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Cruise Performance at Altitude…

Piper J3

Well Known Member
I normally fly round-robin cross-countries of less than 150 miles total so I fly at altitudes of maybe 3500MSL.

Two weeks ago, I flew a couple of hundred-mile legs and ventured up to 7500MSL to achieve a good economy but found that my fuel flow actually increased with altitude. Today I flew 330nm and again at 7500MSL. I copied Van’s POH Cruise Performance and took it with me for comparison. The speeds look to be spot-on but the fuel burn increases with altitude instead of decreases.

Here are two data points from today’s flight:

2500 MSL 115 KTS 5.0 GPH @ 5500 RPM - MAP=25.0 / EGT~1300F

7500 MSL 115 KTS 7.4 GPH @ 5500 RPM - MAP=23.0 / EGT~1300F

Today’s flight was flown in loose formation with a friend’s RV-12. He saw the same speeds but his fuel burn was similar to Van’s published Cruise Performance. Of note is that his EGT’s run about 100F higher than mine (1400F Vs. 1300F).

So, maybe I’m running a little richer mixture? And maybe my carbs are not leaning correctly at altitude?

Any thoughts appreciated. I thought I’d post here first, and then if no luck, I’ll ask in the Rotax forum…


 
Strictly displayed data on D-180 EMS. Either I'm really using more fuel at altitude or the Red Cube flow transducer is acting up. This is a good one...
 
I set my fuel flow K factor in flight to match average fuel consumption based on how much fuel I add after every flight. A little different from setting the K factor on the ground.

I noticed that my cruise fuel flow has crept up from an initial 4.5 GPH to 5 GPH after the first 5 oo hours. That surprised me.
 
The D-180 RV12 fuel totalizer is notoriously wrong at altitude. Mine does the same thing. There are lots of threads on the subject.
 
Oh wow... I just read Service Letter 19-09-23 for RV-12 Rotax 912ULS High Fuel Flow Indication. Figures…. I’m late to the party again… story of my life.

That’s a very interesting phenomenon. Who would have thought that the fuel flow transducer would act up at altitude?

So, my next step is to plan a flight at high altitudes and record indicated fuel flow. I might even try pulling the fuel pump fuse in flight to see if that makes any difference in fuel flow reading. I’ll climb to the service ceiling of 12,000 MSL and report my findings back on this thread.
 
Hey Jim
I didn't know there were mountains above 10,000' in Ohio. Private pilots flying sport rules aren't supposed to go above 10K unless within 2000' of terrain. Right? ;)
 
That restriction is for those with a Sport Pilot license, flying on a Driver's License.

If you have Basic Med, or 3rd class or a General Pilot license, the 10,000 ft limit is lifted, it's then the service ceiling as stated in the POH, about 12,500 ft, higher if you have the 912IS fuel injected motor.,

I'd look at what's left in the tank after 3 hours of flying on the fuel gauge, then go fill up at a pump and see how many gallons it takes. That should give you a more accurate figure, when you save the receipt. Include taxi, warm up, take off, climb, cruise, and decent back to a fuel pump that measures fuel output, and you'll have your answer for one flight with one set of conditions, via pencil and paper.

If your friend is running 1400F on the EGT's, my guess is his prop is pitched a little bit more for cruise than yours. Ask him what RPM and what MAP figures he's getting, too. I would guess his MAP is a little bit higher than yours, but you both won't know for sure, until you log all the data.

I think a Dynon Skyview Classic will log all that data for you, automatically? You just download it from the device onto a memory stick? Someone correct me if I am wrong on this. I've no experience with the Dynon D180 model.
 
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That restriction is for those with a Sport Pilot license, flying on a Driver's License.


Jim Stricker
EAA #499867
PPL/ASEL 1970 - Sport Pilot since 2007
80 hrs Flying Aeronca Chief 11AC N86203
1130 hrs Flying 46 Piper J-3 Cub N6841H
Bought Flying RV-12 #120058 Oct 2015 with 48TT - Hobbs now 607
LSRM-A Certificate 2016 for RV-12 N633CM
Special Thanks... EJ Trucks - USN Crew Chief A-4 Skyhawk
MJ Stricker (Father & CFI) - USAAF 1st Lt. Captain B-17H
 
Cruise Performance at Altitude

To clarify, my understanding that the 10,000 msl restriction is for any pilot flying under Sport Pilot rules. For example, I have an ATP certificate, but I fly under Sport Pilot rules so I consider the 10,000 rule applies to me.

Is that correct?
 
It is about YOUR rating. If have a PPL or better, and is current, IE medical and flight review, sport pilot restrictions do not apply to you.
 
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It is about YOUR rating. If have a PPL or better, and is current, IE medical and flight review, sport pilot restrictions do not apply to you.

Actually it is about what rules you are operating with under your rating.

As already mentioned, if you are a Private Pilot (or higher) rating but are flying as a Sport Pilot with no medical (just a state issued drivers license), then all of the rules that apply to Sport Pilots apply to you. No flight above 10K feet (other than what is allowed for clearing higher altitude terrain) is one of those rules.
 
I didn’t mean to generate so much discussion with the proposed FAR violation. A PPL friend flew with me so I didn’t have to find a 10,000’ mountain in Ohio. Also, I didn’t see any police pulling anybody over at 12000’ to give them a ticket. Just trying to figure out fuel burn at altitude…

So, it appears that Service Letter 19-09-23 holds true for my RV-12 with D-180 EMS. Fuel flow seems to follow a bell-shaped curve with respect to the increasing altitude. I didn’t expect the fuel flow transducer to be extremely accurate, especially with return fuel flow to the tank. What I wasn’t expecting, was increase in fuel flow indicator reading based strictly on the altitude the plane is flying at. I’m going to re-read the service letter a few more times and see if I can make any sense of this…

Today’s Flight Card:

 
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Cruise Performance at Altitude

Thanks Scott for clarifying that. I forgot to mention that although I am ATP rated, I do not have a medical and fly with a drivers license. So I have to follow all the sport pilot rules.
 
OK, I have re-read Service Letter 19-09-23 several times now and still don’t understand how altitude itself affects erroneous fuel flow readings.

This is really weird…

Does anybody have a better explanation?
 
OK, I have re-read Service Letter 19-09-23 several times now and still don’t understand how altitude itself affects erroneous fuel flow readings.

This is really weird…

Does anybody have a better explanation?

I'd ask either Dynon or the makers of that red Cube that's in your plane. I am not aware if the Skyview series does that also, in later RV-12 builds. Perhaps someone else can comment with experience with both.
 
OK, I have re-read Service Letter 19-09-23 several times now and still don’t understand how altitude itself affects erroneous fuel flow readings.

This is really weird…

Does anybody have a better explanation?

It's not that weird.. the fuel flow cube just counts a pulse whenever the little vane breaks the light beam, no matter which direction it's going. If the little wheel that's in the fuel flow vibrates due to pressure fluctuations, it can give you totally whacko readings. So knowing that, picture a little water-wheel type thing with two pulsating diaphragm type pumps, on on each side... you can see how the little wheel could get driven back and forth on very short pulses.

I suspect a different type of fuel pump (non diaphragm) would fix it, OR substantially higher fuel flow, OR a better design of the flow cube, OR maybe higher fuel pressure from the boost pump. I know from experience that shutting off the boost pump will fix it, I just don't like doing that. Maybe something as simple as a couple feet of fuel line coiled and put in line between the boost pump and flow meter would do it, or some flexible hose or something. Those ideas are a little more "experimental" than I'm willing to get, to be honest.
 
We have actually done a lot of experimenting to try and find a solution for the strange phenomena.

- Full transparency follows -
Nothing we tried made any difference (and I assure you we tried a lot of different things), so we eventually gave up since the erroneous indication is conservative in its influence on what the fuel computer shows for remaining fuel, and it only occurs at a very specific set of conditions.

If anyone has any idea (we are always willing to learn something new) we would be happy to hear it, but there is a pretty high probability that we already tried it.......
 
It's not that weird.. the fuel flow cube just counts a pulse whenever the little vane breaks the light beam, no matter which direction it's going. If the little wheel that's in the fuel flow vibrates due to pressure fluctuations, it can give you totally whacko readings. So knowing that, picture a little water-wheel type thing with two pulsating diaphragm type pumps, on on each side... you can see how the little wheel could get driven back and forth on very short pulses.

I suspect a different type of fuel pump (non diaphragm) would fix it, OR substantially higher fuel flow, OR a better design of the flow cube, OR maybe higher fuel pressure from the boost pump. I know from experience that shutting off the boost pump will fix it, I just don't like doing that. Maybe something as simple as a couple feet of fuel line coiled and put in line between the boost pump and flow meter would do it, or some flexible hose or something. Those ideas are a little more "experimental" than I'm willing to get, to be honest.

Right - but you have two pulsating pumps running all the time anyway regardless of altitude or taxiing on the ground. Why extranious reading at 6000'-8000'?
 
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If anyone has any idea (we are always willing to learn something new) we would be happy to hear it, but there is a pretty high probability that we already tried it.......

How about writing an algorithm for the EMS that makes sense of the fuel flow sensor at known bad altitudes? Probably not a great idea for airplane application - but you asked for ideas...
 
How about writing an algorithm for the EMS that makes sense of the fuel flow sensor at known bad altitudes? Probably not a great idea for airplane application - but you asked for ideas...

Because only some airplanes exhibit the condition and of those that do, it is not exactly the same on every one.

Why just at 6000-8000 feet? If we knew that, we would probably have a solution;)

One theory - The diaphragm on the engine driven pump has the non-wetted side exposed to atmospheric pressure. It is possible that the opposing pressure change has an influence on the resonant interaction between the engine driven pump and the electric aux. pump.
 
Scott, have you considered lag time on the fuel pump’s check valve. This will cause a slight back flow until the check seats. The effect may be more prominent at some particular fuel flow rate.

Testing showed it was altitude related, but not fuel flow related, at least not at the Fuel flows that people would typically operate at in the effected altitude range.
 
Testing showed it was altitude related, but not fuel flow related, at least not at the Fuel flows that people would typically operate at in the effected altitude range.

Any relationship as to whether the fuel is E-10 mogas, straight mogas, or 100LL ? I am thinking along the lines of lower vapor pressure of mogas at elevation..., just an idea.
 
Any relationship as to whether the fuel is E-10 mogas, straight mogas, or 100LL ? I am thinking along the lines of lower vapor pressure of mogas at elevation..., just an idea.

My post #13 above is with 93E10 Mogas, and Costco at that. I think most Costco gas doesn't leave the surface of the the planet...
 
My post #13 above is with 93E10 Mogas, and Costco at that. I think most Costco gas doesn't leave the surface of the the planet...

I'm in the group of some of it, that does leave the surface. $3.09 a gallon here in Los Angeles, at Costco, and that's a deal!!! No lead in the fuel is worth it to me, so I can continue to run full synthetic Mobil 4 motor oil. 20 hours on the last oil change, and it hasn't moved on the dipstick yet, at all.
 
How about writing an algorithm for the EMS that makes sense of the fuel flow sensor at known bad altitudes? Probably not a great idea for airplane application - but you asked for ideas...

Do you guys really look at fuel flow much during a flight? About the only time it’s of any use to me is in summer (Phoenix) if shutdown my electric boost pump. High flow and low fuel pressure indicate vapor lock. I just let the electric pump run in summer although recently I have been experimenting with running a 75%/25% auto fuel/AVGAS mix that seems to eliminate most vapor lock.
 
Yes, I do look at fuel flow as part of my instrument scan. I don't fixate on the fuel flow, but look to see if it shows a reasonable value. Same with all other gages to see if in "green" or trending up/down scale. High fuel flow could indicate a leak which would be good to know about soonest...

Just a really weird problem - why would altitude alone be the cause of erroneous high fuel flow indication?
 
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Do you guys really look at fuel flow much during a flight? About the only time it’s of any use to me is in summer (Phoenix) if shutdown my electric boost pump. High flow and low fuel pressure indicate vapor lock. I just let the electric pump run in summer although recently I have been experimenting with running a 75%/25% auto fuel/AVGAS mix that seems to eliminate most vapor lock.


I was reading a post on gearbox wear and oils used (Mobil 1) by Cammie Patch up at Glass Cockpit aviation. In the thread, at the school, one of the RV-12's was having vapor lock problems on winter /spring transition supposedly straight unleaded 91 octane. Engine sputter on the runway trying to take off, happened several times.

The school ended up running 10 to 20% 100LL to clear up the problem until that batch of fuel was consumed.

At least here in CA, the ethanol starts showing up in the fuel effective October 1st, when they switch to winter blend fuel. Before starting annual inspection on my plane, I topped off with 100LL, about 3 gallons worth. i wanted to know that if I wasn't flying the plane for a month, and with a vented fuel tank, that the octane in my mogas wouldn't just vent right out, and I'd be running too low an octane fuel in the 912 ULS.

I'm about to stock up on 100 gallons to hold me over for a bit, since flight lessons 2 to 3x a week have me going through 6 to 9 gallons per lesson, plus at some point I will have to do a cross country solo. I figure that batch of mogas will be gone in 2 months or so.

I do keep a brief eye on fuel pressure and gallons per hour, but my biggest eye watch is on how many gallons are left in the fuel tank. Also Oil temperature, coolant temps, and voltage and amps charging rates.
 
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Gov. of California is going to mandate electric airplanes by 2035 just like he did for automobiles... :D

Don't think I'll still be in California by then. Anywhere but there. I hear some Nevadan's are starting to call portions of Vegas, Reno, and Carson City Eastern California, from all the transplants. Lake Tahoe, Yosemite NP and surrounding areas are being "loved to death."
 
We are seeing a big influx of Californians in AZ too. It has brought up our hangar values since they are used to more expensive prices.
 
Right - but you have two pulsating pumps running all the time anyway regardless of altitude or taxiing on the ground. Why extranious reading at 6000'-8000'?
My guess would be just that, a guess. Vapor pressure? I don't know much about fluid dynamics and whatever other physics come into play here. I do know a little bit about electronics, and about electronics designed to measure movement. It would have been pretty simple and pretty cheap to measure not only motion, but direction in the flow sensor. It's a simple matter of using two optical sensors instead of one, and a dead simple bit of logic to either indicate direction, or ONLY output pulses for one direction and not the reverse.

I would assume that the manufacturer did some due diligence in the design of this device, and determined that the extra dime or quarter's worth of hardware to do this wasn't needed. There's a lot of them out there. I've never heard of anyone with any other airplane complaining about this issue, which suggests it's quite possibly unique to the RV-12.
 
It would be ugly, but you could make a correction card - kind of like a compass correction card. Read 6.5 GPH at 7500' and it's really 4.8 GPH. As I said, it would be ugly...
 
I’m sure the wheel pants do cut drag and give an efficiency boost, but I think it’s not significant. I never really noticed much difference when I removed them. If you are that concerned about speed the other RV designs are a better choice than the 12. I was attracted to the 12 by the LSA qualification.
 
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