The Effects Of Landing Gear Configuration On The Drag Of Homebuilt Aircraft By Ronald Wojnar, EAA 31222 2136 S. 28th St., Milwaukee, Wis. 53215 Ronald Wojnar Age—17 Years

T airplane. Its purpose is to support the airplane while it is at rest on the ground, during take-off, and when HE LANDING GEAR is a very essential part of an

the airplane lands. There are two types for use on the ground only. They are the conventional landing gear and the tricycle landing gear. The conventional landing gear has two wheels placed a short distance from the center of gravity of the airplane and a smaller wheel at the tail (Fig. 1). The tri-

surface as it moves through the air. In order to eliminate this wind resistance, all large airplanes and many smaller ones are provided with retractable landing gears which fold into wells in the wings or fuselage. Retraction eliminates all wind resistance of the landing gear, but due to the actuators and linkages which it requires, it is usually quite a complex mechanism. This

In spite of additional complexity and weight, the tricycle gear does have a few advantages. It permits the airplane to be nearly level when it is at rest on the ground,

complexity makes it generally impractical for small, light homebuilt aircraft. A retractable gear requires very careful designing and building. The time required for the construction of the airplane is increased considerably, and perhaps even doubled when the gear has to be developed from scratch. Naturally, this type of landing gear also adds to the cost of the homebuilt. For these reasons, very few retractable landing gears are found on homebuilt airplanes. The fixed landing gear, which protrudes into the airstream, develops a great resistance to the forward motion of the plane. In a steady airflow, an imaginary straight line can be drawn which indicates the direction of airflow at all points along it.2 This line is called a streamline. Since air possesses mass and inertia, a stream of air moving in a certain direction at a certain velocity will, according to Newton's first law of motion, continue to move in the same direction at the same velocity until some outside force is exerted against it.3 If a body moves in an airstream, the streamlines striking the body must change both their immediate direction and velocity to pass around it. The body must exert a force on the airstream and the airstream, in accordance with Newton's third law, exerts an equal and opposite force against the body.4 To maintain the forward motion of the body, force must be applied on it to overcome the force which the airstream exerts against it. Therefore, it follows that the less abrupt the change in the velocity and direction of the streamlines, the less force the airstream exerts against the body, and less force is required to move the body. The action of viscosity tends to cause the airflow to separate from the surface of the body because of a rapid

whereas the conventional landing gear leaves it with its

deceleration of the streamlines. This causes turbulence

nose pointed well above the horizon. The tricycle gear reduces the length of the take-off run because the tail does not have to be raised during the run. It also promotes excellent ground handling characteristics such as preventing the airplane from nosing over when brakes are applied heavily during the landing roll.1 gear serves no useful purpose. In fact, considerable power

(swirling or eddy flow) in the wake of the body. Eddy formation leads to a reduction in downstream pressure on a body and creates a force which acts opposite the body's motion.s (Fig. 3). In general, the streamlining of a body is the contouring of the body in such a manner as to reduce the wake to a minimum. Because one of the main causes of flow separation, and consequently wake formation, is rapid decel-

is required to overcome the resistance of the wind to its

(Continued on next page)

FIG. 1

cycle landing gear, as its name implies, has a wheel in front and two main wheels located slightly aft of the center of gravity (Fig. 2).

FIG. 2

When the airplane is in flight, however, the landing

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Effects of Landing Gear Configuration . .

(Continued from preceding page)

Obviously, the one element which is common to all

types is the wheel. As previously illustrated, the round shape of the wheel is not streamlined. It can be generally said that at least about 40 percent of the total landing gear drag is due to the wheels8. The remaining landing gear drag is due, of course, to the struts. Common structural shapes used for landing gear struts are shown in Fig. 5.

O Circular tubing

Spring

steel

Streamlined tubing

FIG. 5

FIG. 3 Notice how much smoother the air flows past the tear-drop cross section.

eration of flow along the body, the contouring must make the deceleration gradual. (See preceding diagram). "These considerations lead to the following general rules for streamlining: 1. The forward portion of the body should be well rounded. 2. The body should curve back gradually from the forward section to tapering after-section with the avoidance of sharp corners along the body surface."6 In the language of aeronautics, the resistance which the airflow offers to the motion of a body is called drag. Since the landing gear does not contribute to the lift of the airplane, the drag which it creates is known as parasite drag. Parasite drag is composed of two distinct elements, the drag of skin friction in the boundary layer of air next to the surface (which is very small in the case of a landing gear), and the form, or pressure, drag due to the disruption of streamline flow and the resulting turbulence.? By taking a good look at the design characteristics of different landing gears, it is not difficult to see why they create a considerable amount of drag which tends to retard the performance of an airplane. Fig. 4 shows examples of three gears most commonly used on homebuilt airplanes.

Built-up circular tubing

.Spring steel

Steel struts of

circular tubing

FIG. 4

The first is tubing of circular cross section which is not an aerodynamically "clean" form. The second is the spring steel sheet which is used as a strut. This flat section does not create a considerable amount of drag because of the very low ratio of its thickness to its length. Third is the tubing of streamlined cross section, which is the most desirable aerodynamically. Despite the very great freedom of movement which air possesses, it does behave according to quite distinct rules which may be mathematically predicted. Parasite drag varies with the air density and with the square of the velocity. In addition, a coefficient of drag is needed. The coefficient for parasite drag is based on the drag developed by a flat plate placed at a right angle to the airstream, which has been experimentally determined as 1.28, a constant. To determine the parasite drag, therefore, all that is needed is the area, which is based on the flat plate which would give the same drag under the same conditions. This area is referred to as the equivalent flat plate area.9 The equation1" for calculating parasite drag using these factors is given as Dp = 1.28 f AV2 where Dp = parasite drag in pounds d = density of air in slugs per cubic foot A = equivalent flat plate area in square feet V = velocity in feet per second The parasite drag produced solely by the landing gear constitutes only part of the total drag of the airplane. However, by paying attention to the streamlining of the fixed landing gear, the total drag of the airplane is obviously decreased. In the mid-'30s, a series of full-scale wind tunnel tests were made with various types of wheels and landing gear configurations. These tests showed that the drag of an unfaired wheel 25 in. in diameter and 8Vz in. wide was 6.2 Ibs. at 80 mph, and only 2.9 Ibs. when enclosed in a typical "teardrop" fairing. 11 This amounted to a reduction of drag by a factor of 2.9 to 6.2, or about %. In other words, drag of the wheel was cut almost in half by the use of a wheel fairing. The struts, or structural members of the landing gear can also be streamlined where they are exposed to the airstream to minimize drag. A streamline form is defined by its fineness ratio. The fineness ratio of a body is the ratio of its dimension in the direction of the airstream to the greatest dimension perpendicular to the airstream. 12 For example, the strut section shown in Fig.

(Although conventional installations are shown, any type could be used with another strut and wheel to produce a tricycle arrangement). 8

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6 has a fineness ratio of 3.

Although tubing of circular cross section offers high-

er compressive strength for a given weight than tubing

•3-

FIG. 6

of streamlined cross section, it offers much more resistance. The drag of the circular cross section per foot is expressed by the equation D (per foot) = 0.00012 x Diameter (inches) x V2 The drag per foot of the streamlined cross section tubing is the same except for lower values of the coefficient which decreases with the increase of the fineness ratio. Its value ranges from 0.000009 for a fineness ratio of 2.5 to 0.000008 for a fineness ratio of 4.13 It follows then that the drag of a strut of circular cross section is 0.00012 or 15 times that of a streamlined 0.000008

section with a fineness ratio of 4. It is no wonder that lightweight fairings are frequently used on tubing of circular cross section where the cost of streamlined tubing is not justified. Typical examples of fairings are shown in Fig. 7.

Sheet Metal Fairing

Balsa (Old Style) Fairing FIG. 7

In producing forward thrust, the propeller of the airplane must cause a change in the momentum of the air recording to Newton's laws. The air which is affected is the air which is drawn through the propeller and pushed rearward creating the slipstream. The velocity of the slipstream varies with the throttle setting and the angle of attack, and is generally 10 to 15 percent more than the normal airspeed at full power and 40 percent for a steep

climb at full throttle.1'' As a result, the part of the landing gear which protrudes into the slipstream has its drag increased over that which would be produced if no acceleration had been given by the propeller. On most common homebuilt airplanes, a large portion of the landing gear protrudes into the slipstream and its drag is further increased. From the equation for parasite drag which is given as D p = 1.28 | AV2 notice that the magnitude of the parasite drag, Dp, is in direct proportion to the square of the velocity, V. As the velocity is doubled, the magnitude of the drag is quadrupled. In other words, if all other factors remain con-

.stant, four times as much thrust is required to go only

twice as fast, nine times as much thrust to go three times as fast, etc. Thrust is the force developed by the propeller, and

acts opposite the drag force (Fig. 8). In straight and level flight thrust exactly balances drag, Therefore, the thrust required to balance the parasite drag of the landing gear may be expressed as15 T p ar = D,«, = 1.28 f AV2 The amount of power which a given thrust requires may be determined. One horsepower is equal to 550 foot

pounds per second, and is a generally used unit of pow-

FIG. 8

er. Therefore, to find the horsepower produced in a certain instance we determine the rate of work in foot pounds per second and divide by 550.^ For example, Horsepower = Thrust x Velocity 550 Since thrust equals drag, Horsepower - 1.28 -5 AV2 550

From empirical data determined by NACA tests, the drag of the average aircraft wheel will be roughly 4.5 pounds per square foot of frontal area at 80 mph.17 A typical homebuilt aircraft wheel, 7.00 x 4, has a frontal area of about .7 square feet. By using the formula Horsepower = Thrust x Velocity 550 we may calculate the horsepower required to pull this wheel. (EO mph =118 feet per second). Horsepower = .7 x 4.5 x 118 = 3.12 x 118 = .675 5 5 0 5 5 0

The above calculations show that it takes .675 horsepower to pull the wheel at 80 mph. However, most homebuilts cruise at 110 mph 1 ® at the least. This means that the drag of the wheel will be increased with the square of the speed. The drag of the wheel was about 3.12 pounds at 80 mph, so 3.12 x /110\ 2 = 3.12 x 1.9 = 5.9 pounds \8°V

This drag of 5.9 pounds would require 1.73 horsepower. Many homebuilts cruise at 150 mph or more.1^ This means that the drag which this particular wheel produces is 3.12 x X 1 5 0 N 2 = 3.15 x 3.5 = 10.9 pounds V8°V

10.9 pounds of drag requires almost 4.4 horsepower. The preceding calculations are for one wheel only, so the horsepower required to pull two wheels would be twice as great and the horsepower required to pull three would be three times as great in each instance. And, as previously stated, the wheels account for only about 40 percent of the total parasite drag of the landing gear, so at least twice as much power is required to pull the wheels plus the struts. Parasite drag produced by a fixed landing gear can be effectively reduced by enclosing the wheels in stream!ined, tear-drop shaped fairings (known as "pants"). Struts may be streamlined by using streamlined tubing, or as

thi? is usually impractical, by enclosing unstreamlined tub-

ing in streamlined fairings. Fairings for both the wheels and struts may be made simply and lightly of fiberglas and/or sheet metal. A low parasite drag is of utmost importance to an airplane. Every pound of drag requires a pound of thrust from the propeller to maintain the relative motion necessary for the sustenance of the airplane. Each pound

of thrust used in this way adds to the engine power re(Continued on next page)

i

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These photos were taken in a wind tunnel which I have constructed to show airflow by the use of smoke lines. Comparing the two, notice that the unfaired wheel creates considerably more turbulence. The change in the direction of the streamlines is also more abrupt. The wheel fairing obviously improves the airflow. BIBLIOGRAPHY

Effects of Landing Gear Configuration . . .

(Continued from preceding page)

quired and to the fuel consumption as well.20 Since the landing gear drag represents only a portion of the total drag of an aircraft, its speed is increased only slightly even if the landing gear is effectively improved. However, several miles per hour of cruising speed may be gained. Obviously, the best way to eliminate the drag of the landing gear is to remove it completely from the airstream by retracting it. Retraction eliminates landing gear drag completely, but in most cases it is just not as practical as the simple fixed gear for homebuilt aircraft. Simple lightweight fairings on fixed gear add practically no weight but add significantly to the overall efficiency of the airplane.

Alien, John E., Aerodynamics, New York, Harper and Row, Inc., 1963 Aviation Education Research Group, The Science of PreFlight Aeronautics, New York, The Macmillan Company, 1945 "Directory of Plans: American Aircraft", Air Progress Homebuilt Aircraft, pp. 70-80, Spring-Summer 1966

Kenney, Dave, "What Do You Mean, 'Speed' Fairings?", SPORT AVIATION, 10: 12-13, September, 1961 McGraw-Hill Encyclopedia of Science and Technology,

New York, McGraw-Hill Book Company, Inc., 1960 War Department, Theory of Flight, Glendale, Aero Publishers, 1941

FOOTNOTES 'Aviation Education Research Group, The Science of PreFlight Aeronautics, p. 22. 2 John E. Alien, Aerodynamics, p. 34. 3War Department, Theory of Flight, p 21. •Mbid., p. 22. 5 McGraw-Hill Encyclopedia of Science and Technology, p. 176. eibid., p. 177.

7War Department, p. 83. sibid., p. 92. ^Aviation Education Research Group, p. 85.

•

NEWS

loibid., p. 86.

n

Dave Kenney, p. 12. i2War Department, p. 87.

"Ibid., p. 88. "Ibid., p. 86. 15

Aviation Education Research Group, p. 88.

isibid., p. 90.

17 Dave Kenney, p. 13. i8"Directory of Plans: American Aircraft", p. 70. isibid., p. 72. 20War Department, p. 84.

NOTE

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BUS CHARTER SERVICE Bus rides will be available daily, Monday through Friday, from Rockford to the Museum. Sponsored by Chapter 250 Aero Park, Airport, Menomonee Falls.

You would expect any airplane called Irish 1 "Cougar" to at least be painted solid green! N-1033Z was built by Earl J. Irish, EAA 10498, of 1401 Grand in Leavenworth, Kans., and the late Eldon Luehring. The engine is a Ly-

coming 0-290-G, and the total cost ran around $2,500.00. 10

JULY 1967

Shaping up well is the Thorp T-18 wing under construction by Peter G. Yakoff, EAA 22621, of 3511 W. Harmon Highway in Peoria, III. The fiberglas tips are fitted as are the control sticks.