Does RPM refer to the prop or the engine?

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Sarah

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Sarah
I'm having a bit of trouble piecing together information I read in the Jeppesen PPL, Rod Machado, Gleim and the Private Pilot ground school textbook.

I thought that the RPM set referred to amount of revolutions of the prop but perhaps it is that of the engine as well? It makes sense, the crankshaft is part of the engine and it turns the prop so, shouldn't the RPM of the prop match that of the engine?

If I set the Tach to 1900 during a decent - is that 1900 RPM for the engine and prop?

I think I may have gotten confused by the introduction of a constant speed prop. The tach controls the throttle, or the power supply.. and it displays the RPM right? while MP is the amount of pressure in the intake manifold?

I might be overcomplicating it.. but my understanding was that when using a constant speed prop the amount of MP reduces with a high RPM setting because there is less time for the cylinder to experience a charge.
Meaning, as the cylinder is going through it's cycle, the compression cycle is shorter because of the high RPM.

Am I way off here? Thanks for the help!
 
On most of the GA aircraft the prop is bolted to the crankshaft and prop rpm is equal to engine rpm.

There are some GA aircraft that have a gear box that allows the prop to run at a lower rpm than the power plant. Most likely what ever you would fly does not have the gear box.

I might be overcomplicating it.. but my understanding was that when using a constant speed prop the amount of MP reduces with a high RPM setting because there is less time for the cylinder to experience a charge.
Meaning, as the cylinder is going through it's cycle, the compression cycle is shorter because of the high RPM.

Not following this at all!!!
 
Engine and prop are the same speed on most aircraft, include ones with a constant-speed prop. On aircraft with a gear reduction, RPM is engine RPM.
 
I assume you're talking about a fixed-pitch prop (vs constant speed).

Engine RPM = prop RPM in this case, such as a C172 or Piper Warrior, and most trainers.

For now, thats all you really need to know. If you are curious about constant speed props, this is a good read:
http://www.avweb.com/news/pelican/182082-1.html

regards,
Mike
 
RPM is for engine. Some, notably Rotax, have a gearbox between the engine and the propeller. The Rotax engine is happy running at 5,400 rpm, so the gearbox reduces the propeller RPM down to around 2,400 or so (2.4 ratio). Rotax also usually has a clutch between the two, so it's not a direct connection.
 
Old Geek said:
Engine and prop are the same speed on most aircraft, include ones with a constant-speed prop. On aircraft with a gear reduction, RPM is engine RPM.
Click to expand...

on the only geared airplane that i flew (C-421) the RPM on the tach was the prop RPM
 
Whew! Thanks for clearing that up guys!


tmyers said:
Not following this at all!!!
Click to expand...

What I meant is..

When not in use, atmospheric pressure is registered on MP gauge.
When the engine begins to run and the throttle is decreased, the manifold pressure drops.

The RPM setting is associated with the amount of times the engine goes through the power cycle and the MP is an indicator of the pressure of the fuel and air mixture. So, if a high RPM is set, the amount of time between each cycle is shorter and a smaller charge in the cylinder occurs and the MP will drop.
 
Sarah said:
Whew! Thanks for clearing that up guys!




What I meant is..

When not in use, atmospheric pressure is registered on MP gauge.
When the engine begins to run and the throttle is decreased, the manifold pressure drops.

The RPM setting is associated with the amount of times the engine goes through the power cycle and the MP is an indicator of the pressure of the fuel and air mixture. So, if a high RPM is set, the amount of time between each cycle is shorter and a smaller charge in the cylinder occurs and the MP will drop.
Click to expand...


John Deakin is the master of these types of questions. Here is his take on MP.

http://www.avweb.com/news/pelican/182081-1.html

If you read all his pelican perch articles, you will be better educated than 99% of your instructors.
 
Thank you very much! Those articles were very helpful!
 
kgruber said:
If you read all his pelican perch articles, you will be better educated than 99% of your instructors.
Click to expand...

Don't know about 99%, but definitely some. Quote from one of my first instructors (before I found a great one): "just leave the mixture full rich mostly...you already paid for the fuel when you rented it"
 
Don't overcomplicate it. Changes in cylinder volumetric efficiency, etc vs RPM are small players in this discussion.

The engine is just an air pump. MAP is measured in the intake manifold downstream of the air filter, throttle plate, and other restrictions.

Higher RPM = more airflow = more pressure drop across restrictions = lower observed MAP

Lower RPM = less airflow = less pressure drop across restrictions = higher observed MAP
 
Old Geek said:
Engine and prop are the same speed on most aircraft, include ones with a constant-speed prop. On aircraft with a gear reduction, RPM is engine RPM.
Click to expand...

It can go either way, the GTSIO 520 the tach reads prop as it does with the geared radials I am familiar with.
 
I think you have it backwards, at idle with the throttle plate closed, low RPM, you will have a low MP due to the cylinders trying to suck air through a small opening, creating a vacuum, and a low indication on the MP gauge,

wide open there is no restriction, higher rpm there is little restriction to the airflow and the MP gauge will ride higher, (close to ambient pressure)

JHW said:
Don't overcomplicate it. Changes in cylinder volumetric efficiency, etc vs RPM are small players in this discussion.

The engine is just an air pump. MAP is measured in the intake manifold downstream of the air filter, throttle plate, and other restrictions.

Higher RPM = more airflow = more pressure drop across restrictions = lower observed MAP

Lower RPM = less airflow = less pressure drop across restrictions = higher observed MAP
Click to expand...
 
My limited experience with geared engines:

On a C175 with GO-300 engine the rpm was engine rpm.
On a C421 with GTSO-520, the rpm was Prop rpm.
On a Rotax 912 in a RV-12, the RPM was Engine.
 
VanDy said:
I think you have it backwards, at idle with the throttle plate closed, low RPM, you will have a low MP due to the cylinders trying to suck air through a small opening, creating a vacuum, and a low indication on the MP gauge,

wide open there is no restriction, higher rpm there is little restriction to the airflow and the MP gauge will ride higher, (close to ambient pressure)
Click to expand...

Think he was explaining why MP drops when you push the prop knob forward with out changing the throttle
 
Skylane81E said:
Think he was explaining why MP drops when you push the prop knob forward with out changing the throttle
Click to expand...

Either way, the engine is a vacuum pump. The intake valve opens as the crank is pulling the piston down, so the piston is sucking air in.
The faster the engine is going, the more volume the engine is sucking.
Doesn't matter if the throttle or the prop control is causing the increase in speed.

There are two parts to vacuum (pressure). The amount being pulled, and the amount being provided. The speed of the pump (engine) provides the first part. The intake provides the second part.

If there's no restriction, there's no vacuum. Image the engine was direct injected, and the intake cylinder is open to the atmosphere. There's no vacuum, because there's no restriction.
Another way to imagine this is a shop vac with no hose.

To have vacuum you have to have a restriction. If you put your hand over the hole, now you have vacuum in that the supply of air does not equal the demand. The more you cover the hole, the more vacuum (pressure differential) you have.

In a plane, the throttle plate and the inherent losses in pulling air through the intake provide the second part of the equation. You could pull the carb off, and MP would not equal atmospheric pressure due to the friction loss in the intake tubes, air filter etc.

So, you move the prop control forward, MP goes down. If you open the throttle more (remove restriction) MP goes back up (to the limits of the intake system).
 
ah... man.. so, I guess I had it backward? Maybe I came to the conclusion by rationalizing it but I thought a higher RPM meant less time for the charge to take place in the cylinder and that would result in lower MP?
 
mitchw62 said:
My limited experience with geared engines:

On a C175 with GO-300 engine the rpm was engine rpm.
On a C421 with GTSO-520, the rpm was Prop rpm.
On a Rotax 912 in a RV-12, the RPM was Engine.
Click to expand...

That depends upon the year. they did it both ways.
 
My LSA Rotax 912 time and airship Rotax 912 time has tachometers that measure engine RPM. The Tecnam P2006T (Rotax 912 powered) has tachometers that measure prop RPM.
 
My X3 tach reads engine RPM. No wait, isn't this Bimmerfest about BMW's?

How did I get here?:D

Cheers
 
Sarah said:
ah... man.. so, I guess I had it backward? Maybe I came to the conclusion by rationalizing it but I thought a higher RPM meant less time for the charge to take place in the cylinder and that would result in lower MP?
Click to expand...

The pistons take stuff out of the intake manifold. The faster the engine turns, the more often they take stuff out.

The throttle lets stuff into the intake manifold. The more the throttle is open, the more stuff gets in.

The manifold pressure comes down to the balance between what comes in and what gets taken out.
 
Capt. Geoffrey Thorpe said:
The pistons take stuff out of the intake manifold. The faster the engine turns, the more often they take stuff out.

The throttle lets stuff into the intake manifold. The more the throttle is open, the more stuff gets in.

The manifold pressure comes down to the balance between what comes in and what gets taken out.
Click to expand...

And without regard for why the MP is what it is, the higher the MP the more air will enter the cylinders on each intake stoke, all else being equal. Similarly the higher the MP the more air flows into the engine per unit of time if nothing else is changed.
 
When I had the Gopher engine the tach read the engine RPM. The prop turned just over half that (120:77 reduction).
 
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