CONFUSED about engine/propeller concepts

[magyar]

fixed pitch(e.g. cessna 152)
?is fixed pitch constant speed?

constant speed varible pitch(e.g. piper seminole)
?Varible speed; same as varible pitch?


If the throttle controls power output as registered on the manifold pressure gauge and the propeller control regulates engine RPM then
why do we use rpm as a monitor of throttle output in a cessna 152 but we use MP to monitor throttle output on say a piper seminole(but when on the ground thottle contols rpm)??

side qustion:does a c152 not have MP as well?

[vrrotate]

First off, all piston powered engines have a manifold pressure. Much of the time it is just not measured (like in your 152). Manifold pressure is the pressure of the air at the intake manifold of the engine and (in all the planes that I have flown with MP gauges) is measured in inches of mercury.

Constant speed propeller systems use some type of propeller pitch changing system (governors, counter-weights, springs) so that the propeller will maintain a constant, set RPM. In doing this the propellers are able to better use the power being supplied by the engine. For example a fine pitch for take off and a more coarse pitch for cruise.

When trying to understand the variable pitch, constant speed system, some people find it useful to equate it to a bicycle with multiple gears or the transmission of a cars engine. The gears are changed based on what is required at the given time. So your 152 is like a 1 gear bicycle or a chainsaw engine. The rpm of the wheels on the bicycle or the chain on the chainsaw is directly proportional to the rpm of the engine. This is why the 152 only has an rpm gauge. The Seminole having a variable pitch, constant speed propeller system requires both the MP gauge, to indicate engine power output and the RPM gauge to indicate propeller rpm (HOW the engine power is being used).

Hopefully this answers your questions.

[KAG]

To dumb it down even more...A C152 has a fixed pitch prop. You can't change it's pitch angle, there is no prop control in the throttle quadrant. It's like ridding a peddle bike with no gears.

A seminole has a constant speed prop - meaning you can (and do) change it's pitch through a prop control in the throttle quadrant. Think 21 speed bike now. Different pitch angles for different phases of flight.

Don't worry about the MP guage, it will make sense once you start flying the plane. Your car could have a MP guage if you installed one.

[precision_approach]

Let me confuse things even more:

A variable-pitch propeller is called a "constant-speed" propeller - even though the speed of the propeller obviously changes.

However, once you set the pitch of the propeller the speed of rotation will stay constant even though you make changes to the power. That's the "constant-speed" part.

Contrast the fixed-pitch propeller whose speed changes directly with engine power.

Go to the McCauley website and have a look:

http://www.mccauley.textron.com/prop/propframese.html

[vrrotate]

A variable-pitch propeller is called a "constant-speed" propeller

This is not necessarily correct. A constant speed propeller is a type of variable pitch propeller. Not all variable pitch propellers are constant speed. Although now-a-days most are.

A variable pitch propeller is simply one that has the ability to change the angle of the blades. This can be accomplished mechanically, hydraulically, electrically, etc. While most let you to control the blades while the engine is running, some blades allow you to change the angle only on the ground by hand with the help of some wrenches.

Propellers only become constant speed when there is a governor to accomplish this.

[Invertago]

Your car could have a MP guage if you installed one.

Great, now some pilot geek will do just that to impress the ladies! Actually I could fit it right in below my cars Altimeter!

[Spokes]

At least one car that I know of had a MP guage installed as a factory item. I remember seein gone in a 65 chevy years back.

[albertdesalvo]

Yup, a friend's '63 Pontiac had one.

Pontiac.jpg (19.2 KiB) Viewed 992 times

[sstaurus]

I never fully understood this part...

When the engine is at rest, MP= 29.92 obviously. When you start the engine, it decreases because of the vacuum. So how come when you increase throttle, MP increases? Wouldn't Throttle Increase = more vacuum = lower MP?

[jump154]

I never fully understood this part...

When the engine is at rest, MP= 29.92 obviously. When you start the engine, it decreases because of the vacuum. So how come when you increase throttle, MP increases? Wouldn't Throttle Increase = more vacuum = lower MP?

Remember the throttle is between the atmosphere and the manifold. When the throttle is closed, it restricts the airflow into the manifold - the suction from the piston is the same, less air gets in past the manifold therefore the manifold pressure drops.

Open the throttle, lots of air can get in, manifold pressure is higher. As lots of air can get in, lots of air also gets into the cylider, giving more power. (OK, air/fuel mixture)

[black hole]

Vacuum; as in the car is vacuum measured at the venturi of the carburator. This should not be confused with manifold preasure, which is measured after the carburator or thottle plate( fuel injected engine).

BH

[AuxBatOn]

I never fully understood this part...

When the engine is at rest, MP= 29.92 obviously. When you start the engine, it decreases because of the vacuum. So how come when you increase throttle, MP increases? Wouldn't Throttle Increase = more vacuum = lower MP?

Because you are reading a Negative Relative Pressure (Succion). Negative absolute pressures do not exist.

So, let's define :

P1 : Pressure taken after the throttle plate
Pa : Ambiant pressure
MP: Manifold Pressure as you read it on your gauge

For the purpose of the exercice, let's say Pa is constant at 29.92 in Hg

MP = |P1 - Pa|

(for the non-math crowd, |x| is the absolute value of x, so if x=-5, |x|=5)

You are right, P1 decreases as you introduce more air (throttle opened), and your MP will increase, even though P1-Pa will get smaller.


But, it wouldn't make any sense from a practical point of view to show you MP = P1 - Pa, as it would always be a negative number (Hey bud, set -25 inches doesn't sound right, and it shouldn't, since for a lower number, you'd have more power, which is counter intuitive).

Clear as mud?

[sstaurus]

Ok, so I guess the point here is that MP is taken after the Venturi? If it was inside the venturi more throttle would = smaller in-Hg.