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RocHobby P-39 Airacobra Racing High Speed PNP from Diamond Hobby Review

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Introduction

RocHobby P-39 Airacobra Racing High Speed PNP

Wingspan:38.6" (980mm)
Wing Area:284.4 sq in (18.35 sq dm)
Weight:48 oz (1360g)
Length:36" (912mm)
Construction:Expanded polyolefin airframe; polycarbonate canopy; plastic pilot bust and scale details; plastic wheels with rubber tires; composite propeller with plastic spinner
Wing Loading:0.17 oz/sq. ft. (74.1g/sq dm)
Center of Gravity:70-75mm behind LE of wing
Claimed Speed Range:Up to 100 MPH (160km/h)
Servos:Four FMS FMS-092 9g metal-geared digital; two FMS FMS-093 9g nylon-geared digital
Transmitter:Airtronics SD-10GS ten-channel spread spectrum aircraft
Receiver:Airtronics/Sanwa RX-861 eight-channel spread spectrum aircraft
Battery:Predator 2600mAh 4S 35C lithium polymer with JST-XH balancing plug and Deans Ultra-Plug compatible power plug
Motor:FMS PAEP 3648 outrunner; 770Kv
Propeller:FMS 10.5x8 four-blade composite
ESC:FMS FMS-DT70 70A programmable brushless
Operator Skill Level/Age:Intermediate/advanced; 14+
Manufacturer:FMS Model, 3/F, Building B, 3rd Industry Zone, Matigang, Dalingshan Town, Dongguan City, PRC
Available From:DiamondHobby LLC, 553 Capital Circle SW, Unit 4, Tallahassee Florida 32304
Price (USD):$209.99 with free shipping and applicable tax

A lot of credit is due to the folks at FMS, Diamond Hobby and RCInformer.com.

Their fourth 980mm High Speed warbird is another radical departure from the usual P-51, Corsair or Spitfire. Great subjects all and I have examples of each.

Before now, I've never had an example of the innovative Bell P-39 Airacobra.

All that changed when this latest review subject from Jim Ogorek of Diamond Hobby hit my doorstep.

Dear readers, feast your orbs on the stunning new RocHobby P-39 Airacobra Racing High Speed PNP as developed by FMS, Diamond Hobby and RCInformer.com.

In keeping with tradition, this compact warbird in authentic, historic Thompson Trophy racing livery is packed with a massive 770Kv FMS PAEP brand outrunner spinning a four-blade scale propeller via an FMS 70A ESC nearly the size of a credit card.

This now-proven combination along with one of Diamond's Predator brand 4S li-pos makes for a screaming fast package with a claimed top speed of 100 miles per hour (160km/h).

For those who might prefer to finish the model in a different color scheme, it's time to rejoice. The P-39 is also available in a plain white version ready for finishing by the modeler.

Want a P-39 in military livery? Some airbrushing, perhaps a set of Callie Graphics decals and the result will be sure to wow the pilots at the field with a one-of-a-kind warbird.

Another proven development is the ease of final assembly common to all FMS aircraft, so the time spent going from box to air is greatly minimized.

Let's sneak a peek at the prototype before we begin.

Prototype

The Bell Aircraft P-39 Airacobra was one of the principal American fighter aircraft in service when the United States entered World War II. The P-39 was used with great success by the Soviet Air Force, which scored the highest number of individual kills attributed to any US fighter type.

The P-39 featured an innovative engine layout with the engine installed in the center fuselage behind the pilot and which drove a tractor propeller via a long shaft. This setup also allowed a machine gun which could be fired through the spinner cone. It was also the first fighter fitted with a tricycle undercarriage. Despite the innovations, the P-39 was handicapped by the absence of an efficient turbo-supercharger, limiting it to low-altitude work. As such, it was rejected by the RAF for use over western Europe and passed over to the USSR where performance at high altitude was less important.

The model represents Cobra II, one of a famous pair of heavily modified late-production P-39Q-10 variants as flown after the war by Bell test pilot Alvin M. "Tex" Johnston (1914-1998). Johnston earned the nickname for his habit of wearing Stetson hats and western boots on the flight line. Among the notable modifications were the installation of a more powerful engine and a four-bladed propeller with the latter depicted on the model.

The character of Major T.J. "King" Kong played by Slim Pickens in the 1964 black comedy Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb was inspired by Johnston.

Owned jointly by Johnston and Bell test pilots Chalmers "Slick" Goodin and Jack Woolams, Cobra II was favored to win the 1946 Thompson Trophy race, doing so against race-modified P-51 Mustangs. Cobra I with its pilot, Jack Woolams, was lost in 1946 during a late afternoon test flight over Lake Ontario, possibly at speeds of up to 400 mph (640km/h). The aircraft suddenly and inexplicably crashed into the water, breaking apart upon impact.

Cobra II competed again in the 1947 Thompson Trophy, finishing third. In the 1948 Thompson Trophy race, she was unable to finish due to engine difficulties and sat derelict for many years. Later rebuilt as Cobra III after being displayed as a static museum exhibit, the aircraft was destroyed in a crash on August 10, 1968 during a test flight prior to an attempt at the world piston-engine air speed record. Owner-pilot Mike Carroll bailed but was killed.

Contents

The P-39 comes nearly ready to fly with the following:

  • Wing and fuselage assembled with all electronics
  • Four-blade scale prop and spinner
  • Full hardware package which includes extra hardware
  • Phillips screwdriver
  • Contact cement for installing the scale pitot tube
  • Photo illustrated instruction manual

Needed to complete the model:

  • 2.4GHz computerized aircraft radio system with a minimum of six channels
  • 2200-2600mAh 4S lithium polymer battery rated at 25C or greater

To begin with, the display box is downright beautiful. Some of FMS's recent outings have used boxes representing two distinctly different models. Not so with this RocHobby brand model. It's dedicated solely to the Cobra II.

Inside was the sort of neat, secure packaging I've come to expect from FMS. The contents were also what I've come to expect. In short, a whole lot of beautifully assembled and finished components.

No shortage of color; this bird is yellow. Not a tacky, fluorescent yellow but a hue that screamed "racing." As with the previous warbirds, the P-39 was finished with smooth, even paint and nicely applied decals. Unlike the individually placed decals on the P-47 I'd reviewed earlier, the P-39's decals were one piece each on a matte-finished film which blends well with the paint.

While it's nice having preapplied decals to shorten assembly time, having no decals applied opens the door to some really nice aftermarket decals.

Assembly

Wing

FMS/RocHobby uses the same logical assembly sequence for each of its models starting with the wing.

First up is the installation of the aileron and flap control rods and horns. Once more, I bound the receiver to the transmitter and rigged it to the wing via a spare ESC in order to be able to raise and center the servo arms. Each of the previous warbirds required that the ball on the right aileron's control horn be swapped to the opposite side, but that only took a moment with the aid of a small phillips screwdriver.

From there, it's a simple matter of installing the control horns, adjusting the pushrods, snapping the retainers in place and securing each retainer with a short length of fuel tubing. Working with small screws can be no fun; my screwdriver slipped while I was installing the control horn on the right flap. The result was, of course, a puncture next to the retainer plate.

Someday, I hope to see allen hardware on these models. For now, care is definitely warranted.

One thing which makes these FMS warbirds so special is their electronic module installed in the wing. This module serves to combine servos without the need for Y-harnesses and electronically slows the flap servos for smooth, scalelike deployment.

Attaching the wing to the fuselage starts with removal of the canopy, feeding the servo leads through the opening leading to the battery compartment, seating the wing in the saddle and completing the installation with four machine screws.

The latter was easier said than done; the screws were very hard to turn. Out came my 3mm tap in order to chase the threads; the tap came out of each hole with a bit of yellow foam in the cutting threads.

That did the trick and the screws seated easily after that.

Horizontal Stabilizer

FMS makes installation of the horizontal stab as easy as can be. The fiberglass spar is inserted into the left half which is then inserted into the fuselage. The right half goes on next; the elevator halves are keyed to fit together easily and with perfect accuracy.

Each half is secured from beneath with screws which attach to tabs in the tail section. Once the linkages are attached to the servos, the tail and basic airframe are complete.

Although RCInformer did a superb job of rewriting the manual, this manual and each of the previous 980mm warbirds skip the rudder control horn installation. It'll be clear to anyone assembling this model that the step is necessary and easily done.

Completion

All of the leads coming off of FMS's ingenious servo lead/flap control module are clearly marked as to what goes where both by channel number and function. That's a really good thing where an Airtronics radio is concerned; the elevator and aileron channels are switched around from other brands. The module does a terrific job of reducing the amount of servo leads in a relatively crowded fuselage, much like that on their Ki-61. Y-harnesses come installed for the nose gear and its steering servo; again, the leads are clearly marked.

That nose gear gets special kudos for its terrific engineering. Unlike pull-pull cables which might stretch over time and which may prevent the gear from fully retracting, the FMS setup uses a sliding pushrod system which is far more robust, not to mention accurate.

On the other hand, photos of the receiver and battery installation are small, blurred and nearly impossible to decipher. A closer look showed the battery mounted roughly in the center of the compartment with the straps at either end. The straps are spaced accordingly and this is the only way the battery would fit between the straps.

The Sanwa receiver was tucked into the empannage and the aerials arranged accordingly. One faced rearward, the other at a 90-degree angle taped to the hatch below the canopy retaining magnet.

Installing the battery was no fun. Again, like the Ki-61 in my previous review, there is not a lot of room to work with in the compartment. Making the task really frustrating were the glued-in battery retaining straps. They're simply too short to be fed into their retaining slots. My solution was to snugly strap down the battery without feeding the ends through the slots and with the addition of hook-and-loop on the sides of the batteries and the floor of the hatch.

From here, the manual goes over generic radio binding, proper rotation of the motor, control surface deflection and direction and installation of the scale pitot tube. The manual also instructs the assembler to install machine guns in the wings, but none were provided and Cobra II had no armaments anyway. The box art and the cover of the manual show the P-39 with no guns, so chalk it up to the pre-release version done as a warbird in the video below or to a possible future release in military livery.

Control throws were set per the instructions with the addition of 25% expo on the ailerons, 30% on the elevator and 15% on the rudder:

  • 10mm/14mm aileron
  • 8mm/15mm elevator
  • 7mm/12mm rudder

The suggested throws of 25mm mid down for the flaps and 35mm full down were impossible. There simply isn't enough throw. I eyeballed them to roughly half way down on the mid setting and not quite full down on the full setting.

Speaking of the pitot tube, the foam panel on the wing which helps to hold it in place was really glued down. It took some careful surgery with a #11 X-Acto to slice the tape holding the panel. FMS/RocHobby provides a rather generous tube of contact cement for such a small task, but I'm not complaining. It works well and there's plenty of it for repairs to other foam models. It doesn't set up as quickly as the Beacon Foam-Tac glue I prefer to use, but I was out of it and the result with the factory glue was excellent.

Installation of the propeller completes the model and while FMS recommends balancing the prop before installation, I've never had the need to do so with the previous review models and this model proved to be no exception. Once installed and powered up, it worked smoothly.

It's true that the P-39 was fitted with a three-blade prop for military use, but the four-blade on the model is correct for the Cobra II.

The finished product looked magnificent and seeing three retracts operate instead of only two was icing on the cake.

Time to get airborne!

Flying

My test flights to wring out the P-39 took place at my usual haunt, the Coachella Valley Radio Control Club in the remote Colorado Desert east of Palm Springs. With two prior FMS warbirds under my belt, I had a pretty good idea of what to expect.

Once I completed the range check, I carried the model to the flight line to test the ground handling. It was, not surprisingly, quite good, although the turning radius was roughly that of a school bus. The nose is steered via its own servo and there isn't a lot of throw with its short arm. Still, I got the model lined up with no problem for its first takeoff.

Tracking was straight and true, but it was apparent after it lifted off that the trim was off. It required some down elevator and left aileron, but hoo-wee-baby, did it ever take off! Like its hangar mates, the P-39 goes ballistic in a very short time at full throttle. Once trimmed, I took the P-39 for a few hot laps around the pattern. No aerobatics, just normal flight at low control rates.

Wonderful. Just wonderful.

Cruising speed was at roughly half to three-quarters throttle, much like the previously reviewed P-40, P-47 and Ki-61. Like those models, some fast, low, full-throttle passes over the runway resulted in the coolest sounding Doppler effect imaginable. Diamond Hobby's own video, linked below, perfectly captures the sound. Unfortunately, another pilot at the field on the day we shot video was running up and adjusting a glow model which masked the sound of the P-39. Sadly, I had to mute the audio.

Control certainly wasn't an issue; even at less than breakneck speed, the P-39 tracked beautifully through both coordinated turns and straight and level flight. As far as how it looked in the air, this model is a stunner. The bright yellow paint job and period-correct air racing graphics made for a visual treat straight out of the 1940s.

Down came the gear and down came the flaps to their first position in preparation for landing. While it didn't quite feel as if it would glide in, a bit of power on approach made for a nearly perfect landing.

A few weeks would go by before I was finally able to get video at the club. Club videographer and historian George Muir was on hand to capture the event.

Back out to the runway with the turning circle feeling even wider than before. I dialed in a bit more rudder throw to compensate and off I went.

This is a model which instills real confidence since it flies s well when properly set up. This time, I threw in a few rolls and loops for good measure and the P-39 responded perfectly.

I lowered the gear, brought it around on base and final and touched it down for another perfect landing - without the flaps! I was having such fun that I'd forgotten to lower them. However, I couldn't quite get the model to swing back around for the trip back to the pits and it wound up in the sand at the edge of the runway.

A quick investigation of the nose gear revealed the culprit: It was little more than the grub screw which secures the strut in place. It had come loose and the result was sloppy steering. A borrowed allen wrench solved the issue in moments and a quick ground test showed greatly improved steering.

In went the second battery and up went the P-39 once more. By now, I was perfectly at home with this model, so the next step was a test of vertical climb and a stall turn.

This model is nearly as fast vertically is it is horizontally; it seems to defy the laws of gravity. So, before it went too high, I lowered the throttle in order to pull off a stall turn. Again, beautiful. It came around cleanly and recovered instantly.

The video and subsequent landing on my YouTube video are from the second flight and while the first landing was good with only a small bounce, this one was a perfectly greased three-pointer.

Without flaps.

I forgot to lower them, but no matter. That nose gear makes the P-39 a genuine pleasure to land, flaps or no flaps.

Aerobatics and Special Flight Performance

Anything a four-channel warbird can do, the P-39 can do as well. The only option off the table is knife edge flight due to the scale rudder.

Speed is the name of the game where these FMS and RocHobby models are concerned. This is where the P-39 really shines. It's fast to be sure, but it's also controllable and predictable. Roll it, loop it, stall it and the P-39 can do it.

Is This For a Beginner?

Sadly, no. Like the other models in the FMS/RocHobby 980mm lineup, it's kind of small, extremely fast and will not self-correct. It's typical of almost any low-winged warbird in that respect, but add to that the blazing fast speed and this model would have a lifespan measured in seconds in the hands of a beginner.

Flight Video and Photo Gallery

I had a genuinely good time flying this model despite the hot, humid conditions:

RocHobby P 39 Airacobra Racing High Speed PNP - RCGroups.com (3 min 2 sec)

George snapped a few beauty shots as well providing some terrific screen captures:

Conclusion

As I stated in the description at the top of the page, this truly is another unusual subject and another high performance winner. The RocHobby P-39 Airacobra Racing High Speed PNP from Diamond Hobby looks every inch like an historic racing warbird and performs almost as well as a pylon racer. FMS and RocHobby have done it again.

Two thumbs up as high as I can possibly raise them. This fabulous model belongs in every electric warbird enthusiast's hangar or on their short list at the very least.

My sincerest thanks go once more to Jim Ogorek at Diamond Hobby. It was through him that securing my review sample was possible and believe me, I'm glad I did. My friend Mike Greenshields at Global Hobby Distributors is a man on whom I depend quite often for support gear; he's the one who supplied the fabulous new Airtronics SD-10GS radio and Sanwa receiver.

On top of all the other things he does during the week, George Muir of the Coachella Valley Radio Control Club is only an email away from Sunday morning videos. Thanks, George!

Pluses and Minuses

Plenty of pluses which include:

  • Incredible speed and control
  • Slop-free hardware helps with handling and control
  • Beautifully finished
  • Unusual, fascinating subject
  • Full parts and technical support form a US-based company
  • Historically accurate trim looks terrific in flight
  • FMS's control module eliminates a lot of Y-harnesses and allow for scale operation of the flaps
  • When is the last time anyone saw a P-39 in this scale and with this kind of performance potential?
  • At last, a receiver-ready model available in an unpainted version!
  • FMS, Diamond Hobby and RCInformer.com have knocked it out of the park once more with another must-have warbird

Minuses are few:

  • The manual, though greatly improved, still has some errors, omissions and poorly translated English
  • The ESC comes programmed for a soft start which can cause sudden, unwanted operation while throttling up
  • Battery straps are too short
  • No nav lights like on previous 980mm models
Last edited by DismayingObservation; Jul 07, 2015 at 02:50 PM..
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Old Jul 10, 2015, 02:43 PM
karltrek is online now
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Excellent review. I did not know of the fate of the full scale Cobra 1 & 2. Thx!
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Old Jul 10, 2015, 04:22 PM
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Flys quick on 6S..
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Old Jul 10, 2015, 05:44 PM
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So our local field has a few of these now.

This thing flys on rails. Honestly it's very nice plane. Watching it fly it's more like a well built wood/covered plane than it is foam.

If you really like the look of this plane I'd recommend it.
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Old Jul 10, 2015, 10:42 PM
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I've had mine for 6 weeks or so and love it. I've had one problem when flying from a dirt field with small stones caused the nosewheel to push sideways hard enough to force the plastic retainer off of the axle and the exposed wire dug in and flipped the model on takeoff. I replaced the nosewheel retainer with a metal one and bushed the wheel with brass tubing. I fly mine on 4S 2200s from Hobby People and get 6 minutes at 5/8ths throttle which is as fast as I can keep up with, I love it for being Scale and a super aerobatic performer of Warbird type maneuvers.
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Old Jul 10, 2015, 11:33 PM
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Quote:
Originally Posted by Evan D View Post
Flys quick on 6S..
Good to know Evan. What size mAh, no mod to the SC? Thanks.. N
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Old Jul 11, 2015, 01:42 AM View Post
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Old Jul 11, 2015, 06:20 AM
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I'm using the 3700 compacts. We'll have them and the Strega, voodoo and critical ass out this morning racing. EFX's too. And Aprentices with streamers for combat....
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Old Jul 11, 2015, 03:13 PM
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Quote:
Originally Posted by CNICFLY View Post
Good to know Evan. What size mAh, no mod to the SC? Thanks.. N
I think he's kidding?
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Old Jul 11, 2015, 03:17 PM
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No, look at the Strega/Voodoo (has the exact same motor as the P-39) thread. We've been beating these motors up and many are still going strong that have went to increased voltage. Your mileage may vary as always. Stay within a reasonable current range and prop down accordingly when increasing voltage. This plane can take a larger capacity battery than they recommend for it, just like the Strega/Voodoo.
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Old Jul 11, 2015, 05:27 PM
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I wasn't kidding... I maidened another P-39 today. A buddy repainted it and decked it out as Air A Cutie. On 5s so it was a little slower than my 6s.
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Old Jul 11, 2015, 07:12 PM
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Man, this thing is a rocket on four cells. I can envision it keeping up with an EDF on six! If anyone ever thinks to shoot video, please post it. I'd love to see this thing on six cells.
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Old Jul 11, 2015, 07:17 PM
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On 6S, these planes are veryyyyy fast.
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Old Jul 11, 2015, 07:26 PM
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Way faster than the average EDF, even like my 8s Mirage...
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Old Jul 11, 2015, 11:29 PM
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wow that is nuts in that case! You be crazy! lol.
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