EAA STYLE HERE ARE two weight/balance questions the builder-

pilot must answer: Where is the CG? Where should T it be?

First, measuring the weight and balance of a newly built ship is done with a few simple tools. Scales, naturally, are used, and they must be accurate. The type found in grain and feed stores (and often borrowed from them) are suitable. Also, you need a plumb line and a steel tape, a few scraps of wood, and a level. The ship is mounted on the scales and carefully leveled. This step is critical, since a little friction in the scales will make the whole exercise useless. The platforms must be free, and this may take quite a bit of jockeying by the time you have the wheels chocked and blocked up to level the airplane. Once you're set up, record the weight at each wheel, disturb the set-up by jiggling everything, make another record, then repeat the operation to get at least three observations. They won't be the same, so use the average for each wheel. Be sure to keep all the tare weight (blocks, etc.) at each scale, weigh these after removing the ship, and deduct these weights from the respective wheel loads. Now you have all the numbers, and the weight of the ship is simply the total for all three weigh points. All the moment arms must be measured while the ship is on the scales, using the plumb line to be sure you have the exact horizontal distance between weigh points, and between the weigh points and your reference or datum. To locate the CG, multiply the weight on one of the weigh points — say, the nose or tailwheel — by the distance to the other two. Now divide the total weight into this product. The result is the CG location from the other two points (the main gear if you used the nose or tail weight). To calculate the effects of loads in the airplane is just the same idea extended a little. A tabular form will help to keep things straight: A weight

B arm

C moment

empty fuel oil pilot passenger

A datum should be chosen a few inches forward of the nose. Then all numbers are conveniently positive, even if you later make changes in the airplane. The other question is more complicated, especially because the general comments we can make have strictly limited application. You can form dangerously wrong conclusions by trying to apply these ideas where they are not appropriate. For lightplanes of conventional design, the NACA found that longitudinal stability is pretty well assured if the CG is at least eight percent of the Mean Aerodynamic Chord forward to the airplane Aerodynamic Center. More of a good thing is not necessarily better — too far forward and you run out of elevator control. MAC in the simplest case (a rectangular wing) is almost identical in length and location to the physical chord, near enough for most practical purposes unless the wing is of unusually short span. For tapered wings it is practically the average chord (area divided by span). However, this doesn't work for highly swept wings or other unconventional planforms. If you have such a design, check with someone who has a detailed knowledge of aerodynamics. Aerodynamic Center of the whole airplane, again using an approximate rule for conventional designs, can be estimated as:

the distance from wing MAC quarter-chord point to the Aerodynamic Center

St = tail area 1 = distance between wing and tail MAC quarterchord points S tw = total area of wing and tail Using this general rule frequently shows that the CG should be about at 15 to 20 percent of the wing MAC. Simple rules like these can be very useful in preliminary work particularly. Don't hesitate to consult someone who knows more about it, especially if you are working with an unconventional design.

WEIGHT WATCHING By BILL WELCH (Courtesy EAA Chapter 130)

baggage

total The gross weight CG position is simply B =

C A EPORT A V I A T I O N

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