P-38 - The Ultimate EPO Lightning

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Motor/ESC questions

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  • Motor/ESC questions

    I am hoping the forum can help me with this question. I have two 46 size electric motors, and ESCs that go with them. The first set is one of the Hobby People 46 kits that came with battery, 80 amp ESC with built in BEC and motor. The second is a Rimfire 46 with an Electrifly 60amp opto ESC and seperate UBEC.. The Rimfire set up is in an Avistar Elite, and performs fantastic. Great power on a 4S setup, and what I expected from a model of this size.

    The HP set up is in a Cub with a 71" span, and just doesn't have near the power of the Rimfire setup, even when I go to 5 and 6 cell batteries. Both motors have similar specs, so I expected similar performance. My next step will be to try swapping out ESCs in the Cub to see if that helps since I suspect that is the problem, but my question is this: Can the ESC perform on a minimum level, but fail to allow full power to the motor? And yes, I have calibrated the ESC with the transmitter throttle several times.

    Thanks for any ideas!

  • #2
    Yes the ESC can need to have its throttle range reprogrammed to match the transmitter or the radio may need to have its throttle range reprogrammed to match that of the ESC.

    There are several aspects to motor specs and any being off can make a major difference. Far more than what may seem like a small % change in the number.

    Also not all ESCs of any given amp rating are equal. Castles will FAR outperform any others that I have tried. Sometimes a Castle ESC can give 50% more power from the same motor as a Dynam/Detrum
    I generally end up swapping my RTF/PNP ESCs for Castles. I just changed the ESCs in my Freewing F-14 to Castles and its a large difference.
    FF gliders and rubber power since 1966, CL 1970-1990, RC since 1975.

    current planes from 1/2 oz to 22 lbs

    Comment


    • #3
      Hello LF,
      I can understand your concern and confusion but that is really of no surprise to me.
      The way the electric motors were initially marketed and continue to do so today regarding comparing them size wise to a IC engines has been a bad practice.
      For example in your case the 46's are basically not with similar specs as you mentioned.
      The HP is a 4243-650kV with no specs avail whatsoever which is quite common.
      The GPR is a 4260-800kV which does have a spec sheet as listed on the Tower website
      The lack of motor specs is a main reason that Motion will not list FMS motors in their "Motors and Accessories" of the Electronics section.
      Your motors have a difference of 150kV which on a 4S setup is a 2400 rpm delta.
      A lot of modelers just don't realize that going from 10400 to 12800 in prop RPM's(in your example) which is a 23% increase and would seem great for a power boost but in reality is a good formula for letting out the smoke.
      Power factors on motors is not "linear". Changing prop diameter/pitch and blade count all have "exponential" results on the power consumed.
      Since the specs are not avail for the HP motor nor do you give any specs regarding props or the airframe(ie AUW) it is really unrealistic to provide solid analysis/ideas to your question.
      My biggest recommendation if any is have a Power Meter to test out changes to power configuration and for comparison evaluations.
      Also having access to a power calculator like eCalc to narrow down your options is good so that you aren't shot gunning parts and wasting money.
      This should give you a start in understanding what is going on with your particular concern and good luck.
      Warbird Charlie

      ScaleTech OV-10 Bronco; HSD A1 Skyraider; FlightLine: F7F-2D, P-38 Allied Green, Sea Fury; FMS 1400: P-40B, P-51 Marie, F4U Olathe, F6F, T-28, P-40E, 1500 Razor, 1700 F4U and a Fox glider; Freewing: A-10 Artic Thunder(100+ parts to rise again), A-6, P-51 Iron Ass; VQ: P-39, JU-52; LX PBJ-1(B-25), P-40E; Dynam: ME-262, FW-190, Waco, Catalina; Phoenix Spitfire; Maxford Antonov AN-2; Starmax L-4 Grasshopper; Eflite 1100 T-28 float; Tech1 P40; NP O-2 Skymaster; ASM A-26 Invader

      Comment


      • #4
        Originally posted by Loneflier View Post
        even when I go to 5 and 6 cell batteries. Both motors have similar specs, so I expected similar performance.
        From OV10's post, it seems these motors do not have "similar" specs. For one, the kv's are quite different. Therefore, your idea that they are "similar" specs is incorrect and thus, your expectations of similar performance is also incorrect. Additionally, unless you know for a fact that a motor can handle batteries of higher cell counts, you risk burning something out in the motor. This could have even happened on your first attempt with the 5s and 6s batteries. Something could have degraded in the motor and the extra cells of the battery was of no benefit.
        Combinations of motor, ESC, battery choice is closely related to the size and design of the plane (drag, aerodynamics) and the prop used, along with other factors. They aren't just thrown together in a whimsical manner. Changing one parameter may not necessarily make for an overall desirable or expected change.

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        • #5
          Originally posted by OV10 View Post
          Hello LF,
          The HP is a 4243-650kV with no specs avail whatsoever which is quite common.
          The GPR is a 4260-800kV which does have a spec sheet as listed on the Tower website
          Note also that the HP is physically smaller, assuming those numbers mean something. While there is no standard, that implies the HP motor is 17mm shorter, which means less power off the top, besides the lower Kv rating. To get the prop to near the same rpm as the GPR, you would need a battery that is ~1.23 times (800/650) what you use with the GPR, so the HP on 5S would be nearly the same rpm as the GPR on 4S, assuming it can provide the same power at that rpm. One way to compare power capacity is to use a sensitive gram scale to weigh each, then multiply the result by 3W/g. Some motors can provide more than 3W per gram, but most common ones are close to that.

          Comment


          • #6
            All-thanks for the inputs and suggestions. I was basically going off the fact that both are marketed as 46 size motors, and I have a friend who flies an HP set up just like mine in the same size and weight Cub, and the difference in performance is very noticeable. His flies with excellent authority, mine seems to struggle. I am using a 12x6 prop on both the Rimfire setup, and the HP set up. I have gone as high as a 13x8 prop on the HP motor, thinking that with the slightly lower KV rating, it might help, but with no real change. I didn't really think that 150KV would make that much difference overall. Up until now, I have been flying PNP and BNF planes that are "prepackaged" with motors and ESCs matched to each other, and wanted to build a balsa/ply ARF that would be a bit larger, with more power and stability for windier days. From the looks above, it would seem I need to delve a bit more into the electronic calculations, something I have not done before, and was honestly hoping to avoid!

            Comment


            • #7
              Originally posted by Loneflier View Post
              I am using a 12x6 prop on both the Rimfire setup, and the HP set up. I have gone as high as a 13x8 prop on the HP motor, thinking that with the slightly lower KV rating, it might help, but with no real change.
              What does the stock plane use for a prop? You increased diameter by 1 and increased pitch by 2. That's pretty severe while not changing anything else. Sometimes, increasing diamter and/or pitch will give less performance and may even lessen the life of the motor/ESC.

              Comment


              • #8
                Both the Avistar and Cub are ARFs, and the manuals had a list of various prop sizes that could be used based on the motor and amperage of the ESC.

                For the Avistar, I bought the recommended set up with motor and ESC, and Tower Hobbies also recommended a 12x6 prop.

                The Cub came from Value Hobbies, and they actually recommended their 32 size motor. I went with a larger motor based on what my friend was flying in his Cub. Both 12x6 and 13x8 were listed as options for the combination of motor and ESC.

                I guess I am going to have to find an "RC Electronics for Dummies" book somewhere. I have no background or experience with electrical systems, other that what I have picked up from flying these planes, and really need to get a handle on the basics if I am going to delve into building. I know there are ways to calculate power requirements based on wing loading, I have just never had to mess with it. Time to do some googling on the subject!

                Comment


                • #9
                  The most important number to find for your motor is its max rated watts.

                  This is where you compare their max power output (rated watts is power in, output is usually 85% of that when properly loaded {using the right prop} )

                  Then use a wattmeter and find what prop puts you CLOSE to max rated watts but not over. that is your maximum prop.

                  The 4243-650 is a "power" wind for that motor, high turn count and smaller diameter wire for the windings. These will have a somewhat lower max watts than a speed winding.
                  The 4260-800 is more of a mid-range to speed winding. Heavier wire, fewer turns per pole.
                  You should be able to look into the motors and see this without taking them apart.

                  4260 is decidedly bigger if they used the same reference system for designating the motor.

                  Most makers use overall motor size AABB AA being diameter, BB being length (without the shaft)

                  Some use CCDD where CC is the diameter of the STATOR and DD is the length of the stator. "Cheatah" brand sold by BP Hobbies does this. You get a FAR larger and more powerful motor with the same numbers.
                  If the 4260 is AABB and the 4243 is CCDD, the 4260 will be MUCH bigger.

                  I found out about the Cheatah by ordering one intending to replace a Turnigy (forgot numbers off top of my head) appx 2212-2200. There is a Cheatah with the exact same numbers. I didn't read further. The Cheatah is DOUBLE the diameter and length of the Turnigy and 6X the weight.
                  FF gliders and rubber power since 1966, CL 1970-1990, RC since 1975.

                  current planes from 1/2 oz to 22 lbs

                  Comment


                  • #10
                    I powered a 71" stick-build Sig J3 Cub with a Rimfire .32 4250-800 on 4S with a 12x6 prop. It performed like a Cub, perhaps a bit more power than the prototype, but definitely not sport class. It pulled about 650W peak, which was plenty for that model that weighed just over 5lbs. If you use a 5S battery, you may see similar power levels, but you should already have a Watt meter, if you are experimenting with batteries and prop sizes.

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