Are GAMI Useful for M20J IO-360 Engines?

[A64Pilot]

I’ve not read through all of this, but to jump in, a prop is not just a wing as in all thrust is from the front of the blade, heck a wing doesn’t it truth only get lift from that.

‘You can get lift from a flat piece of plywood, a very significant amount of lift comes from the backside of a prop and the bottom of a wing, if Bernoulli’s principle was the only lift, most helicopters couldn’t hover, not enough wing area.

A prop does decrease efficiency from speed, usually and yes it’s from drag, but it’s not much.

For example a PT-6-34 engine is rated at 750 HP, you can get 750 HP at 58.7 lbs of torque at 2200 RPM or at 64.5 lbs of torque at 2000 RPM, both are exactly 750 HP and within the engines allowed limits, interestingly you can set Tq exactly at 58.7 lbs at 2200 and touch nothing but the prop lever and reduce it to 2000 RPM and tq will increase to 64.5 lbs.

So since the HP is identical and the 106” prop is operating more efficiently at 2000 RPM the airplane should be faster, right?

Its not, apparently the increase in efficiency isn’t enough to make a measurable difference, just like a WHOLE lot of “works on paper” discussions in the airplane the difference is tough to measure it’s so slight. Too bad we don’t fly paper airplanes. Now airlines get serious with this I’m told, they old kill for just a small percentage in fuel burn, but I decided it just wasn’t significant, my biggest gains in mileage was from slowing down, and while I don’t want to start a peeing contest but from my testing the biggest decrease in fuel consumption for me running LOP was from slowing down, if I ran peak to 50 ROP but slowed down to identical airspeed as LOP the MPG just wasn’t nearly as big as I had hoped, but my 540 parallel valve just didn’t like LOP, not like my 360 angle valve does anyway

Go out and try it yourself, fly at some known EGT either LOP or ROP and record fuel flow, LOP may be best as we agree HP is directly proportional to fuel flow within limits, I say within limits because it’s not really flat, BSFC does decrease if you go too deep LOP. Record fuel flow, change prop RPM up or down and maintain same mixture by re-leaning and reset the same fuel flow. Your making the same HP if your fuel flow is identical at 25 LOP at both RPM’s pretty much, see how much faster you go at the same power at lower RPM.

Do the low RPM test first, because if you do the higher RPM you may be at a HP that isn’t achievable at lower RPM.

It should be noted that a recip airplane at lower RPM is getting less drag from both the prop AND the engine, so both added together ought to be more significant than the turbine example that was mostly just prop, I say mostly as the power turbine will change efficiency somewhat with RPM too, but not much.

I did all this in my Maule with a multi point calibrated MVP-50 and there just wasn’t enough difference to matter to me, it was repeatable though, but bottom line even .2 GPH when your burning 12 GPH just isn’t significant, and about .2 was about the best I could measure. I went for smoothest RPM instead, for me in the Maule the decrease in airframe drag at altitude outperformed the better fuel burn down low LOP, At 10 to 12,000 LOP it, well it just wouldn’t we will leave it at that so my LOP was restricted to lower altitudes.

The Maule may not be representative as it was very draggy, in particular it had too much wing for cruise, hence the -7 degree flaps to spoil some excess lift.

Maybe Mooney results will be different.

‘Maybe in a Mooney’s slicker airframe it will.