Calculating Weight and Balance

Tackling weight-and-balance calculations is a task we have all faced with some trepidation, but it is so vital, so important, so fundamental to flight safety that it must be done. It is also a job so well suited to a computer’s talents that it begs to be completed using one.

Let ‘s say it’s time to weigh your newly built airplane and calculate weight and balance. This same procedure can be done the first time and at any time after that , and it is particularly necessary after any modifications arc made that will affect weight and balance. It ‘s also required so I.hat you’ll have the numbers to enter into FAA form 337.

This is a straightforward process even though it’s often approached with fear and loathing. You’ll still have to do all of the grunt work needed to get the plane onto the scales, but this spreadsheet will take most of the computational headaches out of the operation. Here’s how to go about it.

What You Need to Weigh In

1. You’ll need three accurate scales—one for each wheel. What kind of scales? Use any kind, as long as they arc accurate, will support one half the weight of the plane, and aren’t so tall that they ‘ II bump into something important. Photo I shows two heavy -duty platform scales that are rugged enough to support anything short of a 747. (They’re sure up to the job of elevating Roger Allen ‘s Fisher FP-202, which has been modified to look like a vintage L-2.) Or you can do as many folks do and go for the cheaper spread bathroom scales shown in Photo 2. Be aware, though, that these scales usually aren’t accurate. You’ll need to calibrate them too.
2. You’ll also need knowledge of-your plane’s datum location and the station location of each of the three wheels. What’s a datum? The notes at the end of this article detail that bit of lore.
3. You’ll require some way of leveling your airplane. Do this according to the designer’s instructions or, if those aren’t available , try to get the airplane into the attitude that it would be in level flight. This can be approximated accurately by leveling the top longeron. Remember, you’ll need a tail stand for a tailwheel airplane.
4. You’ II also need ramps to get the plane onto the scales.

   

Preparation

1. Remove all of the junk from the inside of the airplane. Don’t forget to check the baggage compartment, wheel pants, tail cone and the cockpit. It may be surprising what you find.
2. Drain as much fuel as possible.
3. Leave the oil in if you like.
4. Inflate the tires to proper values.
5. Weigh separately the stuff that will be put on the scales, such as the chocks.

Those are the tare numbers you’ll need for the spreadsheet.

Leveling

1. Roll the plane onto the scales.
2. Chock the wheels so it won’t roll off.
3. Lift the tail (or nosewheel) until the fuselage is level.

   

Weighing

1. Record the weight on each scale.
2. Drop a plumb bob down from the datum. Mark its spot on the floor as shown in Photo 5.
3. Drop the plumb bob from each wheel’s axle, in turn. Mark each of those spots on the floor.
4. Measure the longitudinal (nose-to-tail) distance from the datum to each of the axles. Since the datum is usually counted as the 0.0 stat ion (called the STA), the distance backward to each of the axles (in inches) will be that wheel’s ST A. ff the nosewheel is in front of the datum, its STA will be negative.

Enter the Data

1. Look at the weight-and-balance spreadsheet.
2. Enter the basic information from steps 1 through 4. This is needed only for good record keeping.
3. Enter the weights and STA numbers for each of the three wheels in Step 5.

What Do We Have Now?

Now you have all of the numbers needed for the FAA form 337. You also have the airplane’s basic empty weight (BEW) on line 17 (Step 6). The BEW tel ls you whether it’s what you had hoped for and if it’s not too much more than the designer said it would be.

You have the location of the e.g. Is it within the range allowed by the designer?

If you don’t have those numbers and if it is between the 23% and 30% chord distances of your wing, you’ ll most likely be OK.

The moment numbers can be used to check whether you’re within the allowable limits in the e.g. moment envelope charts provided by all factory planes and some designers’ plans.

What Else Can I Do?

You’re really only partway home with weight and balance at this point. You now have many of the numbers you’ ll need, but only for an empty airplane—and it’s tough lo fly those. Let’s take another step and see whether our real-life plane complete with pilot, fuel and maybe a passenger or two will perform. This next step is easy to do with the spreadsheet provided below.

The Loaded Plane’s Weight and Balance

First, fill in the numbers asked for in Step 7. There’s a place for oil in case you weighed the plane with the crankcase empty. The computer will do all or the calculations for you, even determining the right weights for oil and gas if you enter the number or quarts/gallons. I’ve also had the computer calculate how the c.g. varies as the fuel is burned off in night. That’s what the flight conditions box is for.

What do you do with these new numbers? You do the same thing you did with the numbers for the empty weight. Check the weights Lo ensure that you don’t ever get over gross. Check the c.g. locations to make sure that you· re al ways wi thin the allowable range specified by the aircraft designer. If you happen to have ,l e.g. moment envelope diagram, you can be certain that you’re always within the envelope as specified by a combination of weight and moment for each of the conditions.

You should also check other flight conditions both with and without passengers and with and without baggage. This can easily be done by changing the weights for these items to zero on the spreadsheet. You can also see what effect a heavier passenger would have on your weigh! and balance. Al  of the c.g. weight and moment numbers will automatically recalculate so that you can quickly look them over.

Notes

The weight numbers l’or each of the items arc straight forward, hut the STA (longitudinal station) numbers might not be intuitively obvious. The STA is the term for the distance between the datum and the c.g. of the item in question.

The STA number is normally measured in inches, but there’s no reason why it couldn’t be metric. It works either way.

We’ re only concerned with longitudinal distances: fore-and-aft, nose-to-tail measurements. The left-to-right balance can usually be compensated for by the ailerons.

Where’ s that elusive datum point? lt’s somewhere along the length of the airplane, and it is almost always chosen by the designer. If it’s not, don’t despair. You can choose any place you want, such as the front of the spinner or 10 feet in front of that. Commonly, the datum point is the wing’s leading edge as shown in Photo 5. lf you choose the leading edge, some things will obviously be in front of it and some wi.11 be behind. To handle that problem, use negative numbers for the STA measurements in front of the datum and positive numbers behind it.

Enter your own numbers and watch the answers magically appear.

Downloads

Spreadsheet instructions
Weight & balance – New aircraft
Weight & balance – Your flying aircraft