Blade Droop Stops
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When a helicopter is at rest, the blades will droop because of a lack of centrifugal
stiffening. To prevent them from hitting the fuselage when starting
up or shutting down (at this time, rotor RPM is low, and so centrifugal stiffening
is also low), we need droop stops. These are used to limit blade travel both up and down. The upwards stops are utilised to restrict excessive blade sailing when starting the
helicopter up in a wind.
Because a blade at rest is very flexible, it can droop quite significantly. This suggests that the droop stops have to limit downwards travel to a great extent.
blade now only has a very limited ability to travel. However, in flight, and
because the blades are stiffened by centrifugal forces, more downwards travel may be needed in order to flap down without being in danger of hitting
(in a non zero offset head, the fuselage also tends to follow disc attitude). To provide
this greater degree of freedom, there are solutions which allow more blade
travel when rotor RPM gets above certain limits. These constructions function by using
centrifugal forces to put the droop stops out of engagement. At a lower RPM,
returns the droop stops.
All articulated and teetering types of rotorheads use some form of droop stops.
Next topic > Underslung Rotor Design
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