On Wed, 18 Mar 2020 22:44:42 -0700, Chris Behm wrote:

Also not correct is the statement that a glider with a lifting tail
would be unstable. Most earlier freeflight model gliders did have
lifting tails, and no in flight controls.

What do you mean by 'early'? :-)

A more correct statement would be 'all current competition free flight
models have lifting tails'.

I used to design my own F1A and F1J/1/2A models as well as building them,
and all had lifting tails.

My F1A towline gliders had their CG at 55% of mean wing chord. The
stabiliser operated at a positive lift coefficient of 0.05, which for the
sections I used (B8403, 7% Clark Y and Woebbeking), put the stabiliser
smack in the middle of its minimum drag bucket. Win-Win!

I used a 10 degree swept back LE on the wing's outer panels, straight TE
and raked Hoerner tips. This combination does two things. The sharp angle
where the tip, raked at 30 degrees with the TE longer than LE, meets the
TE tends to localise the tip vortex. The spanwise flow encouraged by the
swept outer LE and the upper tip surface rolling down to meet the lower
surface at a sharp edge tends. In theory these push the tip vortex
further outboard, so increasing effective aspect ratio, but who knows for
sure? However, the design was easy to fly and trim and won its share of
contests.

My F1J design (small stab, long moment, VIT and autorudder) flew best
with the CG at 65% of mean chord, so it used a similar trim setup to my
F1A gliders, while the 1/2A was a modified traditional model (George
French '1/2A Train'), so it had a shorter moment arm and large (35% of
wing) stab. It was also fitted with VIT and autorudder and liked having
its CG at 80% of mean wing chord.

All three designs were stable in wind and turbulent conditions, easy to
trim and fly, and had good contest records.


--
Martin | martin at
Gregorie | gregorie dot org