I have a formal question about plane designs. You mentioned that you didn't want to get into the design of a 3 meter Chrysalis because of all the time and effort it would take to get the plane and the plans together.

OK, you know your time better than anyone and I won't argue. However, I did a couple of back of the envelope kinda calcs and wonder.

A 3 meter is a bigger 2 meter, right? Wrong? Why can't you simply take the 2 meter, multiply everything by 3/2, readjust for aspect ratio (the linear vs square thing on wing area), cut down on the fuse cross section and go with it? Surely the planform for the wing, airfoil, the general layout for a 3 meter can't be that different from the 2 meter. With very minor adjustments in strengthening here and there I would think that such a 3 meter would be just as grand a plane as the 2 meter.

(Remember now that I'm a glue chemist, ) What am I missing?

From : Don Stackhouse

The basic aerodynamic design is the easy part when it comes to making a built-up kit. That part is to some extent as you describe, but even there the details get complicated. Even though the general dimensions scale fairly easily, other factors get in the way. Some things are proportional to the square of the scale factor, and some with the cube, all of which makes things get messy in both the aero and structural design. Planforms usually can't be scaled directly either, and aero changes due to structural requirements just make things worse. Then there's the problem of shipping overseas. If you go much bigger than about 3 feet long on the box, it gets really difficult to ship. Standard lengths for wood, pushrods, etc., also conspire to force splices in other places, etc.. Aspect ratios inevitably change, which forces changes in the tail areas. Of course the amounts of those changes are different for the horizontal (which is partly a function of wing chord) than they are for the vertical tail (which is partly a funtion of wing span). By the time you're done, it's nowhere near close to just pushing the "enlarge" button on the Xerox machine.

Since the rib spacing is determined by covering sag concerns and by the spacing of the notching machine for the pre-notched trailing edges (trust me, once that cantankerous thing is finally dialed in at a particular setting, the LAST thing you want to do is change it!), the number of ribs is different. This, plus the changes in spar notches, changes in covering sag correction factors, airfoil blending, etc., means a whole new set of rib designs (and on the Chrysalis each rib has to be designed individually, AFTER determining its basic airfoil through non-linear interpolation methods).

Then we get into flight test. Here's where we learn about things like how the changes in aerodynamics and mass distributions force changes in the tail design and even the wing airfoils. Also, nobody scaled up the thermals or the wind, so there are some changes in the proportions and shape of the interactions between the aircraft and its environment to deal with.

This might help to put things in perspective: care to guess how many parts the 2-meter Chrysalis has in common with the HLG Chrysalis?

Exactly ZERO.

In all fairness, it does use the same inboard trailing edges, except that the 2-meter uses four of only the inboard portion (so we start with a shorter length of trailing edge stock), while the HLG uses two of the full-length version.

So much for the easy part.

At this point the design is about frozen, and since this is a beginner's built-up model, we are about 1/3 of the way done. Now, since just about everything is different, the proportions and locations of all sorts of things have changed sufficiently that the instructions have to be almost completely re-done. Not from scratch of course, and there are some drafting symbols, etc., and general instructions for trimming and set-up that can probably be re-used with little or no alteration. For example, I can re-use my little international symbols for different types of glue (for slow C/A I drew a glue bottle with a turtle on it, fast C/A is a glue bottle with a rabbit on it, and white glue is a glue bottle with a cow on it!). However, most of the illustrations will have to be re-done or severely modified, which means changing most of the text and labels as well. The number of steps in the assembly process will probably change, which means a new layout on the instructions sheet. Then there's the new work station in the shop for packing the kits, etc. Like I said, when you're at the point of design freeze, you're about 1/3 of the way done. All composite ARF's like the Monarchs are much better in this regard; at design freeze you're about 1/2 to 2/3 done.

Does that answer your question? (And then folks wonder why we are not eager to give them all sorts of design data for free so they can scratch build a copy without buying the kit!)

DJ Aerotech