rholder(at)avnet.co.uk
Guest
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 Posted: Sun Nov 02, 2008 5:51 am Post subject: Setting prop pitch |
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| Quote: | Set the prop to 5500 WOT flat and level, but that is
not where you cruise. When the prop is set to 5500 you
benefit from the prop pitch during other times such as
in takeoff with better climb performance, lower
throttle setting for cruise means a little better fuel
economy. If it is set for 5500 and lets say you bounce
up off the runway and you go "oh no I'm going to drop
it in", then when you give it full throttle trying to
save yourself the engine will spin up faster to save
you, compared to, if you had too big a bite of air with
the prop and it had to struggle to spin up. If the prop
is pitched too much you will need to give it more
throttle/fuel to achieve your cruise speed. When it is
set for 5500 you will be able to pull back on the
throttle/less fuel and still cruise at the speed you
want. You will have a higher top end speed, too. Mind
you we are comparing 5500 to your 4800 here. A 700 rpm
gain will make a big difference. I know some that
cruise at 5500 WOT just to cruise. This will cause
extra wear over time. My question is why? Do you need
to get somewhere a few miles per hour faster all the
time? Why wear and tear any extra on the engine than
need be. We want it to be reliable for its 1500 hours
and more.
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I think you need to stress (I think this is right) that
the 5500 WOT is static, on the ground. The rpm for WOT in
the cruise will depend on a number of factors.
But i would like to ask if anyone has a 912S/ULS with an
airmaster prop and a MAP guage _AND_ a fuel flow meter.
Could they do an experiment for me ! Chose a specific
airspeed (X) at around your normal cruise speed. I would
use 115 knots on my plane.
Now there are a number of combinations of rpm and MAP that
will achieve that airspeed. I use 25 inches and 4600.
The experiment is to set different rpms; and for each
adjust the MAP so that the airspeed is X (stabilised).
Then for each combination of rpm/MAP record the fuel flow
- ideally to the tenth of a litre but a tenth of a US
gallon would be OK.
As the engine must be producing the same horsepower as it
is travelling at the same speed for each combination, any
variation in the fuel flow is a measure of the efficiency
of the engine. If the efficiency is the same for the whole
rpm range tested then the fuel flow would be the same. So
a "scientific" test will show any variation and maybe
there will be an obvious peak to the efficiency - this
will be where the fuel flow is least.
According to Roger's recent postings the implication is
that this peak efficiency rpm/MAP combination will be at
or around 5000 rpm.
If several people are able to do this, please post results
direct to me so that I can collate independently  The
critical factor here is that the MAP must be adjusted to
get the same airspeed (X) for each rpm figure tested.
So the results might look like the following
at 115 knots
4500 26 15.8 (15.8 would be the litres per hour fuel
flow)
4600 25 15.5
4700 24.5 15.5
4800 24 15.3
4900 23.5 15.1
5000 23.1 15.0 <------ B E S T E F F I C I E
N C Y
5100 22.8 15.2 O N T H E S E T E S T F I
G U R E S
5200 22.4 15.4
5300 22.1 15.5
It would be something to pass the time on your next two
hour flight !
And if you had longer, other speeds could also be tried -
X+10 and X -10 !
I will ask the wear and tear questions in another mail
sometime as many people only answer one question per mail
Thanks
Richard Holder with an efficiency hat on !
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