## Back Calculating Stations

A station is nothing more than a point on an imaginary line that runs from the tip of the spinner to the tip of the tail. We covered in the last issue of Q-Talk that the lower forward surface of the firewall on all Q's is considered to be 14", to give a common reference point. In calculating your empty weight, you determine the front to back distance to the main wheels and the tail or nose wheels relative to the firewall to drive your center of weight calculations. By back calculating, I mean that if you add some weight to the plane at a certain position, like the baggage compartment, then re-level the plane and re-weigh the plane, the scale readings will change in relation to that new weight. The sum of the changes on each of the scales will equal the total weight of the item you added to the plane. The change in weight on each of the scales under each of the wheels tells the stoiy of where the center of weight for that item is located. While that may sound complex, it really isn't and you still use the same type of calculations you used before when you calculated your empty weight.

Follow this process. 1) Read the scales while the plane is level 2) Place the new weight in the plane 3) Level the plane again 4) Read the new weights from the scales 5) Subtract the new scale readings from the prior reading for the same wheel and do the math. For example, if you weigh the level plane, place two 20-pound bags of kitty litter in the baggage area, level the plane and reread the scales, you might get results similar to these:

A station of 86.5 inches sounds reasonable for the baggage area since the split line bulkhead on the Q2 is at 95 inches and the center of the baggage area is forward of that bulkhead.

You can use this exact process for the other items you need to weigh and back calculate stations. One curious thing I noticed when I calculated my pilot/passenger stations was that the station ended up being forward of my hips. I had been told that the center of weight for a person is approximately 2 inches above and 2 inches behind their navel. It stumped me for a while why the pilot station would be forward of my navel until I realized we were not just looking at the center of weight of the pilot, but the weight of the pilot in the plane relative to the center of weight of the plane. If your feet hung directly below you in the plane, ala Fred Flintstone, your center of weight would be closer to the 2" x 2" navel location mentioned above. Since your legs actually go forward of the plane's center of weight, the weight of your shoes, feet, calves, knees and part of your thighs offset a portion of your upper body weight and shift your relative center of weight forward in the plane.

Do not worry if your new weight actually goes negative. For example, if you had 25.5, 25.5 and -11.0 for your weights, then you would follow the sign and the math would look like: 25.5 + 25.5 +(-11.0) = 40.0 That will generally happen when calculating the pilot/passenger or baggage stations on a Tri-Q because those are so far aft of the plane's center of weight. You could also have that happen when calculating the header tank on a tail dragger because it is forward of the center of weight of the plane.

The empty weight calculation should be fairly easy to complete because it is based upon the numbers collected earlier. Be sure to subtract the weight of the oil to show your true empty weight.

You can find examples of these forms in the FAA's Acceptable Methods, Techniques, and Practices guide numbered AC 43.13-1B or contact EAA Aviation Information Services.

Next issue we will go over some fun things you can do with these stations and get into some graphs that make determining your C.G. loading a snap.

Continued in Q-Talk Issue 106

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