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in article 39afe43a_2@plato.netscapeonline.co.uk, Nigel Gray at
nigel210878@netscapeonline DOT CO COT YOU-KAY wrote on 9/1/00 2:23 AM: > I know how it works (sort of!), and I know the settings to use in the PA23 > Apache, but how do you really use the constant speed/ variable pitch prop? > What does it do (apart from the obvious) that the fixed pitch one doesnt? > > Say you want to decrease speed whilst remaining straight and level. I know > that in the PA23 I select 18"/2300 (down from 22"/2300) and maintain > altitude by pitching up. I know that because Ive been taught it by the > venerable George Orive, CFI. But why do I do that? Whats the theory behind > the reduction in power? > > What Im really trying to ask is for a comprehensive guide of what the blue > lever and the black lever really do to your aircraft. > > A fixed pitch prop gives you thrust only as a function of RPM. A constant speed prop gives you thrust as a function of both RPM and propeller pitch. Relative to a fixed pitch prop, the constant speed propeller gives you better static RPM for takeoff, and allows lower cruise RPM. You set the blue lever for 2300 RPM. Reducing MP from 22" to 18" doesnt change the RPM, thanks to the governor, however at the lower power setting the only way the governor can maintain 2300 RPM is to flatten the propeller pitch. So the pitch flattens and the propeller doesnt take as big a bite out of the air and you get less thrust, thus you either descend or slow down. Was that your question? How you can get a power reduction without changing the propeller RPM? |
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