Helium bubbles used to show bird aerodynamics

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.
R,
Chris

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

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?

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

Martin Gregorie wrote on 3/19/2020 8:11 AM:
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?


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.

I flew hand-launch, towed, and power FF in the early '60s. After a detour to race
sports cars, I ended up sitting in gliders instead building them.

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?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

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.


--
Martin | martin at
Gregorie | gregorie dot org