Creating Simplicity

Creating Simplicity

by

Mike Robey

Introduction

After completing and flying the Decathlon for a while I decided to try my hand at something that would fly as well (or better) but didn't require as many 3D printed parts. Something more straight forward and simple. Something I could be brave with and replace easily if need be.  Many times I have found references in model plane magazines to an ARF called the Scanner. Its a very simple low wing monoplane that seems to be extremely popular amongst non-electric fliers.

I took the basic concept from the Scanner (i.e. monoplane, box like fuselage, simple bubble canopy) and then I took the Decathlon wing, tailplane and vertical stabiliser and started making changes. I made the fuselage length the same as the Decathlon because I reasoned, incorrectly as it turned out, that the CG would end up in roughly the same place as the Decathlon. I also decided to make the wing a mid wing to ensure axial aileron rolls. I also made the height of the fuselage such that a 2200 mAh, 3 cell battery would fit under the wing if needed. I increased the size of the vertical stabiliser by quite a bit and changed the shape of the the tailplane and vertical stabiliser to make it different from the Decathlon. I kept the undercarriage as a tail dragger for the sake of simplicity and to save weight (i.e. no nose wheel and no servo for steering the nose wheel).

Design Goals

The idea was to end up with a straight forward build which did not require the 3D printed parts if the builder did not have access to a 3D printer. I wanted to end up with something that I could practice aerobatic sequences with and therefore needed it to be simple to replace if I dumb thumbed it to death. I also hoped it would be a little lighter than the Decathlon because it had fewer 3D parts and also to have less drag because it has a skinny fuselage. It does have less drag but I was surprised to find its weight was pretty much exactly the same as the Decathlon. It is however more streamlined and you can feel the difference when flying.

I followed the same structure as with the Decathlon, including the use of a wooden spar to re-enforce the foam one as well as including two fuselage bulkheads in the same position as in the Decathlon. I did end up removing the front bulkhead to allow for battery placement and realised it was not needed structurally.

Flying Simplicity

Simplicity goes where you point her. The roll rate is not stellar but is plenty for non 3D aerobatics. Inverted requires a tad of down elevator. Basically a very predictable flier with no bad habits. The motor has 2 degrees of down thrust but no right thrust and doesn't seem to need it. The power train I am using is fine but more power would provide unlimited vertical. However the power train I am using can take 4S so I might try a 4S battery in the future. The controls seem to be fairly harmonised. Like the Decathlon but perhaps a bit more so it glides nicely so most landings require little or no power. If you do have to add power on final then one has to be careful not to add too much. In short its a very simple and fun plane to fly. You can see for yourself how she flies yourself at:

A side note regarding the landing in the video. The video shows the first one which was perfect except for the fact that I flared 3 feet too high. I told my cameraman to keep filming and I took off and did exactly the same thing! Previously I had made three perfect 3 point landings so I am not sure why that happened. However on the second landing I actually broke the undercarriage mount. I had countersunk the mounting bolts which meant the plastic was only 2mm thick where the bolts went through. I remedied the situation by not having the mounting bolts countersunk and printing it with 100% infill. The thing will not break now. Mind you perhaps I'll try not to plonk it in from 3 feet!

Specifications

• Wingspan: 1470mm

• Flying Weight: 1385 gms

• Empty Weight: 1172 gms

• Motor: Turnigy NTM 35-36, 910 kv

• ESC: An old unnamed 40 amp ESC that I had lying around
  

• Prop: 10 x 4.7 slow fly

• Battery: 2200 mAh, 3S Turnigy Graphene

• Max Watts: 245 watts

• Max Amps: 24 amps

• Max Thrust: 1150 grams

• Max C: 11

• Flight Time at Full Throttle: 5 minutes

3D Printed Parts

Simplicity employs a number of 3D printed parts which are described below. However, if you are not into 3D printing then there are alternatives which would work.

Cockpit Canopy

The image below shows the three parts to the canopy/battery hatch. Glue them together after printing then sand, fill and paint for a nice finish.

Pretty much any canopy can be used. The fuselage top is a flat surface so if you do not want to print the canopy you can use a canopy made from a plastic bottle or carved from foam or soft balsa.

Motor Mount

The holes are spaced for the NTM motor I used. If you are using a different motor then check the hole spacing and modify the sketchup file to suit. This mount has no down or right thrust so those not into 3D printing could use liteply instead.

To cater for down and right thrust I designed two wedges to sit between the motor and the motor mount. The one I ended up using just has 2 degrees of down thrust but I also designed one with two degrees of down and two degrees of right thrust.

Nose

The nose is a simple affair. In the plans there are foam side cheeks which are removed if using the 3D printed nose. However the nose can be left off and the side cheeks used in their place.

Undercarriage Mount

I decided to use a standard Hobbyking pair of undercarriage legs. They are cheap, light and really strong.

The above image is courtesy of Hobbyking. The legs can be found here:

https://hobbyking.com/en_us/alloy-struts-12cm-high-19g.html

I never worked out what the blue plastic bits are for but they slide off and can be discarded. You could use other legs but the holes in the undercarriage mount are spaced for these. The undercarriage mount simply slots into a rectangular opening in the bottom of the fuselage and the legs are bolted to it.

If you have read my Decathlon build guide then you have heard this before because they are the same as the ones used on the Decathlon.

The mounting plate is simply a 5mm thick printed part with side cheeks to help distribute the load from heavy landings. If you did not want to print this part then a ply plate could be used in its place.

Wheels

I used the same wheels as on the Decathlon. The flying fields I use don't have nice flat surfaces so I need big wheels. The tyres were printed using flexible filament(TPU purchased from sainsmart.com) with 5% infill. The result is tyres which are a bit squishy and have some flex for landing. The hub was printed using ABS and 15% infill.

Tail Skid

I elected to go for the plain tail skid which has served me well on many of my models.

The Build

Those not intending to build their own Simplicity can skip to the conclusion. However if you envisage appreciating the simple joys in life then read on. My build guide assumes you are familiar with the normal Flitetest manner of model aircraft construction. If not then watch pretty much any Flitetest build video before reading further.

Starting with the fuselage we can see all the required parts in the above picture.

Fuselage construction starts with a B fold on the fuselage side. Usual story – make sure side is at right angles to bottom and that you have the side pressed firmly against the table and the bottom. Note we only glue the forward part of the fuselage side to the bottom.

Repeat the process for the other side.

Now glue the rear sides to the bottom. Apply glue to both sides and press the bottom against the table while pushing the sides in place. This will ensure a nice flat join. Slide the fuselage back and fourth while holding it against the table to prevent it accidentally being glued to the table.

Glue the rear bulkhead in place.

I also had this bulkhead in the front but removed it to give me room to place the battery. Its not needed so do not include it.

Now the forward half of the fuselage top is glued on. Make a half cut on the fuselage top where the taper begins and crack it open so the rear fuselage top can be flipped up. Apply the hot glue then place the fuselage top onto the fuselage then flip upside down and hold against the table while sliding the fuselage back and fourth.

Now glue the rear half of the fuselage top onto the fuselage in exactly the same way as the forward half. We do the fuselage top in two steps to avoid taking too long to apply the hot glue and having it start to set before we have the fuselage top in place. Far less stressful this way.

Now that the fuselage is together the wing cutouts can be removed. One mistake I made here was that I partially cut the wing cutouts in the usual way and then covered them in wing tape making them almost impossible to see! This made seeing where to cut really difficult. I should have cut through the wing tape from the inside before construction began so that the partial cuts could be clearly seen from the outside of the fuselage.

This step is only required if you are using wing tape to cover the model. Apply strips about 20 mm wide to each of the four edges to cover the joins. Looks much neater that way.

Now the wing can be commenced. The above image shows the foam parts which should look very familiar to anyone who has built a Flitetest model.

Line the two wing halves together and run tape across the join. Try to get a good close join as the foam can sometimes slip a bit and a small gap can result. Cut the tape so that it extends 50mm or so from the front and rear of the wing but do not fold it over yet.

Flip the wings over, open the join and apply hot glue then lay the wings down flat and wipe away the excess hot glue. Then fold the ends of the tape over. Note I made a mistake on the plans (since corrected). On the decathlon the servo leads exit the bottom of the wing but on this model they need to exit the top of the wing. I have cut the square exit hole on the bottom of the wing. That is an error you will not make as I have corrected the plans. At this point run a barbeque skewer along the wing fold cuts and gently crease the folds in the top of the wing.

Next glue the foam wing spars in place. Make sure you trial fit them as they may need a little trimming here and there first. Also make sure that there is no excess glue on the rear of the spar. Wipe any that you find away before the glue dries. If you are a bit slow and the glue has already dried then gently and carefully cut any excess glue away. Our wooden spar will butt up against the rear of the foam spar and we want a nice, close join. However any excess glue in that area will prevent a good join.

The wooden spar needs to be 1200mm long. It can be dowel if you want. Mine is 12mm square Tasmanian Oak because its strong and reasonably light but whatever you have that is reasonably stiff and light will do. I used weights and clamps to hold the wood against the foam spar. If you use clamps be careful and don't tighten them too much. Remember the foam is soft and you do not want to squash it. I used UHU Por for this join as I wanted the time to ensure I had the spar in exactly the right place. Notice I have closed up the servo lead exit hole in the wing bottom and cut a new one in the wing top.

Apply hot glue to the inside of the leading edge and the top of both the foam and wooden wing spars then fold the wing over and hold it down for at least 3 minutes. Then flip the wing over and check to see that the rear of the wing bottom lines up with the front edge of the ailerons. If you find it overlaps then just trim the wing bottom to correct the problem. Then apply glue to the trailing edge of the bottom of the wing and hold the trailing edge down with weights. Leave for at least 5 minutes to ensure the glue is totally set.

Glue the vertical stabiliser to the tailplane making sure they are at right angles to each other.

Glue the completed tail assembly onto the fuselage making sure that the horizontal stabiliser is at right angles to the fuselage sides. In hindsight this step is better left until after the wing is glued in place so that the tailplane can be checked for alignment with the wing.

Slide the wing into place being very careful to ensure it is positioned correctly. Its hard to eye this visually because the centre wing join is hidden inside the fuselage. Measuring the fuselage to wingtip distance on the leading and trailing edges on both sids and adjusting until the wing is centred and straight is the best way. Once you are sure the wing is correctly placed run hot glue along the fuselage and wing joins left and right and top and bottom. Don't forget to wipe away an excess glue.

I hot glued struts between the fuselage and horizontal stabiliser top and bottom. The main reason being that when the grass is wet the bottom of the tailplane can get damp and when it dries it tends to bow. The struts help prevent this.

Now its undercarriage time. The legs are bolted to the 3D printed undercarriage mount and the completed assembly glued to the fuselage. If you look carefully you can see the countersunk holes in the undercarriage mount. As previously described these have been removed to increase the structural strength of the mount.

The main landing gear and the tail skid ready to glue in place. I used epoxy for this purpose. On previous models I tried UHU Por because it has a little give in it which I thought provided a little shock absorption but I found the undercarriage would pull free after a while.

Glue the tail skid in place.

Then the main landing gear is glued in place. The next step is to cut the battery hatch and glue the cockpit canopy in place. Apologies for forgetting to take pictures of these steps. The battery hatch location can be copied from the plans onto the fuselage and the canopy glued onto the battery hatch wherever you feel looks right. The hatch is large and if cut out when the fuselage is cut out would mean the fuselage top would have been too weak to allow successful fuselage construction. Once the hatch is cut you can also thread your aileron servo leads through the wings and out the servo lead exit hole in the wing. Similarly you can add the elevator and rudder servos at this point.

Whether you are using the larger sheets of foam board from foamboards.com.au or the Flitetest foamboard the fuselage is too long and an extension piece is required. I have included plans for both options and the only real difference is the length of the extension piece. In either case the extension consists of the box which is the fuselage extension and a sleeve piece which is used to join the extension to the rest of the fuselage.

The sleeve piece is glued into the extension piece. Test fit first and make sure that you can slide it in place once the glue has been applied. I used UHU Por to give me more time as the fit was quite tight (which is what we want. Once that is dry run a barbeque skewer along the edges of the sleeve piece so that they are pushed in a little (You can just make that out in the above imager if you look closely). It just makes fitting the completed extension to the fuselage a little easier. Speaking of that...

Glue the extension piece in place.

Next glue the motor mount (with the motor already bolted to it) into the fuselage. Be careful to ensure you get it in straight and do not accidentally build in some unintended up/down or side thrust. When dry glue the nose piece in place. I used UHU Por for both joins.

Finish off by gluing the control horns in place and hooking up your servos. The final task is to position the battery such that the cg lies along the foam wing spar and figure out how you intend to hold the battery down. When balancing place your fingers in the slots cut into the wing bottom for the wing spar tabs. That is your cg balance point.

Before you know it you re done! I set my control throws as large as I could get them and threw in some expo and it all seemed to be fine on the maiden.

Data Files

The sketchup files for the 3D printed parts can be found here.

The STL files for the 3D printed parts can be found here.

The plans include 2 different files for the fuselage.  The one whose name starts with "FT" allows for the size of foam board used by Flitetest. The other one assumes the Foam board is the larger size that I use.  I have not built from the FT sized plans but I see no reason why they should not work as the only significant difference is the length of the nose section.

The full sheet plans as pdf files can be found here.

The tiled plans as pdf files (both A4 and Letter) can be found here.

The dxf files can be found here.

Conclusion

Simplicity is a simple design that flies as intended. Its a simple build which results in a simple and yet attractive model that is fun to throw around. Anyone who builds one will enjoy flying it. From an aerobatics point of view the only improvement I would make would be to use a symmetrical wing section and that is something I am considering for the future. However that's another story....