What ever the manifold pressure gauge says?
celstar said:
What should the MP be on a fixed pitch, carb. runing at 2200 rpm 2800 feet msl O360?
Seriously, my pet peeve, get a MAP gauge.
: Sorry I couldn't resist.
I agree with "Norman CYYJ" answer but that would be ideal. It's about 1" loss of MAP per 1,000ft altitude. I respectfully disagree with "gasman", there is resistance even with wide open throttle, WOT, but I know what he means. However if the throttle is partially closed, forget it. There is no telling or only guessing what the MAP and how much power you're making,
RV's are close to ambient air pressure at wide open throttle but not quite. As Norman CYYJ said there are many variables. The same engine installed in different airframes can difference produce several inches (in-Hg) difference in MAP at WOT! Look at factory plane AFM's with 180hp/fix, eg. Grumman Tiger or Cherokee 180, you'll see RPM only power charts and a difference. I am guessing but I bet you will also see way less than ambient pressure and the minus 1" rule. This is why you need a MAP gauge even with a fixed pitch prop.
The aircraft itself and engine installation affects MAP: air-box, exhaust, prop and airspeed, not to mention non-standard pressure/temp changes aloft. If you want to guess, that is fine, but it's not much real use. You don't know your power (MAP x RPM) with out a MAP gauge. On factory planes with out a MAP (eg C172), they publish power verses RPM only charts after flight-test WITH a MAP gauge; than they reduce flight test data to charts. It still is a guess even than with out a MAP gauge.
Of course all planes with constant speed props have their power charts based on MAP + RPM. To do other wise is guessing. Fixed pitch prop planes should have a MAP gauge as well. Why did Piper and Cessna not install MAP gauges in all planes? Well cost and honestly you do have less control over MAP than you do with constant speed props, but it does not make MAP less important for the FIX Pitch prop.
"Norman CYYJ" is right, albeit very ideal. With out a MAP gauge that's all you can do, is guess.
The MAP gauge is cheap, easy to install w/ no electical connections (using a mechanical gauge). Van's MAP electical MAP gauge w/ transducer is only $73.40.
Vans MAP
All RV's (even fixed pitch prop RV's) should have a MAP gauge in my opinion.
Here is what Lycoming says. You notice they don't have a formula. It says see airframe manufacture data.
-Operating the Direct-drive Fixed-pitch Lycoming Engine
Some pilots have demonstrated a lack of understanding with regard to the operation of the direct-drive, fixed-pitch, normally aspirated power plants. When a power chart is provided, it will indicate that as the airplane is flown at different altitudes above sea level, it is necessary to use a higher RPM for adequate cruise performance with an increase in altitude. A typical example might be the 0-360, 180 HP Lycoming power plant. The power chart by the airframe manufacturer for this fixed-landing gear aircraft lists 75% power at 7,500 feet at 2675 RPM (no manifold pressure gage in their airplane). The pilot throttle at takeoff, the pilot may want to reduce power 100 or 200 RPM and still not see performance suffer.
Those direct-drive normally aspirated engines with a prop governor are also rated indefinitely at full power, and the manuals all recommend full power for takeoff, but specify a small reduction in power, generally to 85% power climb. Study the specific airplane Pilot?s Operating Handbook for detailed power settings.
