On Tue, 08 Oct 2019 17:37:00 +0000, James Lambert wrote:

Funnily enough.... the assumption is made that the tail is acting with a
'downward' force, counterbalancing the mass ahead of the lift... with
neutral stability, the elevator actually exerts no force in pitch. And
it is also possible that the tail generates lift to support a rearward C
of G !!!

Indeed, but every glider I know the W&B for has the CG in front of the CL
at all flying speeds (CL approaches 30% MAC at the stall, but as airspeed
rises it moves further back.

I'm far from certain that I'd want to fly anything with the CL at or
behind the CG. Here's why:

I know quite a lot about trimming small aircraft with rear CG positions -
virtually all competition free flight models are set up this way because
having both wing and tail providing lift adds efficiency, especially if
the tail is flying at its minimum drag AOA. Which is the case for a well-
designed model. Numbers: the F1A gliders (F1A is the international class
for towline gliders - typically 2.2-2.7m span, min weight 420g) all flew
best with the CG at 55% of MAC, a main wing AOA of about 8 degrees and
the flat bottomed lifting tail at about 3.5 dergees AOA. The stability
was good - it has to be to handle rough thermals and the turbulence you
get below 50-100ft on a windy day. But I would not want to fly an
aircraft with that trim set-up because the trim sensitivity was extreme -
my tail had a 90 mm chord and I used a 10BA bolt as the trim adjuster
(thats 1.7mm diameter, with a 0.35mm pitch). I could easily see the
effect of half a turn on the trim (both still air duration and dynamic
recovery from upsets) and so used to fine tune them in terms of 1/4 turn
adjustments. That gives a 0.09mm movement at the TE of a 90mm chord
tailplane, so a very small angular change indeed. 1-2 turns took it from
stalling to an under-elevated over fast glide.

On the other hand, similarly sized models with the CG in front of the CL,
so flying with down force on the tail, are easy to trim by adding or
removing pieces of 0.8mm or 1.6mm balsa under the TE of the tailplane.

Bottom line: I would not want to hand-fly anything with that amount of
pitch sensitivity so am happy to leave anything with its CG behind the CL
in the hands of other, better and braver pilots than myself.


--
Martin | martin at
Gregorie | gregorie dot org