Propeller design – science and experience

The following testing was performed in July, 2012. I just realized I never archived it in my blog so this is old news …

Performance tests of Aymar Demuth vs Catto Prop

Test Platform is my 150hp slow build RV-8. It’s not the cleanest or lightest RV-8 but it’s mine.

Tests using a modified version of the methodology described by Bob Axsom. I chose an 8,000′ reference altitude.

FYI: It turns out that simply substituting 8000 for 6000 from Bob’s method actually results in a normalized test altitude of 8300 ft.

Use the following test method:

  • Set altimeter to 29.92 climb to 6,000ft
  • Check outside air temperature if it is above 3C, subtract 3 and descend 100 feet X the remainder. If it is below 3C, subtract 3 and climb 100 feet X the remainder.
  • Set the throttle wide open, Prop max (if constant speed), mixture for best speed/power ~100 ROP.
  • Fly 360, trim for hands off level flight set the autopilot for the 360 ground track if you have one otherwise fly a GPS ground track (not heading) of 360. Set the altitude hold if you have one at the altitude determined earlier that will provide 6000 ft density altitude.
  • When the speed stabilizes make a GPS ground speed recording every 20 sec. until you have five in a row that are within one knot of each other.
  • Turn to 120 and repeat
  • Turn to 240 and repeat
  • Go home, average the five consistent recorded speeds on each leg.
  • Enter the Average GPS speeds and GPS ground track angles into the three leg worksheet option (saves about 1 gallon of fuel per test and is accurate) of the NTPS spreadsheet.
  • Then do it again (I actually do both test runs on the same flight) average the two results to give you confidence in the test validity)

When I first put my clean piece of paper on my knee board I write the date and what what I am testing at the top. When I get the ATIS I record the altimeter setting and the information identifier there as well just for reference because depending on what altitude I need to fly the test at, I need to have a method to tell Approach the altitude I will be flying at in an altitude reference that they understand (since my altimeter will be set to 29.92 and not the current altimeter reading).

On my 360 leg I have a reasonably level piece of land to fly over, east of Springdale and Rogers. If the speed stabilizes before rogers it is best – it is much easier to meet the 5 consecutive 20 second interval reading consistency requirement. If not the list of recordings can get quite long before 5 consecutive 20 second interval readings are within 1 kt of each other. When I turn to 120 I will initially have a relatively consistent surface but if I don’t get my 5 early I will be flying over mountains and the readings will get less stable. When I turn to 240 there is no efficient way to avoid flying over mountains. When I complete the 240 leg I boundary line my data sheet for test one, turn to 360 and repeat the process. That is the best test method I have been able to come up with to date.

Your observation of difference based on where you are flying is correct but thermals are only part of the inconsistency problem. Changing wind velocity, updrafts and down drafts also affect the GPS ground speed which the NTPS spreadsheet calls for. You are smart enough to understand that so I will not waste any time on explanation. A way to minimize the effect is by doing your testing early in the morning or (less favored) late evening on days when the wind is calm. I have mods to complete and races to go to so I just have to go when I can and try to get good data – its is not perfect.

Because the conditions are not static, even though you fly your tests back to back they will not be exactly the same on two consecutive runs but if you control every thing else correctly they should be within 2 knots of each other. I used to just make one run for time and money economy but now I just bite the bullet and make two.

It is important to establish you airplanes density altitude baseline for testing so that the test results are relevant to one another – once you select 8000 do every single test at 8,000 ft density altitude or the results are not directly comparable.

After I enter my leg speed average numbers into the NTPS spreadsheet and I get my speed and wind numbers I print the sheet or sheets for multiple runs and I staple them to the in flight raw data collection sheet. I write on every sheet the date, what was being tested and I sign them before filing for later reference.

- Bob Axom

 

All tests were WOT and leaned aprox 100 ROP.

The propellers being tested are the original 2-blade Aymar Demuth 68-71 all wood propeller with epoxy pain finish and the Catto composite wood core 2-blade with nickel leading edges.

After 6 runs with the Aymar Demuth propeller and 6 runs with the Catto propeller the average was 157.8 kts vs 161.4 kts. If I throw out the high and low from both groups, the numbers do not change much with 157.6 kts vs 161.4 kts.

The Catto was manufactured as a direct swap for the AD. This resulted in a significant gap at the backer plate as the AD propeller was much rounder than the Catto. Filling this gap may contribute to slightly better performance of the Catto but that was not part of the test.

A few interesting details …

  • Craig had not previously built a 2-blade propeller for a RV-8 with only a 150hp Lycoming. Based on conversations, he predicted a 2750 rpm at WOT and a 4kts speed increase. The WOT max RPM during all tests was 2750. Averaging across all tests, there is approximately 3.75 kts speed increase.
  • The weight is 11 lbs vs 12.5 lbs
  • Static is 2300 rpm vs 2025 rpm
  • WOT is 2725 rpm vs 2750 rpm
  • CHTs are slightly lower with the Catto – likely attributed to the more refined airfoil approaching the hub.
  • Climb is about the same.
  • Fuel burn at cruise RPM is about the same.
  • Craig estimated the 3.75 kts equate to about 12hp more going to thrust.
  • I don’t know propeller aerodynamics but I do know I’m going faster with the same engine.

While a little extra speed is nice, the primary reason for the propeller testing was the nickel leading edges. However, if you can get an Aymar Demuth propeller that matches your engine, its a very good performer for the price.

Addendum: During a regional cross country, the flight is between 9000ft and 11,000 ft DA because there was little to no wind advantage at a lower altitude. My flight was calculated at 142kts and average fuel burn over the 470nm 3.6 hr flight was exactly 7gph. (Fuel burn is around 10-11 gph for the climb and 3-5 gph for decent). Of course, when all the numbers are crunched, the performance increase equated to just 6 minutes.

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