GPH?

[N941WR]

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.)

[AltonD]

My O-300 in my C-172F get 8.0 gph @ 75% on the hobbs, 9.2 gph on the tach.

[dan]

Originally Posted by N941WR

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

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?

[dennis]

help me with that abreviation will you?
thanks dennis

[dan]

WOT = Wide open throttle

[Mel]

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!

[hevansrv7a]

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.

[Ironflight]

Originally Posted by hevansrv7a

.....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......

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!

Paul

[erich weaver]

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

[hevansrv7a]

Originally Posted by Ironflight

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!

Paul

Paul - How are they computing it? Backing into it from Pct. Pwr which is based on MAP and RPM and altitude plus fuel flow?