Skymaster L39 build
 

Thought I'd do a build thread on this as it's a great kit that's worth showing. The kit, turbine and all the other bits have been purchased from Al's Hobbies who have been really helpful with a good price to boot.. Ali has been great and very quick to help on a few little issues with the kit so overall I'm really happy, thanks Al.

So then...........

Skymaster L39

Length: 86" (2200 mm). Wingspan: 68" (1750mm).

Power: 17~27Lb (7 ~12kg) thrust

The ARF pro version of this model is a very comprehensive kit. Included is a very well finished composite airframe, wings, stab fin etc... The control surfaces are all live hinged except the rudder which uses standard pin hinges and is also the only surface that doesn't come pre installed. All parts come pre painted with molded panel lines and rivets. One thing that really impressed me was the fit of the wings, stab and fin to the fuz, its perfect! The wings and horizontal stabilizers are removable for transport. The fin is removable but only for access to the rudder servo.

Its not my favorite L39 scheme but is growing on me, should look great once I've finished with it

There's loads of scope for extra detail but I think I'm only going to give the airframe a wash to bring the detail out. The detail itself has been nicely molded with crisp flush rivets and hatches. The hatches seem a little to prominent for my liking but for an ARF it's very good.

All the retracts, oleo's, wheels, brakes, associated air lines and valves are included. The quality is very good, my only concern is that the spring in the nose oleo is very soft. The first thing I'll do is fit the retracts and check the models stance is OK bearing in mind the extra weight that will be on the mains. I really want it to sit with zero AoA or at least slightly positive one. Air rams and the stuff associated with the gear doors is also included. Just a note on these doors, all will require strengthening as they're a touch on the flimsey side. I'll be using an Airpower digital valve for all the pneumatic functions instead of the supplied mechanical and Jetronic valves. The jet pipe is also included. The extras that I have are the Kevlar fuel cells, full depth cockpit plus pilots and a bypass although I'm undecided if that will be used.

Going into the model is a JetCat P120SX with close to 30lbs of thrust, all control surfaces will have JR DS8411's and the receiver is a Spektrum 12100 with two 3300 nimh's.

There are a few small issues with the kit supplied.

• the bottom of the intake at the turbine end is cracked
• the forward gear door is distorted
• the former the pipe sits in at the back end is too big
• the front oleo spring is too soft

All that's left now is to build the thing................

I'm starting off by installing the retracts and gear doors. This has to be my least favorite part of any build more so with this model as I was expecting them all to be pre hinged. Instead I was confronted with 5 loose fiberglass doors, a pack of offset hinges and no guidance as to how to fit them!

I placed the main retract units and oleo's into the wells and made a temporary fixing with a few self tappers. I had to pack the outside edge of the units with 1/32 ply to get the main gear doors to close, I'll make a hysol fillet so the retract sits on something solid. The retracts will eventually have captive nuts and M4 screws securing them.

It wasn't possible to position the supplied offset hinges pivot point correctly as there isn't enough room between the central reinforcement plate and the gear door. I didn't want to use pin hinges as I don't like the look of them when the doors are closed. Instead I cut the ply plate out and installed two 3mm sts studding at the correct point for the hinges to pivot around. I used some double sided tape to test the inner doors actuated properly. I then added some carbon sheet just above the studding for strength. The center fuz between the gear doors is now rock solid. The outer gear doors were supplied as one plate with no cut lines. I had to add some fixing points for the retract hatch as well as work out the shape of the gear door. I've fixed these outer gear doors via a hardwood block hysoled onto the oleo. I made some plates to fix the air rams and the door gets actuated via the hinge.

The next dilemma was the gear door on the nose. This was supplied as one long gear door, if left like this it would have to remain open when the gear is down this in turn would mean more valves and work. Something I wasn't keen on doing especially as it's not scale. The solution is to cut the gear in two. The area around the oleo will be fixed when the gear is extended with the main nose gear door closed. I've also added two little doors to the fixed part so when the gear is retracted there's no gap. This is the scale arrangement. I've seen loads of pictures of this model with no gear doors I now know why. I'm pleased with the results and I think functioning gear doors add allot to the overall appearance so all in all its worth the effort.

I wanted to fully assemble the model early on in the build to check it's 'sit' when on it's wheels. It became apparent that the nose spring is way too soft and the oleo itself wasn't really long enough. I decided to pack the retract bearer as much as I could to lift it slightly. Ideally it needs a 110 degree retract but I think I'll stick with what I have. I just need to source a new spring. I'm sure the application of thrust would bottom out the oleo creating a negative AoA making the rotation a real jerky jump into the air. I'm also flying off grass so I really want it sorted before It fly's.

Will try and post some pictures of what I've done on my days off.

Cheers, Alex

RE: Skymaster L39 build
 

if it is a arf pro version why are the gear doors and retracts not fitted?
what is so pro about this kit if you have to do everything that you do with any other kit?

RE: Skymaster L39 build
 

Great to see this build thread Alex, I will look forward to seeing how you get on with it. I have just ordered a Flyeagle L39 1:5.5 scale via Jetline models. Should have it in about 3 weeks.
Ian

RE: Skymaster L39 build
 

Hello

The gear doors should have been fitted but the pro also concerns whats included which on the pro version of the kit is everything bar turbine and radio. I purchased my kit at a price that more than made up for the gear doors not being fitted. So I'm not that bothered, I'm a bit fussy too so its probably best I install them.

Hi Ian, hope you find this thread helpful.

Cheers, Alex

RE: Skymaster L39 build
 

Hi Guys

The forward gear door was distorted so I took it to work where I found the perfect radius on the tail cone of a CFM, if I do it 3minutes after shutdown the temperatures just right if I've used more than idle reverse, I did get some strange looks from the engineers. It sorted the gear door out anyhow. Ali did offer to try and get a new one but I wanted to get on with the build.

RE: Skymaster L39 build
 

Test fitting the turbine it became apparent that it's sitting too high so I've bolted the mount onto the bottom of the ply rails lowering it just enough so it sits in the middle of the inlet and pipe. I never like bolting down onto ply as it bites down a long way into the wood and because of the way Ive mounted the bracket when the model is pulling G there will be loads of stress on these four small areas so I made some glass board plates to bolt down onto. For turbine brackets I always use aeronuts these are a bit like nylocks but don't contain any plastic so ideal for high temperature applications such as this. I wanted the turbine as far forward as possible so I trimmed the inlet ducting back to the former and found a position I was happy with allowing a suitable gap between the pipe and turbine. This also means that the pipe doesn't stick out the back end of the Jet. The two fuel lines will go though the ducting and out of two holes as the bend to get round the inlet is a bit tight. I fixed the pipe with it's two steel extensions. I used 2 socket head screws a side which proved to be ample as it's rock solid.

RE: Skymaster L39 build
 

The former in the tail that holds the pipe is a touch on the large side so I added four spacers for a snug fit. There are a few scale mods Im going to make starting off with removing the ridiculous depiction of the nav lights, for some reason they've painted the probe mounts red and green. The pitot probes on the L39 are a big feature in my eyes so these will be added and made removable, while I'm at it I'll paint the nav lights which are pre molded into the tip tanks red and green. I may add landing lights but depends on what time I have. Making the probes was a simple matter of tapping some ali out, sticking it in the hole I made in the leading edge, then using some 3mm studding and different lengths of ali tube made a removable probe. Some of these Hungarian AF L39's had a black strip on the leading edge which I'm going to add helping to cover up the probe mods I've made.

Obviously the below picture isn't a H AF L39 but it shows the probes nicely.

RE: Skymaster L39 build
 

Installing the forward cockpit was a real pain. First off I had cut 9mm off the inside edge of the fuz frame to get the cockpit to fit, as it happens this looked much better anyway so I trimmed the canopy frame too. The addition of some internal supports, two slots to help slide it in and some butchery on the cockpit itself had it positioned nicely plus it's easily removable. When
you place the canopy on it helps secure it further. I also re glued the fixing pegs for a neater look. I'll fix the cockpit with a few screws but waiting for the rear cockpit to arrive from Ali.

RE: Skymaster L39 build
 

Servo installation should have been a matter just screwing the servos on the mounts............ The problem being is no mounts have been installed or provided! The suggested servos for the elevators are JR midi size. I didn't like the idea of this and really wanted to fit standard size high torque servos. This required loads of surgery to get the 8411's to fit, mainly removing excess glue and enlarging the hole slightly. A very sharp wood chisel dealt with the glue and BVM scuff pads did the rest. Hard wood mounts were fitted then it was just a matter of gluing the horns in with Hysol. Flaps and ailerons were simple enough with a little trimming but the rudder was a faff as again no mount was installed so I fabricated my own.

The servo you see in the stab is only my test fitting servo I'm going to use 8411's. Didn't want to get them dusty and scratched

RE: Skymaster L39 build
 

Before I do the the final fitting out I wanted to improve the internal appearance so It was all sprayed grey, this especially helped the appearance of the canopy frame as it's always visible. I also ordered a decent pilot from www.moderlersloft.co.uk not sure of his fit so just got one to start with. Obviously this all adds weight but with a 120SX there won't be problem however I wouldn't recommend it if using a P80 or similar.
I gave the area behind the rear former that the pipe sits in three coats of ceramic paint just as a precaution as the pipe now sits inside of the top back part of the fuz. I used some heat resistant sleeve and cable clips from Als Hobbies to run the elevator and rudder cables in. It's excellent stuff and really neatens the installation, much better than plastering aluminum tape everywhere. The air rams have been installed onto the mounts I made. All pneumatic tubing has been wire locked on for some extra security. I replaced the main gear door rams with some Ultra precision units, Ive never had a problem with these rams unlike some of the Chinese ones.

Cheers, Alex

RE: Skymaster L39 build
 

Alex, looks good so far but the only thing I would change is the intake. You do not have enough room between the motor and the intake to get cooling air around the engine. It will overheat, trust me, I had almost the exact same setup on my F-20 and had bad heating problems. Just cut open the top of the intake and you should be fine.

RE: Skymaster L39 build
 

Ian

I'm starting to build one of these and I'm interested in your comment. What do you mean by overheating? is it the turbine temperature read? or is it the fuselage that is getting too hot?

Can you post a picture of the cut you made to yours?

Thanks
Jack

RE: Skymaster L39 build
 

The exhaust temps in the rear of the fuse will begin to heat up and blister the fiberglass. There needs to be enough cooling air to flow around the turbine and mix with the exhaust in the rear to keep the surrounding area cool. That inlet will not let enough air around the turbine. I will try and post a pic tonight Jack. Simple fix, since the inlet is cut flush with the bulkhead all he has to do is remove some of the top of the inlet and it will be fine. I would also line the fuse near the exhaust with ceramic blanket or metal tape.

RE: Skymaster L39 build
 

Thanks for the input. Wouldn't mind seeing a picture of what you've done..... Would you cut the intake back a bit further, never had this problem before. I do have the Carbon bypass but decided not to fit it maybe I should use it?

Cheers, Alex

RE: Skymaster L39 build
 

I will post a pic of what I did tonight Alex. I personally don't like bypasses so I dont use them but it could be an option. Not sure if it will help in this case though since you are fitting a motor that takes up most of the space between the pipe and the intake. I think you would still run into problems.

RE: Skymaster L39 build
 

thanks for the explanation, I was thinking of using some blanket in the area between the cone and start of the pipe anyhow. I'll take a bit more of the inlet off.

Thanks, Alex

RE: Skymaster L39 build
 

understood!!! thanks a lot for the explanation

By the way, just moved to the bay area a few months ago and have been flying at woodland. Haven't met you yet. Are you planning to go to woodland jets in a couple of weeks?

Jack

RE: Skymaster L39 build
 

Not sure yet Jack. I usually fly at Tomcats although I havent been there in a while. I do most of my flying at the jet events and the Willow. I might be up at Woodland Davis though, if so I will be sure to track you down!

RE: Skymaster L39 build
 

Hi Alex
I like what you have done on your build so far.
Just one question. Is there recommended distance between the turbine exhaust cone and tail pipe?

I’m still waiting for my Fly Eagle L39 to arrive, apparently its having its paint job at the moment.

Ian

RE: Skymaster L39 build
 

Hi Ian, Thanks......

Started building professionally over the winter as my full time job is very part time for that half of the year. Pretty much exclusively jets although I've just finished a 150cc Extra. The only down side to it is I've become even more anal about the details than I already was, just want to give the best model possible to the customer.

To answer your question................JetCat recommend a distance of 3/4 inch between the engine tail cone to tailpipe bell mouth. I used this distance coupled with the length of pipe to position the turbine, I didn't want the pipe poking out the back, luckily this put the turbine at the front most position helping with the CG. Had to heat proof the back end as the pipe sits a little inside the fuz just at the top.

Hope the FE L39 build goes well.

Cheers, Alex

RE: Skymaster L39 build
 

Hello

I neglected to buy any brake tubing which has really held the build up as I'm unable to start doing the final installation. While I'm waiting for the postman I've started to turn my attention to the scale details. The probes have been made as described earlier and Ive applied burnished aluminum tape to all the control surfaces where they exit the wings and stab from. This was initially just a white plastic that had a poor finish so some careful sanding and the application of rivets has really enhanced its appearance. The whole airframe has been flatted back with 0000 wire wool ready for the weathering process and application of some custom decals there's literally hundreds to apply which is going to take hours of intricate work so not looking forward to it.

The flash spoils the photos, it look much better in the flesh.

Cheers, Alex

RE: Skymaster L39 build
 

I cut the inlet back a bit to allow some more air in the compartment, The second photo shows the difference to the finish once flatted back.

RE: Skymaster L39 build
 

Made the wiring up for the flaps and ailerons. Ive used a powerbox lead with a multiplex plug pre installed. I then used ashlock connectors for the servos, should make it easy to assemble at the field with this in mind I installed BVM easyair into both wings which now means its just a matter of sticking the tube in the wing and bolting it up with no mis-connected lines.

RE: Skymaster L39 build
 

Next on the agenda is to dirty the airframe which will highlight all the molded detail. The finish out the box is good but with a few hours work you can really enhance it.

It's very easy, all you need are some water based paints, two rags, a hairdryer and some ultra fine wire wool.

First off you need to decide on the color of the wash, some artistic license is required here as the color depends on the base coat, for example, on this model there are three main colors so three different color washes will need to be applied obviously the real aircraft wouldn't be like this but because of scaling effect etc... this technique gives the best results.

Mix up your paint and then add water to get the desired consistency. I'm going for a subtle look underneath, the grey is very light so anything dark would over power it with this in mind I made up a muddy mixture using Humbrol acrylics. Wet a rag out with the paint and wipe it over the areas you want to highlight. Use the hairdryer for a few seconds, don't over do it or your going to actually dry the paint onto the base coat you want it just before it goes tacky. Working quickly wipe the paint off again with a clean dry rag. Don't worry if it's smeared you can go over the finish with some 0000 wire wool which will blend it in nicely. Once your happy with the effect you can move onto highlighting certain areas. For this mix up a darker wash and apply it to the areas behind panels drag back in the direction of the air flow and do as described above. Blend the streaks in with wire wool. Parts will have to be airbrushed but you can do the larger panels like this, I'll do the whole airframe together as it's a hassle getting all the spray equipment out.

again the flash spoils the photos a bit but you can get the idea.

Cheers, Alex

RE: Skymaster L39 build
 

Hi Alex,
Could you let me know where I can find ashlock connectors.

Thanks
Ian