100% Power?

[dayshift]

Question, is it possible for a nonturbo, piston engine to develop 100% power, say below 3,000 feet, maybe on take-off? I discussed this with a fellow pilot and he said yes, but I disagreed, looking at a Cutlass RG manual under the cruise performance section, the highest percent of power was 77% (bhp), is this it? Thanks.

 

[Singlecoil]

Question, is it possible for a nonturbo, piston engine to develop 100% power, say below 3,000 feet, maybe on take-off? I discussed this with a fellow pilot and he said yes, but I disagreed, looking at a Cutlass RG manual under the cruise performance section, the highest percent of power was 77% (bhp), is this it? Thanks.

If you are referring to rated horsepower, as in a 150 horsepower engine then the answer is no. Propellers are generally in the 85% efficiency range, therefore you lose 15% horsepower right off the bat.

 

[banned username 2]

Re: Re: 100% Power?

If you are referring to rated horsepower, as in a 150 horsepower engine then the answer is no. Propellers are generally in the 85% efficiency range, therefore you lose 15% horsepower right off the bat.

Don't confuse "Power" with "Thrust"...

A 150 horsepower engine may produce 150 horsepower, but the prop will not be 100% efficient resulting in less than 150 horsepower of "work" being done...

Remember "Cruisse" power settings are "Max Continuous Power" settings.... Don't confuse the power settings in the Cruise section with Take-Off power...

 

[averyrm]

Not only can it make 100% of rated power, it can make more given the right conditions (no humidity, cold air) as engines are rated at standard sea-level conditions.

 

[Singlecoil]

Not only can it make 100% of rated power, it can make more given the right conditions (no humidity, cold air) as engines are rated at standard sea-level conditions.

Ok, I guess we all need to define our terms a little more precisely. Brake horsepower is indicated horsepower corrected for friction loss. Brake horsepower is what the engine is rated by, i.e. 150 horse. Thrust horsepower is what you get out of the prop, or roughy 85% of BHP depending on airspeed. The prop's efficiency is different at different airspeeds.
I think we are all correct. You can get more than 100% BHP at density altitudes below sea level, but you are still losing 15% of that to prop inefficiencies. Can you get a cold enough day to get 115% BHP and 100%THP? I wouldn't be surprised if you could. I have flown non-turbo 207's at density altitudes 4000-5000 feet below sea level and they will giddyup and go.

 

[TXCAP4228]

In the Performance section of the POH, when it lists X% power at such and such a manifold pressure and RPM, does that refer to %Brake Horsepower?

How can you get density altitudes 4000-5000 feet below sea level? Really cold air?

 

[Singlecoil]

In the Performance section of the POH, when it lists X% power at such and such a manifold pressure and RPM, does that refer to %Brake Horsepower?

How can you get density altitudes 4000-5000 feet below sea level? Really cold air?

Yes, the POH refers to BHP in the cruise section, at least the 172 manual I'm looking at does.

Yes, really cold air, like 30 below F, and high pressure, at sea level.

 

[dayshift]

Thanks for the respones, but looking at the 172RG manual as an example, at 2,000 feet, 2,500 rpm, 20 degrees below celsius, produces 77% power. The engine reaches max power (180 hp.) at 2,700 rpm which I got from the general section of the manual, so if I increase the rpm by 200 I should get 23% more power?

 

[Singlecoil]

Thanks for the respones, but looking at the 172RG manual as an example, at 2,000 feet, 2,500 rpm, 20 degrees below celsius, produces 77% power. The engine reaches max power (180 hp.) at 2,700 rpm which I got from the general section of the manual, so if I increase the rpm by 200 I should get 23% more power?

Well, shoot, I guess we better take it from the top. I happen to have a Gutlass manual here myself so here we go.
The engine is rated at 180 Brake Horsepower (BHP) at 2700 RPM. That is at sea level on a standard day. This is straight from the engine manufacturer. Now hang a prop on the engine. Since the prop is only 85% efficient, the most Thrust Horsepower (THP) you will ever get out of the sucker is 85% BHP at sea level on a standard day. Go up 2000 feet, and you get an even lower % of BHP for max power. In your example, you said it was 20 below celsius, I think you meant 20 degrees below standard, but whatever. That will increase the available power a little as the air is more dense.
To answer your question, no increasing the rpm by 200 will not get you 23% more power. You can't get 100% of BHP at 2000 feet at 20 below standard. I would give you a wild guess that the most you could get in that condition would be about 81-82%, maybe less. Great question, though.

 

[tarp]

Dayshift:

The engine in your airplane is "rated" at 180hp at 2700. This is the theoretical "best" number (100%) if your engine's driveshaft was connected to a dynamometer or "prony brake" (hence the term BHP or Brake Horsepower).

Unfortunately, in your top of the line Cessna, the engine is connected to a propeller (and a constant speed or variable pitch one at that). Due to the size, shape and twist of the propeller the theorectical "best" performance is to "translate" 80-85% of the BHP into THP or Thrust Horsepower.

However, Props are only at their most efficient for relatively small pitch angles and small ranges of airspeed. i.e. if you tried to make a climb prob (small angle of attack) give you max forward velocity, you are just churning a lot of good air but not getting anywhere fast and you are definitely not efficient. Therefore, the propeller angle of attack must match the speed of the airplane to be as efficient as it can be. Think of how different boat props, cooling fans and your airplane prop look. There are tradeoffs for every design.

The chart, gives you the best approximation of the best power you can get out of your Cutlass. And yes, You might be able to force 25 inches of manifold pressure and 2700 RPM out of the thing and get something like 80-82% of BHP out it.

The engine is working flat out - 100% of the 180 horses - the net result is that you are only translating that into 80% or 144HP worth of THP.

When you follow the chart, it gives you the net result that you are flying at - you will never get 100% THP. And I would gather that there is only a very small window when you get 100% BHP. Obviously as density altitude increases, the engine and prop get less efficient and the only way for the percentage number to go is down.