Webb

Webb

Well Known Member
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Kevin Horton - you might be a good one on this since you are looking at fuel burn, power, hp, etc....
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All this info coming in lately dealing with fuel use, endurance, HP, etc…. makes me wonder how Van’s determined their range numbers.

If you use the 55% and 75% power range and do some math against how much fuel is on board, I wonder what they used for their standard and how it compares with the numbers that are posted by this group.

I have a 7A with an IO-360, 180hp, CS like Van’s using for reporting. If you use the range reported for the 180 HP setup and using that power setting and range, at 8,000 for 55% you get 5.28 hours, for 75%, you get 3.86 hours assuming there is no reserve. If you add 30 minutes for reserve (since these were designed as VFR planes), I increased the flying time by ½ hour and you get 5.78 hours and 4.36 hours.

Also it depends if the stats are determined if you run the tanks dry or allow for non-usable fuel (which I will call 2 gallons per side).

Now comes the comparisons for flow rates vs range by time (using Van's posted speeds for range):

Bone Dry (42 gallons)
55% - No reserve……5.28 hours……..……..8 gal/hr
55% - 30 minute reserve……5.78 hours…..7.26 gal/hr
75% - No reserve……3.86 hours………….10.8 gal/hr
75% - 30 minute reserve….4.36 hours……..9.6 gal/hr

With Un-usable fuel (38 gallons)
55% - No reserve……5.28 hours………….7.2 gal/hr
55% - 30 minute reserve…..5.78 hours……6.6 gal/hr
75% - No reserve…….3.86 hours…………9.8 gal/hr
75% - 30 minute reserve….4.36 hours…….8.7 gal/hr

At 8,000 feet, real numbers I would use for range calcultion is 9.2 gallons/hr at 75F ROP. Allow for 38 gallons usable fuel for a total of 4.13 hours of engine run time, and TAS of mph 190 mph. Take out 30 minutes reserve, and I can fly for 3.63 hours and cover 690 miles. Put the reserve in it and the range becomes 785 miles. According to Van's range for 75% power at 8,000 feet is 765 miles.

Therefore, I consider my range to be 690 miles.

So how does Van's calculate their range numbers?

It's amazing what you can think up when you are sitting at a meeting and don't have a dog in the fight nor do you want to be there after the holiday weekend.

Looking for other opinions or experiences on how Van's determined those numbers.
 
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Use 1 gallon unusable TOTAL and see what you get. It should be close enough to their numbers.
 
751 miles (nm?) with 30 minute reserve is very close to the 765 mile number. It does not answer what their numbers represent or how calculated. In practice, I use 450-500 nm as my refueling/bladder/leg stretch range.
 
Ron Lee said:
Use 1 gallon unusable TOTAL and see what you get. It should be close enough to their numbers.
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Yep. I'd have to check my POH for unusable fuel, but when I measured it (for my plane) it was a surprisingly small number. No where near 2 gal per side - more like less than one pt.
 
Ron Lee said:
751 miles (nm?) with 30 minute reserve is very close to the 765 mile number. It does not answer what their numbers represent or how calculated. In practice, I use 450-500 nm as my refueling/bladder/leg stretch range.
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Statutory miles is what Van's uses so that is what I used for the numbers

And yes, 500 NM is also my upper limit. I totally agree.

KevinH - As far as usable fuel, I wouldn't be comfortable using a pint for unusable unless just straight and level. A few turns or coming into land and I think we would see a pucker factor inversely proportional to the dimenishing level of fuel.
 
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I fly power settings and the stopwatch. My typical cross country profile generates a fuel burn of 8 gph from chock to chock at a cruise power setting of 65%. With 38 gallons, that allows me to fly 4 hours plus a 45 minute reserve at cruise speed.

With zero wind, that's 600 NM or thereabouts plus reserve, including a 20 minute cruise climb to >10k feet and a reduced power 300 fpm descent maintaining about 160 knots TAS. Comparing that to the data Van's publishes for the -6 (and doing some interpolating), I would say Van's information assumes no reserve.

With zero wind, my 600 mile range is roughly Atlanta-Houston, Atlanta-Miami, Atlanta-Oshkosh, etc, which is usually plenty for my derrierre and bladder.
 
4 hrs and 600nm w/o a pit stop ! We old guys are impressed! The Order of the Enlarged Prostate hereby dubs thee, "Iron Bladder." :eek:
 
Rupester said:
4 hrs and 600nm w/o a pit stop ! We old guys are impressed! The Order of the Enlarged Prostate hereby dubs thee, "Iron Bladder." :eek:
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SWMBO might disagree. In all of our X/C trips, I'm the one who caused our only unplanned stop for a potty break. She still gives me grief over that...
 
Kyle Boatright said:
I fly power settings and the stopwatch. My typical cross country profile generates a fuel burn of 8 gph from chock to chock at a cruise power setting of 65%. With 38 gallons, that allows me to fly 4 hours plus a 45 minute reserve at cruise speed.

With zero wind, that's 600 NM or thereabouts plus reserve, including a 20 minute cruise climb to >10k feet and a reduced power 300 fpm descent maintaining about 160 knots TAS. Comparing that to the data Van's publishes for the -6 (and doing some interpolating), I would say Van's information assumes no reserve.

With zero wind, my 600 mile range is roughly Atlanta-Houston, Atlanta-Miami, Atlanta-Oshkosh, etc, which is usually plenty for my derrierre and bladder.
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Somehow I don't think Vans used fannies and bladders for posted range estimates (but in real life they most certainly do).

I agree with you that I don't think Vans is using reserves for the estimate. I also wonder about the amount of unusable fuel.
 
Using 180 hp RV-7A data - Van claims a 75% cruise speed of 200 sm. Assuming you operate the engine in accordance with Lycoming's recommendations, you would be rich of peak EGT. The Lycoming power from fuel flow data I have suggests that 75% power a mixture for best power gives a fuel burn of 10.8 US Gal/hr. If we assume a 42 USG fuel capacity, that gives us 3.89 hrs to dry tanks, or 778 sm range. Van claims 775 sm. So, I conclude that Van's numbers are without reserve, without any allowance for unusable fuel, and do not account for fuel burned during taxi, take-off or climb. This is a marketing brochure number. Nothing more, nothing less. It has no relationship to range achieved in real world conditions, while complying with regulatory requirements for fuel reserve, etc. We all must establish our own perf data, valid for our aircraft.

Unusable fuel - I haven't attacked this test phase in any detail yet. All I've done to date is run one tank dry in level flight (the tank with the flop tube). In those conditions, there were only 0.03 USG (yes, 3/100, only a few drops) unusable fuel. I will be doing more testing later, looking at the effect of typical manoeuvring on unusable fuel, on my aircraft.
 
No reserve

I think Van's even said "no reserve" somewhere on the site.

RV-7A 180 HP 75% = 198 mph. Using their range number I get 10.87 gph.
RV-8 180 HP 75% = 204 mph. Using their range number I get 10.84 gph.

180 x .75 = 135 HP. 10.855 gph x 6.0 #/gallon = 65.13 pounds per hour. That's 0.482 pounds per HP per hour, a very reasonable ROP figure.
 
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