Propeller for the CX4

Discussion in 'Volkswagen' started by Marc W, Jun 16, 2019.

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  1. Jun 16, 2019 #1

    Marc W

    Marc W

    Marc W

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    When I bought my CX4 with a 2180cc VW it had a Tennessee 54x46 prop. I was never favorably impressed by its performance. It would not turn up and the airplane was slower than comparable airplanes. I broke that prop so that took care of that problem. Recently I have been running a borrowed Sensenich 54x44 prop designed for the Sonex. The 54x44 performed well but the engine had to spin up to generate any thrust.

    I own a Sensenich 60x44 propeller. I hadn't flown with it because when I first installed it, static rpm was only 2600 RPM. Since I got the engine pretty well dialed in while I was running the 54x44, I tried the 60x44 again a few days ago. It now turns 2900 RPM static and I have two flights on it. I didn't have good numbers for climb with the 54x44 but I can't really tell any difference. Initial climb was about 700 FPM at 3390 RPM with the 54x44 and it still is about 700 FPM at 2900 RPM with the 60x44. I did a speed run at an estimated density altitude of 7600' today which should be about 75% power. Wide open throttle gave 3200 RPM and 130 MPH IAS. The 54x44 had to turn about 3600 RPM to reach that speed.

    The biggest difference is how much more thrust the 60" prop makes at lower rpms. The props are the same pitch. Speeds are similar with both props but the 60" prop does the same speed at about 400 RPM less. I haven't figured it out yet. Lower RPM means less HP but the numbers don't reflect that. It's a puzzle but that's what keeps this interesting! We will see how it falls out in the long run.
     
  2. Jun 16, 2019 #2

    TFF

    TFF

    TFF

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    You have a lot more diameter. This is just example For load in your head calculations. A 60x50, 59x 51, 58x52,61x49,62x48 and so on will have about the same load on an engine. You can’t go to infinity like that but a baseline prop will give you a range. Add or subtract pitch or length without changing the other will change the baseline. 60x51 will lower the rpm; 59x50 will raise it assuming you are playing with the same prop family. Change family and you may or may not get transferred data. Some measure props different enough it’s a code. Load wise the big prop is the same as a 54x60 in horseshoe talk. All this is not design, it’s tuning.
     
  3. Jun 16, 2019 #3

    Pops

    Pops

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    I agree with TFF. Maybe 1" difference in dia is slightly more effect than 1" difference in pitch but what do I know? I do know you also need a prop to match the airframe along with the engine Hp and rpm. On the SSSC, I talked to Gramps at Culver props and he ask a lot of questions about the drag of my airframe and Hp of the VW and if I wanted a climb, cruise prop. He got it perfect with a 60" x 26" prop. Wouldn't change a thing.

    On the JMR, I also had a long talk with Gramps and he decided on a wide blade 68" x 48" for the C-85. That is the longest for the proper ground prop clearance. So he used the wider blade 69" dia prop and cut it down to 68". We will see.
     
  4. Jun 16, 2019 #4

    TFF

    TFF

    TFF

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    The smart thing is to call the experts like Pops did. Being able to switch the numbers a little here and there is conceptual. Like in Pops’ example with the slightly wider blade, that makes it a design change that only experts understand. Move one way or another and the prop makers will change width, tips, thickness that just a pitch or diameter will no translate. Unlucky you cant get a bag of clubs like in golf and can try each until you get it. On the Biplane Forum one of the members did have about five different props with a couple of brands at close to the same pitch to test on his S1S. It was surprising to see the metal Sensenitch gave the best numbers though it was the oldest design. It was replaced with a fancy Cato for weight loss on the crank and close to same performance. You at the altitude you play at adds to the conundrum. I’m at 250-300 MSL; totally different environment.
     
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  5. Jun 17, 2019 #5

    Marc W

    Marc W

    Marc W

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    I am not always smart! I may do that eventually. Right now I just want to understand what I have a bit better and decide which way I want to go.

    It doesn't look like the Sonex prop is a good match for my airplane. The owner of that prop doesn't think its a good match for his Sonex. Maybe it is because of the altitudes we fly at. My engine spun right up with it but all that spinning didn't translate into more thrust. The big prop doesn't spin near as fast but it makes about the same thrust. Right now I think I would like to see about 200 RPM more with the 60" prop and hopefully better climb. Top speed isn't that important to me but climb at higher altitudes is. I like the diameter of the big prop so I may reduce the pitch a couple inches and see how that works. I am going to test it more before I do any carving.

    Al Schubert has a list of "rules of thumb" in his book How I make Props, which you can download. Three of them are: (1) "Reducing the pitch by 2" increases RPM by about 100 RPM". (2) "Reducing the diameter by 1" usually increases the RPM by 100 RPM." (3) "Reducing the width of the blades by 1/6 usually increases RPM by about 100 RPM." You might know Al designed and built the Fledermas, a little VW powered airplane and carved a number of props for it, so his book is a handy reference for us VW owners. Al did like the smaller diameters.
     
  6. Jun 17, 2019 #6

    TFF

    TFF

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    Picking climbing over speed usually means picking a prop that will be at your redline and over in cruise. In climb an rpm about 200-300 rpm below redline will climb great, but you will have to pull the power back once level. The engine will be working close to max but not over. I would go out and see what these props do at a best clime rate. Note that RPM. It would be nice if you had a manifold pressure gauge. You could see how hard the engine is working. What you don’t want is great climb and then the prop being a brake in level flight; you picked to flat a pitch then.
     
  7. Jun 18, 2019 #7

    Marc W

    Marc W

    Marc W

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    You got it! Initial climb with the 54" prop was at almost 3400 RPM. The 60" prop climbs at a little over 2900 RPM. GPAS specifies cruise RPM of 3200 +/- 200 RPM so I have room to increase the RPM. I did just install a manifold pressure gauge so I have some MAP readings.
     
  8. Jun 18, 2019 #8

    Pops

    Pops

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    The first 32 hrs on the SSSC was with a 1200 cc, 40 hp VW engine with a 54"x 22" Culver prop. 3600 rpm on WOT on climb at about 550 fpm and cruise of 3200 rpm at 65 mph. Poor little engine was working it's heart out. Didn't like working the engine that hard. Built the 60 HP, 1835 cc engine. 1200+ ROC at 3150/3200 rpm at 1200+ FPM and a cruise of 80 mph at 2650/2700 rpm with the 60"x 26" Culver prop. World of difference.
     
  9. Jun 18, 2019 #9

    pictsidhe

    pictsidhe

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    Congratulations, you have a more efficient prop! Your engine will run cooler, live longer and use less fuel while you get the same performance as before.
     

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