On Thu, 19 Mar 2020 06:23:40 -0700, jjdk737 wrote:

Are these lifting tails creating upward lift during low speed flight,
close to stall speed?

Or... are they only providing upward forces at high speed during the
climb, transitioning to a downward force during slow speed flight after
powerloss?

Depends on the model: my F1A gliders used a fixed stabiliser trim for all
phases of the flight: launch, circle towing to find lift, a good, hard[*]
zoom launch and the glide. Rudder setting on tow depends on line tension
(straight with load on the line, circling to check thermals with slack
line, and slight turn into glide circle with the hook open ready for
release.

My power toys had timer controlled vertical trim and rudder as well as
motor stop. Climb is a very steep right hand spiral with some down trim
relative to glide and a bit of left rudder to keep the nose up. At motor
stop the F1J's timer applied a lot more down to bunt over to glide
attitude and then retrimmed up for glide in a right hand circle. The 1/2A
was similar, but without the bunt transition from climb to glide.

So yes, all three types glided with the tailplane providing lift. All
free flight competition models are better thought of as tandem wing
aircraft with both wings providing lift. That was more obvious in the old
days, when very large tailplanes, up to 35-50% of the wing area, with
short moment arms, 3-3.5 times wing chord, were used. Now tailplanes are
around 20% of the wing area and the moment arms are about 5 times the
wing chord. All free flight models are trimmed to fly at minimum sink
trim and to, hopefully, stay in the thermal you launch them into.

Free flight competitions are often flown when gliders belonging to
sensible pilots stay in their trailers. In fact, some of the best
competitions have been flown in overcast, calm conditions with very
little light lift available. However, there's a 9 m/s limit on wind speed
(32 kph, 23 kts) in Internationals and rain seldom stops play unless its
heavy enough to prevent timekeepers from seeing models. On somewhere like
Sculthorpe where runway 05 is 8800ft (9800ft to the boundary fence) and
models are launched from the SW end taxiway, its fairly normal to pick
them up in the next one or two fields out when flying to a 3 minute
maximum: the scoring flight time is 180 seconds and the dethermaliser
timer releases a second or two later. This gives full stabiliser up at
about 45-60 degrees, which stalls the model and holds it stalled,
converting it into a rigid parachute with a 4-5 m/s descent rate.
[*] my F1As, which are now old technology, used carbon D-boxes and spars
and 7mm diameter hardened steel wing joiners. The models were a little
heavy at around 430g (class minimum is 410g), but the tow hook unlatched
at 16kg tension and I would have been pulling around 25-30 kg at release:
they'd gain around 10m in a half-spiral zoom climb when I let go of the
bottom of the line to release the model. With 100 lb Spectra towline
(essentially no stretch) the unlatch tension needed to be at least 16kg
to prevent accidental unlatch when towing on rough ground and/or in gusty
conditions.


Anyway, thats probably far more than you ever wanted to know!


--
Martin | martin at
Gregorie | gregorie dot org