Plane & Pilot
Monday, October 1, 2007

Get The Balance Right

If you think weight and balance are boring and unimportant, you need to read the following

It was 1985, and I was refueling a Cessna 425 Conquest I at Tenerife in the Canary Islands on my way to Johannesburg, South Africa. I’d instructed the fueler to fill the wing tanks first, then begin topping the three 110-gallon internal ferry tanks starting with the front tank. I turned away to fill out the necessary paperwork, heard the pump running for a few minutes and as I finished the fuel request, heard a sickening crunch behind me.
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I once escorted a pilot with a Malibu JetPROP on a round-trip voyage across the Atlantic from Monterey, Calif., to Berlin, Germany. The ferry tank wasn’t large, but it had been beautifully installed in the worst possible place: far back below the rear seat. The result was we had to carry a half-dozen precut, plywood tailstands to allow us to climb aboard, get the door closed, start the engine and taxi away without problems. This, of course, left the tailstand lying on the ramp. When we came back thru Iceland and Greenland, the rampers had saved our tailstands for us.

Another common problem for some airplanes in normal configuration is a tank location that moves the CG aft as fuel is burned off. Bonanzas store most of their fuel in the wing’s leading edge, a station that’s well forward of the typical CG location. This means, by definition, the CG moves aft as fuel is burned off. If you depart at max gross weight with full fuel in most four-place Bonanzas, the CG will slowly move toward the rear as you fly, and it may be beyond the aft limit if the flight is more than an hour or two. This can lead to unusual sensitivity in pitch and, in the worst case, a tendency to phugoid (or porpoise) on landing.

For that reason, you need to make two CG calculations for some Bonanzas, one for takeoff and another for landing. A standard rule for many Bonanza pilots is to keep the heaviest passengers as far forward as possible to prevent the CG from moving aft out of limits.

But don’t overdo it. Load too much weight too far forward, and the airplane may run out of both down elevator and trim and be difficult or impossible to flare. Any flap application may only exacerbate the problem, most often tending to pitch the nose farther down.

Conversely, a CG that’s near the limit isn’t always a bad thing. All airplanes benefit from an aft CG in terms of cruise speed. According to Jim LoPresti of LoPresti Speed Merchants in Vero Beach, Fla., loading the airplane as far aft as possible (but within the allowable limit) cuts drag by reducing the download on the tail, an airfoil designed to fly down as the wing flies up. This doesn’t mean you should fly with any more weight than you have to, by the way. On some airplanes, Jim commented, the difference between a max aft load and a more forward balance point can be three knots.


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