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Old Mar 28, 2012, 08:28 PM
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Irvine, Calif USA
Joined Feb 1999
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Maiden !!

I had the maiden today, wind about 10mph straight down the runway. CG at 75mm. Taxi test to see if it needs nosegear steering trim.

Took off no problem after a 100ft or so takeoff roll, nice climbout, the Neu 1515 is giving plenty of power .

Had to feed quite a few clicks of down (6-7) to trim her out, she liked to climb by herself. A couple of clicks of right aileron and she flies steady hands off.

A lot of turbulence noise ("cool sound" ) coming from the intakes, I think it might be those flow separator plates. But the speed is good since the L-39 is fairly low drag.

Mine was a bit touchy on both elevator and aileron rates, have to reduce those esp. the aileron one by 20% or so.

I cut the power to test thrust incidence, and she dives. So there is some upthrust from the way the thrust tube is pointing - it is pointing "up" too much which makes her climb under power. I have to adjust the thrust angle a bit at the exit, the Aeronaut Rafale initially had a similar problem.

The CG is fine where it is, she is responsive but not touchy. The tendency to climb on mine is clearly an incidence problem, not a CG problem.

After some flying around I line her up for a landing (w/o flaps for now) and she has a nice stable glide, apart from the fact that I had to hold some up elevator.

She lands nicely Now I have to make sure the mains mounts will hold up to grass field in the future, the L-39 is not a lightweight.

.
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Old Mar 31, 2012, 10:09 AM
chillax! It's only a Hobby
1moparman1's Avatar
United States, CA, Millbrae
Joined Apr 2011
413 Posts
nice herb!!
bad weather here in the bayarea
will try to finish the L-39 soon
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Old Mar 31, 2012, 11:43 AM
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United States, CA, Apple Valley
Joined May 2002
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Congrats Herb i dont know but it might be incedence problem on the Sapac L39 also I flew my friends and we had to adjust the CG quite a bit also want it to climb , but it does fly nice , my K & A L39 flew awsome , wish i did`nt sell it
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Old Apr 02, 2012, 12:26 PM
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Irvine, Calif USA
Joined Feb 1999
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Hi, Thanks !

I think the incidence problem is no big deal, but it has to be fixed. I will post some pics later.

The more serious problem, as already shown in the video posted above where the guy punched the retracts through the top sheeting on landing, and then re-covered the wing with what looks like duck tape, is the flimsy retract mounts.

It is very difficult on this L-39 to get a weight rtf of < 8 lbs or so. That means that the wing retract mounts are completely inadequate imo. They seem to be made mostly out of ply, but anchored to thin balsa ribs, and twin balsa (top and bottom) spars. The whole wing is strong enough for flying, but not for landing as that video shows.

I am thinking of ways to get in there and redo the wing retract mounts, but it seems impossible without removing the top covering entirely for most of the wing panels.
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Old Apr 02, 2012, 06:18 PM
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Here's the first set of fixes to the CMP L-39:

Thrust angle adjustment. I am using the fiberglass ducting that is included in the kit to make a "duct straightening" tailcone, with zero angle of incidence with respect to the fuselage:

.
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Old Apr 02, 2012, 06:20 PM
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Irvine, Calif USA
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Here's the second fix

The springs that come with the struts are useless, they are too weak.

Here are the new much stiffer springs.

A steel clip also flew off on the last landing (or takeoff, who knows), and had to be replaced. Thank god I did not lose the pin itself.

.
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Old Apr 02, 2012, 06:49 PM
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Here's a picture of the wing, with covering removed, before the new sheeting and fibeglass cloth will be applied.

The retract mounts have been worked on, and new ply and balsa rib doublers have been installed. Note the flimsy balsa ribs near the retract mounts, no good for an 8+ lbs edf.

Later the whole wing top will be airbrushed to match the fuse.

.
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Old Apr 04, 2012, 11:49 AM
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Irvine, Calif USA
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I used CG / Hangar 9 Ultracote (Oracover) silver polyester film over the fiberglass to save weight. I got most of it on wrinkle-free

If the paint scratches later, the metallic silver will show underneath, which should be ok.

Then I de-greased the surface with alcohol and applied a light coat of Tamiya grey (XF-66).

.
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Old Apr 04, 2012, 11:53 AM
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Irvine, Calif USA
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To match the original camo job, I applied a light coat of dark grey (XF-20) here and there. Then the blue (acryl "cobalt blue") camo over it.

I think it looks pretty good for what I went through

I can tell the difference from the original only with a magnifying glass.

Now I have a wing that's made how it should have been .

.
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Old Apr 04, 2012, 11:59 AM
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Oxnard, Ca
Joined Oct 2008
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Hey herb, is that the same CMP L-39 offered now through nitroplanes?

BTW: Congrats on the maiden!
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Old Apr 04, 2012, 12:05 PM
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Quote:
Originally Posted by Herb View Post
To match the original camo job, I applied a light coat of dark grey (XF-20) here and there. Then the blue (acryl "cobalt blue") camo over it.

I think it looks pretty good for what I went through

I can tell the difference from the original only with a magnifying glass.

Now I have a wing that's made how it should have been .

.
Any idea how much weight you added?
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Old Apr 04, 2012, 12:55 PM
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Hi , the complete wing with retracts etc. now weighs 36.9 oz, before it was 33.2 oz I think.

So I added 3.7 oz with epoxy, ply in the retract area, fiberglass cloth and laminating epoxy on wing top sheeting, ultracote and paint ... which is ok.

Yes same identical L-39 as the Nitroplanes one (all made by cmpairplane.com). CMP should supply a new stronger wing.
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Old Apr 04, 2012, 03:40 PM
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Oxnard, Ca
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Quote:
Originally Posted by Herb View Post
Hi , the complete wing with retracts etc. now weighs 36.9 oz, before it was 33.2 oz I think.

So I added 3.7 oz with epoxy, ply in the retract area, fiberglass cloth and laminating epoxy on wing top sheeting, ultracote and paint ... which is ok.

Yes same identical L-39 as the Nitroplanes one (all made by cmpairplane.com). CMP should supply a new stronger wing.
Well, I think the glassing you did works great and is not a big deal to do. Definitely more time spent, but nothing a couple hours can't take care of.
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Old Apr 04, 2012, 08:31 PM
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I don't know about two hours ... took me a bit longer than that.

Just taking the wing off and unplugging all that stuff takes me half an hour ...
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Old Apr 10, 2012, 02:04 PM
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A bit busy with work and Easter, but ready to fly again at the next opportunity
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