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Power percentages

K

kleindoc

Member
Hi All,

Phase I flight testing goes well. 9 hours on the clock.

Can someone tell me whether it is possible to guesstimate power percentages in an aircraft with a fixed pitch prop and no manifold pressure information? 992SB has an O-320 in 150HP.

If the throttle is fire-walled at 8,000 feet would that be about 80% power? Can I do a proportional equation to calculate the number of RPM's I'd need for 60? power? How can one calculate power output at different altitudes?

kleindoc
 
Fuel Flow?

kleindoc said:
Hi All,

Phase I flight testing goes well. 9 hours on the clock.

Can someone tell me whether it is possible to guesstimate power percentages in an aircraft with a fixed pitch prop and no manifold pressure information? 992SB has an O-320 in 150HP.

If the throttle is fire-walled at 8,000 feet would that be about 80% power? Can I do a proportional equation to calculate the number of RPM's I'd need for 60? power? How can one calculate power output at different altitudes?

kleindoc
Click to expand...
Do you have a fuel flow instrument?
 
kleindoc said:
... Can I do a proportional equation to calculate the number of RPM's I'd need for 60? power? How can one calculate power output at different altitudes?

kleindoc
Click to expand...
RPM is a function of power, pitch, and the airframe. Thus a small engine with the wrong pitch and a draggy airframe can allow the engine to over speed. Conversely, a large engine on a slick airframe with the wrong prop may not spin up enough.

You will need MAP, RPM, OAT, DA, and FF, if ROP. Then look up those numbers in the Lycoming engine operating manual for your engine to find your PP.
 
Last edited:
For WOT, a formula for percentage of engine power based upon density altitude and temperature is (1-6.88E-06 X DA)^4.756. Then you multiply that by the actual rpm divided by the rated rpm. Unfortuneately, because you don't have a MAP you have no way of knowing your induction drop to see any excess drop which reduces power somewhat.
 
I have a spreadsheet that calculates percent power from RPM, MP, FF at both rich and lean settings.

The link is at the top of the page here.

Caveat Emptor

Vern
 
kleindoc said:
No; I don't have fuel flow. I am a firm believer in the KISS principle.
Click to expand...
It sure seems that you are kicking around quite a complex analysis of your power settings if your intentions are to follow a KISS principle. Not sure you will get your answers without getting into quite a bit of complex analysis that may require more than can be handled by rudimentary instrumentation. Perhaps you may be able to make some guesstimates that could get you close. Maybe that will be enough for the KISS system. But it would lend itself for a query into how closely you intend to follow those guesstimates. It may require you to operate with a larger margin of error because of the lack of precision.
 
kleindoc said:
No; I don't have fuel flow. I am a firm believer in the KISS principle.
Click to expand...

Your choice but having fuel flow (and fuel remaining) along with GPS derived time to info makes running out of fuel, or worrying about it, essentially a non-event.
 
kleindoc said:
Can someone tell me whether it is possible to guesstimate power percentages in an aircraft with a fixed pitch prop and no manifold pressure information? 992SB has an O-320 in 150HP.

kleindoc
Click to expand...

I guess the answer is NO; Nobody came close to suggesting RPM values. All I want is for someone to tell me to fly @ ??RPM if I want to save gas or ???if I need to boogie. I really don't care much about imaginary precision.

Suggesting that I need fuel flow information to avoid fuel exhaustion isn't what I was after. God save us from bad decisions, but a dipstick and some common sense is all that is really necessary. (992SB does have a Dynon Flight Dek 180; just no manifold pressure sensor).
 
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If you throttle is wide open you should be able to roughly calculate your MP based on sea level barometric pressure less altitude loss.
If your prop allows the engine to turn 2700 RPM straight and level at 1-2000 feet above sea level,
my guess would be roughly the following based on other 150HP O-320 FP installations:
Press Alt 8K roughly 20 degrees below std temp. decrease power by approx 5% for every 20 degree increase in ambient temp .
2600 76%
2500 68%
2400 61%
2300 55%
2200 50%

6K
2500 72%
2400 65%
2300 57%
2200 50%

4K
2500 76%
2400 68%
2300 60%
2200 55%
2100 48%

2K
2400 72%
2300 65%
2200 56%
2100 50%

Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
Fuel consumption is mostly a function of THP

kleindoc said:
I guess the answer is NO; Nobody came close to suggesting RPM values. All I want is for someone to tell me to fly @ ??RPM if I want to save gas or ???if I need to boogie. I really don't care much about imaginary precision.

Suggesting that I need fuel flow information to avoid fuel exhaustion isn't what I was after. God save us from bad decisions, but a dipstick and some common sense is all that is really necessary. (992SB does have a Dynon Flight Dek 180; just no manifold pressure sensor).
Click to expand...

You could use fuel consumption to estimate power. That is why I asked. You can determine fuel flow at a given RPM, mixture and density altitude by using a tank-switching method and a stop watch then measuring how much it takes to refill the test tank. Those with flow instruments use this same method to calibrate them.

Since your purpose is now restated to be about fuel economy, you would be better off determining the drag curve of your airplane. Your BHP will be controlled by your THP with some of it wasted as prop inefficiency. Prop efficiency will vary, but first you want to know your drag curve. Mixture is obviously relevant, too. Since you don't have FI, a "lean mixture" will give you about 0.44 SFC - at least that's what you can discover from Cessna's performance charts. A best power mixture will probably give you an SFC of about 0.50. Using this information you can estimate power as well as find fuel consumption.

To determine your own drag curve, follow the instructions in the links found on my home page (below). PM or email for coaching if you wish.

In general, the optimal economy cruise will be at 1.32 x V for drag min (CAS). That is called Carson's speed. Your absolute best mpg will be at the V for drag min unless wind is a large factor and then it will vary by only a few mph. Your maximum speed's fuel consumption will be determined (as noted above) by whether you can get to your max rated RPM and by density altitude and temperature.

As you may have noted from this comment, the fuel economy will be more about speed and altitude than about RPM (lacking MP info).
 
Thanks to all!

Special thanks to you, Mahlon; your RPM chart was exactly what I was looking for. I won't hold you to your figures.
 
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