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Weighing Homebuilts

By Tony Bingelis (originally published in EAA Sport Aviation, July 1991)

Contrary to the way some builders talk, weighing a homebuilt is NOT for the specific purpose of finding out how much over the design weight it turned out to be.

I'm sure builders who are pilots know that the mandatory weighing is for the express purpose of determining if the aircraft's center of gravity will be within the prescribed limits for safe flight. Of course, its all up weight is important, too.

In this regard, it seems almost every builder expects his airplane to weigh more than the prototype . . . and for good reason.

The design prototype is generally a "bare bones" airplane with a minimum of instrumentation and avionics, and often lacks all but the most spartan of creature comforts.

For this reason, it is not at all unusual for a well equipped version to weigh 30 to 50 pounds more than the design weight. And that's really too bad because any aircraft's performance suffers with an increase in weight. This translates to an increased takeoff distance and a reduced rate of climb. Also reduced is the aircraft's structural capacity . . . however slight that might be.

There is another correlation. Unfortunately, the added weight always seems to migrate toward the tail and frequently is manifest by a less than desirable aft CG condition.

I assume everyone who has learned to fly has had to learn a little something about weight and balance. A pilot is supposed to be able to figure the effects of various loading conditions on his aircraft's CG situation. I therefore see little need to go into the elementary see-saw balance theory or to get into working typical weight and balance loading problems at this time.

However, I am sure that as a homebuilder who has completed or is soon to complete his first homebuilt you are probably more interested in the options open to you for weighing your airplane the easiest way possible.

What is involved in weighing is easy enough to visualize - that is, get the wheels up on the scales, total the three scale readings, and there you have it - the aircraft's empty weight.

Then, after obtaining that basic information, along with a few measurements and arithmetic figures, you can go on and determine the aircraft's empty, CG and figure the effect various loads will have on the aircraft's CG as well.

Easier said than done, you say? Not really. Actually, I believe that locating a suitable set of scales and weighing the aircraft with them has to be the homebuilder's greatest hurdle.

Obviously, you cannot obtain a trustworthy weight and balance without a fairly accurate set of scales . . . but, where do you start?

Well, you could buy a set of aircraft scales . . . there are such things, you know. The latest of these are low profile electronic marvels. I understand they are so compact that an aircraft can simply be rolled on without the need of special ramps or other paraphernalia. These special purpose scales, as advertised, are expensive and I doubt that a homebuilder would be willing to expend so much money for a one time weighing.

A more attractive option would be to obtain the use of someone else's aircraft scales.

Around larger aircraft maintenance facilities there are aircraft scales used infrequently . . . just sitting idle. Whether the availability of these scales would be extended for your use is another matter. Under favorable conditions, however, you might be able to borrow or rent them.

More than likely, though, the facility would probably prefer to do the weighing for you . . . as a customer service, at regular shop rates, of course.

If that prospect doesn't appeal to you, don't overlook the possibility that some nearby small airport with a maintenance shop might have a set of scales. I'm sure folks like that would be more willing to rent scales out than would a large maintenance facility, especially if you were to offer to put up a deposit. For that matter, you might even luck out and get to use their scales without cost.

Nevertheless, be prepared to look to other sources for your scales, mainly if your aircraft is not based at that airport.

If you live in a rural community you will probably have less trouble locating suitable scales than would a builder who lives in a small city. Most feed stores - and many farmers - have at least one good scale with a 500 pound or greater capacity.

Then there is the EAA. If you belong to an EAA Chapter, check with them. They may have a set of scales. These would probably be the feed store variety that have been cut down and converted specifically for aircraft weighing.

NOTE: In addition to aircraft scales, many EAA Chapters have other equipment for use by their members. This is just one of the benefits of being a member of an EAA Chapter.

You have another, less desirable, option - that of using bathroom scales. Talk with a few of your neighbors to see what they have in the way of bathroom scales. If you have to have extras, try to borrow scales having similar dimensions. This will minimize the problem of blocking and shimming them to a uniform height.

Ideally, your first choice would be 3 calibrated (accurate) scales, two of which would have 500 pound or greater capacities.

One of the three scales could have a lesser capacity as the weight on a tail wheel or nose wheel is generally less than 300 pounds. That's the ideal situation.

Here's a less attractive alternative. In a pinch, you can weigh your airplane with a single (that's right, single) 500 pound capacity feed store scale, or something equivalent to that - and still obtain reasonably accurate results with it.

Be prepared though. Using only one scale to do your weighing will require extra work. That extra work includes the construction of a couple of platform ramps to double as dummy scales. These platforms must be exactly the same height as the scale you have so that the aircraft will be supported in a level attitude no matter which wheel the scale happens to be under.

As you might suspect, weighing an airplane with a single scale entails a number of repetitive aircraft movements. Don't attempt to do this scale switching, from wheel to wheel, alone.

You should have at least 3 helpers to help you roll the aircraft up the ramps and onto the scale and platform.

With the scale first under one wheel, take that reading. Next, off-load the airplane from the scale and switch the platform and scale to the opposite wheel. Read the weight of that other wheel and make a note of it. Weigh the tail wheel or nose wheel in the same manner.

Record which weight is for the right wheel, which is for the left one and which one is for the tail (don't forget to subtract the weight of any blocking or chocks used on the scale).

Another way to weigh the airplane, but only if you cannot arrange for the use of one or two good high capacity scales, is by using several bathroom scales.

Before putting the bathroom scales you have acquired to work, weigh yourself on each of the scales (and get a couple of other guys to do the same). This will give you some idea of their accuracy and a clue as to the amount the readings will need to be corrected, if any.

Most bathroom scales have a fairly low capacity, a little more than 300 pounds, usually. Since the average homebuilt Weighs between 800 pounds and 1200 pounds, this means that the weight on each wheel will most likely exceed 400 pounds . . . and the capacity of a single bathroom scale.

The trick is to use two bathroom scales under each wheel to obtain the necessary capacity. This remarkable feat can be accomplished by positioning two scales side by side and bridging over them with a plank.

The airplane can then be rolled up onto the plank with the wheel approximately centered on the plank between the two scales.

Note the reading on each scale. Add the two readings together to obtain the total weight for that wheel. This may not be the most accurate way to do it, but you can use it when you have no other recourse.

1. Your aircraft should be completely equipped and ready to fly. It will be weighed with full oil but no fuel except, perhaps, for the small amount of unusable fuel that cannot be drained.

This means, also, that the wheel pants should be in place as well as the cowling, spinner, upholstery, battery and anything else that constitutes your aircraft's normal flight configuration.

2. Select a Datum - this will be the starting point from which all your measurements will be made. Most builders use the tip of the spinner for this reference. That way, all the measurements (distances) will be aft of the spinner and all figures will, therefore, have plus values.

NOTE - If, on the other hand, the Datum point you select is the leading edge of the wing or some other similar location, you will have to work with plus and minus figures. This usually results in unnecessary complexity and increases the risk of making an arithmetic error.

3. Measure and make a note of the distance from the Datum of the following:

    Nose wheel (or tailwheel)
    Main gear (wheel axle centerline)
    Fuel tank(s)
    Oil supply/oil tank
    Baggage compartment

4. Find a conveniently located LEVEL point (top longeron, cockpit floor, or whatever). Use it to level the aircraft.

5. Build, or have available, a ramp for each scale. These must be the same height as the scales and should be about 3 feet long to provide a reasonably shallow incline for the wheels.

6. Obtain several 2 x 4's about 12 inches long for use as chocks.

7. Have available and use a plumb bob, steel tape, and carpenter's square to accurately establish the location of the Datum, and to measure distances between it, the wheels and other fixed, locations.

Although I don't recommend it (because it can be a bit risky), here's how it's done.

If you have wing jacks, you can jack the airplane up, roll the scales in position below the wheels. Lower the jacks slowly. You then have the airplane up on the scales.

This method is sometimes used for retractable gear jobs and aircraft equipped with vertical shock struts.

It is not a practical procedure to use for spring gear equipped aircraft. There is a good reason for this. When a spring gear is lowered onto the scales the gear legs will spread as the weight of the airplane bears down on the scales. The scale surfaces will be loaded with abnormal side loads, and the weight reading obtained is likely to be unreliable.

Well, all this should alert you to some of the weighing problems you can encounter. Although the actual weighing takes but a few minutes, the results will become part of your aircraft records forever . . . unless, that is, you decide one day to re-weigh the airplane.

Why would anyone re-weigh his airplane in a year or two? Because it is a good idea.

Aircraft, like people, seem to pick up weight with time.

When an airplane is first weighed it will be lighter than it will ever be again.

Where does this additional weight come from? Well, there are mud daubers, and there are accumulations of dust and dirt. But that is not all. Your airplane will gradually acquire and be carrying around more "onboard equipment" than you would suspect. How about maps and charts, sick bags, a small tool kit, tie downs, airport directory and loran manuals, log books, cleaning rags, and maybe an added radio or transponder/encoder, an intercom, ELT, fire extinguisher, a couple of new instruments, extra cushions, etc. Now, do you believe it?

A well equipped high performance aircraft is more likely to suffer from this gradual weight growth than a simple homebuilt.

For example, a local Glasair when reweighed recently was found to have picked up 147 pounds over a span of 3 years. That increased its empty weight from the original 1,174 pounds to 1,321!

How does such a weight increase affect the CG? That could be a worrisome thing under some load combinations of fuel, passenger and baggage.

It is something you should know, isn't it?

In short, it is most important that you do weigh your airplane, and do it as accurately as you can with the scales you have to work with.

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