[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]

RV6A O-320 160HP

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]

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]

Lycoming Power Chart

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]

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?

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.

 

[Don]

Dan (Rocket Bob) - great advice. I've been pouring over the charts. Clearly I'm running 50-55% power, which is good to know. I now have 2 or 3 new questions, which I think will be best answered in the air. I know there will be a lag in my flying over Christmas vacation but when I get back I'll start collecting data. It will probably be helpful having a few weeks to think about what I want to do in the air. At this point I am planning a series of runs to measure speed at various engine settings using the three direction method I've seen posted here several times (perhaps by you). Then I want to run those same settings in cruise and measure fuel burn. Then I'll know, for my plane, for my engine, and for all the equipment errors I have, what speed vs fuel burn I actually have. Maybe I can get that from the charts but a) it won't be as much fun, b) my knowledge will be first hand and more intimate - I'll know it, not just remember it, and c) it will be form my plane and instruments and their idiosyncrasies.

If I still have questions, I'll post them.

Woodmanrog - To answer your question, I think we all fly for our enjoyment but we enjoy different things. I see DR, Sam Buchannon, and others talking about flying Cubs, the Legal Eagle and other low and slow planes. It's different than flying an RV but at least for them, just as enjoyable. I see folks like Bob Axsom (anyone heard from him lately?) and for him its about speed and he has posted he sees no other reason to fly. I am probably somewhere in the middle and identify most with a guy like Kent Passer who wrote Speed With Economy and speaks at Oshkosh.

As I look a my mission profiles, most of my XC flights are between 200 and 500 nm. Just going to visit a friend in NC (215 nm), I can save 4 gallons of fuel by slowing up 20 knots and the difference in flight time is a shade less than 10 minutes. I don't notice either the 20 kts at 6,500' nor the 10 minutes (which I look at as bonus flight time) but I do notice the $20 saving in fuel.

I am not, and never would say I'm right and your wrong on this topic. I am saying we all fly for different reasons and for me, I want to know the full profile of my plane - fast, slow and in between and then I like finding the speed and economy that works for me...and let me add, I like fast, too (just not exclusively).

 

[erich weaver]

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.

Bob: I'm a big fan of the Deakin articles as well as Mike Busch's articles and webinars. What Dan stated was a slight refinement of what Mike Busch is reported to have told the original poster, and is consistent with previously published Deakin/Busch teachings. However, what you say in the last sentence of the quote above entirely contradicts what Deakin /Busch preach. Power is a function of fuel flow when LOP, not at best power setting, which is ROP. Do you really disagree with this?
Regards,
erich

 

[DanH]

What Dan stated was a slight refinement of what Mike Busch is reported to have told the original poster......

No, it was a polite correction. The reported statement was....

"When operating ROP, you've got more fuel than necessary (by definition of ROP)"

....which the impolite would call bull( ).

The same can be said for purple zones depicting "ultra abusive mixtures"......but that would be thread drift.

 

[erich weaver]

No, it was a polite correction. The reported statement was....

"When operating ROP, you've got more fuel than necessary (by definition of ROP)"

....which the impolite would call bull( ).

Well, you've now gone from " not entirely correct" to "bull". Stop sugar coating it and tell us what you really think. . Seems to be a touchy subject. I think you may be hanging an awful lot of importance on the use of the word "necessary" in the quote from Busch. I tend to think Mike does understand what ROP means and we should hold off on the crucifixion for now.
Best regards
Erich

 

[rocketbob]

Bob: I'm a big fan of the Deakin articles as well as Mike Busch's articles and webinars. What Dan stated was a slight refinement of what Mike Busch is reported to have told the original poster, and is consistent with previously published Deakin/Busch teachings. However, what you say in the last sentence of the quote above entirely contradicts what Deakin /Busch preach. Power is a function of fuel flow when LOP, not at best power setting, which is ROP. Do you really disagree with this?
Regards,
erich

Yes I do disagree. Its pretty simple. At LOP you are running below best power at that prop/throttle setting (hence a reduction in speed), so its not accurate to say that your fuel flow at some LOP number is a percentage of best power (100% power).

If you read Deakin's articles, he always prefaces the LOP fuel flow #'s by fuel flow multiplied by some number "about" X. "About" is also known as fudge factor. That why I believe there is a more accurate way to determine % power.

 

[humptybump]

Don, one thing you will want to do along with all of the engine power math is to check you TAS. I can see running 50%-55% power and getting 6 GPH. But the numbers are not adding up for me that you recorded 145kts.

It's probably my math but given these RV's easily climb to 7000' in less than 10 minutes, and even assuming you were flying about 5.5 of the 6.3 hr on your recent cross country, the distance, time, speed, and fuel burn do not all work together. Like I said, I may well be my numbers.

You may want to compare you settings to the "cross country Cossack"

Running multiple performance circuits is both a good way to gain some aircraft specific data and to realize nailing down exact numbers is tougher than it sounds.

 

[hevansrv7a]

fuel flow times sfc is power

If your LOP SFC is, for example, .42 or .43 which is a reasonable guess for an O-320 with normal mags, then fuel flow is a valid indication of HP. This is not an opinion; it's just a fact. If ROP then the SFC is higher and the HP per gallon is lower, but it still works. Best power is around 0.50 pounds per HP per hour, for example. If you are LOP and smooth then you will have a very consistent SFC and the correct number can easily be developed.

Yes I do disagree. Its pretty simple. At LOP you are running below best power at that prop/throttle setting (hence a reduction in speed), so its not accurate to say that your fuel flow at some LOP number is a percentage of best power (100% power).

If you read Deakin's articles, he always prefaces the LOP fuel flow #'s by fuel flow multiplied by some number "about" X. "About" is also known as fudge factor. That why I believe there is a more accurate way to determine % power.

 

[erich weaver]

Yes I do disagree. Its pretty simple. At LOP you are running below best power at that prop/throttle setting (hence a reduction in speed), so its not accurate to say that your fuel flow at some LOP number is a percentage of best power (100% power).

Hmmm. Not following the logic here, but perhaps there is a fudge factor to the 15.whatever times fuel flow rule for LOP operations as you say, and power can be calculated more accurately. How big does this fudge factor get or are we already at "close enough" using the 15x rule?
Erich

 

[rocketbob]

If your LOP SFC is, for example, .42 or .43 which is a reasonable guess for an O-320 with normal mags, then fuel flow is a valid indication of HP. This is not an opinion; it's just a fact. If ROP then the SFC is higher and the HP per gallon is lower, but it still works. Best power is around 0.50 pounds per HP per hour, for example. If you are LOP and smooth then you will have a very consistent SFC and the correct number can easily be developed.

Actually you are proving my point in a roundabout way. The BSFC numbers vary from engine-to-engine configuration, and as you point out they are a "reasonable guess", in this case based on the fact that the engine is a O-320 with mags. And the BSFC numbers change at best power. So to say that % power is based on FF x some number (in the case of the original question, compression ratio) is inaccurate.

My point is someone can't just say if you run LOP and your compression ratio is 8.7, the fuel flow * 15 is your computed percent power. That is an educated guess and it varies.

 

[hevansrv7a]

We agree in part

That number, 15, is inexact but is close to correct for that compression ratio if you are looking for BHP. You are right that it will not give percent of power because that would make no sense if the engine were a 360 or a 540, for example. The correct number per the Advanced Pilot guys is a few decimal points lower than 15.0 if I recall correctly.

It is also true that compression ratio is a limiting factor on engine efficiency with higher ratios able to reach higher efficiencies. The simple explanation is that a higher compression ratio is also a higher expansion ratio and that expansion is where the heat energy is converted to mechanical energy. That is one reason that Diesels typically run at lower SFC than our normal spark engines.

So yes, a given compression ratio in a known design can be assigned a number that when multiplied by fuel flow in gallons can tell BHP with reasonable precision - again assuming you are LOP or at another identifiable point on the SFC curve such as best power.

You are correct that BSFC varies, but when you are running LOP with that engine, assuming it is in good condition, you will be within .01 of .42 and within .01 of .50 for best power. They will be equally accurate or inaccurate.

Example: 8 gallons per hour in my IO-360. Use 6 pounds per gallon. That is 48 pounds per hour. Use .42 for SFC. So, 48 pounds / .42 gives 114 HP which is about 63.5%. 114 HP / 8 gph = 14.25 HP per gph. That is my multiplier. Or use 8 for my multiplier for pct. power (8 x 8 = 64). If my SFC is .41 the answer changes by 3 HP or 2.5%.

I just don't see why this is so controversial.

Actually you are proving my point in a roundabout way. The BSFC numbers vary from engine-to-engine configuration, and as you point out they are a "reasonable guess", in this case based on the fact that the engine is a O-320 with mags. And the BSFC numbers change at best power. So to say that % power is based on FF x some number (in the case of the original question, compression ratio) is inaccurate.

My point is someone can't just say if you run LOP and your compression ratio is 8.7, the fuel flow * 15 is your computed percent power. That is an educated guess and it varies.

 

[erikpmort]

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".

because we can always go faster if we want to!!! economy is just as cool as going fast sometimes.

also, i buzz around my local valley a lot and when i'm not going anywhere, i don't need to go all out fast to nowhere. 140 mph on 5.5 gph is just fine with me. end rant.

 

[humptybump]

Don, if you are interested, here is a link to my test process. It is heavily based on recommendations by Bob Axom.