166 S. Schuster and H. Schwanecke 
acc. to (7) 
Fig. 20. Polar of a dihedral foil in a symmetrical flow 
2. Lateral stabilizing can be brought about much more effectively by the moment due 
to sideslip than by that due to rolling. While the rolling is less dangerous for the lift, a 
range of only +8 degrees can be admitted for a sideslip angle. 
3. Additional rolling of a dihedral foil in a sideslip motion acting in the same sense 
as the moment due to sideslip will raise the risk of a lift breakdown. 
EXAMINATION OF ROTARY INSTABILITY 
With a model of a dihedral foil the relation between rolling and sideslip has been tested 
thoroughly in a small VWS tank using a special device (Fig. 24). For a constant lift, the 
model at u, = 4 m/sec could immerse freely. Relative to rotary motions both about the lon- 
gitudinal and the vertical axis the model could either be fixed or set free. 
The sideslip angle was measured each time when the lift collapsed. For free rolling, 
sideslip angles up to 11 degrees could be brought about; rolling angles then amounted to 
about 10 degrees acting in the same direction as the moment due to the sideslip. Enlarging 
the sideslip angle to 11 degrees caused a lift breakdown at the foil which had been more 
emerging and a rotary instability was the result. This procedure is not reversible, i.e., the 
sideslip angle has to be totally reduced to regain the flow and to start with sideslip again. 
For a fixed rolling of 20 degrees, sideslip angles of 16 degrees and 9 degrees could be ob- 
tained; the smaller sideslip angle is conjugated to rolling in the same sense as the moment 
due to sideslip, the other one is conjugated to rolling in the opposite direction. 
