Tip Speed

Low tip speeds have the advantage of low noise and good hovering performance. High tip speeds have the advantage of low rotor and drive system weights and high stored energy for autorotative entries and flares.

One of the primary considerations that limits tip speeds on the high side is rotor noise. Tip speeds of more than about 750 ft/sec are considered to be excessively noisy. A lower limit is set by the requirement to store kinetic energy in the rotor in case of a power failure.

Avoiding advancing-tip compressibility and retreating-tip stall also limits the choices of rotor speed. It is generally accepted that advancing tip Mach numbers of more than about 0.92 will produce high blade loads as a result of the Mach tuck

900 800 700

6001— Acceptable Tip Speed Choices 500 400 300 200 100

0 20 40 60 80 100 120 140 160 180 200 220

Forward Speed (knots)

FIGURE 10.4 Constraints on Choice of Tip Speeds

phenomenon described in Chapter 6. It is also generally accepted that for conventional helicopters at maximum speed, the tip speed ratio limit should not exceed 0.5 to avoid retreating blade stall. Figure 10.4 shows how all these constraints limit the tip speed options available and why the maximum speed of "pure” helicopters is about 200 knots (as of this writing).

Solidity

Once values of disc loading and tip speed are selected, the solidity is the primary main rotor physical parameter to be chosen. There are three possible flight conditions that might establish solidity:

1. Hover at high altitude and temperature: The solidity is selected to achieve the maximum Figure of Merit. This criterion applies primarily to compound helicopters and flying cranes.

2. Maximum speed: The solidity is selected to prevent retreating blade stall at the design maximum speed.

3. High hadfactors: The solidity is selected to prevent retreating blade stall at the design maximum maneuverability requirement.

The basis for all these considerations will be found in Chapters 1, 3, and 5.