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#1
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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 |
#2
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Helium bubbles used to show bird aerodynamics
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. |
#3
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Helium bubbles used to show bird aerodynamics
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 |
#4
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Helium bubbles used to show bird aerodynamics
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 |
#5
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Helium bubbles used to show bird aerodynamics
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 |
#6
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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 |
#7
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Helium bubbles used to show bird aerodynamics
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 |
#8
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Helium bubbles used to show bird aerodynamics
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? |
#9
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Helium bubbles used to show bird aerodynamics
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 |
#10
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Helium bubbles used to show bird aerodynamics
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 |
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