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A Realistic Look
Not only does FAA require a weight and balance check as part of an aircraft’s permanent records, but an airplane that is extremely tail heavy (i.e., exceeds Center of Gravity limits) is a potential disaster machine — a set up for a short-lived stall-spin career. So, too, is an airplane so nose heavy that it can’t be flared for a safe landing. Of the two, the nose-heavy condition is the least likely to lead to total disaster, only because you may not be able to get it off the ground.
Whatever you do, don’t abandon the idea of weighing your aircraft simply because you cannot find calibrated scales … or any other reason for that matter. You must know what your weight and center of gravity (cg) situation is before you attempt to fly.
Scales Are Where You Find Them
The ease of any weighing effort, naturally, is determined by the equipment used. The scales ideally should be of a low platform variety and accurately calibrated. But, how many of us can really obtain such equipment, even for a once-in-a-lifetime project? Nevertheless, you objective is to get the best scales you can.
Feed stores in many parts of the country have accurate platform scales which often can be borrowed over the weekend for such an outlandish purpose as weighing a homemade airplane. Unfortunately, you can usually only obtain one platform scale at the feed store. You could, perhaps, go to another feed store somewhere and borrow a second scale. It would make your job a lot easier. But, one is better than none.
Here’s another lead. The Department of Public Safety in at least one friendly state utilizes special low-profile scales for weight trucks. These fine folks have been known to drop by when asked and allow the use of their scales in the interest of public safety … and curiosity. Besides, they couldn’t permit you to haul that airplane along the public highway if its wheel load were so high it might damage the highway pavement.
Equipment rental shops in larger cities sometimes have scales for rent, although the smaller shops are not likely to have something usable.
Perhaps a university laboratory or an airport maintenance shop can accommodate your needs. Ask someone at your local airport how their weight and balance requirements are handled.
The last common source and last resort, in most cases, is bathroom scales. I’d like to have a dollar for every airplane that has been weighed on bathroom scales.
I don’t know which presents the bigger problem, using only a single, accurate platform scale for the weighing, or using five bathroom scales of doubtful performance. Either way, you have your work cut out for you.
The Single Scale Problem
If you can only obtain a single platform scale, don’t be too concerned. You can still do a fairly good job of weighing the aircraft. More work will be involved, of course; not only will you have to build a small inclined ramp for each wheel to roll up onto the scale, but you will also need to build a leveling platform for the other main wheel This ramp platform will have to be built to the same height as the surface of the scale. Remember, the airplane must be level in all directions for weighing. This is particularly important when only one scale is used. That one scale can be used for each of the three weighing points, however, a bathroom scale used under the tail wheel or nose wheel will do much to simplify the problem with little or no sacrifice in accuracy.
The Multiple Bathroom Scale Problem
Bathroom scales are notoriously inaccurate … ask anyone on a diet. Most bathroom scales can only weigh up to approximately 300 pounds. The range of bathroom scales is usually insufficient to accommodate the weight of a single wheel of a standard-size aircraft. They may work well enough, however for designs such as Volksplanes, miniMAXes, Titan Tornados, Challengers, gyrocopters and other light planes with empty weights around 500 pounds (250 pounds or so per wheel).
To weigh lighter aircraft, three scales will do. Larger aircraft also can be weighed with bathroom scales, in a rather primitive fashion, although five scales will be needed. I suggest you solicit all the scales you can without built-in handles on top. Six or seven will do. By comparing your own, or some known weight on each of them, you can pick the scales that provide the most uniform readings. Don’t forget to set each of them to zero before checking.
If your airplane weighs much over 500 pounds, the use of two properly bridged bathroom scales under a wheel will be required. In other words, the weight of the wheel will be shared by both scales. Each scale will read approximately half of the total weight of the wheel.
Put two of the scales side by side and make sure that they are of uniform height. Bridge across them with a single board to accommodate one wheel. The opposite wheel will need a similar set up. A separate ramp will also be necessary for each main wheel to enable you to push the airplane up onto the platform scales.
Preparing the Aircraft For Weighing
Remove all the junk — tools, hats, rags, etc. from inside the cockpit and baggage compartment. If you haven’t done a lot of hangar flying lately, the aircraft may still be clean and orderly. When weighing older aircraft, however, be sure the aircraft is clean (dirt is heavy). Check the wheel pants — they may have accumulated a few pounds of airport mud.
Drain all of the fuel that will come out. If you have been making taxi tests, you will undoubtedly have an unknown quantity of fuel aboard. Also drain whatever oil will run out. Don’t bother to run up the engine to warm the oil unless it is wintertime and the oil won’t come out of its own accord. Be sure all of the parts are on the aircraft (cowling, windows, propeller, fairings, engine baffles, etc.)
You can either weigh the airplane with the spinner and wheel pants or without. In either instance, they should be listed in the equipment list as should other removable equipment, like radios and installed ballast. This will make future calculations of any equipment changes involving weight and balance easier.
Getting it on the Scales
Those feed store platform scales you may be using are on wheels. Be sure to chock them before trying to push the airplane up the ramps and onto the scales. The risk involved in pushing or pulling the airplane up the ramps is possible abuse to the aircraft. Don’t ever permit anyone to pull on the propeller blades. If you do permit anyone to push, be sure to tell them where they can apply force. One safe way is to muster a couple of helpers whose jobs will be to grasp each tire with both hands and manually rotate the wheels up the ramps. If the aircraft is large and heavy, it would be a good precaution to have somebody with a chock following each wheel.
Actually, getting the plane onto the scales is not a big deal. Simply assure yourself that the aircraft is not being damaged and that the push is not so energetic that the plane will roll off the opposite end.
Lifting an airplane bodily onto the scales is not recommended if the landing gear is of the slap-spring or tapered-rod type. Those landing gears will flex and exert a springy side force on the scales. This, in most cases, causes improper readings.
Leveling the Aircraft
Often, in the process of preparing for the weighing, the builder completely forgets that a tail stand or platform will be needed to hold the tail of the aircraft in a level attitude (taildraggers) This stand should have a large enough surface to support a bathroom scale. It might be difficult to establish exactly how high that platform must be before hand, because you must allow for the height of the scales under the main gear. Don’t forget to allow for the height of the scale to be placed under the tail wheel, too.
Tricycle gear aircraft will also require some sort of elevated stand for the nose wheel and scale.
Most aircraft are leveled by using the top longerons as a reference for determining the fore and aft level attitude. Sometimes a large carpenter’s square can be laid across the longerons to check the lateral level. At any rate, use whatever level points the designer has identified in the plans.
When you weigh the airplane using a single scale under one wheel at a time, it is more important than ever to get the airplane level each time the scales are switched to another wheel. Level means level, not almost.
If the aircraft is not level laterally, air can be released from the tire to lower the wing on the high side. To a degree you can take the same corrective action if the nose is slightly high when leveling a tricycle-gear job.
Check everything one more time. The rest is anticlimactic now.
The Datum Reference Line
Before proceeding any further, you must determine the location of the datum. The datum is simply an imaginary vertical reference line from which you make all horizontal measurements for balance purposes The datum line can be at any easily identified location such as the leading edge of the wing (most commonly used), nose of the aircraft, firewall, or most anywhere you like. A lot of people say a datum line at the nose of the aircraft, or even at some location ahead of the aircraft is best and simplest to use because all of your figures are then positive numbers. I don’t think working with a few minus figures is enough of a problem for me to give up the convenience of using the wing’s leading edge as an easily located permanent datum.
Before removing the aircraft from its level position, you must determine the exact location for certain key elements in the aircraft.
Use a plumb bob to establish centerline marks on the floor between the nose and the tail wheel. Snap a chalk line between these two points, or stretch a string to provide a centerline from which to work. Again using a plumb bob, mark the location of the datum on the centerline already marked on the floor. The distance (or arm) of the different locations from the datum such as the oil tank, fuel tank, wheels, pilot, passenger, baggage compartment, and installed equipment can now be measured accurately. Record their location as plus or minus numbers. Use metric measurements if you like. It will make no difference. Some builders have difficulty in determining the location of the center of gravity of a seated pilot. I understand that it is at the belly button (navel).
Now for the Numbers
The essential information you have been anxiously awaiting is the empty weight. You arrive at that figure by adding the left and right main wheel weight readings to that of the nose (or tail) wheel reading. Don’t forget to subtract the weight of the tare (all of the chock blocks and shim material used on top of the scales). This is your empty weight, like it or not.
And, Finally, the Paperwork
All that remains now is a simple arithmetic problem whereby you obtain certain essential information. A pocket calculator is very helpful, but so is an assistant who remembers their multiplication tables.
Here is a very rough rule of thumb for determining if your cg location is all right. With the airplane loaded for flight, the cg should fall somewhere near one-fourth of the total distance back from the leading edge to the trailing edge of the wing. That is a cg location at approximately 25 percent of the wing chord for most airfoils. This is just right. Under no circumstances should it fall further aft than 30 percent of the wing chord. If the information is provided in your plans or with your kit, use those limits as your guide.
Complete a couple copies of the weight and balance calculations. Give one to the FAA inspector and retain a copy for your aircraft records.