# General equations for powering a model aircraft

1. Sep 4, 2015

### ammtst

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. Sep 4, 2015

### 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?

3. Sep 4, 2015

### ammtst

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

4. Sep 4, 2015

### rcgldr

5. Sep 13, 2015

### NTW

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

6. Sep 14, 2015

### rcgldr

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.