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#21
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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 |
#22
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Helium bubbles used to show bird aerodynamics
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 |
#23
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Helium bubbles used to show bird aerodynamics
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 |
#24
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Helium bubbles used to show bird aerodynamics
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 |
#25
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Helium bubbles used to show bird aerodynamics
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 |
#26
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Helium bubbles used to show bird aerodynamics
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 |
#27
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Helium bubbles used to show bird aerodynamics
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 |
#28
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Helium bubbles used to show bird aerodynamics
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 |
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