On Sat, 21 Mar 2020 07:31:14 -0700, Eric Greenwell wrote:

Martin Gregorie wrote on 3/21/2020 7:04 AM:
On Sat, 21 Mar 2020 06:40:45 -0700, Eric Greenwell wrote:

It's called a "lifting tail" even though it is producing very little
lift, and is producing that lift with a high drag penalty from the
parasitic drag? Confusing...

Recap: the tail has to lifting if the model's CG is behind the wing's
centre of pressure (CP). We also know from wind tunnel studies etc, that
the CP of a wing operating near min sink is close to 33% of the chord
behind the LE. All the fixed trim FF competition models I've built or
flown have their CG at around 55% (towline gliders and rubber powered) or
in the 80-90% range ('chuck' or catapult gliders, power models). VIT
models (those where a timer reduces the AOA of the tail when it stops the
motor) will have the CG 10-20% further forward than those with fixed trim.

In all these cases the CG is behind the CP when the model is gliding, so
the tail most be lifting or the model would simply pitch up and stall.

- What is the advantage for trimming it with a small positive lift
instead of zero lift?

Good question. Its due to launch requirements. If fixed trim model glides
with downforce on its tail, then simply speeding it up will make it pitch
up and stall. Thats an unavoidable consequence of speeding up a fixed
trim model.

If the model has a lifting tail it will pitch up relatively slowly
because thats what the combination of wing and tail sections combined
with decalage (the difference in geometric AOA between wing and tail) is
designed and trimmed to do. This, combined with a small amount of wing
twist (all FF models use wash-in on the wing on the inside of the glide
circle) and rudder setting can be arranged to convert excess speed into a
spiral climb rather than a straight pitch-up or loop and, as the model
slows down the turn opens out and it settles into a circling glide at min.
sink speed.

The glider turn is fairly open. Around 40 seconds per circle is about
right. Too tight a turn raises the sinking speed while too open a circle
may let it wander out of the thermal.

Launch behaviour is a major design input because, with the exception of
large, open class rubber powered models which seen to climb and glide at
very much the same speed (I've never flown these, so don't know for sure,
but climb and glide speeds look very similar), all FF models are launched
a lot faster than they glide to get them nice and high in the thermal
you've just picked. Power models climbing speed is at least 2-3 times
faster than they glide, hand launched gliders are thrown as hard as
possible and F1A towline gliders are towed as fast as you can run while
the model flies a catch-up arc at the top of a 50m towline, so must be
travelling into the wind about twice as fast as the person launching it.
As a result it will pulling 15-40 kg line tension at launch: you really
hammer them off to gain as much height as possible. The maximum permitted
towline length is 50m under 5kg tension and, depending on the model and
how hard you can launch, it will gain another 10m to 60m before settling
into its glide.


--
Martin | martin at
Gregorie | gregorie dot org