On Fig. 1, Go Along the Horizontal Weight Scale and Find the Weight That You Have Selected

We are going to design a wing for an aircraft yet to be created. The first step is to decide how much weight the plane will carry (because in cruise flight, the lift force must be equal to the weight force). We also need to decide what its cruising speed will be and what its landing (stall) speed shall be. For this example, let’s use the numbers below.

Maximum weight / 1320 lbs (1430 lbs seaplane)
Maximum speed / 120kts
Maximum stalling speed / 45 kts

On Fig. 1, go along the horizontal weight scale and find the weight that you have selected. Next find the line which represents the landing speed (1.3 times stall) that you want. Now go vertically from the weight to this landing speed line. Now go horizontally from this point to the vertical (wing area) scale. The point where this horizontal line crosses the wing area gives the required wing area.

Fig. 1

Required wing area ______ft2

On Fig. 2, go along the weight axis to the weight that you specified. Next find the line representing the wing areas we found on Fig. 1. Now, go vertically from your weight to the line representing your wing area. Once again, go horizontally from this point to the vertical scale and mark this intersection. This value is the wing loading-the weight supported by each square foot of the wing.

Fig. 2

Wing loading ______lbs/ft2

On Fig. 3a, locate on the horizontal scale the wing loading value that you have just determined. Next, find the line representing the cruising speed (0.8-0.85 of max. speed) that you selected. Now go vertically from your wing loading o the cruising speed line. Once again, go horizontally from this point to the vertical scale and mark this intersection. This value is the lift coefficient at cruising speed.

Fig. 3a

Lift coefficient at cruising speed ______

Using Fig. 3b, find the stalling speed on the vertical axis. Find the wing loading on the horizontal axis. Mark where these two values meet. Follow the closest lift coefficient line up and to the right. This is the maximum lift coefficient your wing will need to generate.

Fig. 3b

Maximum lift coefficient ______

Select an airfoil that has good L/D ratios at those two lift coefficients for your aircraft.

Airfoil selected ______

To get the dimensions of your wing, you need to use the term Aspect Ratio (AR).

AR=wing span2/wing area

Fig. 4a shows the relationship between the drag coefficient and the lift coefficient for various values of aspect ratio. A slow plane will fly at a higher lift coefficient and thus, to keep the induced drag down to a reasonable value, slow planes will have higher AR values. AR values over 15 are common on slow flying gliders! Fig. 4b shows the tradeoffs between high and low AR values and different performance characteristics.

Fig. 4a

Fig. 4b

Even though high AR wings are more efficient, they do come at a cost. Long, narrow wings are not as strong as shorter, wider wings. Designers must strike a balance between efficient wings and strong wings. AR values between 5 and 7 are common on small airplanes.

AR you choose ______

To determine the wing span on Fig. 5, find the value of wing area that you determined from Fig. 1. Then find the line representing the aspect ratio which will give a reasonable value of induced drag as shown on Fig. 4. Now move vertically from this wing area to the proper aspect ratio line and move horizontally from this point to the wing span scale on the left.

Fig. 5Fig. 6

Wing span ______ft

To determine the wing chord, repeat the previous steps with Fig. 6.

Wing chord ______ft

Your wing span multiplied by your wing chord should give you the wing area you determined from Fig. 1.