Explain Constant speed props to me

[Henning]

What will blow your mind is what limits the RPM is not a speed control. It's the fastest that the engine can spin a prop that is taking that much of a bite into the air. What's limiting the RPM is the power of the engine.

On my 235 a flat prop spins at about 2550 RPM always. I control power in the pattern with MP. Reduce to 20" on crosswind, 16" for slow flight, 11-12" to descend.

Exactly it limits RPM by using all the available torque at that RPM by the application of drag.

 

[mtuomi]

This is managed by running high RPM when in high power modes, and reducing torque (Internal Cylinder Pressure) to operate in low RPM modes. You can reduce torque one of two ways, restrict airflow with the throttle or restrict fuel flow with the mixture, I prefer to use the mixture.

What range of AFR is your engine running, if you can control the BMEP by leaning mixture without hitting the knock threshold?

About engine torque control methods, by far the best way would be ignition angle control. Do any airplane engine types offer this method of control?

(Not saying this again, there is no detonation, ever, in any condition, in internal combustion engines).

 

[Henning]

What range of AFR is your engine running, if you can control the BMEP by leaning mixture without hitting the knock threshold?

About engine torque control methods, by far the best way would be ignition angle control. Do any airplane engine types offer this method of control?

(Not saying this again, there is no detonation, ever, in any condition, in internal combustion engines).

Detonation is a result of multiple factors, not just mixture. Load is also a major factor. If you stay out of a conducive load, you don't get detonation, you never get ping or knock, it just fades out. I had no way to measure the F/A ratio, however it's going to be leaner than 13.7:1.

There is no method of ignition advance on an aircraft engine, they are static/single time magnetos. This isn't a big deal really since they are slow turning and operate in 400 rpm range typically when making power.

 

[sidneyfw]

What a great question Dallas, and some awesome information, I've been struggling to understand throttle pitch and mixture myself. What a great forum, can't believe I've been away so long. Sidneyfw

 

[Jaybird180]

What a great question Dallas, and some awesome information, I've been struggling to understand throttle pitch and mixture myself. What a great forum, can't believe I've been away so long. Sidneyfw

We'll change that opinion soon enough

 

[RotorDude]

I really wish people would quit making the analogy between a CS prop and a transmission, it's nothing of the sort.

Look, these are propellers, you are pilots, you should understand them correctly.

A propeller is a screw, the steeper the pitch, the further the screw will try to pull through the medium it is in per revolution. Horsepower is a measurement of how fast you can move something in a given time. The further you moved it in a given time, the more horsepower it required.

Propellers are an airfoil, not a gear. When you increase Angle of Attack, you increase drag, when you decrease angle of attack, you decrease drag; simultaneously you coarsen or fine the pitch of the screw.

This is the basics of a Variable Pitch Prop. When you use a Constant Speed Propeller, you use this ability to vary the drag profile in order to govern the speed of the engine by using up all of the available torque.

The propellor blades are mounter so they can rotate. In the middle of the hub is a piston that drives out with oil pressure and returns either through a spring in the hub, nitrogen charge, or some counterweights on the blades. There is also a linkage between the piston and the blade to adjust the pitch. That prop hub is bolted onto the end of a crankshaft with a hollow nose and an oil port through the front main bearing to allow pressurized oil to be fed into the hub, or released from the hub.

To control this oil pressure and thereby the pitch of the blades and drag of the propellor, we employ a variation of James Watt's centrifugal governor. This consists of a set of flyweights counterpoised with a spring whose rate can be adjusted. These drive a spool valve which controls the flow of oil in and out of the hub. The flyweight shaft is driven off the crankshaft.

What you do when you move the blue handle is adjust the tension of the counterpois spring. As the engine speeds up, the flyweights want to go out and open the valve to put oil into the hub and add drag. When rpm drops, the counterpois spring brings the flyweights back in again releasing oil from the hub and reducing blade drag.

When you move the blue handle and adjust the counterpois spring, you adjust what RPM the spool valve opens and closes at. Here is the really important part to understand: When you select a lower RPM, you are selecting to increase load on the engine, that is why you see a rise in MP when you pull back the prop.

You can get yourself into trouble with detonation if you are making too much power when you pull back. Pulling back and reducing RPM also changes the timing of the flame front vs Top Dead Center which will also move you closer to detonation.

This is managed by running high RPM when in high power modes, and reducing torque (Internal Cylinder Pressure) to operate in low RPM modes. You can reduce torque one of two ways, restrict airflow with the throttle or restrict fuel flow with the mixture, I prefer to use the mixture.

Also, since Power=Torque*Time, an engine turning more RPM will be able to make more power, so when climbing is necessary, I increase RPM rather than increasing torque with mixture or throttle.

I am sure you meant Power = Torque*RPM

 

[Henning]

I am sure you meant Power = Torque*RPM

Rpm=time.

 

[timwinters]

I really wish people would quit making the analogy between a CS prop and a transmission, it's nothing of the sort.

Look, these are propellers, you are pilots, you should understand them correctly.

I know how a CS prop works...at least mine...but I couldn't care less if the next guy doesn't. Who cares? The CS prop and CV transmission might function totally differently but they produce the same results so comparing them is appropriate.

Not everyone who flies a plane is mechanically minded and they don't necessarily need to know HOW something works. They just need to know WHAT it does for them and to USE it.

I don't know HOW all the internal circuits in my GPS or VOR receiver work but I know WHAT they do for me and how to USE them. And I really don't think I need to get an electronics degree so I can know how they work. I don't care how they work.

If someone WANTS to know how a CS prop works then fine but they don't HAVE to know to be a good pilot.

 

[RotorDude]

Rpm=time.

Care to explain?

 

[Henning]

Care to explain?

Revolutions per minute, the time component of the equation. Nothing wrong with using RPM to provide the time component, but it is not required. Think about the original 'horse power' calculation, there is no rotation at all. In fact torque isn't required either, just force.

 

[mtuomi]

Detonation is a result of multiple factors, not just mixture. Load is also a major factor. If you stay out of a conducive load, you don't get detonation, you never get ping or knock, it just fades out. I had no way to measure the F/A ratio, however it's going to be leaner than 13.7:1.

There is no method of ignition advance on an aircraft engine, they are static/single time magnetos. This isn't a big deal really since they are slow turning and operate in 400 rpm range typically when making power.

(Detonation never ever happens)
Just wanted to say that, sorry. I think I've said this before, I know it's being anal, but still, it never does. Wrong word. <- fighting against the windmills.

What's the CA50 of a normal airplane engine at max rated power? All these numbers sound very odd to me.

 

[RotorDude]

Also, since Power=Torque*Time, an engine turning more RPM will be able to make more power, so when climbing is necessary, I increase RPM rather than increasing torque with mixture or throttle.

Rpm=time.

Revolutions per minute, the time component of the equation. Nothing wrong with using RPM to provide the time component, but it is not required. Think about the original 'horse power' calculation, there is no rotation at all. In fact torque isn't required either, just force.

It doesn't sound like the Physics definitions I know and use.
(E.g. see: http://www.engineeringtoolbox.com/angular-velocity-acceleration-power-torque-d_1397.html )

Some well-known definitions:
Power = Energy / Time
Energy = Force * Distance
Torque = Force * Arm

Substituting:
Power = Force * Distance / Time

In our case, the torque's "Arm" is the radius of the prop's center of drag from the shaft, and the circular distance covered by that point on the prop per minute is given by:
Distance per minute = RPM * Arm * 2 * Pi

Therefore:
Power = Force * Distance per minute = Force * RPM * Arm * 2 * Pi

Substituting for Force * Arm:

Power = Torque * RPM * 2 * Pi

And if we assume radians per minute for RPM:
Power = Torque * RPM

In other words, the more torque, the more power, and the more RPM, the more power. Time is not a part of the equation.

 

[Doggtyred]

And.. as expected... for a guy looking for a simple explanation (not necessarily the purest, most correct with regards to engineering and physics explanation) the audience has really lost sight of the target.

Keep it simple and focus on learning rather than proving who the smartest person with a keyboard is. A good teacher explains in terms the learner understands. This kind of discourse/****ing contest belongs elsewhere, like at Valhalla, the graduate student bar in the basement of the physics building at Rice University..

 

[StinkBug]

Thanks Dave. I had all but walked away from this post when it started veering away from the real purpose.

 

[Dan Thomas]

In other words, the more torque, the more power, and the more RPM, the more power. Time is not a part of the equation.

So James Watt was out to lunch when he defined one horsepower as 33,000 foot-pounds per minute?

Dan