"Bryan.G. Wilkinsonn" wrote:

Can I seek some advice?

I have been advised to fit a manifold pressure gauge when a variable pitch
prop. is used.

I am completing the construction of the instrument panel so now is the time
to make room for one.

Can anyone explain the possible reasoning behind this, and how the pressure
gauge is used in flight.

Also, does anyone know of a supplier for a fuel pressure transducer.

#333 is a tri-gear with a 912S.

best regards
Dennis

Bill Freeman wrote:

Possibly a more basic answer might help.  I'll try.  With a fixed pitch prop,
there is a fixed relationaship between the horsepower and RPM because the
prop absorbs a fixed HP at a given RPM.  Varying the throttle position changes
the HP and the RPM.  So, you set up a cruise condition by setting the RPM
recommended for a particular % power from the POH.

With a constant speed prop, you set the engine  RPM with the prop control
and vary the MAP with the throttle.  Since the throttle has no effect (above
very low RPMS) on the RPM, you need a MAP guage to tell you what throttle
setting you are using.  Of course, you also need to have the RPM to know
what % power setting you are using.  Imagine a car with 3rd 4th and 5th
gears very
close to the same ratio.  If you were driving at 60 mph in 3rd, you will have
a higher RPM and lower MAP than driving 60 mph on 4th.  60 mph in 5th
will require a yet lower RPM and higher MAP.  All three cases are consuming
the same HP, since you are driving at a steady 60 mph, but you are getting
it a different way in each gear.

So, the MAP is your 'throttle guage' and the tach is your 'gear' (altho the
prop governor holds a constant RPM, so it is still different than a car).
You look up power settings by both RPM and MAP, again from the POH.

Hope this helps a bit.

Bill

 
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