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2 blade vs 3 blade prop

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  • 2 blade vs 3 blade prop

    I know all the theories about this, mainly 3 blades are less efficient, but has anyone out there done actual flying comparisons? Has any one done static tests with a digital fishing scale? No speculations, please. Just the facts. Thank you, the doctor

  • #2
    Re: 2 blade vs 3 blade prop

    I have not done digital fishing scale tests or whatever the heck that is. I have recently went from two to three blade on two of my nitro planes. First plane: Hangar 9 50 size corsair, OS 55 nitro two stroke. Went from 12x7 two blade to a 12x7 three blade. Results: Slower level flight speeds, better climb performance, gets off the ground faster, slows better for landings. Second plane: Top-Flite 60 Corsair ARF. Went from 13x6 two blade to 13x7 three blade. Results: Same as above for other corsair with the exception I did not lose as much level flight speed but I did lose some. I also did the drop a size in blade when going from two to three blade at first. I did not like how small the props looked in relation to scale. Since I was already on the lower end of the scale for prop size for those engines I went ahead and went to the same size three blades and it has worked great as far as I am concerned.

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    • #3
      Re: 2 blade vs 3 blade prop

      Something to be aware of is length and pitch of the prop has a lot to do with it. There can be many variables when comparing props. And there are a few ways to look at things. So your question can stir up many ways to explain the answer.

      Yes, the more blades you have the less effeciency it will be with regards to power consumption. Because we are dealing with electric motors (not gas, nitro, or real ones) it should be known that an electric motor will spin up to the same RPM that the Kv is rated for at a given voltage. So given the same motor, esc, and battery, going from a 2-blade to a 3-blade with the same prop length and pitch you will use more amps. Why? Because the motor is spooling to the same RPM but has to do it with more blades (resistance). But what is that resistance providing? Usually more thrust.

      I've yet to see an electric motor thrust equation..... <smiley image="smiley_what.gif"/>
      but normally you will see more thrust with the 3-blade prop, but with that usually a loss in overall speed. Because of the increase in power consumption you may lose some flight time. Remember, larger changes in prop size/type will have more dramatic results then others.

      If you want more precise answer we would need to know the prop's specifications. Like is it a 10x7? 9x9?
      Hangar: 90mm F16, 90mm F-15, 80mm Mirage, 90mm F-18, 1400mm P-40, 64mm F-22

      RIP: 80mm F-14

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      • #4
        Re: 2 blade vs 3 blade prop

        You are correct on that. Also the airframe (plane) its self will make quite the difference. I have a 60 size topflite p-47, I run a 13-8 two blade with an os 75. That is at the lowest end suggested by os, I went to a 14-6 and it pulled the engine down tremendously to the extent I thought I was going to do bearing damage, and that size is in the middle of what os suggests. Same size engine, this time topflite corsair, and it pulls a 14-7 with no problem.

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        • #5
          Re: 2 blade vs 3 blade prop

          Update. I put a 3 blade 10x6 prop on one of my Art-Tech pitts. It comes with a 10x4.5 2 blade. I clipped 3/8 inch off each blade. No other changes. The thing flies like a rocket now. Same amount of flight time. No over heating of esc, battery or motor. It takes off faster, flies faster, amazing climb, extremely aerobatic. They can say 3 blades are less efficient . I say the proof is in the pudding! The doctor.

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          • #6
            Re: 2 blade vs 3 blade prop

            It's probably good that you shortened the blades a bit since you went up in pitch. Saying that the 3-blade is less efficient is correct. By how much..... only your battery can tell us.

            If you flew the exact amount of time, how many Mah's would you put into your pack with the 2-blade? And now with the 3-blade? The difference may not be huge, but there should be a difference that's all.

            Glad the change was good for you. The increase in thrust is why you have improved verticle and aerobatics.
            Hangar: 90mm F16, 90mm F-15, 80mm Mirage, 90mm F-18, 1400mm P-40, 64mm F-22

            RIP: 80mm F-14

            Comment


            • #7
              Re: 2 blade vs 3 blade prop

              Update. DO NOT put a 3 blade prop on your Art-Tech pitts! I was happily flying some great aerobatics when I lost power (not good with a pitts,glide ratio of a brick) and had a forced landing. The 20 amp ESC couldn't handle the prop. The battery was still well charged and the motor was fine. Continuing my experiments, I am going to install a 40 amp ESC and a 480 outrunner with a shortened 3 blade prop. If this doesn't work, I don't know what will. I guess 2 blades. This will be a winter project. Weather permitting, after a few fight tests I will be back with the results. The Doctor

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              • #8
                RE: 2 blade vs 3 blade prop

                I know this is an old thread but I just bought a Flyzone Corsair and it comes with a 2 blade, 12x6 prop. Looking at several conversion charts and comments on-line it seemed the general consensus was to just go DOWN in diameter only and leave the pitch alone. Makes sense, so I went to a MA 11x6 3 blade. I have yet to fly it so time will tell.
                Addicted to planes!

                Comment


                • #9
                  RE: 2 blade vs 3 blade prop

                  This was a classic example of not controlling all the variables in the experiment.

                  So to the subject of "prop efficiency" there are papers on papers on the subject with well controlled wind tunnel data from NACA. They all conclude that 2 bladed props are more efficient (power in verses useful power out) than higher blade counts as long as the tips are kept out of the mach rise (start of super sonic air flow). This is mainly due to tip losses (tip vortices) just like on wing tips.

                  Now props do not work by themselves so in the real world of moving airplanes other factors come to bear. Such as ground and airframe clearance, noise and airframe wake turbulence.

                  Like the Flyzone Corsair or any airplane per the lift equation we know that a wing produces lift by the square of the velocity. We also know that the speed of a prop driven model is limited by the pitch speed of the prop.
                  Now with electric motors it is easy to measure the power consumed. So with that in mind take a reading (amp or watts) with the 2 bladed prop and an RPM reading. Then multiply the RPM reading by the prop pitch to get pitch speed. With the 3 bladed prop, keeping the pitch the same, trim the prop until you get the same rpm (it will be the same power, amps). This way you will have controlled the power and the speed of the airframe. Much to your horror you will find that your rate of climb and acceleration are down with the 3 bladed prop.

                  You can rework the prop tips a bit to allow you to use a bit larger diameter prop and gain an ever so slight amount of climb rate. SO for our models the only reasons to fly more than 2 bladed props is ground clearance and cool factor. But as to efficiency if more than 2 blades are use on a prop it is a game killer.

                  Real world model experience you will note that in the ROC 980 series MotionRC recommends going with 2 bladed props to get all the performance potential out of the airframe and power set up. Some of you have noticed that FMS Pitts flies so much better in the vertical with the larger disk area afforded by a properly fitted 2 bladed APC prop. (Ok, you need to land better than normal to keep from having a blade strike on landing) And in my discipline FAI F3D Pylon we carved our own 2 bladed props, These were for small 6.5cc engines that ran at 32K rpm and aircraft that went 300 kph on the race course. Yes, we tried single blades but the dynamic loads became very difficult to control.

                  Just a note here, in engineering and science 'Theory', 'Laws' and 'Rules' are the same thing. That is they are well understood and explained. I think folks are confused with the term hypotheses, this means an idea which is an assumption or as of yet unproven.

                  So the short of this is if you want improved flight performance from your models for any given power change from a high blade count prop to one with a lower blade count. I should add properly sized 2 bladed prop. I stayed away from discussions of pitch to diameter ratios and blade profiles and the like.

                  All the best,
                  Konrad

                  Comment


                  • #10
                    RE: 2 blade vs 3 blade prop

                    Konrad,
                    * What an awesome reply. *Thanks.* And, YES, with my new Corsair I entirely care about the "cool" factor alone.* If I give up some speed and rate of climb, then so be it.* I just wanted to get close to the original, at least as close as physics allow and not burn out the ESC or motor unnecessarily.* I think I can glean out of your reply*that basically I had the idea correct in going down on the diameter and keeping the pitch.* I know this is not the ideal as far and the physics go, but for my aesthetically pleasing Corsair would I be safe?
                    Addicted to planes!

                    Comment


                    • #11
                      RE: 2 blade vs 3 blade prop

                      Yep, close is correct. I'd like to see folks use an Amp meter whenever changing things that effect the power system. It will save you a lot of time money and frustration. I like inductance meters as they don't have any impact on the system. Amp meters that break into the line can cause issues. Read up on ESC battery lead length.

                      All the best,
                      Konrad

                      Comment


                      • #12
                        RE: 2 blade vs 3 blade prop

                        Originally posted by retiredflyer
                        I know this is an old thread but I just bought a Flyzone Corsair and it comes with a 2 blade, 12x6 prop. *Looking at several conversion charts and comments on-line it seemed the general consensus was to just go DOWN in diameter only and leave the pitch alone. *Makes sense, so I went to a MA 11x6 3 blade. *I have yet to fly it so time will tell.
                        The math works out that the 11x6x3 *should* draw somewhat more power than the 12x6x2, but there are other factors involved, like blade stiffness, blade shape, etc., so buy a watt meter, and make sure you don't let the magic smoke out, don't just go by approximations. Any time you change the power system, it is a good idea, IMO, to have a watt meter on hand. Even a cheap one will keep you flying instead of replacing parts.

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                        • #13
                          RE: 2 blade vs 3 blade prop

                          What are you using for the formula? I assume it is a rendition of the Abbott equation first published in Model Aviation.

                          All the best,
                          Konrad

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                          • #14
                            RE: 2 blade vs 3 blade prop

                            I found an equation for prop load factor, which is load = [D^3 * P * sqrt(B-1)], where D = diameter, P = pitch, and B = blade count. If I recall, it was for a 'perfect' prop, in that all the other variables cancel out. I put the equation in a 'what if' spreadsheet, so I can substitute any one variable, and let it calculate either of the other two. I don't recall where I found it, but it's been accurate enough to choose the nearest size when changing blade counts; but, I always verify with a Watt meter. That last step is very important, as I've found, many times, that props with the same diameter and pitch can vary as much as 10% in power and thrust, on the same motor and battery.

                            Comment


                            • #15
                              RE: 2 blade vs 3 blade prop

                              Well I just got my watt meter at everyones suggestion, and I hooked it up with the stock 2 blade 12X6 prop that came with my Corsair. Ran it up and it registered 26.9 amps and 271 watts. I then put on the 11X6 3 blade master airscrew prop and it registered 27.7 amps and 276 watts. Almost identical! So I guess I can run with the 3 blade with no worries to any of the electronics. I have no idea if they will be a major performance drop, I'm not a speed demon anyway. Will have to wait till Spring unfortunately to actually try it out.
                              Addicted to planes!

                              Comment


                              • #16
                                RE: 2 blade vs 3 blade prop

                                There will be no "major" change in performance.

                                You haven't defined what you are looking for when it comes to performance. But as you have mentioned speed you won't notice any. As both props have the same pitch and they are both drawing about the same power from the same motor/battery combo the speed will be about the same. You might notice a drop in the vertical but you would need another aircraft flying next to you to notice it or have your model fully rigged with telemetry to notice. I doubt you will even notice the drop in your flight duration

                                What watt meter did you use?

                                All the best,
                                Konrad

                                Comment


                                • #17
                                  RE: 2 blade vs 3 blade prop

                                  Originally posted by Konrad
                                  There will be no "major" change in performance. *

                                  You haven't defined what you are looking for when it comes to performance. *But as you have mentioned speed you won't notice any. As both props have the same pitch and they are both drawing about the same power from the same motor/battery combo the speed will be about the same. *You might notice a drop in the vertical but you would need another aircraft flying next to you to notice it or have your model fully rigged with telemetry to notice. I doubt you will even notice the drop in your flight duration *

                                  What watt meter did you use?

                                  All the best,
                                  Konrad
                                  I actually got an Electrify PowerMatch meter.* Seems decent, about 45 bucks at TH.*
                                  Addicted to planes!

                                  Comment


                                  • #18
                                    RE: 2 blade vs 3 blade prop

                                    Looks like a versatile tool.

                                    I couldn't find a spec sheet, but I'm always concerned with "break in" meter effecting the input impedance (stressing the ESC's input caps).
                                    Since that meter adds a lot of wire length to the battery side of the ESC. I'd be very tempted to add around 440 micro farads of capacitance with low ESR caps to the power leads.

                                    Inductance meter don't have this risk.

                                    All the best,
                                    Konrad

                                    Comment


                                    • #19
                                      RE: 2 blade vs 3 blade prop

                                      Originally posted by Konrad
                                      Looks like a versatile tool.

                                      I couldn't find a spec sheet, but I'm always concerned with "break in" meter effecting the input impedance (stressing the ESC's input caps).
                                      Since that meter adds a lot of wire length to the battery side of the ESC. I'd be very tempted to add around 440 micro farads of capacitance with low ESR caps to the power leads.

                                      Inductance meter don't have this risk.

                                      All the best,
                                      Konrad
                                      While the extra length does add some stress to the input caps, the short duration of a power test isn't that significant. It takes a lot of time with the extra ripple to heat up the internals of the caps, perhaps the time it takes to fully discharge the battery at ~50% power to the motor. Just don't use the motor to discharge the battery with the meter in line, and you'll be fine.

                                      Comment


                                      • #20
                                        RE: 2 blade vs 3 blade prop

                                        The ripple effect is worse at medium throttle than at full.. so if just doing a brief full power test with the wattmeter you shouldn't have an issue.
                                        FF gliders and rubber power since 1966, CL 1970-1990, RC since 1975.

                                        current planes from 1/2 oz to 22 lbs

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