Hello, I purchased a Chrysalis HLG kit the other week and was reading through the directions with a friend when I saw that the angle of the V-tail is set to 74 degrees.

My friend said he thought that may be a typo because he usually sets them for about 100-110 degrees. Any advice? Thanks in advance!

From : Don Stackhouse

The angle is correct, but it's the complement of the angle your friend was thinking of. An angle of 74 deg., measured according to the drawing shown in the upper right corner of the Chrysalis plans, equates to an included angle of 106 degrees between the two panels. This is the angle your friend was thinking of. It also equates to a dihedral angle per panel, measured from horizontal (the same way you would measure wing dihedral) of 37 degrees per side.

That said, there is no "one size fits all" dihedral angle for V-tails. The dihedral angle of a V-tail is what determines how much of its authority is devoted to pitch effects, and how much to yaw effects. Different airplanes need different ratios of these two, so the required V-tail dihedral varies, just as the required areas for the fin+rudder and stab+elevator of a conventional tail vary for different airplanes. No one would claim that the area ratios of a conventional tail should always fit within some narrow arbitrary limits; likewise, it does not make sense to try to impose similar draconian "rules of thumb" on V-tail dihedral. We have kits in our product line that fall well outside of the 100-110 degree range, in both directions.

v The amount of horizontal tail effects (pitch effects) involved in a given design depends (among other things) on the chord of the wing. The yaw effects are related more to span. Therefore, a very high aspect ratio airplane (such as our Spectre VR) will need less stab and more fin area, or in the case of a V-tail, more tail dihedral (in this case, that means the included angle measured between the panels would be less than your friend's 100-110 deg.). For a low aspect ratio model (such as our Nymph), there's more wing chord in comparison to the span, so it needs a bigger stab, or in the case of a V-tail, a flatter tail angle (less V-tail dihedral).

Other factors come into play as well. For example, the additional aerodynamic pitching moments from large landing flaps with larger deflection angles require additional elevator authority. This is why the Spectre 120 has a flatter tail angle than the Spectre VR, even though their aspect ratios are similar.

That old 100-110 degree included angle rule of thumb only works within the typical range of aspect ratios commonly found on fairly typical model sailplanes. As you can see from the above, once you get outside of those common R/C sailplane aspect ratios, it no longer works.

Don Stackhouse
DJ Aerotech