition at fairly high speed.

This is my picture of the fin of the AD Skyraider at the National Naval Aviation Museum that shows the airfoil and the angle to the left relative to the dorsal fin that can be seen forward of the red anti-collision beacon.

Byron (SpadGuy) Hukee (see http://skyraider.org/) provided this picture of a Skyraider's rudder.
If you look closely, you'll see a kink in the trailing edge of the rudder just above the location of the horizontal stabilizer. You'll also note a difference in the fairing of the fuselage into the fin between the left and right sides of the airplane.

An even more striking example was provided by Ed Barthelmes (see https://www.amazon.com/1-Skyraider-Walk-Around-No/dp/0897474295) of the AD-5's vertical fin leading edge. Its air inlet and dorsal fin provide an excellent perspective of the fin's offset to the left side of the fuselage.

The old Airfix 1/72 kit of the AD Skyraider incorporated this feature. Some modelers have erroneously gone to the trouble of removing it...

1 comment:

  1. Interesting take on the offset left fin with the cambered right side which seems at first to be antithetical until you think about compromising for both ends of the speed range. Kind of like having one's cake and eating it too...Airplanes are compromises.

    ReplyDelete