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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maintenance & restoration case. When the CG moves behind the center of lift, the airplane becomes unstable; this uncontrollable situation will always end in an aircraft mishap. 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. For the designer, the challenge is mating wing and fuselage to acquire a CG near the aft limits of accepted range. For the pilot, it is the challenge of loading the aircraft to maintain a safe CG location near the rear limit of the range. For the pilot to do this, an understanding of W&B terms and the ability to work weight and balance problems is necessary. Terms: 1) Empty weight is the weight of the airplane empty with unusable fluids. 2) Payload refers to everything loaded into the airplane. 3) Gross weight is empty weight plus payload. 4) Weight, usually in pounds (in the United States). 5) Datum, the point from which measurements are made; everything aft of the datum is a positive number, everything forward is negative. 6) Station, a point at which a component balances or a movable object is loaded. 7) Arm, the distance from the datum to the station to the object. 8) Moment, the force of the object, determined by multiplying the weight of the object by the arm (WxA). On our fictitious airplane, let’s assume an empty weight of 1,062 pounds with a moment of 75,402. The pilot’s seat is located at 72 inches aft of the datum, the passenger seat is at 103 inches, fuel is 74 inches, and the baggage compartment is at 125 inches. Remember that your given information is weight and arm, and you derive moment by multiplying individual weights by their respective arms. This will allow for a loading that looks like this: controlling the CG. The more forward the CG, the more down force required to keep the airplane in level flight. With more of an aft CG, the horizontal requires less of a down force (Figure 4). Placement of the CG is important for many considerations. One important factor is the stability of the aircraft. The center of lift must always be behind the CG for the reason of stability. With gravity acting on the airplane through the CG coupled with the down force of the horizontal, it is easy to understand why the CG must never exceed the aft limits (Figure 4). The aft CG limit on all designs will be somewhere forward of the lifting force of the wing. This always allows for a down force on the horizontal, which maintains a stable W&B condition. If the aircraft were loaded in a manner allowing the CG and the lift force to align vertically, the horizontal down force would equal 0. Stability would be neutral in this 102
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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.
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