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  • #16
    Very interesting discussion, now for my 2 cents worth!
    I built a scratch built cub a few years back. It was light(750 empty) with a c90 with c-85 pistons and a Catto prop.
    Hal Stockton came through 2 summers ago with his RANS s-7 with the big bore 912 uls, and we designed a custom exhaust for him. Now of course we had to have a fly off and see who could take off and climb the fastest. Hands down the cub outclimbed him, by 200 fpm or so I would guess.
    Of course he could flat walk away in cruise but that's another story!
    In my opinion, the big bore rotax would make a great engine for the lsa, but a hot rodded c-85/c-90/0-200 while considerably heavier, are capable of making more power.

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    • #17
      Is your profile pict of the cub you are referring to above? Great looking plane.

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      • #18
        Yes that's it.

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        • #19
          Thats very close to the engine I would like to build. Do you recall the specs on the prop and what you turned static?

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          • #20
            The prop was a Catto 78"X38" and it would turn 2390 static. It would do 90-93 IAS at 2400. I think you would want at least a 42 pitch if not a little more, as the LSA is so much cleaner.

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            • #21
              Catto ran numbers for me when I was looking for a prop upgrade on my Luscombe with the stroked C85. He recommended a 78x38 but could increase the pitch to 40 if I wanted a little more speed. If we build a LSA we will use a C85 and a 78x40 Catto.
              Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

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              • Mark Goldberg
                Mark Goldberg commented
                Editing a comment
                We have found the Catto 76 x 44 to be excellent for the LSA. Mark

              • whee
                whee commented
                Editing a comment
                Awesome. Thanks Mark!

            • #22
              As Mark mentioned, Catto recommended the 76 x 44 for my Bob built 8.5 x 1 O200.

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              • #23
                Catto's recommendation is somewhat dependent on what you want to do with the airplane. I told him I wanted max STOL but to keep cruise speed at 100mph or better. The C85 has a lower max rpm than the O200 which will also affect Catto's recommendation. The prop selection for playing around on local gravel bars and a max rpm of 2575 won't be the same as a prop for general flying and a max rpm of 2800.
                Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

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                • #24
                  If I remember correctly from the EAA video, Bob said that he used the O-200 cylinders, crank, and cam in his C-85-8. This would effectively make it more like a high compression O-200 with a C-85 case. He is saving weight with no starter/alternator, and by using a lighter case. Most C-85s with O-200 cranks retain the C-85 cam and are limited to stock C-85 rpm. With a C-85 case supporting the loads of the high compression and rpm, I'm curious if Bob machined the case for O-200 through bolts. It sounds like raising the compression, and turning O-200 rpm may be pushing the strength limits of a C-85 case.

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                  • #25
                    Just recently ran my corvair engine at Barswell SC college. I built the 120 Hp (3Liter). They now have a 3.3 that makes 130hp. Now I gotta get building the plane!

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                    • #26
                      William Wynne builds the engine mount that he and Bob Barrows designed at Barnwell, SC

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                      • #27
                        There is a relatively easy formula to calculate the max speed a given prop could generate at a given RPM:
                        Speed (in MPH) = Pitch (in inches) X RPM X 60 (minutes per hour) / 63360 (inches per mile)

                        That's because the "pitch" indicates the theoretical (100% efficiency) maximum distance that the prop would "screw" itself through the air in a single revolution. So a prop pitched to 44-inches would move forward 44 inches with each revolution. At 1000 rpm (assuming 100% efficiency) the prop would pull itself 44000 inches through the air (that's 3667 feet, or just under .7 miles). Thus a 44-inch prop (76x44) turning at 2750 RPM (redline for an O-200) has a maximum theoretical airspeed of 114 mph.

                        The "length" of the prop (76" in this case) is pretty much a function of two things: ensuring prop clearance (max allowable length) and ensuring the torque of the engine is sufficient to turn the prop. So even if you have room for a longer prop to have sufficient clearance, most guidelines generally recommend no longer than 76" for the O-200. (Longer props are typically more efficient than shorter props - but only until the tip speeds near the speed of sound. After than, the thrust degrades quickly.

                        Catto props are said to be extremely efficient, and I suspect they pitch them so that the "rated" pitch already accounts for any aerodynamic loss (usually at least 5%). That should make the following table reasonably accurate for the 76x44 Catto prop. Based on this data, the 44" prop is clearly not optimized for best cruise performance, so it's likely a "climb prop" that would result in a higher "static RPM", and better takeoff and climb performance, albeit at the expense of cruise performance. The formula shows:
                        RPM MPH
                        2750 114
                        2650 110
                        2550 106
                        2450 102
                        2350 98
                        A 76x52 prop would calculate out as follows:
                        RPM MPH
                        2750 135
                        2650 130
                        2550 125
                        2450 120
                        2350 116
                        Of course, that might limit your static RPM too much, and you would almost certainly have longer takeoffs, and it would climb slower in the bargain, so you have to decide which is more important to you. A 48" prop would be a compromise between these two, with numbers that would split the difference exactly: (125 @ 2750, 120 @ 2650, 116 @ 2550, 111 @ 2450, and 107 @ 2350)

                        And I think this is exactly why a ground-adjustable fixed-pitch prop would make a lot of sense for the LSA, where you cannot legally use a constant-speed prop in order to qualify to fly as a Light Sport pilot. When you set out on a long cross-country, you can change the pitch to a "cruise" setting, and when you want to go play around on short strips or visit the back country, you re-set the pitch to a "climb" setting. Modern adjustable-pitch props make it pretty simple to change the pitch. I watched a guy change the pitch on a Sensenich prop at OSH (with the spinner already removed), and it took him less than 5 minutes, start to finish. Of course, he knew what he was doing and had practice at doing it, but still... Sensenich and Whirlwind both make good ground-adjustable props, and there are probably others out there as well...

                        As an aside, and to help convince myself that the formula was correct, I asked the Catto folks what pitch they would recommend for an O-360 powered Patrol to ensure a top speed of around 140 mph at 2700 RPM. They said they would recommend a 54" prop to deliver almost 140 mph, while still providing decent takeoff and climb performance. (That's where the 180 HP comes in handy!) The table for a 54" pitch prop shows:
                        RPM MPH
                        2700 138
                        2600 132
                        2500 128
                        2400 122
                        2300 117
                        And that matches up pretty closely with what Nicole Catto told me I could expect from the 54" prop.
                        Jim Parker
                        Farmersville, TX (NE of Dallas)
                        RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)

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                        • #28
                          After a lot of discussion with Nicole at Catto, I finally finally decided to go with a 74x40 prop as recommended. I told them initially that I wanted a 76x44 like Bob and Mark are using. But the difference is I have a stock 0-200, not the higher compression (8.5 x 1) version that Bob builds. From experience, the folks at Catto don't think that the stock 0-200 is putting out a true 100 HP...maybe somewhere around 92 ish.. Guess we'll see how it works out once I get it flying...

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                          • Mark Goldberg
                            Mark Goldberg commented
                            Editing a comment
                            I am running a 76"x44 Cato. Bob is using something completely different. Mark

                        • #29
                          My bad...I thought at one time Bob had tried the Catto and liked/recommended it. I also forgot to add that there is a difference between the 0-200A and the 0-200D.

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                          • Mark Goldberg
                            Mark Goldberg commented
                            Editing a comment
                            Collin - you are correct that Bob tried the Catto and does recommend it. But Bob likes to wind his LSA engine up above 3,000 RPM. Not many builders like that and want a more normal redline. The Catto on my LSA will get to 2850 RPM WOT in level flight. Not enough for Bob.
                            Bob used a McCauley prop that produced very good thrust. Then he made a carbon fiber prop from a mold he made from the McCauley. That is what he is running now I believe. Mark

                        • #30
                          I have a 76"x36" Catto(didn't formally choose it, it came with a core C-90 and is labeled for a Zenith CH-750).

                          I have a stock 0-200 in my LSA.

                          Full throttle level flight at around 3000' and around standard temp is 3000 RPM and around 115 Mph IAS

                          Cruise under the same conditions at 2650 is about 105 mph

                          Climb at full throttle/60 Mph IAS is about 2550 RPM.

                          All fine with me as I an more worried about density altitude here in AZ then cruise speed. Reality is I don't see over 2750 RPM anyway as I'm not in a hurry to get somewhere and just use that as my max cruising RPM.

                          Catto told me they can adjust about +/- 3" in pitch. If I ever get around to rebuilding my C-90 core with high compression pistons I likely will have it re-pitched as coarse as they can go with it.

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