# Calculating Prop Thrust

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#### Atomic_Sheep

##### Well-Known Member
What you are thinking of on the certified airplanes like Cessnas is, as Himat pointed out, called a constant speed prop. The reason is the use of a governor. When you adjust the prop pitch, you are actually just changing the setting on the governor. Say you have leveled off, and are ready to set up for cruise. You increase prop pitch. Now you can adjust your throttle. Unlike a fixed pitch prop though, your manifold pressure, but not rpm, will change. The governor automatically changes the prop pitch to keep the desired rpm. You don't really see them on less than 180 hp engines that much. They typically use engine oil pressure to operate the prop and have to have a hollow drilled crank. In the small side of the homebuilt world, you may find electrically operated variable pitch props on pretty small planes. These are not really "constant speed" props though, because they do not have governers (usually). You have to adjust the pitch yourself with each throttle change. No biggie, just different. Hope this helps clear up some of the confusion
I think this a bit confusing for me because a new part has been instroduced into the equation, namely manifold pressure. No idea what it is in a physical sense. The way I understand it is... your engine has a compression ratio which is the same thing as manifold pressure - well I suppose compression ratio is the maximum manifold pressure that the engine is designed for? So at constnt RPM, by changing the load, you're effectively increasing/decreasing the manifold pressure and if we reach the max manfold pressure as per max compression ratio for the engine, that's when the engine starts to sputter and stalls if you keep pushing and put too much load on it?

If my understanding is correct, the next logical question is, what does manifold pressure translate to in terms of why even have it? If you have a constant speed engine like the TPE331, you've got a governor which maintains constant RPM and I'm not sure how a turboprop engine differs to a piston from a manifold pressure perspective given that there is no manifold pressure in a turboprob, but I would imagine that turbines have compression equivalents and given that the RPM is constant and the governor works off hydraulic pressure, you're effectively going to always maintain a constant manifold/turbine pressure and this will be managed by the rotating prop blade angles.

Having said all this, why does it even matter what the prop thrust is at static or dynamic? I'm obviously an aircraft design guru asking questions such as this, but I"m guessing the answer is as simple as - it depends on the intended 'mission' of the aircraft and the desired characteristics of this mission.

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