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On your last question, maneuvering speed Va increases with weight because
its defined as the speed at which the banked stall curve crossed the
(usually +3.8g) load factor limit. The stall speed curve moves to higher
speeds as the airplane weighs more. And so the intersection with the load
factor limit also moves up in speed.

John.
--
John T. Lowry, PhD
Flight Physics; 724 Alderson Ave.; Billings MT 59101
Voice: 406-248-2606
Web site: http://www.mcn.net/~jlowry
E-mail: jlowry@mcn.net

Christof Edel wrote in message
news:39b01bad.1452189@news.supernews.com...
> In article <39afe43a_2@plato.netscapeonline.co.uk>, "Nigel Gray"
> wrote:
>
> >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.
> >
>
> You want to know how manifold pressure and pitch relate to power,
> dont you?
>
> First of all it is important to understand that "manifold pressure"
> has nothing to do with the variable pitch prop - you have that with
> any piston engine. It is the pressure whith which air/fuel mixture
> enters the cylinders. Higher the pressure means more air entering the
> cylinder means more power.
>
> With a fixed pitch prop, higher power always results in higher RPM.
> Hence the RPM gauge is all you need to know how much power you are
> generating. A MP gauge is not needed.
>
> Now enter the VP prop. The "black lever" does exactly the same as it
> does on a fixed prop - it opens the throttle, hence increases manifold
> pressure and power. With the "blue lever" full forward there is no
> difference between a VP and a fixed prop. More power, higher RPM. You
> just have the additional MP gauge to look at and impress passengers
> with.
>
> When you reduce power, lets say to a 22/23 cruise, this is what
> happens:
>
> - you pull back the black lever --> manifold decreases --> less power
> If this is all you do, you essentially fly it like a fixed pitch
> prop at very high RPM.
>
> - you pull back the blue lever --> this tells the prop governor to
> reduce RPM --> prop governor increases pitch of the prop -->
> prop has higher angle of attack --> higher resistance -->
> RPM decreases --> even less power
>
> Why bother? The decreased prop pitch will result in a more efficient
> operation at cruise speed. Even better - by setting a specific RPM
> rather than having a lever to directly control the pitch the prop
> adjusts itself to higher cruise speed. You accelerate - prop moves
> through air more quickly - prop angle of attack decreases - prop
> governor will reduce pitch to keep prop speed constant --> pitch more
> appropriate for the higher speed.
>
> This is similar to an automatic gearbox changing gears as you speed
> up, just smoother.
>
> When you want to fly at 65% power there are normally several settings
> to achieve this - e.g. 21/24 or 22/22. Remember, higher RPM - higher
> power, higher manifold - higher power - you can reduce one and
> increase the other and still have the same power output.
>
> Now back to your deceleration. You reduce MP to 18". The prop governor
> will decrease pitch to maintain RPM, but you still have reduced the
> engine power. Its just the governor making it impossible to notice
> this on engine RPM so you need that MP gauge.
>
> You could reduce RPM aggressively to the same effect. But if you do
> that you will probably blow the Engine up - this is equivalent to
> trying to accelerate from 10 to 20 mph in fifth gear with full
> throttle - the power output is reduced by brutally squeezing the RPM
> down on an engine that "wants" to produce more power.
>
> Phew, this has gotten rather long. I hope that this is understandable.
> Maybe somebody can explain to me why maneouvering speed (Va) increases
> with higher weight - this is something I *never* understood.
>
> Keep curious!
>
> Christof
>
> PS. I assumed that you know the relationship between power, attitude,
> speed and rate of climb/descent - if not I have just made a fool of
> myself for replying to an obvious troll... ;-)
>

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