Why not start with a high efficiency 5V PSU (like the one that currently poweers the Pi or perhaps a higher current one) feeding a 2.3V switching converter feeding the cell feeding a 5V switching converter feeding the Pi ?
-- Steve O'Hara-Smith | Directable Mirror Arrays C:>WIN | A better way to focus the sun The computer obeys and wins. | licences available see You lose and Bill collects. | http://www.sohara.org/
The weight difference isn't significant: a factor of two at the most. My glider is a Standard Libelle. It is a single seat type, 15m span, weighs
206kg ready to fly less pilot. More details and pics here in case you're interested:Understood. We, the gliding community, have always been wary of Li-ion batteries since a lot of us are or have been model flyers and are well aware of their reputation for catching fire while being charged and in crashes. The Boeing 787 fun and games hasn't helped their rep. either. They were using SAFT Li-ion batteries.
I don't need the extra capacity these chemistries offer since even one of my pair of 7.2Ah SLAs can run my panel for 14 hours plus. Besides, a new
7.2AH SLA currently sells for GBP 16.80 as against GBP 86.56 for a Lithium iron phosphate drop-in replacement. This price does include the charger but its still only a 7AH battery. The alternative US sourced K2 12 LFP 7, another 12v 7AH SLA drop-in replacement, is $US 140 without the charger, so that looks like a lot more money for very little benefit.Admittedly the LiFePO4 chemistry is *much* safer than the SAFT cells that Boeing was using but I still don't need something behind my seat that can spew toxic smoke after a bad accident: in a glider the pilot is the nose weight that puts the balance point, known as the CG (centre of gravity) where it needs to be for safe flight: in this case about 30-35 cm behind the seat back. Heavy, removable items such as batteries tend to be fitted as close to the CG as possible. Yes, we do need to know what we weigh and the acceptable weight is always written on a placard in the cockpit - and checked by weighing and recalculating it every 7 years.
----- The only glider type that does get serious benefit from lithium battery chemistry is something like the Antares 20E, an electric self-launching motor glider. It is a single seat 20m span beast with wings full of batteries. It carries enough power to take off and climb 10,000 ft or to motor 150-200km if the weather turns bad. Its an excellent, very fast and efficient glider too. One has flown 1500km in Scottish wave. The pilot had declared a 750 km triangle for his day's flight, but got round it so fast he decided it would be rude to stop so early and went round a second time before heading home.
-- martin@ | Martin Gregorie gregorie. | Essex, UK org |
I guess the batteries are a small part of the weight of the aircraft so not so important. I worked on a radio design for a drone (powered, not a glider) which was probably about the same size as your glider but with a smaller wingspan. The radio unit (an aluminum box) had a stud for a ground cable. In the design review they spend some 20 minutes discussing (arguing) over whether the stud should be shortened a half inch to save weight. lol
So I'm thinking weight is important in any aircraft and especially so in a glider.
But I understand the concern for safety. I believe that many Li batteries are not allowed on commercial passenger flights as cargo. Sort of like the oxygen generators on that flight that crashed some years back because of the fire.
I have thought of getting into flying, but I have many, many other things in the way at the moment. I am very into kayaking, but need a hip replacement before I make any plans.
-- Rick
Yes. Yuasa NP12-7 batteries are 2.65Kg, so the pair of them are less than
2% of the flying weight.
That depends. Paradoxically, weight its less important in gliders, both model and full size. In any aircraft in steady flight, lift from the wing exactly equals the flying weight and lift varies as the square of the speed, so doubling the weight of a glider only makes it fly 40% faster, though the sink speed will rise by more than 40% because drag will also increase. It can be useful to increase a competition glider's weight because that increases its cruise speed, but you'd only do that on a day with strong lift because its rate of climb in thermals. Consequently, all modern racing gliders are set up with water tanks or bladders in their wings so they can carry water ballast on strong days.
Gliders are faster than you might expect too: I never fly mine, a 45 year old design, at less than 42 kts (48 mph) when climbing carefully in a very weak thermal) and its interthermal speed will be somewhere between
53 and 85 kts (60-98 mph) depending on the air mass I'm flying through and hence what speed to fly the glide computer is suggesting. The redline is 118 kts (136 mph). Modern gliders are considerably faster than mine: I know that in 2012 at the Woprd Champs in Texas the better Open Class gliders, e.g. the Antares 23, were cruising at over 100 kts between thermals. By contrast a powered aircraft is much more weight sensitive, since increasing the weight increases the flying speed and so maintaining level flight requires more power as a direct result. This has a direct effect on the length of the take-off run, take-off speed and on climb rate after take-off. This has a bearing on the steady trickle of light aircraft take- off crashes which turn out to be carrying four heavy people plus their golf clubs or hunting/fishing gear and where the pilot didn't weigh people and stuff and check the take-off weight.Altitude also doesn't affect gliders much: again they just fly faster to compensate for the thinner air, but it had a double wammy for powered planes. Thinner air requires the plane to fly faster, so more power is needed for level flight, BUT the thinner air also adversely affects the engine, reducing available power, and propeller efficiency, reducing the thrust it produces. This is generally not a problem in the UK, but is a regular cause of accidents in the USA, where the combination of high mountain airfields and high temperature (which also reduces air density) can be lethal.
Was the drone you worked on a high altitude type? If so I can understand why there was such extreme concentration on weight reduction. The other aircraft that is famous for the concentration on weight is the U-2 reconnaissance plane. Despite its size and speed it had no power assisted controls and a single wheel undercarriage - both, among other features, were entirely for weight reduction.
Exactly so. Rip that thin plastic envelope and wait for the fire...
You might want to take a trial flight at your local gliding club just to see what its like. Who knows, you may get hooked. I've done both. Last summer I finally got round to having a flight in a Tiger Moth biplane, which was nice but I realised that, apart from an infinitely better all- round view, the other thing I really like about gliding is the continual awareness you must maintain to find your next climb if you want to stay airborne. Besides, every winch launch is a real blast: 35 seconds from standstill to releasing at 1200 ft or more above the winch can't be all bad.
Sooner or later you'll have to stop kayacking and glider pilots can fly into their 80s if they remain reasonably fit and alert. In the UK (and, I think, Europe) its very much cheaper than power flying, especially if you enjoy winch launching. I've also flown in the USA and, talking to pilots there, it seems gliding is still cheaper, but only about 80% cheaper. But then again, almost all gliding over there is off aero-tows, so there is the cost of tug operation to cover.
-- martin@ | Martin Gregorie gregorie. | Essex, UK org |
On Sat, 7 Feb 2015 14:15:52 +0000 (UTC), Martin Gregorie declaimed the following:
It also has the minor problem that, at cruise altitude, the difference between stalling from going too slow, and ripping the wings off from going supersonic, was around 20 knots. The pilot had to concentrate on airspeed control.
But once you got into lower air, it was just a big glider with an engine attached. The first flight was an accident -- it was being taxied around and a head wind (gust) was sufficient to lift it off the ground.
--
Wulfraed Dennis Lee Bieber AF6VN
wlfraed@ix.netcom.com HTTP://wlfraed.home.netcom.com/
That's a good question. Indeed, since we're not _too_ concerned with the mains charger efficiency at sub KW power levels (circa 10 to
25W?), that sort of pragmatic approach will do the trick without adding too much to a Raspberry-Pi owner's electricity bill. Besides, if it ever became a more important consideration, you still have the option to upgrade the single cell charger at a later date.Using a high efficiency laptop charger to feed a switcing converter to float the single cell at the 2.3v mark will probably represent the next best solution to highest efficiency short of buying an expensive dedicated high efficiency single cell charger.
The additional losses in the extra switching converters in this case, are simply the price you pay for a more reliable energy storage system based on the electrochemistry of a single cell which doesn't have to be restricted to lead acid technology.
All other issues aside, a 3.7v lithium cell would be a better choice but it takes a lot to beat the charm of the simplicity of lead acid battery technology which has been around even longer than CRT technolgy (both now over a hundred years old).
One thing to watch out for regarding the charging circuit is the possible need for an extra blocking diode to stop reverse current flow from the battery into the output terminals of a home made charger (one, perhaps based on a 12v mains PSU driving a 2.3v output switching regulator). A dedicated high efficiency 2.3v mains charger, if they exist at all, should have the need for such a blocking diode designed out. Careful selection of your chosen 10v to 24v input, 2.3v output switching regulator may eliminate the need of a blocking diode altogether.
A typical homebrewed single cell UPS solution would most probably involve a cheap, but high efficiency laptop charging brick (which has the charm of accepting mains voltage inputs over the range 90v to 265v rms ac - you could, in the case of nominal 230v ac mains supplies, use an autotransformer to step the mains voltage down to 150v rms to protect against overvolting faults in the mains supply (up to a max voltage of 406v and a cut out voltage of 138v rms).
Your laptop charging brick (with or without the optional step down transformer) would then feed a 2.3v output switching regulator, preferably one that doesn't need a seperate blocking diode, to float charge the battery _and_ supply current to the 2.3v to 5v 1 or 2 amp converter(s). You could choose to use a 3.7v lithium cell if you're feeling adventurous but I suspect the power management circuitry would be significantly more complex than the charmingly simple constant voltage requirements of a single SLA cell.
It's the relative simplicity of constant voltage charging of SLAs that contributes to the ongoing popularity of lead acid battery technology despite the advent of more exotic battery types. Also, since pretty well all batteries are "Consumable" items, their relative cheapness also works in favour of SLAs.
The biggest downside in any multicell battery is the issue of maintaining charge state equilibrium between the cells. Such imbalances can be corrected in the case of open wet lead acid batteries by using a "Gassing Charge" once every few months. However, this technique can't be used for the AGM and Gel SLAs since it will produce hydrogen/oxygen bubbles trapped in the Gel or the glass fibres of such cells. There's a lot of merit in opting for a 'single cell solution' where relatively low power requirements make this a viable and reasonably efficient option.
-- J B Good
I'm not so sure of that. I bought my 17 foot boat from a 70+ guy who had cancer. He was still kayaking until he got sick. Plus it is such great exercise. You may *need* to be fit to fly, but you will *become* fit by kayaking. It fixed my lower back trouble by strengthening my torso (core muscles). In fact, kayaking is a great social activity with most participants in their later years. Flat water kayaking is a bit tame for the 20 year old crowd I guess.
-- Rick
The "Coffin Corner". See:
Rob.
difference
Ah, the so-called coffin corner, where stalling speed and the IAS at that altitude (actually the TAS (True Air Speed) coincide. Get fractionally fast and the you're beyond airframe limits: fractionally slow and stall/ spin is the likely result. I believe U2 pilots know it well.
That is what happens when the required cruising altitude is "as high as she'll go".
-- martin@ | Martin Gregorie gregorie. | Essex, UK org |
Yeah, I assumed you meant white water or ocean kayacking. I'd forgotten how popular flat-water kayacking is in the US and Canada and know very little about it.
Good point about the social aspect. I think those who don't take part in what are inherently group sports don't understand that side of the game.
Its the main difference between gliding and GA flying: since you need at least three people to launch a glider (tug pilot/winch driver, wing runner and glider pilot) almost by definition its a group game, unlike GA, where you can wander onto the field, do the walk round, climb in, start up and fly.
-- martin@ | Martin Gregorie gregorie. | Essex, UK org |
Being at the launch point and helping is part of the fun! (At least it was for me when I was actively gliding.)
And there's also a lot of waiting around the launchpoint for suitable weather - perhaps depending on where you are in the world!
// Christian
-- Graham. %Profound_observation%
GA?
-- Rick
General Aviation: see
-- martin@ | Martin Gregorie gregorie. | Essex, UK org |
Quite. Most clubs have a pretty comfortable club-house. The social aspect includes the point that it will tend to collect a group of like-minded individuals which means that its a nice place to hang out. Many (all?) pilots tend to have a fairly black sense of humour, so its nice to know that the associated jokes and viewpoints aren't likely to be appreciated.
For instance, I like Sir Charles Kingston-Smiths comment that "The only time you can have too much petrol in board is when the plane is on fire".
Look him up if you don't recognise the name.
Also true, but forecasting is getting better: take a look at Blipspot / RASP (respectively US/American terms for the same software or XCWeather. There's also a growing network of ground-based FLARM receivers in the UK and Europe, so you can see if your friends are flying and where they're going. All this stuff is available on the 'Net and many of the tracking sites the use radar feeds are now taking FLARM feeds as well.
-- martin@ | Martin Gregorie gregorie. | Essex, UK org |
Ditto.
-- Alex
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