Would there be any advantage in using a Robart incidence meter on the v-tail, and if so - how can this be done accurately?

From : Don Stackhouse

No.

Low Reynolds number models tend to operate within an extremely narrow range of angles of attack. The same is true to an even greater extent for the typical tail. A fraction of a degree can make a big difference in lift coefficient and the resulting trimmed airspeed. Because of the very high dihedral angle, the slightest error in positioning the incidence meter exactly parallel to the chord of the tail will introduce significant errors into the measurement. Even if you do get it exactly parallel to the chord, the notches in the grippers on the meter that seat on the leading and trailing edges do not seat at the same spanwise location on the top of the notch as they do on the bottom, and because of taper in the surface, the gripper at the leading edge doesn't seat the same as the gripper at the trailing edge. The grippers will not seat the same on the cambered airfoil of the wing as they do on the symmetrical airfoil of the tail, and variations in the shape of the leading edge radius can introduce even more errors. Yes, you can measure a very precise looking number, but does it really measure anything of significance with all those potential errors?

When I generate the wing and tail saddles for a new composite V-tail fuselage, I design jigs for the wing and tail on the computer, then use computer plots to manufacture the parts for the jigs. The wing, tail and fuselage jigs are all mounted to a perfectly flat metal table (the table I use is precision ground cast iron, about 12 feet long, 2 inches thick not counting the cast-in stiffening ribs, and weighs about the same as a Volkwagen), then the jigs are used to hold the wing, fuselage and tail in position while I cast the wing and tail saddles onto the fuselage plug. I'll use my incidence meter as a quick cross-check to make sure things are in the right place, but I rely on the jigs to provide the real accuracy. They can provide greater accuracy than the incidence meter can measure.

In the Chrysalis, we use the machined fuselage sides, and the laser-cut tail bulkheads to perform this same alignment function. They act as your jig. As long as the table you build your fuselage on is flat, and the straightedge you use for the centerline on the fuselage bottom sheeting is straight, the incidence should be controlled more closely than you can reliably measure with your incidence meter.

A curious thing, incidence. Wing-to-fuselage incidence can substantially influence performance at a given flight condition, yet there is no clear criterion for which operating point to optimize, and even if you build it differently than specified, odds are that it will still be able to find some set of operating points where it flies well. Meanwhile, wing-to-tail incidence ("decalage") is incredibly sensitive to small changes, but it is also sensitive to variations in C/G and other builder-determined effects. In any case, the precision with which we can check it during assembly is often not nearly as good as the precision with which a well-designed and manufactured kit can position it. The bottom line is: If you have a well-designed and precisely made kit, be thankful; build your model as carefully as you can, but don't get overly obsessed with things you can't measure precisely and reliably during construction anyway; and let the model tell you if it's happy, when you take it out to fly.

Don Stackhouse
DJ Aerotech