# Manifold pressure vs rpm vs power obtained and more about constant speed prop

I know that there are similar topics here, one is mine even, but still I have few questions about constant speed propeller. My questions:

1. Why if we decrease RPM and maintain throttle lever setting the power output decrease?
2. Sounds like very basic knowledge but I heard a lot of versions and I want to reassure. Is tachometer on constant speed prop reading engine rpm or prop rpm?
3. If we move prop lever we are changing blade pitch and then rpm adjust to it or we are changing engine rpm and blade angle adjust to new rpm
• I'm not sure your presumption in #1 is entirely correct. When prop RPM is decreased the torque load on the engine goes up. You can see this on the torque gauge of a turboprop. I believe manifold pressure of a recip would also increase. I'm not sure how this mathematically affects the total power output, but the engine is definitely working harder. – Michael Hall Mar 18 '20 at 16:53
• Why not take a ground lesson with an instructor to get this sorted out? Power output is one thing (rate of fuel burn), thrust output is another. I would study the power output vs rpm curve (don't forget engine friction). The peak efficiency rpm is what you want in cruise (engine efficiency x prop efficiency) along with optimal prop pitch. For take off, we get a little more $thrust$ by using higher rpm and less prop pitch, it's just not as efficient. – Robert DiGiovanni Mar 18 '20 at 22:31
• Surely I will, but I haven’t flow any aircraft with variable pitch prop yet so all I know is from theory. – Konrad Mar 19 '20 at 7:54

1. Power is energy per unit of time, and since there is fewer revolutions, there is fewer power strokes to release that energy. There is less contention for the air from intake manifold so the manifold pressure will increase a bit and each power stroke will deliver a bit more energy, but not nearly enough to compensate the overall lower number of power strokes.

For this reason, manifold power can only be related to power for constant RPM, and it's still complicated by other things, especially the density altitude. Fuel flow is a better indicator of power as long as you keep the engine properly leaned.

2. Usually propeller, but may depend on the aircraft. But it does not really matter, because the gear ratio is always fixed and because the pilot does not have any other use for the value than comparing it to some nominal values in the operating handbook, which are aircraft-specific anyway.

In one aircraft the POH might say that maximum RPM (more can't be selected anyway) is 2500 and most efficient for cruise is 2100, but on another aircraft it might say that 2750 is for take off, but for 2 minutes only, 2600 is maximum for continuous operation and 2300 is most efficient. If it instead said 3130 and 2625, it would make no difference.

Because of this, turbine RPM is only reported in percent, which sometimes extends to the propeller on turboprops too. It is still aircraft-dependent, because some engines go up to values like 102.5% and the spread between maximum and most efficient also varies.

3. You are setting the desired RPM, which the governor will try to attain by adjusting the propeller pitch. If the propeller is too slow, the governor decreases the pitch, thus reducing the thrust and associated resistance until the engine can keep up, if it is too fast, it increases the pitch until the propeller absorbs all the power the engine is providing. If the throttle is too closed, even the finest possible pitch might have too much drag and the RPM will be lower than selected.

There were some variable pitch propellers where the pilot directly controlled the pitch, but the governor is a fairly simple hydromechanical device (weight on spring measures the centrifugal force and directly controls hydraulic valve that adjusts the pitch) and results in so much more practical handling that it quickly prevailed.

This is further complicated in beta range (reverse thrust) where the power lever switches to controlling pitch directly while throttle is controlled by another governor to maintain RPM. But you'll mostly encounter that on turboprops.

• Thank you! Few more questions to your answer: 1. According to my first question, so the blade angle change is not sufficient to compensate all power loss due to lower rpm and therefore new net power output is lower right? And for example if I am flying with constant throttle setting and I decrease rpm, to maintain same power output I need to advance throttle a bit after reducing rpm? – Konrad Mar 18 '20 at 16:20
• On most Lycoming and Continental engines, the RPMs for the Tach are actually measured off of the engine. There is a cable running from the engine, through the firewall, to the panel. The cable connects to the back of the engine near the magnetos – Dean F. Mar 18 '20 at 16:25
• And one more question: so power output is also regulated by rpm? For example if I fly at constant throttle setting and increase rpm despite having more fine pitch propeller I have a bit higher power input and inversely when I decrease rpm with the same throttle setting? – Konrad Mar 18 '20 at 16:28
• @Konrad, what do you mean by power "input" in the second half of your question? – Michael Hall Mar 18 '20 at 17:09
• @Konrad, don't forget the conservation of energy. Increasing the blade pitch does not increase power, it only increases the coefficient of lift, so the propeller will try to move more air with each revolution, but that requires more torque, and if you don't provide it by opening throttle, the propeller will just slow down to the available power (power = torque × rotation speed). And the engine can't develop as much power at lower rotation speed, so it decreases. – Jan Hudec Mar 18 '20 at 20:14
1. Horsepower is torque x RPM. Reduce RPM, but maintain same torque (manifold pressure), HP goes down. Reduce torque, maintain RPM, and HP goes down. You'll find say, that 2300 RPM and 25" of MP make the same HP as 23" of MP and 2500 RPM.

2. If it's a direct drive engine, prop RPM and engine RPM are the same. If it's a geared engine, prop RPM is prop RPM and engine RPM is engine RPM, and they are usually indicated with their own separate tachs.

3. When you move the prop lever you are adjusting the preload on a spring in the prop governor. The spring controls the force on a set of flyweights (like on a steam engine governor, but super compact). The flyweights control oil to the prop to seek an RPM that that puts the flyweights in an equilibrium state (not moving out, not moving in). The governor moves the blades to wherever it takes to get to that RPM with a given torque being applied by the engine.

I am really not sure of the wording of question #1. Taking a stab at it, I would say that reducing the RPMs at a constant throttle setting would increase the both the pitch and the AOA of the prop blades.

For question #2, it depends on the engine. Most certificated engines, the prop is connected to the flywheel/crankshaft in a direct drive configuration. So, the prop and engine move as one unit at the same RPMs.

For question #3, moving the prop lever adjusts the flow of engine oil being diverted to the prop hub. The pressure from this oil drives a piston which adjusts the prop pitch angle. The adjustment is done by controlling the speed governor attached to the engine. This creates a sort of feedback loop. I tried finding a picture of the L shaped weights of the governor and am unable to at this time.

See the following AviationStackExchange question, How does a constant speed propeller work?

The only picture of a governor that I have. It is from a Lycoming IO-540.

• I changed wording of question 1, maybe now it is clear. 2. But what if there is a gear between prop and engine. In that case engine rpm are displayed? – Konrad Mar 18 '20 at 15:15
• In the only geared engine that I fly (the Rotax 912), the tachometer measures engine RPMs. The prop is connected to a reduction gearbox. I believe the Austroengine diesel in the Diamond brand of Aircraft also uses a reduction gearbox. – Dean F. Mar 18 '20 at 15:38