Helium bubbles used to show bird aerodynamics

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 08:28:33 -0700, Eric Greenwell wrote:

I flew hand launch gliders in early60's. My best glider had a planform
identical to the original Discus. It was called the "Sweepette". The
cover on this article shows the 1960 version:

https://indoornewsandviews.files.wor...0/inav-113.pdf

I know it well: Lee Hines has been a good friend ever since 1983, when I
was on the way back from the Australian WC and stopped over on my way
home to fly in some Californian comps. I did the MaxMen's comp at Taft
and the Sierra Cup at Sacramento, travelling with Lee.

I got 2nd in in the Sierra Cup that year - an epic event with 10 rounds
flown Saturday and Sunday morning. Flyoffs started at 1:30 PM on a hot,
thermally afternoon. We flew 4,5,6 and 7 minute flyoffs before those with
clockwork timers dropped out en mass. By the 10 minute round it was
cooling off and only Walt Ghio and I were left. We both blobbed it, but I
dropped more than he did.

Halcyon Days!

I wonder if Wil Schumann was inspired by the Sweepette, or some earlier
version of that planform?

Lee seems to have developed the Sweepette in the late 60s and got the
Model Of The Year award for it in 1970.

I have a copy of Wil's article on modifying an ASW-12 so he could fly it
in Standard Class, and that modification included his new planform, but
annoyingly, the article doesn't show a date. The ASW-12 was introduced in
1965, so its at least possible that Wil was first with that planform.
AFAIK Lee has never followed developments in fullsize gliding in any
detail. He was always more interested in Formula 1 GP racing, and spent a
season as a Goodyear gofer (and got into the partys) when Jimmy Clark,
Graham Hill, etc were to top drivers.

Do you know when Wil was flying that modified ASW-12?


--
Martin | martin at
Gregorie | gregorie dot org

"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.

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

On Thu, 19 Mar 2020 09:33:40 -0700, jjdk737 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 don't have access to a wind tunnel, but Mark Drela does and may will
have done that, However, I have photos that have a bearing on it.

When I drew up my glider series I made calculations of where the wing
wake should be at the tail location and how thick the wake was at that
point. I wanted to be sure that the tailplane was outside the wing wake
because my flying mates and I had already discovered that an F1A isn't
trimmable if the tailplane is inside the wing wake at its normal flight
attitude.

Anyway, I did the calcs, drew the lines on the CAD screen and adjusted
the fuselage height and wing AOA to put the tailplane comfortably below
the wing wake - I like zero dihedral inner wing panels with all the
dihedral on the shorter tips, which occupy about 1/3 of the semispan, so
wing/tail wake interference would be a problem if it ocurred.

A long time later I checked this by making a rake to let me see the
airflow behind the wing and onto the tailplane. Here's a description and
pictures:

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

Notice in both pictures that the streamers are deflected up in front of
the tail - I think that's a fairly clear indication that lift is being
generated.

Here are some wind tunnel pics showing similar effects, but the flow
deflection is a lot less because flow velocities will much higher than a
FF models gliding speed. An F1A glider's gliding speed is quite low: in
flat calm its easy to run alongside and just behind one and then reach in
and grab the tail boom just behind the wing is it sinks past your waist.

In case it isn't obvious, these models weigh just over 410 grams. They
are generally 2.2 to 2.5 metres in span and fly are about 10 mph (16 kph,
4.5 m/s). Sinking speed is around 0.3 m/s, giving a glide ratio of about
1:16.


Anyway, here are the wind tunnel links:

https://www.aa.washington.edu/AERL/K...hguide/flowviz

https://www.youtube.com/watch?v=IIHFkp1aAOI




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



--
Martin | martin at
Gregorie | gregorie dot org

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 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

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