Great points and agree 100%
Kevin Horton said:
I wouldn't discount flight test data just because it disagrees with a prop efficiency map.
Excellent points and agree 100%. Thanks for the torque data. I called Thunderbolt engines (Lycs custom engine product). They are at Reno and than Red Bull San Diego. The answer was there are no documents with torque, but they where going to get me something when I get back. I suspect they will do what you did, derive it.
Reading between the lines, Jim seems to be saying running at 2,200 RPM is more efficient. He very well be right, but more efficient and fast are two different things. Also more efficient compared to what? I don't think the MT is inherently more efficient at 2,200 rpm than say a metal Hartzell at 2,200 rpm. In general the slower you turn the more prop, the greater the efficiency, but than we get into less HP. I have some data below.
Also, I agree 100% about Hartzell's prop data and actual flight test being different. I was told the same thing, engine and airframe have an affect on the theoretical predictions, certainitly. That is why we do flight test. However I'm talking about large differences between predicted and reported prop efficiency. Since Jims data some what takes out the engine/airframe, it was the same for each prop test.
Going to higher altitudes you expect to go slower (with out a turbo). Climbing = less HP; Less HP = less speed; Forget that the airframe has less drag at altitude or the prop has different efficiency, disregard it. I estimate you lose 10 mph due to less HP alone. Including the reduced airframe drag with altitude, you would not lose that much TAS.
To lose 15.5 mph, assuming the extra 5.5 mph loss was from the prop, would mean a huge increase in prop inefficiency. Looking at the Hartzell data, at 2,700 rpm changing only (estimated) HP, airspeed & air density, reflecting the change in altitude, prop efficiency loss is less than 0.25%; overall prop efficiency is about 83.3%. That is 1/4th % change in efficiency.
To lose 5.5 mph (or more) due to the prop alone, means an 8% efficiency loss, or less than 75% overall. That's not going to happen, at least from the prop predictions from Hartzell. Considering airframe drag was disregarded that means the prop would have to go into the tank. A 2 by 4 would do better. I know for a fact Hartzell, in the real world, the C2YK/F7666 is a better prop than that; thus my contention the data is not correct.
That's why I say, an efficient prop at sea-level is still going to be efficient at altitude, in the range of RPM, airspeed and tip Mach# we run. If a prop is on fire at sea level, its not going to suck suddenly at 12,500 ft
Kevin I see no magic or gremlins in a Hartzell prop or any prop that makes it significantly better or worse at altitude. It's just an efficient prop and can't wrap my mind around the prop dying at 12,500 feet at any RPM.
RPM & prop diameter does have an affect on efficiency. Diameter being fixed we only control RPM (and pitch). From Jim's data you see the general correlation, as you climb speed drops and higher RPM = more speed. However there inlays my complaint, all the changes in slope (a line with lots of curves, hills and valleys) and cross-overs, when the data is plotted It should be a fairly straight line with may be a slight overall curve but not a sine wave.
RPM affect?
Lowering RPM from 2,700 RPM to 2,500 RPM, efficiency prediction says you gain 1/2%. That's about 3/4 HP more thrust say at 8,000ft. Efficiency is simply prop thrust (HP) divided by HP at crankshaft into prop. 20% loss is the generic number people, but it varies based on flight condition (air density, airspeed, rpm, HP at crank). The down side of lower RPM, is most engines make less HP. In the case in point about 6 HP less power to gain 1/2% prop efficiency. You expect speed to decrease with RPM. 2 mph less speed in exchange for better miles-per-gal is fine, but again efficiency and speed are different. If you could keep the HP up at lower speed that would nice.
If a prop is optimized for lower RPM, like at 2,200 rpm which was mentioned to take advantage of peak torque, you may gain more efficiency. However efficiency and speed are not synonyms. Going faster is often not efficient, but who wants to go slow.
As Ross has pointed out many times,
gear reduction and turning the prop slower has efficiency advantage. Of course a
gear reduction has some disadvantages. Regardless if you want to "gear down" and run slow prop RPM's, the prop has to be optimized for lower RPM for a given HP & airspeed. Hartzell's are not made to run real slow. Turbo-prop props do turn as slow as 1,900 rpm. Of course their fwd speed is higher, so slower RPM keeps tip speed's down. Over Mach 0.90 tip efficiency goes to heck.
Hartzell prop guys know how to balance efficiency and speed for a direct drive Lycoming for a plane with RV performance. The above was in reference to the older but still efficient HC-C2YK/F7666-4, not the newer Hartzell blended airfoil prop, which is optimized even more for the RV. It's 3 mph faster than the C2YK/F7666. That's pretty amazing. (Are you listening Hartzell where is my free prop, give me give me.
.)