Flying or Servo and Linked Tabs

Orville R. Dunn (1949) gave 30,000 pounds as a rule-of-thumb upper limit for the weight of transport airplanes using leading-edge aerodynamic balance. Dunn considered that airplanes larger than that would require some form of tab control, or else hydraulically boosted controls. The first really large airplane to rely on tab controls was the Douglas B-19 bomber, which flew first in 1941. The B-19 used pure flying or servo tab control on the rudder and elevator and a plain-linked tab on the ailerons. In a flying tab the pilot’s controls are connected only to the tab itself. The main control surfaces float freely; no portion of the pilot’s efforts go into moving them. A plain-linked tab on the other hand divides the pilot’s efforts in some proportion between the tab and the main surface. The rudder of the Douglas C-54 Skymaster transport uses a linked tab.

Roger D. Schaufele recalls some anxious moments at the time of the B-19’s first flight out of Clover Field, California. The pilot was Air Corps pilot Stanley Olmstead, an experienced hand with large airplanes. This experience almost led to disaster, as Olmstead “grabbed the yoke and rotated hard” at liftoff, as he had been accustomed to doing on other large airplanes. With the flying tab providing really light elevator forces, the B-19 rotated nose up to an estimated 15 to 18 degrees, in danger of stalling, before Olmstead reacted with forward control motion.

Flying tabs are quite effective in allowing large airplanes to be flown by pilot effort alone, although the B-19 actually carried along a backup hydraulic system. A strong disadvantage

is the lack of control over the main control surfaces at very low airspeeds, such as in taxi, the early part of takeoffs, and the rollout after landing. The linked tab is not much better in that the pilot gets control over the main surface only after the tab has gone to its stop. Still, by pro­viding control for the B-19, the world’s largest bomber in its time, flying and linked tabs, and the Douglas Aircraft Company engineers who applied them, deserve notice in this history.

An apocryphal story about the B-19 flying tab system illustrates the need for a skeptical view of flying tales. MIT’s Otto Koppen was said to have told of a B-19 vertical tail fitted to a B-23 bomber, an airplane the size of a DC-3, to check on the flying tab scheme. The point of the story is that the B-23 flew well with its huge vertical tail. Koppen said this proved that a vertical tail could not be made too large. Unfortunately, this never occurred. Orville Dunn pointed out that (1) the B-23 came years after the B-19, and (2) it didn’t happen.