General equations for powering a model aircraft

[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.

 

[berkeman]

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?

 

[ammtst]

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?

yeah first person view. Nothing really around that size and that weight range that's 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

 

[rcgldr]

Do a web search for "radio control model watts per pound" and you'll find links similar to this one:

http://www.rc-airplane-world.com/watts-per-pound.html

Divide watts by 745.7 to get horsepower.

 

[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

 

[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.