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General equations for powering a model aircraft

  1. Sep 4, 2015 #1
    I was looking at building a fpv plane. I was wondering if anyone could help me with the basic equations i would need for motor sizing etc. Just how I would go about finding power required. Looking at about a loaded weight of about 5kg wingspan 2.5-3m. Not sure of speed maybe around 40-70kph.
  2. jcsd
  3. Sep 4, 2015 #2


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    Staff: Mentor

    Welcome to the PF.

    fpv = "First Person View"?

    When you look at model airplanes that are around the size you want to build, what size engines do they typically use?
  4. Sep 4, 2015 #3
    yeah first person view. Nothing really around that size and that weight range thats all. And most are using an electric was thinking of using a petrol. Like could i use a direct ratio in terms of weight to power and just scale up or down as required. Or do I need to go through dimensional analysis and all that stuff
  5. Sep 4, 2015 #4


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    Homework Helper

  6. Sep 13, 2015 #5


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    As a first approximation, you can scale down the power required for a 'big' airplane. We may assume that the weight scales as L3 and that the airspeed scales as L1, so the power required would scale as L4. The glide ratio L/D is assumed to be equal, which may not be correct, due to the Reynolds number being different.

    If your model is built -for example- at 1/6 scale, and the 'big' airplane needs 1000 hp, the power required for your model would be 1000 x (1/6)4 = 0,77 hp
  7. Sep 14, 2015 #6


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    A lot of model aircraft fly faster than scale speed, and typically, they have a higher power to weight or drag ratio than a full scale aircraft. At the upper end of the watts per pound range, you have models than can hover and climb while vertically oriented.
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