John Olsen 1

Posted by Panther on 26/02/2013 10:44:18: Queensland can be a real hot place. When it is not underwater?

I have a 65Ah deep cycle battery and a Turnigy Fat Boy charger, the latter can do up to 1344 Watts, although only if I connect to a 24 to 30 Volt supply...I do have two of the batteries but so far haven't needed that much capacity or wanted to carry them both the 100 or so yards from the parking to the pits. One is enough, it lets me charge a 6S 4200mAh Lipo in about quarter of an hour, a charge being the 3000mAh that the timer is letting me use on a typical flight. This seems about right, leaves a bit for an extra circuit or two if the strip is blocked, and avoids fully discharging the Lipos. I'm charging at 2.5C on batteries that are rated for 6C so should be well within what they can stand. Alternating two Lipo packs and the usual standing around talking gives the batteries time for a rest between times too. I'd rather have two Lipos and charge at the field in these larger sizes, as the capital cost of more gets a bit high. I wouldn't want to go much bigger with electric, the main limitation being the size of the ground equipment needed to charge them, eg bigger lead acid cells. I don't really want a generator hammering away in the background all the time. Now if we could cover the roof of the shed with solar cells and put a good size bank of batteries in there it would be nice. I do own chargers smaller than this, the one for my Ember indoor model takes four AA cells and charges the 1S 70mAh lipo in about ten minutes or so. The Fat Boy can in theory charge those OK, if I could be bothered organising the leads. I also fly IC, but will try to avoid poking any wasps nests. John

I have a version 2 but it has not gone into a plane yet. I do have a version 1 in a foamy and a Fyetech unit in a Superstik, both seem to steady the model OK. They don't actually fly it for you...well the Fyetech unit can on heading hold but I haven't tried that yet, not having the need to maintain a fixed heading for more thaqn one leg of a circuit. Regarding the original post and the cause of the crash, did you check that both the control movement from the TX controls and the control movement for correction from the gyro are correct? It is possible for them to disagree. You get the gyro movement in the correct sense by means of the controls on the gyro and you get the Tx movement in the correct sense by the programming or reversing switch on TX. The movement from the gyro can be quite fleeting so it is hard to check. If it is wrong it will of course be destabilising the aircraft. Other than that...well, all the usual suspects can still go wrong! John

Well, an ordinary DC motor with three armature windings and a commutator is effectively a three phase motor with a mechanical inverter synchronised to the armature position. If you added three sliprings and connected them to each commutator segment you could pick up a somewhat spiky and squarish three phase AC. The frequency would of course depend on the rotational speed which will vary with applied Voltage and load. Similarly the ESC circuit is generating a rather spiky three phase waveform, and attempting to synchronise it to the rotor position by sensing the back EMF generated by the windings. That this is not always successful is evidenced by the need to change the timing sometimes, to suit the type of load etc. However it is a very useful feature since if we just apply a fixed frequency to a permanent magnet motor, it is liable to drop out of phase lock quite readily when a load is applied. The sensing effectively determines that the rotor is lagging behind and so alters the frequency to suit. There are some similarities to the inverters used for three phase motors in industry and in my workshop, but a big difference is that most such motors do not have permanent magnets in the rotors...although the Gentle Annie washing machine made by Fisher and Paykel here in NZ did. Without the permanent magnets you can't use the sensing trick, but the squirrel cage motor does not need it. When the motor is approaching maximum speed, the back emf generated by the magnets passing the coil will oppose the Voltage applied by the ESC, so the current will fall off...and so the torque will fall off. So unless the load is very small, the rotor will not reach synchronous speed. So it won't be able to do that with a propellor connected. So with a load applied the ESC and motor will find a speed where the current being taken is sufficent to match the load, in pretty much the same way as a commutated DC motor will. If the load increases it will slow down a little. One of those previous threads had a link to an application note with a lot of useful stuff about how the ESC works. Another John

For a fully charged Lipo I would expect about 4.2 Volts per cell, so 3.8 might be about right for storage mode. The idea of storage mode is that lipos can be damaged if they are fully charged and then the temperature drops. On the other hand, they can also be damaged if they are over discharged. So the idea is that if they are going to be stored for some time in possibly varying temperature conditions, you charge them to a medium state...typically 50 to 60 % charge. Some chargers will discharge them to a suitable state if they are already fully charged when you use storage mode. If you are pretty sure that you will be using them again soon, and that the temperature is not going to drop, you are probably OK to just fully charge them. On the other hand, if they are going to be stored in an unheated shed in a frosty climate, it might pay not to fully charge them, especially if you charge them in a nice warm room beforehand. One thing to remember is that you can never know whether you would have got a longer life out of your batteries by treating them more kindly. John

I accidently bought a low kv version of a motor for a larger plane, a 60 equivalent. To get enough power I had to go up in prop size, which then meant a few minor mods to the undercarriage to get enough clearance. The result is that the prop is doing about 7000 rpm where a more normal choice would have been doing about 10,000, and the plane is whisper quiet. You can hear the airframe noise. The downside is that you don't get so much indication of how fast it is going. However the plant is going to go into a Fokker triplane under construction here, where the large prop size will be nice and scale, so the mistake turns out to be all to the good. I've bought the normal motor to substitute into this plane later. One thing to avoid if you want to be really quiet is a pusher prop. The airflow coming off the wings and into the blades gives a louder noise than with the prop at the front. There is Dornier 335 with the rear motor on a youtube clip that gives quite a siren effect. That one is IC, but you will hear the same effect with the popular electric flying wings. John

While it is true that a stall corresponds to an angle of attack rather than a speed, what generally concerns people is the conflict between landing at a low enough speed for the undercarriage to remain intact, and remaining in control while approaching that landing. The control requires a high enough speed for the controls to work, while the hardness of the ground dictates a low speed. So what people mean when they talk about "stalling speed" is the speed at which the angle of attack to maintain level flight just reaches the stalling angle. Below that speed the airfoil will not be able to generate enough lift to match the weight, so the aircraft will accelerate downwards out of control, frequently by stalling one tip first. Above that speed control should be able be maintained. And yes, it is a speed which will vary with the weight of the aircraft, as well as with the density of the air. (Hence both altitude and temperature will affect it, as will high lift devices like slots and flaps.) Full size pilots go to a lot of trouble to calculate it before takeoff, since rotating a big jet will not get you off the ground if you are not going fast enough...if the wing cannot generate enough lift at the maximum angle of attack to lift the aircraft then you will stay on the ground, and if you rotate further the increase in drag beyond that angle will reduce the force available to accelerate the aircraft, tending to cause damage to the hedge at the end of the runway. So although it is not a fixed speed under all atmospheric conditions, for a given aircraft with a given all up weight, there is a minimum approach speed below which you will be in trouble. It is a bit pedantic to care that the same aircraft could stall at twice the speed if it was pulling four Gs when the approach and landing phase is what is under discussion. We can worry about the high speed stall when we get into aerobatics, which we will generally attempt high enough up to allow some recovery space. John

Mine did a couple of runs on glow fuel before I fitted the shim, so it certainly works Ok. It has since run on petrol although I have not flown it yet, the fuselage still needs covering. You can get a good slow idle with spark too. That could be useful on the plane I am putting mine in, since it is an old style free flighter with radio added, so it is likely to be a bit of a floater. Well, it was when it last flew over forty years ago! John

Hmm, I can''t count. Having looked at the drawing there are actually five webs to clear on the Saito. Anyway the files are on their way to Andy right now. If we were allowed to put pdfs in our albums I could put one there too. Most of the work is pretty simple turning, but the clearance for the webs needs a mill. Both of course kind of tricky if you don't have the gear, but maybe you can find a local model engineer who can do them for you. The gasket can be done by hand work if necessary, just cut one out of some 0.6mm shim. John

Balsa USA are generally good, although probably not the absolute top. We (my son and I) have put together a 1/6 scale Taube from the BUSA kit. It is not too hard to build, and should maiden any day now. Sheet parts are die cut rather than laser cut, I think this is true with all their kits. The die cutting is of good quality, but of course not as good as laser cutting. It builds up into a nice model, although it would be sport scale rather than a scale mans dream, unless you put in a lot of extra detailing. The Taube does not have ailerons, and should be a stable flyer. It does not have the usual short nose of a WWI job. Ours has an ASP 60 four stroke, but it could be electrified without too much trouble. John

Hi Andy, I have converted a Saito 62 to spark using the RCXL CDI. There is a photo in my album. I could provide a CAD drawing of what I did if it would be helpful. The idea is that you turn a round part that just fits over the front of the engine, then mill clearance for the three webs. This lets the round part sit back against the gear housing. A single grubscrew is enough to clamp it in place. The magnet mounts on an oversized prop driver. I reduced the compression ratio with a shim under the cylinder. This requires adjusting the tappets afterwards. Chris, Saito don't make two strokes for the same reason that Rolls Royce do not make mopeds. John

Ok, a couple of points related to motorcycles...my little Honda 100 (four stroke) used to start smoking if it had to creep in traffic for too long. Yes, it got too hot. Like a lot of air cooled four strokes, some of the cooling was from the oil, which is why you will see fins on the outside of the sump, and even bigger fins inside the sump of those old air cooled Hondas. I haven't looked inside any recent ones. Back in the seventies there was a phase when they made a lot of largish single cylinder two stroke trail bikes. Up to five hundred cc was not uncommon. The Americans discovered that if you ran them hard across the desert and then shut them down for a rest, the motor would seize while stopped. The reason was that the piston was still hot, but the fins on the barrel would quickly cool it down, so it would grip on the piston. Once it has done so, further cooling does not seem to ungrip it, resulting in a long walk. You might also notice that a lot of modern high performance two stroke motorbikes, even for motorcross racing, ahve water cooling. A major advantage is that the temperature can be more easily kept at the optimum point. So cooling on an air cooled engine is not always as well controlled as we would like it to be. Actually if it is right for the hottest conditions it will be overcooled for the coldest, and vice versa. This is why a lot of radial engine aircraft have those adjustable flaps on the cowling outlet. But it is true that the inlet does not need to be enormous. There is a scale effect that means that the fins on a small cylinder don't need to have as much area as on a big cylinder, the volume varies as the cube while the area varies as the square. So if you halve the linear dimensions you get 1/8 of the volume in the combustion chamber with 1/4 of the area to get rid of the heat through. So less heat through more area means it will run cooler. (Actually it will be able to rev higher, which means more fuel proportionally can be burned, but there will still tend to be less heat per unit area.) John

Telemetry is not for the pilot, it is for the flight enginer who should be a separate person. That is at least if you are using a visual display. We have used a tachometer sensor to help get the prop size right, I had to peer over my sons shoulder while he flew the plane. For indications that the pilot might want you should use an audible warning, which some setups allow. John

I thought you guys didn't need a passport anywhere within the EU these days. Or do they want one when you go by air? But as Peter says, a light plane flight would be more the thing for a first experience anyway. Gliders are good too, and you don't have to worry about the engine stopping. As for me...my ambition at the moment would be for the seemingly interminable Auckland winds to stop whistling around my eaves long enough for me to get a few flights in. We just had a three day long weekend, lovely summer weather, apart from the wind. Maybe I should get into slope soaring. John

Total loss lubrication was not uncommon in the early years of motoring, especially on motorcycles, but the usual means was a pump, either operated by hand whenever the rider thought of it, or else a pump driven by the engine and arranged to vary its flow with the throttle setting. This sort of thing was still common into the thirties. The pump delivered into the crankcase rather than the inlet, I think usually via the big end. From there oil would splatter around and lubricate the cylinder walls. One of our engines here actually has a boss in the crankcase backplate, looks like the ones that used to be provided for tapping for fuel tank pressure on some two strokes. It has the usual nipple under the front where the bearings are, so I am not sure what the purpose is. Might be just a design afterthought. It's an ASP four stroke. John

With a full size spark ignition engine, the point to measure is right beside the spark plug boss. At a high enough temperature the spark plug points will get hot enough to cause ignition before the spark arrives, which is of course undesirable. The other temperature of interest is what the cylinder walls are running at, since the oil will stop working if this gets too hot...with water cooled engines this would be covered well enough by moniitoring coolant temperature. If we had materials and lubricants that could stand the pace we would actually avoid cooling altogether, it is a loss of heat and so reduces efficiency. However, seizing up solid would be even worse for efficiency, so we have to cool them. One of the gains over the years has been lubricants that will allow higher temperatures. Four stroke engines will attain higher gas temperatures in the combustion chamber, because they do not suffer as much from exhaust gas mixed in the fresh mixture. This gives higher thermal efficiency, and results in higher exhaust temperature. Ideally we would expand the gases more and get a lower exhaust temperature with higher efficiency, but doing so would greatly increase the size of the engine. Exhaust turbines are one way to capture some of the energy still contained in the hot gases, but hardly applicable in our sizes of course. John

I got those batteries charged up OK just after you left. I managed to get six flights in before the sun slipped down on the last one. John

Tim, if you opened the needle that would actually be a little richer rather than leaner, but still a good thing and probably also easier on glow plug life too. We tried one on an OS 56 four stroke, but the sensor was not a good fit between fins so I suspect was reading the air temperature more than the engine temp. John

Readers of Terry Pratchetts books will be familiar with the King of the Golden River and what his business involved. I don't think the Chinese are very much into design rights, you want an Armani suit, they will sell you one, probably from the exact same factory that the real ones come from. (I saw some of these recently.) Get them to build a few thousand widgets for you and they will build a few thousand more for themselves. John

I'm working on a contract for computer networking with a large local electric power distribution company. I happen to be sitting next to the Health and Safety people there, so naturally get to overhear quite a lot of their discussions. Obviously some of what I hear about individual cases would be confidential, so I will avoid too much detail, but I am finding it quite interesting. One of the guys there is English, been here in NZ a few years, but it does appear that one reason he valued is that the English Health and Safety legislation is seen here as being a good model to work towards in improving our somewhat cowboy system. One of the mistakes that has been made here is in allowing too much self regulation, the capitalists having argued that too much regulation is expensive etc. One result of that sort of approach was the Pike River mine disaster, which was severe enough that it surely made the news even in the UK...29 dead. The report of the Royal Commission is on line and makes sobering reading. Another disaster that would be more familiar in its implications to readers here was the plane crash on the West coast. A Fletcher had been modified for skydiving use...it took off, reared up in a stall and crashed and burned with severe loss of life. Turns out the Centre of Gravity was too far aft, but nobody competent had taken a look at it. The accidents and screw ups they are dealing with next to me are mostly much less severe, but have convinced me that an awful lot of people are complete idiots. A recent example was a team working down a tunnel...the rule is they are supposed to communicate at fixed time intervals. For various reasons this didn't happen, and as a result they made the newspapers by having a rather large callout of emergency services. The guys down the tunnel seem to have been a bit bemused by the attention, probably they had forgotten that one of the cockups at the above mentioned Pike River mine disaster was that nobody even realised that there had been an explosion for about 40 minutes. So from what I have seen, the people actually involved in heath and safety are actually quite sensible, and are trying to come up with arrangements that are safe for the worker while allowing the job to get done. Just bear in mind that there are still people out there who would be quite happy to have your ten year old kids working down the mines, provided they can make a quid out of it. John

It is not really all that high tech, apart from making the LEDs. The generator part only needs to be a small dynamo like the ones in the wind up torches with a simple gear train, and presumably there will be a solid state regulator of some sort. The electronics side of things will give little trouble. Quite a lot of these sorts of places can manage to look after a cell phone, even if there is only one in the whole village. The wind up torches are fine for many things, but something that you can set going and not have to touch for half an hour has its good points too. I've had to tinker with things in the dark using one of the capacitor based wind up ones, and it is always going dim at a bad moment. So Ok, the electronics part can't be made locally and is imported, but then the kerosene that the oil lamps use will also be imported. If this unit is paying for itself in three months, then after that time it is saving its own cost in imports every three months. Once there are enough of these about, there will be a cottage industry of making good ones out of the parts of broken ones. John

Well, to roll the Stinson at all nicely I have to apply full aileron and rudder in the same direction, then add in down elevator as she comes inverted. That hasn't changed with the gyro. Of course I could turn the gains up a bit and see what differnce it makes but it seems pretty good at the moment, so I am inclined to let things be. It does seem to me to have great possibilities for those awkward scale subjects that have bad habits, like say the Fokker triplane. Well, the full size one had bad habits so I guess we can't expect the models to be too much better. There will be limits to what the gyro can acheive, since one ot the problems with the Fokker is that on landing the wings blanket the rudder and tailplane, so you lose control authority. I just splashed out and bought one of those little UMX ducted fan Mig 15, with the built in gyro. It will be interesting to see how that goes. John

Yes that was an impressive if short flight. I thought Mark had got it under control at one stage, after it had gone inverted about four feet up. The breeze did get a little stronger after you left Mark, the Stinson was having to fly quite crabwise to do a cross wind leg. John

Well, some months later I have finally been able to try out a three axis unit in a model. (The summer here has been a bit dodgy so far, what with tornadoes and tropical cyclones...) The unit is the hobby king mk 1, now superseded by a better unit that has provision for V tails and for on/off via a spare channel. Mine is being tried out in my foamy Stinson. I started out with the gains set quite low, but enough to get some response from the surfaces when you move the plane. I haven't tried increasing them yet. It flies fine. It was quite calm this evening when we started, but the wind gradually got up. It does seem to reduce the tendency to nose over that is a feature with this plane. Having spats, and on grass, it is normally a bit tricky to avoid nosing over when you open the throttle, and then if you are using the elevator to hold the tail down she will take off too soon and want to tip stall. So I think that it is helping in that phase. General flying was fine, it can still loop easily. Doing a decent roll is not easy, but it never was, it is not really an aerobatic design. It does give the impression that a little rudder in the turns could be good, but that may be because this style of aircraft really should have a bit anyway, so I'm not sure if it is because of the gyro. By the last flight I was flying in a breeze that was evidently about 1/4 to 1/3 of the flying speed, going by the angle had to fly to make good a course at 90 degrees to the wind direction. This is quite a good breeze, although it was not especially gusty. Anyway, the model flew very steadily in this. So it certainly works. I'll experiment some more later. I also have the Fytech unit in another plane, and it has flown four times, but so far the gyro has not been enabled. John

1. True, although they are unlikely to have made a sandwich, so you can probably scratch through the glue to the conducting layer if it is on the inside. 2. There is no rule against putting a cup of water in there along with the dielectric under test.