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85hp Upgrade Carb & Prop Question

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  • #16
    I was able to do 104 knots in level flight with a 7443 Sensenich...
    N29787
    '41 BC12-65

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    • #17
      That's moving! Thanks for the info.
      Stu

      '46 BC12D N96234
      Jefferson County (0S9)

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      • #18
        I can too but the cows get big fast!

        Gary
        N36007 1941 BF12-65 STC'd as BC12D-4-85

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        • #19
          You're going to need a little shorter prop than 74". To get the 85HP the engine needs to see 2575 static and the 74" prop is too long. It will run on the engine but down at 2300 the engine is only making about 75HP.
          If you have a bunch of props you can use, slave in a manifold pressure gauge and select the prop that gives you the rated RPM (2575) at the maximum manifold pressure. That's about the best you can do.
          EO

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          • #20
            It's all about choices and compromise with fixed pitch propellers.

            Static rpm on the ground or water while stationary for a C-85 (in TCDS A-696) is listed by Taylorcraft as 2350 for Landplane and 2310 for Seaplane. Continuous rated engine limits for all operations 2575, (85 hp; there's some small % latitude for rpm) by Continental. Where in the flight regime the rated rpm occurs partially depends on propeller diameter and pitch (density altitude and available power are others). Generally a short prop needs more pitch and a long prop less pitch to absorb and transfer the given level of available power. At sea level and dry air the engine will make more power than at altitude and humid air. And so on.

            If takeoff and initial climb are important then a propeller diameter is usually chosen to maximize disc area which affects the amount of air blown, and pitch chosen to minimize stalled portions of the prop blade during that phase of flight. The efficiency of that (or any) fixed pitch prop is limited over a range of rpm and airspeed. Takeoff thrust (important on floats and skis in deep snow), climb rate (important during obstacle clearance), and cruise speed (efficient for miles per gallon and generally not to exceed rated rpm), often require different propeller specs. Constant speed props which vary pitch over a broader range are less of a compromise.

            The reason I wrote this is there's choices to be made as Edwin notes and manifold pressure can help with that choice (I install them for that and to detect carb ice). Mine's to maximize takeoff thrust on floats and skis. Obstacle clearance or rate of climb has never been a concern or priority, and cruise is secondary to takeoff for a 300 mile round trip with four hours of fuel onboard excluding winds aloft.

            Gary

            N36007 1941 BF12-65 STC'd as BC12D-4-85

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            • #21
              Propeller shops have experience and data to recommend a propeller manufacturer, diameter, and pitch to optimize performance in a phase of flight. Larger diameter and flat pitch for takeoff, smaller diameter and more pitch for the later phase of climb or cruise. One I've used is Dominion Propeller in Anchorage, AK. (http://dominionpropeller.com). There's others of course. They would know what works.

              Gary
              N36007 1941 BF12-65 STC'd as BC12D-4-85

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              • #22
                There is no "perfect propeller", only the perfect propeller for your application. Tip speed and clearance limit the length a propeller for the installation. Factors that effect the thrust produced are many. Airfoil shape, area of the blade, angle of attack (not pitch angle), density or the air and speed of the blades through the air. The propeller changes its cross section and pitch angle as you move out on the blade, so in one revolution the root and tip move forward the same distance, thats why re-pitched props can have some weird issues. Geometric pitch and effective pitch ratio will change with the drag of the aircraft. That's why the same prop on different planes differ in performance. The AOA is changed as the engine speeds up or airspeed changes. There is too many variables to list to be able to select the correct prop, put one on and try it. That's really the best way to find the correct prop for you.
                EO

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                • #23
                  Yes Edwin AOA affects performance, but pitch and its distribution are the only direct propeller dimension we can adjust at a given diameter. Propeller airfoils vary so there's choices between manufacturers as well. I guess we could also speed up or slow down to change AOA and efficiency but that's beyond reality for most.

                  The problem with the install and try method is....where in the range of options does that particular propeller lie? Having a variety of propellers to match the airframe and engine helps. Without that option then experienced propeller shops can help focus on a range of options for the owner's flight profile.

                  In years past we had prop swap days that involved testing various props for static thrust against a pull scale. There is a range between propellers. Obviously not every Taylorcraft owner wants to fly loaded on floats or skis or off short strips and instead enjoys the moment of flight and efficiency of cruise. Knowing which group you're in helps.

                  Like many I've flown fixed and constant speed props of varying types, some on the same aircraft back to back. It comes down to choices.

                  Here's what Stu wants: "Lastly any climb prop recommendations as my 7442 isn't going to cut it. I am just trying to get an idea of what to look for to start and then can dial it in."

                  Gary
                  N36007 1941 BF12-65 STC'd as BC12D-4-85

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                  • #24
                    Rather than more dancing around the subject here's my choice for my flying: For takeoff and climb install the longest propeller that's approved pitched to a compromise between maximum static rpm and still capable of no more than +5% over speed WFO in level flight. I can control overspeed so max static is critical for me. Propellers and performance vary by manufacturer so consider that as well.

                    Gary
                    N36007 1941 BF12-65 STC'd as BC12D-4-85

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                    • #25
                      Fly on any propeller you want to. I have my preference also for the longest prop I can fit on the aircraft. All fixed pitch metal props are now manufactured by Sensenich, even McCauley.
                      EO

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                      • #26
                        Thanks for all the info, I am learning a lot and it's been very informative. I had heard before and think its funny that Sensenich manufactures McCauley but I suppose that's how it goes these days.
                        Stu

                        '46 BC12D N96234
                        Jefferson County (0S9)

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                        • #27
                          This topic brings up some old memories. Back in the early '70s I had been in Alaska for 10 years and ridden as a passenger in Part 135 aircraft on the job. I took an interest in learning and owning an airplane. I asked before and after purchase for advice on this and that.

                          "It depends" was the typical reply from those that had time or interest. Mechanics were often most helpful as long as it didn't interfere with their work, but pilots rarely bothered. I guess they figured they'd survived without much help and so should others.

                          Today when someone asks or a discussion develops I try to be better than those I knew. It's not possible to know or pick every right answer and these forums can help broaden the range of experiences.

                          Still> Ask an expert at a prop shop for what they've seen work. They get immediate feedback if it doesn't.

                          Gary
                          N36007 1941 BF12-65 STC'd as BC12D-4-85

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                          • #28
                            Originally posted by Edwin Otha View Post
                            Fly on any propeller you want to....
                            I'm not convinced that is sound advice.. Airframes and engines have Type Certificate legal limits as to what maintains your airworthiness (and hence legality). In this instance, prop type, pitch and diameter.

                            Experimental aircraft may be different (legally) but TC data is always a good guide for what I call "Jane Eyre" (Sense and Sensibility)

                            Rob
                            Last edited by Robert Lees; 05-18-2019, 14:05.

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                            • #29
                              Originally posted by Kerbs View Post
                              Thanks for all the info, I am learning a lot and it's been very informative. I had heard before and think its funny that Sensenich manufactures McCauley but I suppose that's how it goes these days.
                              Uhm, where did you hear that? Both have machine shops and foundries to forge them...unless things have changed in the last 10 years...
                              N29787
                              '41 BC12-65

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                              • #30
                                Sensenich, does indeed manufacture all metal McCauley propellers basically as a supplier at their Lititz, Pa plant. Their Florida plant does the wood, composite and wood propeller rebuilds. Source, Sensenich President during plant visit last year.
                                Mark
                                1945 BC12-D
                                N39911, #6564

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