Did you know that experienced DC-3 pilots claim they can tell if the airplane is loaded within c.g. limits based on how it taxis? I have no doubt this is true. I know one fellow who says the same thing about his Beechcraft D-18 (a twin-engine Beechcraft taildragger that looks like a shrunk DC-3). Would you believe there are others who claim they can detect similar problems in their A-36 Bonanzas? Anyone who has excessively aft-loaded their Bonanza and hit the brakes too hard knows what I mean. That airplane will recoil off the compressed nose gear and settle on its tail. It will look like a bird ready for takeoff at zero airspeed. For the rest of us, anything we can do to make computing a weight and balance easier works to our advantage. After all, if calculating the weight and balance is easier, we're more likely to do it in the first place. In this article, we'll look at an easy way to construct a fuel-usage line on a weight vs. c.g./envelope. Then we'll examine a few other interesting ideas about weight and balance.
The Weight vs. C.G./Envelope
Last month I demonstrated how to create a fuel-usage line on airplanes having the typical c.g./moment envelope. Suppose, however, you fly an aircraft like a Piper Saratoga that doesn't have a c.g./moment envelope? Instead, it has a weight vs. c.g envelope like the one shown in Figure 1. Constructing a fuel-usage line using this type of chart is quite easy.
The first thing to do is to compute a weight and balance for a typical flight. When I did this on the Saratoga, I came up with weight of 3,600 pounds and a c.g. of 91.9 inches aft datum (I won't go over how to compute this c.g. since I'm assuming you already know how to do such things).
In Figure 2 (point A) I've placed a red dot at the point where the weight and c.g. mentioned above meet on the weight vs. c.g. envelope. Next, I compute the c.g. with no usable fuel on board. In other words, I assume that all the usable fuel is gone and subtract its weight (612 lbs.) and its moment (57,528 lb-in) from the previous weight and moment. This gives me a new weight of 2,988 lbs. and a new moment of 273,145 lb-in. Dividing this new moment by the new weight gives me a c.g. of 91.4 inches aft datum. I placed a red dot at point B in Figure 2 to represent this point.
Constructing a fuel usage line is simply a matter of connecting these two dots together as shown by the red line in Figure 2. Now all you have to do is transfer that line onto a rectangular piece of clear plastic as shown in Figure 3 (it's wise to label the top red dot as TOP).
Draw a similar line of equal length and at a similar angle to the horizontal weight lines shown on the graph. Then, place the plastic in such a way that the red dot at the top of the fuel-usage line is on the intersection of the airplane's departure weight and departure c.g. While keeping the bottom of the plastic rectangle parallel to the horizontal chart lines, you can immediately see if a fuel burn will move you beyond the c.g. envelope.
Figure 4 shows several fuel-usage lines. As you can clearly see, fuel burn on this model of Saratoga has very little effect on the airplane's c.g. Therefore, unless you take off with a c.g. near the forward or aft limits of the envelope, it's unlikely you'll experience a c.g. problem as a result of a fuel burn.
The Truth About the Forward C.G. Limit
On most airplanes, it's very difficult to exceed or even approach the forward c.g. limit. Sure, it can be done, especially on an airplane with a baggage compartment forward of the pilot (like the Saratoga). Often, when certificating airplanes, test pilots must place removable ballast at the forward end of the cockpit just to reach these forward c.g. limits. One of the certification requirements for small airplanes is that there must be enough elevator authority to rotate the nose at a speed of 80% of the stall speed. This would be hard to do if the c.g. were beyond the forward limit.
The Truth About the Aft C.G. Limit
Unlike the forward c.g. limit, exceeding the aft c.g. limit is easy. It's easy because the aft part of the airplane is the area available to store passengers, baggage and all kinds of heavy things. Therefore, computing a fuel-usage line becomes even more important in an aft-loaded airplane.
Comparisons
Not all airplanes have similar center of gravity tolerances. Figure 5 is a comparison between the c.g./moment envelopes of a Cessna 210 and an A-36 Bonanza (In order to make this comparison, I modified the Bonanza's c.g. chart to look like the Cessna's).
It's obvious that the Cessna 210's c.g. envelope is wider. In other words, the 210's forward and aft c.g. limits aren't as restrictive as that of the A-36. Therefore, we can assume that the A-36 is more sensitive to where weight is placed. It's reasonable to say that it's a little easier to improperly load an A36 than a Cessna 210. Anyone who's flown the A-36 knows how easy it is to aft-load this airplane.
Just for fun, I constructed a few fuel-usage lines for the A-36 in Figure 6.
Notice how line A is nearly parallel to the A-36's forward c.g. limit. This means that you're unlikely to have a forward c.g. problem as you burn fuel in this airplane. Fuel-usage lines C and D, however, tell a different story. In these instances, assuming you're aft-loaded to begin with, it's possible to encounter an aft c.g. condition as fuel burns. As a final note, please don't use any of these graphs to compute a weight and balance for your airplane. I've used generic weights for these airplanes which may not reflect the weights of the specific airplanes you fly.
For more information on this subject, see "Hot and Heavy Performance: Computing Weight and Balance and Density Altitude."
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