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RPM and Brake Horse Power % On Constant Speed

Cardiff Kook

PATRON
Sisters, OR
You push the throttle in. Manifold pressure goes up. % bhp increases. Makes sense.

Why does adjusting RPM change BHP? Doesnt that just change the pitch of the prop which makes it harder/easier for the prop to turn?

RPM is prop rpm only, correct? The crankshaft speed is completely independent? Or is that where I have things wrong?

At a certain mp the crankshaft spins at some speed. The speed the prop spins is then governed by the prop governor.

Decreasing RPM creates more load on the engine- but why would it decrease BHP?

At a fixed mp, decreasing rpm decreases % BHP- but isnt it bad to run at say 25 MP and 2100 RPM in a 185 with a 550, say? But the %bhp i think is pretty low at that setting?
 
"RPM is prop rpm only, correct? The crankshaft speed is completely independent? Or is that where I have things wrong?"

On most engines the prop is bolted to the crankshaft. They rotate at the same rpm unless there's a gearbox in between them.

Gary
 
"RPM is prop rpm only, correct? The crankshaft speed is completely independent? Or is that where I have things wrong?"

On most engines the prop is bolted to the crankshaft. They rotate at the same rpm unless there's a gearbox in between them.

Gary

Even on a constant speed?
 
Yes connected mechanically. Some engines like Rotax and a few geared engines from days past have a gear box or belt drive to reduce RPM of the propeller but still a direct correlation between propeller speed and crankshaft speed.

https://www.faa.gov/sites/faa.gov/files/07_phak_ch5_0.pdf

Around page 5-28 explains the concept

And if you want a much more in depth of the mechanical workings of it see the propeller section of the faa powerplant handbook.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/amt_powerplant_handbook.pdf


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Yes, prop is bolted to crankshaft except as noted above. The basic concept of power is torque x rotational speed (RPM, for example). There are some constants as part of that multiplication, depending on desired measurement units (HP, KW, etc)
 
Yes and the lower the RPM the lower the HP and less fuel burn. It is a balancing act between speed/power vs fuel burn. Adjust for phase of flight and mission. Look up the setting in the engine operations manual. Most everything else (only run 24 square, never run over square, ETC)is just something someone was told and they took as law for all engines.
DENNY
 
"RPM is prop rpm only, correct? The crankshaft speed is completely independent? Or is that where I have things wrong?"

On most engines the prop is bolted to the crankshaft. They rotate at the same rpm unless there's a gearbox in between them.

Gary

Free Turbine anyone?:lol:
 
Free Turbine anyone?:lol:

Where not only is there not necessarily a direct correlation between prop and shaft, but there may not even be a direct correlation between one half of the engine and the other, let alone the prop :eek:. Not that I've ever seen one started with a prop tie on :bang

Take care, Rob
 
All good questions CK. That's how we learned and are still at it. I found with a CS prop the rpm that ran smoothest made me happy. Never heard back from the engines, but the three in one C-185 ran full TBO w/o cylinder changes or blowing up.

Gary
 
You push the throttle in. Manifold pressure goes up. % bhp increases. Makes sense.

Why does adjusting RPM change BHP? Doesnt that just change the pitch of the prop which makes it harder/easier for the prop to turn?

RPM is prop rpm only, correct? The crankshaft speed is completely independent? Or is that where I have things wrong?

At a certain mp the crankshaft spins at some speed. The speed the prop spins is then governed by the prop governor.

Decreasing RPM creates more load on the engine- but why would it decrease BHP?

At a fixed mp, decreasing rpm decreases % BHP- but isnt it bad to run at say 25 MP and 2100 RPM in a 185 with a 550, say? But the %bhp i think is pretty low at that setting?

Easiest way to think about is basically the faster an engine turns the more power it makes. If you take the prop off and keep increasing the RPM the engine will keep developing more power until at some point the valves will start to float, porting in the cylinders wont allow a supply any more fuel or the the engine cant exhaust any more air, ignition starts to fail or the whole engine blows up. The constant speed propeller can only harness the power that the engine is developing at a given RPM. Thats why when you cycle the prop at say 1700 RPM the engine slows down as the propeller is harnessing more power than the engine is developing. The engine manufacturer sets the max RPM and the HP based on reliability etc.
 
All good questions CK. That's how we learned and are still at it. I found with a CS prop the rpm that ran smoothest made me happy. Never heard back from the engines, but the three in one C-185 ran full TBO w/o cylinder changes or blowing up.

Gary

So Gary,

Did you run lop or just rich. A spray and 185 pilot friend of mine strictly runs rich and fuel flow of 15 gal/ hr in his 185.Thoughts?

Thanks Brian
 
So Gary,

Did you run lop or just rich. A spray and 185 pilot friend of mine strictly runs rich and fuel flow of 15 gal/ hr in his 185.Thoughts?

Thanks Brian

Three IO-520's TBO'd in 1979 C-185 wheels. skis, and floats. One with long 2-blade Mac, two with 3-blade Hartzell. Flew 23 squared 100 rich of peak, but only one cylinder monitored on whatever Continental recommended I assume. Avg. consumption was 13.8-14.0/hour when checked. Good baffling and cowl flaps used to keep CHT in green fast or slow. Lots of slow 17-20" MP.

What's LOP? :-? Never knew about that last Century.

Edit: Some might recall that before the Internet and Google, there was the aircraft Flight Manual if available, then Light Plane Maintenance/Kas Thomas' periodic, then Sacramento Sky Ranch/John Schwaner's manual on engines. Today it's wide open info and opinion.

Gary
 
One more last hurrah on the above post. Smoothest rpm was 2300 but still vibes. The panel shook like a dog passing worms at others. New engines and props gave same vibes. The next operator of that C-185 took exception and had $$$$, and after discovery, had a Seaplanes West engine mount installed. Vibes apparently went away at most power settings I was told. 20 years of my flying life in that shaker was replaced by an engine mount change. Whatever.

Gary
 
One more last hurrah on the above post. Smoothest rpm was 2300 but still vibes. The panel shook like a dog passing worms at others. New engines and props gave same vibes. The next operator of that C-185 took exception and had $$$$, and after discovery, had a Seaplanes West engine mount installed. Vibes apparently went away at most power settings I was told. 20 years of my flying life in that shaker was replaced by an engine mount change. Whatever.

Gary
Starting in 1978 Cessna changed the engine mount with a three bladed prop. It was very smooth. As good as or better than the Seaplanes west mount. I've flown both.
 
You push the throttle in. Manifold pressure goes up. % bhp increases. Makes sense.
Why does adjusting RPM change BHP? Doesnt that just change the pitch of the prop which makes it harder/easier for the prop to turn?....

MP & RPM are related.
See for yourself, set your 185 up in flight at say 22 squared.
Now wind the prop knob out to 2000 rpm, your MP pressure will go up to approx 24" (maybe).
Youre making roughly the same amount of power.
Do you have a power chart in your POH?
Compare for example 22 sq, 23/2100, 21/2300-- it's gonna be about the same horsepower.
Here's the power chart for my 470 (below),
at 2500 feet altitude 23" / 2200 & 22" / 2300 are both producing 153 bhp,
making 149 mph, and burning just over 12 gph.
C180 power & range chart.jpg
 

Attachments

  • C180 power & range chart.jpg
    C180 power & range chart.jpg
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Why does adjusting RPM change BHP? Doesnt that just change the pitch of the prop which makes it harder/easier for the prop to turn?

RPM is prop rpm only, correct? The crankshaft speed is completely independent? Or is that where I have things wrong?

At a certain mp the crankshaft spins at some speed. The speed the prop spins is then governed by the prop governor.

Decreasing RPM creates more load on the engine- but why would it decrease BHP?

Yes and no. HP, and Torque BTW are power measurements. If you aren't loading your engine with fuel, you aren't producing any power. This is why prop selection is so much more important than we give credit for. And why the pumped up figures custom engine builders sell there wares with often leave unknowing folks bewildered. What an enginebis capable of putting out, and what it puts out when governed by other parameters like your prop or mission, are two different things.

Adjusting the rpm does just change the blade pitch. The reason the hp changes is because when you adjust it you are not monitoring that change by blade angle, or power, you are monitoring it by rpm.

The rest of what you said is correct. Since it took less power to spin a flat pitch, and you are only monitoring prop speed, you are now pulling less power. RPM alone is an incomplete formula in HP. You either need torque, or fuel burn to complete it. Look at it this way, put your truck or airplane in low gear and point it down hill with the engine off. RPM will sky rocket, HP output? Not so much.

As a side, this is why many engine/prop/ship combinations see a marked improvement in how they leap off the ground with the prop dialed back a shade. This is very common in many MT installs.

Take care, Rob
 
I believe you are running a 550 so make sure you check the engine operations manual and prop for any RPM limitations.
DENNY
 
If we havent confused the OP enough here is more….

Horse power = (torque X RPM )/5252

Power = work/time

Work = force x distance

So horsepower is force applied over distance in a given time. The 5252 is a constant to make the units come out as horsepower. Think of manifold pressure as an indirect measure of torque or force applied and the rpm as the distance and timeframe that it is applied over. In other words both the amount of force being applied by the engine to the air and the timeframe that it is applied over are what determines how much power is being applied to the environment by the engine. Power is directly tied to fuel consumption in piston engines

Another thought is either increasing the manifold pressure or the rpm will increase the amount of fuel turned into mechanical energy in a given time period, therefore increasing power output and fuel consumption.

To go further thrust (a force only) is directly correlated to fuel consumption in a jet engine. Their power output can vary for the same amount of thrust so power in these engines is not a good indicator of fuel consumption. This is why power is talked about in regards to piston planes and thrust for jet planes. It is also hard to measure thrust in a piston aircraft.

Power is also what determines a rate of climb, excess thrust determines your angle of climb.

Back to constant speed prop’s… decreasing rpm is done by taking a bigger bite of air which provides more resistance to the engine. Increasing manifold pressure will cause the propeller governor to drive the prop to a coarser pitch to maintain the rpm. In this case power will increase because more fuel/air is now going through the engine for each cycle of the pistons, this is transmitted to the environment by moving more air with each swing of the prop. Increasing rpm will increase power because now the engine will cycle the pistons and therefore more fuel/air faster. This is transmitted to the environment by moving smaller bites of air faster with the prop. Either would result in more fuel being burned faster and a faster aircraft speed. The advantages of lower rpm are efficiency from less loss from friction in the engine and also less form drag from the prop. The constant speed prop is also an advantage for takeoff because it can maintain the maximum engine/prop speed throughout your whole takeoff with throttle wide open (max manifold pressure) therefore creating maximum power. A fixed pitch prop is a compromise, it is too coarse to maintain maximum rpm at slow speed.

I may have oversimplified some things but I believe what Ive stated is essentially true as far as us pilots are concerned. There are other factors like prop efficiency/slip etc but this is the basic idea of a constant speed prop.



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