Effect of Induced Velocities

The induced velocity in the wake of a hovering helicopter can produce operational problems if the hovering is done close to dust, sand, snow, or other loose surfaces. The higher the disc loading, the stronger is the placer mining capability of the wake. Helicopter disc loadings range from 4 to 12 lb/ft2, and the corresponding downwash velocities in the remote wake range from 35 to 58 knots. Even the lower velocities can lift enough dust or snow to effectively cut off the pilot’s view of the ground, as is shown in Figure 1.2. At the higher disc loadings, gravel can be entrained in the wake and forced to circulate through the rotor and the engine intake.

A high rotor downwash velocity also makes it difficult to work under a hovering helicopter while hooking up a sling load or guiding the pilot to a precision landing.

Thus it may be seen that the higher the disc loading, the more severe are the operational problems. For nonhelicopter hovering aircraft that use very high disc­loading devices such as propellers or jet engines for lift, these problems become severe enough to limit landings and takeoffs to hard-surfaced, prepared areas. In light of these problems, one might ask: "Why use high disc loadings?” The answer is that high disc loadings permit design of compact helicopters with low empty weight, which for many applications are the most efficient aircraft.

Another effect of the rotor-induced velocities in hover is to produce a download on the fuselage and any other aircraft components that are located under the rotor. The rotor wake contracts from the diameter of the rotor to its remote wake size in about a quarter of a rotor radius, as shown in Figure 1.3, which was obtained using smoke for flow visualization. For most helicopters, the fuselage can be considered to be immersed in the remote wake and to receive the full effect of the downwash. The download, or vertical drag, on the fuselage can be computed from the standard equation for drag:

D = CDqS

The dynamic pressure in the remote wake, q2), is equal to the disc loading of the rotor. This statement may be proved by using the relationships:

v 2 — 2vl = 2

and

Thus

or

q2 = D. L. lb/ft2

A first estimate of the download may be obtained by assuming an effective drag coefficient of 0.3 for all the aircraft components in the remote wake. (A more elegant procedure will be found in Chapter 4.) For this estimate, the vertical drag, Dyf is:

Dv= (0.3) (D. L.) (Projected area of all affected components)

It is often convenient to express the vertical drag as a fraction of the gross weight:

Dv (0.3)(D. L.)(Projected area)

G. W. = (D. L.)(Disc area)

or:

Dv 0.3(Projected area)

G. W. Disc area

The example helicopter has a projected area of 380 ft2 and a disc area of 2,827 ft2. It thus has a download of 4% of its gross weight. The rotor thrust required to support a hovering helicopter and its vertical drag is: