Easymini

Tools, materials, construction and flying.

Boxed Easyminis

Plans

The plans will all fit on an A4 sheet of paper. They are all identical but use different file formats. Right click to save any of them, but only the JPEG is likely to be displayed by left clicking it:

easymini.jpg This is a JPEG image that should be printable by almost any application that can print a picture. It will be exactly the right size if the application prints it at 68.816 pixels/inch (2.7093 picels/mm).
easymini.dwg This is a 2D CAD drawing in Autodesk DWG format. It should be printable at the correct scale by any CAD system that accepts this format.
easymini.dxf This is a 2D CAD drawing in the DXF CAD interchange format. Any CAD system that accepts this format should be able to import and print it.

The model's fuselage is exactly 250mm long, so if necessary you can tweak the printer or plotter set-up to correct your hard copy. Keep the width and height factors the same: pixels must be square for an accurate plan to be printed.

Tools

To build this model you'll need:

If you want a cutting mat, get it from a graphic arts supplier or eBay. Sheets of sandpaper come from your local friendly DIY store, though the knives they sell are usually too thick-bladed and/or blunt. All the rest can be picked up from a model aircraft shop (hobby store) along with the glue, balsa and plastecine.

Materials

Balsa wood

You'll need balsa of various thicknesses. All balsa sheets are the same length, 890mm. They come in two widths, 75mm and 100mm, and a variety of thicknesses. Finally, balsa is often classified as soft, medium and hard, with soft balsa being light, reasonably flexible and fragile and hard balsa being strong, stiff and fairly tough. You'll need:

Thickness (mm) Width (mm) Hardness
2.5 75 Medium to hard
1.6 100 Medium
0.8 75 Medium

With reasonable care one 100mm wide sheet of 1.6mm balsa will make wings for eight of these gliders. The other two sheets will make tail surfaces and fuselages for many more than that.

Glues

The whole model can be assembled using thin cyanoacrylate for fast assembly. If you use this type of glue, assembly should take under 15 minutes once the parts have been cut out, but it should not be used by the young because its very good at instantly sticking fingers together. Don't bother with the commonly available Superglue: you'll get much better results if you use Zap-CA or Hot Stuff. These are available from model aircraft shops or online model suppliers. Note that cyanocrylate glues are weakened by water, so if the model is expected to last it is a good idea to give it a light coat of 50:50 thinned clear nitrate dope after assembly to protect the glue from getting wet.

White or aliphatic wood glue is also suitable, particularly for young builders, because it is water-soluble before it has set and will form an easily-removed skin if it gets on fingers. However, assembly will take longer because these glues take 15-30 minutes to be hard enough for gentle handling and should be left overnight to set fully before flying the model.

Balsa cement isn't recommended for this design because it shrinks a lot as it sets and can easily warp the thin tail surfaces. That said, using it would allow the model to be flown an hour or two after it is finished.

Thin wire

This is used to make the catapult hook that fits under the nose of the glider. It can be bent from a paperclip or you can use 18swg (0.5mm) piano wire, available from the model aircraft shop.

Thread

Some strong cotton or linen thread is used to bind the catapult hook onto the nose of the glider.

Ballast weight

A small amount of plastecine (modelling clay) will be added to the nose of the finished model to balance it. You can also use thin electronic solder if you have any: it wraps nicely round the nose and is retained with a smear of glue.

Waterproof paint

The model will fly perfectly well if left bare, but you may want to make it a bit more durable and waterproof by painting it. Here I recommend using clear nitrate dope thinned 50:50 with dope thinners. These are available from most model aircraft shops and give a fast drying, lightweight finish.

Similarly, it will be much easier to find after a flight if the wingtips and tail are given a very light spray with dayglo hazard warning paint, or it can be decorated with various coloured felt-tip pens, though these colours do fade in sunlight.

If you use both colour and dope, put the dope on first.

Hardwood

You need a hardwood handle for the launch catapult. I used a 150mm stick with a cross section of 6mm by 12mm.

Catapult rubber

I used a 300mm length of good quality 6mm wide aero strip and knotted the ends together to form a loop. Aero strip is sold by some model aircraft shops. It is specially made for driving the propellers of rubber powered models but can be hard to find in some towns, especially when all you need is a metre or so.

If you can't find any aero strip, check your local office supplies shop for large rubber bands. Any bands where the rubber is about 6mm wide and the loop is around 150mm long should be fine.

Construction

Wing

The wing is cut out as one piece of wood.

The center of the fuselage and the lines where the tips bend up must be accurately at right angles to the trailing edge of the centre section, so use a setsquare and a soft pencil to mark them.

Leave the tips and trailing edge square. Round the leading edge.

Score the top of the dihedral joints with a ballpoint pen, bend the tips up to the correct dihedral using a 20mm piece of scrap balsa under the tip as a guide.

Glue the dihedral joints: run thin cyanoacrylate into it or, if you're using white glue, rub it in and check that the dihedral angle is still correct.

Tailplane

Fin

Fuselage

The trailing edge of tailplane and fin are 13mm from the rear of the fuselage. The rear end of the fuselage is left sticking out to form a grip for launching. If the wood for the fuselage is a bit soft this grip may break off after a few flights, so its best to err on the hard side when selecting the 2.5mm balsa.

Assembly

  1. Cut out the wing, bend and glue the dihedralled tips as described, and glue the wing to the top of the fuselage with its trailing edge exactly 105mm from the nose. The wing must be exactly square with the fuselage. Use the centre line you marked on the wing to get it right and dry fit it before gluing to make sure that its also square when viewed from the front. Sand the top of the fuselage to correct any lack of squareness.
  2. Glue the fin to the side of the fuselage so its trailing edge is 13mm from the rear end of the fuselage and its lower edge is aligned with the bottom of the fuselage. the fin is best glued on the side that the model will turn toward when it is gliding. This means putting it on the left side for a right-hander and on the right side for a left hander but this is not critical.
  3. Before gluing it on, hold the tailplane in place to check that it will be accurately parallel to the wing because this affects the way the model will fly. Sand the underside of the fuselage as needed to correct any discrepancy. Glue the tailplane onto the underside of the fuselage so its trailing edge lines up with the fin's trailing edge and is square to the fuselage viewed from above.
  4. Bend the launch hook from a paper clip and bind it on with thread and glue. Its rear end goes into the bottom of the fuselage about 3mm so it can't pull out of the binding.
  5. Now apply any paint, ID or decoration to the model, but be sure to keep it light. I give my models a single coat of clear nitrate dope thinned 50:50 with dope thinners and sand lightly with fine sandpaper when the dope is dry for a nice, smooth finish. A misting of orange dayglo hazard warning paint on the wing tips and the fin makes the model much easier to find after a flight.
  6. The final step is to make sure the model balances at the designed place. The correct balance point is 20mm in front of the wing trailing edge. Mark it on the underside of the wing on both sides of the fuselage using a soft pencil or (better) a fibretip pen.
  7. The final step is to balance the model. It must be complete, decorated, and ready fo fly before it is balanced. Cut the cardboard cradle from cardboard that's around 1mm thick and bend its base as shown on the plan. Adjust the model's balance point by adding small amounts of plastecine or thin electrical solder to the nose or tail until the model balances with the cradle exactly on the balance mark on both wings.

EasyMini-H being balanced

Tips

Since this little model can be lost easily, I made wing, tail and fin templates from plywood so I can build them really fast.

These models are very light. Mine weigh around 5 grams ready to fly. They will be fragile if you can't find hard balsa for the fuselage and so will require careful handling.

If you prefer to make them stronger and don't mind them coming out a bit heavier, replace the balsa fuselage and tail surfaces with stronger materials. Using 2.5mm spruce strip or sheet for the fuselage and 0.4mm ply for the tail surfaces will make the model much more durable though it won't glide quite so well due to the increased flying weight. I made one this way, which increased its weight to 9.6 grams. This doesn't affect the height it will reach on launch. In fact it may even get a bit higher, but it will glide faster and so won't stay up as long or be lost upward in a thermal quite so easily. Thin ply and spruce are available from all good model aircraft shops and online sources.

The model shown on the right having its balance checked has a spruce fuselage and 0.4mm ply tail surfaces. Notice that by comparison with the models in the box, shown above, quite a lot more nose weight was needed to balance the heavier tail surfaces and rear fuselage, but it still flies well.

The best way to assemble these models depends on which glue you use:

Launcher detail

Launcher

This is a 150mm stick (mine is 6mm x 12mm cross section) with a 150 mm loop of 6mm rubber attached to its end.

I've found a good way to anchor the rubber is to drill a 6mm hole in the stick, feed the rubber through it, put a 3mm dowel through the rubber loop and pull the rubber snug around it. I wrapped masking tape round stick and dowel so the dowel can't fall out.


Flying

Right handers should fly these gliders right-left - that means it should glide in a gentle left-hand turn and will fly in a right-turning spiral climb when launched. I'm left handed so mine flies left-right.

Do a few gentle hand launches first to get the basic trim right. The model should be launched just fast enough to float away from your hand. Push it in a straight line rather than throwing it and make sure it is pointing in exactly the same direction as you launching. This is quite easy to learn and is essential to get right for successfully trimming a model. You're looking for a steady glide with a fairly wide turn in the correct direction for your handedness. Correct the turn by gently bending the rear of the fin. Bend the rear of the tailplane up or down to correct any tendency to dive or stall. Its worth spending a little time getting the initial trim right because if you catapult a badly trimmed model it can crash so fast you won't see what happened.

To catapult the model, hook the loop of rubber into the wire hook under the nose and, holding the grip at the rear of the fuselage between finger and thumb of your dominant hand, and the catapult stick with the other hand, keep the model near your waist while moving the stick away from the model. To launch, simply let go of the grip at the rear of the model while looking well ahead in the direction it will go: if you're looking at the stick you'll instantly loose sight of the model, which will be travelling very fast indeed. Always point it steeply up and never at anybody else.

Launch at a 60 degree angle with the model in a vertical bank with its underside toward you. It should make one turn on the climb and then roll out into its glide turn. Start with low power launches and, if its climb and glide pattern look ok, increase the power next time. If, instead the model loops or arcs over and down when it should be spiralling upward, or doesn't have a steady, floating glide after the climb you should correct the trim before trying a harder launch. As the launch power increases the model flies faster and all adjustments become more sensitive.

The ones I've made have all been pretty much on trim straight from the bench, requiring just a gentle rudder tweak to make them turn: from a shoulder height hand launch the model should give a smooth glide to land about 5-6 m away and have turned through about 10 degrees. If it stalls, bend the tailplane's trailing edge down a little. If your tail surfaces are balsa be gentle with them or they may crack: breathing on them first makes then bend more easily. When you're happy, try a catapult launch. Always launch up at 60 degrees in a vertical bank, but the first launch should not be at full power: watch what it does and adjust the trim before using stronger launches. A good climb is one where it model rolls the opposite way to its turn at the same rate. This gives a steady spiral climb. Correct the pattern by bending one side of the tailplane: if it arcs over and dives in the opposite direction to the expected spiral climb, bending the side of the tailplane up on the inside of the spiral will help it roll into the spiral as well as keeping its nose up. If it spirals up nicely, rolls out and shows a stally glide, bending the side of the tailplane on the outside of the spiral down will often cure the stall with very little effect on the climb. Always remember that these changes affect both climb and glide and that trim changes affect the high speed climb more than the slow glide.


If you manage to launch your model into a thermal on a hot, sunny day and it climbs up and away until its out of sight, don't be sad: enjoy the sight of it flying really well and see if you can make another that good. If you've put your name and phone number on it you may even get it back. Don't forget to ask where it landed so you can find out how far it went.

Have fun!


Credits

The Easymini was designed by Chuck Marcos and originally published as a plan and construction article in Model Aviation, May, 1999.

Those wanting to see the original article, plan and more flying tips should look here:

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Original plan