On Oct 9, 8:27 am, Orval Fairbairn wrote:
In article om,



wrote:
On Oct 8, 11:27 pm, wrote:
I own a Cessna 150 with a 150 HP engine installed (0-320- E2D)

The STC states the static RPM is not to be over 2250 RPM.

My airplane meets the Static RPM Requirement.

My question is am I getting the full 150 HP on takeoff??? turning
2250 RPM

A variable pitch prop (expensive) can keep constant RPM near the max
power output at takoff. But your fixed pitch prop is designed for
overall average operation. If it was pitched for near full power 2700
rpm at takeoff it would go beyond the max RPM at cruise and ruin the
engine.

The placarding probably relates to the STC, which "limits you to 100
hp", for certification purposes.

It is easier to get an STC for a higher power engine if you "limit, via
placards, the max power" to that called out in the original type
certificate.

Baloney. The STC, and any TCDS, will specify a static RPM range.
Yours probably says "Not under 2150 RPM, not over 2250 RPM." The
O-320E2D in a 172M will spec not under 2270, not over 2370 RPM. The
figure relates to brakes locked, full throttle, zero wind RPM and
won't change much with density altitude. The static RPM is used for
engine and prop health purposes. A fixed-pitch prop will never give
you full engine hp until you are in flight at standard conditions (sea
level, 59°F) so you'd have to be at or near full throttle with the
wheels just above the waves.
Your O-320 in the 152 is NOT derated any more than the O-320s
in our 172s are. We don't get 2700 RPM unless we're in flight and the
throttle is wide open. Many aircraft with fixed-pitch props use
exactly that criteria for propeller pitch: max throttle in level
flight gives redline RPM. Too much pitch would prevent reaching full
hp, too little would leave unusable throttle travel in level flight.
Since the O-320 is certified to produce full power with no time
limitation, you could, with the correct prop, cruise at 2700 and go
places fast. Your range would be a lot shorter since the fuel burn
would be pretty bad. We have to break in new engines over a 3-hour
flight, with the last half-hour being at full throttle, which gives us
2700 RPM. The old 172 goes nearly 140 mph at that setting. Without
wheel pants, too.

Dan