AIR CURRENT PROTECTION.

TM 55-4920-201-14

may be constructed to enclose the blades for

1-34. AIR CURRENT PROTECTION.

localized protection from air currents. (See figure 1-

1-35. Balance checking operations shall be

19). The shroud may consist of transparent plastic

performed whenever practicable in a draft-free

sections formed so as to enclose the blade around

area, as aid currents generated in drafty rooms or

the sides, top, and outboard end, and be supported

by the nearby movement of personnel may cause

by a carriage which can be rolled on casters into

motion in the assembly being checked which will

position under the blade. The shroud, however

prevent accurate, stabilized balance indications.

constructed, should provide sufficient rigidity so

This is a particularly important consideration in the

that surrounding air currents cannot collapse or

case of a helicopter rotor head and blade assembly

curve the sides, thereby creating a bellows which

in which the span of the blades makes the assembly

would cause air disturbances within.

especially susceptible to air current distrubances.

In the absence of draft-free conditions, a shroud

Figure 1-29. Shroud for Protection of Assembly Being Balance Checked in Drafty

Area.

1-40