The Development of Satisfactory Flying Qualities on the Douglas Dive Bomber, Model SBD-1 Through Flight Testing Successive Modifications in Control-Surface Area, Hinge-Line Location, and Aerodynamic-Balance Nose Shape Page: 37 of 73
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36
Comparison of the aerodynamic-balance nose shapes shown in
figure I for the elevator and rudder shoTr the change from blunt
to modified elliptical shape. The blunt nose usually gives dif-
ficulty because of its adverse hirnge-moment characteristics at
large surface throws, resulting in a tendency for overbalance.
The shapes given in figures 3a and 3b have proven quite satis-
factory for a number of Douglas designs, giving a satisfactory
compromise between loss in control-free stability and reduction
in control-surface hinge moments.
5, Balance tab.- The use of the balance tab with uniform
ratio to the main-control-surface travel in combination with the
overhang type of aerodynamic balance, usually should be discouraged.
Although the expected. reduction in hinge moment can be obtained,
the accomp-onying charge in free-floating angle of the movable
control surface effects a considerable reduction in stability.
In cases where no overhang-type aerodynamic balance is used, the
balance tab offers an effective method of hinge-moment reduction.
As indicated in the flight tests on longitudinal stability with
the SBD.-1 model (table VI, phase VI), difficulty was experienced
in obtaining the desired elevator control forces over the required
speed range. The use of a balance tab would probably have reduced
the pull-out forces, but would have further aggravated the low-
speed overbalance.
6. Control-surface gap.- In general, it can be said that the
gap between the fixed and the movable control surfaces should be
kept to the minimum possible clearance for manufacture. From the
XBT-2 tests, where the gap between the elevator and the stabilizer
was increased, a flat spot in the variation of control force with
angle occurred. In the case of the ailerons, there was some indi-
cation that the control forces became lighter. From a performance
point of view, the additional drag caused by a, large control-surface
gap is not desirable. The ideal arrangement from all points of
view except manufacture is the pressure-seal type of aerodynamic
balance.
7. Tail cone.- On the XBT-2 it was found inadvisable to have
the tail cone move with the rudder. No essential differences
were obtained with the tail cone fixed to the fuselage or movable
with the rudder concerning ability for spin recovery, and consid-
erable difficulty was found in effectively balancing this portion
when connected to the rudder. With the tail cone attached to the
fuselage, there was less tendency for rudder-force reversal with
essentially the same control effectiveness.
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Root, L. E. The Development of Satisfactory Flying Qualities on the Douglas Dive Bomber, Model SBD-1 Through Flight Testing Successive Modifications in Control-Surface Area, Hinge-Line Location, and Aerodynamic-Balance Nose Shape, report, May 1942; (https://digital.library.unt.edu/ark:/67531/metadc61066/m1/37/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.