| Free-to-Roll Testing of Airplane Models in Wind Tunnels |
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| Langley Research Center, Hampton, Virginia | |
| Oct 31 2007 | |
Causes of, and cures for, wing-drop/rock behavior can be evaluated.
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A free-to-roll (FTR) test technique and test rig make it possible to evaluate both the transonic performance and the wing-drop/rock behavior of a high-strength airplane model in a single wind-tunnel entry. The free-to-roll test technique is a single degree-of-motion method in which the model is free to roll about the longitudinal axis. The rolling motion is observed, recorded, and analyzed to gain insight into wing-drop/rock behavior.  The FTR Test Rig allows free rotation of the airplane modelaround the centerline. Optionally, brakes can hold the modelat a commanded roll angle, or the rig can be locked againstrotation for conventional static testing. The FTR test rig was designed to replace an older sting attachment butt, such that a model with its force balance and support sting could freely rotate about the longitudinal axis. The rig (see figure) includes a rotary head supported in a stationary head with a forward spherical roller bearing and an aft needle bearing. Rotation is amplified by a set of gears and measured by a shaft-angle resolver; the roll angle can be resolved to within 0.067° at a rotational speed up to 1,000°/s. An assembly of electrically actuated brakes between the rotary and stationary heads can be used to hold the model against a rolling torque at a commanded roll angle. When static testing is required, a locking bar is used to fix the rotating head rigidly to the stationary head. Switching between the static and FTR test modes takes only about 30 minutes. The FTR test rig was originally mounted in a 16-ft (≈4.0-m) transonic wind tunnel, but could just as well be adapted to use in any large wind tunnel. In one series of tests on the FTR rig, static and dynamic characteristics of models of four different fighter airplanes were measured. Two of the models exhibited uncommanded lateral motions; the other two did not. A figure of merit was developed to discern the severity of lateral motions. Using this figure of merit, it was shown that the FTR test technique enabled identification of conditions under which the uncommanded lateral motions occurred. The wind-tunnel conditions thus identified were found to be correlated with flight conditions under which the corresponding full-size airplanes exhibited uncommanded lateral motions. This work was done by Francis J. Capone, D. Bruce Owens, and Robert M. Hall of Langley Research Center. This Brief includes a Technical Support Package (TSP).
Free-to-Roll Testing of Airplane Models in Wind Tunnels (reference LAR-17153-1) is currently available for download from the TSP library. Login first to download.
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