designing a wing

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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
Maximum speed
Maximum stalling speed
1320 lbs (1430 lbs seaplane)
120 kts
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
400
350
Wing Area (ft2)
300
250
30 kts
35 kts
200
40 kts
45 kts
150
50 kts
100
55 kts
50
0
0
500
1000
1500
2000
Weight (lbs)
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
40
35
Wing Loading (lbs/ft2)
40 ft2
30
60 ft2
25
80 ft2
20
100 ft2
15
120 ft2
140 ft2
10
160 ft2
5
180 ft2
0
200 ft2
0
500
1000
1500
2000
2500
Weight (lbs)
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
2
1.8
50 kts
1.6
60 kts
1.4
70 kts
1.2
80 kts
1
90 kts
0.8
96 kts
0.6
105 kts
0.4
120 kts
0.2
140 kts
0
160 kts
0
5
10
Wing Loading (lbs/ft2)
Lift coefficient at cruising speed __________________
15
20
180 kts
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
Aspect Ratio Trade Study
2500
nmi, kts, or fpm
2000
1500
Range
Max Speed *10
1000
Cruise Speed *10
500
Rate of Climb
0
6
8
10
Aspect Ratio
12
14
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. 5
Fig. 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.
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