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Aug 13, 2013, 06:08 PM
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New Product

Finally a new foamy Airacobra -- Skyangel P-39 "funfighter"


This plane is a new model in the "funfighter" series of little (around 24" wingspan) EPO warbirds. I bought the ARF/airframe kit, which is all that I have seen available so far. Unlike some of the other Skyangel airframe kits I have bought previously, this one did not come with the motor.

The plane arrived with the two other Skyangel planes that I ordered at the same time, in a big, beat-up looking package. Turns out, two of the Skyangel boxes inside were nearly flattened. The P-39 damage was a bent and cracked fuse as well as a bent wing (polyhedral added as a bonus, I guess you could say). After some work with a hairdryer, the shape is restored, with only appearance-damage (wrinkles) remaining. These aren't the first Skyangel planes I have received in flattened boxes. They really should rethink their packaging.

This kit actually came with two sets of prop/spinner parts -- two-blade and three-blade. On the package with the 3-blade parts, it says: "If use 3-blades propellers we suggested data as below: ESC:30A-40A, Motor kv: 1500-1700, Battery: 1000-1500mah 3S 30c." Both versions have some nice yellow accent paint added, as per the full-scale scheme that is modeled (see link below).

The interior elevator control rod routing, and the nice long equipment hatch along the top follow suit with other funfighters. How much room is inside the P-39 for bigger batteries and equipment placement? See photo for a comparison with the Corsair and Rare Bear, with a stock Skyangel 850 mAh battery placed inside the P-39 for reference (caution: I don't know yet if that is where the battery will need to be placed for proper CG). The P-39 interior is skinnier (the battery won't fit transversely), but it is also deeper -- about 1-3/4" on average. So there's plenty of room.

Strangely (and unfortunately), they don't give you a nice molded servo pocket for the elevator servo like on other funfighters. Instead, there's just a big oversize "well." You could fit two and a half servos in there. And the length of the well doesn't even correspond to the length of a typical 9g servo (including a stock Skyangel servo)...it's too big. And there's no channel for the servo wire. So it's going to take extra work to get the servo secured in there.

I hadn't previously seen any photos of the underside of this new model. The top is a visibility concern because of the mostly dark-green color. The bottom, as it turns out, is light gray. The paint job is modeled after "Brooklyn Bum"...see for example:
http://www.lewisairlegends.com/aircraft/brooklyn-bum

Photos attached. Time to get out the tools and glue...
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Aug 17, 2013, 08:12 AM
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Finished and ready for maiden


I used HS-55 servos all around (two aileron, one elevator). A photo below shows how I secured the elevator servo by gluing scraps of Depron and balsa into the big "well." Then for the elevator control horn, I used a screw-down backplate-type horn instead of the stock flat-plastic ones which I don't trust as much. I threw out the stock clevis keeper bands (one shown in the photo lying on a piece of Depron), because they aren't stretchy and were already so loose they would just slip off. I used slices of fuel tubing instead.

For a motor, I was going to use a Turnigy 2826/2200kv motor that is near-identical to the stock Skyangel motor in size (and the screw pattern matches)...but from my experience with that motor I didn't expect it to be both 3S and 4S capable. So I cannibalized the stock Skyangel motor from my Skyangel Corsair funfighter and swapped it over to the Airacobra, then reconfigured the Corsair with the Turnigy motor. That way I can try the new Airacobra on both 3S and 4S. I tried the Corsair on 4S and it wasn't much improvement (probably because of the Corsair's big fat draggy cowl), so I don't need 4S capability on that one.

For an ESC, I am using a Pentium 40A ESC with switchmode BEC. So my Airacobra will be fully 4S-ready.

My doubts about the battery placement expressed in the first post were confirmed when I tested for CG. Turns out that the battery needs to be in the most aft of the three compartments. No doubt that is because of the Airacobra's long nose, with the motor relatively far out in front of the wing leading edge compared to the Corsair and Rare Bear. The CG worked out perfectly with a stock Skyangel 850 mAh battery exactly centered within the three pairs of retaining "ribs" of the aft compartment (photo).
Aug 20, 2013, 02:22 PM
Redacted per NSA "suggestion"
dedStik's Avatar
The P-39 has always been one of my favorite warbirds.
Aug 22, 2013, 08:03 AM
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maiden report


Difficult launch on maiden. Needed a lot of down-trim and aileron trim...it wanted to nose-up and stall. I had the elevator visually faired with the stab before flight. But after the flight with the plane all trimmed up, the elevator is visually just a hair nose-down from being faired. Once trimmed up, it was fast and agile...even for a funfighter. And the rolls were beautifully axial...in contrast to the Corsair which corkscrews around.

On the negative side, that mostly dark-green paint job is tough to stay oriented on. At least with the Corsair's dark-colored paint-job, you have the gull-wings to help with orientation...but not so with the P-39. Problem is, even though the plane has a contrasting light-colored underside, when the plane is flying upright, that side is in shadow. So it's just all one dark blob.

The other negative is that there is NIL for equipment ventilation in the P-39...especially with the motor -- which came down too hot to hold a finger on. The motor comes down as hot on 3S as it does on 4S with the Corsair and Rare Bear. Those two other planes have air being pushed across the motor inflight (and an air exit hole in the lower aft fuse). So unfortunately, I consider 4S to be out of the question on the P-39. Unless I can figure a way to mod the cowl to scoop air across the motor...without deterring too much from the sleek semi-scale looks.
Last edited by herk1; Aug 29, 2013 at 04:14 PM.
Aug 22, 2013, 04:26 PM
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kelloggair's Avatar
where did you order from?
Aug 22, 2013, 05:15 PM
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I got it from hobbyb.com, which is the only place I've seen this newer model so far. I'm sure that sooner or later it'll show up at other places that offer the Skyangel series, like Toysonics and Hobbyking.

I put a couple more flights on the P-39 today. I am still finding this model difficult to launch. The same type of underhand push that works just fine with the Corsair and Rare Bear is not working well for me with the P-39. After release, it wants to roll to the left, and the nose drop too hard. On the second battery today I had my first failed launch on one of these Skyangel planes in months...it went full inverted too close to the ground, so I just throttled back and let it dork in on the nose. No damage fortunately, and my reinforced motor mount glue-job held. So I spent the rest of that battery doing repeated hand-launch-and-land for practice. I moved the battery back a bit and put in a couple clicks of uptrim (but not too much because I didn't want to repeat the problem on the maiden launch)...and that helped with the nose-drop...but every launch still had that left roll. Next time I will practice different power settings and maybe try to launch with some right bank.

If it wasn't for the low-wing configuration of this plane, I would launch it "English-dart" style like I do with some of the Skyangel jets, in order to avoid that rolling tendency. Kind of hard to do that when there's no good way to get a good grip on it from underneath near CG though.
Aug 31, 2013, 08:32 AM
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Landing gear mod for the P-39


On my latest outing with the P-39, the winds were calm...which made the P-39 even more difficult to launch. After three straight failed launch attempts (trying different launch tricks each time), with a broken prop and fuse on the third, I had to call it quits. And guess what? after the third failed launch, I took the battery out of the P-39 and put it in the Corsair funfighter, then launched the Corsair with the same underhanded shove that I was using on the P-39...and the Corsair sailed out straight and true with wings rock-solid and level, like it always does. The P-39 funfighter is just a bear to launch.

So I decided to put landing gear on the P-39.

The P-39 funfighter kit does not come with landing gear. And the model has no provision for gear designed in -- it's designed as a strictly hand-launch model. However, one thing the model maker did do that helps a little, is put decorative panel lines on the underside of the wings showing the outline of the main landing gear doors in the scale position. So I used that as a guide for where to make the cuts for my landing gear mount blocks (photo). For gear hardware, I went to my parts bin and recycled some parts from a previous kit (the main gear from my old GWS A-10 was just the right size).

One thing I like about the real P-39 is that it is a trike-gear plane. So I followed suit and put trike-gear on my P-39. (of course, those MLG panel lines were aft of CG not forward like on tail-dragger WWII planes) I made the nosewheel steerable. There was room in the front interior compartment for a steering servo and linkage. So it was a clean install with no exterior linkage showing (photo).
Sep 02, 2013, 04:09 PM
Once you taste flight...
skydvejam's Avatar
So I suppose my next question, how does she fly with the gear on her?
Sep 02, 2013, 06:36 PM
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It flies just fine with the gear once in the air and up to speed, but there's still a problem getting there. Because it has been squirrely on takeoff roll for some reason. So I have to goose the power, then yank the plane off the ground before it veers off the pavement and hits the edge of the grass. But yanking it in the air like that before it's up to speed results in the same problem as with hand launch...the plane is a total handful until it gets "on step."

I've put tricycle landing gear on two other Skyangel planes using similar methods and had no such problem. It might be due to the short distance from the mains to the nosewheel on this plane. I might try yanking the gear mount blocks out and turning them around to stretch that length an inch. There should still be enough strut-CG distance to achieve rotation.
Sep 03, 2013, 07:56 PM
Hooper, full throttle!
Tommy D's Avatar
Regarding hand launch (yes I see you already took care of that/added wheels)... I'm wondering if that big prop way out in front of the leading edge is really killing you with P factor.

Did you ever try to handlaunch say straight and level at 1/2 throttle or so?

Just Wondering. You must like the model some.. or you wouldnt have bothered with the wheels!

Tommy D
Sep 04, 2013, 09:46 AM
yank and bank!!
What timing.... I want to buy a better plane and I love the p-39!!!

Thank you so much for posting!!
Sep 04, 2013, 09:50 AM
yank and bank!!
I suspect the wing loading is a significant issue; my cox extra 300 has virtually the same wingspan but one third of the weight.... I wish the p-39 weighed around 9oz but maybe the battery weight can be played with...


BTW, whats the AUW of your plane? I only ask to plan how I could outfit this model....
Last edited by failboat; Sep 04, 2013 at 11:28 AM.
Sep 04, 2013, 05:07 PM
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Quote:
Originally Posted by Tommy D
Regarding hand launch (yes I see you already took care of that/added wheels)... I'm wondering if that big prop way out in front of the leading edge is really killing you with P factor.

Did you ever try to handlaunch say straight and level at 1/2 throttle or so?
Yes I suspected torque roll...so on one of my launch attempts I launched it with only about one-third throttle. Same result (left roll to inverted). So because of that, I don't think it's necessarily torque roll that is the problem and my next test will be a hand-launch attempt with a prop of bigger diameter and less pitch, with high power. I already ripped off the gear and put on a 6x4 prop.

I already have a 6x4 prop on my Skyangel Corsair now, together with that higher-kv (2200 kv) motor that I mentioned uptopic, and my Corsair is a total blast with that setup...better than ever! Super-quick acceleration, crazy fast vertical...and top speed still pretty much equal to the stock motor and prop. And still rock-steady on launch.

I wonder if my P-39 might have residual effect from the shipping damage...i.e. that something still isn't quite straight, even though it looks pretty straight/symmetrical. I'll be interested to hear what sort of experience other people have with getting this plane airborne.

failboat - AUW without the gear and with the stock Skyangel 850 mAh battery is about 12.9 oz. A little over stock weight because of the bigger ESC I put in.
Sep 07, 2013, 12:58 PM
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Well I think I've figured out the launch issue with this plane. It's not torque-rolling, it's top-heavy. It wants to swing around so that the top-side is underneath.

I tried my test-launch with the 6x4 prop on high power setting like I said I was going to in the previous post of this thread. The plane still just quickly rolled inverted after release like it always does. But this time I noticed that after overshooting full-inverted a little, it swung back, pendulum-style to full inverted. So my theory is that, until the plane speeds up and gets enough air flowing over the wings and tailplanes to hold it in position in the roll axis, it's going to flop over so that the "heavy" side is down.

To test that theory, I decided to try to launch the plane inverted -- intentionally. So I repaired the damage from the first (failed) launch, then put in several clicks of forward stick trim. Then I held the plane inverted, ran up to about half power, and did a regular overhand push-launch. Guess what? For the first time ever, the plane did not roll after launch. It was still wobbly until it got on step, but at least I didn't have to deal with the roulette-wheel split-second guessing game as to what the attitude of the plane was going to be when I got my hand on the right stick. So it was a successful launch. And that was my first-ever attempt to launch a plane inverted!
Sep 07, 2013, 10:55 PM
Hooper, full throttle!
Tommy D's Avatar
Now I don't feel so bad.. I stomped mine to death @ the field today!

First 2 hand launches were "ok", but cant say I was thrilled how it flew.

Next 5 or so hand launches was complete frustration. I cut to the chase and let my sneakers take care of this dog!


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