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11-11-2011, 06:32 PM
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Join Date: Apr 2009
Location: Propwash Airport (16X), Texas
Posts: 136
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Quote:
Originally Posted by AlanTN
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. :-)
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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.
Last edited by RDOG : 11-11-2011 at 06:37 PM.
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11-11-2011, 07:44 PM
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Join Date: Apr 2011
Location: Linden, TN
Posts: 79
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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.
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11-11-2011, 08:20 PM
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Join Date: Oct 2007
Location: Pensacola, FL & 2R4, Loveland, CO
Posts: 222
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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.
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120500
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11-11-2011, 08:29 PM
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Join Date: Oct 2007
Location: Pensacola, FL & 2R4, Loveland, CO
Posts: 222
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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.
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11-11-2011, 11:05 PM
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Join Date: Apr 2011
Location: Linden, TN
Posts: 79
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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.
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11-12-2011, 03:37 AM
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Join Date: Jan 2008
Location: KTCY
Posts: 643
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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.....
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Dave & Trina
RV-9A Flying  - 330 Hrs. Painted  Finishing the interior.
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11-12-2011, 05:40 AM
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Join Date: Jul 2005
Location: Detroit, MI
Posts: 1,587
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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.
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H. Evan's RV-7A N17HH 240+ hours
"We can lift ourselves out of ignorance, we can find ourselves as creatures of excellence and intelligence and skill. We can be free! We can learn to fly!" -J.L. Seagull
Paid $25.00 "dues" net of PayPal cost for 2015, 2016, 2017 and 2018 (December).
This airplane is for sale: see website. my website
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11-12-2011, 05:12 PM
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Join Date: Apr 2011
Location: Linden, TN
Posts: 79
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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.
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11-12-2011, 06:15 PM
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Join Date: Oct 2007
Location: Pensacola, FL & 2R4, Loveland, CO
Posts: 222
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No Alan, but I plan to readress it after my 12 is completed.
It will go int a display described here - http://www.ihmc.us/groups/oz/
Dave
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11-12-2011, 07:59 PM
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Join Date: Mar 2008
Location: Texas
Posts: 1,378
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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.
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