ON THE SAFE SIDE
Where is Your Center of Gravity?
By Jim Tiller
The Swan Sings
As I’ve mentioned in the past, in June 2009 my wife and I became full-time RVers. Since then, we’ve covered some 32,000 miles in 30 states and five provinces in Canada. In a few days we will be off again from our native Texas ... off to see the Wizard, perhaps.
At times we have had pretty good Internet access; other times not so great, or not at all. But that’s largely an excuse. To be honest, I’ve run dry on ideas for the column and have asked my editors to allow me to “retire.”
Knowing a good thing when they see it, they have graciously agreed, and as of this month Jim Tiller of District IX will be taking the reins. I’m sure you will enjoy his efforts. In the meantime, fly safely, and thanks to the many who did so graciously feed me input and ideas. Oh, yes … MERRY CHRISTMAS AND HAPPY NEW YEAR!
—Don Nix, This e-mail address is being protected from spambots. You need JavaScript enabled to view it.
Where is your Center of Gravity?
A modeler reported to me recently that he crashed a new airplane on its first flight. After some research he found that the Center of Gravity (CG) location printed in the manual was off by more than two inches and this tail-heavy condition contributed to the crash. When he spoke to the distributor about the incident, the company told him the proper CG was in an addendum on its Web site—an addendum that, obviously, was unknown to him. He reported this to me, not only out of personal frustration, but also as a safety issue. The errant model could have caused a lot of damage on its short, first flight.
In its remarks to the modeler, the distributor reported it was not at fault, because it had published the addendum and, additionally, the builder is solely responsible for what happens to his model. As you know, most manuals actually go to great lengths stating the manufacturer and distributor are not responsible for the airplane after it is sold.
There are two questions here. First, should the distributor have made a greater effort to correct the misstated CG location? That answer is simple. Yes it should have. The correct center of gravity is a critical measurement for safe flight. It is boldly stated in many manuals, but in some manuals, mostly those intended for trainers and other beginner airplanes, the critical CG is not even mentioned. It assumed to be in the correct spot if the builder follows instructions.
I think modelers, magazines, and reviewers should make every effort to report inadequate instruction manuals. The phrase “this is a builder’s kit” or “designed for the experienced modeler” is the innocuous tagline that alerts us to manual deficiencies in most reviews. We are far too tolerant of poor instructions. Many online sales sites allow the buyer to post reviews or comments. We should complain loudly about inadequate instructions or, in the case of many Chinese imports, the total absence of coherent instructions at all. The squeaky wheel gets the grease—demand better instructions. If the comments affect the sales dollars, they will listen.
And, in like fashion, the companies that take the time to provide great manuals should be commended for their efforts.
The second question: Is the modeler ultimately responsible for the CG of his airplane? The answer is, again, yes.
Builders often replace parts from their ARFs they believe are sub-standard or too weak to hold a load. They do that to protect their investment, but also in the interest of safety. In light of this, and other incidents like it, we should also be skeptical of the CG reported in manual. Don’t blindly trust that it is correct. Do a rough calculation yourself.
The CG is not hard to figure and there are many references out there that explain the calculation. Google: “calculating CG on an airplane wing” and you get pages of results. The general rule of thumb is that the Center of Gravity is about 25-30% of the wing cord (wing width) at the Mean Aerodynamic Chord (MAC). The MAC is the center of a rectangular wing or its equivalent in a tapered or swept wing.
That sounds very technical, but for a rectangular wing (constant cord) the CG is 25-30% back from the leading edge anywhere on the wing. If the wing is 10 inches wide, the CG should be about 2.5 inches in from the leading edge. Many model designers put the main spar at this point, making it even easier to locate.
On a tapered or swept wing it is a little harder to find the MAC, but not impossible. For those mathematically impaired, like I am, it is simplest to go to a web-based CG calculator like this one: www.scaleaero.com/CG_Calculator.htm.
Before you techie guys get too critical, this is an oversimplification of the CG calculation, but focus on the message. All modelers, beginning and experienced, should know how important the calculation is, and check it, before the first flight. Also, the definition of the CG and its importance should be part of the instruction manual for new fliers.
Before the first flight, mark the manufacturer’s CG location on the root of each wing. Put one finger on each side of the fuselage at the CG and lift the finished airplane (minus fuel) off the work bench. It should stay level or tip slowly nose down. If it falls quickly toward the tail, you need to do your own CG calculation. Be skeptical—get it right. There is an old saying “A nose heavy airplane doesn’t fly very well; a tail heavy airplane doesn’t fly very long.”
We are ultimately responsible for the models we build, but manufactures and distributors should step up too. This is a safety issue. Safety should be as big a driving factor as sales. Q