Help Understanding Piston Engines w/Constant Speed Propellers

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It’s very hard for me to admit this and swallow my pride given my massive knowledge as a builder of sports/racing car engines, but I’m struggling to understand some basic concepts surrounding the following:

As I fly the DC-6 and, now, the Staggerwing, I’m trying to understand what, specifically, the Manifold Pressure and RPM indicators are telling me in regards to what the engine is doing and how that differs or is the same as to what the propeller is doing.

Let me, first, put out my assumptions fully expecting to be wrong here.

• The manifold pressure gauge is, essentially a vacuum/boost gauge. Readings on the gauge below Barometric pressure is a vacuum and anything above is boost. In the case of the aforementioned aircraft, boost is provided from a mechanical supercharger rather than an exhaust turbocharger. Closing the throttle induces more and more vacuum the more closed it gets. Opening it further and further induces boost the more open it gets. All, of course, working in harmony with engine RPM — less RPM, less supercharger speed, less boost, and vice versa, etc.

• The tachometer is the one I’m mystified by. The POH gives engine RPM values for various power settings, yet I get the sense what I’m seeing reflected in the gauge is the propeller RPM not the engine RPM. I don’t understand how you can have the throttle wide open, see boost even and have low RPM, if this isn’t the propeller speed.

If I have the throttle wide open and the propeller lever pulled way back, what is preventing the engine from running at a high RPM? The throttle is open, aren’t fuel and air being fed into the engine for go fast like on an automobile? Wouldn’t you hear the engine running fast, but the propeller running slowly? Instead, I hear a relatively mild engine speed and can’t understand how that can be.

I’m just totally confused by what I see and hear reflected by these behaviors and their relationship.

Help a brother out here.

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This might help? Unless you are talking about the inner working of propeller governors etc. I need to come back on that.

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Because the propeller blade angle is changed to a course angle, effectively taking a bigger “bite” of air and therefore creating more thrust at the same RPM. There could be a difference between engine and propeller thrust, some aircraft have a gearbox.

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You’re speaking about thrust, which I’m understanding in theory, what I’m speaking to is how I hear the engine running slowly via its exhaust note and see a low RPM value on the tachometer, yet I’ve got the throttle firewalled.

Where is all that fuel and air going? Isn’t it being burned and, therefore, shouldn’t the engine be running at high RPM?

Sure, I get if the propeller’s RPM is adjustable, then it would be turning slowly and making less noise, but wouldn’t the engine be making a ton of noise?

I guess, I’d have expected two tachometers — one to indicate the RPM of the engine and another to indicate the RPM of the propeller.

Of course, if I not understanding any of this prop to engine relationship then that’s what I’m seeking clarification on.

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If you are going uphill in a big truck and you keep the engine RPM steady, you will still have to increase fuel to the engine to do that.
Adjusting the prop pitch takes a bigger bite of air requiring more power, like the truck

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As I’ve been sitting here mulling this over, I was thinking something similar.

Am I being too ignorant of physics? Is the effort required to move a propeller through the air as much effort, say, as pulling a grade in a truck with your foot to the floor?

Like you said, the truck engine’s RPM isn’t going to start racing, because it’s under load due to gearing and the hill it’s climbing.

Is moving the propeller through the air that kind of effort?

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Exactly. The greater the angle to the air, the more air it pulls through for each RPM, and that needs more power.

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Man, I love aviation!

There is so much to learn and understand.

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And bear in mind, the prop moves a lot of air.
Just think of it in terms of size when compared to a household fan.
So yes, moving that much air requires a lot of energy.

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I feel so dumb saying this, but since you cannot see the air, it doesn’t seem like there’s anything to it.

Of course, I say that even fully grasping how important aerodynamics are and how drag is a massive thing when selecting wings in a race car for downforce vs. top speed.

It’s funny how our brains just miss making connections.

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I was in a similar thread today. Here is a good article explaining it: How A Constant Speed Propeller Works | Boldmethod

Hope that helps, happy flying!

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Let me pose this question then.

I thought altering the speed of the propeller was to keep hypersonic speeds at the tip in check to reduce noise.

If the constant speed propeller is that — constant — how is this noise management a thing then? Is it all just in the pitch of the propeller that is generating all that noise rather than solely the speed it’s moving through the air?

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I’ve seen that before and read it, but for some crazy reason I’d still really struggled to understand it all in practice.

Again, I’m used to engines and gearboxes and the grades and corners of a road race track.

The entire concept of air (well, and gravity) being the force against the engine/prop has proved difficult for me to fully grasp.

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I’m not so sure it is noise, I would think it’s more due to prevent cavitation on the blades
Again, I’m not a prop person, I very seldom fly props, and really haven’t studied them that much.

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Ah, good point. I’d forgotten about cavitation.

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If I understand the theory of the Constant speed prop, it is to allow the engine to operate in its peak power curve.
Adding fuel increases the power output of the engine and would normally increase the engine RPM. This would normally speed the propeller up. To prevent that from happening the blade angle increases to absorb the additional torque that the engine is now producing allowing the engine RPM to remain at steady. The opposite applies for a reduction in power, as less torque is available, the engine RPM decreases allowing the propeller RPM to remain more or less constant.

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How do you mean? The engine RPM is equal to engine RPM, at least on piston engines. On some piston engines and all turboprops there is a gearbox between the engine and the propeller, this gearbox is not like your car, its just a reduction gearbox. The ratio remains the same.

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Hang on a second though.

On a turboprop, we see a Propeller RPM readout, that changes with a propeller lever. When we talk about “constant speed” propellers is there still some range of variation in their speed as seen reflected in the propeller RPM on, say, a G1000?

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I think what you don’t get (yet) is the propeller blade angle is variable. They move the same way the propeller of a ship is usually adjustable.

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Exactly. So you set for the prop RPM you want and as the engine output increases, the RPM of the prop stays constant, but the blade pitch changes due to the governor.

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