Decathlon Build

While I was putting my Decathlon together I took lots of photographs, some just to record the build, and others to show to people when I was unsure how to deal with a particular bit (normally when something didn't quite fit!)

The first problem that became apparent was that I hadn't realised that the batteries I have bought were so long ! The picture shows that even with a battery pushed right up to the firewall, it reached back over the servo tray. Various options were considered, such as building a sloping tray to raise the rear up clear of the servos.

I have already bought two batteries so that I wouldn't have to wait for a recharge between flights, so I wanted to be able to easily change the battery without having to take the wings off. Plus, I didn't particularly like the idea of charging Lipos with them mounted inside the fuselarge Luckily the batteries' width will allow then to fit in the space normally occupied by the i/c motor's silencer.

By cutting a hole in the back of this space the battery can be slid in from the front, and will sit a bit further forward and thus not interfere with the servo tray. Also the battery is positioned slightly lower and thus sits under the servo tray.

Alister built a tray to for the battery to slide into inside the cockpit. A slight enlargement of the hole in the front of the cowl allows the battery to be easily inserted and removed.

The first construction problem surfaced when it came to installing the fin.

The solution was to sand down the front edge of the fin until it was a good fit and allowed the fin to fit correctly into the slot in the fuselage  Note the sheet of paper to protect the fuselage covering from the sand paper.

And then when I came to try and fit the wings for the first time I discovered they wouldn't seat properly.  The picture shows the gap between the top edge of the fuselage and the bottom surface of the wing .

And as you can see the same problem on the other side of the fuselage  It appeared that the trailing edge was not directly being held "up", but that the leading edge was being held "down" by the position of the dowels that locate into the  holes shown in the next picture. 

As you can see, rather than try and move the dowels on the wings, I decided to reshape the holes.  In the picture the holes have been enlarged slightly on their upper edges, and small pieces of balsa have been glued into them.  The one on the right has already been worked away to allow the dowel to enter and be held in the correct position.  When both had been done the wing nearly sat correctly on the saddle, but a small amount of material needed to be removed from the saddle to make it match the profile of the bottom of the wing.  Also the wing rocked slightly across the diagonal, so the material was removed from the leading edge on one side and the trailing edge on the other side.  I was concerned that these modifications may have altered the angle of incidence of the wing.  Another builder had repositioned the dowels in the wings, and in doing so had decreased the angle to such an extent that the model would hardly fly,  but since my modifications could have only slightly increased the angle I wasn't too concerned.

Now that the wings were in place I mounted the aileron servos and control horns.  I mounted the horns where the instructions suggested, but when I came to test the servos I realised that placing the  linkage into the outermost hole in the servo arm  gave too  much control surface throw.   I could have just  used  "low rates" on the transmitter to overcome this, but using the innermost hole but one gave throws very close to the figures in the manual.  However I was unhappy with the clevis coming off the horn at an angle.

So I built a jig to put a couple of bends into the control rods.  Of course on the first attempt I put the bends in the wrong way and had to go and buy two new rods and start again.

When it came to mounting the motor, we decided to try and use the existing i/c motor mounting holes in the firewall even though they were at an odd angle and were spaced too wide.  A cardboard "mock up" proved the principle but we realised it would be hard to get a screwdriver onto the bolts into the firewall. 

So for the final version we did it slightly differently.  The picture is misleading as it looks like the motor has some left thrust but it is an optical illusion.  You can also see the platform in the silencer space that supports the front of the battery.

The balsa tray in the cockpit was made the right size to hold the battery in place, but I added two further retaining methods.   First I mounted a simple sliding "finger" on the firewall that stops the battery sliding forward.  Secondly when we installed the platform we put slots in to allow a strap to be threaded over the battery and pulled tight.

When it came to mounting the cowl I wanted to put the screws into the edge of the firewall rather than just into the thin covering at the back edge of the cowl.  In order to mark where I needed to drill holes in the cowl  I put a 12V "brake light" bulb inside and used the shadow of the front edge of the firewall to position the holes.

Here's a picture of the first time I mounted the prop and spinner and ran the motor!

This was taken just before the maiden flight, just in case it didn't survive ! 
The range check proved problematical but some repositioning of servo leads, receiver and antenna seemed to sort out the interference that caused the motor speed to "hunt" at greater than half throttle.  However the first flight showed the problem still persisted.   A change of ESC (From CC to Hacker) and  moving the receiver/servo battery away from ESC and putting ferrite rings on several of the leads cured the problems.  A range check before the next flight was "all clear" so.......

After getting the trims sorted out during the flight, I took this picture.  During the flight I had let another club member fly it for a while, and he commented that a bit of "right thrust" would help the handling. 

It was quite easy to add a bit of right thrust by putting a couple of washers under the left hand side of the motor mount.  Because the "X" mount is at an angle this also added a little bit of down thrust as well but this doesn't seem to have caused any problems.

Then disaster struck !   A practice dead stick landing didn't quite go to plan, and the undercarriage was neatly ripped out of the bottom of the fuselage :(  The only other damage was to the wing spars on one side which were slightly bent.   The undercarriage was reattached with some epoxy resin and the small amount of damage to the side of the fuselage above the undercarriage was covered with some "hacker" stickers.

The damage was all repaired within a week or so of the accident, but then the model sat for several weeks over Christmas until the weather was good enough to fly again.  I realised the weight of the model resting on the soft foam wheels was putting "flats" on the wheels, so it now rests on supports with the wheel clear of the ground.
When a few days of good weather did come I took advantage of them and got five flights in two days.


After flying the Decathlon for while I decided something wasn't quite right.  It always seemed to have to fly a bit fast to maintain altitude, and although adding some "up trim" gave a slower cruise it produced undesirable trim changes with speed/throttle setting.   I set up the fuselarge with a spirit level to measure the incidence between the wing and the elevator , and every time I measured it I got the same answer.  There was zero degrees of incidence between the wing and the horizontal stabilizer. 

Looking at the profile of the wing it is slightly semi-symetric, so even with zero angle of attack it would produce enough lift at a high enough airspeed.  So, I decided to  make a  modification to the wing  saddle  to drop the wing railing edge a little to increase the incidence. 

The Yellow line show the original top edge of the fuselarge side where the wing rests.

The Black line shows the original edge which went underneath the wing's trailing edge.  The blue lines show the removed material.

The  Green lines show the 1mm  of material that has been removed.

And the results....

I've only flown it four times since making the above modifications, but they certainly seem to have had a beneficial effect.  The model now flies slower without the former slight nose up attitude and with the elevator trim set neutral.  The last time I flew it was on a dead calm day, and I took some time to try out gliding the model to see how it behaved.  This has given me more confidence to make slower approaches without fear of stalling, and my landings have improved as a consequence.

Modifications 2

I've always been a little concerned that after a flight the rear most cells in the battery pack are a little warmer than those at the front.  The battery pack stretches from the fire wall back into the cockpit, and the cells at the front get cooled by the air flowing over them in the enclosed part of the battery tray.  The rearward cells are not enclosed and the air flow around them is thus lessend.   Air can also enter the cockpit through the "fuel bung hole" in the firewall (see pictures of the motor mount above) but the is no obvious place for it to exit.  So I've made a exit vent.

First of all I cut a hole in the underside of the fuslarge just behind the servso tray.

Then I made up a wedge shaped cover for the hole, the idea being that it would suck the air out of the cockpit.

Here it is glued inplace over the hole.

Hopefully I'll get a chance to fly tomorrow morning and then I'll decide if any thing else is needed to increase the flow of air over the back part of the battery pack.
UPDATE: I had three good flights with the Decathlon this morning, and the extra vent does seem to have had a beneficial effect because after the flights the rear most battery cells were only a little warmer than the front cells where as before the temperature difference was much greater.  Now I just have to paint or cover it.