Helium bubbles used to show bird aerodynamics

https://youtu.be/2sh8_3-R90I

At 0:58 in the video the bird's wingtip vortices are described as "This helps provide lift". This statement caught my eye.

However, Wikipedia (https://en.wikipedia.org/wiki/Wingtip_device) mentions that "Wingtip devices increase the lift generated at the wingtip (by smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio.

A contradiction?

- John non-aero-e DeRosa

These are journalists describing science. I think more accurate would be "these vortices are a consequence of the process of generating lift with that wing". They create drag, which we don't want, but they come with the lift which we want.

An analogy would be to say "the rubber you leave on the road from your tires creates all the thrust for your car". Tearing up the tires and heating the road from the slipping rubber rob some of the engine's energy, but you can't go forward without them.

On Mon, 24 Feb 2020 23:21:59 -0800, Bret Hess wrote:

These are journalists describing science. I think more accurate would be
"these vortices are a consequence of the process of generating lift with
that wing". They create drag, which we don't want, but they come with
the lift which we want.


Yes, that's fair. A wing can't generate lift without generating tip
vortices. The energy used in spinning them up adds to the aircraft's
sinking speed, but clever wing design, which may include winglets and/or
wingtip shaping as well as a carefully designed wing planform can reduce
the energy that goes into spinning up tip vortices and hence will reduce
the sinking speed of the aircraft.

For a very readable account, try "Affandi Darlington on winglets", which
first appeared on PPrune but may have vanished unless The Wayback Machine
has a copy. I have a local copy I can send if you can't find it anyplace
else.

Peter Masak's "Winglet Design for Sailplanes" and Wil Schuemann's "A new
wing planform with improved low-speed performance" are both worth
reading. A web search will find the Masak paper and Wil Schuemann's paper
is in the Soaring Symposium archive.

When I developed my 'Delta-G' series of F1A class competition free flight
gliders, I used a combination of Wil Schuemann's planform ideas, first
seen by glider pilots on the S-H Discus, combined with Hoerner wingtips,
also common on gliders from the first glass airframes until winglets took
over, and had decent competition results with this series of models.


--
Martin | martin at
Gregorie | gregorie dot org

On Monday, February 24, 2020 at 10:33:46 PM UTC-6, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
https://youtu.be/2sh8_3-R90I

At 0:58 in the video the bird's wingtip vortices are described as "This helps provide lift". This statement caught my eye.

However, Wikipedia (https://en.wikipedia.org/wiki/Wingtip_device) mentions that "Wingtip devices increase the lift generated at the wingtip (by smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio.

A contradiction?

- John non-aero-e DeRosa

There are always wing tip vortices when a wing is generating lift, but they can be tight and concentrated, like a horizontal tornado, or broad and diffuse. The tight and concentrated carry more energy that the diffuse vortices, even though the vorticity (and the generated lift) is the same.

Rich L

On Wednesday, February 26, 2020 at 6:26:53 AM UTC-8, Richard Livingston wrote:
On Monday, February 24, 2020 at 10:33:46 PM UTC-6, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
https://youtu.be/2sh8_3-R90I

At 0:58 in the video the bird's wingtip vortices are described as "This helps provide lift". This statement caught my eye.

However, Wikipedia (https://en.wikipedia.org/wiki/Wingtip_device) mentions that "Wingtip devices increase the lift generated at the wingtip (by smoothing the airflow across the upper wing near the tip) and reduce the lift-induced drag caused by wingtip vortices, improving lift-to-drag ratio.

A contradiction?

- John non-aero-e DeRosa

There are always wing tip vortices when a wing is generating lift, but they can be tight and concentrated, like a horizontal tornado, or broad and diffuse. The tight and concentrated carry more energy that the diffuse vortices, even though the vorticity (and the generated lift) is the same.

Rich L

It is unclear here whether it was the journalist reaching the wrong conclusion or the scientists feeding her a bad conclusion. Obviously, vortices don't create lift for the bird (although flocks use the vortices of the leading bird for additional lift).

Tom

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 2/25/2020 2:10 AM:
When I developed my 'Delta-G' series of F1A class competition free flight
gliders, I used a combination of Wil Schuemann's planform ideas, first
seen by glider pilots on the S-H Discus, combined with Hoerner wingtips,
also common on gliders from the first glass airframes until winglets took
over, and had decent competition results with this series of models.

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