Thread: Mini-HowTo Beginners Guide to Motor and Prop Selection View Single Post
Nov 03, 2009, 03:48 PM
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Joined Jul 2006
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Making the wing give up it's secrets...

Let's take a typical park flyer type plane and examine it's wing. In the pic below, we can measure the wingspan from wingtip to wingtip and find that it's pretty close to 48", and the average width of the wing is about 7 3/16". By the way, the technical term for 'average wing width' is the word 'chord'. The weight of the plane below, as it sits on the runway, battery installed, and ready to take off is 25 ounces.

What we'll do now is figure out the wing loading of this plane. And once we know what the wing loading is, we can calculate the stall speed, and the stall speed will automatically tell us how fast we want the plane to go when it comes times to pick a motor and prop.

This is the only time we will ever get into any arithmetic, so bear with me here, and we'll see it's not really all that hard to do. Remember that wing loading is simply the weight of the plane in ounces divided by the area of the wing in square feet.

Step 1) Multiply the wingspan of 48" x 7 3/16"
48" x 7.187" = about 345 square inches.

Step 2) Find out how many square feet this is by dividing our answer by 144.
345 / 144 = about 2.4 square feet.

Step 3) Find the wing loading by dividing the weight of the plane by the wing area in square feet.
25 ounces / 2.4 square feet = about 10.4 ounces per square foot.

That's it. That's the wing loading of our pictured plane. 10.4 ounces per square foot. It tells us that each square foot of wing area has to carry 10.4 ounces. Notice I use the word 'about' a lot. That's because there's no need at all to be super accurate and exact. This is not rocket science! In the next post, we'll see how to calculate the stall speed of our plane.... using it's wing loading only.