Helium bubbles used to show bird aerodynamics

So Martin, as a totally “model� ignorant observer lol, question, the key is all about getting minsink rate the absolute lowest possible irregardless of l/d?
Dan

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

Curiously, that is not (we're told) how the model gliders are trimmed:
the tail is lifting at minimum sink. That seems inefficient to have a
small wing producing lift instead the big wing, with it's lower drag
from a larger aspect ratio.

Its not as inefficient as you might think: several people, as well as
myself, have found that a well designed model with a long tail moment has
a fairly low coefficient of lift on the tail. At a normal glide trim the
parasitic drag of the tail is greater then its drag due to lift. The
tailplane is working at a Cl of around 0.05, which puts it pretty much in
the centre of its minimum drag bucket, while the wing will be operating
at a Cl of 1.1 - 1.2.

We don't care what the glide slope of a gliding model is like since its
not going anyplace, just circling in the thermal it was launched into.
All we care about trimming it to glide at min. sink speed. Contests are
won and lost on total airtime recorded during the event.

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...

- What is the advantage for trimming it with a small positive lift instead of zero
lift?
- How about using a smaller horizontal trimmed for a higher L/D? That would lower
the parasitic drag and the overall drag of the horizontal, while still producing
the lift needed for 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

Martin Gregorie wrote on 3/21/2020 6:42 AM:
All you need is:

- a copy of "Circular Airflow" by Frank Zaic ABE Books
https://www.abebooks.com
have a few copies if you don't have one (and you won't unless you were
a keen FF model flyer). Other second hand bookstores should also have
copies

That was my bible over 60 years ago. Twenty years ago, I finally gave it to a
college friend that's still building models, even now. What is the "bible" for
modelers now?

--
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 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

On Sat, 21 Mar 2020 07:14:21 -0700, uneekcowgirl wrote:

So Martin, as a totally “model� ignorant observer lol, question, the key
is all about getting minsink rate the absolute lowest possible
irregardless of l/d?

Spot on!

Flying a competition consists of making a previously announced number of
flights during the day. Each flight is timed from when its launched until
it lands. The winner is the person with the highest total flight time.

Simple!

Modifiers:
To avoid losing models, there is a predetermined maximum scoring time for
each flight. All models carry a 'dethermaliser', universally called a
'd/t'. This is typically a timer that, then it trips, moves the entire
tailplane to a 45 degree 'up elevator' angle. This stalls the model and
keeps it stalled so it descends vertically relative to the air: its
normal gliding sink speed will be around 0.3 m/s but after the d/t has
popped its descent rate will be in the 4-5 m/s range, which is usually
enough to drop it out of even a strong thermal.

The 'max' flight time gives the possibility of, on a good day, having
several people who have all maxed out, i.e. flew for the maximum time in
all flights. This is sorted out with a flyoff involving just the flyers
with full scores, held in the evening when both wind and lift should be
weaker. In a small competition this is a single unlimited duration
flight, timed to the ground.

Internationals and World Championship events are a little different.
Theese fly 7 rounds during the day, each being a 50-60 minute period
during which every competitor makes one flight. Its usual to have several
people maxing out, so flyoff rounds are flown in the evening the
determine the winner. These are flown in much shorter rounds, typically
20 minutes each, and with the max time going up, starting from 5 minutes
and increasing by 2 minutes for each successive flyoff. If there are
still several flyers with full scores when it gets too dark to fly, a
final 10 minute round is flown at dawn the next day.



--
Martin | martin at
Gregorie | gregorie dot org

On Sat, 21 Mar 2020 07:38:34 -0700, Eric Greenwell wrote:

Martin Gregorie wrote on 3/21/2020 6:42 AM:
All you need is:

- a copy of "Circular Airflow" by Frank Zaic ABE Books
https://www.abebooks.com have a few copies if you don't have one
(and you won't unless you were a keen FF model flyer). Other second
hand bookstores should also have copies

That was my bible over 60 years ago. Twenty years ago, I finally gave it
to a college friend that's still building models, even now. What is the
"bible" for modelers now?

Thats still worth reading if you want to design your own models, but you
do need to remember that materials and ideas have moved on since that was
written: nobody had glass or carbon back than and everything was still
covered with modelling tissue and lightweight silk and nylon.

Jim Baguley, a Brit, wrote an excellent set of articles in Aeromodeller
magazine back in the '60s and one of the Swiss flyers wrote a good series
on FreeFlight News in, I think, the late 70s. Both are still worth
finding and reading.

NFFS (American) has a seminal series of annual Symposia, starting in 1968
and still going strong - I have a complete set. Its packed with good
ideas and the Models Of The Year are worth studying.

Articles in Free Flight News, SCAT News (Californian - formerly on paper,
now online as SCAT Electronic News) and Vol Libre (French, stopped a
decade ago) are all good sources, but the best source of design ideas are
still in the bars and various International and World-class competitions.



--
Martin | martin at
Gregorie | gregorie dot org

Thanks Martin, from what I am reading, this type of experimenting can actually be a lifetime pursuit playing with all of the multiple variables that combine for success, or that don’t lol.
Dan

On Sat, 21 Mar 2020 18:15:55 -0700, uneekcowgirl wrote:

Thanks Martin, from what I am reading, this type of experimenting can
actually be a lifetime pursuit playing with all of the multiple
variables that combine for success, or that don’t lol.

Yes, this often happens. Many of us started flying models, usually
remarkably badly, at age 10 or so, enjoyed the challenge and stuck with
it. I dropped out in 2000 when the soaring bug bit, but most of the
people I was flying and competing with in the 70s are still flying unless
they've gotten ill or dropped off the perch. I'm thinking I might have a
crack at building and flying FF scale, which don't go nearly as far, when
its time to hang up my parachute.

Competitive FF is a good way of keeping fit. Flying in typical UK
conditions means you can easily cover 10+ miles a day, and that's just
retrieving the model after a flight, so all the galloping round with a
towline model on the line, looking for a thermal, is an extra bit that
the rubber and power guys don't do.


--
Martin | martin at
Gregorie | gregorie dot org