Masters Theses

Date of Award

6-1981

Degree Type

Thesis

Degree Name

Master of Science

Major

Aerospace Engineering

Major Professor

W. F. Jacobs

Committee Members

Ralph D. Kimberlin, R. L. Young

Abstract

Presenting the performance of airplanes which employ powered-lift requires unique methods of flight testing and data reduction. The purpose of this study was to (1) show why the theory of conventional jet airplane performance is inadequate for powered-lift airplanes, (2) present a method by which the performance of powered-lift airplanes may be presented, and (3) apply this method to the Ball-Bartoe Jetwing STOL aircraft.

This study begins with a brief presentation of flight test theory followed by a discussion of the performance determination of conventional jet airplanes. This approach, which results in thrust required and thrust available curves, is based on airplane drag as determined from the classic drag equation. This theory is shown to be invalid for powered-lift airplanes because drag is no longer a meaningful variable in the determination of powered-lift performance. Furthermore, the thrust required curves, which normalize to one plot for conventional airplanes, no longer do so for powered-lift.

The problem of presenting the performance of powered-lift airplanes was solved by using excess thrust in place of drag. In like manner, the thrust performance curves were replaced by a mapping of airspeed versus flight path angle (V-γ). By using a theoretical relationship for the excess thrust coefficient, a generalized plot of normalized airspeed versus flight path angle was obtained. The generalized V-γ plot maps the entire flight envelope of an aircraft onto a single graph. This one plot is valid for any altitude, temperature and weight. Additional performance information, such as rate of climb capability, may be obtained from the generalized V-γ mapping.

By applying this procedure to the Ball-Bartoe Jetwing, the STOL, cruise and climb characteristics were readily determined. Three configurations were tested: gear and flaps up, gear down and flaps 15° and gear down, flaps 30°. The rate of climb capability was determined for each of these configurations; corresponding to maximum rate of climb, take-off climb and balked landing climb, respectively. Also, the service ceiling and absolute ceiling were determined.

Test data were obtained by performing sawtooth climbs at several airspeeds and power settings. Plots of altitude versus time were used to determine rate of climb and the flight path angle for all test airspeeds. Thrust was then plotted versus flight path angle from which the V-γ plots followed directly.

The accuracy of the flight test results was verified by comparison with wind tunnel data. Except for the condition of zero thrust, results compared very favorably; in some cases the error fell within the data scatter and was not discernible. Angle-of-attack data, however, was unreliable due to the location of the angle-of-attack indicator in the upwash flow.

The accuracy of the V-γ plots could be improved if more data points are obtained throughout the total range of airspeeds in the flight envelope. A tail stall problem and an inherent lack of longitudinal stability prevented full exploration of the low-speed flight envelope.

The conclusion is drawn that the V-γ mapping is a necessary tool in evaluating the performance of powered-lift aircraft. It clearly showed the unique STOL and climb characteristics of the Jetwing. In order to fully realize the STOL capabilities of the Jetwing, the tail stall and stability problems should be addressed. A flight test manual which describes the theory and procedures for determining the performance of powered-lift airplanes would be helpful in the evaluation of future designs.

Download
Files over 3MB may be slow to open. For best results, right-click and select "save as..."

Share

COinS