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Why is manifold pressure important?


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A simple explanation:


On an aircraft with a fixed pitch prop, there generally is no manifold pressure gauge - so it can't be that important. When you're flying you can look at the engine RPM to get an idea of how hard the engine is working.


If you have a constant speed prop however, the prop will try and keep the engine at the same speed no matter the power setting. So with half throttle or full throttle, you may see 5000RPM in a Rotax 912. So you can't use the tacho to see how hard the engine is working.


You CAN however look at the pressure of the intake manifold. At full throttle, you should see slightly less than 1 atmosphere. With half throttle you see less pressure. As you climb and the pressure drops, your intake manifold pressure will drop, showing the engine isn't working as well (assuming you have no turbocharger). Aircraft manuals tell you what combination of RPM and manifold pressure to use for various stages of flight.


Does that help?





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Guest basscheffers

Al: Click! the whole "full throttle is 1 atmosphere" made me realise why it is in inches of mercury going up to about 30 on the scale... (for those were wondering as well: think about it for a bit. Hint: QNH on American altimeters)


Sloper: you can't tell by ear; the engine is always running at the same RPM, so droning at the same frequency. It might be a bit louder at times, but how are you to know with other noises, ANR headphones, etc.? (unlike fixed-pitched, where you will immediately hear you have gone into a dive and are speeding up)



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My silly "click" moment was when I realised the throttle did just that: it slowed the engine down, and without it you'd get full power. For some reason, probably from the term 'accelerator' with cars, I'd pictured it the other way around.


RE noise: It might be a bit hard for the writers of aircraft flight manuals to say: "For an economy cruise, set the engine to 4500RPM, then play with the throttle until the engine makes a kind of dull roaring noise, but not TOO dull, or not with too much roar"



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In flight adjustable props are a bit different to constant speed props and make MPG even more important as pilot makes all adjustments to prevent overloading engine.


For instance manufacturer lists (Rotax 914) figures for different RPM such as 5000RPM/31inHg/20.4L/hr. You adjust prop and throttle to get near these numbers. I personally keep MPG under the figure by a bit so minor alterations in the aircrafts flight will not put the MPG above this figure as I consider it the maximum work/stress recommended figure for the engine. If you drop to 4800RPM, its 29inHg...easy to do if you hit a bit of sink when on alt hold on auto pilot.


As 5500RPM is 35inHg, I personally find around 5100RPM with around 29-30inHg and 21L/hr most relaxing/least subjest to inflight movements when on auto pilot. What you don't get in airspeed is not worth the hassle.


It is no real problem balancing the numbers if you have a regular instrument review technique.



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Another important feature of manifold pressure is the ability to detect carby or throttle icing before it becomes a problem. If you wait until the engine is running rough it's a bit late! With a constant speed propeller you won't see a RPM drop at the onset of carb icing, and even with fixed pitch it is sometimes a while before you realise that the engine is losing RPM. For example, if you are cruising at a constant altitude at a steady RPM with a constant MAP and you note a half inch Hg drop in MAP, you are probably picking up ice and need to do something about it.


When checking carb heat function before takeoff or on approach, you expect to see a drop in manifold pressure. In flight, when applying carb heat to prevent or clear icing, you can increase throttle to restore the MAP to what you need to maintain the speed you want (within the ability of the engine).


Not related to the above, but another one for those that have mixture idle cut-off. Set, say 900 rpm, pull the mixture control out and watch the tacho closely. The RPM should RISE momentarily (by about 25 to 50 RPM) before it dies. If it does not, the idle fuel mixture is set too lean.



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Yes David the 914 is a turbo.


I was trying to hilight with the figures the importance of manifold pressure especially in turbo engine as the potential for damage is higher than normally asperated I THINK.


Would like to know if this is true or just an urban mith.


At full throttle 914 is around 39 in Hg, but will "flick"to around 42 in Hg.



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Stress on engine.Supercharging.


Two main problems arise with supercharging (boosting) aero engines. Getting the heat out of them and detonation. 2 can be a consequence of 1.


Fuel type becomes more critical. You have to be careful of the fuel specs. (Octane number).


There are two occasions when more power is good. Take-off (because you do it better with more). and when you want to get to a high altitude. (Compensate and boost ).(sometimes). Some direct drive superchargers "change gears" as they climb, to compensate for the thinner air.


Engines that use "boost",( ie running MP above ambient pressure),for take-off, usually have a time limit at that power.(Usually five minutes) The Rotax has a time limit also. This is because the engine capacity to get rid of heat is not sufficient to control temps. and the engine is used as a heat sink for a short time. Nev



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when flying say a continental 75 hp in an old taylorcraft what kind of balance should i look for, i have figured out that 95 mph is the max for the bird but need to figure out the balance between my beechman roby adj prop, revs and manifold pressure, anyone got any ideas on this.







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Pitch on prop.


IF you get the min revs for achieving rated power on take-off at sea level, and just reach your max permitted revs at 6-7000' at full throttle you have your prop pitched OK. You don't need a MAP gauge on this engine as it will run at max power all the time. You may wish to limit your revs a bit after take-off once you have cleared obstacles, by pulling the throttle back a bit, but it would not be a requirement as long as oil and head temps were OK. Leaning out (if fitted) is usually done at below 75% power. Your MP and revs will tell you this but as a rough rule when you are above 3000' you can lean the engine even with full throttle. ON THE ENGINE YOU REFER TO.


OOPS. I see you have an adjustable prop. What I have said still applies but with the adjustable facility there are opportunities to do a fuel and wear saving exercise.on X-country. Some engines have a RECOMMENDED MINIMUM rev setting for continuous cruise and I think it was about 2300 for the engine you mention. (Check that out on Cub and Bellanca sites).Nev



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