Designing an airplane may seem at first to be a rocket science. After some exploration and thinking, it turns out only a few rules you would have to follow. Among them, the first is the wing-loading, then the power-weight-ratio. Below I list the details of the most important ones.
A DIY model airplane by me looks like this:
Its body can be assembled by simply gluing together wooden and metal parts like in this picture:
The first thing to do is to estimate how much weight the plane will be. As a reference I list the actual weight of my plane here.
The body without battery is about 4oz. The wing is 1oz.
With 4 AAA NiMH cells at 2oz, it totals 7oz.
With 6 AAA NiMH cells at 3oz, it totals 8oz.
With 6 1/2AAA NiMH cells at 1.5oz, it totals 6.5oz.
The design of the plane goes as follows.
The wing loading of a plane for beginners should be around or below 5oz/sqft. Start by imaging how big your plane shall be. Then estimate its weight. We want to limit the weight to about 8oz. By using a wing of 24 inch long, and 8 inch wide, we get a wing loading of 8/(24*8/144)=6. This is a bit over 5, but ok.
To be more precise, with different battery configuration the loading would be (compare them to 5oz/sqft):
- 4 AAA, 7oz: 7/(24*8/144) = 5.25.
- 6 AAA, 8oz: 8/(24*8/144) = 6.
- 6 1/2AAA, 6.5oz: 6.5/(24*8/144) = 4.9.
The output power required by a plane for beginners should be around or over 1.5W/oz. With the motor and propeller used by the plane, it drains max 7A at 7V with a 2S Lipo (see test comments under the motor on HobbyKing), that is 49W. Or a 49/8=6W/oz. With an efficiency of 50%, it would be 3W/oz. That’s very powerful for a beginner plane.
If we lower the power supply to 4.4V, we estimate the current would be sqrt(7*(4.4/7)). That would be 2.1A. The power would be 2.1*4.4= 9.5W. Or we get a 9.2/8=1.2W/oz. That will be a bit low. When we tested with 4 cell NiMH battery, rarely the plane can fly up, more often it keeps flying lower and lower. It’s a sign of lack of power. Thus we have to raise the number of NiMH cells to 6.
More accurate calculations (compare to 1.5W/oz):
- 4 AAA, 7oz, 4.4V: 9.5W/7 = 1.36.
- 6 AAA, 8oz, 6.6V: sqrt(7*6.6/7)=2.6A. 2.6*6.6=17W. 17W/8=2.1.
- 6 1/2AAA, 6.5oz: 17W/6.5 = 2.6.