85 kts = 98 mph .... So that would be 4gph at 98 mph...Is it better to loaf along at 4gph at 85 mph
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.
OOOpps... .. Yep...First, it's Carson's speed, not Compton. ....
Technically true... But for most of us... Vy is at or very near L/D max.Yes, the L/D max should be minimum endurance x 1.32. Vy has nothing to do with it. ...
I'm sorry but this is just wrong.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.
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.
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.
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!
I went flying tonight and set the throttle to 97 mph. Knots are for ships.
Fuel burn was 3.2 gph.
Some builders have been known to drink corn "squeezins"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!
Some builders have been known to drink corn "squeezins"
Joe Gores
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?
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!