Weight and Balance:


WB BASIC.JPG (25431 bytes)

Basically my plane is heavier than most Glasair I FT aircraft. I tend to over-do some things. The fact that the plane is all electric, water-cooled, electric flaps, electric prop, etc... just doesn't help. However, I'm quite happy with how things turned out.

Here's how I figured my weight and balance:

Who Is MAC Anyway?

First I had to calculate the Mean Aerodynamic Chord (MAC). You may ask "Why? Isn't it a kit?". Well this was an early kit and, more so, I'm no perfectionist when it comes to using a tape measure. Besides, I'm glad I did, because my numbers came out slightly different than the factory's. 

WB MAC.JPG (38390 bytes)My understanding on a tapered wing is that the MAC is the chord, front to back at half of the wing lifting area. I learned it easier with a drawing. Click on the picture to the right to see my rendition. First, I had to thoroughly measure my wing; Tip chord length, leading edge length to center of wing, trailing edge length to center of wing, wing center chord length and the sweep of the leading edge center to tip. With this information I made a drawing of the wing. On this drawing I marked 50% of the chord, fore and aft, at the tip. Next I marked 50% of the chord, fore and aft, at the center of the wing. From each of these 50% chord points I measured a distance both fore and aft equal to the opposite ends chord length. This means that the center of the wing has a line running fore and aft equal to two times the tip chord length. The tip has a line running fore and aft equal to two times the center chord length. From here draw an "X" between the most forward and aft points of these extended tip and center chord lines.

Still with me?  It took me a while to figure it out, so if you're not, print the picture and read it again while looking at the picture.

What does all this have to do with my weight and balance? Well, the factory, and most aircraft designers, give the center of gravity (CG) range in a percent of the MAC. My Glasair wing has a forward CG limit of 13.5% of the MAC from the leading edge MAC (LEMAC) and a rear CG limit of 28.5% from the LEMAC. Ok, wait a minute, I forgot to finish telling you how I found the MAC. Well, it's pretty easy. Where the giant "X" crosses, draw a line from this point to the leading edge and one to the trailing edge, both being parallel to the line of flight. Measure the length of this line. This is the MAC. The front of this line is the LEMAC. 

The CG Equation:

WB GEAR.JPG (34549 bytes)WB STATIONS.JPG (41124 bytes)The Glasair uses a datum point 60" ahead of the firewall. Now that I was armed with the MAC I was able to start calculating the CG. To have correct CG on my Glasair, it has to fall between 13.5% and 28.5% from the leading edge toward the trailing edge at the MAC. I measured from the datum to the forward and aft CG limits of the MAC. This came out to 82.36" for forward and 88.96" for aft. Next I measured and marked the remaining stations and built this Excel spreadsheet to make flight planning easier.

6/2/2007: Corrected calculation error for "Flight Planning Results", "% of MAC" on The Excel Spreadsheet.

9/30/2007: Completed final weight and balance in preparation for the airworthiness inspection. To do this I used a single digital scale and five pads that I built the same height as the scale. First I placed the airplane onto three of the pads. Next I placed pads behind two of the pads in use. The scale went behind the third and I rolled the plane back onto the scale. By rolling the plane forward and moving the scale to another position I was ready to weigh the next gear point.




The results look like this: