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Nov 02, 2018, 04:39 AM
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Build Log

L-39 Albatros - Freewing 1053mm span 80mm EDF


I was thinking I might need another plane.... another EDF jet....
Hmm, I like the look of that Freewing L-39 Albatros 80mm EDF jet!!! So on the spur of the moment I ordered it from HOBBY PARADISE in China..... and TWO DAYS LATER it arrived from China!! (to Australia!). Incredible....

I had quickly read up on it, to be sure it would be at least 'good' or 'OK'. Everything I could find was positive, thus I just went and did it!
Even though I prefer 'larger' 90mm jets really, I decided to give this slightly smaller 80mm class a try.

The L-39 is the 'latest generation' manufacturing ideas and parts etc. It is just all done SO WELL !! Every aspect of it is done 'properly' and you really can just assemble it and fly, and it is very likely just going to KEEP WORKING.
Well, maybe some servos/electronics could still be not totally trustworthy.....

The painting is excellent, the decals are excellent, the engineering ideas and implementations are excellent, the landing gear setup is excellent.... it is just all so well done!

Assembly took about one hour.....
Painting it fully in WBPU took another hour. The reason for WBPU is many fold:
1) Add a gloss finish
2) Add a small amount of surface strength
3) Slicker... lower drag.
4) Easier to clean!
So using WBPU on ALL foamies is highly recommended!

I am not sure when i will get to fly it as the weather outlook is not good, but it will be ready to fly as of tonight.
Ordered.... sent from China to Australia..... ready to fly on Day 3.... !!!

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Nov 02, 2018, 04:51 AM
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Some aspects


Being a PNF, and one that is truly so close to ready to fly as you get it (!!) I probably won't have a real lot to have to outline about the L-39.

The pics below show some aspects.

Freewing gave a long wire "servo lead capture" thing - nothing special but nice that they include that.
HobbyParadise threw in a free Lipo checker/alarm thingy..... Well, it is better than nothing! LOL
There extremely fast shipping sequence is probably NOT the norm, but the few things I have bought from them have always been in expected/good time. This L-39 was just AMAZING.... incredible... unbelievable.. to have been so FAST to arrive!

The tip of the nose (nose cone) and the fuselage tail end have to be glued on. Freewing give the typical foam contact adhesive type glue and I decided to USE that for this aircraft! Let's do what THEY suggest/provide..... and we will see what happens over time.....
I did clean the tail joint with Isopropyl Alcohol, plus roughed up the foam a bit, then glued it with the recommended "apply it, join the parts, take them apart for 90seconds or so, then make the final joint" manner. You are basically using a CONTACT Adhesive, so that is how to use that.

The wing wiring uses 7S type JST-XH connectors, like batteyr balance leads have. To do all the connections they needed TWO of those.A benefit of JST-XH is they are Keyed, and they also Click/LOCK together so they should be reliable connections.

WBPU just shines an aircraft to look like it just came out of the spray shop.... lowers drag.... toughens the surface a BIT.... and makes it easy to clean off any mess that tries to stick to the surfaces.

The servo pushrods use ball links on all connections, so that is a nice step up from the old days of cheap, weak, plastic quick link type connectors!
All hinges are pin pivots (not foam hinging!) and the hinge qualities are all very good.

The L-39 has a LOT of decal detail! Lots of decals! So that adds a lot to the overall scale look of it. On top of the really well done paint-job, and the scheme they chose to do.

The landing gear are all trailing link with good sized wheels, so flying from grass will be EASY! This was another very important aspect of the L-39 that made this choice quick and easy!

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Nov 02, 2018, 04:59 AM
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RX/Flight Controller


I am using the FRsky S8R combined Receiver and Flight Controller. Not for any real need or specific reason, just that I only buy S6R/S8R's now.
It will allow setting up some 'semi automated' flight sequences at some stage. Plus the plain Stabilisation functionality if that proves useful some days (windy).

To make the mounting position of the S8R more suitable I had to move/remount the Freewing control unit (MCB-E). I rotated it to be lengthwise and to the left side of the fuselage. This also allowed a narrow path fot the ESC/Battery leads to run along the fuselage inner edge nicely.

The S8R was mounted to a 1mm thick alloy plate so that it can be stuck on with 'gyro isolating tape' but then the mounting plate is screwed to a mounting block, which allows you to remove the RX by unscrewing the mounting plate.

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Last edited by PeterVRC; Nov 03, 2018 at 07:14 AM.
Nov 02, 2018, 05:03 AM
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Battery Tray


Whilst the stock battery tray setup is quite good, I still saw a more useful, 'better', way to set that up.....

I am using two 3S 5000MAH 60C batteries connected in series - not one single 6S - so those are wider than the single battery would be. The stock strap on their tray is not suited to that wider span. But also having a longer battery tray means more ability to move those around (even staggered) to set the CofG well, and with good flexibility.

The stock tray was removed and some narrow rails added to the stock rails - this is to bring the mounting level up to match the foam levels fore and aft of the stock tray length. Then a new longer tray is epoxied in onto those, and allow batteries to mount all the way to the rear bulkhead.
I cut the stock strap to be two halves that mount to the fuselage sides so that they are outside the battery width.

Unfortunately even with the batteries as far rearwards as they can do the balance point (CofG) is over 10mm too far forwards than required! This needs further measures to correct....

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Last edited by PeterVRC; Nov 03, 2018 at 07:18 AM.
Nov 03, 2018, 07:24 AM
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ESC/Battery leads extensions.


To get the batteries as far rearwards as possible the battery lead end must be forwards - which is much tidier to run the leads anyway. But this also meant the stock ESC leads need to be extended to get them to the forwards area.
I always use a Unisense-E Telemetry sensor for Voltage, Current, Capacity, Barometric Altitude, so the positive lead is set up to include that device.

The remounted control unit (MCB-E) means the ESC leads could be run past that right against the left fuselage side and that keeps it neat and in place. So the wiring all worked out really nice and easy to place when the batteries are installed.

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Nov 03, 2018, 07:28 AM
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Flap pushrod


Freewing mounted the Flap servos 'symmetrically' (not asymmetrically/opposing) to save using a reversed servo for one side. This is a good idea, but they did not place it well and that means if the ball joints on the flaps are both to the outer wing end direction then that right wing Flap servo directs its pushrod way past the balljoint. If you were to attach it the joint would be under high sidewards loading and try to pop the balljoint off! You could BEND the pushrod to correct for that but that looks lousy.

I moved the balljoint to the other side of the control horn and then the pushrod can run straight, with zero side load, and also then looks equal to the left side, and how ALL pushrods run 'straight' and parallel to the fuselage centre line.

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Nov 03, 2018, 07:37 AM
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Tail weight... Strakes!!??


With twin 3S 5000mAH batteries, they weigh a LOT more than a single 6S battery.
The L-39 was really designed for a 6S 4000mAh battery, and being just an 80mm EDF, that is all they really need. Versus 2650/3000mAH for 70mm EDF's, or 5000MAH or so for 90mm EDF's. So using 5000mAH is already a bit much for it, and gets worse if done via twin 3S 5000MAH!!
All in all, twin 3S 5000mAH is NOT a good idea.....

So the AUW ends up at 3250g which to me feels a little bit too heavy. For its size it is more likely suited to 3000g, or a bit less.Which 4000MAh would do. There are so many plastic addons and 'good' stuff' that it has no more leeway for extra weight - and no true need for any. it is just that I HAVE the batteries, so wanted to use those.....
The real answer is "NO - do not use those twin 3S 5000MAh batteries, as the total AUW is too much!"
But I will use them.... and maybe later I will decide I MUST get some 4000/45000MAH single 6S batteries for it. (Boo !!). I will wait and see....

This high AUW, and the heavy battery weight, even with them places as far rearwards as they can possibly ever go(!), means it cannot even be balanced at the recommended CofG... it is over 10mm too nose heavy!! That is a lot for this size of plane.

I tested tail end weight and it need 90g back there to balance out!! 3340g AUW then.... OUCH !!

How to add the weight? Well I had two ideas....
One was to embed the lead into the tail end underside.
The other idea was to add ALLOY Strakes to the tail end underside. As per a number of aircraft types have - but the L-39 never does.....
(Viper Jet, BAE Hawk, F-16)
For some reason I decided to add the Strakes!!! And later decided I preferred to NOT have any there..... DOH!

I can remove them and switch to the embedded lead weight method easy enough anyway, so I will just fly it with the strakes first anyway.

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Nov 04, 2018, 04:38 AM
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Landing Light mod


A few people have modified the power setup for the landing lights - which in stock form only come on when the gear is down - to have them on ALL the time. The reason for this is that many full scale L-39's DO have those wing tip landing lights on all the time..... though I am sure they have it SWITCHABLE anyway, but it is just that many turn them on right away and leave them on.
We could add an RC Switch to make them controllable..... but it is easier to just wire them up to be on all the time.


The Control Unit (MCB-E) provides the voltage source for the all LED's and it has the in line resistors built into it. Each lighting channel - which it has many of (!) - has its own resistor.
There are wingtip navigation lights also (Red/Green) and they are permanently on, so you might think you could parallel up the wingtip landing lights with those. But no.... because the resistance driving TWO led's will then either make them duller OR the voltage will be too low to even 'start' one LED or both! So you need to feed the landing lights their own individual voltage sources, from the MCB-E 'Light" output ports that it has spare in the L-39.
To do this you need to make up two extension leads for the 0.1" header pin spacing connectors and run them from each wing landing light lead that you disconnect from the wing wiring circuit board. Running these into the plane and into the MCB-E 'light' ports.


I forgot to take pics, but it is a simple extension to make up. I used some modded servo leads using just the male plug end - to have just two wires, Pos & Neg, plus then a 2 pin header soldered to the other end. Thus forming a 2 pin extension.
Nov 04, 2018, 04:51 AM
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AUW and Thrust....


I did a thrust test today.
2200g of Thrust using 100 Amps current level. That was not quite the peak and more of a sustained 20 seconds sort of value.

With a 3340g AUW, having 2200g of thrust is a bit weak! Mind you the full scale L-39 has about a 0.37 : 1 Thrust to Weight ratio.... BUT at its high LIFT to Weight ratio, that is not too bad. In the 1053mm wingspan Freewing L-39, at 3340g, which is NOT a high lift to weight ratio then(!), the 2200g is somewhat anemic!
This is just another reason pointing to needing LESS AUW. Something like 2800g AUW !
This would give a good Thrust to Weight Ratio, plus have the aircraft much more nimble, light, as per the full scale is. The full scale weighs around 4000Kg. Versus an F-18 at 20,000Kg. So modern day fighters ARE heavy..... but L-39's Trainers are not!

I will fly it with my heavy batteries, tail end weight, and thus anemic thrust to Weight ratio, but I can already tell now that it needs to be LIGHTER!
So I will all buy certainly need to buy some sub 800g batteries. Preferably no more than 750g really!
That allows for a 6S 4500mAH 30C, or some '60C' brands - but if it is 750g really then you are really only able to have a 4000mAh 60C - which is really going to be under a true 30C anyway. And will need to be a 'normal' type Lipo also... some run of the mill 'nothing special' cheaper type.

One thing to remember is that this is an "80mm jet"......
70mm use 2650mAH to 3000mAH
90mm use 4500mAH to 5000mAH, and tend to need "60C"
80mm use 4000mAH region. And being more down the size ladder, where lift levels per wing area falls down exponentially, it means they don't have 'tons' of lift available.
70mm struggle to do 'much stuff' in terms of good features, good landing gear (oleos etc), plastic parts, as the wing/lift levels at that size are pretty poor. And 80mm only move up the scale enough to give you all those good bits with a bit of extra leeway.
it is only really by 90mm EDF's that you get quite a bit more leeway for all those good things - but even those are only just able to do them all with a SMALL leeway!
Thus the Freewing 80mm's, which come with quite a lot of 'good bits', need to keep their battery weights down as there is no leeway left!

I will not buy another 80mm jet again! Even though this L-39 is FANTASTIC quality and everything really - they are just that little bit limited due to size still. 90mm and UP is where the returns start being much greater!
I had been going to get the Freewing twin 80mm A-10 which is 1700mm !! And now I wish I just had done that.....
Nov 14, 2018, 01:11 AM
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Maiden Flight


Well the Maiden Flight went very well.... sort of.
It took off... flew around nicely, for the most.... was easy to land.....
Then I ran it into a tree on the landing overshoot.... which occurred because I cruised on by in ground effect to take longer to land.... which was because I had forgotten to deploy the landing gear (!!!) on approach.....
The landing looked pretty cool really! VERY low, cruising on by with no landing gear out. LOL

It broke the nose off, and had a few other very minor things amiss but all things considered it was a good outcome - from one that could have ended up much worse!!
It is all repaired and has flown 5 more times since.

The original video quality is very good in 2K, but not so good on YouTube, I am trying to work out why it is so 'average'/lousy and will fix that when I work that out.

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L-39 Albatros - Freewing 80mm EDF - Maiden Flight (5 min 16 sec)
Nov 14, 2018, 01:25 AM
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Rocket Pods, New Small Tanks, Gunpod


This is a Slovakian Air Force L-39
In the form it comes.... I have never even seen any pictures of any Slovakian Air Force L-39 with the Large Tanks fitted. Some other Air Forces do use them and there are pictures of those. Only the Small Tanks appear in Slovakian AF pictures.... though that does not mean they NEVER use them.
But anyway.....

I decided to make up some UB-16-57 Rocket Pods (two), plus Mounting Rails to add onto the Wing for those.
Well really, the Rocket Pods often go on the Outboard Rails which is where the stock large Tanks go..... but if they ALSO have Tanks, then those go on Inboard Rails, so you need those then.

I also decided that a common mounting system would be best, so that ANY items can be placed on ANY Rail positions. So I removed the stock Outboard Rails and replaced those with 3D printed ones also. Plus modified the stock Large Tanks to use my mounting system.

The Small Tanks.... well they are smaller tanks.....
They weigh less than the stock Large Tanks, even with the Rails for them. 60g for the pair versus 90g for the pair of Large Tanks (and Rails included in all cases).

The Mounting Rails are screwed into 'pegs' inserted into the Wing underside, so that all Rails can be removed if you want to.
Sometimes the Slovakian AF L-39's have NO Ordinance, NO Tanks, And/or NO Rails.....

The Gunpod was designed by VFR201 (RCGroups) so I just used that in the form it came - it seems accurate enough.

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Nov 16, 2018, 08:22 PM
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Gun Pod


Not painted up yet....
Nov 16, 2018, 08:29 PM
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Ready for Combat Flights


Too windy today.... war has to wait!

I still need to fully finish off the nose repairs - fill/paint.
And paint the Underwing Tanks in White... and the Rocket Pods in some form of Grey. Though they are probably passable in their stock printed colors really. The Gun Pod had to be painted correctly though.

The Strakes need to be painted Red - though I might design/print a mounting system for the tail down there, so Strakes or lead weights can be fitted/removed easily as required.
But I will probably NOT use twin 3S 5000mAH anymore, so I don't really need tail weight options to balance those. Easier was to buy more 6S 4500mAH batteries! Which I will need for the F-14 Tomcat anyway, so...... the Strakes will come off soon.

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