Q-talk 105 - Weight & Balance
- Category: Q-Talk Articles
- Published: Sunday, 06 March 2011 19:00
- Written by Dave Richardson
- Hits: 5712
Last issue we talked about using a scale to calculate the center of gravity, or center of weight, of your Q. This issue we will show some examples of calculating the specific stations for the items that are not in the empty weight of your plane. These items include the pilot and passenger, header tank fuel, main tank fuel, engine oil and baggage compartment locations. These positions can be loaded with variable amounts of weight and changes in that weight during flight can affect your center of weight. I am sure there are a number of Q's flying out there that simply used the numbers calculated the Q manuals. After all, they built their plane per the plans, right? Minor variations in where the stations are located can have a big impact upon the final center of weight calculations for your plane. Take the time and do the math so you do not have to find out during your first flight how different your plane is when loaded. The math is simple and can be done on any calculator capable of adding, subtracting, multiplying and dividing.
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.
Calculating the center of weight for the main fuel tank and header tank actually gives you an opportunity to also determine their maximum quantity as well as calibrating their fuel level gauges. The process is the same as before. Add the weight, level the plane, read the scales, subtract the weight from the prior weight and do your calculations. On my Tri-Q2,1 worked with my header tank first. I added precisely measured fuel in one-gallon increments to the main tank and transferred the fuel to the header tank. When the fuel transfer pump could not pull any more fuel from the main tank to the header, I switched off the pump and placed a mark on a piece of tape next to the header tank site gauge for each gallon. I added the last gallon in Vi then 'A gallon increments until I could start to see the fuel returning back down the overflow tube, then I stopped. This meant that the header was full and the main tank only contained unusable fuel. I knew how much fuel the header held and what affect each gallon had on my sight gauge. I leveled and re-weighed the plane again for the header tank station calculations. With the header tank full, this was a great time to measure my fuel flow rate to be sure I met or exceeded the 150% expected full throttle requirement. I measured mine at both level and 14-degree inclination, simulating a steep climb, for full header, 75%, 50% and 25%. To do this, I disconnected the fuel line at the carburetor and timed how long it took to draw off two-cups. Then I used the formula of 450 /(seconds to fill two cup container) to get my gallons per hour. See Q-Talk issue 91 for an explanation of the formula.
With the stations calculated for the empty weight of the plane and the stations for the variable weight items, you are ready to fill out the weight and balance sheet required by the FAA during your inspection. The first thing you need to know about this form is that this is a standardized approach to representing weights and may not represent how you will be loading your plane. You will show your math for your empty weight calculations, as well as extreme forward and aft loading conditions. For the purpose of the standards, you will use 170 lbs. for the pilot and passenger weights. The minimum and maximum amounts of fuel and oil are based upon the horsepower rating of your engine.
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.
Next you need to complete the extreme forward and rearward loading section of the form. For calculating the forward extremes, you want to "load up" as much as you can forward of the center of gravity and unload items aft of the C.G.. For the forward extreme loading, I used the pilot of 170 lbs, a full header, Vi gallon of fuel in the main tank and no baggage. For the aft loading, I used both a pilot and passenger at 170 lbs. each, 1/3 full header tank (not a good situation), 10 gallons in the main tank and about 30 lbs. in the baggage compartment. That was close to gross and almost to the aft C.G. location.
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
You can order a printed copy of Q-talk #105 by using the Q-talk Back Issue Order Page.