[hevansrv7a]

In a previous post on a perhaps-hijacked thread, I began to estimate the efficiency of a hypothetical prop that just happened to have the same numbers as the Ellipse on a 150 HP RV6. I have now completed a more thorough examination.

The original numbers (before wingtip changes) were:
8407 Density Altitude
1440 pounds
150 nominal HP
187 mph TAS (using two-way GPS) at 2700 RPM

I compared that airplane to the CAFE 6A and to Van's numbers for 150 HP. Van's says the 6, at gross weight and 8000' will do 186 mph on 150 HP. Van's also says that the 6 is 2 mph faster than the 6A. I used my spreadsheet iteratively to find that the drag would be reduced from 134.32 pounds to 130.93 pounds. I used ambient air density which is 78.6% of sea-level and got 117.9 HP for a 150 HP nominal.

Without any compensation for weight, that would reduce the prop efficiency to 79.13%. But, like the salad shooter, there's more.

The CAFE airplane was tested at 1650 pounds. Weight means drag. I used my other spreadsheeet - the glide angle one - to find that the glide ratio of the RV6A at 106 mph and 1650 pounds and 134.32 pounds of drag is 12.250. Now, we all know that weight does not change that, so the speed must decrease. The glide speed has to decrease by the square root of the weight difference. Skipping the math, the drag is now 117.16 pounds. This is not much different from Paul's estimate for the flat plate differences between the subject airplane and the CAFE airplane. There is a small error in here because I don't have the perfect L/D speed for the -6. The error favors the Ellipse prop because the glide ratio of a -6 should be slightly better than for a 6A, meaning lower drag.

Now we can go back to the Power&Drag spreadsheet to compute that the prop efficiency would be 78.3% for the given test conditions to match the estimated HP.

 

[sonny junell]

Thank you, i know that took some time thanks for sharing

 

[SHIPCHIEF]

Why are you using data from the 180 HP CAFE RV-6A against an unknown, non evaluated 150 HP RV-6 to derive efficiency numbers against one propeller, when the better propeller efficiency test is two or more props on one aircraft. This is how VAN evaluated props many years ago, and I would suggest that to be the accepted way.
Are you flogging a dead horse here?
Do you have a fixed pitch prop to compare to others in a meaninful verifiable way on a single aircraft with sufficient data collection?
Better yet, has your spreadsheet been verified against a sufficient number of applicable aircraft? (In this case, several RV-6 of different manufacturer )
The reason I ask, is because I've been reading your posts, and they a starting to look like an attack on one prop designer with a 'different looking' prop. You have gone to the point of putting words in his mouth asserting a drag figure he used to be a fact, rather than his estimate for use in designing the prop. That does not make a 'fact' so do not use it.
His props have excelled in some applications, and seem to be gaining some acceptance in others. Like everyone else' an increase in the population of his product over time will determine if it is a winner.

 

[Kyle Boatright]

Why are you using data from the 180 HP CAFE RV-6A against an unknown, non evaluated 150 HP RV-6 to derive efficiency numbers against one propeller, when the better propeller efficiency test is two or more props on one aircraft. This is how VAN evaluated props many years ago, and I would suggest that to be the accepted way.
Are you flogging a dead horse here?
Do you have a fixed pitch prop to compare to others in a meaninful verifiable way on a single aircraft with sufficient data collection?
Better yet, has your spreadsheet been verified against a sufficient number of applicable aircraft?
The reason I ask, is because I've been reading your posts, and they a starting to look like an attack on one prop designer with a 'different looking' prop. You have gone to the point of putting words in his mouth asserting a drag figure he used to be a fact, rather than his estimate for use in designing the prop. That does not make a 'fact' so do not use it.
His props have excelled in some applications, and seem to be gaining some acceptance in others. Like everyone else' an increase in the population of his product over time will determine if it is a winner.

IMO, Elippse largely makes a paper argument. It may be accurate or not, I don't know. Not enough back to back comparison information has been presented to prove that the claims are repeatable, consistant, verifiable, etc.

I think the original poster in this thread's intent is to present an alternate view of the theoretical discussion.

Personally, I have a hard time believing that after 100 years of propeller development, there are big gains to be made in prop design *unless* someone comes up with better materials.

I'm one of the guys on here who poo-poo's alternative engines too, until there is enough performance and reliability data to provide a meaningful comparison.

 

[hevansrv7a]

I will try to explain why and how

Why are you using data from the 180 HP CAFE RV-6A against an unknown, non evaluated 150 HP RV-6 to derive efficiency numbers against one propeller, when the better propeller efficiency test is two or more props on one aircraft. This is how VAN evaluated props many years ago, and I would suggest that to be the accepted way.
Are you flogging a dead horse here?

I'm trying to do the best comparison that can be done. I know that it is not perfect. I also refer to Van's data on the 150 HP RV6. The CAFE data itself is not perfect. That said, it's the best we have. The physics don't change. Paul says the airplane in question has comparable or lower drag than the CAFE 6A. What I'm doing is a valid technique when you have such good data. I don't agree that comparing two unknown props is all there is to it. Efficiency is not properly measured by using a comparison that is lighter by 210 pounds and does not have a nosewheel. Is the Ellipse faster on a given airplane than the A-D prop. Yes, but that's not a measure of actual efficiency, merely relative efficiency.

Do you have a fixed pitch prop to compare to others in a meaninful verifiable way on a single aircraft with sufficient data collection? No, but that is not the point, here.

Better yet, has your spreadsheet been verified against a sufficient number of applicable aircraft? (In this case, several RV-6 of different manufacturer )

The spreadsheet, to begin with, uses standard calculations and they are not hidden. Anyone can check them. Further, it balances to the observed data for all the CAFE airplanes that I used it on. The spreadsheet is a quick, reliable, repeatable way to solve some calculations that otherwise might be tedious or difficult. But, it does not do any un-tried math or math relationships.

The reason I ask, is because I've been reading your posts, and they a starting to look like an attack on one prop designer with a 'different looking' prop.

The very last thing I would want would be to criticize anyone for a different looking prop! A Norris prop doesn't/won't look conventional, either. I know of a prop that will be made by someone else altogether and it will be very strange to look at.

You have gone to the point of putting words in his mouth asserting a drag figure he used to be a fact, rather than his estimate for use in designing the prop. That does not make a 'fact' so do not use it.

I'm not attacking Paul and I have no investment in my BGT prop. Do I believe Jack Norris more than Paul? Yes. Is an honest look at the math an attack? No! Are we entitled to question and evaluate postings about how well that prop worked? Heck yes! When I have comparable data I will post it and everyone will be invited to evaluate it, too. I don't think I am putting words in anyone's mouth. I use what was said, but I don't make it up.

His props have excelled in some applications, and seem to be gaining some acceptance in others. Like everyone else' an increase in the population of his product over time will determine if it is a winner.

Nobody can argue with the fact that some of his props have done exceptionally well at Reno and I have openly admired Paul's successes. We have had misunderstandings for which I blame both my writing and his reading, but winning is winning. None of that changes what I wrote in this thread. 78% is what it is. If you think it's good or bad, it's your opinion. All I did was the best I could with data and accepted mathematical relationships. There are no "ad hominem" attacks here. You make a good point that some of his props do really well and some maybe not so terrific. I was only looking here at one where there is enough information to do the numbers. I have argued in other posts that Paul's results are inconsistent. He offers an explanation. That's a fair discussion, but there is room for growth and learning.

My responses are inserted above.

 

[SHIPCHIEF]

Fair enough.
Now I'll be fair to you, sit back and read some more of your work.
Thanks for contributing.