Scudrunners

Hey all, I am struggling a bit with manifold pressure and constant speed props. Wondering if anyone happens to know a good resource to understand the basics?

Think of a manual transmission car.

The gas pedal controls the manifold pressure.

For any given speed, the manual transmission gear in your car selects
the RPM. You can choose very high RPM (first gear - prop control all the
way in) or you can choose very low RPM (top gear - prop control out all the way).

If you've ever driven a manual transmission car or motorcycle or truck,
you already know how to control manifold pressure (with the gas pedal)
and RPM (with the stick shift).

Starting out in a car, you probably don't want top gear (low RPM) and
wide open throttle (high MP) aka oversquare. You probably want low
year (prop knob all the way in)

But when you are going fast, you probably want to bring the RPM down
for reduced noise and increased efficiency in cruise: top gear. You bring
the prop knob back for that.

Note that a manual transmission in a car is akin to a variable pitch prop.
You probably will have some added shenanigans on top of the variable
pitch prop which will try to maintain a given RPM regardless of your
airspeed changes. Some older airplanes do not have this fancy RPM
maintaining stuff.

Hope this helps. It's always easy to learn something new, if you can relate
it to something you already understand.

Ugh. I hate the car analogy. It's so fundamentally different as to what a constant speed prop does, that it really prevents you from properly understanding what exactly is going on.

There is no good analogy for that.

On a basic level, while you are flying in cruise, you can think of the manifold pressure as an indicator of how much air goes into the engine. Generally, more air means more power.

The RPM lever is the amount if rpm you ask the engine to rotate at. Let's say 2200 rpm. When you go from idle power to full power, the propeller will go from idle rpm, let's say 800 rpm to your requested 2200 rpm in a fine pitch. (Fine pitch is almost a flat blade. If you look at it standing in front of the plane it will look very wide). The rpm will go up all the way to your requested 2200 rpm. At that point, the engine still has extra power. Instead of accelerating further (which you don't want, you want 2200 rpm), it will rotate the blade of the propeller so it becomes coarser (less flat). That means the propeller is 'eating' more air with every rotation. This also creates more drag, but you will fly faster.

Once you are now established in cruise, and you want to fly at 2100 rpm, the blades will go even coarser. If you want 2300 rpm, the engine won't have enough power to rotate at 2300 rpm in the same coarse pitch, so it will save some drag by turning the blade finer, allowing the props to spin faster.


If you really really want to compare it to a car, then imagine a mythical car in which the wheels turn at a fixed rpm (chosen by the rpm lever), and the wheel diameter changes as you drive and press on the gas pedal to make use of the extra available power.

I hate the car analogy

A better one is always appreciated.

A car has an internal combustion four stroke engine with a throttle. So does an airplane.

A car has a transmission which uses the torque from the engine to propel the car. An airplane has a prop.

A fixed-pitch prop is like a bicycle with only one gear. We have all ridden those. They are not very good -
it cranks too slowly at slow speed, and too fast at high speed. But it's cheap and simple.

A constant speed prop is just a variable pitch prop with a governor.

A variable pitch prop is different from a fixed pitch prop, because the blades can move in flight,
which is a LOT like a transmission in a car, or different speeds on a bicycle.

If you can understand the above, we just need to add the idea of a servomechanism which automatically
changes the blade angle in flight, within the pitch stops of the blades, in order to try to maintain a selected
RPM.

Teaching stuff to people, entirely unrelated to anything they know, is very difficult. Did you study
much in the way of mathematics at school? Advanced abstract algebra? Complex analysis?
Partial differential equations? Control theory and estimation? I went to school with some very
very bright people, and it was a challenge for all of us, when we had something thrown at us that
was complicated, completely abstract and unrelated to anything we knew.

Automatically changing blade angle is pretty neat ....

Helicopters do this with their main rotor. The advancing blade must have it's AOA
reduced every rotation - it is going faster, and must have it's AOA reduced to stop
producing so much lift.

On the other side, the retreating blade must have it's AOA increased every rotation,
to make more lift, because it is going slower and must have it's AOA increased to
produce as much lift as the advancing blade.

As the helicopter goes faster and faster, you run out of travel on the main blades,
and you encounter retreating blade stall, and the aircraft will roll to that side. That
speed is what determines Vne.

This is pretty complicated compared to a constant speed prop on a Cessna.

Colonel wrote: ↑

Tue Feb 01, 2022 2:46 pm

I hate the car analogy

A better one is always appreciated.

A car has an internal combustion four stroke engine with a throttle. So does an airplane.

A car has a transmission which uses the torque from the engine to propel the car. An airplane has a prop.

A fixed-pitch prop is like a bicycle with only one gear. We have all ridden those. They are not very good -
it cranks too slowly at slow speed, and too fast at high speed. But it's cheap and simple.

A constant speed prop is just a variable pitch prop with a governor.

A variable pitch prop is different from a fixed pitch prop, because the blades can move in flight,
which is a LOT like a transmission in a car, or different speeds on a bicycle.

It gives an extremely rough but incorrect explanation as to how a constant speed prop works. 'The rpm lever is like a gear'. Ok. That might help someone to operate the plane, but if they are trying to actually understand how the constant speed prop works, then it's fundamentally different.

A car doesn't keep a constant RPM when you press the pedals. A constant speed prop does. That's the crux of the matter.

I think the car analogy although technically not completely accurate does convey some good info, it at least gets the ball rolling on relating high and low gears to fine and course props and how they affect engine loading. there might be some middle ground to find with one of Honda's CV transmissions but we can discuss that next.

What I am struggling with is I guess really understanding what the MP gauge is showing me and why say if you reduce RPM why does the MP go up? or the opposite?

Oh oh, I was writing this novel about propeller pitch, blabbing about it being the angle of attack for a propeller.

The propeller on our light singles are often fixed pitch, also called constant pitch, sometimes they are fine and people call them a climb prop, sometimes they are course and they call it a cruise prop. Most of the time they are trying for the sweet spot in between climb and cruise. With a constant speed propeller we get a lever to change the pitch.

I don’t hate the gear analogy because it’s a nice basic starting point. I get what digits is saying. When you go up or down a gear it’s a fairly big engine speed to forward speed change. In a big truck it’s tricky to shift gears. No synchros so getting the rpm of the gears close enough to mesh with the change of speeds is a bit of an art.

Conversely when you move the pitch lever for a propeller it will be a nice smooth rpm change as the blades turn at the hub. The pitch changes a little slower and smoothly.

Fill the sink with water.

If you move your hand through the water like a knife it’s full fine pitch. Easy to move but not much water gets pushed.

If you move your hand palm first like a canoe paddle it’s full course pitch, because it’s a wider surface pushing, it’s more difficult but moves more water.

Because we want to create thrust, the propeller blades would be at an angle somewhere between knife edge and fully 90 degrees. Plus one of the things the Wright brothers figured out was the part of a prop near the hub moves slower so it is courser than the outer tips which are going much faster. Later propellers were shaped to maximize thrust and minimize drag.

Once a propeller becomes too course it’s not moving enough air back, full course is a feathered propeller. The purpose of that is to reduce drag if the engine is shut down or not functioning.

I still like opening the window on the highway sometimes and flying my hand. If we’re imagining the hand is a propeller it’s going straight up like a main rotor of a helicopter. If a helicopter goes fast enough the rotor side going forward creates more lift than the side going back towards the tail.

Similar to the sink of water. Knife edge hand is a low angle of attack which is a fine pitch, as you increase the angle of attack by turning your hand, that increases angle of attack, course pitch like pulling the propeller pitch lever back. The pressure increases like a higher gear, the rpm decreases, yet more air is pushed back and at higher speed you maintain more thrust.

What I am struggling with is I guess really understanding what the MP gauge is showing me

You could put a MP gauge on your car. Some do, to indicate turbo boost.

When the engine is turned off, MP is 30 inches of mercury or ambient. 14.7 lbs or the weight
of the atmosphere.

When the engine is idling, and the throttle blades are closed, you will see quite a vacuum drop
across the throttle blades - they are closed, after all - and you might see perhaps 10" MP.

When you stomp on the gas and the throttle blades open, you won't see much of a vacuum drop
from ambient across the throttle blades - they are wide open, and you will see perhaps 29" MP.

Turbo boost is when we increase the MP above atmospheric pressure and pump air into the
engine, and make LOTS of power!

Hope this helps. I find you need to break stuff down. First look at an aircraft with a fixed pitch
prop, then look at one with a variable pitch prop (but no governor), then finally look at a constant
speed prop, which has a governor which automatically changes the blade angle.