Fuel burn

[Olarry]

As a follow up to my earlier post today, Ft Myers Fl to Jackson Mi, ( long cross country) I stated my fuel burn was 5 gal/ hr. After calculating fuel used and time flown, the actual number is 4.8 gal per hour which jives with what I was seeing on my fuel flow gauge at about 5350 RPM'S. True air speeds were 117 to 120 kts. Fuel usage would have been better but I made several climbs and descents trying to find smoother air.

My plane is a Jubilee edition delivered in Jan 2014 and now has 134.4 hrs TT. I had the sport engine upgrade done by Lockwood during it's annual in December. They repitched the prop and at gross weight you can really feel the increase in climb performance when comparing to the factory setting. This probably increases the fuel usage a little relative to speed but I like it much better.

 

As for flying in Florida during the winter month's, you better get going early if wanting to fly low because as the ground heats up and the winds pick up, it's like the the gentleman from Georgia stated, it gets real bumpy until you get above 3 to 4000 feet most of the time.

[WmInce]

They repitched the prop and at gross weight you can really feel the increase in climb performance when comparing to the factory setting. This probably increases the fuel usage a little relative to speed but I like it much better.

Thanks for your post.

Can you expand on the repitching of your prop?

Specifically, what was it before (RPM, DA, true airspeed) and what did you end up and at what operating weight?

[Doug G.]

I calculate on 5 gph but get about 4.5 at 5000 rpm.

[FlyingMonkey]

I calculate on 5 gph but get about 4.5 at 5000 rpm.

 

That sounds very similar to what I see with a 912ULS.

 

I'd like some clarification on what Charlie Tango said in the other thread, about how comparisons are irrelevant without knowing percentage of throttle used.  Is that true?  

 

On the one hand, I can see how an engine loaded up at WOT and getting 5000rpm is doing more work than one at 5000rpm and at only 75% throttle, but it's also puzzling.  The Rotax fuel burn tables say X rpm =  Y fuel burn, and make no allowance for variance of throttle position.  After all, to turn 5000rpm requires the cylinders to fill with fuel/air mix a certain number of times in a minute (5000/4?) and does not change.  Thus the metered fuel used is constant.  Also, horsepower available is directly related to rpm regardless of throttle setting -- something those of us who have re-pitched our props have seen firsthand.

 

I can see cruise, climb,  and other aspects of performance changing with throttle position at a given rpm, but I'm confused about fuel burn.

 

So the question is:  Does 5000rpm at 75% throttle use less fuel than 5000rpm at WOT?  If so...WHY?!?

 

Pardon my ignorance if just asking this question makes me seem like a dumbass.  

[Jim Meade]

I calculate on 5 gph but get about 4.5 at 5000 rpm.

Why do you estimate 5 gph when your experience is 4.5 ghp at a given rpm? Do you add a buffer factor (e.g. an extra 30 minutes) on top of your generous estimate?

I guess my question to all (not just Doug) is why would one estimate on the high side, then pad that a bit, plus include the FAA's 30 minutes VFR, to the point where one won't leave the ground for a 40 minute flight without 3 hours of fuel on board?

Why not make the most accurate estimate possible and then deliberately add whatever safety factor is required or one prefers as a separate calculation?

I would think the latter course of action would lead to better confidence when it came to planning longer trips.

[FastEddieB]

General concept is: " Err on the side of caution".

 

But I get your concern.

 

Cirrus pilots would sometimes enter a few less gallons into the Avidyne on a fill-up for the same reason.

 

Like some set their watches a few minutes fast.

 

Not my M.O. - like you I'd like the most accurate info to act on. But seems to work for others.

[Roger Lee]

"Does 5000rpm at 75% throttle use less fuel than 5000rpm at WOT?  If so...WHY?!?"

 

Hi Andy,

 

The short answer is yes it does. If you are running 5000 at WOT and then you are either over pitched with your prop or in a WOT climb.

Wide open throttle is WOT. The carbs don't know what you are doing. They only know where you have the throttle positioned and supply fuel accordingly. At WOT in any setting you are pouring as much fuel as the system allows through the carbs which at WOT can be anywhere from 6.2-7 gph. 

Prop pitch in our use can play a big part in fuel use. I have seen at least 1.5 gph more use with an over pitched prop. 

 

If you are throttled back at 5000 rpm at 75% throttle you have reduced that fuel flow.

 

THe engine doesn't care what you are doing the fuel flow is governed by throttle position on our engine's.

 

 

I figure like most 5 gph. You never know what winds aloft will be or what throttle position you may choose. If you burn less fuel than 5 gph then great, so you land with a couple extra gallons. You use more than 5 gph on climb outs and then it may be a short climb or a long extended climb at or above 6 gph. You may run more rpm on a flight than 4.5 gph or have an extended flight due to winds aloft that weren't correct. It is just averaging since trying to be exact has gotten many into trouble. On all of my cross countrys I average 5 gph. landing with only 30 min. of fuel means you only have 2.5 or less gallons in the tank. Then you have unequal tank drain which may put you in a tank empty and usable fuel to the outside part of the tank. When I fly I ALWAYS have extra fuel and have never once had to worry if I was going to make it. Why would anyone ever put their self on the short side of fuel on purpose. To put yourself this low on fuel puts just begs for an off field landing.

[Ed Cesnalis]

That sounds very similar to what I see with a 912ULS.

 

I'd like some clarification on what Charlie Tango said in the other thread, about how comparisons are irrelevant without knowing percentage of throttle used.  Is that true?  

 

On the one hand, I can see how an engine loaded up at WOT and getting 5000rpm is doing more work than one at 5000rpm and at only 75% throttle, but it's also puzzling.  The Rotax fuel burn tables say X rpm =  Y fuel burn, and make no allowance for variance of throttle position.  After all, to turn 5000rpm requires the cylinders to fill with fuel/air mix a certain number of times in a minute (5000/4?) and does not change.  Thus the metered fuel used is constant.  Also, horsepower available is directly related to rpm regardless of throttle setting -- something those of us who have re-pitched our props have seen firsthand.

 

I can see cruise, climb,  and other aspects of performance changing with throttle position at a given rpm, but I'm confused about fuel burn.

 

So the question is:  Does 5000rpm at 75% throttle use less fuel than 5000rpm at WOT?  If so...WHY?!?

 

Pardon my ignorance if just asking this question makes me seem like a dumbass.  

 

 

Take it to extremes to demonstrate the point.  We leave my CT as is and it takes WOT to see 5,500 RPM @ 11,000' 

 

We make your prop so flat that we see 5,500 RPM at 40% throttle @ 11,000'  ( we may have to tow you up to 11,000' because you wouldn't be producing enough power to climb up )

 

I will be burning 5 gph and you will be burning only about 2 because your throttle is only partially open but both engines will be turning the same speed and pumping the same volume of air.

 

IOW  to realize your power you would have to advance your throttle to full and your RPM would go to something like 10,000 (if it didn't blow up)  your power would be in a different RPM range due to your pitch.

 

I'm sure it would be dangerous to run a 912 without a prop attached because without resistance it would so easily over speed.

[FlyingMonkey]

I use the best information I have available, but in my airplane that information is a little sketchy.  I only have the fuel sight tubes and a watch.  No fuel computer or fuel flow meter.  The longest cross country flight I took was about 400nm, and not knowing what the fuel burn would be I filled the tanks.  I flight planned for 5.5gph @ 5400rpm, and I found on landing that that that was actually *very* close to correct, IIRC the actual fuel burn was something like 5.55 - 5.6gph. 

 

The one tool I wish I had is a fuel flow meter, and I may add one in the future to allow more accurate fuel planning.  Until then, carrying more fuel than needed and using the rule of thumb I posted in the other thread seems to work well:

 

5000rpm = 4.5gph

5200rpm = 5.0gph

5400rpm = 5.5gph

[Ed Cesnalis]

Altitude makes a difference as well.  When cruising above 10,000' I cannot burn more than 5gph, There is not that much air and the mixture can't get any richer than the main jet permits at full throttle.

[FlyingMonkey]

"Does 5000rpm at 75% throttle use less fuel than 5000rpm at WOT?  If so...WHY?!?"

 

Hi Andy,

 

The short answer is yes it does. If you are running 5000 at WOT and then you are either over pitched with your prop or in a WOT climb.

Wide open throttle is WOT. The carbs don't know what you are doing. They only know where you have the throttle positioned and supply fuel accordingly. At WOT in any setting you are pouring as much fuel as the system allows through the carbs which at WOT can be anywhere from 6.2-7 gph.

 

If you are throttled back at 5000 rpm at 75% throttle you have reduced that fuel flow.

 

THe engine doesn't care what you are doing the fuel flow is governed by throttle position on our engine's.

 

Then that must mean that 5000rpm at 75% throttle is a leaner mixture than at 5000rpm at WOT...correct?  After all, you are dumping more fuel into the system for the same number of combustion cycles.  Though I guess the carb is also metering more air into the system.  Which begs the question:  where does the extra fuel/air mix go?  The cylinders hold the same amount of gasses in each cycle, you can't force more into them.  Does it just go out the tailpipe?

 

Just trying to wrap my head around this.

[FlyingMonkey]

Altitude makes a difference as well.  When cruising above 10,000' I cannot burn more than 5gph, There is not that much air and the mixture can't get any richer than the main jet permits at full throttle.

 

Yeah, I was assuming the same altitudes with the same aircraft, just re-pitching the prop for different cruise rpm.

[Ed Cesnalis]

Yeah, I was assuming the same altitudes with the same aircraft, just re-pitching the prop for different cruise rpm.

 

The statements in these threads only consider RPM pitch and altitude play a part.

[Roger Lee]

Andy,

 

Prop pitch figures into this along with throttle position. Ed's prop pitch is flatter than ours and he flys much higher than most of us on a regular basis. His numbers will be different than yours and because of his regional altitude he tends to fly at WOT most of the time.

WOT is set to be rich because during the usual WOT at climb out is where detonation is more likely and not at a reduced throttle setting in cruise. Rotax purposely sets it up this way. If you fly WOT in cruise then the engine is more unloaded, but this is where you still want the richer mixture to protect the engine and since we can't adjust our mixture and then altitude then makes us run richer still. That said since we can't adjust our mixture, WOT will deliver more fuel than a reduced throttle setting no matter what. 

 

Just so this thread doesn't head off into never never land with all kinds of engines and planes, We can confine this to a CT with a Rotax 912ULS and as Ed states: The statements in these threads only consider RPM, pitch and altitude.

[FlyingMonkey]

So, two planes at same altitude and the only difference is prop pitch, one at 5000rpm WOT and one at 5000rpm 75% throttle, the one at WOT is running much richer and thus burning more fuel?  If that statement is true, then I think I understand what's happening.

[Roger Lee]

Andy,

 

In the case above with no other factors it's all about the mechanical throttle position and the prop pitch.

[FastEddieB]

Roger,

 

Is there an altitude where even full throttle there is not enough vacuum to open the slide all the way? At that point would not the needle would come into play and restrict fuel flow to be proportional to the slide position, not the throttle position?

 

In most planes its a given that WOT is WOT. In ours, with no direct linkage between the throttle plate and the slide, can't WOT end up being less that WOT?

[Top Cat]

Why do you estimate 5 gph when your experience is 4.5 ghp at a given rpm? Do you add a buffer factor (e.g. an extra 30 minutes) on top of your generous estimate?

I guess my question to all (not just Doug) is why would one estimate on the high side, then pad that a bit, plus include the FAA's 30 minutes VFR, to the point where one won't leave the ground for a 40 minute flight without 3 hours of fuel on board?

Why not make the most accurate estimate possible and then deliberately add whatever safety factor is required or one prefers as a separate calculation?

I would think the latter course of action would lead to better confidence when it came to planning longer trips.

Jim,

 

  Excellent points.

 

  For me personally, I feel after 120 hours of flying my RV-12, I've got a pretty good feel  for an accurate fuel burn. I compare it to fuel I've put in, the reading on the tank gauge, the reading reflected on the Skyview, and the actual in flight burn.

  That said, rather than use 30 mins VFR reserve, which is a minimum, I use at least 5 gallons because the POH warns that high aircraft attitudes with 4 gallons or less can cause engine stoppage. I think about sudden climbs and go-arounds at destination with less than that. The POH also warns about taking off with 4 gallons or less.

 

  Personally I do pad the arrival fuel on the conservative side. Fuel below me at the field, is like runway behind…not much good. However, as I deal with actual conditions I can and do review and revise my fuel needs.

 

 In that regard I follow your last comment:-

 

"Why not make the most accurate estimate possible and then deliberately add whatever safety factor is required or one prefers as a separate calculation?

I would think the latter course of action would lead to better confidence when it came to planning longer trips."

[Jim Meade]

I appreciate everyone's opinion. It seems that most would prefer to use a high swag on fuel burn rate rather than know it exactly. That is, they would rather feel any fuel calculation they make is comfortably optimistic rather than know precisely how much fuel they burn. Comfort is a good feeling.

 

None of the FAA knowledge exams take that tack, but after all, one only has to pass each of those once and then can continue to fly with a happy feeling rather than be burdened by exact knowledge, which is boring and bothersome.

[Ed Cesnalis]

Roger,

 

Is there an altitude where even full throttle there is not enough vacuum to open the slide all the way? At that point would not the needle would come into play and restrict fuel flow to be proportional to the slide position, not the throttle position?

 

In most planes its a given that WOT is WOT. In ours, with no direct linkage between the throttle plate and the slide, can't WOT end up being less that WOT?

 

By design WOT uses the main jet.  If the pressure differential isn't sufficient to fully raise the diaphragm then your point is valid.

[FlyingMonkey]

I appreciate everyone's opinion. It seems that most would prefer to use a high swag on fuel burn rate rather than know it exactly. That is, they would rather feel any fuel calculation they make is comfortably optimistic rather than know precisely how much fuel they burn. Comfort is a good feeling.

 

None of the FAA knowledge exams take that tack, but after all, one only has to pass each of those once and then can continue to fly with a happy feeling rather than be burdened by exact knowledge, which is boring and bothersome.

 

Words, like fuel, should be metered in use.  You could have just said "you guys are a bunch of idiots" and saved a bunch of words.

 

But what do I know, I'm "unburdened by exact knowledge"...  

[Ed Cesnalis]

Andy,

 

Prop pitch figures into this along with throttle position. Ed's prop pitch is flatter than ours and he flys much higher than most of us on a regular basis. His numbers will be different than yours and because of his regional altitude he tends to fly at WOT most of the time.

WOT is set to be rich because during the usual WOT at climb out is where detonation is more likely and not at a reduced throttle setting in cruise. Rotax purposely sets it up this way. If you fly WOT in cruise then the engine is more unloaded, but this is where you still want the richer mixture to protect the engine and since we can't adjust our mixture and then altitude then makes us run richer still. That said since we can't adjust our mixture, WOT will deliver more fuel than a reduced throttle setting no matter what. 

 

Just so this thread doesn't head off into never never land with all kinds of engines and planes, We can confine this to a CT with a Rotax 912ULS and as Ed states: The statements in these threads only consider RPM, pitch and altitude.

 

RPM, altitude and prop pitch determine power setting

 

Power setting and mixture determine fuel burn.

 

We do not control mixture so we don't control fuel burn precisely.

 

Mixture may or may not lean with altitude depending on which circuit ( needle or main ) has control.

 

Air / fuel mixture mass reduces with altitude.

 

RPM alone does not determine power setting and power setting alone does not determine fuel burn rate.

[Top Cat]

I appreciate everyone's opinion. It seems that most would prefer to use a high swag on fuel burn rate rather than know it exactly. That is, they would rather feel any fuel calculation they make is comfortably optimistic rather than know precisely how much fuel they burn. Comfort is a good feeling.

 

None of the FAA knowledge exams take that tack, but after all, one only has to pass each of those once and then can continue to fly with a happy feeling rather than be burdened by exact knowledge, which is boring and bothersome.

Jim,

 

  The FAA exams are just that…exams. They may help you qualify for the license, but they don't make you an 'aviator'. That's as in…using your skill, knowledge and judgement to make prudent decisions in the air.

 

  You'll always have a happy feeling if you land with reserve and more. You'll always be unhappy if you landed in a field without enough. The FAA could fault you for failing the exam but I doubt it will fault you if you mismanaged your fuel and landed off airport.

 

 I agree that if you have figured and used accurate burn rates for planning and in flight then fudging your reserves to the point that you curtail the length of your cruise is being overly conservative. That being said, I'd never fault anyone for always  being prudently cautious.

[Tom Baker]

I appreciate everyone's opinion. It seems that most would prefer to use a high swag on fuel burn rate rather than know it exactly. That is, they would rather feel any fuel calculation they make is comfortably optimistic rather than know precisely how much fuel they burn. Comfort is a good feeling.

 

None of the FAA knowledge exams take that tack, but after all, one only has to pass each of those once and then can continue to fly with a happy feeling rather than be burdened by exact knowledge, which is boring and bothersome.

 

The examiner that I use for checkrides likes the idea of extra cushion and added reserves when it comes to fuel. In his view the FAA minimums are just that, minimums. He likes to see and hear that the applicant errors on the side of caution when fuel and weather minimums are concerned.

[ralarcon]

Jim,

 

  The FAA exams are just that…exams. They may help you qualify for the license, but they don't make you an 'aviator'. That's as in…using your skill, knowledge and judgement to make prudent decisions in the air.

 

  You'll always have a happy feeling if you land with reserve and more. You'll always be unhappy if you landed in a field without enough. The FAA could fault you for failing the exam but I doubt it will fault you if you mismanaged your fuel and landed off airport.

 

 I agree that if you have figured and used accurate burn rates for planning and in flight then fudging your reserves to the point that you curtail the length of your cruise is being overly conservative. That being said, I'd never fault anyone for always  being prudently cautious.

 

Agree, I try to take off, with the maximum amount of fuel, circumstances allow for. Recently my usual 1 hr 20 min flight took 2 hrs. Which shows it is hard to predict weather and prevailing winds, especially in Fla. The whole conversation is very useful though. Also sometimes , down here, you have to detour significantly just to avoid thunderstorms.

 

Cheers