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Max Endurance Fuel Burn?

A

AlanTN

Active Member
I understand from the RV-12 POH that the max glide ratio speed is 85 kts. I assume that this is also the max endurance speed. For those who have tested it, what have you found the fuel burn rate 85 kts to be?
 
I think he was looking for max endurance speed. I wish there were a table that showed this. Is it better to loaf along at 4gph at 85 mph (21.25 mpg), or to open it up a bit and go for 5gph at 100 (20 mpg) for instance. Easy to do the math for those two, but what about the rest of the range.
 
DonFromTX said:
Is it better to loaf along at 4gph at 85 mph
Click to expand...
85 kts = 98 mph .... So that would be 4gph at 98 mph...
But the most efficient way to spend money is at the Carson's speed which is ~1.32*L/Dmax. (L/Dmax~= Vy)
Above the Compton speed fuel burn increases significantly with a disproportional small increase in speed.
 
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I confused the knots with mph, my error. Never heard of the "compton" stuff, will have to investigate that.
 
Max endurance should be at best glide or max L/D. That's where total drag is the least. Faster and parisite drag kills effeciency. Slower, and induced (AOA) drag kills it.
 
OK, that is helpful. With that knowledge then, if we cruise at 98 mph we should get the best fuel mileage then?
So when I add wheel pants, intersection fairings and every speed mod I can find, will that INCREASE that most miserly speed since there is less drag? By how much?
 
Max Endurance Fuel Burn

Max endurance, i.e. max time in air for given fuel quantity, is surely the speed obtained when flying at minimum fuel flow.
So providing T's & P's keep in limits, simply fly at lowest RPM in green RPM arc that allows level flight.
You will use the least fuel per hour and so stay airbourne for the most hours.
 
confusion

I think perhaps I asked the question the wrong way. One source says that Max Endurance Speed (how to stay in the air the longest) is Best Glide Speed/1.316 (65 knots in this case)

It also defines V optimum cruise as the lowest fuel consumption per knot. that is (Vbg) best glide speed * 1.316, 112 kts in the case of the RV-12. That is referred to as Carson's Speed in the source I am reading.

What I was really looking for is the Maximum Range (Vbr) fuel consumption. According to the specs on Van's web site the Max Range is 614 miles.

They use 5000 RPM for the Max Range. My suspicion is that you would be traveling faster than 85 kts (98 mph) at 5000 RPM. If 5000 RPM does produce 98 mph and the fuel capacity (usable) is 19.8 gal then the fuel consumption is about 3.2 gal/hr., which is about 31 miles per gallon. (If it is true, that would be very good.)

Since 85 kts. appears to be the best L/D ratio, it would likely also be the most efficient cruise speed. I was just curious if anyone had verified the fuel consumption at 85 kts. in flight.

I am considering an RV-12 and trying to understand it better. Sorry for the engineer talk. :)
 
Wind is a factor too. In a head wind, going faster is better because the wind will not be pushing the plane backwards for as long. In a tail wind, going slower will increase the time that the wind has to push the plane forward. In my case, there is almost always a head wind. :D
None of the above theory matters because my tank does not last as long as the aircraft tank, if you know what I mean. :D
Joe Gores
 
AlanTN said:
I think perhaps I asked the question the wrong way. One source says that Max Endurance Speed (how to stay in the air the longest) is Best Glide Speed/1.316 (65 knots in this case)

It also defines V optimum cruise as the lowest fuel consumption per knot. that is (Vbg) best glide speed * 1.316, 112 kts in the case of the RV-12. That is referred to as Carson's Speed in the source I am reading.

What I was really looking for is the Maximum Range (Vbr) fuel consumption. According to the specs on Van's web site the Max Range is 614 miles.

They use 5000 RPM for the Max Range. My suspicion is that you would be traveling faster than 85 kts (98 mph) at 5000 RPM. If 5000 RPM does produce 98 mph and the fuel capacity (usable) is 19.8 gal then the fuel consumption is about 3.2 gal/hr., which is about 31 miles per gallon. (If it is true, that would be very good.)

Since 85 kts. appears to be the best L/D ratio, it would likely also be the most efficient cruise speed. I was just curious if anyone had verified the fuel consumption at 85 kts. in flight.

I am considering an RV-12 and trying to understand it better. Sorry for the engineer talk. :)
Click to expand...


5000 RPM will give you about 4.8-5.0 fuel burn and in our aircraft that's about
108kts indicated or around 115kts true at 3,500 feet.
 
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So, obviously no one is going to be able to go 614 miles at 5,000 RPM. I was curious if it is even possible at the most efficient speed of 85 knots. Any body have a burn rate at that speed?

BTW, I am not negative about the RV-12. At this point I think it is the best option.
 
I believe the following is correct. If wrong please correct. The following is based on my memory of "Aerodynamics for Naval Aviators."

Maximum enderance speed equals minimum decent speed, which according to page 6-10 of the POH is 60 kts. It is not maximum range speed.

Max glide speed is equal to maximum range speed and minimum lift to drag speed. Again according to page 6-10 of the POH this is 85 kts.

Most efficient cruise speed (considers time to get somewhere) is an entirely different question but is usually 1.33 times Lift/Drag speed (85 kts). So 1.33 times 85 kts which is 113 kts. Again this is not maximum range speed.

So for max range, set the speed at 85 kts, find the fuel flow at this speed and see how far 20 gallons will take you.

Sometimes engine and propeller issues can change these numbers.

Hope this helps.

Dave

Fuselage kit just received.
 
To follow up, the math would produce the following, rounded to two places:

Assuming 614 nm, 85 kts is correct, and 20 gallons of gas, no reserve.
Flight time: 7.22 hours
Fuel flow: 2.77 gal per hour

I do not know what the actual fuel flow at 85 knots actually is.
 
Dave, what you stated in #13 is pretty much what I was saying in #9. The reason you got a lower fuel burn and a longer time in #14 than I did is that you were assuming 614 nm and I think that Vans RV-12 specs on their web site are speaking of 614 statute miles. (It simple says 'miles.')

In any case what I am wondering is if anyone who is flying an RV-12 can verify the burn rate at 85 knots.
 
The RV-12 has a Dynon 180?
Set the power where the MPG, flight time is highest.... Problem solved.....

But it has a ground adjustable prop....
So you'd have to do that exercise a number of times with different prop settings.......

Oh... and extracting meaningful data out of the vast amount that is generated by the Dynon is more work and effort than one would think...
Make you appreciate professional test pilots much more and explains why they need sooooo many test flights to get useful statistical data.....
 
FWIW

First, it's Carson's speed, not Compton. Yes, it's 1.32 x L/D max.

Yes, the speed for minimum fuel burn in level flight should be the same as the maximum endurance speed. Yes, the L/D max should be minimum endurance x 1.32. Vy has nothing to do with it.

Best MPG on the Dynon should work if it's based on airspeed and you keep a constant altitude. The TAS will vary, but the IAS should be the same at various altitudes. If it's based on GPS, you have to test that way in either zero wind or 90 degrees to a very light wind.

Please see all the spreadsheets on my site for how to determine speeds ad how to adjust for wind and a lot more.
 
Dave, did you ever refine the drag polar that you were working on a couple of years ago for the RV-12?

By now there ought to be enough people flying to test the reality of the theories about the RV-12 performance.
 
Fortunately or unfortunately, as mentioned...even small changes to the pitch of the prop have a significant effect. Van's recommendation is 4950 to 5000 RPM WOT on the ground/no wind.
 
Marty, I am just trying to find someone who can tell me what their fuel burn rate is at 85 knots.

Someone asked me why I asked the question. That's when it got complicated. :) Do you know the answer?

I suppose you are shut down for the winter now because of the cold weather. Too bad!
 
Go fly

Alan, I experimented with my -10 yesterday, doing low fuel burn runs. I know it isn't a -12 but the same principle applies....fly it at 85 knots for a half hour and get your own accurate answer....X 2.

I ran LOP and saw around 174 MPH and 11 GPH.

Best,
 
Hey Alan - -

I have stayed out of this one but - I fly year around. I have sealed up the cabin, and greatly improved the cabin heating system. Works great. Almost never miss an entire month. When the snow gets cleared from the runway, and the wind is not too high, I go flying.

John Bender
329 hours
 
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"I am just trying to find someone who can tell me what their fuel burn rate is at 85 knots. Do you know the answer?"

I will be happy to post an answer to this question as soon as the weather cooperates in N.E. Texas. Looks like another week or so before weather will allow a flight for me.
At 95kts IAS, my fuel burn is 4.3 GPH.
 
My Dynon said 3.8 at 85kts IAS today, but then I don't put much faith in the 180 when it comes to fuel...not their strong point, but I hear they're working on it. When I built mine there had been no EAB attempts (Van was the first). If I did over, I would go EAB and put in mechanical fuel gauges...and a 180 for all the other wonderful things it does do.

Pete
 
If thrust does not vary with airspeed (i.e. Jet) then Vy equals L/Dmax. Propeller efficiency/engine issues make thrust vary with airspeed.

Taking Van's numbers for the RV-12
@ 7,500 feet & 5,000 rpm, range 614 miles (assume statute) @ 116 mph (assume true), and using all 20 gallons (no reserve) would result in:

Flight time of 5.29 hours
Fuel burn rate of 3.78 gallons per hour
30.7 statute miles per gallon

These numbers seem, to me, to be in general agreement with what pilots are reporting in other RV-12 threads.

Note that these numbers are not optimized for max range (85 kts indicated) or endurance (60 kts indicated).

However, 85 kts indicated at 7,500 feet is to 113 mph true - pretty close to 116.

-Dave
 
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Thanks, Peter. Your 3.8 gph agrees closely with Dave's calculated results.

Dave using the same calculations and working backwards for cruise at 5500 RPM I arrive at 99 knots for the IAS and 4.7 gph. And at the 5000 RPM cruise I get 88 knots for the airspeed. I am assuming 2% increase in speed per 1,000 feet.

It seems as though the RPM is higher than I would expect for both of these cruise levels.

I must not be understanding something correctly.

Also the cruise speed suggested here is slower than I would have expected.
 
Correcting Vy statement

DaveLS said:
If thrust does not vary with airspeed (i.e. Jet) then Vy equals L/Dmax. Propeller efficiency/engine issues make thrust vary with airspeed.
(snip)


-Dave
Click to expand...
I'm sorry but this is just wrong.

Vy is best rate of climb. Best rate of climb is achieved when available horsepower is at its maximum. By definition, this would be at the speed which requires the least power for level flight and that is about 76% of L/D max. For a graphical understanding, please see: http://home.cogeco.ca/~n17hh/Models/ClimbGraph.jpg

But let's be clear that these are theoretical and often have nothing to do with our actual numbers, especially on FP but even with CS props. Engine power is not a constant. In reality, if Vy and L/D max are equal it is a co-incidence. On my airplane, Vx and Vy are pretty much the same at 110 kts indicated, which is well above my minimum power speed of around 75 kts indicated. And don't forget the weight affects these numbers.
 
hevansrv7a said:
I'm sorry but this is just wrong.

Vy is best rate of climb. Best rate of climb is achieved when available horsepower is at its maximum. By definition, this would be at the speed which requires the least power for level flight and that is about 76% of L/D max. For a graphical understanding, please see: http://home.cogeco.ca/~n17hh/Models/ClimbGraph.jpg

But let's be clear that these are theoretical and often have nothing to do with our actual numbers, especially on FP but even with CS props. Engine power is not a constant. In reality, if Vy and L/D max are equal it is a co-incidence. On my airplane, Vx and Vy are pretty much the same at 110 kts indicated, which is well above my minimum power speed of around 75 kts indicated. And don't forget the weight affects these numbers.
Click to expand...


sorry but this is just wrong too...

Vy occurs at Maximum Specific Excess power and rarely occurs at minimum power required because of prop efficiency losses at lower airspeeds.
 
I agree that Vy and max L/D are not necessarily the same. 'Stick and Rudder' page 364 paragraph 1 makes the statement, "Of all the working speeds of the airplane this is the one whose exact location on the speed scale is hardest to fix; too much depends on the exact characteristics of the power plant---r.p.m., propeller pitch and diameter and so on."

That whole section on page 363 and 364 is worth reading.

My original idea was that the RV-12 POH defines the speed of best glide as 85 knots, and the speed of best distance (most miles per gallon) should occur at the speed of best glide or possibly slightly higher because you don't have the prop drag that you would have in a glide. Vy is much more complicated.

I was attempting to discover under what conditions Van's was getting 614 miles on 20 gallons of fuel. (Actually I think it is 19.8.)

The comments here have been quite helpful.
 
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Can we admit we agree?

RV8R999 said:
sorry but this is just wrong too...

Vy occurs at Maximum Specific Excess power and rarely occurs at minimum power required because of prop efficiency losses at lower airspeeds.
Click to expand...

We seem to agree that in real life, the maximum available power over that required to hold the airplane up is what makes it climb and that the rate of climb is the weight times the rate divided by the numbers for unit of power.

For a theoretical engine, my statements are correct because they are based on airframe behavior (curves) and an assumed constant thrust HP. That is what the graph shows.

As I pointed out in the example of my airplane, the various factors (rpm, torque curve, high AOA, altered cooling, prop efficiencies (advance curve, etc.) and others) in our engines mean that the actual V speeds for climbing can be just about anywhere with no way to know except to experiment. My minimum power speed for level flight is about 75 and my best climb (x or y) is about 110. That's even higher than my best L/D of about 96. If I had a constant speed prop I could probably get my best ROC well below 96.

The only important take-away here is that neither can be determined from the other.

The nice thing about EFIS's such as my GRT is that Vx is easy to find. Anyone with a VSI can find Vy.
 
Oh, btw, one more thing...

AlanTN said:
I agree that Vy and max L/D are not necessarily the same. 'Stick and Rudder' page 364 paragraph 1 makes the statement, "Of all the working speeds of the airplane this is the one whose exact location on the speed scale is hardest to fix; too much depends on the exact characteristics of the power plant---r.p.m., propeller pitch and diameter and so on."

That whole section on page 363 and 364 is worth reading.

My original idea was that the RV-12 POH defines the speed of best glide as 85 knots, and the speed of best distance (most miles per gallon) should occur at the speed of best glide or possibly slightly higher because you don't have the prop drag that you would have in a glide. Vy is much more complicated.

I was attempting to discover under what conditions Van's was getting 614 miles on 20 gallons of fuel. (Actually I think it is 19.8.)

The comments here have been quite helpful.
Click to expand...


I don't know who said that the best glide for the -12 is at 85, but it's important to understand what is being said.

The speed at which the airplane will get its best mpg in zero wind is unlikely to be the same as its best glide with a zero power engine and also unlikely to be the same as a glide at minimum idle. Further, zero power on the engine can be with stopped prop or windmilling. The simplest way to understand this is to understand that a zero power engine means there is prop drag whereas your best mpg will be with the engine running and thus no prop drag. In the case of the C-152 that Jack Norris tested, the difference between a prop-less glide and a glide per the POH was very significant.

If you want to know your best glide with a dead engine you have to accept the risk, do it carefully, and just do it (hopefully over an airport).

But if you want to know your best speeds with power on there are many methods and my two Oshkosh presentations (on the website) explain that in detail.
 
hevansrv7a said:
We seem to agree that in real life, the maximum available power over that required to hold the airplane up is what makes it climb and that the rate of climb is the weight times the rate divided by the numbers for unit of power.

For a theoretical engine, my statements are correct because they are based on airframe behavior (curves) and an assumed constant thrust HP. That is what the graph shows.

As I pointed out in the example of my airplane, the various factors (rpm, torque curve, high AOA, altered cooling, prop efficiencies (advance curve, etc.) and others) in our engines mean that the actual V speeds for climbing can be just about anywhere with no way to know except to experiment. My minimum power speed for level flight is about 75 and my best climb (x or y) is about 110. That's even higher than my best L/D of about 96. If I had a constant speed prop I could probably get my best ROC well below 96.

The only important take-away here is that neither can be determined from the other.

The nice thing about EFIS's such as my GRT is that Vx is easy to find. Anyone with a VSI can find Vy.
Click to expand...

But in real life the THP is not constant throughout the operating envelope
 
The RV-12 Pilot Operating Handbook on page 6-10 published by Vans states the the RV-12 best glide speed is 85 kts.

'Aerodynamics for Naval Aviators' states that the best rate of climb (Vy) for JETS is L/Dmax. This is not necessarily true for propeller aircraft. I just double checked this.

Alan, I think your math works well, starting at 116 mph true and ending up with 88 KIAS, where the other example started at 85 KIAS and ended up with 112 mph true, all at 7,500 feet.

-Dave
 
I went flying tonight and set the throttle to 97 mph. Knots are for ships. ;)

Fuel burn was 3.2 gph.
 
Thanks, Larry. Do you have any idea what your RPM was? It is interesting that Peter used 3.8 gph and you only used 3.2 gph. It must be those rivets that you filled in! :)
 
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Eating crow

In the intrest of setting the record straight, I posted information that is just plain wrong. Thanks 'heavensrv7a' for pointing out my error. After rereading (for the third time) here is what I should have said:

L/Dmax is not equal to Vy for a jet, it is equal to Vx, and maximum endurance for jets only. P94 and p154 AforNA

L/Dmax is best glide (max distance in a glide) for both jets and props (if prop drag is ignored, a big if). p94

And,

L/Dmax is the max range speed for props. P94

L/Dmax is a great number because going faster or slower will increase drag. Flaps, retractable landing gear, and weight will all change the speed at which L/Dmax is found. Increasing drag will lower it, decreasing weight will increase it. When flying slower than L/Dmax, raising the nose will eventually lower the aircraft.

-Dave
 
Lower nose --- glide farther!

"When flying slower than L/Dmax, raising the nose will eventually lower the aircraft."

Dave's last statement needs to be burned into the brain of every pilot. In an approach at below the best glide speed, to glide farther you have to lower the nose. Totally non-intuitive!
 
AlanTN said:
Thanks, Larry. Do you have any idea what your RPM was? It is interesting that Peter used 3.8 gph and you only used 3.2 gph. It must be those rivets that you filled in! :)
Click to expand...

Actually Alan...since Larry lives in corn country, he is allowed to use fuel without ethanol (go figure) while those of us that live in civilization are forced to burn corn "squeezins". There ya go!

Pete
 
Actually Alan...since Larry lives in corn country, he is allowed to use fuel without ethanol (go figure) while those of us that live in civilization are forced to burn corn "squeezins". There ya go!
Click to expand...
Some builders have been known to drink corn "squeezins" :D
Joe Gores
 
600

So 19.8 galls divided by 3.2 multiplied by 97 mph gives 600 statute miles as the absolute range.
 
Here in Tennessee we can also got non-ethanol gasoline. One source I have says that E10 has 97% of the energy that gasoline has. So that would only account for about 0.1 gallon per hour difference.

Actually if Larry burns 3.2 gallons per hour at 97 mph indicated, that would be about 112 mph TAS at 7500. So 3.2 gallons will get you about 693 miles.

What is your secret, Larry?
 
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gblwy said:
So 19.8 galls divided by 3.2 multiplied by 97 mph gives 600 statute miles as the absolute range.
Click to expand...

Wait a minute. If you climb while flying (according to your calculations) you could get an extra 20-40 miles of glide at 13 to 1! My weak attempt at humor is meant to say you better calculate a fuel reserve. ;)
 
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AlanTN said:
Here in Tennessee we can also got non-ethanol gasoline. One source I have says that E10 has 97% of the energy that gasoline has. So that would only account for about 0.1 gallon per hour difference.

Actually if Larry burns 3.2 gallons per hour at 97 mph indicated, that would be about 112 mph TAS at 7500. So 3.2 gallons will get you about 693 miles.

What is your secret, Larry?
Click to expand...

Tail winds. ;)
 
AlanTN said:
Thanks, Larry. Do you have any idea what your RPM was? It is interesting that Peter used 3.8 gph and you only used 3.2 gph. It must be those rivets that you filled in! :)
Click to expand...

RPM was 4260. I took a picture of it. :D

I would lean toward the 3.8 actually. I just sent my Dynon in and they may have recalibrate my K factor. I think it is fairly close, but I am not 100% sure until I do some verifying.

I would like to think it was superior flying skills. :rolleyes:
 
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