It has been a long time since the last Inside Story edition and for that I'm afraid I am to blame. However, we have a good issue here with plenty of interesting bits and there is a lot going on in micro RC right now. It is certainly a good time to be alive and more so if you are a modeller. Before introducing the article fully let me remind you of the excellent E Zone micro forum, it is THE place to be on the web for micros and is frequented by many excellent modellers. These include Aerovirenment's Matt Keennon, the creator of the Pixel Helicopter Alexander Van de Rostyne, the leader of the Aeronutz Mark Denham and many other talented modellers you may of heard of. On top of the "la de da" famous types we have many more modellers on-line each with their own set of skills and interests. So, if you are new to micros check it out. Make sure you peruse the thread index first and be sure to remember the search facility, this helps avoid repetition and keeps the conversation fresh and interesting. Gordon Johnson and myself moderate the forum and we do are best to provide guidance and useful resources to the users but it is they who make it so good. Thanks guys!
Here are a few recent topics that have been favourites on the forum:
This thread was started by Bob Bailey recently and describes his excellent dual muscle wire actuator. Muscle wire is a nickel titanium alloy that reduces in length when heated. This can be done by passing a current through it giving contraction proportional to the current. Surprisingly although the change in length is small, the force is large. The next problem is that muscle wire doesn't return to it's origional length very quickly and this hysterisis can cause the control surface to not return to centre. Add to this the fact that the wire used is only 25 microns thick (half a human hair or 0.025mm) then producing a working actuator is "quite" difficult.
Bob's excellent design uses PCB, etched brass parts and comes to a total weight of 0.1g. This is for two channels of actuation and it IS proportional! He solves the problem of centering by spring loading the wire and by careful design of the crank geometry. Matt Keennon has tested the actuator and also presents his results in the thread.
An enormous thread all about some of the guys who have made their own infrared control systems. They are fully proportional 3 channel systems mainly including speed control and two actuator drives. Far too much detail to list here but if you are interested then check it out. Everything you need is described and there are plenty willing to help.
Chris has kindly drawn up some of his models as plans, in fact not plans but full construction articles. They are quite old threads now but I think they are worth mentioning as the models and plans are so nice and many may not of heard of them. The first model is called the Micromoth and the second is the MicroPitts, based on Tiger Moth and Pitts Special respectively.
Thought I should share this one with you, I have a digital camera and although it is quite a good one with fully manual modes and hot shoes (whatever they are) and a fair few zillion pixels I have never got a decent in-flight photo, Ever! So to buck the trend amongst about 20 blurred shots came this one, a picture of Ray holiday's Swordfish dropping a bluefoam torpeado. The release mechanism is similar to the actuator described in Peter Frostick's mini article this month and simply pulls a pin out of a loop supporting the torpedo. I can't over emphasise the luck in getting this photo. I clicked as Ray gathered the spit to say now and the annoying camera delay did the rest. These pics were taken at a recent Nottingham meeting. Ray's model uses a cut down GWS RX and pulse converter (built by Moi) driving some homemade actuators.
Oh yes, the column. We have a nice mix this month, firstly, Gordon Johnson presents his quick junior model. A three-channel sports model powered by a 6mm pager motor. All up weight 15g. An excellent introduction to pager motor powered models. He also describes some handy hints on finishing and constructing the model. I have expanded his suppliers list to help folks around the world to build one if they feel that way inclined. You can of course often subsititute materials when building micro models and it is to be encouraged if it gets you in the air. Plus, with such small models the buildng costs are normally tiny if you get it wrong.
Next, a real melting pot article by Peter Frostick. Peter is an excellent modeller of both free flight and RC models. His Mustang that was featured in the last issue in his mini on hot-wire cutting won the Pistachio class at this years UK indoor freeflight nationals. In this article, he delivers some of his wisdom on trimming small models before giving helpful notes on actuator use. He finishes with a case study of a recent model; a 9" profile Fike controlled using the radio gear from a tiny RC car. I saw this model flying recently and the lack of proportional control was only evident by the expression on Peter's face and the machine gun sound emanating from his thumb.
I have also managed a mini. I describe a simple machine to make really light and flexible wire, which is very suitable for actuator and even some power hook-ups.
Finally a nice little mini article from newcomer Robert Throssell. He shows how he got himself out of a sticky situation (prepare to sigh) by making some non-magnetic tweezers.
Again, if you want to see "The inside Story" out on a more regular basis we need content. Please get in touch. Mini articles especially are easy to do and of course, I am keen to hear about your models and projects.
By Gordon Johnson
After a long wait for my sample E-Tec 90mAh cell, I finally had one. However, I needed a plane to test it in. I have a series of planes I call “Quick” planes because they are quick to build and quick to repair. I had a set of tracings for Ralph Bradley’s Homeboy pusher plane lying on my workbench. But, I knew from my static tests that it would not do for a geared pager motor propulsion set as the prop needed would to be too large in diameter for the clearance between the motor pylon and the fuselage. I liked the outlines of the flying surfaces in Ralph’s design. So, I started cutting based on those outlines and the high wing tractor configuration Quick Junior was born. Thanks Ralph! Also thanks to Billy Stiltner for drawing up these plans.
What I like about this plane is that a 15g plane is possible without resorting to extreme equipment. If you have an RFFS-100 or JMP Combo receiver and actuators already, the cost of making this plane is probably about $35 and a lot of that is the CF tubing for the fuselage.
Almost everything needed was obtained from Bob Selman Designs except the CF tubing and rod that was bought from from Dave Lewis. Here follows a list of materials and possible suppliers:
A suitable RX:
*If you have Dynamics Unlimited actuators in another plane mounted BIRD style in the tail you can upgrade them to remote actuators with Bob’s housings
You will also need some 2mm Depron or some 1/32 and 1/16 contest grade balsa.
For a propeller I used a carbon fibre unit I molded myself, see Inside Story for details. A kit to make your own CF props is available from Bob Selman. Alternatively, you can use a heavier plastic prop. To aid in the selection of materials and the building process I’ve included a detailed weight breakdown.
What if you don't want to use the lightest equipment? I’ve flown the Quick Junior with the heavier Kokam 145 cell and it was fine. Similarly if you wanted to use a light balsa fuselage stick instead of carbon fiber that would also work. But, every time you use a heavier component, the flying speed will increase. Since the wing loading as built is about 1.8 oz./sq. ft., a number of heavier components could be substituted and the plane would still have a very low wing loading.
Why did I use the wire cut Gary Jones undercambered foam wings from Bob Selman (also available from DWE)? I wanted good lift with relatively low drag. The picture below shows that these wings have a nice undercambered airfoil, which should provide more lift than a curved flat plate or an undercambered stick and tissue wing with covering on the top. But, the wire cut wings I used will also have less drag than the other two wing alternatives, especially the stick and tissue wing. This means the pager motor won’t have to work as hard to pull the plane through the air. I can’t quantify these effects but, the plane weighs 15g and a propulsion set that generates 11g static thrust at full throttle will pull this plane around very nicely at about 60% throttle. However, the wire cut wings are slightly heavier due to the material in the middle that gives the airfoil its camber. There are tradeoffs with anything.
Trim about 1/8 inch off the trailing edge of the wings with a brand new knife blade and a straight edge. Then, trim the wings to the shape on the plans. Sand the wing tips rounded and the LE rounded. It’s also a good idea to lightly sand the wing with a very fine grade sand paper in a sanding block just to smooth it out. Sand the dihedral joint at an angle by angling your sanding block as the wing root is extended to the edge of your workbench. Keep sanding one wing and the other until you get a perfect fit when one wing is blocked up 2 inches as shown in the plan. Glue the wings together with foam safe CA.
I like some color on my planes if at all possible. I painted the wings by dusting them with Krylon “Short Cuts” enamel paint, available in most hardware stores. Using the pattern shown on the plan, cut out the scalloped shape on two sheets of printer paper. Tape together so you have one complete set for the LE color, and another for the main color, as in the picture above. Spray one side of the one that will cover the LE with 3M photo mount adhesive. Let it air dry for a minute or two, and then place it carefully over the LE region of the wing. If you don’t get it right, it peels off easily and can be repositioned. Measure forward from the TE to make sure it is positioned straight. Dust on the color for the main part of the wing. I let mine fade to the color of the wing towards the TE to save weight. After this coat of paint is dry, peel off the paper template. Spray the template that will cover the main body of the wing and after letting it air dry slightly; position it over the main part of the wing, and slightly back from the paint line (so the LE color will overlap the main color slightly and the wing color won’t show through). Dust on the LE color, which can be marginally heavier since there is less area to be covered. Note that for the entire wing and tail the paint added just 0.12g to my plane. To achieve this you must “dust” the paint on.
Cut out the wing pylon from 2mm thick Depron foam. Sand it on the top until it is a perfect fit with the foam wings at the dihedral joint. If you don’t have foam, use contest grade balsa about 2mm thick. Using a sharpened brass 3/8 inch tube, bore two holes as shown on the plans for the BSD MiniAct with housings. Cut out two small square notches as shown on the plan. At the front edge of each hole to clear 1/16x1/32 wonder magnets for centering. Cut a length of 1.3mm carbon fiber tubing 9.5 inches long. If you’ve never used this CF tube from Dave Lewis or Westechnik, you are in for a treat. It is simply the lightest CF tube available anywhere. It’s hand made by a Dr Elder in Germany. If you don’t have the CF tubing, use a light balsa stick. Compare the weight of your stick to that of the CF tube as given in the weight breakdown. Try to match the CF tube weight by tapering and sanding your balsa stick fuselage. Place the fuselage stick down on wax paper. Apply thick foam-safe CA to the bottom edge of the pylon, and glue it to the fuselage one inch from the end of the tube as shown on the plans.
When I built the Quick Junior, I weighed the CF tube fuselage and Depron wing pylon. Then, I made a simple 2mm Depron profile fuselage and found that it weighed about the same. I went with the stick fuselage to make changing motors and gearboxes easy for testing purposes. However, an alternative would be to simply make a profile fuselage from Depron for about the same weight.
Cut the tail out of either 2mm Depron or 1/32 inch contest-grade balsa. For the Depron I sanded it down to 1mm thick, which makes the weight equal that of 1/32 contest balsa. I used Depron for two reasons. First, it does not warp from humidity. Second, when applying only a dusting of paint it will cover easier than brown balsa. I didn’t want a brown tail to go with my nicely painted wings. So, I sanded. No aerodynamic counter balancing is required for these size surfaces when the BSD MiniActs are used. Paint using the same technique as for the wings.
Bevel the elevator hinge lines with a sanding block so the beveled sides face down as shown on the plan. Do the same for the rudder so the bevel faces to the left side of the plane. Lay a piece of Scotch tape down on wax paper and cut four strips about ¾ inch long and 1/8 inch wide for hinges. Hinge both surfaces on the side with no bevel. Cut 1/64 light ply control horns as shown on the plans, cut a small slit in the control surface and insert the horns and angle forward until the hole is directly over the hinge. Wick some thick CA into the joint with a pin.
If you have any Lego blocks, it makes getting your tail aligned and glued in place much easier. Stack some scrap sheets of balsa on both sides of the fuselage stick untill they are the thickness of the stick. Next, make two stacks of Lego blocks about 1.5 inches high, where the bottom layer is set back to not touch the fuselage pylon. These will allow you to keep the pylon perfectly upright and when the elevator is glued to the tail end of the fuselage stick, it will be perfectly perpendicular to the pylon because of the shims on either side supporting it to the fuselage stick thickness. After the elevator glue has dried use the two stacks of Lego blocks to hold the rudder perpendicular to the elevator and glue it in place. The only drawback of this method is explaining to your kids why you need to permanently keep some of their Lego in your workshop.
The elevator shown in the pictures has the elevator on only the left side. In the plans it is drawn going all the way across. Either will work. The partial elevator is simpler, but the full elevator will give a bit more authority.
The wing installation is simple, but should be done before the landing gear. Verify that the top of the wing pylon mates perfectly with the wing. With the elevator of the plane still shimmed to the thickness of the fuselage stick (a couple of weights on the elevator are a good idea) use some sort of blocks to block up the wing tips on each side till you are satisfied it is level. Apply glue to the pylon and glue the wing in place.
Landing gear is dead weight, but I like rolling take offs and landings and usually include them on my planes. So, I keep its weight to a minimum. The Quick Junior only weighs 15g, and the landing gear does not need to be very strong. Use a very fine piano wire for the axles at the bottom of the 0.5mm CF rod. Use only about ¼ inch overlap between the wire and the CF rod at the bottom to reduce weight. After making the bent wire pieces for the bottom, lay the rod and wire down on wax paper and sparingly apply thick CA with a pin. It doesn’t take much in the joint where the two meet to make a joint more than strong enough for a plane this light. No wrapping of the joint is needed. Use a couple of pieces of tape to tape the axles with a slight toe-in on your building board. A model vice is useful for supporting the fuselage while you get the top of the CF rods to overlap in an upside down V on the fuselage tube. Apply a small amount of thick CA with a pin at the joint. After it is dry use a single “X” of Kevlar thread or fine un-waxed dental floss two wrap the landing gear/fuselage joint, and then sparingly apply thin CA.
For the wheels cut out some pieces of 1/32 contest balsa sufficient to make 5/8-inch diameter wheels after turning down on a Dremel tool, glue the two sheets cross-grain to each other with thick CA (Super Phatic glue is better if you have it since it is lighter). You need to make a holder for your wheels in the Dremel. I use a couple of 1/8 ply washers with a CF rod glued in one of them. I sandwich both sets of wheel blanks between these washers, chuck it up in my Dremel, and sand them round with a sanding block. For axles, either small id aluminum or plastic tubing works well. For wheel keepers tiny wood blocks work well. The wheels you see in the pictures are laser cut for me by my friend Carl Martin.
For the propulsion set I used the new Didel 6x12mm 4.5 ohm pager motor, geared 6.7:1 (60/9t) and a 4.8x3.1 CF prop. This motor sees to have just hot enough a wind to give more thrust than the 10 ohm pagers, but not so much that it gets hot and burns out. Only time will tell how long the motor will last as used here. But, I have about an hour on mine and Billy Stiltner has two hours on his. The price is low enough that it can be replaced periodically if it is not too long lived. I used a gearbox Carl Martin laser cut for me. You could make your own gearbox or use a Didel gearbox or one of the new Falcon gearboxes, which come with a 6mm 10 ohm pager motor. The Falcon gearbox is available from Falcon and in the US from Bob Selman Designs.
Installing the 9t pinion on the 4.5 ohm pager requires some care. You may need to ream out the bore in the pinion using a tapered reamer. Keep reaming the pinion and testing repeatedly. If it is too hard to push on even half way, remove and ream a little more. When pressing the pinion on you must support the back bell. The edge of a stick about ¼ inch thick will fit between the wires on the motor and support the thin plastic bell. If you simply hold the motor in your fingers and press the pinion on you will likely cause the motor shaft to break through the back bell and ruin the motor.
Slip the actuators through the holes in the wing pylon. Both push rods are installed on the right side of the fuselage. I apply a bit of CA where the flange around the coil sticks out a bit so it will be glued to the pylon. Make two push rods using fine ~0.27mm diameter brass wire Z-bends. Brass is used to avoid interfering with the actuator. Glue them one each on the ends of the CF push rods using the same technique as for the landing gear. Make two ~0.38mm diameter piano wire Z-bends for the tail ends of the push rods. Sand the long end pointed to aid in inserting it in shrink tubing. Your push rods should be cut slightly long and will be trimmed later. Use the smallest shrink tubing you can get (the blue color from Dubro is available in many hobby shops and will work fine). Slide a length of shrink about 3/8 inch long over the piano wire Z-bend and CF push rod, and then shrink it with your heat gun (not too much, just till it is snug). Now work the Z-bend in and out a couple of times until is just snug, but will move without too much force for adjustments. Insert the brass Z-bend into the actuator horn, and lay the other end against the tail control horn, which has the piano wire Z-bend in place. Measure where it needs to be cut so it overlaps the Z-bend and allows for some adjustment. After cutting the CF rod, sharpen it with sand paper to make it easier to insert into the heat shrink.
With the plane held securely in a model vice by the fuselage, use two needle nose pliers to hold the Z-bend at the tail and insert into the heat shrink at on the CF rod. Adjust until it looks about right. Now put one of the tiny 1/16x1/32 Wonder magnets on the outside of the actuator coil. You can use a toothpick to move the Wonder magnet back and forth untll the actuator control horn is exactly in the center. Go back and adjust the Z-bend at the tail until the control surface is neutral. For the elevator this might not be possible. But, by pushing the Wonder magnet to one side on the coil, you can get the elevator to droop only a minimal amount. Use a toothpick to apply some CA or other glue to affix the magnet on the outside of the coil.
The receiver is simply attached with double-sided foam tape. The ET-90 cell can be attached with a small strip of Velcro, double sided tape, magnets, or the method of choice. For an antenna I used magnet wire like used in actuators to keep the weight down and save nearly half a gram, depending on what antenna would have been used.
The Quick Junior is a very stable flyer. It will fly at about 60% throttle. But, at full throttle will climb aggressively towards the ceiling. It can sort of prop hang for short moments. I can generally get about 20 minutes of flying time from one ET-90 cell. I use a very light voltage monitor from Dave Lewis to insure I don’t discharge the cell too far. Best of all, in addition to getting into the almost half-ounce range, the plane is fun to fly.
You can download the plan in either jpg or Pdf format. If printed from the Pdf version it should appear at the correct size whether printed onto A4 or Letter. The plan includes a scale so photocopying or adjusting the scale when printing is another option for the jpg version.
Quick Junior Plan
|PDF Version (991KB)|
|Quick Junior Plan|
By Peter Frostick
Many fliers seem to find little low wing planes a box of tricks to trim(!) so here are a few basic tweaks learned over the years; some the hard way!, but most by asking some very talented people a lot of questions. Flying with a zero-zero incidence set up for indoor RC is possible, but for those like me preferring a quieter life, a general trim based on free flight suits most folk a lot better! Typically, the Mustang pictured can be best tamed by treating it as a flying wing! This involves a CG set well forward and plenty of incidence, or angular difference between wing and tail. Long nosed subjects are very sensitive to thrust angles, so please make thrustline adjustments a little bit at a time.
Something all low wingers love to do is tip-stall, therefore some "washout" or progressive raising of the trailing edge in both wing panels is essential, and if your wing is made from foam a touch of leading edge downward "droop" bent in near the tips works wonders: This is a Mark Drela detail from his "Upstart" indoor chuckie plan instructions of 1980s vintage, and has cured several of my previously "unflyables" of their wicked ways! Motor/prop torque is another problem to be addressed, and slightly more washout on the right wing is pretty effective. As indoor RC flights are always made under power this seems not to affect general handling at all. Higher revving direct drive or geared props are far less bother than big slow turners regarding torque, but they generate more gyroscopic reaction: this is a very good reason to keep prop weights down to the practical minimum. There's one big item we haven't covered yet? ---- elevators!! I never use a down elevator function, but have the control surface spring loaded downwards onto a neutral stop; this allows it to be shimmed for neutral trim accurately. With the usual undercambered curved plate wing section indoor flyers use, a little up elevator movement goes an awfully long way!!; for a 13"span model a 4mm movement at the trailing edge is generous. Remote actuators do seem the best way to go for this task, as you can multiply the torque with different control horn lengths, and this coupled with firm return springing of the control surface produces really good servo-like proportional control. Increasing the scale model's dihedral will help coordinate rudder turns, but don't use too much as it looks terrible and makes the plane too stable to fly realistically. The settings described are not "set in stone" but I hope this advice will make starting the complex trimming task a bit easier for you ---- good luck.
All this micro RC technology has come upon us very quickly, but one of the main problems to be seen at indoor meetings is a lack of understanding of actuator limitations--- The newer tiny units now required are super light and not too expensive, but they are not feedback servos, and cannot really be used as a trimmable function, for the simple reason they don't know where neutral is!! This matters little when the model has a large fixed fin area like a vintage style power model; but if it's a war bird with a rudder 50% of fin area we're in deep trouble because a rudder with no sense of its own neutral position takes the line of least resistance --- if the plane stalls and spirals to the left, the easy going old rudder is more than happy to follow! Several solutions have been found to provide a "neutral groove", among them highly effective mutual magnetic centring of two closely spaced actuators, as in the Micromag and Bob Selman paired units: also popular is the pivoting of control surfaces with flight rubber strips: excellent for moderate sized rudders. If however you have opted for faster, smaller, and more taxing planes the neutral groove needs to be precise or else all those fancy trimming twists and warps will have no effect. My approach is to centre the rudder with a "hairpin" spring of steel wire which achieves the centre detent "groove" in much the same way as a transmitter stick return spring. In smaller models, a 0.010" top E steel guitar string works fine, but needs a bit of experimental bending to deliver the goods. Coupled to this a fairly linear output from actuator coil/magnet helps no end: this is where it gets tricky! Long thin magnets within a short actuator coil give the highest Small angle torque, but this quickly dies away at larger angles, thus is no match for the progressively increasing tension of the spring. My favourite is a very linear magnetic setup designed by old friend and techno-wizard Eric Hook, who came up with "Tabmag"; so named because the magnet was shaped lik an aspirin tablet --- the drawing says it all really, showing a flattish disc magnet within quite a long coil.
This magnet proportion seems to produce a big "fluffy" and loosely focused field which grabs all the opposing magnetic force going: the mechanical linkage for remote use is very light, verging on crude!, but a tad fiddly for some modellers; using a 4mm x 3mm Neodymium magnet, this unit equals the Didel Millibird's torque and weight figures. This magnet/coil set up can be used in a B.I.R.D. configuration at a good weight saving. If your pockets are deep of course, Didel's ready built offerings are a total delight; so just enjoy!
Like so many flyers, I'd been amazed at the diminutive size of Nick Leichty's RC models seen recently on some websites, so personal ambitions started to turn to the "very small. Realising that I could never achieve flying models at these tiny sizes quickly, a "small but possible" option was needed. This spark of interest was soon fuelled by the arrival of much lighter, but high current draw LiPoly cells from the usual suppliers (bless'em all) --- the new 90Mah, 2.3 gram version looked perfect!
The general plan was to scale down the "Fike E" 13" span Peanut scale monoplane kit from "Micro X", to nine inches span: For those of you unfamiliar with this aircraft, it has a very wide wing chord measuring one third of the span -- a nice way to get a lot of wing area into a small model plane. I should also add that the Peanut scale model built from the kit is a very good indoor radio flyer in its own right ---- The wood selection is good, likewise the tissue: and a real bonus is that the rigging angles and CG position shown on the plan are about right too. This scale plane looks a bit odd at first glance, but flies really well with zero dihedral and is very stable with rudder control: also its stall manners are about the best you'll ever meet!!
My usual (chicken hearted!) approach to indoor free flight scale models is to build a foam flying test model first, so this was also done for the proposed radio model. For foam builders there is just one drawback to the Fike and its similarly proportioned stable mate the Lacey M10 --- neither will fly with simple curved plate wing sections: a flat bottomed aerofoils are the key to success! so the question was how to build it light enough? I tried doing the calculations to work out the weight of a solid wire-cut blue/yellow foam wing of scale thickness; this looked unpromising, so a double skinned wing with vertical balsa spar webs might be the best way to go ----- quite wrong as it later turned out!!
Motor, prop and RC weights were the next items to be assessed, and a brief listing of major items produced the following.
The last item seemed heavy!! but my knowledge of pager motors was pretty hazy, also the Techmax was a "known quantity" with a suitable power draw for the tiny cell; also plenty of thrust ---- might just have to fly a bit faster!
Even with carefully wire cut foam wing skins only 1.0 mm thick it was soon apparent that the weight saving over a solid wing was going to be marginal at this very small scale, so a thinner section solid version was tried; this came out at 1.7 grams for 27 square inches area, and felt reassuringly durable ---- after cutting some 4 and 1.5 mm foam sheet for the profile fuselage and tail feathers it was time to glue it all together.
The inbuilt trim was set to a well tried standard free flight formula adapted for simple rudder/motor controlled indoor radio models, and in this case comprised a 28% chord CG, zero wing incidence; five degrees negative on the stabilizer, also six degrees down thrust and three right ---- note that the wing on the Fike monoplane is set at zero to the top longeron line; quite an unusual approach, and the aircraft flies relatively nose-high. When all the foam pieces were assembled the total weight was 3.75 grams, including a light but tough undercarriage; we were in business!
Now, the original idea was to use a very light remote actuator for the rudder, but the CG correction needed with that heavy motor up front soon knocked that notion on the head! so a 0.5 gram 50 ohm B.I.R.D. was installed in the rudder hinge line with a sub-min series resistor to cut the current draw to sensible proportions. The next quite major mistake was made by installing a well tried "BitCharger" micro racer car radio conversion --- these are lovely little things for virtually no money, but proportional they're not! therefore a far from ideal choice for fast and sensitive models!!! On the great day of testing the diminutive aircraft whizzed up to take off speed very quickly and left the floor, to reveal an unimaginable excess of rudder reaction!! ---- The very twitchy little beast was soon tamed by reducing rudder movement to a scant 3mm either way, and twisting in about 1.5 degrees of right aileron warp (trailing edge higher) to the right wing to counter the motor torque: in this mode I reckoned it flew rather well, but without an up elevator function needed a lot of flying site space to manoeuvre reliably: not to mention "machine gun" speed thumb blipping of the mentally confusing transmitter buttons by the perspiring pilot! I do recommend that you always build simple test model of any project; even if it's only a basic profile "chuckie," at least you'll learn quickly where to put the CG and what rigging angles might work!.
Since the tiny Fike's rather traumatic transition to controlled flight, I've come to appreciate its virtues a lot more, and have even re-learnt the joys of primitive control systems!!. Those of you who can remember the late Ken Willard's wild little "Gasser" single channel pylon racer from 1960 will instantly identify with the "pure adrenalin" flying style required!! These super small RC craft now seem a quite reachable goal, given a degree of finesse in the building stage: anyway it's so nice to build something you can't buy ready made!!!
By Graham Stabler
When I started in micro RC proper (about 3 years ago) one of the very first things I did was to learn how to make my own actuator coils. I did this because you couldn't really buy them at the time and I'm a skinflint. After a while, I started winding them for others but the one thing I hand trouble with were the leads to the coil. Initially I used the wire that was on the coil and just left it long. However, one stormy night when I managed to make 12 coils and broke the leads on every one of them I decided that a single strand of 0.05mm wire was not strong enough. My customers agreed! I managed to find a way to attach a small PCB to the coils to act as terminals but what to use for wire? I started with single strands of enamelled wire but after using them, myself I found it too stiff and still quite easy to break. So, I decided to make my own multi-strand litz wire. Litz wire is used for RF applications normally, it is really just multiple strands of enamelled copper wire twisted or braided together to form a wire. Sometimes this it then wrapped with silk or similar to make an extra layer of insulation but this was not needed in my case as the enamel coat is enough insulation.
I had made wire a bit like this before but with much thicker. I’d just take a few lengths of wire clamp one end in a vice and use a hand drill or electric screwdriver to twist the wires together. For these thinner conductors I wanted something simple, reliable and a little faster. Hence the Whirl-Litzer was born. It is possibly the most simple machine ever made and I’m sure doesn’t warrant all the wittering I have just penned but it would be no fun otherwise.
The basic construction of the machine can be seen in the following diagram. It consists of a wooden batten to which is attached a geared motor with hook on the output shaft and another fixed hook.
To use the machine the wire is held with a suitable holder (such as a lump of “bluetac” or a hair clip with simple rubber jaws), fed through the fixed hook and wrapped multiple times between the two hooks.
Then the motor is turned on and the wire twisted to the preferred twistyness, not wishing to be too technical. It is now that the springy nature of the hook helps, as it will tend to avoid breakages and is generally more forgiving.
Now you have some homemade litz wire between the hooks of the machine. I tended to make fixed lengths of wire of about 12” long. To speed the process up I marked 12” intervals on the batten and tinned short sections of the wire above these marks. Note, you MUST buy solderable enamelled copper wire otherwise this cannot be done. This wire has a coating that melts at soldering temperature, otherwise you will have to cut the wire, separate the conductors and scrape off the enamel (life’s too short).
Once these sections have been tinned then the wire can be cut in to sections leaving ready tinned lengths of wire.
You can tailor the wire you produce to your application by varying the number and thickness of conductors. For example, many super thin conductors will be more flexible than a few thick ones. You can also alter the properties by varying the amount of twist. So there you have it a simple machine for a simple task but infinitely useful.
By Robert Throssell
Have you ever tried to pick up a little magnet that is the final piece of a long evenings micro engineering when .... SNAP it's stuck its self to your steel pliers or scalpel blade. What do you do? let the kids learn some new choice phrases by overhearing your verbal therapy? Kick the cat? No! You invest in a pair (or pairs) of non-magnetic tweezers! Easy, huh. Well it would be if your local model shop hadn't closed hours ago and it's the start of the owner's annual holiday. If you are anything like me, you want to finish this work of masterpiece NOW. Hmmmmm......
This is what I did...
At this point, you may have realised the potential of this cheap, non-magnetic stainless steel - especially if you get the disks from the bin at work!
Hope you enjoyed it!
Graham I have been making Litz wire like you describe and
After I get it twisted I throw a coat of testors enamel paint on
to give it some color. The paint is not really needed I just like black and red wires for some reason. But anyways I use a homemade winder made form an old analog clock for winding.
I think it is 6.67:1 ratio. It is what I use to wind rubber on my rubber powered planes. Alejandro Garcia has some pictures on his website of a similar winder.
My winder just uses 2 of the gears and I wrapped the hook on like Graham's ornithopter like a spring. At first I had used .015 music wire for the hook and it broke while winding 2 strands of 3/32" rubber
So I have now replaced with .031" wire. Seems to be holding up well.
Anyways it is a good tip Graham about the Litz wire. Thanks.
About the technical end of it now electricity flows through a wire
on the outer surface mostly on the skin so multi strand litz wire will handle more current than normal multi strand wire.
How much more current can it handle though. Lets say with 7 wires what awg or mm would we have to have for a lv M20 at 1 amp current draw? How about for 350ma like for the Didel 4.5 ohm motor on the Quick Junior?
Ok for 1 amp we would need about 143ma per strand using
The copper.xls that Michael posted
as a guide for wire sizes and current handling capabilities.
We would have to have more than .07mm wire which can handle about 138ma so the next size up would be .0799 or .08mm at 220ma per wire which should handle 1.54A ok I bet the .07mm wire could be used though because of the skin effect. 7 wires of .0711mm could handle 970ma.
Ok for the 4.5 ohm motor at 350ma max well we would only need 50ma per wire and that would be .0564mm wire at 54.9 ma per wire. I bet with the skin effect that even .03mm wire could be used with no problem. Now can I get someone to back me up on this. I really have not researched how much more we can get by with the skin effect.
In my opinion, there is no skin effect as we use DC current to feed our motors (at full throttle). When not at full speed, the pulsed rate of the current delivered by the ESC is not high enough to generate a noticeable skin effect.
So DC current does not mostly flow on the outer surface of the wire? Oh well I thought I had thought of some weight saving idea there but to be safe I guess you should use
7 strands of of .0799 mm for the m20
7 strands of .0564mm for the Didel 4.5ohm 6mm motor.
I thought about making the ends of actuator coils litz wire for added strength. Maybe I read it somewhere I cant remember but the idea is good regardless if it was my idea or not. Instead of soldering the Litz wire to the ends of the coil wires build it in. Leave enough for say 7 strands of about
10" a peice or however long you need so 70" on each end for 10" of reinforced wire. If they are remote then you probably only need 5" or so.
"fill" taught me alot about winding coils. He had an idea that at the end of the winding process, to make four or five loops of the wire an inch or two long. Then twist them together and wrap them around the coil and seal with epoxy. Since this is done with the coil wire without cutting it, it is a bit tricky, but you have a built in litz wire lead to terminate the coil without any soldering. I never got around to trying it, but he said it worked great. Then DU dropped the price of their machine wound coils and it has become a bit of a lost art, but it is still fun.
The process of looping and then twisting the wires from a coil is called skeining and is used in proper coil winding alot. I taught fill alot about coil winding
In practise leaving long pieces of wire dangling from coils for later skeining can be a recipe for disaster, you have to be very careful not to snap the wires. I had considered adding a skeiner to my winder such that two would wrap the wire around two hooks before beginning the wind, then one would spin to skein the wire. This wire would be stowed in the windiner head (wrapped around say) and the coil would be wound. Then at the end with the coil still in the winder the same process would be completed for the other wire. I just never got around to it.
Billy, the point of is being multistrand in our case is just for flexibility, strength and tunability, you can make wire for all occasions.
I found that the short lengths holding the end with fingers can help in keeping the wire from snapping. Practice with wires not used to make coils beforehand to get aquainted with force that will break the wires.
I made some tweezers as described in the article. They work really great for magnets. I also made a pair with really sharp
tips for holding surface mount components while soldering.
They work really great as well. On one pair I glued some 1/32" ply to the sides and wrapped with thread then glued the thread.
The ply helps stiffen up the action and makes them feel more like normal tweezers. The thread holds the ply in place and makes for a nice grip. I have 3 store bought pairs of tweezers I use for modeling all the time they are handy for placing small parts while glueing. One pair was from a Swiss Army knife well I think the homemade tweezers will replace the swiss army knife tweezers as the action is better and the tips can be cut to desired size
and the handles can be made big to get a better grip.
Thanks for the tip Robert.
finaly got around to taking some pictures of my tweezers.
From left to right
1 Tweezers from Swiss Army Knife
2 Pair with ply and thread added to sides
3 short wide pair these are like an extension of the fingers.
4 longer narrow pair with very fine tip for surface mount soldering.
Thanks again for the article.
I visited Alejandro Garcia's site regarding his rubber winder. WOW what a great site! Just goes to prove that this really is a world wide hobby/sport! Do you have an email address for Alejandro? He is a very creative and involved individual
Jon B. Shereshaw
Gladstone, NJ. USA
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