FOX CUB with TJ100 E-Conversion (Video and build)

[dennisp]

The Horizontal and Vertical stabilisers

One thing I forgot to mention from the start is the kit does come with CD which contains the building pictures and also video which my PC just could not understand... perhaps it was in Chinese...! The build pictures really are not that helpful and focuses on the turbine build and the fitting of the heat reflecting tape and a few other items that really do not help you.

So back to the build, there is nothing difficult about assembling the two stabilisers, but do not glue the hinges yet otherwise you will be in real trouble just make sure that everything fits OK. As DT always remarks one day Dennisp will build a model without any modifications....!, well this one is no exception and the first modification is about to take place.

As I mentioned at the start of this part of the build there is a lot of silver heat material applied to the back of the model to ensure the turbine does not set the model on fire..!, so the manufacturer provides a “U” wire coupler to join the halves of the elevator together.





This picture shows heat reflecting silver tape and the elevator and rudder servo’s for the turbine installation.



As we are using a EDF for power there is no issue with heat, so it possible to go for a dual elevator servo one on each side of the fuselage rather than a shared servo with that not so nice “Y” junction. Most of us now have a computer radio so the use of two elevator servo’s should not be an issue to set up.

So two holes were made in the fuselage sides to mount the HS-225MG servo’s.



This leaves lot us lots of space to mount the rudder servo so that there is no chance of any binding of the push rod wire with the two elevator servo’s.

Now it is time to focus our attention on the fuselage....

The Fuselage

This part of the build is to assemble the landing gear ready for the next set of dennisp modifications....! The main landing gear was initially fitted to the fuselage using #6 x ¾” self tapping screws which I had some reservations about, but all the same I pressed on.



The nose gear is very simple a cast bearing which is secured to the fuselage with 4 x self tapping screws and the job is done...!



Now the last part of the fuselage work is the cutting of the holes for the vertical stabiliser , horizontal stabiliser and the wing fixing holes, my preference for this is to use a hot soldering iron (Precision Aerobatics method) as it seals the edge of the covering on the hole nicely.





Now it is time to see if the electrocution of the Fox Cub is going to work with a quick test fit of the fan (not secured just held in place), all seems good so time for a cupper and burn one.



Installing the servo’s in the Fuselage

The NLG is easy, insert the supplied rod in the landing gear tighten the screws and fix the horn in place, but do not forget to grind flats on the rod for the screws to bit into, this is going to be 11lb + model and they will sure slip if you do not do this.



In the picture you can see I have used ball joints at both ends of the steering push rod to allow for the slight angle that the push rod takes up.

Moving to the back of the model it is time to fit the three servo’s the two HS-225MG’s for the elevator and a high torque standard servo for the rudder. This picture clearly shows the safety washer between the ball joint fixing screw and the ball joint.



Next step of the build will be initial installing of the fan......

[dennisp]

Preparing the Fan for installation

I am sure that most of us agree we learn by experience, well at least I do.... the Boomerang and Velox electrocutions did not perform quite as I had hoped, so I had done an investigation into why LAH..?

The TJ-100 fan comes with the ESC in the tail cone great but not great as you need a long tail pipe, I changed the tail pipe on the Velox so the fan tail cone was not protruding beyond the exhaust pipe and the thrust shot up by over 50%. Having learnt this from experience time to consider what to do to the Cub especially as the fan hangs under the fuselage and that real long tail cone would required a long exhaust pipe. Needles to say a simple solution remove the ESC from the tail cone of the fan and shorten same...., the picture shows the normal tail cone and the shortened version.



With the fan preparations completed time to do some simple assembly work again..

Completing the Vertical and Horizontal stabilisers

Nothing special here just like any other model, make sure that you remove the covering where the horizontal and vertical stabilisers join the fuselage.



I use slow 60 minute Epoxy as it will give you time to get the alignment to the fuselage correct, once all the glue has dried then the elevators and rudder can be fitted. DO NOT chop the bottom rudder hinge short, use a drill so the hinge point goes through the rudder bearing mount and into the fuselage, this will save any chance of a bottom hinge failure at high speed if the rudder is used.



Back end is now completed and servo cover in place.



Odds and Bobs tasks

Before entering the electrocution phase and all the relatively small modifications just a few minor things to be done, the MLG wheels are fitted to the MLG.



Next the Chinese puzzle that comes with the model....?



These are the supplied wing bolts AIYO me no like, so 4 x M3 socket screws and wings nut a much nice solution....



Next step the electrocution modifications.....

[dennisp]

Electrocution modifications

I mentioned a few pictures back I was not that impressed with the idea of using self tapping screws for he fixing of the MLG. There are two rails going from the front to back of this bay of the fuselage which are quite long (about 5 to 6”’) I could see my enemy Mr. Gravity assisting the ripping off the MLG one of my special returns to mother earth on landing (actually mine are more like un-controlled crashes...!) and the rails breaking.



So we need a little extra strength at a low weight increase, you will spot in the picture there is a 1/8” piece maple ply glued between the two rails and the fuselage bottom and a small piece at the front edge of this. Next another piece of 1/8” maple ply closes the box in with two top strips for the M3 blind nuts and we have the classic low weight box section. Lightening holes will be added before this is all glued in place.



Time for a bit of light relief from all the saw dust I had created with a trial fit of the office ...



Next step of the process is to make the systems tray a nice simple job with one support at each end to allow the tray to be removed.



Fitting the ESC

I had determined early on (well at least I thought so....!) that the ESC would sit nicely under the fan with lots of nice Singapore warm air passing by to keep it cool.



Needless to say my sums had failed by a tad and the fan would not sit on the mounts properly grrr...!, you will note that I had marked the fan mounting plates and drilled holes so that the fan mounting plates could be moved from the front to the back in two 1 Cm positions to allow other fans to be fitted.

Time to get into trouble with the MOHA again and a hole was cut to allow the ESC to sit in the fan mounting plate.



Not very elegant so only 3 out of 10 for that hole, but this was the hardest part of the modification, the fan mounting plate is almost ½” thick and the Dremel was not a happy bunny doing this task. I had to use tungsten carbide cutting disk to do this task LOL one expensive cutting wheel, still once done the result was as required with the ESC sitting nicely in the expensive hole.



Now it was the fun part of the conversion and a full test fit of the fan.



Next step fitting all the bits and bobs that allow the model to fly..

[ERJets]

I always enjoy reading your build thread.
Well done Sir.

[dennisp]

Thanks Ernest, once the build blurb and pictures is completed, I will then do a small section on the hits and misses of the kit and flight data gathered and the MRO required from the first flights, so please stay tuned.

[dennisp]

Bits and Bobs fitting

It is nice to tackle something easy and the obvious choice of the few parts remaining was to find a home for the air tank, and this just happened to fit with some anti-slip perfectly in the rear bay of the fuselage.



Next on the agenda was to complete this area of the installation with a small plate to locate the receiver and servo pack, power switch for the receiver and servo pack, air pressure gauge and the air fill valve.



One more tray to fitted out and that is the centre tray for the receiver, 20Amp BEC, the flight data recorder and associated 2.4 GHz transmitter and the air valve for the brakes. There is loads of room on this tray so plenty of choices and options can be made.



Still following the process of moving from back to the front of the model the next step was the ”Office” or in usual terms the cockpit, this is really large and deserves some attention but the initial thing to address was how to retain this item on the model. The kit includes what I would classify as a nasty all plastic canopy catch which if you are brave you could glue to the Perspex canopy, this may work but it is not a good solution in my book. So more sawdust and a very simple support for a Fly Fly Models catch which is a Rolls Royce compared to what is provided so that when the glue is applied to the catch the loads are not all on the Perspex which has a tendency to split and crack over time.





Time to fit the fan in its home which is so simple using the jubilee clip securing method, once again some anti slip mat to stop it moving. The jubilee clip MUST go over the rear of the fan where the stators are, as the stators and motor tube make the fan case solid. If you fit the clip over the front of fan you will distort the fan casing and find the impeller rubbing the fan casing. The intake ring was fabricated in the same manner as the Boomerang and the Velox using soft balsa sanded to shape, filled with car body stopper, sanded and then sprayed. In the picture below you see some small 1/64” ply plates (x 4)which are glued to the intake ring and will be held in place with Capt. Harn’s sticky tape solution just to stop the intake ring falling off.



With most of the bits that had been scattered around the bench now installed the MLG was fitted and just behind are two Robart air line disconnects so that the MLG could be removed with the hope of the beast fitting inside my Volvo.



With almost everything installed it is time to fit the nose on..... oh boy this model is getting big.....!, the nose is fitted with the pitot that provides the pressure feeds for the flight data recorder for altimeter (height) and air speed data.



The entire nose cone is secured by 3 x M3 screws to the fuselage, which is a small challenge as the access is from the front tray (battery area) so you either need 12” long fingers which I do not have or in my case a long hex drivers for the screws. Some time was spent doing this and a few English rugby songs sung and other not so nice statements before I found a way forward. I used some 25mm long M3 socket screws which I fitted with the nose cone off from the front of the fuselage, Mr. Gravity applied some help here and these sort of hinged down towards the floor. Next with the long hexagonal screw driver the first screw ( bottom one) was fitted just a few turns so the nose was held in place by this screw. The process was repeated for the other bottom screw and finally the top retaining screw, the just tighten away and job done.



Time for the plumbing and wiring and the major part is the wiring and the usual spaghetti of wires which due the low location of the middle tray made it very hard to tidy up.









So as always the last job I do as I am not an expert at making cockpits like Ernest and Paul, I need to do something with this big open space at the front...! During one of my occasional visits to the Fook Hai Hobby centre I came across a nice looking instrument panel in one of the LHS and by chance it just fitted nicely with of course a few minor modifications. A balsa bulkhead was fabricated with some support strips to the front of the canopy which would eventually have a piece of artists black paper glued to same to form a cowl.







Pilot seated in place for his hot rides and canopy secured with canopy glue all set for the final trimming.





So we are finally there a completed electrocuted Fox Cub.



With the model assembled and the MOHA out shopping time for the weigh in and also a check of the wing angle incidence, we need a positive value on level ground otherwise we will have the have a very big EDF buggy, the meter shows about 2 degrees which is fine for semi symmetrical wing.



Some brass tube was bonded to the MLG to hold the brake air pipes in place and then it was time to burn one and have a cupper.



Next will be the evaluation of the kit (likes / dislikes) and the analysis of the first two flights

[dennisp]

Analysis of the kit

Let me first state these are my personal views and may not be same as other builders of this model, with that out of the way let us move along....

Value for money

This is always subjective, but it is cheaper than a Boomerang Sprint (similar size model and construction) and about the same as a normal jet model (P-60 size) without the retracts and you can buy it in Singapore no shipping costs.

Ease of conversion

This has to be the best one so far as the amount of modifications required is minimal, no ducting to make or alter just a simple balsa intake ring in the case of the TJ-100.

Dislikes

Oh yes this always the fun part.....!!

As always the quality of the hardware provided is the usual Chinese standard, in my world that means file in the circular bin in the workshop.

Push rods provided are far too small for a turbine model, more filling required.

The provided instruction CD would have been a lot more use if it had shown that the hinge point holes were all pre-drilled.

The covering material is aka Chinese and tends to lift, so a good iron (not with the MOHA one please) is required and even a blast with the heat gun. As a side after the second flight a complete piece had decided to move along to pastures new. With the speed this model can achieve this really is important and worth the time spent to ensure all the covering is secured.

The front tray is glued in position so a little fiddly to fit the pack tie down’s and I am sure a nightmare for the turbine guys and all the bits and bobs that they mount here.

Likes

The fan mounting, so simple and allows the rapid changing of the fan for either MRO or testing different makes of fans.

My mistakes

Yes even I screw up.....!!!

The centre tray should be raised higher to allow easy access to the receiver and air valve, my original logic was if the packs need to move back then they could move on to the tray as needed.

Needless to say not finding those holes for the hinge pins and having to make good one flap.

References for the build.

RC Jets International – June / July 2007
RC groups URL - www.rcuniverse.com/forum/m_5325153/tm.htm

Next I will review the flight data and some additional comments....

[dennisp]

Flight Data

We have now gone full circle form the videos of the flight, to the build and data that was captured by the Flight Data Recorder (FDR). There were a few technical problems with both of the flights in that I am a mode 2 pilot and the CTP (DT) is a mode 1 pilot the sticks are all vickey verky....! We had some challenges in getting the buddy cord system working as the Futaba transmitter type data converted would not convert the Fox Cub settings on either the 14MZ or 12Z to CTP’s 12FG. We later discovered that this could be that the 12FG does not have as many wing and tail types as the other two transmitters, after a lot of programming we achieved the goal and CTP was able to fly the model.

Due to this odd ball way of doing things the trims were not doing a lot on the 12FG and I set the trim levels on the 12Z, still all these computer issues will be resolved for the next series of test flights.

The pack configuration for both flights was 1 x 5S - 5,000mA 30C Poly Quest and 1 x 6S - 5,000mA 30C Poly Quest giving us a 11S configuration which I believe is the maximum the TJ-100 with the Neu 1515-3D motor can take.

1st Flight

Here are the data graphs for the flight.



With the Power Watts being consumed by the motor in Orange




Points to note is that we used almost all of the 5,000mA capacity the packs had available, which was later confirmed on the re-charge of the packs which required over 5,000mA to be put back in.

2nd Flight



With the Power Watts being consumed by the motor in Orange



Although the CTP gave her more boot so to speak we seem to have used less power, I suspect that this due to moving the Cg back about 1 Cm and not requiring so much up trim for level flight. I need to do some further checks in this area......

For both flights the maximum Amp draw is over the 100 Amps which I had set as my target, so I have done a prediction of how the model will behave on 10S.



The Amp draw has now dropped to 87Amps which is lower than the recorder 101A for both flights and should preserve the packs, there is always a trade off and we seem to lose about 5 mph and a 20% reduction in our maximum rate of climb which would still be very respectable at 29.69 Ft/sec.

Things I need to do from a FDR perspective is try to get a better X axis scale so we get more clarity which should be possible and if possible change the colours of the plots so that they are more visible.

So was it worth it...??, for me yes in so far as I was pretty confident in the 1st place that there would be no issues with the electrocution of the Fox Cub, however now that I know that the FDR provides the data I require then when fitted to the Velox we will have a much better idea of its odd ball to say the least flying characteristics.

In the case of the Fox Cub the FDR only recorded the throttle positions but has the capability to record the aileron, rudder and elevator positions which again will be useful for the Velox and other projects that I have stored at the SSW waiting to be attacked by me.

The last landing......

The failure of the ball race in the MLG was not expected but I had been warned that the brakes as supplied were not a lot of use and leaked air like crazy.



The cage of the ball race can be seen in this picture and the separation of the balls to casing, it is also interesting to note the large gap the each side of the ”O” brake seal so any pressure applied would just leak past the sides of the ring.

MRO.....

MRO is in progress and I hope to have some Jet 1A wheels for the MLG, so we should have brakes next time CTP...!! DT has provided me with a new MLG leg which has a wider track so that will make the model more stable on the ground, so a mixture of fixes and improvements in progress.

As part of the flight test program I would like to try the lower cost fans like RCL Lander 120mm fan (when are you getting these Capt..?) and the JP-1000 and JP-1200 fans, the objective here is to prove you do not need to have a expensive Schubler, Tam Jets or Stumax to have fun with HP EDF flying

Well that is the story so far, I will provide updates as and when they become available on this thread.

If DT (CTP) has a spare 5 it would be great to hear comments on the flight of the Fox, he seemed quite happy as he wanted me to fly the bird on the 2nd flight...!!

Thanks once again to all those who provided the support on the big day.

Happy and Safe flying in Singapore..

[dt8666]

Hi Dennis... this is by far the best and most comprehensive build log.. Welldone sir...

OK.. I was typing the whole flight report for almost 20mins and a wrong key press wipe out all my work! sorry,,,, need to redo again later.. this time learn the lesson and will not type direct to the post reply!

[dennisp]

Oh dear sorry to hear that sir...it is that hot "buy button" action