First, we should all start naming our planes "The Compromise", because that's what a stunt plane is -- it's a giant compromise between competing requirements, and any time you completely solve one of them then (unless you're hard up against some limit imposed by the rules) you create another. That would make our naming perfectly honest -- except then you wouldn't be able to tell the planes apart. Ahh, compromises...
As an example that's pertinent to your question, if you go with a 1:1:1 crank, flap and elevator ratio then you get a nice mostly-linear ratio (but not quite, alas, because the bellcrank is working in a different plane than the flaps are). However, that means that you give up a whole lot of leverage on the bellcrank. That means that the lines don't have to move as much for a given control surface motion (nice!), which in turn means that the force on the lines for that given control surface motion is increased. Because your lines are curved in flight, not straight, they act like springs -- and by reducing the leverage that they have on the control surface, you increase the effect that their springiness has on the control system. This is an effect that I've experienced myself, on a Ringmaster which I built with the pushrod too far out on the bellcrank; when I made a super-long elevator horn for the thing, that dragged in the mud every time I landed on grass, the airplane performance perceptively improved.
I think that as pertains to your question, the things you want to achive are -- sort of in order of importance -- the following:
- Good workmanship -- make it so it doesn't wear out until over 1000 flights, passes pull test, etc.
- Make the controls free-moving and free of slop (already, two competing requirements)
- Use the biggest bellcrank that'll reasonably fit. For a Vector 40, that's probably a 4" bellcrank. "Reasonably" is a judgement call because the bigger the bellcrank, the more room you need for it and the more the leadouts will bend at the wingtip, which is bad, but a bigger bellcrank means that the lines have more leverage, which is good.
- You can go overboard on this -- at one point people were experimenting with pulleys instead of bellcranks (Ted Fancher either originated this or was the most famous guy doing it) it turned out to be more trouble than it was worth
- Get the elevator travel correct with respect to the bellcrank. +/- 20 degrees at full bellcrank travel is probably as much as you'll ever need, but I usually make plenty of room for the bellcrank to move and set the travel at +/- 30 degrees on the bench, before the fuselage is buttoned up. I'm probably giving up line leverage -- but my planes always have plenty of turn.
- A 1:1 flap:elevator ratio isn't always best -- and "best" depends on a number of factors, including pilot preference and how heavy the plane comes out. You won't find out until you start trimming. So, if you're serious, make the ratio adjustable. Most people do this with a slotted elevator horn and a carrier for the pivot pin that can be loosened, adjusted, and tightened.
- You won't know the best flap:elevator bias until you trim. So while you're making the elevator horn slotted, use a ball link on the end of the pushrod that can be screwed in and out (and note that this presupposes an access door unless you're using a take-apart plane)
- Other stuff I'm forgetting at the moment
- Oh, yes -- a perfect relationship between handle and elevator/flap motion. Note that I don't say "perfectly linear" because -- especially for heavier planes -- folks are going to logarithmic flap control, where the flaps tend to move a lot around center, but slow down after a while with respect to the elevator. The idea is that in level flight and in round maneuvers you want to vary the lift for smooth groovy level and round which is aided by lots of flap, but in square corners you want to turn the airplane -- having more elevator than flap at that point helps
Howard Rush has gone to the trouble of making an Excel spreadsheet that lets you enter in all sorts of parameters of your control system, then calculates the actual flap and elevator motion. This won't let you make that perfect control system, but it lets you figure out your own personal best compromise -- especially if you're fabricating your own bellcranks so that you can do things like offset the pivot point or the flap rod pick-off point. It's available for the asking.[/list]