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  1. #1

    motor-esc-battery specs for 1/2A Cessna 170A

    Hi, I'm new here and I have started finishing a Guillow Cessna 170A model (45" wingspan) with my son that I started around 30 years ago . I have built it and its ready to cover, need to put in the electrics now and could use some advice on motor specs & prop choice to sufficiently power this one if you have a moment, would certainly appreciate it, thanks much.
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  2. #2
    Pond Skipper's Avatar
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    Look for brushless motor - 60 to 100w weight 22 to 34g motor and 480 - 860mah 2S lipo size batteries 10 -12amp esc.

  3. #3

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    Really needs a PAW .06 diesel.

  4. #4
    Pond Skipper's Avatar
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    Lol true that or a .07 / 4Cyl. wish someone would mass produce even a .10 would be shaweet.
    Last edited by Pond Skipper; 04-14-2014 at 12:08 AM.

  5. #5
    Moderator BMatthews's Avatar
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    Handling and tuning TWO contra pistons and a needle valve is bad enough. Can you imagine trying to match up FOUR! ? ! ? ! ? You'd get it running just right just about the same time it ran out of fuel......

    Dtarking, for RC flying I'd suggest an inside doubler strip of wood around the wing saddle opening to strengthen up that area.

    Success or failure for electric power relies on knowing the final expected all up weight of the model. So giving us the airframe weight combined with RC gear as it sits now would aid us in giving you a better idea of a motor.

    My thought is that Pond Skipper's lower end suggestion of 60 watts might be a little light. For sporty flying I would suggest that you're looking at 70 watts per lb as a nice target for power. And at a 45 inch span and seeing the pictures so far I'm going to guess that your final ready to fly weight will be more in the 2 pound to 2.5 lb range. So you'd want a 120 to 140 watt max power motor. And because this is a little more than Pond Skipper's suggestions I'd suggest a 850 to 1200mah 3S pack over a 2S pack.

    I'm guessing that you're also pretty new to this stuff. So you'll also need to pick the Kv value of the motor to suit the model. The Kv value is sort of like the gear ratio. Higher Kv values spin smaller props faster. Lower Kv motors turn big props slowly. For a model of this sort you'd want an inbetween sort of motor to turn a roughly 9x5 or 9x6 prop.

    Keeping it cheap I'd suggest the following stuff from Hobby King;

    http://www.hobbyking.com/hobbyking/s...00KV_235W.html
    This one looks like a nice option to me. Running it at around 130 to 140 watts will keep it comfortably below it's max continuous rating of 173watts so it'll tend to run cooler and live longer. Don't forget the accessory pack for the mount and prop adapter. And get a couple of extra replacement motor shafts at the same time.

    You'll find LOTS of other options. But if you look for motors with the same sort of maximum watts and max current and the same Kv value as this one they will all be roughly equivalent. It's OK to go with higher wattage and max current but don't alter the Kv value or you'll need to use something other than a 9 inch prop.

    I'd run this motor on a 9x5 prop and a 3S (3 lipo cells in Series, hence the "3S") using a 25amp to 30 amp ESC. This means the ESC is over rated but that's OK. We do it that way so the ESC isn't pushed hard and ends up heating up. It's pretty common to use ESC's rated for 1.5x to 2x the expected maximum power draw for this reason.

    Note that the power that the motor will pull is almost totally dependant on the prop. Need more power? Go up in diameter or pitch or both. Motor or battery running hot to the touch? Reduce the diameter, pitch or both to make the motor pull less power.

    To test the motor to ensure it's not pulling too much power and thus will try to smoke the motor the normal method is to use a watt meter. But if you don't want to buy one you can do a quick and dirty test to make sure you're not too close to letting out the smoke. With everything set up ground run the motor at full throttle for a minute. Then shut it down and disconnect the pack. By that time check the motor for warmth over the next minute. It'll take roughly that long for the heat from the core windings to make it out through the magnets and outer bell. It will likely become quite warm. But if it gets anywhere near to anything like uncomfortable to hold then it's too hot and you need to go down a size on the prop.
    Witty saying to be plagarized shortly.....

  6. #6
    Thanks all for all the great explanations there and tips! The diesels sound awesome, just wouldn't go over at any of the fields where I can fly.

    Yes, the weight is something I'll have to do tonight as I just finished my ailerons this morning. Haven't any servos, etc., installed yet, so will update what weight I have in terms of the uncovered airframe parts as they are.

    I also haven't bought a radio/receiver yet...I see there's a ton of choices and prices since my adolescent days in this. Would you recommend anything in the 6-7 channel range on the Hobbyking site? Some seem way too cheap to be relying on...was probably going to go with a DX6i or DX7i otherwise. Also, what is mode 1 vs. mode 2?

    And what servo size would I be looking at for my rudder/elevator/aileron setup? I'd like to throw all this stuff on my Hobbyking shopping cart and get it all in one go, however unlikely doing that will be...

    More n00b questions to come, thanks for your patience...

  7. #7
    Moderator BMatthews's Avatar
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    The 6i would be fine. But if you and your son plan on moving on and up you may as well go for the 7i right out of the gate. And go with a compatible 7 or more channel output receiver to complement the Tx.

    By far the massive majority flies in Mode 2 with the ailerons and elevator on the right stick. So no point in not following the crowd. And as a result the massive majority of the radios also only come in Mode 2. In fact the very fact that you know of the two modes and asked is a good sign that you're an old timer that has come back to the fold....

    For servos on this size of model I'd suggest you stick to the 14 to 18 gram mid to mini size of servos. The extra power helps. It should not be an excuse to accept tight or binding in the control systems but it will give you a little more leeway.

    Clearly if you did fly at all before it's been many years at this point. I'd strongly suggest you find a club in the area and sign up then get the local hot shot pilot to do the test flights. It'll give you and your son a MUCH higher chance of coming home with the plane still in one piece.
    Witty saying to be plagarized shortly.....

  8. #8
    Pond Skipper's Avatar
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    Yes opps on me...this is a 45in span. I hear Guilow's and think of this size. Moderator is corrrrectomundo.

    Most 1/2A engines produce 80w of power ish in your case to match a 45in wing you would want 100w min.
    Here is a rule of thumb chart.

    50-70 watts per pound; Minimum level of power for decent performance, park flyer/slow flyer models
    70-90 watts per pound; Trainers and slow flying scale models
    90-110 watts per pound; Sport aerobatic and fast flying scale models
    110-130 watts per pound; Advanced aerobatic and high-speed models
    130-150 watts per pound; Lightly loaded 3D models and ducted fans
    150-200+ watts per pound; Unlimited performance 3D and aerobatic models




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  9. #9
    Thanks everyone.. Well I finally received my orders of stuff from HK, that took a while... And now have a question about motor/prop mounting. Does the pic below look correct? I oversized the hole in the cowling to let the aluminum prop mount come thru since it has to have clearance to spin. Any minimum distance from the cowling to the prop you would recommend?

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