## Q-talk 104 - Weight and Balance

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- Category: Q-Talk Articles
- Published: Wednesday, 23 December 2009 21:24
- Written by Dave Richardson
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When it is time to certify, change the engine or just fly your Q, you need to pay particular attention to the plane's weight and balance. It is important that you understand where the center of gravity is located for both the items carried on the plane and quantities that will be consumed during the flight. I have never liked the term "Center of Gravity" because it sounds like you are talking about a massive object, like the earth has gravity. I prefer the term "Center of Weight" for our discussion because that is really what we are talking about.

As kids, we all practiced finding the center of gravity, or weight, of an object by attempting to balance the object on our finger. For example, you may recall placing your finger under the 6-inch mark of your trusty 12-inch ruler and balancing it there. You may have later found you could do the same thing with a yardstick when you understood that 18 inches was the midpoint of the stick. The consistent weight and measurements of these sticks made the process of finding the center of weight fairly easy. Irregularly shaped objects, however, required more trial and error than anything else to find the balancing point. Actually, there is a fairly simple way to find the center of weight of even odd shaped solid objects. Attach a string to any point on the object and suspend it in the air. From that attach point on the object, draw a line down and imagine that line continuing to the floor. Now attach the string to another point on the object, suspend it and draw a new line down relative to that position. Where the two lines cross is the center of weight. This is great for small items, but I doubt many people would be willing to use tins method when locating the center of weight of their airplane.

Rather than use the string technique, scales can be placed under the three ground points of a level plane. A little simple math can be used to gain the same information. To make this method work, you will need to know the distance between the three wheels, when looking at the plane from the side, as well as where the wheels are positioned relative to a fixed or known point on the plane. For all Q's, the fixed point is the lower forward surface of the firewall. By convention, that reference point or datum is considered to be 14 inches. Another way of saying that is to imagine viewing the plane from the side with a line on the ground beneath extending from the spinner to the tail. If a dot was placed on that line below the lower forward surface of the firewall, it would be considered a reference measurement of 14. Using this fixed point can help with calculations because deviations from the plans by the builder can change the relative measurements between the wheels. Determining these relative distances is fairly easy. First, you need to level your plane. Generally this is done by placing a level on the armrest or center console and adjusting the tail until the level indicates the plane is in position. Next, you need to draw some points on the floor or on some paper securely taped to the floor. The easiest way to determine the points precisely is to use a plumb bob suspended by a string. First, place the string against the forward surface of the lower firewall and allow the bob to swing freely just above the floor without dragging. When the plumb bob stops moving, draw an X directly below it on the floor. Repeat this process at least two or three times from various spots on the lower area of the firewall. When done, draw a connecting line through the X's forming a reference line. With the plane still level and unmoved, use the plumb bob again to draw an X on the floor indicating the center of each of the three axles of your wheels. Then draw a line connecting the X's of the two main wheels. A chalk line is helpful here, if you have one available. You have everything you need at this point. Precisely measure the distance from the reference line below the firewall to the line drawn between the two main wheels. Add 14 inches to that measurement to get the position of your main wheels relative to the firewall reference point. Next, you need to precisely measure the distance from the firewall reference point to the X you placed next to the tail wheel or in the case of a tri-gear, the nose wheel axle. If you have the tail wheel configuration, add 14 to the measurement to calculate your tail wheel position. If you have a nose wheel, subtract your measurement from 14. For example, if you measured 5 inches from the reference point to the nose gear axel, you will figure 14 minus 5 or 9 inches as your nose gear position. It is important that you measure these positions accurately because any error will be magnified in the calculations that will come later when calculating the center of weight.

With these positions, you will now be ready to weigh your plane. You will use the measurements in the same way you used the 6-inch mark on the ruler to make it balance. This is a good time to talk about scales. Bathroom scales are not recommended to weigh your plane. Certified scales are best because they are known to be accurate. Any scale will have a limited weight range. Be sure your scales are designed to handle the expected gross weight of your plane plus extra pounds for a safety margin. Another factor you need to consider while weighing your plane is called tare. This term refers to shims, chocks or blocks used to level the plane that also will be placed on top of the scales. Better quality electronic scales have a zero button that will let you electronically ignore any tare weight. Only the weight of the plane will be displayed. To do this, you simply place the tare items on the scale by themselves and press the zero button. The scale will reset itself to 0 for you. Just remember, if an item is on the scale but will not be flying with you, it can be subtracted from the weight read. Another thing to consider when locating a good scale is how precisely it will report the weight to you. Large scales designed to measure semi-trucks may round the weight to the nearest 5 or 10 pounds. This would not be accurate enough to do the calculations for your plane. Measuring the weight in whole pounds would be adequate but tenths of a pound is ideal.

In preparing your weight and balance report for your plane, you will be calculating the center of weight for the following items:

? The Airplane (Empty?defined below)

? Motor Oil

? Header Fuel Tank (If installed)

? Main Fuel Tank

? Pilot & Passenger

? Baggage

? Additional fuel (Optional)

So, what is the empty weight of the plane?

The empty weight of the plane is everything that will be in the plane when it flies, like seat cushions, radios, instruments, etc., except for you and your passenger, usable fuel, oil and baggage. I bet your first question was, "Do I have to take the oil out of the plane to measure the empty weight?" To be precise, you do. The engine manufacturer, however, may provide a specific location of the oil's center of weight. Your second question might be, "Do I have to remove all the fuel from the plane?" The design of the main fuel tank is such that you will never be able to get all the fuel out of the main tank while flying the airplane. To be able to determine the true weight of your plane, you must remember that it includes this unusable fuel. For example, in Q2xx's, you could add a couple of quarts of fuel or more to the main fuel tank and then pump it to the header until the pump cannot transfer any more fuel. The fuel should then be drained at the carburetor until it stops flowing. On the Quickie, it can be done in one step, draining the fuel directly from the carburetor. In both types of planes, any remaining fuel left in the main tanks after fuel was purged is considered unusable and part of the plane's empty weight. .

The plane should be ready at this point to determine its empty weight. This is probably the most important step because everything will be determined relative to this calculation. It is important be accurate. The weighing process can be condensed into four easy steps; 1) Load 2) Level 3) Read Scales 4) Calculate.

Load

In reference to the empty weight, "Unload" would probably be a better term. Follow the definition of the empty weight of the plane as defined above. Place the plane on the scales. Be sure to close the canopy in the proper position, too.

Level

Every weight measurement will require that your plane be level to get an accurate reading on the scales. If you forget this step, your numbers will be skewed and your calculations will not reflect the real situation with your plane.

Read Scales

It is just as simple as it sounds. Write down the weights reported from the scales on all three wheels, noting each wheel's location. Also subtract any tare weights from the numbers shown, if your scale does not have a zero button as mentioned above.

Calculate

The math used to calculate of the center of weight is very simple. Here is an example. Please note these numbers are not based on actual measurements and should not be used to calculate the center of weight on your plane.

Measurements relative to firewall reference point for Q2xx tail dragger configuration

Step 1) Add all the measured weights (less any tare) to get the empty weight (517.0 lb in this example.)

Step 2) Multiply each weight by the measurement from the reference point and sum those results.

Step 3) Simply divide the number you get in step 2 by the number you get in step 1 and you get the center of weight.

Like the ruler example mentioned in the beginning of this article, the number 40.16 above means that if you could put your finger under that point of the plane and lift it up, it could balance there. Now, 40.16 is actually 26.16 inches aft of the lower forward surface of the firewall, since the firewall is considered to be 14 inches.

That is as tough as it gets. No PhD is required to make those numbers come out right, either. Once you have done this a few times, you will feel comfortable with the pattern and it will become second nature to you.

The next step is to calculate the center of weight for the remaining items; oil, header fuel tank, main fuel tank, pilot and passenger, baggage and additional fuel. The process for calculating these positions is almost identical to the empty weight calculation but with one minor change. For these measurements, you need to record the weight of the plane before and after you load the particular item you want to measure and level the plane. So that means the weight you will be using in the calculations will be the difference from the prior weight instead of the total weight. For example, if you want to measure the pilot and passenger position, you would determine what each of the scales read prior to sitting in the plane. Then sit in the plane and close the canopy again. Be sure to level the plane before you read the scales. Limit your movements when sitting inside the plane to reduce changes in the scales as much as possible. Read the new scale measurements once the scales settle down. Subtract the old weight from the new weight to see how the numbers have changed on each of the wheels. Be sure to deal with any tare, too. With the tail wheel configuration, you will notice that all three weights will go up on the scales while sitting in the plane. With a Tri-Q, you will notice that the weight on the nose gear will actually go down and the sum of the weight on the mains will be higher than your weight.

Next issue we'll work through more examples of calculating the center of weight positions of other items in your plane. We'll also look at what needs to be included in your weight and balance report for your airworthiness certificate inspection. Once we have all the numbers computed, we'll go over some graphs and other uses of this information to help you prepare for each flight.

Continued in Issue 105.

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