Hartzell F7666 blades vs. F7496 blades

[Bob Axsom]

I ran a couple of tests over three months apart with the plane (RV-6A) in the long range race configuration to determine a top speed that can be compared with a new prop in exactly the same configuration. In July at 6,000 ft density altitude and everything maxed out for speed I got 176.0. Today without toping the tanks again I ran the same test and calculated 177.5. So the average is 176.8. Without changing anything except the prop I will someday run the same test again and report the results. Let's hope it is 3 kts faster.

Bob Axsom

[hydroguy2]

got my fingers crossed for you

[Bob Axsom]

I topped off the tanks and test flew the new prop today at 6,000 ft density altitude using the U.S. Air Race Handicap Procedure and the National Test Pilot School spreadsheet for three directions of flight. The speed in the long range race configuration (same as the baseline test - no changes) was 179.2 kts so it is approximately 2.5 kts faster than the average of the two previous tests before changing the prop and 3.2 kts faster than the other test with all tanks full. OK so I was secretely hoping for more but it is exactly as the Hartzell engineer told me it would be the results agree with Van's test results.

On the negative side it is noiser and rougher than the F7666 bladed prop. I will get it dynamically balanced and see if that provides any improvement.

Bob Axsom

[dedgemon]

So how do you think it would compare to the F7497?

[Bob Axsom]

I received some confidential charts produced for me by Hartzell engineering that indicate that the efficiency of the F7496 doesn't peak until around 185 kts and then decays very slowly. The F9497 never reaches the efficiency of the F7496 and it decays in a curve similar to the F7666 although higher. However at around 180 kts the F7496 is only 3 kts faster than the F7666 so the difference between the "96" and "97" should be less than that. Currently I am being limited by power I think more than the prop so the difference might be higher with a more powerful engine.

Bob Axsom

[hydroguy2]

Since you were due for a prop overhaul and you are always looking for speed, I'd say good job on both!

[Bob Axsom]

I have other mods that I have been waiting to install so the 3kt difference related to the propeller change alone will have to stand as fact. This seems reasonable since Hartzell engineering prediction and Van's testing also state the difference is 3 kts. There is a possibility that the difference in another configuration will be greater because of the higher efficiency of the F7496 blades at higher true air speed. The directly comparable test configuration was my long range race configuration. This configuration has four fuel tanks and a 23 ft. wingspan. The short range race configuration is approximately 150 lbs lighter with two fuel tanks and a 21.5 ft. wingspan. My earlier testing using the same test methods showed this configuration to fly at 184.4 kts. If these results are accurate and the difference in efficiency of the F7496 blade is increasingly higher than the F7666 as TAS increases then the short range racing configuration top speed may be greater than 187.4 kts. I'm pessimistic but I will look at it at some later time after the prop is dynamically balanced and the tip tanks are removed before the Taylor 100 Air Race next April. Before that happens I will hopefully install the "cruise" stock wing tips with lights, etc. and I may make some flat tips that will reduce the wingspan to 21 ft. for short range racing to copy Tom Martin and Wayne Hadath's experiments with their Rockets. And the beat goes on as Sonny & Cher used to say.

Bob Axsom

[osxuser]

I'd imagine the smoothness thing is a dynamic balance issue. We went from a 7666 to a 7497 on a customers 6a, and got ~5mph and it was smoother .

[David-aviator]

Bob,

I take it these props are all 2 blades.

Are you convinced a 2 blade prop is more efficient than a 3 blade prop is this HP range?

[Bob Axsom]

Originally Posted by David-aviator

Bob,

I take it these props are all 2 blades.

Are you convinced a 2 blade prop is more efficient than a 3 blade prop is this HP range?

I am convinced that they are but that is based on many years of having it impressed upon me through reading and limited experience reported by one friend. Hardly a scientific fact. I am interested in straight and level maximum speed with practical climb performance. In the back of my mind lies a nagging suspicion that if one was to optimize a fixed pitch prop for speed in my airplane the shape and configuration would be surprising in diameter, pitch, airfoil, blade profile and perhaps the number of blades. On a retired migrant aerospace worker's income I will never find it. This plunge has been very expensive and the value of a 3 knot gain is meaningful in my situation only because I want to do better in the races I participate in.

Bob Axsom