Re: Propellor "reverse thrust"

Date:         29 Aug 97 00:53:41 
From:         k_ish <kenish@ix.netcom..com>
Organization: Netcom
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showie@uo.guelph.ca wrote:
> On the subject of reverse thrust, I took a flight on a Manx Airlines
> Shorts 330 a few years back.

Almost every high-performance piston or turprop aircraft has a variable
pitch propellor.  A coarse (high) pitch is more efficient at low
(climbout) airspeeds, a finer (low) blade pitch is efficient at cruise
speeds.  On descent, a zero or almost zero pitch creates lots of drag to
help control descent airspeed.  Most turboprop aircraft have a "beta" or
reverse-pitch position which works exactly as you speculate.  Lastly,
most multi-engine planes can "feather" their props (blades aligned with
the airstream).  If an engine fails, its prop is feathered to minimize
drag.

Depending on the sophistication of the aircraft, prop pitch is either
manually controlled by the pilot or automatically controlled.  Once the
basic blade pitch is set, a governor mechanism controls prop pitch to
keep it turning at a constant speed, even if airspeed or power setting
is changed.

Pitch adjustment is usually done with hydraulic pressure and a piston
mechanism; some are electrically controlled.  "Ground adjustable" props
can be set to either a climb or cruise setting during preflight,
depending on the type of flying you plan to do. Lastly, some props are
designed to flex and change pitch as the loads on the prop change.

Prop design is an art all in itself.  I know just enough to know I have
barely addressed the topic!

Ken Ishiguro