Simviation Forum - Questions about prop pitch

Nov 2^nd, 2008 at 3:26pm

brettt777 Offline
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I am a maintenance tech at a small but growing company that specializes in L-39s. I don't have my A&P yet becuse I specialize in avionics but I will have it as soon as my time is in. Anyway, since we are starting to get into more GA aircraft and we will be purchasing a small single engine airplane for our own use and training, I want to understand more about how they work which finaly brings me to my question(s)...I know what prop pitch is but not sure how it works. What I mean is, when you have an rpm gauge in the plane, is that prop or engine rpm? I was told it's engine rpm but then in a simulator I adjust the throttle and that rpm stays the same even though the engine is obvioulsy changing rpm. I adjust the prop pitch and the rpm changes. So is this engine or prop rpm? Does it depend on the plane, say either a 182 or a P-51? If I can adjust engine speed with the throttle but prop rpm stays the same, does that mean the prop isn't physicaly attatched to the engine? I pray these questions don't sound too ignorant but most of my aviation experience is with jets so I am not intimately familiar with props.

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Reply #1 - Nov 2^nd, 2008 at 5:25pm

specter177 Offline
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Short answer, it's prop RPM. The RPM gauge shows how many RPMs the propeller is doing, but it depends on the aircraft, as some aircraft have fixed pitch propellers (like a Cub). This means that, as opposed to a constant speed prop, the propeller pitch doesn't change to keep the RPM the same with different engine power, and changes with throttle settings. Propellers are geared down from the engine pistons, which spin at much higher RPMs. Hope that made sense.

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Reply #2 - Nov 2^nd, 2008 at 6:18pm

Brett_Henderson Offline
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This topic is probably as confusing as any in aviation. It comes up pretty often, so I dedicated a thread to it a long time ago..

Quote:

I know what prop pitch is but not sure how it works. What I mean is, when you have an rpm gauge in the plane, is that prop or engine rpm?

Prop RPM and engine RPM are one in the same for all piston powered aircraft.. and even if there IS gear reduction (like on small aircraft using a Rotax engine, or large radial engined aircraft), they might as well be the same, because their relationship is linear. They are liked together mechanically. Neither can change RPM independent of the other.

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I was told it's engine rpm but then in a simulator I adjust the throttle and that rpm stays the same even though the engine is obvioulsy changing rpm.

The sim replicates constant-speed props very well. What you're HEARING, is the engine changing power output (and noise) when you change the throttle, but the RPM are not changing. That's one of the first things that threw me while getting my complex endorsement. I'd pull the power back.. the engine SOUNDED like it was slowing down, but a glance at the tachometer confirmed that both the engine and prop had not changes RPM. That's the whole point of a constant-speed propellor.. constant speed.

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I adjust the prop pitch and the rpm changes. So is this engine or prop rpm?

It's both (they are mechanically linked).

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Does it depend on the plane, say either a 182 or a P-51?

I cant speak for old war-birds, but I know that some were fitted with "adjustable pitch" propllers.. where the pilot literally could change the pitch of the blades, regardless of throttle setting.. but as a rule; big plane or small; old or new... a constant-speed propeller functions the same.

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I pray these questions don't sound too ignorant but most of my aviation experience is with jets so I am not intimately familiar with props.

Pray not.. these are excellent questions. It shows that you want to learn. It's a confusing concept to get your head around.. and believe it or not, about 1/2 the AP guys you'll meet do not properly understand constant-speep props, or at least have trouble explaining them.

Here's that thread.. feel free to ask more there, or add comments..

http://www.simviation.com/cgi-bin/yabb2/YaBB.pl?num=1212879811

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Reply #3 - Nov 2^nd, 2008 at 11:00pm

brettt777 Offline
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Okay I read all this and the other article so let me see if I understand...when I go to a power setting the rpm comes up to whatever rpm the prop will allow it to spin, correct? As the power setting is adjusted more or less oil pressure is applied to the prop so that the pitch changes to a more or less aggressive setting, keeping the engine at the same rpm. So when the prop control is adjusted it is only changing the amount of oil pressure the prop sees and if the power setting isn't changed and oil pressure stays the same, then the prop rpm will change because the pitch will change according to how much oil pressure is allowed to the prop. If the prop control isn't changed and the power setting is, the rpm will change momentarily until the change in oil pressure adjusts prop pitch which puts rpm back where it was. Is that anywhere near correct?

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Reply #4 - Nov 3^rd, 2008 at 7:20am

Brett_Henderson Offline
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brettt777 wrote on Nov 2^nd, 2008 at 11:00pm:

Okay I read all this and the other article so let me see if I understand...when I go to a power setting the rpm comes up to whatever rpm the prop will allow it to spin, correct? As the power setting is adjusted more or less oil pressure is applied to the prop so that the pitch changes to a more or less aggressive setting, keeping the engine at the same rpm. So when the prop control is adjusted it is only changing the amount of oil pressure the prop sees and if the power setting isn't changed and oil pressure stays the same, then the prop rpm will change because the pitch will change according to how much oil pressure is allowed to the prop. If the prop control isn't changed and the power setting is, the rpm will change momentarily until the change in oil pressure adjusts prop pitch which puts rpm back where it was. Is that anywhere near correct?

That about sums it up !

Key to keep in mind is that you are not selecting a blade-pitch with the prop control. You're selecting and RPM .

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Reply #5 - Nov 3^rd, 2008 at 8:36am

brettt777 Offline
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Great! Well I learned something today then. Actualy what I did is take what you said and go into the simulator and while still on the ground I just played around with the throttle and prop controls and watched the rpm and MP gauges and then it all fell into place. Thanks again Brett. You have obviously been an AMT for a while...?

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