Airfoils for Autogiros

Cierva was well aware of the importance of airfoil shape in improving the per­formance of his autogiros [see Cierva (1930c) and Cierva & Rose (1931)]. He wrote in reference to the twisting moment produced on autogiro blades by the use of a cambered airfoil versus a symmetric airfoil: “It [the Gottingen-429] is a reasonably efficient airfoil, although others give greater lift and a great many different curves are used for designing [fixed-wing] airplanes. But, the important advantage of this particular type is that its center cf lift or pressure is approximately the same at all angles which it may assume in flight. This is not true of other types of airfoil, so that center of pressure travel is a factor to be reckoned with in using them.” In essence, Cierva is referring here to the connection between aerodynamic performance (better maximum lift coefficient and improved lift-to-drag ratios) through the use of camber and the corresponding increase in pitching moments caused by that camber – see Section 7.7.3. Compressibility issues on the advancing blade can be an issue for an autogiro, although somewhat less so than for a helicopter because the autogiro operates at lower mean lift coefficients, but there is still a need to use airfoils with good aerodynamic characteristics at high subsonic Mach numbers.

Cierva had many airfoil sections to choose from, but the aerodynamic characteristics of most were not well documented. On the C-4, Cierva used the Eiffel 106 airfoil section, later

switching to the Gottingen-429 airfoil. Some years later, Cierva was again to reconsider the choice of the airfoil section for his Autogiros, but, limiting his study to ten candidate airfoil sections, he decided to replace the symmetric Gottingen-429 airfoil, which had “abrupt stalling” characteristics with the reflexed-cambered RAF-34 airfoil of 17% thickness-to – chord ratio (see Fig. 12.12). The new blades were first tested on the C-19 Mk-IV, which became one of the most successful Cierva Autogiro designs. On the C-30 Autogiro, Cierva switched the airfoil again, this time to the cambered Gottingen-606 airfoil. In some flight conditions, mainly at high airspeeds, the higher pitching moments resulted in the blades twisting elastically and so this produced control problems. These aeroelastic effects arose because of the generally low torsional stiffness of early wood and fabric covered rotor blades. A crash of a C-30 Autogiro was tied to the use of this cambered airfoil sec­tion – see Beavan & Lock (1939). The NACA had also noticed such aeroelastic problems and had analyzed the effects of blade twisting – see Wheatley (1937a, b). On the Kellett YG-1 (which also used the Gottingen-606 airfoil) but with an upward deflected trailing edge tab, the NACA eventually replaced the blades with a reflexed airfoil based on the NACA 230 series. Yet, these airfoils were not successful and were found to have relatively poor characteristics at high lift and at high speeds – see Bailey & Gustafson (1939) and Gustafson (1971).