## Maintenance and Restoration: Aerodynamic Considerations

when it comes to acquiring a private pilot certificate is weight and ... equations to airplane pilots while relating all to loading an .... or call EAA at 1-800-236-1025.
nuts & bolts

maintenance & restoration Aerodynamic Considerations Weight and Balance for the Homebuilder JOE CL AR K

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or whatever reason, one problem area of learning when it comes to acquiring a private pilot certiﬁcate is weight and balance (W&B). Homebuilders usually have control over W&B concepts by the time they are ﬁnishing their projects, but even then, sometimes misconceptions may persist. In many ways, the problems lie with age-old fears of math and physics left over from school days. Almost every ﬂight instructor begins teaching W&B using the old technique of a simple seesaw and children. This approach is ﬁne, as long as the instructor carefully and appropriately relates the example to airplanes. In many cases it is not, resulting in new private pilots misunderstanding important concepts of W&B. Most of us can grasp the idea of two children on a seesaw. If they weigh the same and are equidistant from the fulcrum, the seesaw balances. The problem comes when trying to introduce the ideas of datums, stations, moments, and equations to airplane pilots while relating all to loading an airplane. Then we have to determine the exact location of the fulcrum and correlate the position to aircraft behavior and performance. For the builder, a full understanding of W&B may play an important role in the selection of aircraft. For example, certain aircraft may have weight or moment limitations. A pilot who is tall and heavy, for instance, may be limited

to certain airplanes that have the load-carrying capacity for his frame. Or, in the instance of airplanes powerful enough to carry heavier loads, the concern might be placement of the load in the aircraft to keep the airplane safely balanced. When it comes to W&B, a few important considerations for builders include fundamental understanding of airfoils, placement of the wing with relation to the fuselage, and knowing how levers work. They also need to know the relationship between the airfoil, the center of gravity (CG), and the ﬂight controls. Each airfoil is different, possessing unique characteristics. Each has different limitations regarding how far forward or aft the airfoil can tolerate CG placement. The measurement of these limitations may be in feet, inches, or percent mean aerodynamic chord (MAC). In addition to airfoil considerations, designer/builders must also think about the distance between the CG and the force exerted by the different ﬂight controls. This is where the concept of levers comes into play. This is particularly important regarding the elevator surface area, pitch force and control, and aircraft loading. Dynamic force, or true airspeed, also plays an important role. For example, an airplane traveling at high speeds requires more input force to move the elevator, thus moving the nose. An aircraft ﬂying slowly, however, requires little

When it comes to weight and balance, a few important considerations for builders include fundamental understanding of airfoils, placement of the wing with relation to the fuselage, and knowing how levers work

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effort to reposition the elevator the same amount. Speed also factors into how much and how quickly the aircraft will rotate about the CG. Another issue is the size of the ﬂight control affecting the movement. All of these concerns relate to one another in both the design and position of the CG. Additionally, aircraft performance depends on the loading—particularly in relation to airspeed and fuel efﬁciency. To understand why, let’s go through a short refresher on some aerodynamic considerations. In the beginning of ﬂight training, ﬂight instructors dutifully teach each of their student pilots the concept of the four forces. Every student pilot in America will tell you that in level, unaccelerated ﬂight, thrust equals drag (T=D) and lift equals weight (L=W). Or does it?

Certainly, if the power and airspeed are constant, T=D. The airplane will neither slow down nor accelerate. The lift and weight equation, however, is a little more complicated. For the airplane to maintain level ﬂight, indeed the wing must lift the weight of the airplane. However, there is more to this than meets the eye. The lift and weight equation involves more than just L=D; it also involves the balance of the airplane while keeping stability in mind. Here is part of the secret to understanding weight and balance: the wing must lift the weight of the airplane, plus the weight of the down force of the horizontal stabilizer and elevator (Figure 1). In other words, the wing of an airplane weighing 1,617 pounds must lift 1,687 pounds if the elevator down force is 70 pounds. Another part of the secret is that the designer or pilot ﬂying the aircraft has some control over the amount of the down force exerted by the horizontal tail. This control over the down force stems from the design of the aircraft and the way it carries its payload. In other words, the down force is dependent on the CG location. The ﬁrst thing regarding CG placement is the airfoil itself. Some wings can tolerate extremes of CG placement more so than other airfoils. Generally, most CG limits fall into the realm of 15 percent to 30 percent of the MAC. Many pilots have a hard time visualizing this because their instructors, while explaining W&B adequately, did a poor job of illustrating the important details of the concept. For example, a wing with a MAC of 52 inches and a forward and aft limit of 17 percent and 30 percent as described by the geometry and characteristics of our ﬁctitious airfoil, will have a forward CG limit of 8.8 inches (52 x .17 = 8.8). The aft limit of the airfoil will fall at 15.6 inches (52 x .30 = 15.6). This gives a range of 6.8 inches (15.6 - 8.8 = 6.8). Illustrated appropriately, this is not a lot of movement allowed for this particular ﬁctitious airfoil (Figure 2). The CG range the pilot is concerned with also involves the placement of the wing on the fuselage in reference to the datum. In other words, if the datum is located at the tip of the spinner, all components are measured aft of that point. If the leading edge of the MAC is located 62 inches behind the datum, the actual CG range for our ﬁctitious aircraft becomes 70.8 to 77.6 (Figure 3). The method by which the designer or pilot can control the amount of down force on the horizontal is by EAA Sport Aviation

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WEIGHT

ARM

MOMENT

1,062

71

75,402

Pilot

180

72

12,960

Passenger

160

103

16,480

Fuel

180

74

13,320

35

125

4,375

Empty weight

Baggage Gross weight

1,617

122,537

In the example above, the sum of the total moments divided by the gross weight determines the actual CG. This would be 122,537/1,617 = 75.8 inches aft of the datum. That is in the heart of the range of 70.8 to

Where all of this comes to play regarding performance is that with less lift, the wing creates less induced drag. With less induced drag, the pilot can choose between lower horsepower for the same cruise speed or cruise faster with the same power setting. 77.6 for the airfoil. The equations to remember are 1) moment = weight x arm, and 2) total moments divided by gross weight = CG. In this particular example, the crew has few options to move the CG. Keep in mind, if you intend to design your own aircraft, you need to place the wing appropriately to gain full advantage of the airfoil and the loading. Also, place the fuel tanks in such a position so that as the fuel is used, the CG stays in place or moves only slightly.

Memorial Wall

 A Place for Remembrance and Reflection  EAA’s Memorial Wall provides a place to remember important people in a very special way. Located behind the EAA Aviation Center in Oshkosh,Wisconsin, the EAA Memorial Wall honors departed EAA members and aviation enthusiasts among park-like surroundings provided by ponds and trees and Compass Hill.

Joe Clark teaches at Embry-Riddle Aeronautical University, has given 6,000 hours of dual instruction, is a former U.S. Navy A-7 pilot, and owns a 1952 Cessna 170.

SOURCES For a better understanding of weight and balance and design considerations, reading recommendations include: Aerodynamics for Naval Aviators, by Hugh H. Hurt; Ofﬁce of the Chief of Naval Operations, Aviation Training Division, U.S. Navy, 1960. The Design of the Aeroplane, by Darrol Stinton; New York, Van Nostrand Reinhold, 1983. Design for Flying, by David B. Thurston; New York, TAB Books, 1995. Design for Safety, by David B. Thurston; New York, TAB Books, 1995.

Your loved one’s inscription will be cast on a bronze plaque. New plaques are installed every year listing all honorees whose names were submitted before March 31st each year. The official ceremony and first viewing of the new installation takes place during AirVenture. Your memorial contribution of \$350.00 covers engraving and installation costs, administration of biographical data for each individual, a video of the ceremony and permanent maintenance of the site. To submit your special person’s name, or to learn more about EAA’s Memorial Wall, please visit www.airventure.org/memorialwall or call EAA at 1-800-236-1025.

www.eaa.org EAA Sport Aviation

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