On Thu, 19 Mar 2020 08:43:45 -0700, Eric Greenwell wrote:

How do you determine the tail is lifting in gliding flight? And wouldn't
be more efficient to have the larger wing provide all the lift, and just
use the tailplane to provide stability?

Good question, but I think flight stability gives the answer for FF
models. They're all trimmed for minimum sink, it being a duration event
with distance covered being a matter of wind and thermal strength and
what time you put on the d/t timer, so they all glide slowly at min.sink
to maximise flight time.

We also know from wind tunnel tests, etc. that the Centre of Pressure (CP)
of almost all airfoils is around 33% chord at slow speed.

So, for FF models, if your CG is behind 33%, then the tail *must* be
producing lift for stable flight.

Similarly, we know that while aircraft trimmed that way can be extremely
stable, they aren't necessarily controllable, but that they are if the CG
is in front of the CP, which requires downforce from the tail for stable
flight, so all manned aircraft are set up like that. This is particularly
obvious if you look at any of the earlier Boeing airliners: the tailplane
has quite a noticeable negative incidence *and* has an inverted cambered
airfoil, to the amount of downforce it produces will be considerable.

Back to gliders, yes, the less downforce you need from the tailplane, the
more efficient the glide becomes, but the more squirrelly it becomes as
you move the CG back.

The other way of getting efficiency is through leverage. If you lengthen
the tail boom you need progressively less downforce at its rear end to
balance the nose-down tendency. This alone means the tail needs to
produce less downforce, and so reduces the drag the goes with producing
it. It also means you can make the tailplane smaller, so reducing its
surface drag.

Putting all the surface in the wing seems to produce stability issues,
which I won't pretend to understand. All you can say is that tailless
gliders have all had issues, mostly connected with high speed stability.
I remember Rudy Opitz reporting that his father found that the Horten
SIV.b developed a nasty high speed pitch oscillation well below Vne and
that this affected his on-task speed. The Akaflieg Karlsruhe's AK-10 also
had this problem. It has affected powered tailless aircraft too - that's
what killed Geoffrey De Havilland in the DH.108 and nearly got Eric
"Winkle" Brown as well.


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Martin | martin at
Gregorie | gregorie dot org