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Old Apr 11, 2012, 09:24 PM
chillax! It's only a Hobby
1moparman1's Avatar
United States, CA, Millbrae
Joined Apr 2011
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Quote:
Originally Posted by Herb View Post
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:

.
Herb, did you just tape that on the rear of the tail??
i see mine has the same, the end is pointing up
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Old Apr 12, 2012, 11:56 AM
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Irvine, Calif USA
Joined Feb 1999
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No I glued the fiberglass correction exhaust with epoxy. I sanded both sides thoroughly before gluing them together - I don't want this thing to fly off in flight.

Hopefully this will correct most of the thrust angle issues.

The problem is more serious for 90mm fans that have a larger outer diameter (eg haoye or cs), because the fan then ends up sitting lower in the fuse. Unless of course you entirely redo the ply fan mount (which I did not).

.
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Old Apr 12, 2012, 03:09 PM
chillax! It's only a Hobby
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United States, CA, Millbrae
Joined Apr 2011
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Thanks, i see that now
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Old Apr 13, 2012, 01:34 PM
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Generally I think it might be a good idea on the L-39 not to cut the thrust tube too short until you've had the first flight

That way you can modify things a bit more easily. I am glad I did so on mine ...
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Old Apr 13, 2012, 04:33 PM
chillax! It's only a Hobby
1moparman1's Avatar
United States, CA, Millbrae
Joined Apr 2011
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i was lucky, i did not cut it too much yet
i will fly it like that first, i am also doing 4000 8s up front that will help with the nose high flying
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Old Apr 15, 2012, 02:12 PM
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Well I flew her again yesterday (15mph winds ) and she still needs downthrust.

I programmed some throttle-to-elevator mixing now, we'll see if that does the trick. About five extra clicks of down at full throttle ...

I snapped the flimsy plastic steering servo arm off yesterday, so that had to be replaced.
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Old Apr 15, 2012, 10:41 PM
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Flew her again today, three times

I have now about five clicks of down mixed into the throttle, and she flies a LOT better.

I flew her on 8S and she flew well, but still a bit pitch sensitive (slightly rearward CG), then on 9S and she was slightly faster - but now the slightly forward CG (+3 oz in batt weight) makes her track nicely.

She really gains speed when she flies straight and level, faster than the T-45 for roughly the same amount of power. I did dremel out the intakes quite a bit and that seems to "ram" more air into the fan now.

I think I will stick with 9S 3000's from now on, and get rid of the 3oz of lead in the nose.

Red line (ca 75mm) is the farthest you want to go back on the CG. It flies but it is still quite pitch sensitive. Blue tape line is CMP wrong CG.

.
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Old Apr 16, 2012, 12:28 AM
DELTAS RULE
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tehachapi, CA
Joined Jan 2006
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great build and mods herb!! i love the flap hinge mod. cool struts too. do they finnaly sell them? lol nitrplanes never seems to have had them in stock in the past.
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Old Apr 17, 2012, 01:40 PM
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I got the struts from scaleflying.com , NP never had them

They are ok but do have too much play imo, one of the steel clips flew off on a landing (or takeoff). I hope the struts won't disintegrate too soon ...

CMP did a really bad job on that wing & retract mounts , they should have known better The mains mounts are a joke.
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Old Apr 20, 2012, 12:17 PM
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The struts and retracts are now on sale at HK as well (not for long ) for $120

http://www.hobbyking.com/hobbyking/s...dProduct=19949
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Old Apr 20, 2012, 11:37 PM
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Nice is to late though they are out for now i want them bad hopefully they will be back in stock soon
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Old Apr 20, 2012, 11:42 PM
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It's a nice flier - once all the various issues (wrong CG, weak main mounts, upthrust, flimsy struts, tailheavy etc ) have been sorted out

Short video from today :

Aero L-39 Albatros 90mm EDF 1200mm by CMP flyby (0 min 48 sec)
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Old Apr 20, 2012, 11:45 PM
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Nice Herb congrats
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Old Apr 25, 2012, 03:17 PM
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Thanks ... the 9S pack helps with the CG a bit but the L-39 still feels a bit tailheavy.

.
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Old Apr 25, 2012, 03:25 PM
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Have been working now on a way to secure the canopy.

I found that the pilots (esp. the front one) protrude too much into the fuselage space, which is a problem as the batteries take up a lot of space.

So I removed the two pilots with their little individual cockpit tubs; they can be inserted later for static display, when the battery is out of the way.

To secure the cockpit, I am using a ply tongue at the front, and a spring-loaded canopy latch at the back. That way there's no unsightly stuff showing up.

The canopy assembly has to be made rigid, with a good fit so it won't fly off. I use jb-weld since it is is very rigid when it dries (I learned this from keith sparks).

Now is also a good time to do some extra detailing.

.
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