07-09-2016, 02:41 PM
EVALUATION OF A TECHNIQUE FOR DETERMINING AIRPLANE
AILERON EFFECTIVENESS AND ROLL RATE BY USING
AN AEROELASTICALLY SCALED MODEL
1453537430-nasareport.pdf (Size: 1.6 MB / Downloads: 6)
SUMMARY
Aileron effectiveness, damping in roll, and predicted full-scale roll rates at
selected dynamic pressures below the aileron reversal boundary have been experimentally
determined on an aeroelastically scaled model of a recently developed, multijet
cargo airplane. The studies were conducted in the Langley transonic dynamics tunnel
at Mach numbers up to 0.93.
Results of this wind-tunnel investigation, compared with full-scale flight data, have
demonstrated both a successful method for determining full-scale aileron effectiveness
including the reversal boundary and a new dynamic technique for predicting flight roll
rates at dynamic pressures below the reversal boundary based on measured values of
aileron effectiveness and damping in roll.
INTRODUCTION
Loss of aileron control on an airplane can occur due to deformation of the wing
under the action of aerodynamic loads resulting from aileron deflections. When the aerodynamic
forces due to wing deformation and aileron displacement combine to produce a
value of zero rolling moment, the condition is referred to as aileron reversal. For
dynamic pressures (constant Mach number) slightly above this point, the effect of the
ailerons is reversed from the normal effect.
Theoretical prediction of the reversal boundary becomes very difficult in the transonic
speed range where existing aerodynamic theories are inadequate. For example,
during the early design stages of a recently developed, multijet cargo airplane, aileroneffectiveness
studies were conducted on a low-speed flutter model. (See ref. 1.) These
studies indicated sufficient operating margin. Analytical studies based on later aerodynamic
data (also discussed in ref. 1) indicated a lower reversal boundary than the previous
low-speed studies had indicated. In view of these results, transonic wind-tunnel
studies of a complete aeroelastically scaled high-speed flutter model were initiated.
(See refs. 1 and 2.) It is interesting to note that Guyett in reference 3 states that flexible
models of this type have not been widely used in the design stage to estimate aileron
effectiveness even though control power in roll has dictated the wing stiffness on many
airplanes.
The static experimental technique presented in references 1 and 2 is useful in
determining the aileron effectiveness, including the reversal boundary. However, the
airplane designer is also interested in knowing the roll rate produced by a static deflection
of the ailerons. Since this problem is dynamic in nature, it cannot be experimentally
determined from static measurements alone.
The purpose of this report is twofold: first, to evaluate by comparison with flight
data the use of an aeroelastically scaled model for determining the aileron reversal
boundary; and, second, to present and evaluate a new dynamic technique for establishing
aileron effectiveness and roll damping at conditions below the reversal boundary so that
full-scale roll rate can be experimentally predicted from an aeroelastically scaled model.
The basic wind-tunnel approach to determine the aileron effectiveness consists of
measuring the static rolling moment generated by the model for a known aileron deflection.
Two static experimental techniques are presented which differ in the manner that
the model is mounted in the wind tunnel and the rolling moments determined. One method
uses the sting-pylon-spring mount discussed by Grosser in reference 2, and the second
method uses the two-cable mount system presented by Reed and Abbott in reference 4.
The dynamic technique used to determine the aileron effectiveness C^, and
damping in roll C^ utilizes the two-cable mount system. The technique is, in principle,
similar to that used in flight tests, namely, measuring the response of the model to
known control inputs. In this study, the dynamic response of the model to a sinusoidal
deflection of the ailerons was measured. Reference 5 shows that, by properly selecting
a suitable elastic-axis location for the mount system, the dynamic response of the configuration
to a sinusoidal deflection of the ailerons can be approximated by a single degree
of freedom at dynamic pressures somewhat below the reversal boundary. In order to
verify both the static and dynamic techniques, the aileron reversal boundary and the predicted
roll rates at dynamic pressures below the boundary, based on model measurements,
are compared with full-scale flight data.
SYMBOLS
Measurements for this investigation were taken in the U.S. Customary System of
Units. Equivalent values are indicated herein in the International System (SI). Details
concerning the use of SI units, together with physical constants and conversion factors,
are given in reference 6.