Figure 5-3

CHAPTER 5

HELICOPTER AERODYNAMICS WORKBOOK

COMPRESSIBILITY EFFECT

Another factor, which limits forward speed in helicopters, is the compressibility effect on the

advancing blade. The airspeed the advancing blade "sees" increases as forward helicopter speed

increases. At low speeds, air compression is not a problem because the air experiences relatively

small changes in pressure with only negligible changes in density. At high speeds, the pressure

changes taking place are larger and result in significant air density changes.

The dominant factor in high speed airflow is the speed of sound. Speed of sound is found at

the point where pressure disturbances are no longer propagated through the air, and this

propagation speed is a function of air temperature. As altitude increases and temperature

decreases, the speed of sound decreases.

If the airflow is traveling at some speed above the speed of sound, the airflow ahead of it will

not be influenced by the pressure field, because pressure disturbances cannot be propagated

ahead of the airfoil. As the speed nears the speed of sound, a compression wave forms at the

leading edge of the blade and all changes in velocity and pressure take place sharply and

suddenly. The airflow ahead of the airfoil is not influenced until the air particles are suddenly

forced out of the way by the concentrated pressure wave formed by the airfoil. Typical

supersonic airflow is shown in figure 5-3.

Figure 5-3

The principal effects of compressibility on the advancing blade are: 1) a large increase in

blade drag and 2) rearward shift of the airfoil aerodynamic center. This increases power

required to maintain rotor rpm, vibrations, cyclic feedback, and an undesirable high twisting

moment on the blade.

Compressibility effects become more severe at higher lift coefficients (higher AOA) and

higher speeds. The following operating conditions represent the most adverse compressibility

conditions:

1. High airspeed

2. High rotor rpm

3. High gross weight

5-4 FLIGHT PHENOMENA