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From: ths@lanl.ARPA
Newsgroups: net.aviation
Subject: Re: T.O. in Cessna 172
Message-ID: <32838@lanl.ARPA>
Date: Mon, 4-Nov-85 13:19:57 EST
Article-I.D.: lanl.32838
Posted: Mon Nov  4 13:19:57 1985
Date-Received: Fri, 8-Nov-85 05:06:36 EST
References: <769@bgsuvax.UUCP>
Organization: Los Alamos National Laboratory
Lines: 42

> In the 172 that I rent I noticed my take-off r.p.m. was only 2200 which is
> just the beginning of the green arc....  I'd like to know what other's
> take-off r.p.m.s.  Is it this particular engine or characteristic of all
> 0-320-H2ADs?

I recall very vividly the first time that I noticed the RPM during take-off
in a C-172 (O-320-E?).  I too was at gross and was a bit concerned at the
long take-off roll.  I glanced at the RPM and.....WOW ONLY 2200!  Since I
was on the LOONNGG runway at Stockton Ca.  I continued the take-off
(amazing how the neophite pilot will go where angels fear to tread!).

When I returned to Reid-Hillview I asked my instructor "how come"? He said
it is that way all the time but he either failed to point it out to me
or I failed to note his comment during the check-out. I had about 70 hours
at this point and it was my third or fourth flight in the C-172.

As with most experiences of this type it stayed with me. Today I make a
definate point of having my students check the rpm early in the take-off
roll for 2200 to verify that they are getting the available power at that
point.  Carburator ice, bad mixture setting, fouled plugs, or carb heat
left on will rob you of critical power.

During the climb, at 70 to 80 knots the RPM will usually come up to 2300 to
2450 depending on altitude, climb speed and type of prop and engine
condition.  I don't know if there is a "climb" prop approved for the C-172
but that could increase the rpm by a 100.

The only time you may see RED LINE of 2600 to 2700 (which represents 100%
power at sea level) is during operations at low altitudes and full power or
during high speed descents.

The moral of this story Anne, is that fixed pitch propellors do not permit
the engine to turn 100% RPM during take-off or climb operations.
As a result the engine can not develop full power, a rather unfortunate
situation since these are the two flight operations where you really NEED
all the power you can get.  This is why the "variable pitch" propellor
came into existance.

SEE THE ARTICLE IN AOPA PILOT MAGAZINE LAST MONTH - IT DESCRIBES THIS
SITUATION VERY WELL.

Ted Spitzmiller