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Overhang or Leading-Edge Balances
When control surface area ahead of the hinge line is distributed along the span of the control surface, instead of in a horn at the tip, the balance is called an overhang or a leading-edge balance. Overhang design parameters are the percentage of area ahead of the hinge line relative to the total control surface area and the cross-sectional shape of the overhang (Figure 5.4).
Experimental data on the effects of overhang balances on hinge moments and control effectiveness started to be collected as far back as the late 1920s. Some of these early data are given by Abe Silverstein and S. Katzoff (1940). Airplane manufacturers made their own correlations of the effects of overhang balances, notably at the Douglas Aircraft Company (Root, 1939). As in many other disciplines, the pressure of World War II accelerated these developments. Root and hisgroup at Douglasfound optimized overhang balance proportions for the SBD-1 Dauntless dive bomber by providing for adjustments on hinge line location and overhang nose shape on the SBD-1 prototype, known as the XBT-2.
Root wrote a NACA Advance Confidential Report in May 1942 to document a long series of control surface and other modifications leading to flying qualities that satisfied Navy test pilots. For example, in 1 of 12 horizontal tail modifications that were flight tested, the elevator overhang was changed from an elliptical to a “radial,” or more blunt, crosssection, to provide more aerodynamic balancing for small elevator movements. This was to reduce control forces at high airspeeds.
Overhang aerodynamic balance, in combination with spring tabs, continue in use in Douglas transport airplanes, from the DC-6 and DC-7 series right up to the elevators and ailerons of the jet-powered DC-8. The DC-8’s elevator is balanced by a 35-percent elliptical nose overhang balance. Remarkably constant hinge moment coefficient variations with elevator deflection are obtained up to a Mach number of 0.96.
George S. Schairer came to the Boeing Company with an extensive control surface development background at Convair and in the Cal Tech GALCIT 10-foot wind tunnel. Although early B-17s had used spring tabs, Schairer decided to switch to leading-edge
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Figure 5.4 Typical hinge momentparametervariation with size for leading-edge or overhang aerodynamic balances. The round nose is more effective in reducing hinge moment due to surface deflection Ch& than the elliptical nose, which does not protrude into the airstream as much when the surface is deflected. (From Phillips, NACA Rept. 927, 1948) |
balances for the B-17E and the B-29 bombers. The rounded nose overhang balances on the B-29s worked generally well, except for an elevator overbalance tendency at large deflection angles. Large elevator angles were used in push-overs into dives for evasive action. William Cook remarks, “A World War II B-29 pilot friend of mine was quite familiar with this characteristic, so the fact that he got back meant this must have been tolerable.” However, overhang balance was not effective for the B-29 ailerons. Forces were excessive.
The wartime and other work on overhang aerodynamic balance was summarized by the NACA Langley Research Department (Toll, 1947). The Toll report remains a useful reference for modern stability and control designers working with overhang aerodynamic balances and other aerodynamic balance types as well.
