Testing a New Prop Design, first installment

[Howard Handelman]

http://home.cogeco.ca/~n17hh/PrinceEllipticalProp.html

I welcome your comments, of course.

[Chad Jensen]

Great write up Howard! Keep the reports coming!

[Howard Handelman]

I did some more testing today (a nearly perfect day for it) while using my Catto 3-blade. I will write that up as an addendum to the first installment. But here's some HP information which is largely independent of the prop.

I was at 10,000' density altitude, 8540 pressure altitude, temp about 51 degF. My power meter said 75% while going 170 KTAS. My engine was at 2690 RPM x 22.6". My atmosphere table says that 22.6" is equivalent to about 7500'. Or said another way, I was getting a net 0.84" of boost (over ambient) from the ram effect of my front-facing snorkel. The density of the air at 10,000' is 73.85% of standard. The pressure at 22.6" is 75.5%. Let's ignore the -10 RPM. That means that my power reading of 75% (best power mixture) is very reasonable. At least so it seems to me.

My fuel flow was 10.5 gph or maybe a little higher (readings are not as stable as I'd like). Superior says my best SFC is .48. Maybe .50 is reasonable. Let's use 6 pounds per gallon (this varies more than I like but you have to use something; CAFE says 6.02). So my 10.5 gph is 63 pounds. If my SFC is .50 then my HP is 126 which is 70% of 180. If it is 0.48 then the HP is 131.25 or 73%. Close enough?

Note this was at 10,000, not 8000'. My readings at DA 8000 were 2720 x 24.3, 81%. 12.45 gph, approx. 56 degF, PA 6680. TAS was about the same. An SFC of .48 gives 86%. SFC of 0.50 gives 83%.

Note that I am ignoring all those altitude-compensated power charts from the manufacturer. Superior admitted to me that theirs is copied from a Lycoming chart and turned 90 degrees, but there are some errors in it. Anyhow, Lycoming doesn't specify how they configure the motor for the test. The use of fuel flow and SFC was suggested by Walter Atkinson of GAMI and I agree with him that it's a more reliable method but still not completely precise. Superior's SFC number is not a constant so we don't really know what the SFC is at a given moment, only a reasonable range of values. At least one of Kevin Horton's spreadsheets which are supposed to nearly match Lycoming data does say 81% at 8000'. Walter and Kevin have been very generous with their time and expertise. I hope I'm not misinterpreting what they told me.

Lonnie Prince has agreed to build one test prop per Jan's design - to start. I checked with a leading aero expert and the formula Jan is using is essentially correct. My other expert does point out that the "devil is in the details" when looking at the corners of the prop and the constants in the formula. I plan to validate the first test prop using aviation scales to measure torque and will report that when I can.

So far, lots of data, some of it highly suggestive, but no rock-solid proof, yet. More to come. As always, I'd welcome and appreciate comments from the guys who actually understand this stuff!

[Max Torque]

Great job, Howard! Glad you're doing this and writing about it.
Good to see what Jack Norris has been advocating for many years is finally catching on! (Jack Norris' book: Propellers Explained http://www.propellersexplained.com/ )
Looking forward to your updates!

[Howard Handelman]

Max, thanks. Jack Norris is a "client" of mine. I did the technical work on his website. The writing is Jack's. I own a copy of the book. Lonnie knows him, too. However, the Prince Elliptical is NOT a Norris designed prop. Lonnie designed it independently and as stated earlier has sold many of them, especially to military customers. I am not sure, but I think that Prince's prop predates Jack's publication of the book. There are no fixed pitch Norris designed props for sale that I know of. The Whirlwind RV series IS based on the Norris/Bauer work, but that is a CS prop. If you look at the details, there are some significant differences between Jack's design and Lonnie's'. Still, Jack advocates the tapered tip concept and he is aware of the Prince props from a visit to the Prince Aircraft factory a while back. Jack will tell you that you cannot just eyeball a prop and know how correctly it is designed.

For those that would like a Norris designed prop, Jack will license the design for a nominal fee which goes to Andy Bauer's widow. Jack will design the prop to your specifications. That may translate into my doing it for Jack to pass along to you, but that's not my call. Jack's contact info is on his website. Be aware that Jack's program designs for only one airfoil. He can explain that in more detail. Also be aware that in order to get the design right, you have to know the drag curve of your airplane. That's where Jack's pioneering work in 1995 comes in and where my more recent work (see my website) comes in, too. This is not easy stuff.

[Max Torque]

Thanks for the info, Howard! I'm overseas and wondering...how would the optimum t.o./climb prop design for an homebuilt bush plane - similar to a 135 hp Super Cub - be shaped? Airfoil? Length? etc.? That's what I need to find out!

[Howard Handelman]

I don't design them, only test them once in a while. Thus, I suggest asking the people who design props. Jack Norris, Craig Catto, Lonnie Prince in no particular order. Of those, the last two build them. You could also talk to Paul Lipps, but I'm not completely sold on Paul's designs (but many other people are). Also, since you are overseas, you could contact Jan Carlsson at:
( http://www.homebuiltairplanes.com/fo...-carlsson.html ).

In a few weeks time you could look into a new manufacturer in New Zealand. The product Launch is next week and it will be unusual, but I can't say more than that, yet. See: http://www.nzflair.com/flair-brochure.pdf, Friday, 14 Oct., 1:30 pm.

Finally, I'll just beat this drum one more time: You can't design the prop correctly if you don't know the drag curve of the airplane. For example, your best rate of climb is at your airframe's speed for minimum sink (no, not L/D max!) except that the engine may not perform well at that speed. That works better for CS props, of course. Your maximum rate of climb is the excess HP available (1 HP = 33,000 foot-pounds/minute) divided by the weight of the plane (in pounds) giving feet-per-minute. But what's your HP and what's your airframe's THP requirement at least sink? Aha! And, btw, the speed for best angle of climb is even slower (again, power-plant modifies this). Notice that I use HP and THP. The difference is the prop's efficiency which is not a constant.

Please refer to my presentations at AirVenture last year and this year for more on that and feel free to contact me for 1:1 discussion. I enjoy obsessing on this subject!

[David J. Gall]

Howard,

Nice work! Regarding your horsepower measurement issues, may I recommend Arthur P. Fraas' "Aircraft Power Plants" (1943) as an excellent reference.

See you at Oshkosh next year,

David J. Gall
P.S. I design propellers, too!

[Howard Handelman]

David, nice to hear from you. Everyone: Dave knows his stuff. I don't know anything about his props, yet, but his knowledge and understanding are awesome. His scholarship is beyond that. By all means, include him in your short list of "go to" guys. I am looking forward to some posts about Dave's props, next.

Howard,

Nice work! Regarding your horsepower measurement issues, may I recommend Arthur P. Fraas' "Aircraft Power Plants" (1943) as an excellent reference.

See you at Oshkosh next year,

David J. Gall
P.S. I design propellers, too!

[David J. Gall]

Thanks for the kind words, Howard. I applaud your real-world efforts to measure and quantify. Please carry on!