As slow as you can go.
The slower you turn the prop the better. The less power the less fuel burn. Also the airframe is closer to best L/D, Lift over Drag. However not many pilots want to cruise at 110-130 mph either, but you can and will save fuel. (I think L/D is 100 mph (or 110 as the previous post said) at sea level, but it increase as you climb.)
Altitude is a player. Assuming zero wind, usually 7,500-12,500 feet, for most of us, is a good compromise altitude for a "going somewhere" flight, giving the good flight time to fuel economy compromise (about 65% power). An altitude that allows wide open throttle and gives 75% power is better for time while still giving good fuel econ. For most RV's with good induction, 75% is around 8,500'-9,500', with a moderate RPM. If you really are racing and there are no winds, than WOT / MAX RPM / down low is the way to go (if there are no winds). This will hurt the wallet.
Winds. This is the game to play when you can. I have climbed real high on a few occasion to get awesome tail winds. That's as close to a free lunch as you can get. I have seen consistent ground speeds of 230-240 mph in my RV-4. At work my record gnd speed, I have seen, was about 700-720 mph. It also works the other way; 170 kts on the nose is no fun.
The other "theory" is to climb well into the mid teens and run down at 50-55% power or less. Aircraft weight plays a roll in optimal cruise altitude. Also the time and fuel used to climb may be a detriment depending on leg length and winds. At some point it is diminishing returns, HP is to low to maintain best L/D. If you have a 150HP engine you will be altitude limited. If you are over powered with 180-200HP you can obviously fly higher. I have not crunched the numbers but it depends on the plane and weight. I guess FL180 is a good performance limit altitude for a light weight RV with a 180/200HP engine, about 15,000'/16,000' for a 150/160HP RV. Above that you are just getting near stall.
Practicality comes into play. How slow do you want to go. Most are happy with an econ cruise below 11,500 ft, 65-75%, making good time, wide open throttle (21.0 in-hg or less) and RPM as low as you can go. I say as low as you can go based on type and brand prop. I run 2300 rpm because it gives a smooth feel and is above the Hartzell 2250 RPM continuous restiriction. Also I don't want to go slower than that.
If you have a fixed pitch prop throttling back to get lower RPM can be counter productive because you are forcing the engine to SUCK past a restriction which cost HP (called pumping losses).
The best you can do is fly to altitude, WOT and take what ever the RPM is. Depending on how you "SIZE" you fixed prop it will be optimal only at one speed and altitude.
This is the Nitty-Gritty where the C/S prop shines and is more efficient for a wider range of conditions. Many point to just better takeoff and climb for the C/S prop, but they can be more "efficient" in cruise. Also the Hartzell BA prop for example is metal which allows thinner blades, which makes it more efficient than thicker fiberglass over wood blades.
C/S props also let you run the RPM where its smoother, more quite. This does help a little with engine friction losses. For example, 75% power with 2,500 RPM and 75% at 2350 RPM (differnt MAP's), the 2350 RPM will actually have less fuel burn for the same power and approx airspeed. Constant speed props excel in these optiamal low RPM conditions. You can set the RPM independent of throttle setting.
If I did not have the 2000-2250 RPM restriction (which the BA Hartzell does not) and wanted to set some personal cruise MPG record, I would climb till my MAP was about 18.5-19" (or 12,500'), wide open throttle, set RPM to about 1900-2000 RPM. That would give me about 55% power, and of course lean the heck out of it. Its practical and requires no O2 legally. It still is going to give speeds near the 180's (RV-7, 180HP). For better MPG, if you don't mind sucking O2 to climb into the teens, climb higher, but your TAS will decrease, saving a little more fuel. As you climb into the teens you will need to add RPM, which is fine, but prop efficiency goes down and you get more engine friction losses.
At some point practical, efficiency and hey I want to get there today come together. Most of use rarely fly at optimal efficency. Look at the numbers above. By flying technique you can increases fuel econ by 50% that is pretty good, but most are happy with may be 20% better econ for a little loss in speed. I just want to go up and have fun and look at the country side when I go X-C. All this optimization is too much work.
The whole topic of what is best econ can't be discussed in a vacuum. Aircraft weight, sea level HP, prop, cruise density altitude, cruise winds, airframe drag and range (leg length) all factor into the mix.
With gas over $3.00/gal may be we will all fly at 130 mph. There is no one answer, but its fair to say the closer you fly to best L/D the better econ you will get. IN general less RPM and the higher you go the better.
George
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