Fuel Burn/Percent Power Question

[Don]

I don't think this is a stupid question but it may be an ignorant one. If it is, ignorance is curable and I'd appreciate the education.

I've been flying my 9A for about 6 weeks now and trying to explore just what the plane can do both in the air and in terms of performance. I've never flown with such great technology (D-100 and a VM1000C). The economy of the plane is mind boggling compared to my Cherokee 140. In the Cherokee I always expected between 8.5 and 9 gph fuel burn and 97 knots. I'm finding the RV-9A is giving me about 145 knots (I need to nail this number down better) at 6,500-7,500' with a fuel burn *under* 6 gph. My 9 has an O-320 and a FP Sensenich prop and I am leaning LOP.

Let's be clear - the 9A is a bit better than 50% faster on 30% less fuel. If that isn't clear enough, my wife who pays the fuel bills can tell the difference!

According to the POH I was supplied, 65% power is achieved when MP + (RPM/100) = 45. Every change of 3 in the number adds or subtracts 10% power (42=55%, 48+75%, etc). So, my first question is to learn if this is a reasonable rule of thumb for figuring % power, or not.

My second question regards how MP is measured and whether there needs to be any calibration of the equipment. The plane came with a VM1000C installed. The MP and RPM values are reported on the instrument and this is what I'm using to calculate % power. When I turn the VM1000 on with the engine off, it is close to barometric pressure but so far never exactly what ASOS is reporting. One odd thing that I don't know what to make of is the VM1000C manual says I can switch between MP and HP by holding button one down. My VM1000C only shows MP and I can't get HP to show. I'm not sure if this means anything or not.

If I were to guess what % power I'm using looking at the Lycoming charts in the Cherokee POH, which is also based on an O-320, albeit a 150 hp not 160 hp, I'd guess I'm seeing 55 or maybe 60% power. The fuel burn is a bit better than 55% power with the Cherokee 140 and if you compare engine RPM to density altitude, I'd guess I'm closer to 60% power. When I look at Van's performance specs I see 167 mph (darn near exactly 145 knots) is what you should get at 55% with the O-320 (160 hp) engine.

I'm basing my fuel burn by dividing gallons added (starting and ending with full tanks) by my Hobbs meter, which seems remarkably close to actual clock time. For example, my last 5 flights consisted of 7.3 hours of flying and I burned 43.74 gallons of fuel. One hour was practicing landings and the rest was XC flown mostly at 6,500-7,500 and at 65% as determined above (MP+RPM). A bit over an hour was flown at 75-80% (MP +RPM). 43.74/7.3 = 5.99 GPH. On several occasions I have seen 5.5 GPH (or even a bit less) when just doing XC flying.

I guess the bottom line question is, how do I figure what % of power I'm actually using. I am beginning to doubt my POH's method of calculating power or, if the method is correct, how do I verify the sensor?

Whatever the result, I am simply astounded at the economy of this plane.

[IowaRV9Dreamer]

Hi Don - you'll get tons of responses. Your post is timely as I've been emailing with Mike Busch about just this question. BTW - the EAA has a bunch of his webinars online and they are very interesting.

Here are some words that Mike sent me. Note that these are for a larger engine (IO-360 200 HP):

The POH charts are predicated on the assumption that you're operating the engine ROP. When operating ROP, you've got more fuel than necessary (by definition of ROP), and so power is a function of mass airflow through the engine (i.e., pounds of air per minute). In turn, mass airflow is the product of MP and RPM. MP determines how much air goes into a cylinder during each combustion event. RPM determines how many such combustion events occur per unit of time. So MP x RPM determines power output. And your POH tables are based on MP and RPM.

When running LOP, things are totally different. You've got more mass airflow than necessary (by definition of LOP), and so power is strictly a function of fuel flow. MP and RPM have nothing to do with it any more. Your Lycoming IO-360-A2B or -A2D has a compression ratio of 8.7-to-1, so when LOP HP is simply FF in GPH multiplied by 15. At 9.5 GPH LOP, you're making 9.5 x 15 = 142.5 HP, or 71% power (142.5/200).

I asked Mike where the "15" came from. It is basically horsepower / fuel flow. Here is what he told me:

The 15 was a quick estimate. For 7.5-to-1 engines, the number is 13.7, and for 8.5-to-1 engines the number is 14.9. Yours is an 8.7-to-1 based on the TCDS, so I tossed out 15, although it might really be 15.2 or 15.3.

I'm not sure what the compression ratio is of your engine, but I sure like the idea of a simple relationship between HP and Fuel Flow when lean of peak.

BTW - I envy your fuel flows! I've been flying that engine LOP and 8.5 gph is about as low as I want to go. Of course I had been burning 12!

Good Luck and I hope this helps you.

[captainrichhill]

I have an RV6A with O-320 160 HP Fix Prop
I filled up just out side of Atlantic City, NJ and landed in Jacksonville, FL 4:40 mins later and burned 33 Gals (i had 56gals to start with tip tank mod). I could have made it on regular Thats 7.07/hr at 2200rpm at 5000msl.

These planes are very efficient machines. Better gas mileage then my Jeep Grand Cherokee at 3x the speed and 50x the fun.

I hope this helps some.

Rich

[BobTurner]

MP with the engine off should be less than ASOS altimeter setting, unless the airport is at sea level. Roughly 1" of mercury less for every 1000' of elevation.

[Don]

Thanks Dave. That's another data point for saying I'm running about 45% power.

I'm still working through the ideas you posted but years ago I heard something to the effect that most gas powered engines burn 0.42 (or 0.43 - not certain) pounds of fuel per horsepower per hour. So if 104 hp is 65% of 160, you'd expect the engine to burn 104 x 0.42 = 43.7 pounds of fuel per hp @ 65%. 43.7 lbs. /6 lbs./gal = 7.3 GPH. I think Mike Busch is using this figure of pounds of fuel per hp to figure percent power....but it's been a long day and I need to chew on it a bit more.

[Don]

Originally Posted by BobTurner

MP with the engine off should be less than ASOS altimeter setting, unless the airport is at sea level. Roughly 1" of mercury less for every 1000' of elevation.

KXSA is at 135' MSL, so I'm guessing the difference with MP will be about .14" off and from memory, that's close to order of magnitude of the difference I've seen. I'll pay attention the next time I'm out.

Thanks.

[Vac]

Don,

Lycoming publishes power charts for some of the different versions of O-320. These are in the engine owner/operator's manual and will provide the data you're looking for. Low power cruise 45-50% is doable in RV's. Average (historic) fuel burn for the O-320 in our -4 is just a shade under 7 GPH. I record data for each sortie based on actual consumption.

Lycoming refers to their graphic data presentations as "curves." If you can find the curve for your engine or a similarly configured O-320, it's where you'll want to start. There are also curves that depict SFC.

Incidently, I've found "the rule of 48" to be fairly accurate. If you look in the POH for our plane posted in the POH tab of the site, it has the curve for our engine and it's also got tabular data derived from the Lycoming curve. Good starting point if you've got a carbureted -320 B or D. If you have any questions, drop a PM.

Enjoy the low fuel bills!

Vac

[woodmanrog]

I love these discussions about fuel burn/economy etc. But, as my hanger partner asked me the other day, why do you own an airplane that is made to go fast and then go slow? Even at 2500 rpm with a fixed pitch prop we are only burning 8-9 gph while cruising at about 155 knots. I am now flying faster. I also find that the engine (0360 carburated) likes it better. Less vibration and I hear it saying "wooopeee".

[DanH]

Originally Posted by Don

So, my first question is to learn if this is a reasonable rule of thumb for figuring % power, or not.

I'd suggest obtaining the Lycoming power chart for your engine and comparing factory settings to the rule of thumb. There should also be a fuel flow vs power chart.

My second question regards how MP is measured and whether there needs to be any calibration of the equipment.

Engine off, VM1000 on. Set an accurate field elevation on your altimeter, note the altimeter setting in Hg, compare with indicated manifold pressure. That said, realize your altimeter is subject to calibration error just like the manifold pressure gauge. Was your altimeter accurate at the recent pitot-static and transponder check?

Originally Posted by IowaRV9Dreamer

Here are some words that Mike sent me. Note that these are for a larger engine (IO-360 200 HP):
The POH charts are predicated on the assumption that you're operating the engine ROP. When operating ROP, you've got more fuel than necessary (by definition of ROP) ......

That's not an entirely accurate statement.

The Lycoming power chart is based on best power mixture, not merely rich of peak. ROP is a very wide range of possible mixtures. Best power is a specific setting, roughly 100 ROP. At this setting you have exactly the amount of fuel necessary for maximum output.

[rocketbob]

Dan is spot-on. What Mike Busch is telling you as far as LOP fuel consumption is not accurate since you are losing a bit of power to gain a bit more efficiency. Power is only a function of fuel flow running at the best power setting.

I think one could come up with a simple formula based on fuel flow if it was based on a coefficient from the best power setting. In other words, fuel flow * some number * how far away from best power you are in EGT.