Tinkering & Learning, with
Simple Model Aeroplanes.
30th April 2015. Last Christmas, my daughter bought my son and I a toy rubber-powered plane each, along with other stocking fillers. Yes, my daughter knows ‘boys love toys’! My son is actually a member of his local Model Flying Club, and flies electric Radio Control planes. But as for me, the last model plane I built must have been thirty years ago. That was also rubber powered, & flew into a tree in the local park on its third or fourth flight, and hung there for quite some time. After an unaccountable delay of four months, the plane was assembled, and test glides in the (small) back garden were satisfactory.
My friend Dennis and I were going for lunch that day, so we called in at the very same local park to test the model properly. Here you see Dennis releasing it, while I videoed the event for posterity. The Skytouch II whirred rapidly, gained speed and height, did a kind of back-flip, and flew straight into a tree behind Dennis, quite high up, where it resolutely remained. Now honestly, I had no idea whatever that might happen. Remember, it had been 30 years since I flew a model plane; I thought it would just float along in a power-assisted straight and level path until the rubber wound down, then it would simply glide to earth. It was clear that much thought needed to be given to the problem. Happily, the Internet in general, and YouTube in particular, offer a wealth of information on simple model aircraft. We should have looked there first!
Borrowed from www.theplanpage.com with grateful acknowledgements.
We soon found this glider, which is an illustration from a series of articles by Charles H Grant, which appeared in the U.S. publication Model Airplane News in the earlier 1940s. It is a long series, called ‘Modelling Your Future in Aviation – Official Air Youth course in elementary aeronautics’. The way the dimensions are derived from the fundamental unit – the wingspan – was fascinating. Some balsa sheet was hurriedly obtained, and we built the above glider. Actually, the wing should have been 3/16" (4.75mm) sheet, but we could only get 1/4" (6.35mm) – so after we had sanded it to a rough aerofoil shape, it was a bit thicker and heavier than it should have been. Also, we made the nose rather longer, so not so much mass would need to be added. We mounted the wing on a short flat strip so that it could be held onto the stick by a rubber band, and moved to different positions. After some trials, it produced a nice long flat glide. So far, so good.
The next step was obviously to add some rubber power. A propeller from a cheap sheet balsa toy plane was fitted, as above. We had ended up with about 4 grams of soldering wire as a front weight, but the propeller bracket and undercarriage was about 6 grams (plus some of the rubber band) – so the model would be nose-heavy. Accordingly, the stick was extended by about 1" (25cm) behind the tailplane, so that soldering wire could be added there. We spent half an hour in a local park (not the one in the photo above with all the trees) until the model would glide quite flat in its new configuration. Then, little by little, turns were put on the rubber. The result was very disappointing. Again, I was expecting to extend the smooth glide, but even with as little as 30 or 40 turns, it would ‘run away’ and behave erratically. With more turns still, it would climb steeply, but not have height enough to drop down into a proper glide. In fact, it would nosedive into the ground. The stick was shortened at the front half an inch at a time, until the rear ballast had been almost completely removed. The model was then as light as it could be; but the performance did not improve. Still, we had spent a nice hour and a half in bright, sunny springtime weather. But another re-think was clearly necessary. Our model was probably too heavy. Its final weight was 22 grams; the undercarriage had been abandoned by then, with no effect. The motor would not run long enough, and was altogether too fierce.
Scouring the Internet, it was pleasant to learn that our amateur diagnosis was largely correct. The fierceness of the motor was down to two factors: the propeller was too small at 4.5" (11.4cm), and the ordinary, unlubricated rubber band was not really suitable as a power source. Proper rubber strip, coated in a lubricant of equal parts of glycerine and green liquid soap, was required to give a steady power output; while a larger, e.g. 7" (17.8cm) propeller would turn more slowly, and prevent the wildness of our flights that day. The rearing up & nose-diving, we learned, is called a Power Stall. And certainly, at 22 grams, our powered glider was a heavyweight, what with its solid wing. An alternative model was sought on-line. There are scores of them up there! The Denny Dart II, shown above, was obviously a classic, so that was adopted. It was designed by Neil Dennis, and it is used all over the world by young schoolchildren, as a class project to acquaint them with model flight, and also a number of abilities & skills that will be of benefit to them. I fit into the group called ‘children of all ages’, so I have taken to the Denny Dart II like a duck to water, if you will excuse the rather inappropriate metaphor. 8^) It has a 12" wingspan, and the same length of stick. We are anxiously awaiting the delivery of suitable rubber and propeller! The airframe you see above was quite easy to make, and weighs only three grams. Aha – surely, the game is afoot?
2nd May 2015. Our eagerly awaited parcel of propellers, tissue & rubber arrived. We had got a 6" prop mounted in a nose block, and it was good to see that this exact type was widely being used by experienced folk on-line. In addition, the nose block fitting was exactly right for the 3/8" x 1/8" stick of the Denny Dart II. (Or perhaps the other way around?) The commonest sizes of rubber for these small, light planes are 3/32" (.25cm) and 1/8" (.32cm), so we obtained both. To make a rear hook for the motor, a pin was inserted at an angle, through the stick, near the tail. The model glided quite well in our small back garden when the wing position was correctly adjusted. The wing is secured to the stick by small thin rubber O rings, which can be moved to get the wing in the right place. A 14" (35cm) length of lubricated 3/32" rubber gave a nice steady motor run of 10 seconds for 200 turns. Naturally, the weather was too windy today – Grrh! – though we did venture out when it got a bit calmer, and sallied forth to the local park for a trial. As soon as we had walked into an open space, a light rain began to fall. This will not be an unusual event for aero modellers! We tried a few longer test glides, but the softness of the balsa stick and the weakness caused by inserting the motor hook caused the stick to break after a few glides, so nothing could be done under power.
But if so, which way? The upperside of the wing must be chosen carefully…
3rd May 2015. Rain; but by about 1400 it and the wind had abated, so we took the first Dart II – wch, now being slightly battered, we regarded as expendable. There was something of a breeze but it had lulls in it, and we were able to make a large number of test flights with gradually increasing turns on the motor. At last we had a promising model! It still turned to the left quite tightly, but this was fine with us. The sun got quite strong, so any water drops the model picked up from the grass, dried very quickly indeed. We had forgotten a watch, but counted seconds, and more than one flight lasted around 20 or 25 seconds, so that was good. Later we finished the second Dart II. The stick on this was made of three 1/8" squares glued together. It was much stronger, but the airframe was twice as heavy as our first essay, at 5.9 grams! Virtually all this extra weight must have come from the laminated stick we had made, as the wing, tailplane & fin were made from the same materials as in the original. Perhaps a light stick from medium 1/8 x 3/8 balsa is best, after all…
4th May 2015. Unlike the traditional British Bank Holiday Monday, we had bright sun, and less wind than yesterday. Still working on the expendable Dart II, the wing was pinned down and given a very light spray of clear acrylic lacquer on both sides to see if this would improve the lift.
Back at the park, starting with the Expendable, it seemed that the sealing of the tissue was beneficial, as we got over 20 seconds on the first flight of only 200 or so turns. Unfortunately, there was a residual warp in the wing, which offset to some extent the tendency of the model to turn hard left, opening up the turn quite a bit. After a few flights, we made a bad launch, and it went in straight away. Picking it up, without really thinking, we put some more turns on without starting from scratch. We must have ended up with a very large number of turn indeed, for the Dart then circled several times, gaining height at each, which it had never done before. Moreover, a breeze arose, and suddenly, the trees with which the park is adorned, assumed a sinister aspect. We had hardly flown far enough to worry about them before… Yes, you guessed: a couple of hundred yards away, three magnificent chestnuts were busy coming into flower. So two more things about flying even ‘minimalist’ rubber powered planes were quickly learned. 1. We need a much bigger treeless space to fly in; and 2. You are always advised to somehow render your model visible against the sky – by having brightly coloured wing-tips, tailplane & such. In my defence, I can only say that it had never occurred to me that a battered Denny Dart II could suddenly come into its own in such a fashion & become invisible. O Thou, of Little Faith! We looked for it, but without avail. I’m sure it’s in one of those chestnuts. The second, orthodox (but heavier) Dart was then cautiously tried, but the weight did seem a drawback, and then the wing holdfast soon came un-glued, so we retired home, but still very satisfied at the sight of the brave little Dart II calmly circling as it went away...
6th May 2015. Very windy & rainy. So we just planned a more logical sequence of model trials. The Denny Dart II would be thoroughly tested, and also provided with a small card rudder to help it circle to the left. Also, a scaled up version would be made. There don’t seem to be any scaled up DD IIs on YouTube, and hardly any mention of scaling them up online, though it seems to be an obvious thing to try. We thought that increasing the flying surfaces by 100% would be a good start – that requires all dimensions to be increased by a factor of ~1.4. We also prepared a 7" prop. These came ‘as is’, so a wire axle was made, and it was decided to epoxy the prop assy to the front of the stick on the scaled up version. We also ordered some round-jawed pliers to facilitate making loops in wire.
7th May 2015. A cool day, but wind lighter & OK for tests. We were up early, and started on the scaled up Dart, finishing the airframe & leaving it pinned down to set thoroughly. We’re using aliphatic resin glue by the way. Then to Queens Park. Damp on arrival but very little wind & periods of dead calm. The tests were quickly completed in about 10 minutes. A small card rudder induced the plane to circle left, as desired. Increasing numbers of turns were applied, but it was very reluctant to climb much, though it would circle for 20 seconds or so. This we attribute to the weight of this second Dart, as mentioned already. The all-up weight is 10.7 grams; the first one, that flew away, would only have been about 7.2 grams.
8th May 2015. The weather is set to deteriorate & preclude tests for several days. The large DD II was completed. The wingspan before putting in the dihedral was 17" (43cm). A wire hook was made with our new pliers.
The wire hook is epoxied to a small piece of basswood, the end of the wire being turned up. The block is epoxied to the stick.
The 7" (~18cm) prop is mounted on 1mm piano wire which passes through a 1mm hole in a small piece of copper clad fibreglass printed circuit board - PCB. This is epoxied to the front of the stick. The spacer is simply a couple of tiny bits of PCB. A small aluminium bush could be fitted later. It would be good to contrive a nice light removable nose-block - we forgot to install the two O rings to secure the wing & had to saw the prop mount off to fit them, then re-epoxy it in place. By the way, the prop mount is angled down probably about 4°, which seems to be standard practice.
Here are the two Darts. The small one is heavy at 10.7 grams all-up, while the scaled-up one, DD II-3, which has twice the wing area, is only 14.9 grams all-up – a step in the right direction! Even as rain & wind increasingly threatened, it was just possible to try some test glides on the Big One. These showed that the wing needed to be quite far back. Power was applied, the motor being a single band of 1/8" lubricated rubber. It would fly OK, in quite a straight line, but not climb much. A rudder is needed, and I guess a little more power. It’s now 1630 and the rain has set in, so that’s it for the next few days as regards flight tests!
10th May 2015. Getting up early certainly has its advantages. A small window of light wind occurred this morning, so at 0730 we set off with the large Dart II-3, plus the heavy small Dart II-2-with-a-difference – more on this below. A four-strand 3/32" motor was made up. The results were promising, but an unexpected problem arose. There was heavy dew, and when the plane landed on the grass, the untreated tissue became flabby. In dry air & sunshine, it would have dried out very quickly, but not at this time of day. The model got heavier & heavier. We put the big Dart in the car to dry out, and took out the small, heavy Dart-with-a-difference. The difference was, that we had obtained a ‘Power Up’ module from Amazon, at £10.99.
These are sold as an electrical power unit for attachment to a paper aeroplane, to give them the ability for longer flight. At one end of the carbon fibre stick, the red unit is a ‘super capacitor’ – one that will hold quite a lot of power at a low voltage, (typically 2 or 3 V), and slowly release it into a suitable load – such as the tiny motor at the other end of the stick. The device is 8" (20cm) long and weighs only 6.5 grams. The capacitor has a clip to fasten it to the nose of a paper aeroplane, so the motor is at the rear and has a pusher prop. To save weight, there is no switch, nor is one necessary. Three AA cells are contained in the plastic box, which has contacts to transfer power into the capacitor. The recommended maximum charging period is 20 seconds. The prop begins to rotate immediately, and gradually builds up revs during the charge. There is also a discharging contact on the battery box; in case of a failed launch, the motor can be reset to zero or just a few rpm, & the charging begun again. There are many YouTube videos of these in action, mostly with paper planes, but relatively few so far, with them attached to small balsa models. We crudely attached a Power Up to our small, heavy Dart II-2 with a rubber band, and to our delight there was ample power to fly it. Of course, it got wet on the grass & heavier still, but electric power must be investigated as soon as possible. 8^)
Electric Denny Dart II nicely dried out after its soaking!
A nice new light, waterproof Dart was required. It occurred to us to cover it with plastic, many sorts being available. It further occurred to us that Clingfilm would probably do. Another airframe was knocked up.
The uncoated wing weighed 0.8 g. Had we coated it in thick tissue, that would have got it up to 1.2 g. With Clingfilm it was only 1 g. The tailplane & fin added 0.4 g. So the total weight of the flying surfaces was 1.4 g. Taking advantage of this low figure, it was decided to cut a 12" long 1/8" square piece of basswood for the stick. This is at least twice as dense as balsa, but even though it weighed 1.2 g, the total covered airframe would only be 2.6 g, and the stick would be very strong.
Mounting the Power Up on the stick. This is the underside of the capacitor. It incorporates a plastic clip to fit over the nose of a paper plane, which is made broad for that purpose. To align the carbon rod along the axis of the plane, a vertical tab is provided. This goes down into the centre fold of the paper plane. There was no obvious way of using these, so they were cut off with a razor saw.
13th May 2015. Here is the new DD II. To sort out the balance, test glides on the plane allowed us to get the right wing position. Happily, the centres of gravity of the dart and power module coincided very well – as I suppose, one might expect. A balsa wedge raises the arm so that the prop clears the stick. The capacitor is held on with a rubber band. Primitive, but if the plane nose-dives into the deck, the module should simply come off, minimising damage to the airframe. It is almost impossible to fly a model plane & video it at the same time, but we could not resist trying, as the weather was so calm today. The video is awful, but the plane is very promising!
The work is ongoing; some separate ‘Super Capacitors’ have been obtained. They are not expensive. We also await anxiously a batch of tiny electric motors from Hong Kong, which we are pretty sure are the same as those in the ‘Power Up’ module. Those are very cheap, and, being DC motors, may be made to rotate in whichever direction one chooses. We have nothing against the pusher system; indeed, it is commendable simply because it is so rare these days. More later!
13th May 2015. It’s raining today, so we just increased the visibility of the plane with a very light coat of cellulose car spray paint on the clingfilm. This actually tightened up the coating, but without warping the wing. As a further bonus, the added weight is almost undetectable on our scales, which read down to 0.1 g.
14th May 2015. Windy & cold. On the basis of the above, a new large Denny Dart II was started, to be coated with clingfilm.
Large (17" span) Denny Dart II flying with the wing backwards – oops!
15th May 2015. At last a calm morning. We have found a much more suitable site to test these models: grass, no trees & ample space for test flights. The small electric red Dart and the large DD II were taken there. A small rudder of card had been fitted to the rad Dart II, and we were unprepared for how powerful its action was, what with the pusher prop only about 3" in front of it, so there were a couple of steep left-hand dives into the deck, the second of which broke off the tail fin, so that was that. Attention was turned to the big Dart II-3. Above you see it flying quite well. We tried some video but it was no good, so just grabbed the above still. It is indeed a tribute to the design of Neil Dennis, as not too many aircraft – even model ones – will fly with their wing on backwards – we hadn’t noticed our error in fitting it.
But the stick of the above Dart was now bendy & floppy, so construction of the new DD II-5 was pushed ahead.
Wing, tail & fin, covered & sprayed red: 6.1 g
Stick, 1/8 x 3/8 basswood for rigidity: 3.6 g
8" prop: 4.8 g
Four-strand motor: 2.5 g
Prop mount & wire: 0.6 g
Total weight: 17.6 g
H’mm. The original scaled-up Dart II was 14.9 g. The next one will be a tad heavier, but the old Dart had undoped heavyweight tissue. There should be more lift from clingfilm. Let’s go for it!
(Memo. Since clingfilm is so light, let us cover the underside of the next Dart wing as well as the upperside, and also fit ribs with an aerofoil section! Lift is, to a large extent, the name of the game?)
16th May 2015. Blast! The Weather Forecast is predicting winds of 10 to 16 mph for the next 10 days. So we took the repaired electric Dart II to the park with the trees, to see if there were any quiet spots in their lee. To our delight, this was the case, and by limiting the charge time to 10 or 12 seconds, the model flew very well, and was in no danger of ascending high enough to get blown away, or climb into a tree. So that’s looking pretty good; we just need some calm weather to extend the flight time. Then we can try to apply electricity to the 17" span Dart.
17th May 2015. Too windy. Spent a lot of time on-line looking for small electric motors to power the large Dart; the ones from Hong Kong have not yet arrived, and it is unlikely they will be strong enough. Eventually, a thought took shape: what about a further scaling up of the Denny Dart – say to 24" (61 cm)? Why not? Add a few more ribs with a proper aerofoil section; cover the wings (& tail) with clingfilm on both sides; retain the small but rigid stick? There would surely be quite a bit of lift? The project can occupy us during this windy spell...
19th May 2015. Still windy, plus rain yesterday & more forecast this afternoon. Yesterday we began the wing of the new Giant Denny Dart – by far the most difficult part of it, of course. Wingspan to be 24". What aerofoil section to use? Much reading done on-line. There seems to be a general consensus that of the vast number of them, just a few – perhaps 15 or 20 – will suffice for most types of model aircraft. One obvious candidate was the ‘Clark Y’ section, which has been a general workhorse for 90 years. It was noted also, that for slow flying, which is what we want, the thicker the wing, the better. The basic Clark Y thickness is 11.7% of the length. We jacked that up to 17%. If it doesn’t work, we can always start again!
Photoshop makes that sort of re-sizing very easy. We also remembered a dodge for transferring a simple design. Print it out on a laser printer onto glossy paper, then iron it onto the balsa. It worked well enough. The ordinary printing ink on the paper does not come over, only the laser toner.
Cutting out ribs with a scalpel has to be done carefully for obvious reasons. We only need seven, but they’ve all come out slightly different. The usual method is to pin them together for sanding until they are uniform, which worked OK.
The centre section is the easiest part…
…while jigging the tips into the right place is fiddly. A scaled-down rib helps to strengthen the tip.
By mid-day today we had got the wing done, apart from two short extensions to the bottom spar, and a small gusset inside each tip. It weighs 11.3 g.
And oh yes, the ten small motors arrived from Hong Kong. I must have misread the eBay advert; I thought they were about 8 millimetres in diameter. No, they’re 8 millimetres long! So unless I go mad & decide to make a tiny version of the Bristol Brabazon (which will definitely not happen) they’re not much use for the current project. I connected one to 3 cells wch were down to 3.9V, and it went weeeeee!!
24" span Giant Denny Dart II ready for test glides & perhaps even a little power.
Giant Dart underside, showing motor taped on & capacitor below wing.
23rd May 2015. At last a calm day. However, we had not been idle in the meantime. The wing had been covered with clingfilm – rather clumsily, but OK for a trial. Also, a motor had been obtained. This was not terribly expensive, but we had bought it on spec, so were not too disappointed when it turned out to be far too large and heavy at 46 g. It is a twin-motor and gearbox combination. Gearboxes are good, we had learned, because these small electric motors rotate at very high speeds. This is OK for a small, fine pitch prop, but not for a big prop – they don’t have the power for that. Adding a gearbox means more weight, but this is eclipsed by the extra thrust available from a large prop with a relatively coarse pitch, rotating slower. It was decided to break up this double motor, because we could take off one motor and try it by itself, and also test the other one still attached to the gearing.
Having done this, we then tested them, from 3 to 5 Volts, both direct from cells, and also from a charged-up 5 Farad, 5 Volt capacitor. They were both given 5" props. The motor alone was fitted with a fine pitch one, and when driven with 4.5 Volts, produced so much draught that papers six feet away were blown off the bench onto the floor! However, the charged capacitor would only run this motor for a few seconds. That would do for a test though. The motor remaining on the gearbox was fitted with a coarse pitch, and totalled 34 g – quite a bit. This produced less thrust, but would run for much longer on one charge of the capacitor.
The most urgent thing, though, was to test our ‘invented’ wing, by test glides. So we simply used a round stick of balsa, on which was mounted a standard Denny Dart tail, but without any elevator incidence. We were compelled to fit the single motor, in order to balance the model, if nothing else. The centre of gravity of most models is supposed to be a quarter or perhaps a third of the wing chord back from the leading edge. The new Giant Dart balanced easily, so we even fitted the capacitor. To our extreme gratification, the first test glide was excellent. Moving the wing forward & back did not produce any significant improvement. For once, we evidently got something right first time, even if by luck rather than judgement. We even dared to apply some power, about 3 Volts from some rather tired cells, and at last the long-sought result was obtained: increasing the amount of charge simply extended the glide, making it longer & flatter. This is what we always thought things should be like. On this basis, we returned home and fixed the wing more rigidly on the stick. This increased the all-up weight to 54 g. Three freshly charged AA NiMH cells were put in the charger. Back at the park, it was almost an anti-climax. In near-calm, the Dart flew as straight as a die (we must have put the fin on squarely; whatever next…) for two or three seconds, then descended in a nice flat glide as the power waned. After six or eight identical flights there was simply no point in continuing the tests, gratifying as they were. Back home again to fit the other motor, and a rudder. This was done quite roughly, as it is to rain tomorrow & the wind will rise again. To cut a long story short, a 7" prop was fitted to the geared motor, and as a result of the metal & tiny roller bearings in the gearbox &c., the new weight of the Giant Dart rose to a whopping 76 g. We doubted whether it would fly at all, but it did, almost exactly duplicating its earlier performance with the motor and the slim prop in the image above. In other words, we had more power, but it was negated by the increase in weight; so no advantage accrued. Indeed, it was worse, because our Design Specification 8^) for this project is a very light, very slow-flying plane. The 76 g Dart flew appreciably faster, so was a retrograde step.
What we need to do, is to go back to basics and use a small, high-revving motor & a small fine prop, just like the ‘Power Up’ module. Only on a rather larger scale. There will be more sustained power then. The dodge will be, to control it so that the power run is not too long & so allow the model to fly away.
Scroll courtesy cliparts.co (q.v.)
26th August 2015. Sorry; there has been a gap since May. The reason is very simple, and was indeed foretold by the very last sentence above:
The dodge will be, to control it so that the power run is not too long & so allow the model to fly away.
Yes; the dodge turned out, would you believe, to be Radio Control. And for the last three months, like Toad of Toad Hall, this new Craze has consumed us. We ought to have seen it coming; the connection between simple model aircraft and my life-long interest in the history of radio, amateur radio &c. was, in retrospect, inevitable. We have been so busy that there didn’t seem time to write it up – so just a brief summary will be given…
Needless to say, a very great deal has changed since my first attempt at radio control planes, which was in the late 1960s & early 1970s! Even single-channel control was very expensive, and I must have gone without food & other minor necessities to purchase the Staveley single channel transmitter and receiver. An Elmic rubber-driven escapement was used, to give left or right rudder. There was a little handle let into the side of the plane which allowed you to wind up the rubber to power the escapement. If the rubber ran down, it stopped working; but equally, if you wound it up too tight, the over-tension might prevent the armature from clicking round. To cut a long story short, we spent ages very carefully building a Veron Deacon, and installed the gear. The engine was an E.D. ‘Bee’ 1cc diesel, (second-hand of course) which was quite potent for a very light model with plenty of lift like the Deacon. After checking that everything worked, including a range check for the radio, off it went. We clicked the button on the transmitter, and the model duly turned down-wind. We clicked the button again, and nothing happened. The plane continued straight as a die, downwind, until it became a speck in the distance. Of course, we hadn’t painted it to save weight, so it was just doped white tissue & became invisible against the sky much sooner that it would have had it been black, or red. We hastened in the direction of flight for half a mile or so, to no avail. We hadn’t even put our name and address on it! It must have been a month’s wages that flew away… >8^( I need hardly add that such events were quite commonplace in those days; a simple little mistake or omission in building or assembly, and something would drop off, or jam the rudder.
Forward to 2015. This time it would be different! YouTube was ransacked for simple radio-control models. They are there in abundance, but we thought we would first carry on with our plan of motorising the Denny Dart II. We got a middle-of-the road Spektrum transmitter, and a micro receiver with two ultra-micro servos. The result pleased us no end, until we tried to fly it.
To be continued…