Some things to consider.

1)  Legally, *we* are the manufacturers, not Lancair.  We can select any gross weight we want to ... legally.  So that part is correct.

2)  There are several factors which may enter into a gross weight determination.  The most obvious is the strength of the wing. The wing is designed for a certain total lift, and the prudent designer/builder does a static test.  This was, in fact, done for the Lancair 320.  They had pictures in their advertisements at "a 9 G load".  I do not know the precise load applied, but I suspect that it was 9 times the at-the-time stated gross weight of 1685 pounds.  (1685 * 9 = 15165 pounds)  (We will ignore second order effects such as subtracting the empty weight of the wing from this calculation.)

3)  Was the wing stronger?  Could they have piled more weight on until it ultimately broke?  What was the load at which the first permanent deformation occured?  I do not know these things, but somebody does.  But for now, let's assume the wing broke at 15165 pounds.

4)  Now you have to decide what safety factor you will use.  Normally, composite structures use a safety factor of 2.0, and metallic structures use 1.5 and sometimes less.  Assume we use a safety factor of 2.0.  This means that the wing is good for 4.5 G at the gross weight of 1685 pounds with a safety factor of 2, or more weight at a lower G (such as 3 G at 2527 pounds).  If we use a safety factor of 1.5, then we can go to 4.5 G at a weight of 2250 pounds. 

5)  So you see, the ultimate load capability as determined by testing sets a maximum load limit that can be distributed between gross weight, desired safety factor, and maximum G loading.

6)  There are other structural factors then just the wing, although that is usually the most limiting factor.  For example, there is shear and bending of the fuselage (snapping the fuselage off the wing).  I doubt that's going to happen before the wing breaks.

7)  Sometimes the designer makes the wing stronger to account for manufacturing defects.  Prudent homebuilder designers do this.  For example, Don Goetz told me that the Lancair structural joints, which are Hysoled and then covered with BID tape, are strong enough with only the hysol or only the BID tape.  He said they did that "to account for butter-fingered builders like you..."  (He was talking to me at the time.)  So there's some extra margin which may help.

8)  Next we get into performance issues.  We all know that performance goes down as weight goes up, but I doubt that adding a couple of hundred pounds would push us off some low-performance cliff.  Besides, the experimentals do not have to abide by the FAR minimum performance criteria.  At some point, however, the airplane will be too heavy to fly safely from a P&FQ (performance and flying qualities) standpoint.

9)  The Lancair 320 landing gear is not all that beefy.  That's why the maximum landing weight was not raised.

10)  Maneuvering speed -- the maximum speed at which a vertical gust will stall the wing rather than break it -- is a function of gross weight.  This relationship does not change when you arbitrarily raise the gross weight by accepting a lower maximum G.  (It does, however, change when you increase the gross weight by accepting a lower safety factor.)  I would not recommend changing the selection of manuvering speed.

All of the above shows why raising the gross weight can be done as long as the inherent limitations are understood.  If the factory said 1790 pounds was okay, that's good enough for me.

Hope this helps.

- Rob Wolf