It's not a Radian but....

Introduction

I lay the blame for this design squarely in the lap of the FT-After Hours guys. I am not a glider guider but each time I listen to an FT-After Hours pod cast, sooner or later, the Parkzone Radian will come up. Chris and the guys clearly LOVE the Radian. I have never flown one but I have seen them flown. I admit to being intrigued when watching them land as they come in so slowly that you have to resist the temptation to walk out and pluck the thing out of the air.  Each time I listen to the guys talk about the Radian a strong sense of curiosity is stirred within me.

I haven't bought a model in ages. I have enjoyed designing and scratch building so much I haven't wanted to. Now when I look at the price of a model all I can think of is the huge amount of foamboard I could buy for the same price.  Then there is the question of whether or not I will enjoy a Radian as much as the FT After Hours guys. What if I spent the money and didn't like it?  I had also seen a glider called the Gentle Lady fly and that seemed to have similar, although not quite as floaty, flying characteristics.

However, being constructed from balsa and lite ply, the Gentle Lady's shape was a lot squarer and started me wondering if I could design a glider in foamboard that would fly like the Gentle Lady or the Radian. I looked at both models and decided I would have a go and see what happened. The result I call "Serenity".

The Design

The radian has a 2 metre wingspan. It also has wings which can be easily detached for transport. I decided to keep the Serenity's wingspan down to 1.4m as that is about as big as you can go and still fit it into a car without having to take it apart. I looked at the wing sections for both the Radian and the gentle Lady. I also looked at the wing section for my Hobbyking Walrus (which I love flying) and then sketched out a wing section that I thought would be appropriate. The Radian's wing has this gentle curve from the wing root to the wing tip. I have read that this is the most optimum way of including dihedral in a wing.

Obviously I could not emulate that in foam board so I decided to approximate by using Polyhedral. I used the above image and worked out wing section lengths and dihedral angles which approximated the equivalent amount of dihedral.

The next step was to break out QCad and start designing. I started with the wings. I have a centre section with no dihedral and then two outer sections, each of which has around 15 degrees of dihedral. I used a box spar (thank you Peter) and the outer wing panel was cambered Flitetest style. I know that means drag and its a glider but I wanted the stability that it provides.

The fuselage turned out a bit trickier. I approximated the size of the elevator, tailplane, rudder and vertical stabiliser from my HobbyKing Walrus and then came up with my own shapes for them.  I increased the size of the rudder by a lot because I have no ailerons and so I needed to be sure the rudder is responsive enough. I gave the fuselage and vertical stabiliser an aquatic shape for no other reason than I liked the look.

I then realised that the fuselage was too long for anyone using Adams Foamboard and so I shortened the fuselage length. This was a big mistake which I only realised when I mated the first fuselage to the wings and realised that Serenity would be very short coupled.  After some head scratching I came up with the idea of building the fuselage in two sections (front and rear) and then joining them together. That way the fuselage could be a sensible length and building from Adams foamboard would not be an issue. I am building using foamboard from foamboards.com.au which is 30% heavier than Adams foamboard.

The result is seen below:

I covered the wings in wing tape so that the wing bottom would go all the way to the trailing edge instead of stopping earlier (as in the normal Flitetest wing). I thought it would cut down on the drag a bit.

The Maiden

I have been reading a book on model plane aerodynamics and, the day before the maiden, I had read about the dreaded dutch roll and how can be caused by too much dihedral. I glanced over at Serenity and my heart sank. Too much dihedral I thought. Its going to dutch roll and crash.  What was I thinking? The next day dawned sunny but a bit windy. I wasn't particularly nervous because I thought it wasn't going to end well anyway.  I got my friend Shaun to hand launch for me so I could be ready on the sticks to fight the dutch roll that I was sure would start as soon as Serenity was in the air.  I steeled myself, advanced the throttle and nodded to Shaun who then threw the plane forward and......

The Serenity gently floated up into the sky. No Dutch Roll. My heart soared!  Despite the wind she seemed quite stable. The only thing wrong was the brake was not set on my ESC so the propellor wouldn't stop windmilling and fold.  Therefore on that day I could not really get a sense of what it glides like. The landing was vertical! With the wind blowing I pointed her into the wind for landing and she literally stopped moving forward over the ground. With the nose pointed slightly down she floated down onto the ground at zero ground speed. The memory of that makes me smile everytime I think of it.

The Following Weekend

The next weekend the sun was again shining but there was very little wind, my ESC brake was now turned on and I had painted the underside of the wing tips black to help my old eyes figure out which way up it was when up high. All was perfect.  I discovered the Serenity does indeed glide well. I am no glider guider but Shaun assures me it will thermal nicely.  I found you could potter around at ground level with just enough throttle to keep it level. That was a lot of fun because it flew so slowly. Not as slow as a Radian but pretty slow. I also discovered the Serenity has a party trick. If you get some height, point her into the wind, cut the throttle and slowly bring on full up elevator and hold it you do not get a stall. Instead she maintains a horizontal attitude and gently floats down almost vertically. At first I used rudder to keep her into the wind but then I realised I was interfering with her built in ability to stabilse herself so I left the rudder alone and found her descent was more graceful.  Applying throttle makes her nose up so you have to hold in down elevator to counter that. From memory that is pretty standard for motor gliders.

Is the Serenity a replacement for the Radian? I don't think so. The Radian has less drag and its 2 metre wing span would yield a lighter wing loading. Its gently curving wings give it a distinct advantage over Serenity. However I really don't care as the Serenity is so much fun to fly and is reasonable glider in her own right. Especially considering she is made from foamboard.  See for yourself:

Specs

Here are the basic Specifications for Serenity:

Wingspan: 142cm

Fuselage Length: 98cm

Weight without battery: 692 grams

Weight with battery 874 grams

Battery used: 2200 mAh, 3S, Zippy Compact

Motor: Emax BL2220/07

ESC: 30 amp Turnigy Plush

Propellor: 9 x 6 Folding

Receiver: Lemon DSM2

Centre of Gravity: 63mm from wing leading edge. We started about 10 mm further forward but kept moving it back until we were happy. I would suggest starting at 60mm and adjust until happy.

The Build

I designed and used a number of 3D printed parts for this model. However they can all be easily done without 3D printing. The canopy can be made from foamboard and the wing joiners from lite ply. The templates for both are included on the plans. The Motor mount can be lite ply and the nose cone can be fashioned from either balsa or foam (with a bit of sanding).

The build is pretty standard Flitetest style stuff. However it  is a little more involved due to the polyhedral and the two section fuselage. What follows is a description of the build with pictures to show what is being described. I do apologise for the photo quality but, at the end of the day, you can see what you need to see. Finally at the very end of this article are the files for 3D printing and the plans.

The Fuselage Assembly

The picture above shows all the fuselage parts. The front fuselage is constructed in the normal way and then the rear sides and bottom are joined to the front using short doublers.

We start with the front fuselage by gluing one side. It is vital to ensure the sides are at right angles to each other. After the side is glued we glue the short tapered bottom to the side bits at the front. These are all B folds.

Next we repeat the process for the other side, again making sure we have our sides at 90 degrees from the bottom.

Next we glue the front top onto the fuselage.

Now we assemble the doublers which will be used to join the rear fuselage sections to the front fuselage. The one with short sides (on the right in the image) goes above the wing and the other below the wing. In both cases B folds are used.

Next we glue the lower doubler in place.

Followed by the top doubler. Note - be careful to ensure the top doubler is aligned properly so that the fuselage top will sit nicely over the top of it.

 The next step is to glue one of the rear sides onto the front fuselage and the doubler.

Followed by the other rear side section.

We next glue the rear bottom section onto the front fuselage and doubler. Now we have a partially built fuselage which is as it would be if it had been cut out as one piece. Make sure the glue is totally dry before moving onto the next step (have a nice coffee or something).

Now we glue the rear bottom and sides together. The fuselage is curved and the sides are long so you need to test fit first and make sure your glue gun is really hot. The benchtop is not your friend in this case because of the curved shape of the fuselage sides. However the benchtop edge is your friend. Apply the glue, fit the sides and then run the bottom of the fuselage across the benchtop edge and back again. Keep doing this until the glue dries. Also, as you can see in the picture, do not remove the cutout for the tailplane until after this step.

The next thing to do is to glue the top of the fuselage on. Same as with the sides except you have to be even quicker as there are two sides to glue at the same time. Once again the benchtop edge is your friend.

Once the glue is dry remove any excess beads that might have formed along the join. If some of the joins didn't turn out great don't worry. It will still look okay and will fly fine. When viewed up close mine are not that great but it all works fine.

Now cut away the wing slots as per a normal Flitetest build.

The last thing prior to fitting the wings and tail feathers is to install the motor and esc.  I bolted the motor to the motor mount and glued on the nose cone before gluing the whole lot to the front of the fuselage.

If you are using my 3D printed motor mount then be aware that it has 2 degrees of downthrust and two degrees of right thrust built into it so make sure you know which way is up! Its hard to see but the black mark at the top of the motor mount is an arrow telling me which way is up.

I do not have any photos of the tail feathers being assembled and glued to the fuselage but its standard Flitetest stuff. Glue the vertical stabiliser to the tailplane making sure the two surfaces are 90 degress to each other. After the wing centre section has been glued in (see later), slide the tail feathers on, line the tailplane up with the wing centre section and apply glue.

The Wing Assembly

Wing assembly begins with the centre section.  This consists of the box spar and the wing as well as the four wing joiners (2 per side). These are either 3D printed or cut from lite ply.

We start by gluing the box spar sides onto the bottom of the spar. These are A folds. The spar is reasonably long so make sure you get everything in place all the way along before the glue dries. Also wipe away any excess glue inside the spar. Later we will slide the wing joiners in at each end and we don't want any glue to block their passage.

Next glue the box spar to the underside of the wing's upper surface. The spar should be aligned along the two folds which give the wing its curve.

Apply glue to the leading edge and spar bottom and then...

Fold the wing over and...

Hold it down until the glue dries. If alone sing a jaunty tune. If not then sing it in your head.

Next apply glue to the trailing edge on hold down until dry.

Finally glue the wing joiners in on one side only. Make sure you trial fit first. You might have to clear away any hot glue that stops the joiner from lying flat against the inside of the spar wall.

Apply the wing tape to the centre section.

Now we assemble the outer wing panels. On the plan I have drawn the wing panel as one piece but when it came to putting it altogether it was easier to cut off the wingtip and treat that as a separate piece. Do not mix the parts of the two outer wing panels. If you keep the wing tip part with the wing section it was cut from then you won't get confused about which side goes where. Assemble the box spar first. Again its an A fold. Note the wing dihedral guage on the right in the photo.

Glue the spar on in the same manner as with the centre section. Again make sure there is no excess hot glue where the wing joiners will slot in.

Glue and fold the outer wing panel in the same way as with the centre section. When dry bend the wing tip section so that the curve matches the end of the other outer wing panel and then run glue into the folds holding them at the correct curvature until the glue is dry.

Finally glue the wing tip onto the outer wing panel. Use the wing dihedral angle support included on the plans to ensure the correct dihedral between the two wing panels. After the glue is dry then run a bead across the top wing surface and smear it flat. Finally run some tape over the join.

Repeat the same process for the other wing panel.

Now do you see why we didn't glue the wing joiners on both sides of the centre section? Slide the centre section through and use a ruler to ensure it is as central as you can get it. You need to measure along the leading and trailing edges on both sides and keep adjusting until everything is centred. Run some hot glue along the fuselage wing joins, wiping away excess glue as per a normal Fltetest build.

Cover the outer wing panels with wing tape. Do not cover where the outer wing panel will join the wing centre section.

Glue the other two wing joiners into the wing centre section. test fit one of the outer wing panels onto the wing joiners and apply hot glue when happy. Repeat the process for the other side. Finally cover the joins with wing tape.

All that is left now is to install your electronics, program your radio and set your battery position to get the correct cg. There is plenty of room inside the cockpit to slide the battery back and forth.

Now you are ready to fly!!

I have often been told that when I hand launch I throw like a girl. Luckily the Serenity doesn't need much of a push.

Below are the data files including the plans and 3D models. Enjoy!

Data Files

Tiled Plans (A4 and Letter) are here.

Full Plans (A4 and Letter) are here.

Sketchup files for 3D printable parts are here.

STL files for 3D printable parts are here.

The dxf files for the plans are here.

Conclusion

Serenity isn't a Radian but it flies slowly, has gentle flight chracteristics and is very floaty. If you want a cheap foamboard glider then give Serenity a go.  I have really enjoyed the design process and I love the excitement you feel when it all goes well.  I would also urge anyone thinking of trying to design their own model to give it a go.  Even if they don't fly as expected you still learn something which can be used in the next design.  I have pretty much lost interest in ARF models simply because I do not want to deprive myself of the fun and enjoyment you can get out of bringing your own design from an idea in your head to floating through the sky above it.  I was thinking of an aileron/flaps version to complement Serenity but then I thought what the heck would I do with my HK Walrus which is pretty much the same thing. I will just have to spend some time thinking of something else to design. Meanwhile enjoy Serenity.