When building from a kit or plans there is usually a weight range. How does the designer arrive at this weight? Does each componet,like flaps,fuse,stab have a desired weight? Can this weight be expressed in percentage of the weight range, so when you are building, you can say the part should be X percentage of the total weight range of x to y and this weight is x grams and then when you weight it you know that the weight of the part is in range or is too heavy and must be rebuilt? Hope i am making sense here. Many thanks for any input. Ron.

Less is better, in general you can get away with more loading on a bigger plane, and span loading is important in the turns.  The span loading bit is why there's any impetus at all toward higher aspect ratio designs.  The _wind_ is the same no matter the plane size, and a heavier loaded plane will bore through a given set of turbulence better than a light one, which is one reason why (IMHO) bigger planes do better in competition.

I'm not to the point yet where I do a good job of estimating weight by major component, but I _do_ find that estimating the density of the wood I'm using helps a lot.  I weigh each piece of wood when it comes off the pile, and write it's density on the piece.  Then I try to choose wood wisely -- weight bearing bits like spars come from heavier wood, full ribs come from lighter wood, sliced ribs come from heavier wood, depending on the section thickness and their spacing, sheeting comes from lighter wood, etc., etc., etc.  Just being aware of wood density and _thinking_ about where I'm going to use it (and sometimes judicious use of a Forstner bit for lightening holes) adds a lot of lightness to a structure.

At a guess I'd say that you probably want to aim at 5-15% of the weight in the stab, maybe 30-40% of the weight in the wing, and maybe 40-60% of the weight in the fuselage.  I'm not even sure this adds up, and as I said I'm not at the point where I know this -- I'm at the point where when I remember I weigh all the bits at all the major completion steps, write it down, and use it for target weights on the next model.

Knowing this would be a Very Good Thing -- I know from watching mechanical designers engineering systems for aerospace applications that you don't keep the weight down and the strength up by "just building it as light and as strong as you can", because George will build out of aluminum lace for weight, then Bob will build out of good strong stainless steel, and when you bolt the structure together it'll be heavy and weak.  Instead, you have to establish a weight budget (and a strength budget), apportion it out to the various bits, then design (and build) to that.

For control line stunt, this also means that you need to go fly and break a plane or two -- if you want to be harsh about it, if you haven't had an airplane break in normal use, then you haven't successfully explored the light-weight end of the envelope.  Once you do that a few times, you'll get a better idea of how to apportion the weight.

All the above info is great, but do not forget stiffness is also a factor. Light may be just fine, but if something like the flaps or elevator bend under load or the tail twists, then all is for naught.  Using a built up structure at times is better than a solid piece of super light balsa, as in the flaps or stab and elevator.  On three of mine I found by replacing the above with built up structures, I saved 3.6 ounces, average, per plane..  Many built up structures, using very light balsa, can come out very strong and very light, much more so than solid wood.  Worth the effort.

The one single author that I think has helped me the most with my model building is Roy Underhill, who has nothing to do with model airplanes.  I have a copy of his "The Woodwright's Eclectic Workshop", and when I get the chance I used to watch "The Woodwright's Shop" on TV with the kids.  The show and books are about woodworking in the colonial US.  Because of the level of sophistication of the tools (and wood) available, you have to know your wood a lot more than you do when all you use is power tools -- so he gives a lot of theory along with his obsolete practices.

The one thing he says, over and over -- when you cut wood, take advantages of its weaknesses; when you use wood, take advantage of its strengths.

A big ol' slab of balsa, twisted along its length, is not very torsionally rigid.  Why?  Because when you twist it the top and bottom surfaces compress and stretch easily, because you're stressing it across the grain.  On the other hand, a leading and trailing edge, with diagonal ribs, is far stiffer in torsion because when you twist it the stress is all running along the length of the grain in the ribs, where the wood is much stronger.  If you start thinking about each piece of wood, and what the stresses are that the wood must carry, then you can start making amazing improvements by careful wood selection and grain orientation.

Hi,

I agree with Ty on this one.  I'm building a Silhavy Gypsy and the plans show solid wood stab and elevators.  Well, in an effort to do better on weight than my buddy Mike Thompson, i'm going to build them up out of 3/8 with 3/32 planking.  Hopefully that will knock about 1/2 ounce off the stab/elevator combo.

Jim Pollock