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RV-12 RPM, Speed and Efficiency

ScottSchmidt

Well Known Member
Today I did some flying and wanted to see how the -12 was in efficiency at different power settings. I need to try this again at a higher altitude where I would typically cruise. I know this is not perfect data and there are more accurate ways of accomplishing this.

I was flying at 5200 ft with an OAT of 32F, only one person and 6 gallons of fuel.

Prop pitch setting would change these numbers between -12's. I would be interested in seeing what others are seeing.
I only measured TAS from the Dynon. WOT (wide open throttle) had a max RPM of 5770.

I will try lower RPM settings to see how efficient the -12 can be.

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Playing with numbers

Let's say you were going to take a 200 nautical mile trip. The time difference between running at 4800 vs 5770 RPM is 25 minutes ( 1.9 vs. 1.5 hours) and the fuel burn difference is .4 gallons (8.7 vs. 10.3 gallons).

Interestingly, if I take that same 200 nautical mile trip in my -9, it will take me 1.3 hours at 150 KTS and only 9.5 gallons burning 7.1 GPH, slightly better than the -12 running full out.
 
For the altitude Scott was flying at, if you run the engine at right near the max continuous of 5500 RPM, they will typically burn just shy of 6 GPH, so his chart looks pretty close.
 
Pretty amazing.

To put it in automotive terms, even at the worse point on your chart you are getting 22 mpg at 150 mph...:)
 
A couple more points

As it happens, I noted a couple of parameters last week. Slightly lower temperature, one-up, but with full fuel, at 4000 feet, with spats. I've had to translate into your funny fuel units ;-)

rpm fuel speed
4700 4.2 100kt 115mph
5000 4.8 109kt 125mph
5500 5.6 120kt 138mph

Cheers...Keith
 
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Well, for an ex-Lycoming driver it's much too fast in any currency ;-)

Having said that I wouldn't swap my engine - starts in a heartbeat, sounds fine in the cockpit, and people on the ground have commented how quiet the plane is in the air.

Cheers...Keith
 
150 mph? That's just a few ticks shy of Vne. Do you RV12ers routinely fly that fast?

I think Scott probably did it for a short period, for data comparison only.
Max continuous RPM on teh Rotax 912 is 5500, so typical max true airspeed is usually in the 135 - 140 MPH range.
 
At max continuous rpm, it's at 24.5 mpg at 140 mph on Scotts chart.

Still quite amazing in automotive terms.
 
I thought people on the ground comment that someone is flying a sewing machine!:D Then again maybe I was wrong.
Well, for an ex-Lycoming driver it's much too fast in any currency ;-)

Having said that I wouldn't swap my engine - starts in a heartbeat, sounds fine in the cockpit, and people on the ground have commented how quiet the plane is in the air.

Cheers...Keith
 
Scott, that's interesting. My flying in AZ is done anywhere from 4500 to 12500 Ft, and I don't think I've ever seen an average fuel burn over 5 GPH. I usually cruise at 5300 RPM, and most flights are at least two hours.
 
Fuel Burn

My longest leg flown was 3.3 hours at 5000 ft and 5100 RPM. Added 14.6 gal, and was full when I left. Temp was -7C when I took off and 0C when I landed. Fuel Flow was showing right at 5 GPH so need to tweak the K factor one more time.
 
Keep in mind that actual power produced is a function of both engine RPM and manifold pressure. The two together correlate to a specific fuel flow.
So, considering that RV-12's have ground adjustable props, stating that I get X fuel flow at Y RPM doesn't tell the whole story, without also giving manifold pressure. Prop pitch can have a big influence on what the fuel flow Vs RPM ratio is.
 
True, Scott, but for planning purposes on a long cross country 4.5 GPH seems to work out within a few tenths of a gallon. The few 12 owners I've met at the airfield say that's what they are seeing too.
 
Fuel burn

My fuel burn is pretty consistently 4.3 - 4.4 gph at 5200 rpm and 110-112 knots. I flight plan for 4.5 and have never had any problem with that in 225 hours

Wayne 12041/143WM "Little Bird II"
 
True, Scott, but for planning purposes on a long cross country 4.5 GPH seems to work out within a few tenths of a gallon. The few 12 owners I've met at the airfield say that's what they are seeing too.

With experience flying many 3000+ mile (round trip distance) cross countrys in RV-12's, that has not been my experience.
As I already said, fuel flow all depends on how fast you go / how much power you are producing. Power produced is a function of MP to RPM.
If you cruise cross country at anything near 120 TAS (which the RV-12 is capable of doing), the fuel burn wont be anywhere close to 4.5 GPH.
 
I am seeing an average of 5 to 5.5 GPH. that is usually cruise of 5300 Rpm, 116-118 KTS TAS. I always PLAN on 6 GPH. cruise at 5100-5200 RPM does not change the TAS that much.
 
Haven't done any 3000 mile trips but, just beating around Oregon in my CT in loose formation with my neighbor in his -12, I seem to burn about 4.7 gph at 5200 whereas he burns a little less. Probably because he runs a little less rpm so I can keep up.
 
You guys seem to fail to get the point:rolleyes:
I could easily fly cross country in an RV-12 at 3.5 gallons per hour. So do I win?

The point is, telling the world that my airplane burns bla bla GPH is not providing information of any value beyond to demonstrate that the airplane can stay in the air if the engine is burning at least 4.5 GPH.

I believe in providing factual useful information. I have always tried to do so on this forum, and Van's does so with all advertising literature.

I think it only fair that prospective builders know that an RV-12 doesn't fly at the top cruise speed as advertised by Van's, while burning only 4.5 GPH.
 
You guys seem to fail to get the point:rolleyes:
I could easily fly cross country in an RV-12 at 3.5 gallons per hour. So do I win?

The point is, telling the world that my airplane burns bla bla GPH is not providing information of any value beyond to demonstrate that the airplane can stay in the air if the engine is burning at least 4.5 GPH.

I believe in providing factual useful information. I have always tried to do so on this forum, and Van's does so with all advertising literature.

I think it only fair that prospective builders know that an RV-12 doesn't fly at the top cruise speed as advertised by Van's, while burning only 4.5 GPH.

Co - rrect! The laws of physics have not changed. Fly fast burn more fuel. Drag increases as a square of the IAS. Your motor has to overcome the drag. There was an interesting article in the RVator some time ago when Van's flew their RV-9 next to their RV-12. The fuel burn on the RV-9 was slightly lower than on the RV-12 probably because of the flush rivets on the 9 and no wheel pants on the 12. So whether Rotax, Lycoming, Continental or Subaru, piston engines burning gasoline have very much the same fuel efficiency. Of course the airframe does play a major role as we see the CT being more draggy than an RV-12 and an RV-9 being cleaner than the RV-12 without wheel pants.
To quote Van; "every aircraft is a compromise". The RV-12 is obviously an excellent design in its class. Good speed, excellent useful load and the legendary RV handling qualities. You wanna go fast! You will burn more fuel - there are no free meals! The fuel burn numbers as published will stay as they are with this airframe.
The bottom line is the RV-12 is pretty efficient but flying 120kts IAS will see you burning around 6gph. If you want better economy, fly high and you might see see 120kts TAS at a much lower fuel burn as you will have a much lower IAS.
 
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Fuel Flow

I do think my fuel flow is slightly high. I am still tweaking the k factor. I don't think it is off by much though.
It is much easier calibrating the -10 when you can burn 40 gallons and fill exactly to the same spot.

No matter how you look at it, the -12 is a great plane. Fun, responsive, fuel efficient and cheap to fly. Yes, the -9 may burn less for the same speed but the savings burning auto gas will probably make it cheaper. We typically save over $2/gallon compared to 100LL. As a comparison to the -10, I can fly my -10 with 4 people, at 158kts and 10.5gph, right around 17 mpg (zero wind). So 10.5 gph times $5.50/gallon = $57.75 divided by 4 people = $14.43/person. Not too bad. About $14.50 /person to go 180 miles.

The -12 is 5.5gph times $3.20 = $17.60 divided by 2 people = $8.80 / person.
To go the same 180 miles it would be around $11.50/person in the -12 buying auto gas. But if you have to buy 100LL for $5.50/gallon, the -12 is about $40 in fuel or $20/person.
 
You guys seem to fail to get the point:rolleyes:
I could easily fly cross country in an RV-12 at 3.5 gallons per hour. So do I win?

The point is, telling the world that my airplane burns bla bla GPH is not providing information of any value beyond to demonstrate that the airplane can stay in the air if the engine is burning at least 4.5 GPH.

I believe in providing factual useful information. I have always tried to do so on this forum, and Van's does so with all advertising literature.

I think it only fair that prospective builders know that an RV-12 doesn't fly at the top cruise speed as advertised by Van's, while burning only 4.5 GPH.


Assuming summer time heat, and flying at 7500 to 9500 ft, 110kt TAS, with the prop set for cruise not climbing performance, what type of GPH do you see, real world, in your plane.


I understand the relative density of air is less at elevation and in summer heat... Assume 50F air temps at 9500 feet.

And next question is GPH in the new 912 iS FI motor without the Bing Carbs.
 
Hello Scott;
Did you record your static runup RPM at full throttle?, or rpm achieved during climb out?.
I'm always interested in other folks numbers. I am trying to satisfy the Rotax service letter SL-912-016R1 (no less than 5200rpm at 29.1" hg MAP), while also allowing max rpm in level flight of no more than 5800rpm.
I am getting a static full throttle runup of about 4920rpm, DA -1386', 43 deg f, 29.9" MAP

Thanks
 
Assuming summer time heat, and flying at 7500 to 9500 ft, 110kt TAS, with the prop set for cruise not climbing performance, what type of GPH do you see, real world, in your plane.


I understand the relative density of air is less at elevation and in summer heat... Assume 50F air temps at 9500 feet.

And next question is GPH in the new 912 iS FI motor without the Bing Carbs.

There are numerous variables from one flight to another but on average Van's prototype/demonstrator RV-12 will true out at about 120 Kts at about 5.2 GPH at altitudes above 10K
The RV-12 and RV-12iS has been flown to OSH and back together. Flying the same flight profiles and speeds, the is airplane burned about 30% less fuel on any given leg.

Hello Scott;
Did you record your static runup RPM at full throttle?, or rpm achieved during climb out?.
I'm always interested in other folks numbers. I am trying to satisfy the Rotax service letter SL-912-016R1 (no less than 5200rpm at 29.1" hg MAP), while also allowing max rpm in level flight of no more than 5800rpm.
I am getting a static full throttle runup of about 4920rpm, DA -1386', 43 deg f, 29.9" MAP

Thanks

Maximum continous RPM for the 912ULS is 5500, not 5800. 5800 is the limit for take-off, and I think is restricted for 5 minutes.

Your static RPM sounds a bit low. It should be in the neighborhood of 5000

Because of the min:max speed ration of the RV-12, you will not be able to achieve meeting the 5200 RPM in climb and get the cruise speeds that most people do.
Climb RPM is typically in the 5100-5125 range with the the prop pitch that most RV-12 owners are using.
 
There are numerous variables from one flight to another but on average Van's prototype/demonstrator RV-12 will true out at about 120 Kts at about 5.2 GPH at altitudes above 10K
The RV-12 and RV-12iS has been flown to OSH and back together. Flying the same flight profiles and speeds, the is airplane burned about 30% less fuel on any given leg.

About 3.64 gallons per hour? Anyone done the break even point analysis on the iS vs the ULS in fuel expenses at $4.00 a gallon? About 800 hours on the motor?
 
Our RV-12 was always slower and burned a lot more fuel than others typically reported. I could get 120 K true out of it, but at the cost of burning 6.5 GPH or more. Climb was a little anemic for an RV-12, too. The original builder had it way over-pitched, maybe trying to squeeze out a little more speed. I got the pitch close to recommended settings, which helped, but we were still burning too much gas and my friend's 12 would easily walk away from me.

I tried getting it closer when we did the prop hub inspection last summer. We're slightly over-pitched still; static RPM is just a little below 5000. We're around 4980, if I remember right. During the carb teardown process we found a bad choke cable on the left side that was probably not allowing the left side starting carb to completely close off, which would probably explain our previous high fuel consumption. The damage was not visible until the cable was pulled out of the sheath -- there was a broken strand wrapped around the cable, up inside the sheath. Getting the carbs balanced was a WHOLE LOT faster and easier once that was fixed! We're still getting the K-factor zeroed in, which is more of a challenge when you're filling the plane from 5 gallon cans and not a pump.

We are seeing 120 knots at around 4500 MSL, 25" MAP, between 5300 and 5400 RPM, burning around 5.5 GPH. Numbers are sketchy at this point, since so much has changed. Flying 110 or even 115 kt TAS saves a lot of fuel, though. I think during the upcoming CI we'll de-pitch the prop just a little bit to get the static RPM up to 5000 and see how much good that does. We've also got some areodynamic cleanup to do with the gear leg intersection fairings and such. It's an ongoing project.

Oh, and we've pretty much never paid $4/gal for fuel. We burn ethanol free premium MOGAS at under $3/gal.
 
Here are a few data points?

Shoot for 5100/5200 RPM in initial climb. My field elevation is 800MSL.

One person w/ half fuel = 1100+ FPM initial climb.

Prop pitched per above should yield 110 knots at 3000 MSL and 5500 RPM.

Cruise flight at 5500RPM should have throttle positioned about 1? pulled out from full throttle.

My fuel burn per above is right around 6 GPH, which pretty much agrees with published Rotax specs.

No fun having an airplane if your not going to go fast? :D

Another data point... E9310 at Costco is $2.12/Gal today. :D:D

https://www.rotaxservice.com/wp-content/uploads/2017/05/912Sperf.pdf

 
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