I'll spend a couple of paragraphs addressing my theory about why inline designs may be sensitive and tend toward instability in level flight (see remarks about the in-line Beringer airplanes from many who have built them and found them to have the same "balancing on the head of a pin" feeling in level flight that requires constant attention).
I know I am coming in very late on this one, but since I did have a comment about in-liners in response to Tom Dixon's rather critical/hostile letter to SN a few years ago about tail incidence, and tangentially about in-liners, I suppose I can comment.
I don't think there's any doubt that most "in-liners" are very "wafty" or funny-feeling around neutral, compared to regular stunt planes. I probably even saw Ted's Avanti flight way back when, well before we met. I was incrediby impressed with the Bob flew it, in fact, in many ways, my mind was set on what good stunt patterns looked like by the way Bob flew it.
But - as anyone who read my first shot at a design column could see, the concept is fundamentally flawed. The idea is that since the airplane should turn the same inside and outside, it ought to be "symmetrical" and, of course, that means that you should put the engine, wing, and tail all in a line so that there is nothing that "favors" up or down. Great idea, unfortunately, unless you have drop gear, fin/rudder that looks like the tail on a largemouth bass (symmetrical from the side view), and contra-rotating propellor and engines, you have plenty of other assymetry already, so putting the engine, wing, and tail in line doesn't accomplish the goal of making the airplane neutral in terms of up and down response. In fact, an in-liner will probably need quite a bit more downthrust and/or positive stab incidence, since you don't have the *advantage* of having the engine trying to nose you down to compensate for the precession like you did with a higher thrust line. I haven't thought it through whether or not the higher stab helps or hurts with this. I suspect it *probably* helps compensate, too, but I couldn't explain it in words. Point being, the goal of equal/neutral inside/outside response is NOT accomplished by making an in-liner, in fact it probably hurts.
But there is one whoppingly obvious (to me, anyway) disadvantage, and it's most evident in level or low-g flight. The turbulence off the wing in level flight streams almost straight out behind it. There's very little lift (just the weight) and it doesn't take much Cl to create it. Therefore the "downwash" is very minimal.
So, bingo, the stab/elevator is running in the wing turbulence in level flight. And very tiny changes in the Cl from slight corrections at the controls move the turbulent area up and down around the stab. So not only is it in turbulence, slight corrections drastically change the turbulence, and thus the stab effectiveness. So you make a slight correction, or a slight perturbation comes along and loads or unloads the wing, and all of a suddenly your control input works 50% better or 50% worse, and you never have any idea what it's going to do next. Leading to that "vague" feeling.
Of course, it clears itself up pretty well in harder maneuvers, and may be more equal in terms of stab effectiveness when you start getting to higher Cl as the downwash is moved away from the stab. But now, you have a situation where it works very weakly and unpredictably around neutral and really comes on strong in maneuvering. That might *sound* good, but I think that something that drastically changes charactersitics depending on the conditions is a formula for unpredictablity and inconsistency. Maybe you can practice a lot every day and get used to it, and used to how it's different in different conditions, but you are gonna have a bit of a time trying to just pick it up and fly it well the first time, or anytime the engine runs a little different, or when the air density changes, or the wind changes, or (etc).
Never mind that almost everything that we have done in the past 20 years in terms of design and trim is geared towards making the airplane
more positive and responsive around neutral. This arrangement has the precisely opposite effect.
And the Avanti in particular has another, quite intentional, feature that encourages this "vague" feeling, and that's the old "stab bigger than elevator" trick. In fact, the plans show a rather exaggerated case, with the stab TE about 1/2" and the elevator about 3/16 (or something like that). That works on the same principle, creating turbulence that makes the stab less effective around neutral to "smooth it out". So in this case it's a sort of double whammy.
The very last thing I think you want is a system where you move the handle a lot, and not much happens, then you move it a little more, and all of a sudden it "grabs" and really responds.
Once again, a lot of people have been trying to *prevent* exatly this effect. Paul Walker started it, and I have subsequently ripped off the idea. We are actually rounding off the edges at the stab/elevator (and wing/flap) hinge line to reduce the turbulence and make the controls *more* effective around neutral than they would be otherwise. The 3 airplanes upon which I have done this have been *very very positive* around neutral and nonetheless track very well. With some remarkably aft CGs at time. Of course, I am also willing to build in or dial in the necessary positive incidence, and not feel "guilty" about it.
So it's no wonder to me that the Avanti was a little tricky to fly. Even without knowing the historical record, you could pretty well predict that a very superior pilot (and make no mistake, Bob Baron was in general a very superior pilot) could fly it well, on some occasions - but probably not consistently from month to month. Which pretty much was what happened.
Brett