Helium bubbles used to show bird aerodynamics

On Thursday, March 19, 2020 at 9:33:44 AM UTC-7, wrote:
"How do you determine the tail is lifting in gliding flight?"

Agree. Do we have wind tunnel testing to prove the theory that your fixed stab is always providing upwards forces on the aft end of the fuselage?

I used do some free flight modeling.
Mostly Dick Mathis designs.

There is a misconception that a requirement for stability is that the tail must be providing a downward load. In fact all that is required is that the lift slope of the tail be higher than the wing with increasing angle of attack. That guarantees a righting moment with any angle of attack disturbance.

All this talk about free flight models reminds me of family connections. My brother won the world competition in F1B rubber powered free flight a few years back (first US win since the mid '50s). These aircraft have surprisingly sophisticated powertrains using secret and carefully hoarded rubber motors driving folding, feathering, torque sensing prop hubs. The latest have wings that are folded in half for the high speed climb, and unfold for the glide. They typically have lifting tails.

Absolutely. On a modern glider, this slope goes through zero at the angle of attack of best L/D (zero lift = minimum drag). Below that speed, the tailplane produces lift, above it produces downward force.

Tango Whisky wrote on 3/21/2020 1:37 AM:
Absolutely. On a modern glider, this slope goes through zero at the angle of attack of best L/D (zero lift = minimum drag). Below that speed, the tailplane produces lift, above it produces downward force.

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.

--
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 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.


--
Martin | martin at
Gregorie | gregorie dot org

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

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

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

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

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 01:37:35 -0700, Tango Whisky wrote:

Absolutely. On a modern glider, this slope goes through zero at the
angle of attack of best L/D (zero lift = minimum drag). Below that
speed, the tailplane produces lift, above it produces downward force.

At a slight tangent:

If you're in a COVID-19 lockdown and need something to do with your
hands, you can do a lot worse that teach yourself some basic flight
stability rules while having fun making stuff.

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

- a sheet or two of 1/16" balsa

- modelling clay or electrical solder to use as nose weight

- some glue (white PVA, cellulose cement or a decent brand of
cyanoacrylate such as Zap)

- a suitable knife. I like the snap-off blade sort you can find in DIY
stores.

- maybe some glass-headed pins to hold stuff in place while glue dries

The section of the book called "Spiral Stability Demonstration" (starts
on page 49 in my copy) shows how to make very simple hand launch gliders
and use them to see the effect of side area, vertical and horizontal tail
size, etc. The models are small, light and slow enough to be flown in any
reasonable sized room or in your garden on a calm day. Its easy enough to
extend this to an investigation of CG position relative to wing chord and
the associated up or download on the tailplane.


Couple of other links:

- if you liked experimenting with the Zaic test models, you might like
Easy Mini, a small, light catapult launched balsa glider. Its interesting
because, when correctly set up, that same fixed trim works at high speed
in a spiral climb and equally well after it slows down into a slow
floating glide. You can lose these upwards of you fire them into a
thermal. They are suitable to use with scout groups, after-school hobby
groups, and, of course, young gliding club associates.

https://www.gregorie.org/freeflight/easymini/

- a debunking job I did to prove that an ancient Egyptian wooden bird was
probably a kids toy or a decoration and certainly not anything that
actually flew. Proving that was a lot of fun:

https://www.gregorie.org/freeflight/saqqara_bird



--
Martin | martin at
Gregorie | gregorie dot org