PART TWO

WING DESIGN By Ray Borst, EAA 1526 There is another important factor that we must yet consider. This is, what wing span shall we use? The aeronautical engineer uses the term Aspect Ratio. Aspect Ratio is equal to the wing span divided by the wing chord. Since the wing span multiplied by the chord equals the wing area and we have already found our wing area, we don't have much trouble determining what span to use.

Figure 7 shows the relationship between the total drag coefficient—surface drag plus a drag that is caused by the lift and is called induced drag—and the lift co-

To determine the wing span on Figure 8 find the value of wing area that you determined from Figure 4. Then find the line representing the aspect ratio which will give a reasonable value of induced drag as shown on Figure 7. Now move vertically from this wing area to the proper aspect ratio line and circle the intersection. Next draw a horizontal line to the vertical scale of the chart and mark the intersection of this horizontal line on the scale. Now on the left scale read the value of wing span.

-r 3o

____/*:

5 zc a

v>

-4—y-

200

Fig. 8 Wing Area Sq. Ft.

Example

Wing area =137 ft2 Aspect Ratio=6 Move vertically from 137 on the bottom scale to the line representing an aspect ratio of 6 and circle the intersection. Next draw a horizontal line from this point to the vertical scale and read a value of 28.5 feet. Fig. 7

efficient for various values of aspect ratio. Note that the drag increases for a given aspect ratio as the lif' coefficient increases. Also, the drag coefficient increases

as the aspect ratio decreases.

From this graph you can easily see that for low values of lift coefficient the aspect ratio doesn't make a whole lot of difference. Then you expect to find a rather lo^ aspect ratio on fast planes. This is usually true and values down near 5 are common. 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 values. This is usually true and values of over 7 are common. Indeed, for thd ultimate in slow planes—gliders—values of 15 are common. 10

AUGUST-1961

To determine the wing chord, on Figure 9 find the value of wing area that you determined on Figure 5. Then find the line representing the aspect ratio that you have selected. Now, move vertically from this value of wing area to the proper aspect ratio line and circle the intersection. Next draw a horizontal line to the vertical scale of the chart and mark the intersection of this horizontal line on the scale and read the value of wing chord. Example

Wing area=137 ft2

Aspect Ratio=6

Move vertically from 137 on the bottom scale to the line representing an aspect ratio of 6 and circle the intersection. Next draw a horizontal line from this point to the vertical scale and read a value of 4.75 ft.

To check the values of the wing span and wing chord that we have obtained, we will multiply them together and they should give {be wing area.

1-l

s/ -** j>/„

k

V2

i'

^ ^^> s \ _— ^

\

\s

\

^^

—V

^ \7 /

-/ f

1 /*

i

4

6

ff

/»

/Z

/+ /L

/9 l

Fig. 9 Wing Area Sq. Ft.

Example

Fig. 11 Angle of attack

Wing span = 28.5 ft. 28.5 X 4.75=136 ft?

Chord = 4.75 ft.

This value compares quite closely with the value of 137 ft 2 that was originally obtained from Figure 5. There is but one more thing that we must determine. This is, at what angle of incidence should the wing be with the fuselage? Figure 10 shows A ^ , the angle of

I urge each of you to get a copy—at the price of $2.95 per copy you can't make a better investment. We have decided to use a 2412 or a 63-212 airfoil. Let's pick the 2412. The NACA reports give data for this airfoil. Figure 11 shows their data for the 2412. Find our design lift coefficient on the vertical scale. Now move horizontally to the average curve and circle the intersection. Next move downward vertically from this circle to the horizontal scale and mark the intersection. This value is the angle of attack that a large aspect ratio wing would have to have to develop the required lift. To correct this angle for the aspect ratio that we are using, we must add the A O^, that we found on Figure 10. Thus our total angle of attack and hence the angle be tween the fuselage horizontal (water lines) and the wing is the sum of our answers from Figure 10 and Figure 11. Example

For our example plane, find a lift coefficient of

0

.2

.4

.6

Fig. 10 Lift Coefficient

attack increase, that is necessary because of these induced effects. At low values of lift coefficient, it may

be ignored. However, for a puddle jumper, this angle can amount to an appreciable amount.

Next, on Figure 11, find the angle of attack required to obtain our lift coefficient from a wing with a large aspect ratio—this curve is the published NACA data.

Test data for most of the modern airfoils was published in NACA Report 824. This Report is out of print now. However, Dover Publications, Inc. Dept. EAA 180 Varick Street New York 14, New York

has published a paperback called Theory of Wing Sections by Abbott and von Doenhoff who wrote Report 824. This paperback is virtually a reprint of Report 824.

.18 on Figure 10. Next find the curve representing an aspect ratio of 6. Now move vertically from the .18 lift coefficient to the curve representing aspect ratio of 6 and circle this intersection. Now move horizontally from this circle to the vertical scale and mark this intersection.

The value read on this vertical scale, +.50 degrees is

the amount that we must add to allow for aspect ratio effects.

Next on Figure 11, find on the vertical scale the value .18. Now move horizontally to the curve representing our 2412 airfoil and circle this intersection. Now move vertically downward from this circle and mark the intersection with the horizontal axis. Read a value of —.45 degrees. Now adding these two values +.50 and —.45 gives +.05 degrees. Thus our wing should have an angle of attack of +.05 degrees at cruise. If the fuselage is to remain level, the wing must be at an angle with the fuselage and hence the wing will point upward .05 degrees. We will neglect this small angle. NOTE: Usually this angle is positive and the wing points upward slightly.

Another item that effects the flying properties is

dihedral. There are planes flying that have no dihedral and have supposedly good stability. It is possible that a continued on next page SPORT AVIATION

11

WING DESIGN . . -

Continued from preceding page high wing plane might have reasonable stability with no dihedral. However, I think that the plane would be more pleasant to fly with 2 degrees of dihedral. It is quite difficult to imagine that a low wing plane could have much stability without using dihedral. A low wing plane will require 3 degrees or so dihedral. The last item to be discussed is, what planform should I use? Should the wing be elliptical, tapered or rectangular? John Thorp says rectangular, and Prof. Raspet two years ago said there is no reason for a plane weighing less than 12,000 pounds for having anything other than a rectangular wing. Let me illustrate with one final graph why these authorities say this. Figure 12 shows several planforms—a rectangular and various taper ratios. I have shaded the region where the initial stalling will occur for these various types of wings. Note that the rectangular wing stalls first at the root and progresses slowly out toward the tip. Each line shows the stalled region for additional increases in the angle of attack. A wing with a slight taper ratio will also stall at the root first but the stall will progress faster toward the tip. As the taper ratio is increased, the point of initial stall will move away from the root as shown in Sketch 3. The extreme position is shown in Sketch 4 where the high taper ratio wing stalls first at the tip. This, of course, is to be avoided unless you like to spin in from one hundred feet. Use a rectangular wing — play it safe and live to enjoy your plane. In conclusion, let me say that the answers you obtain using my procedure are not exact. I think you can come within 5 percent, however. Thus I have not intended to make professional designers out of you. But, if you follow my step-by-step procedure, you will obtain somewhat better flying properties and we will have eliminated the "widow makers". Good luck with your design and let's get them flying. A GYRO-COPTER IN FINLAND . . . Continued irom page 9 Test flights we have made with this gyro-copter

seem very successful. Stability in flight is amazingly good and takeoff is stable too. Fuselage is steel tube construction, rudder and stabilizer are fabric-covered. Rotor blades are of pine, balsa and plywood. Instrument panel: altimeter, air-speed indicator, variometer and tachometer. We are very charmed to fly our gyro-copter and under construction is a new ship with cabin -- which is very comfortable here, near to the polar circle. Kalevi Kokkola lives at Pajalahdentie 21 A 15, Helsinki, Finland and Seppo Kokkola lives at Ulvilantie 3 D

32, Helsinki, Finland.

Specifications KOKKOLA Ko-03 GYRO-COPTER "UPSTART" Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

diameter . . . . . . . . . . . . . . . . . . . . . . . . . . 26 ft. 6 in. type . . . . . . . . . . . . . . . . semirigid with lag hinges section . . . . . . . . . . . . . . . . . . . . . . NACA 23012 chord . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 in. rpm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258

Weight empty . . . . . . . . . . . . . . . . . . . . . . . . . . 270 Ibs. Weight loaded . . . . . . . . . . . . . . . . . . . . . . . . . . 485 Ibs. Min. speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 mph

Cruising speed . . . . . . . . . . . . . . . . . . . . . . . . . . 53 mph Min. sinking speed . . . . . . . . . . . . . . . . . . . . 10 ft./sec. Endurance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 h Engine . . . . . . . . . . . . . . . . Poincard 28 hp at 2350 rpm Control . . . . . . . . . . . . . . . . Conventional control stick 12

AUGUST 1961

OF Fig. 12

Civil Aviation Is Being Forced Into 'Pioneer Age,' Jane's Says As printed in the December 21, 1960 New York Times The challenges of cost, noise, air traffic control and airport accessibility are pressing civil aviation into a new "pioneer age," according to the editor of "Jane's All the World's Aircraft." The editor, John W. R. Taylor, writes in a preface to the 1960-61 edition of the authoritative yearbook, to be published tomorrow, that performances in speed, altitude and range "are no longer the sole yardsticks by which the worth of an aeroplane is judged." "In civil aviation," he continues, "the cost of achieving them has been so great in terms of two-mile concrete runways, the noise problem, the nightmare of air traffic control and the inaccessibility of major airports that we are witnessing the first years of a tremendously exciting new pioneer age." The volume contains pictures of a three-horsepower butterfly-like device, produced in the Soviet Union, "that would not look out of place in the 450-year-old sketchbooks of Leonardo da Vinci," and a British man-powered aircraft weighing only seventy-four pounds. "These aircraft represent a reaction against the noise and fury, high cost and remoteness of modern aviation," Mr. Taylor said. "Nor is this the aim only of back-yard, do-it-yourself aviators, for the far more sophisticated and practical Hovercraft, jet-lift and other aircraft have the same broad objective of making air travel more convenient and accessible for everyone, everywhere," he said.