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-   -   Max Endurance Fuel Burn? (https://vansairforce.net/community/showthread.php?t=78799)

MartySantic 11-12-2011 08:48 PM

After reading this thread, my head hurts real BAD! Fly the RV-12! Enjoy it!
:):):)

AlanTN 11-12-2011 10:21 PM

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!

pierre smith 11-13-2011 04:43 AM

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,

JBPILOT 11-13-2011 07:05 AM

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

Paris12Man 11-13-2011 09:39 AM

"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.

Peterk 11-13-2011 10:11 AM

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

MauiLvrs 11-13-2011 10:14 AM

Quote:

Originally Posted by hevansrv7a (Post 595016)
First, it's Carson's speed, not Compton. ....

OOOpps...:o .. Yep...

Quote:

Originally Posted by hevansrv7a (Post 595016)
Yes, the L/D max should be minimum endurance x 1.32. Vy has nothing to do with it. ...

Technically true... But for most of us... Vy is at or very near L/D max.

DaveLS 11-13-2011 12:14 PM

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

AlanTN 11-13-2011 09:10 PM

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.

hevansrv7a 11-14-2011 07:51 AM

Correcting Vy statement
 
Quote:

Originally Posted by DaveLS (Post 595423)
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

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.


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