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Cary Malott, Flight Advisor contributed the following article from the January issue of Experimenter. Thanks Cary!

WHAT DOES IT WEIGH?
by Charles Golden

A couple months ago at one of our Chapter meetings, we had some discussions concerning aircraft weight and balance. As an ex-Loadmaster from the Marines, this subject is always dear to my heart. Therefore, I thought there was enough interest for an article....

Weight is one of the greatest enemies of pilots, most likely second only to weather. Therefore, it must be respected and not taken lightly. Surprisingly, it does not seem to be high on the list of most pilots. The force of gravity acts on an aircraft constantly trying to pull it out of flight. The available lift from the airfoils is the only force to counteract gravity and keep the aircraft in flight. If the available lift is not equal to the aircraft weight, level flight cannot be maintained and the aircraft will descend.

Aircraft builders like us know better than anyone that it is essential to keep weight to a minimum. It is always easier to fly when the aircraft is light, and more difficult and dangerous when the aircraft is heavy. Therefore, it is always our primary concern to build our aircraft as light as possible without sacrificing strength or safety.

Excessive weight reduces the flying ability of an aircraft in almost every respect. Some of the most important performance deficiencies of an overweight aircraft are: Higher take off speeds, longer take off run, reduced rate and angle of climb, shorter range, reduced speed, reduced maneuverability, higher stalling speed — and I'm certain you can come up with several more. Typically, fuel burn is the only weight change that takes place in our type of aircraft during flight. As fuel is burned, weight is reduced and the aircraft performance improves. This is the only good thing I can think about for the consumption of fuel.

Balance refers to the location of the center of gravity (cg) of an aircraft. It is of primary importance to aircraft stability and safety of flight. The cg is the point at which an aircraft would balance if it were possible to support the aircraft on a point. The cg must be within specific limits for safe flight.

The primary concern of aircraft balancing for small private aircraft is longitudinal balance. Certainly lateral balance is critical to flight but in our case, it is not typically measured regularly. More important to lateral balance for us is: Don't let your wings run out of petrol. So for our discussions I will only discuss longitudinal cg.

The cg of an aircraft is not typically a fixed point. Its location depends on where items are loaded, their weights and if they are shift in flight. Again, we typically don't shift items around in flight in private aircraft other than fuel burn off.

Abnormal cg conditions affect the flying ability of an airplane with basically the same flight characteristics as those mentioned for excess weight earlier. Additionally, two essentia1 airplane conditions will be reduced by improper cg: Stability & control. Forward cg causes problems with control and raising the nose, particularly during take off and landing. Rear cg causes the most serious effect on stability to the extent of reducing the airplane's ability to recover from stalls and spins. Who doesn't remember the cg tests run on the Velocity a few years back? Don't load an aircraft to a tail-heavy condition exceeding the manufacturers limits.

Limits for the location of the cg are estab1ished by the individual manufacturer. The forward cg is often determined by the landing characteristics of the aircraft. It may be possible to maintain safe cruising flight with a forward cg, but when it comes to that part about landing, you are going to want it right. Prop strikes are embarrassing. Forward cg limits are also placed due to the amount of elevator travel available at low speeds.

The aft cg is the most critical. As the cg moves aft, a less stable condition exists and lessens the airplane's ability to right itself after maneuvering

The actual location of the cg can be altered by many factors, all usually under the control of the pilot. Placement of the baggage or cargo, the assignment of passengers to various seats and fuel load. Fuel load is pretty straightforward unless you have a swept wing aircraft and then things get mighty difficult. If your wings are swept I can recommend you either study your weight and balance carefully or better yet, get rid of that airplane. You might want to check your plans and make sure they were supposed to be swept.

To continue further with this discussion we must understand several terms associated with weight and balance. I will list several terms and definitions and then continue with how to actually weigh your aircraft and compute your actual weight and balance. If you are interested in weight and balance, several good books are available to pilots to study. Several excerpts in this article are taken from FAA Advisory Circular 91-23A.

Arm (moment arm)—is the horizontal distance in inches from the reference datum line -to the cg of the item.

Center of Gravity (cg)—is the point at which an aircraft would balance if it were possible to suspend it at that point.

Center of gravity limits-are the specified forward and aft points beyond which the cg must not be located during takeoff, flight or landing.

Datum (reference datum line)—is an imaginary vertical plane from which all measurements of arm are taken.

LEMAC—is the leading edge of the Mean Aerodynamic Chord

Moment—is the product of the weight of an item multiplied by its arm.

Mean aerodynamic chord (MAC)—is the average distance from the leading edge to the trailing edge of the wing.

Station—is a location in the aircraft identified by the distance from the reference datum line.

Useful load—is the weight of the pilot, copilot, passengers, baggage, usable fuel, and drainable oil It is the empty weight subtracted from the maximum allowable takeoff weight This term applies only to general aviation aircraft.

Unless you know a way of balancing an aircraft on a point, accurately weighing your aircraft is the only reliable method of computing your center of gravity. Every aircraft requires an accurate empty weight and cg location to compute actual flight characteristics and ensure the safety of each flight. Over a period of time, aircraft should actually be reweighed if any difficulty is observed or numerous changes in equipment have taken place. The use of reliable, calibrated scales is a must. I got mine from weekend racers who must have exact weights to win. Check around at local race tracks and someone may rent you a set

Typical aircraft weighing procedures include:

1. The aircraft should be clean, inside and out. It should also be dry.

2. The aircraft equipment should be checked against the equipment list for the aircraft, or a list should be developed if one does not exist.

3. Fuel tanks should be drained. Remaining fuel in tanks, fuel lines and the engine is termed "residual fuels" and is included in the empty weight.

4. Typically, oil should be drained. Again, remaining oil is considered residual and included in the aircraft empty weight.

5. All other reservoirs containing hydraulic fluid, anti-icing fluid or other liquids should be filled to capacity and are considered as part of the aircraft empty weight.

6. Generally, all aircraft are weighed in a flight level attitude. Weighing in any other than level position will give you an accurate weight but not necessarily an accurate load on each wheel, from which we will compute the cg.

Finding the Center of Gravity

After the correct weights on each wheel have been obtained and recorded, the empty weight and empty cg can be calculated. Empty weight refers to the sum of the three scale readings you have just obtained. Total the three and record this weight as the empty weight.

The center of gravity is the point at which all the weights of the aircraft can be considered to be concentrated. Remember, if you could somehow place this aircraft on a pinpoint and it would be balanced, that would be the empty cg. The average location of the weights can, therefore, be obtained by dividing the total moments (weight x arm) by the total weight. The process then involves multiplying each measured weight by its arm to obtain a moment and then adding the moments. The result will give you an inch line aft the reference datum line where the aircraft center of gravity is located as in the example below.

Expression of the cg relative to the MAC is a common practice and relative to most small, general aviation (GA) aircraft. Knowing the cg relative to the reference datum line is only appropriate if a chart is available and on hand.

The relative position of the cg and the aerodynamic center or center of lift of the wing have critical effects on the flight characteristics of the aircraft. Therefore, relating the cg to the chord of the wing is convenient from an operations standpoint. Typically, an aircraft will have acceptable flight characteristics if the cg is located at or near the 25 percent average chord point. The MAC is established by the manufacturer. You might find typical GA aircraft to have usable cg's of 15-30% MAC.

vIn summary, the MAC is established by the manufacturer who defines its leading edge (LEMAC) and its trailing edge in terms of inches from datum. The cg location and various limits are then expressed in percentages of the MAC.

This article was meant to discuss the basics of aircraft weight and balance without going into extreme details … and to address the importance of weight and balance. Although we are typically building and flying small, experimental aircraft, weight and balance is just as critical as it is for the heavy iron. It must be taken seriously, as all parts of aviation should be.

Obviously, each and every aircraft has individual characteristics which are typically defined by the manufacturer. Please refer to your manufacturers manuals for information concerning your particular aircraft. Many books and manuals concerning weight and balance are available to learn more about this area. As always, have fun and fly safe

1 Experimenter, January 1999 (Reprinted by permission from the newsletter of EAA Chapter223)