Aerodynamic Characteristics of a Wing With Fowler Flaps Including Flap Loads, Downwash, and Calculated Effect on Take-Off

This report presents the results of an investigation in the N. A. C. A. 7- by 10-foot wind tunnel on a wing in combination with each of three sizes of Fowler flap. The purpose of the investigation was to determine the aerodynamic characteristics as affected by flap chord and position, the air loads on the flaps, and the effect of the flaps on the downwash. The flap position for maximum lift; polars for arrangements considered favorable for take-off; and complete lift, drag, and pitching-moment characteristics for selected optimum arrangements were determined. A Clark Y wing model was tested with 20 percent c, 30 percent c, and 40 percent c Fowler flaps of Clark Y section. Certain additional data from earlier tests on a similar model equipped with the 40 percent c Clark Y flap are included for comparison. Results of calculations made to find the effect of the Fowler flap on take-off, based on data from these tests, are included in an appendix.

The maximum lift coefficient obtainable, based on original wing area, had a nearly linear increase with flap chord up to 50 percent, but the maximum lift force per unit of total area increased very little beyond the value obtained with the 30 percent c flap. The maximum load on the flap occurred very nearly at the maximum lift of the wing-flap combination and was nearly 1 1/2 times the load that would result form uniform distribution of the total load over the total area. In general, the flap appeared to carry a large proportion of the additional lift caused by its presence and to have its center of pressure much nearer the leading edge than it would normally bein free air. The addition of the Fowler flap to a wing appeared to have no appreciable effect on the relation between lift coefficient and angle of downwash. The calculations in the appendix show that, by proper use of the Fowler flap, the take-off of an airplane having wing and power loadings in the range normally encountered in transport airplanes should be considerably improved.