Calculating Prop Thrust

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Atomic_Sheep

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Nov 6, 2008
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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.
 

akwrencher

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Oct 16, 2012
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A little off topic, but to answer your questions about manifold pressure, all piston engines have manifold pressure. Your car has it, you just don't have a gauge for it, the computer does though, and uses it to know how much load (in conjunction with rpm) is on the engine. Manifold pressure is not related to compression ratio, that is to do with how the ratio of the volume of air contained in the cylinder between top dead center and bottom dead center. The reason I mentioned it is because if you fly a prop governed plane, manifold pressure will change with throttle changes, but not rpm (to a point). It tells you how much load you have on your engine. It is really "manifold absolute pressure" and will always be lower than atmospheric pressure unless you have a turbocharger. It changes because you have a throttle plate restricting the airflow in your engine, and your pistons are sucking air past the plate, which is a restriction. The term may be confusing because on regular non turbocharged engines, "manifold pressure" is really a partial vacuum until you get to wide open throttle. Take a shop vac and put your hand over the hose end while it is running, but not all the way. Let some air past, and imagine what is happening inside the hose. This is what is going on in your engine manifold.

Turboprops are turbine engines with output shafts, and not worth trying to compare to piston engines in any meaningful way. About as different as cat's and catterpillers :)

I'll let the prop experts answer the thrust questions :)
 
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