Calculating Engine Performance

AKBill

AKBill

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AKBill
I have totally confused myself. OK POH states standard day 2500ft, 2700rpm my engine produces 82% of rated bhp. So 82% of 150bhp would be 123bhp.

Today barometer is 30.46in, temp is -11C, dew point is -18C. That makes the pressure altitude -494.5ft and density altitude -3853.4ft.

How do I calculate the rated horsepower for the current conditions at 2500ft AGL?
 
Figure out the density altitude at 2500 pressure altitude. Then go into the tables. It might be slightly higher than sea level but.
 
AKBill said:
I have totally confused myself. OK POH states standard day 2500ft, 2700rpm my engine produces 82% of rated bhp. So 82% of 150bhp would be 123bhp.

Today barometer is 30.46in, temp is -11C, dew point is -18C. That makes the pressure altitude -494.5ft and density altitude -3853.4ft.

How do I calculate the rated horsepower for the current conditions at 2500ft AGL?
Click to expand...

At your field, DA = -495'. So at 2500 agl, you're flying at 2500 - 495 + Field Elevation. Look up the next hughest a d next lowest altitudes in your Performance Tables and interpolate.
 
Found this from Lycoming. Still scratching my head a little..:)
 

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Don't worry about it. Firewall the engine and be happy with the extra power you get, it won't hurt anything.
 
Most GPS units have a density altitude calculator. Use it or a whiz wheel or an electronic E6B to calculate density altitude. Then use the POH charts to figure your horsepower at that density altitude. I dont think its good for an engine to run it at full throttle if the density altitude is below sealevel. So dont firewall it in those conditions, even for takeoff.
 
My assumption is that the first altitude on his chart is for 2,500’.

We know that at SL on a standard day a normally aspirated engine can produce 100% rated power.

I think that establishes a trend line, a loss of 18% power every 2,500’, and implies a similar gain “below” SL DA’s.

Assuming a roughly straight line increase in power, at minus 2,500’ DA you would have 118% power, at minus 5,000’ DA you would have 136% power. Interpolating for your calculated DA of minus 3,853’ should give you your answer.

All figures in MSL. You asked about AGL. You’d have to convert that to MSL but the same idea applies.

I think. This is just off the top of my head.
 
AKBill said:
How do I calculate the rated horsepower for the current conditions at 2500ft AGL?
Click to expand...

Rated horsepower is a number in a book. It doesn't change with altitude or pressure.
Actual delivered horsepower depends on manifold pressure, RPM, mixture, temperature in the intake manifold, and humidity. The book doesn't give you enough information to do an accurate calculation.
 
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FastEddieB said:
My assumption is that the first altitude on his chart is for 2,500’.
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The possibility of a manufacturer doing so is mind-boggling, since more than 1/3 of the U.S. population lives along the coast (East, West and Gulf). To say nothjng of the millions more who live further inland but still at low msl altitudes (my airport in WV, on the bank of the Ohio River, is at 567 msl).

To not include a page like this would be estupido!

Screenshot_20180204-111203.jpg
 
Hank S said:
To not include a page like this would be estupido!
Click to expand...

I had to check...

My Sky Arrow has SL cruise performance data.

The Cirrus does not:

cruise1.png
 
coloradobluesky said:
I dont think its good for an engine to run it at full throttle if the density altitude is below sealevel. So dont firewall it in those conditions, even for takeoff.
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Curios why you would not use full throttle with DA below seal level. All good reading thanks. Just wondering with a fixed pitch prop, would it be a waste of time to install a manifold pressure gauge? I would think it would give some information as to how efficient the engine is running.

edit: here in SE AK a lot of my flying is with DA below sea level, don't know anyone that does not use full throttle
 
coloradobluesky said:
I dont think its good for an engine to run it at full throttle if the density altitude is below sealevel. So dont firewall it in those conditions, even for takeoff.
Click to expand...
I actually asked this on the Beech Aero Club site the other day. I was re-calibrating my JPI engine monitor recently, and setting up warning triggers and noticed that on takeoff I was exceeding the operating range for manifold pressure shown in the POH (I live at sea-level). It seemed like most of the pilots there weren't concerned about it because it goes down so quickly after climbing to altitude, but I'd be interested in other opinions. I think I'm seeing "30.1" on takeoff, but the max operating range is 28.7, I believe.
 
Challenged said:
I think I'm seeing "30.1" on takeoff, but the max operating range is 28.7, I believe.
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Do you have a constant speed prop? What do you fly Sundowner/Serra? I bet there are a few BAC members in your area. I've never been able to hook up with the club events, just to far for me to travel right now.
 
The only way I know to calculate horsepower is from fuel burn. A rule of thumb (for non turbo gasoline engines) is 12hp/gallon/hour.
So 180 hp engine burns 15 gph (at 180 hp).
At 75% power, it is (180/12) * .75 = 135hp, which burns 15*.75 = 11.25 gph.
It is close, but not exact.
 
Challenged said:
I actually asked this on the Beech Aero Club site the other day. I was re-calibrating my JPI engine monitor recently, and setting up warning triggers and noticed that on takeoff I was exceeding the operating range for manifold pressure shown in the POH (I live at sea-level). It seemed like most of the pilots there weren't concerned about it because it goes down so quickly after climbing to altitude, but I'd be interested in other opinions. I think I'm seeing "30.1" on takeoff, but the max operating range is 28.7, I believe.
Click to expand...

If this were a classic car and a British engine, then anything above 95% of max power will definitely cause it to blow up.
If it were a German engine then 100% is OK and 100.0000000001% will definitely cause it to blow up.
If it were an American engine then 100% is OK and 110% may cause it to blow up.
If it were an Italian engine then 100% is OK and anything over that will cause the manufacturer to rewrite the spec.

I'm guessing Lycoming/Continetal engines are like classic American engines in that infrequently brief periods of 4.8% over max operating range doesn't have much effect.
 
coloradobluesky said:
The only way I know to calculate horsepower is from fuel burn. A rule of thumb (for non turbo gasoline engines) is 12hp/gallon/hour.
So 180 hp engine burns 15 gph (at 180 hp).
At 75% power, it is (180/12) * .75 = 135hp, which burns 15*.75 = 11.25 gph.
It is close, but not exact.
Click to expand...

Scroll up a few posts: my 180 hp engine burns 18.2 gals at 100% . . . 10 hp/gal.

Looking at the 2400 RPM section, it shows 14.1 gph for 79.8% (143.6 hp), and 9.5 gph for 75.5% (136 hp) to go 3-4mph less. So its 10 hp/gal at high power, and over 14 hp/gal at cruise power.

So yeah, 12 hp/gal is a good ballpark "average", but I don't know people who run >75% power for very long . . . And without that, the estimate is rather high on fuel burn.
 
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