Piggy18

Looks really good, I'm also going to install a fuse locking bolt at the bottom middle, does the rear cockpit fit with the fuel tank upgrade?

Agrav8ed

I had to trim the bottom of the rear cockpit to make room for the tank but it works fine.
Tone

SAP_2000

Hi guys. I am just finishing up mine and have a few questions.

The CG. Manual says 145-155mm back from LE at the root. Earlier in the thread, one guy said he removed some lead and said it was an improvement with CG around 160mm.
Have any of you guys experimented with a more aft CG? Looking at the wing and from experience with other L-39´s, the CG on this one seems to be way forward.
I remember my old SM L-39. Early on they came out with a CG that was way nose heavy. I experimented a bit and ended up around 40 mm back from the original CG with meant back of wing tube and just ahead of the LG mounts. It became a totally different plane. Elevator throws was reduced a lot, landings and rotation much more scale and could even keep the nose up after touch down.

Have anyone flown it with CG at center of wing tube or aft of that, and if so how was it?

I read somewhere that the neutral position of elevators are a bit up for the plane to fly straight, around 3-4mm.
is this correct? Could it be a hint of that the CG is too forward on this bird?

Any flap to elevator mix required?

What are your opinion on the throws in the manual. Elevator travel of up to 2" seems excessive and might be another hint of a nose heavy plane.

Any other hits or tips prior to the maiden is appreciated

Agrav8ed

The CG. Manual says 145-155mm back from LE at the root. Earlier in the thread, one guy said he removed some lead and said it was an improvement with CG around 160mm.
I think my CG is around 160mm. Not 100% sure because I used the nose drop method and it flew fine there so I just left it.

Have anyone flown it with CG at center of wing tube or aft of that, and if so how was it? I have not

I read somewhere that the neutral position of elevators are a bit up for the plane to fly straight, around 3-4mm.
is this correct? Could it be a hint of that the CG is too forward on this bird? I needed a slight bit of up trim to fly but nothing to bad.

Any flap to elevator mix required? I fly with condition modes so no mixes needed.

What are your opinion on the throws in the manual. Elevator travel of up to 2" seems excessive and might be another hint of a nose heavy plane. I fly with the manual throws on everything except the ailerons with I add a bit more as I thought the roll rate was a bit too slow.

Any other hits or tips prior to the maiden is appreciated. Flies great just like most L39's. I thought the springs in the gear were a tad soft so I added a small spacer inside to increase tension. I also adjusted my elevator a bit as on my jet there was a bit of a gap between the elevator and the fuse. Any other changes are documented in this thread. Otherwise, its a pretty good kit.
Thanks,
Tone

SAP_2000

Thanks Agrav8ed

I maddened it yesterday. CG was at 170 mm or center of wing tube. I will keep it there but don't recommend to go further back. Rotates and lands well, and as i guessed, around half of the elevator throws was needed. I'm at about 1" elevator throw now and even that is on the responsive side.
Ailerons was ok too.

The plane flew very well. 4-point rolls and inverted it behaved beautifully and very neutral as i like it.

But the brakes, has someone gotten them to work at landing speeds? I am very happy i flew at a full size airport yesterday with plenty of runway.
The brakes grip fairly well at stand still and low speed (up to 10-15 mph). But above that the braking force is zero (and i mean zero).

Pulling the wheels off i see that there is no physical contact between the brake "drum" and the wheel itself. Not like other designs using a dick with or without brake pad material.
This brake purely relies on electromagnetism with no physical friction and i can't understand how this could work at higher speeds even in theory. Yes magnets are powerful, but in my experience they need to be close and not moving/ rotating.
This surprises me, with JP who makes a lot of LG sets for many popular jets, and i guess we should be reading about JP brake issues on every forum and FB group. For me, this is the first i seen or heard about it.

And my question is what have you guys done? Or did your brakes work well? If so, are they the same design as mine?
I have the JP brake controller with the 2 adjustable pots and its running on 2S (2-6S specified)

Please see the attached pics.
Thanks

alasdair

I have heard of 2 kinds of "electric" brakes.
One kind has an electro-magnet push or pull a brake pad against a drum or disc, and the friction slows the wheel.
This must be the other kind, in which a row of magnets attached to the wheel is interfered with by a coil of wire with a variable current flowing through. The retarding, without physical contact, is something to do with magnets, movement and coils of wire that I don't quite follow. My physics classes were much too long ago.
Question though. Where does the energy go? Brake discs and pads heat up, but how is the energy dissipated in the contactless electronic brake? Does it make electricity? (so where does that go?) Or does it make current that heats up a resistance wire? Or what? Anyone familiar with the physics care to explain?

SAP_2000

Would like to hear someone explain the physics in this also.
But I have a feeling its a reason why we don't see contactless electromagnetic brakes in common applications (cars, planes, trains) and so on. I imagine if these types of brakes was effective it would save the car and airline industry billions not having to replace brake discs and pads.

LN-JET

Well, regenerative braking is a thing, you know.

Agrav8ed

I use the Xicoy Lgc 15 retract controller. No complaints and it works well. I like the jp retracts but I am not a fan of their controller.
thanks,
Tone

more

JP brake system is bad designed because the material for brake friction is its own the metal on electromagnetic and metal pad inside the wheel (you have to change whole magnetic when brake force reduced).That's why the brake force will be appeared in only low speed. Meanwhile,Electron's designed used the changed-able brake disc like old style air brake.
I think many manufacturers know about the problem but they still use JP products because of the low price.

SAP_2000

I know very well as i driven EV´s for many years, but regenerative braking requires somewhere to put that energy - back to the battery and that's not possible in this case..

SAP_2000

Agreed about manufacturers using JP because of price even there are known issues with brake and retract controller and obviously the brakes design.

But there is nonphysical contact here, so tell me how could anything be wearing out?

SAP_2000

Very interesting, but you not say specifically about the braking force.
Do you have the same design brakes as me and get good braking force at high speeds after touch down?
And if so, did you first use the JB brake controller and then switched to Xicoy LGC 15 and was there any improvement on the brakes at high speed?

I still don't understand in how these brakes physically can provide any significant grip at speeds given their contactless design.
Im looking for a way to make this this work, so input from you guys that have flown this plane with the same brake setup is very valuable.

Im happy to buy a different brake controller or even wheels and brakes as long as it will work.

more

The physical contact is from magnetic case and the pieces of metal in inner radius of the wheel(you can see the wearing in your wheel picture)
The pieces of metal will be pulled down to direct contact with the case when magnetic is activated.The friction between metal is not good compare to brake material that is used in automobile.(All air brake use automobile brake material and it works very good in any speed.)

Agrav8ed

I have used the JP controller in the past and have never been a fan. Even with smaller jets. There is a fairly long thread on here about JP brakes and trying to tune them or get them to work evenly. It is hit or miss and the brakes will often pull to 1 side. Not a fan. Therefore I just started using the Xicoy LGC 15 controller. It has gyro controlled braking, gyro controlled steering, and anti-locked that will pulse. The also have a smaller version without the gyro that will do antilock brakes and is much cheaper. I have mine set to 100% braking power. I have flown this jet most of last season and I never had any problems with the brakes. I am running it all on 2s lipo power. Jp brakes and retracts are not bad overall, their brake controller leaves much to be desired however.
Thanks,
Tone

SAP_2000

Thanks again. I think i will go the route of Xicoy controller. Either the gyrostabilised gear and brake controller, or just the normal one.

But if replace the gear controller with the LGC15, i understand you have to enter max amp draw for each gear as well as the time? Does anyone have these data for the ER200 L-39 gear?

Still i think these brakes are a poor design and the friction created between the magnets in the wheel and the 3 thin metal surfaces on the "drum" is not ideal. But as long as it works, I'm ok. In my case the braking force was zero at high speeds and if i hadn't flown of a full size runway but at my normal RC field it would have cost me the plane. Brakes has to work 100% every time at speeds and not just at stand still

Agrav8ed

I do not remember the times for the controller, but I looked it up in my text and according to Shane from JP the cut off voltage is ER-150:1.25a, ER200 1.8a. Pretty sure the retracts in the L39 is the ER200.
Thanks,
Tone

Todd D

My Feedback: (51)

The lubrication has run out of the wheel bearing too. Be prepared for the bearings to fail. I replaced my main wheel bearings with bronze bushings made by local machine shop.

Westwindpilot86

Anyone have a cockpit they want to sell? Thanks

Agrav8ed

So due to time, weather and circumstance I finally got my new L39 into the air. I have to tell you, man I love this jet. I forgot how smooth and easily it flies and yet how agile it can be at the same time. It really is a great flyer. And low passes with the smoke on just look incredible. I forgot how much fun it is. I also forgot how much fuel I can burn through in a day lol, especially at these prices. On my original L39 I would have a bit of an issue with the nose gear door not closing unless I am straight and level on take off. So when I built this one I changed the servo geometry for the nose gear door and reinforced the vertical plates. Problem solved. However, on this jet, I had a problem with the right main door not closing all the way. Initially I thought this may be a geometry issue also but now I just think it may be the servo as it did not feel very smooth when I cycled it by hand. I will play with it and see. I did not really have any issues with the brakes but I am using a LG15 controller and as I flew on grass it is a bit easier to stop. Anyone on the fence for a large L39 should strongly give this one some consideration.
Thanks,
Tone

Piggy18

Hi all,
Im stuck on the best way to install the cockpit, would others mind sharing their ideas and or photos?

Agrav8ed

At the front of the cockpit near the nose and under the clear canopy I glued some basswood blocks to support the front part of the lip (second to last pic in post #149). I also had to remove a bit or material from the side of the same front canopy to allow it to rotate a bit (see the first pic in post #170). Besides making a bit of a relief for the latches that was it (see pic in post #154). Takes a little trial and error for the correct fit but it was in no means difficult.
Thanks,
Tone

seaside535

Piggy18

Thanks Tone,
That makes sense, I was a little nervous about cutting it, the balsa block supports are an excellent idea for the front canopy I will also do the same.

I noticed you added extra carbon inside the fuse, would you recommend doing this?

Agrav8ed

There are plenty flying without this upgrade. I personally like the added security and strength. I also added a few more attachment points between the front and rear canopy. The front part of the canopy holds alot of things and hs a fecent amount of weight in the model. The four stock screws with no carbon reinforcement just does not seem like enough to me. The weight gain is negligible and most of us had to add a little nose weight any way. So to answer you question, it is not strictly necessary, but I like to build hard and fly hard.
Thanks,
Tone