Let's recap what we've done so far.

We've taken an average plane from the shelf, measured it with a tape, and weighed it. Based on that, we came up with the plane's stall speed, and that gave us a good ballpark figure for the top speed of our plane. We then learned that the manner in which we want to fly our plane is going to determine how much thrust we will need.

The only thing left for us to do now is measure how much ground clearance we have for our plane's prop. And this is going to determine just how big a prop we can stick on our plane. Why? Because we want the largest diameter prop that will fit on our plane! Ideally, we want a prop with a diameter that is about 1/4 the wingspan of our plane. In our example plane above, this means we want to pick a prop that is about 12" in diameter (1/4 of our 48" wingspan).

That's what we will shoot for whenever we pick a motor/prop combo for our plane. As it turns out, many years of testing and real world experience and fancy theories have shown this to be true. This "1/4" rule gives us the greatest system efficiency, and maximizes the potential of our planes. And now, before we start having fun with a special piece of software I'll introduce you to, we'll wrap up everything we've learned:

1) Pick a top speed of about 2.5 to 3 times your plane's stall speed.

2) Pick the biggest diameter prop possible (up to 25% wingspan) and spin it

**just** fast enough to get you your desired top speed.

That's it, we've learned all the background knowledge we'll ever need to know. There is no more to learn. We did it without fancy equations, boring math, discussions of motor kV, watts per pound, and all the other confusing nonsense that's thrown at beginners when they ask how to pick motor/prop combos for their planes.

Now we get to start having fun with a piece of software that will make us all experts at picking combos...