Verifying the design CG range of an existing airplane

[zaitcev]

Suppose I came about an experimental airplane. I know that it is reasonably safe to fly, being flown for a while by several owners, and of course a DAR issued it an airworthiness certificate. Still, I'm thinking about re-measuring and weighting it, then re-doing the W+B form. Given the factual weights and dimensions, how do I find if the CG is within a safe envelope?

I figure that first I need to know where the center of lift is. Can it be done by measuring the wing and possibly guessing what common airfoil was used? If yes, how?

After that, I guess that I need to know where the permissible envelope is (knowing that I can see easily if factual CG falls within it). How do I find out?

[martymayes]

The desirable cg range for a conventional, powered lightplane is ~ 15 to 28% MAC. Of course, the forward and aft limit must always be verified by flight testing to ensure adequate stability and control exists at those extremes.

If you start from zero and adjust the loaded c.g. to ~22% MAC, the plane should be safe to fly.

[WLIU]

What you are asking about is stability testing. Ed Kolano has written some great articles for Sport Aviation and Experimenter magazine. There are also some other good published references.

The short version is that you make a series of flights with sand bag ballast to configure different CG locations at your stated max gross weight. Start loading where you think the middle of the CG envelope is, then carefully fly with more aft loadings. Fly in calm air. On each flight you do stability checks at various speeds. Slow flight and stalls are especially interesting as you have to be able to land safely. Be very careful. I will let Mr Kolano's writing explain the rest.

Best of luck,

Wes

[zaitcev]

Thanks a lot for the suggestions. As it happens, the airplane is at 27% as it is, in part due to my weight. Worse, due to lap tank CG goes aft as fuel burns off. I made the first flight today, and did power-off stalls to learn what it feels like. The stalls are soft, but it noses down definitely, so it appears safe in this respect (tail stops flying later than wing). I'll look into Mr. Kolano article and other sources.

[jam0552@msn.com]

Remember that your center of gravity must be ahead of the wing center of pressure - that ensures that the wing and aircraft pitch nose down when the wing stops producing lift. With this pitching moment, the aircraft will naturally regain flying airspeeed with the elevator controls neutral. Usually the center of pressure on a typical low speed airfoil (such as the one used on most long- and short-wing Pipers) is 33% mean aerodynamic cord (MAC). So 15-20% MAC is usually a safe CG range.
-Joel Marketello

[1600vw]

The airplane I fly today I did not build. I did as the poster and being as this airplane had already 100 or so hrs flown off on her I flew her. After flying her I put her on scales thinking she was nose heavy. I then get out the paper work for this airplane and using the numbers from this I redid or reweighed the airplane.

I then did the math and found I needed a little tail weight. So I add a little tail weight that was needed and go fly her again. I swear she is still nose heavy. I land and put her away and break out the paper work again and go over all the numbers. According to the plans and paper work she should be with in CG.

So I go ahead and fly her. Then one day I have a partial engine out and found indeed she is nose heavy. So back into the shop the plane goes. This time I myself broke out the tape measure and measure all the datums. I found what was in my paper work was not correct. I stood there scratching my head wondering how could the builder get these number wrong?

I go ahead and use these new numbers I came up with and set the cg within the range stated. Today she flies so much better. I had to remove almost 20 some pounds off the nose to bring her within CG. I was flying her something like 4-5 inches forward of the CG. Now I am around 1 inch from the forward most point of the CG. with full tank of gas. Empty she is almost right in the middle of the CG.

[martymayes]

As it happens, the airplane is at 27% as it is, in part due to my weight.

I would use ballast and bring the c.g. to ~20% MAC and test fly the airplane. Higher stick forces and increased longitudinal stability may make the plane much more enjoyable to fly. Plus it would negate the c.g. change with fuel burn off. Of course, a permanent solution might require more effort.

[WLIU]

What type of airplane?

Then the question is can a heavy component like the battery be moved forward? Or a much lighter new technology battery be installed to improve the CG?

And of course, can this situation be used as motivation to lighten the pilot?

Best of luck,

Wes