Fuel burn

[gbigs]

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.  

 

The 912iS with sport upgrade is claimed to be 30% more fuel efficient than the 912ULS  all in-flight variables being equal.  Weight, wind, throttle position, DA etc. 

 

Thus, if you get a 7gph burn at 5400 rpm with the 912ULS the 912iS engine with sport upgrade will get 4.9 gph.   And that seems to be what is being reported.  At least it is for my plane.

 

I am getting 4.2gph at 5450rpm...I have seen 3.5gph but only at around 4800rpm or so.  My prop pitch is coarse and my fuel/air flow managed by a computer at any given throttle position.

[FastEddieB]

1) I have proven myself capable of not running out of fuel.*

 

2) My flights are often "back of the envelope" rough calculations. The flight should take about 2:30. I have at least 3:30 fuel. Should be enough. Let's fly! If the fuel gets lower than I'd like, I'll stop. Easy peasy.

 

3) For those who's situation let's them always take off with max fuel, great! But in the larger aviation world, "tankering" fuel is frowned upon as inefficient. Yes, it's better to have too much than too little, but in commercial operations "just right" is the goal.

 

 

*Though a couple of times I've been very, very close. Hopefully, lesson learned.

[Top Cat]

The 912iS with sport upgrade is claimed to be 30% more fuel efficient than the 912ULS  all in-flight variables being equal.  Weight, wind, throttle position, DA etc. 

 

Thus, if you get a 7gph burn at 5400 rpm with the 912ULS the 912iS engine with sport upgrade will get 4.9 gph.   And that seems to be what is being reported.  At least it is for my plane.

 

I am getting 4.2gph at 5450rpm...I have seen 3.5gph but only at around 4800rpm or so.  My prop pitch is coarse and my fuel/air flow managed by a computer at any given throttle position.

 I don't know who is reporting 7gph at 5400 with a 912 ULS.

 

At 5400-5500 rpm my 912ULS burns 5-5.2 gph.

 

I plan on 4.8 gph. If the Sport upgrade engine gets 10% better burn rate than the 912ULS then using my number it would be around 4.3gph.

[FlyingMonkey]

 I don't know who is reporting 7gph at 5400 with a 912 ULS.

 

 

Yeah, IIRC Rotax tables show ~6.5gph at WOT at 5800rpm for the 912S/ULS

[Doug G.]

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.

Yup, I am very conservative (as a pilot). I almost always plan an extra hour of fuel based on 5 gal. per hour. I only have a 2-3 hour bladder so that sets the legs. The reserve doesn't change many flight plans, even with more weight.

[gbigs]

 I don't know who is reporting 7gph at 5400 with a 912 ULS.

 

At 5400-5500 rpm my 912ULS burns 5-5.2 gph.

 

I plan on 4.8 gph. If the Sport upgrade engine gets 10% better burn rate than the 912ULS then using my number it would be around 4.3gph.

 

Like I said, the 912iS engine with sport upgrade is spec'd at 30% better in fuel efficiency by Rotax.  So if you get 5gph in your 912ULS then a 912iS will get 3.5gph which I have only seen at 4800 rpm or about 100IAS in cruise.  Since you gave no reference for your fuel burn...I assume you mean 4800 rpm at sea level DA.

[Ed Cesnalis]

The 30% applies to cruising at 92% throttle or less.  Its called something like econo-mode.  You won't see much if any improvement at WOT.

[Jim Meade]

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.

The problem I have with this whole line of thought is there is enough sloppy padding that no one really knows the fuel situation. I will bet a lot of money that airline and charter operations do not do a lot of swag type padding. There is nothing wrong with having a lot of reserve, but let's calculate it and not have it be the product of guessing.

[Jim Meade]

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.

I disagree. The FAA exams for the question we are discussing here are for the purpose of you demonstrating that you have the competence to calculate fuel usage. This is totally absent from the "let's round it off at 5 gph, well, shucks, let's make it 5.5" that we see in this discussion.

[Doug G.]

I never guess on what fuel I have, but there is no such thing as accuracy on distance per fuel burn. You can plan for winds, but the reality is you won't know until you are up there.

[Doug G.]

If I ever have a problem because of my planning, you'll be the first one I tell Jim. I know how to use the CTLS Performance info, but other than some sort of math exercise, why? Do you always use the .26 gal. taxi burn figure in your calculations?

[Doug G.]

I would also add that my calculations are not inaccurate, just purposefully more than needed.

[FlyingMonkey]

The problem I have with this whole line of thought is there is enough sloppy padding that no one really knows the fuel situation. I will bet a lot of money that airline and charter operations do not do a lot of swag type padding. There is nothing wrong with having a lot of reserve, but let's calculate it and not have it be the product of guessing.

 

But airline and charter operations work for profit.  Every gallon of fuel more than they absolutely need hurts their bottom line and eats into profitability.  I'm flying for fun.  The downside of me carrying more fuel than I need is practically nil.  The upside if I have to divert or encounter unforeseen circumstances is comparatively very large.  I'm pretty sure if there was almost zero cost downside to commercial ops flying full tanks all the time, they'd do it.

 

There are lots of things about commercial operations that are worthy of emulation.  Carrying exactly the fuel needed for the flight is not one of them, IMO.  Remember also the big boys have precise calculations for fuel burns at various weights and conditions.  We have a very general set of manufacturer information and much more rudimentary fuel management tools.  Even if you knew down to the tenth of a gallon *exactly* how much fuel you'd use on a given flight, why would you not carry extra if possible and the downside is minimal?  It only equates to more available options.

[FlyingMonkey]

I prefer to use as precise numbers as possible with no rounding or padding.  At the same time, I don't push things to the FAA minimums.  I have my own minimums that are much more conservative.

 

I think that's the point most of us agree with.  Nobody is advocating not knowing how much fuel you will probably use based on the best information available.  We're just saying if you have the capability for more beyond what is required for the flight plus VFR reserves, why wouldn't you take the extra fuel (assuming you don't run into weight issues)?

 

In my airplane, we can call 30 minute VFR reserves 2.5 gallons.  Add a gallon for unusable fuel, so say 3.5 gallons.  That is an amount of fuel I can barely even see in my sight tubes, which are the only means I have in flight of verifying fuel state.  There is simply no WAY I am flying down to that level.  The red marks on my tubes are my "must be on the ground" level, and they are at 5 gallons total (2.5g per side).  I've never gotten anywhere close to that fuel level, and don't really plan to.  with 34 gallons capacity there is just no need to do that.

[Top Cat]

I disagree. The FAA exams for the question we are discussing here are for the purpose of you demonstrating that you have the competence to calculate fuel usage. This is totally absent from the "let's round it off at 5 gph, well, shucks, let's make it 5.5" that we see in this discussion.

I don't agree with you. Passing the FAA exam demonstrates on the day that you have the competence to answer the question.

  After that, in the real world, a pilot should follow the FAA requirement for sure but that means also using judgement and personal limits.

 

 In my case I certainly don't follow your exaggerated comment 'aw shucks let's round it off' etc. Instead I've spent a lot of time figuring what my ACTUAL burn is. Allowing for what the book says it should be, my ACTUAL airplane/engine constantly burn 4.8 gph. That's what I plan on.

 

  If I were to use the FAA 'minimum' of 30 mins VFR at cruise burn rate I'd be legal to land with 2.4gph. Not happening. At the very least MY personal minimum is 4.8. That also exceeds my airplane's POH which warns against arriving with 4 gals because the engine could quit if high angles of attack were conducted, as in a go around. So here's an example of me following the FAA minimum and also the POH requirement, and taking the more conservative number. Arriving with no less that 4.8 gals in the tank is my personal absolute minimum.

 

At the other end, my maximum flight time at 4.8 gph is 3.25 hours. Depending on conditions on the day I might get 3.5 hours but by this time I'm deciding when to land in order to have that above mentioned minimum of 5 gals in the tank.

 

 In this regard I feel comfortable that I'm not only meeting the FAA requirement, I'm also showing why I answered the FAA exam question correctly, and I'm acting as the pilot in command by making decisions on the side of caution, prudence and common sense.

[Top Cat]

Like I said, the 912iS engine with sport upgrade is spec'd at 30% better in fuel efficiency by Rotax.  So if you get 5gph in your 912ULS then a 912iS will get 3.5gph which I have only seen at 4800 rpm or about 100IAS in cruise.  Since you gave no reference for your fuel burn...I assume you mean 4800 rpm at sea level DA.

 

I 'plan' on a 4.8gph burn..for the flight. including start up, warming oil, climb, cruise descent. I would 'plan' on arriving with 15 gals if I left with a full tank of 19.8.

That's the 'plan'. Of course once in the air the plan will be revised. Climb to a higher altitude for smoother air will cost greater burn for climb power but should burn slightly less at the higher altitude. The longer descent at above idle power would offset the increased burn in climb. Again, with experience, I've found, with the type of flying I do, 2500-4500feet, at 5300rpm typically gives me 4.8 gph.

[FlyingMonkey]

I disagree. The FAA exams for the question we are discussing here are for the purpose of you demonstrating that you have the competence to calculate fuel usage. This is totally absent from the "let's round it off at 5 gph, well, shucks, let's make it 5.5" that we see in this discussion.

 

Implying that everybody who doesn't do fuel calculations for every flight your way (or the FAA exam way, if you prefer) is incompetent is not helping to sway people to your point of view.

 

Everybody here who is flying has passed a checkride, and thus been shown competent to calculate fuel use.

 

Frankly your tone is insulting, and I think intentionally so.

[ralarcon]

I never guess on what fuel I have, but there is no such thing as accuracy on distance per fuel burn. You can plan for winds, but the reality is you won't know until you are up there.

 

Amen !, fuel calculations (I use Garmin Pilot, which compensates for weather) are a rough estimate, sometimes accurate ,sometimes not so much. Commercial passenger aircraft have run out of fuel. And then there is this guy !

 

 

https://www.youtube.com/watch?v=2G9qXASYusU

 

 

Cheers

[gbigs]

The 30% applies to cruising at 92% throttle or less.  Its called something like econo-mode.  You won't see much if any improvement at WOT.

 

I don't fly at WOT.  The claim is not for any particular configuration of flight, it is for ALL configurations of flight.  If their claim is not true, then they are liable for fraud in advertising, a pretty serious legal situation.

 

Here they claim a 38% to 78% better fuel burn than any other engine in the LSA class:  http://www.flyrotax.com/desktopdefault.aspx/tabid-410/551_read-1361/

 

Here they claim a 38% better fuel burn than the 912ULS engine, even more than they did before:  http://www.flyrotax.com/desktopdefault.aspx/tabid-410/551_read-1303/

[IrishAl]

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.  

 

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

 

I’ll try to clarify the issues that Andy has raised in this thread.

 

I like to write to clarify my own thinking and this may be helpful to others. (Sorry if it’s boring or repetitive to more experienced heads.)

 

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

 

It depends on the load.  Load equals the power generated at a steady rpm..  So an engine under a very small load will spin at 5000rpm at 5 or 10% throttle.  Add a little more load, and you’ll need 25% throttle to maintain 5000 rpm, etc.

 

In an aircraft, engine load is related to prop pitch in that a coarse pitch takes more power to spin at 5000 rpm than a finely pitched prop, all other things being equal.  

 

So a coarse pitch prop will need more throttle, or more gasoline, to produce the extra energy to maintain a certain rpm,and so the fuel burn will be higher.

 

CT’s point is that, because every aircraft is pitched differently, you can’t make direct comparisons of fuel consumption at any particular RPM, because it's not a level playing field.  

[IrishAl]

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.

 

Then that must mean that 5000rpm at 75% throttle is a leaner mixture than at 5000rpm at WOT...correct?  

Not so.  When you open the throttle, you don’t just put more fuel in, you also let more air in, so mixture is not affected.  

 

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?  

It goes into more power - more air and more fuel together produces a bigger explosion.

 

 

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?

Not so.  When you open the throttle, you allow more air in. 

 

At idle, the engine is virtually strangled – there’s a very small hole for air to get in when the throttle is closed - that’s why inlet manifold vacuum is at its highest when the engine is idling. 

 

Opening the throttle makes the hole bigger, so more air rushes in, (manifold pressure increases) and more fuel is sucked up into the venturi.  The cylinders receive more air & fuel, producing a bigger explosion and more power.

 

 

 

A further variable is introduced by altitude.  Air is thinner at altitude, so less air gets in for the same settings as on the ground.  The Bing’s automatic mixture control adjusts the needle so that fuel flow is reduced by a similar rate so that the mixture remains correct.

 

Thus, at 7500’, less fuel is allowed into the engine at WOT than what is allowed in at MSL due to the auto mixture control. 

 

 I believe CT said that WOT at 7500’ is roughly equal to 75% throttle at sea level.  

[Ed Cesnalis]

Andy,

 

5,000 RPM and lower manifold pressure means less power and less burn.

 

5,000 RPM and higher manifold pressure means more power and more burn.

 

More / less manifold pressure means more / less fuel air mixture, the pressure effects the mass.

[FastEddieB]

A further variable is introduced by altitude. Air is thinner at altitude, so less air gets in for the same settings as on the ground. The Bing’s automatic mixture control adjusts the needle so that fuel flow is reduced by a similar rate so that the mixture remains correct.

Well, the needle that's accomplishing this is attached to the slide.

 

So if in fact you have the throttle fully advanced, but the needle is restricting flow, that MUST mean the slide is not open all the way.

 

Doesn't it? I may have to go back and find/read that pdf on BING operating principles!

 

Linkies: http://contrails.free.fr/temp/type64e.pdf

 

http://www.omnilex.com/public/bmw78/cvcarb.pdf

[Ed Cesnalis]

Well, the needle that's accomplishing this is attached to the slide.

 

So if in fact you have the throttle fully advanced, but the needle is restricting flow, that MUST mean the slide is not open all the way.

 

Doesn't it? I may have to go back and find/read that pdf on BING operating principles!

 

Linkies: http://contrails.free.fr/temp/type64e.pdf

 

http://www.omnilex.com/public/bmw78/cvcarb.pdf

 

When the throttle is advanced beyond 92% control of mixture is supposed to be transferred from the  needle jet circuit to the main jet circuit resulting in a full rich mixture.

 

Ii in fact you have the throttle fully advanced, but the needle is restricting flow you have a carb that is not performing as designed.

[FastEddieB]

That seems right.

 

Does this help?