N941WR

N941WR

Legacy Member
I'm very confused by the GPH numbers I have read on this forum.

Dan (and other's), please add your $.02.

My O-290-D2 manual states it has a 75% power fuel burn of something like 7.1 GPH. I seem to remember an O-320 powered C-172 I used to fly burning 9.6 GPH.

Am I being naive in that the engine doesn't know what airframe it is in and that 75% power and the resulting fuel burn should be the same for the same engine?

Are people not flying at a 75% power, but 65 to 70%?

I understand the advantages of fuel injection and electronic ignition over carbs and mags, is that what is causing the differences? (My rule of thumb is 5% for each.)
 
N941WR said:
Are people not flying at a 75% power, but 65 to 70%?
Click to expand...
Exactly. And to be honest, I don't give a **** what "PERCENT" power my engine is putting out.

When I travel (not in formation), I run WOT. I climb up to an altitude that presents the least amount of compromise, if not the most advantage. For me that means finding a "sweet spot" in terms of MPH & MPG, in that order, but the priority of each is very close to equal.

So on any given day that might mean 10,500' MSL, maybe 16,500' MSL. Whatever. Whatever gives me the best "performance" in my mind. And the priority of each flight might be different. If I intend to meet somebody at the destination at a specific time, controlling MPH is the priority. If Jen is with me, then MPH is usually a relatively high priority. Otherwise, MPG often becomes a higher priority, and I start seeking out optimal conditions. If I have my O2 on board, I might climb up in search of the sweet spot.

I run LOP literally ALL of the time in cruise when I travel. I can count the exceptions (ROP flights) during the past let's say 500 hours on one hand.

At any given MP/RPM setting, in my airplane the difference in fuel burn between ROP & LOP is on the order of 2-2.5 gph. Which at a minimum is a 20% improvement (often more), at the sake of roughly 10% speed loss.

If I'm running 65% ROP vs. 65% LOP, the difference in fuel burn is less...on the order of 0.5 gph like you mentioned.

But like I said, I always cruise at WOT. So it's not like I'm going LOP, then throttling back up to recover the lost power/speed. I run WOT and LOP and come what may speed-wise...which is why I employ "altitude hunting" to my advantage.

Is this answering your question at all, or am I just rambling?
 
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One minor detail. Most of my cross country flying is above 8,000' where 75% is not avialable. I usually cruise at between 60 & 65%. I'm cheap and like to fly. I'm not in that big of a hurry. If I need to get somewhere in a hurry, then I can push it up. That's one of the beauties of the RV. You can cruise by the airspeed or by the fuel flow. BTW, the numbers in the Lycoming manual are pretty much right. You will burn "X" amount of fuel for "Y"% power. There's no free lunch!
 
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Let me be a little pedantic..

Much of this is swiped from experts on this forum, all of whom are invited to correct me. I am trying here to succinctly restate what has already been well covered.

Percent power is a concept with poor definition especially when we try to infer it from MAP and RPM. Let's assume, however, that it is a HP number that can be computed from the rated HP. So a 180 HP engine is at 75% when producing 135 HP (assuming it really produces 180). I have noticed that various sources will give different MAP and RPM numbers for the same pct. power on the same configuration such as IO-360. I have no clear explanation for that.

How much fuel will it burn at 135 HP or any HP? That depends entirely on BSFC - Brake Specific Fuel Consumption. The BSFC is usually expressed on these pages in terms of pounds of fuel per horsepower per hour. My Superior XP-360 manual says the number is 0.50 for 50 degrees Rich of Peak, .43 for peak power (slightly rich of peak). Walter Atkins says my 8.5:1 pistons will give me 0.40 BSFC when running 40-50 degrees lean of peak. Of course, if your engine is not working at design efficiency, the BSFC will be higher to some unknown extent. My 180 HP engine at 75% at 50 LOP will/should burn, therefore, 9GPH approximately. I used the rough value of 6 pounds of AvGas per gallon. Walter also says that when LOP, only fuel flow determines HP, not RPM nor MAP.

Note that none of this has anything to do with airframe or prop. However, the airframe and prop, along with environmentals determine how fast you will go on a given HP because that determines how well HP is converted to thrust and how much drag opposes it.
 
Data confirms....

hevansrv7a said:
.....The BSFC is usually expressed on these pages in terms of pounds of fuel per horsepower per hour. My Superior XP-360 manual says the number is 0.50 for 50 degrees Rich of Peak, .43 for peak power (slightly rich of peak). Walter Atkins says my 8.5:1 pistons will give me 0.40 BSFC when running 40-50 degrees lean of peak......
Click to expand...

I will add that one of the cool features we got in the latest GRT software release was real-time computation and display of SFC! I have found that I generally get it stable at 0.39 - 0.41 in cruise, leaned to roughness and then smoothed out just a tad. This corresponds to the point where all my EGT's (graphically represented) are within a few degrees of each other.

In ohter words....the science and theory actually works! :D

Paul
 
Dan C's philosophy above is layed out in detail in John Deakin's article "Where Should I run My Engine, Part 3, Cruise", which is available on the Avweb site (go to the "News" tab, then "columns", then find "Pelicans Perch" on the right hand side, scroll down and go to "more Pelicans Perch", and you will see the article listed there. A lot of other great stuff in his articles as well. You may have to sign up on Avweb to download the articles, but you can decline to receive their various e-mail services, as desired.

I have read and re-read these articles and highly recommend them. Deakin's bottom line is: Forget about power settings. Figure out what your mission is with respect to speed vs economy for this specific flight, then set your RPM and MAP to accomplish this goal while respecting engine operating limits.

erich weaver
 
Ironflight said:
I will add that one of the cool features we got in the latest GRT software release was real-time computation and display of SFC! I have found that I generally get it stable at 0.39 - 0.41 in cruise, leaned to roughness and then smoothed out just a tad. This corresponds to the point where all my EGT's (graphically represented) are within a few degrees of each other.

In ohter words....the science and theory actually works! :D

Paul
Click to expand...
Paul - How are they computing it? Backing into it from Pct. Pwr which is based on MAP and RPM and altitude plus fuel flow?
 
Without a dyno test of a particular engine down to the same pistons, CR, cam, ignition, timing etc. at different AFRs or EGTs, no reliable hp figure and resultant SFC number is available. Maybe a ballpark figure SFC. Hp falls off considerably LOP say 16 to 1 AFR compared to best power mixture of around 13 to 1 AFR.
 
How They do it?

Well, to be honest, I don't know exactly how GRT is computing Percent HP and SFC, but based on the fact that we input a series of HP/MAP/RPM numbers into a table, I have always assumed that it is basically a curve-fitting exercise. Fuel flow is, of course, measured using the fuel flow transducer.

The data for the engine is based on the Lycoming graphs, and since my engine is 100% stock (from a performance standpoint), and the Lycosaurus has been around long enough to have a lot of operational experience, I expect that the numbers are about as good as the instrumentation accuracy (and one's ability to read the data of the graph, given line thickness).

The Fuel Flow transducer is pretty accurate, based on over 300 hours of flying and very careful record-keeping of fuel loads.

Now Ross, you are correct if you are looking for absolute precision - you need laboratory conditions. But within the accuracy of production equipment and field measurements, and with HP values being verified by flight test data (the RV-8 airframe performance is pretty well known), I'd say that the numbers aren't bad...I usually assume that %HP is good to about +/- 2 or so, and the the resultant SFC's are about what I would expect.

Not laboratory precise - but then, the sky isn't the laboratory. Cross-country experience has shown the values to be good to a few percent.

Paul
 
Paul - Does the HP displayed on the GRT EFIS change when you change the mixture, without changing the rpm or MAP? If not, then they are not accounting for the power changes when you go leaner or richer than mixture for best power, and the SFC numbers are garbage.
 
N941WR said:
I'm very confused by the GPH numbers I have read on this forum.

Dan (and other's), please add your $.02.

My O-290-D2 manual states it has a 75% power fuel burn of something like 7.1 GPH. I seem to remember an O-320 powered C-172 I used to fly burning 9.6 GPH.

Am I being naive in that the engine doesn't know what airframe it is in and that 75% power and the resulting fuel burn should be the same for the same engine?

Are people not flying at a 75% power, but 65 to 70%?

I understand the advantages of fuel injection and electronic ignition over carbs and mags, is that what is causing the differences? (My rule of thumb is 5% for each.)
Click to expand...

Bill, if your question has not been answered so far, here is another cut on the subject.

There has to be 2 base assumptions to come close to clearifying and answering the questions.

Brake Specific Fuel Consumption (BSFC) has to be stated in the formula. BSFC = lbs of fuel per hour per HP. For Lycoming engines, you can find from .390 to .450 stated on the internet by various authors. The number is about the same be it a 0235 or a 0540 or a 0290 or 0320.

The other number is the weight of the fuel, which varies by grade, density, and probably other factors I am not aware of.

For the sake of auguement, lets use a BSFC of .435 and weight at 6 lbs per gallon.

If your 0290 is burning 7.1 gph at 75% power, it is producing 97.9 HP.
Formula: Lbs fuel per hour per HP/BSFC = HP. (42.6/.435 = 97.9)

If your 0320 is burinig 9.6 gph at 75% power, it is producing 132.4 HP.
Same formula, different numbers. (57.6/.435 = 132.4)

At 100% power, your 0290 will produce 129.2 HP and burn 9.3 gph.

At 100% power, your 0320 will produce 176 HP and burn 12.8 gph.

(The fuel burn you state for the 0320 looks more like a 0360 at 75%. That's not to say the 0320 won't burn 9.6 gph, but not at 75% if properly leaned. The 0320 burn would be more like 8.7 gph at 75%. Fuel injection or carbs have no bearing on BSFC)

The reason the engines are not producing the same power or burning the same amount of fuel at 75% is that they are not of the same displacement. The 75% power rating is different because the 0320 is a larger engine, capable of more HP, and requires more fuel to produce that HP.

dd


 
Kevin Horton said:
Paul - Does the HP displayed on the GRT EFIS change when you change the mixture, without changing the rpm or MAP? If not, then they are not accounting for the power changes when you go leaner or richer than mixture for best power, and the SFC numbers are garbage.
Click to expand...


Yes Kevin, %HP does change if you hold RPM and throttle position constant. Setting a particular power for cruise means zeroing in a little bit at a time with both throttle and mixture.

I am not going to get into a long discussion on how they do it - I've already said that I don't know the details of the algorithms, and can't intelligently add any more facts to the discussion. What I can tell you is that the numbers over many cross-country flights work out to be very close - at least close enough for flight planning and the like.

Cheers!

Paul
 
David-aviator said:
Bill, if your question has not been answered so far, here is another cut on the subject.
Click to expand...
Actually, I knew the answer, I was just provoking the discussion based on something Dan posted on another thread.

In short, all these crazy low GPH numbers that have been tossed out are BS. It all comes down to how you fly your engine, which is what I had thought.

Running an F.I. engine LOP seems to really help the burn rate, which is something I will not be able to do with my "little" engine.

As for the % power on the O-290-D2, I'm need to re-read the book. This engine is good for 140 hp @ 2800 RPM w/ a five minute limit and 135 HP at 2600 RPM. Thus 75% power should be a little over 101 HP (Hey, I gotta take every single HP I can get!) based on the 135 HP number. Still, those few HP are not really going to be noticeable.

My big question will be, how much of an impact on power and GPH will the duel P-mags have. Based on my experience dyno tuning programmable ECU's in cars, I'm expecting around a 5% gain.
 
Bill,

I sent you an e-mail with an excel spreadsheet that should let you play around with some of these questions and get quick answers. I didn't write it, but have no reason to doubt it. This one came from a more mathmatically enlightened RV-er than me. Just sharing the love.

Best,
 
Kevin Horton said:
Paul - Does the HP displayed on the GRT EFIS change when you change the mixture, without changing the rpm or MAP? If not, then they are not accounting for the power changes when you go leaner or richer than mixture for best power, and the SFC numbers are garbage.
Click to expand...

Now here is a smart lad.

Sounds like maybe they've used pretty valid data then and since you are stock, the SFCs displayed might be pretty close. Cool!
 
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