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Jul 22, 2016, 10:47 AM
http://www.sgvhumane.org/
cmdl's Avatar
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Mini-Review

FMS P-39 Hells Bells


The P-39 is a legendary plane. While it wasn't appreciated by American pilots, it was of much use to Soviet pilots, during WWII. Five of the 10 highest scoring Soviets aces logged the majority of their kills in P-39s . My own interest in this plane is that it has the clean lines of sleek European fighters rather than the brutish looks of American fighters. Thanks to Gearbest, I have the opportunity to experience this marvelous plane.

Arrival
The plane arrived from Gearbest with plenty of packing tape I appreciated this since it kept the package secure and only involves minimal extra effort or cost.

Unboxing
unboxingHD (1 min 53 sec)

The plane's paint seems to sit better compared to some other companies more prevalent in the US. It gives it a more scale look rather than having it feel like a hastily painted toy.

Assembly
The assembly is where the FMS attention-to-detail really shines. First, the hardware is clearly superior to both Horizon or Hobbico. The screws used by FMS are sturdy in the heads and not "butter-headed" where the slightest touch of a phillips-head screwdriver would ruin them.
The use of pictures, rather than diagrams, in the FMS manual, is also a superior mode of instruction. Moving on, among other pros, I was thankful for the consolidated connector interface for the landing gear, flaps, and ailerons. Having the wing servo connectors not only accessible but properly identified and aligned precludes headaches for the assembler.
And it gets better. The pushrods are strong and firm, not thin and bendy as some may have experience with in the American manufacturer market. Having rudder and elevator servos aft, rather than fore in the battery compartment, is a great idea, in my opinion. This minimizes the dependability on long bendy pushrods and allows for much better control of elevator and rudder.
Perhaps the one issue I may have with the plane is the foam cowl. While a cowl integrated with the fuselage certainly has better looks, my concern is the heat dissipation. Foam, being an insulation material, would likely trap more heat around the motor and convey it back. Then again, this may not be an issue. But it is something that caught my eye and worth noting.
Among other components, I do appreciate the extra capacitance added to the speed controller to account for the lengthened battery<->ESC wire. It is good to know that FMS pays attention to electronics. More so, since I measured amp draw at full throttle to be 47A. On a 60A ESC, that is wise headroom.
I now come to my favorite part. While I was hooking-up the electronics, I noticed that the ESC and motor were not connected. I determined that the cleanest way to connect them would be by taking off the motor mount. It is then when I discovered that the motor mount firewall is actually a cylindrical ring that runs the inner perimeter of the cowl fuselage. An elegant design vastly superior to the typical cover mounting firewall with unnecessary mesh blocking central axial access. The FMS firewall allowed easy passing through of ESC and drop-in of the motor and remounting. Further, the motor has a metal motor mount, not the plastic jobs you see on Horizon and Hobbico models. Superior for heat dissipation and torqueing.
The wing mounting and horizontal-stabilizer mounting were effortless and optimal. They both used metal screws rather than unreliable plastic pins and saddles.
If there is one con I noted in this model, it is that FMS did not account for the differences in screw-pillar lengths for the flap control horns. Each flap control horn should have had at least one long screw so as to accommodate for the pillar farther from the trailing edge (where the foam is thicker).
As the assembly wrapped up, I checked CG. At recommended 65mm, the plane was very slightly nose heavy with gear retracted and quite a bit nose heavy with gear deployed. This was using a GForce 2200 mah battery.

Flight
This part of the review is best described as "not boring".

Maiden: Plane looks pristine and pretty. After reading every single page of the sage advice posted at: https://www.rcgroups.com/forums/show....php?t=2265815, I settle for 60% Travel Adjust on aileron and elevator and 80% on rudder this equates to reduced throw linear rates. The choice of rates is personal and subjective.
Plane set on runway, takes off beautifully, and I lose control seconds later. No response on any of the controls and she plummets down. At the crash site, I notice that the nosegear retract is completely retracted but the maingear/wing retracts did not fully retract.
Went home, tested all electronics, they functioned fine. I got the maingear retracts to completely retract but they wouldn't cease their movement seemed they registered as not having fully retracted. So they hummed endlessly while retracted. My approach was to leave all landing gear deployed and not push my luck. She's fixed up with JB Weld, and since at this point she won't look any worse, I go with my beloved packing tape. The spinner is damaged but I can use the backplate. The prop is busted so I go with a 9x9x2. This has higher pitch than the stock 6" pitch. While I'm not looking for a speed demon, it should be noted that due to higher pitch, this prop will result in higher speed than the stock prop. Full throttle pull is 31A.

Second maiden: Takeoff, she stalls just as she lifts off and cartwheels. She has a minor crack in the fuselage, and more importantly, the ESC is not working. There isn't a scratch on the ESC but it just quit.

Third maiden: I replaced the ESC with an E-flite 60A. This time, I get to finish what I started. This airframe handles wonderfully in the air. She has a high stall speed when inverted (inverted flight begins 2:10 into the video), as I found out at the end I'd say more Spitfire than P-47.

p39flight (3 min 45 sec)


Because she's been battered, the fuselage is a bit warped and I have a lot of right trim added just to get her to fly straight. But she flies "on rails" once trimmed. And, of course, that last part in the flight video was my fault, nothing to do with the plane.

Conclusion
An awesome plane. She'll be back up in the air, next week. Love the airframe, and I'm glad I got the chance to figure out that the ESC was causing the mishaps. Sometimes, we aren't so lucky to determine what went wrong. Finally, many thanks to my cameraman, Ben Cheung.
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Jul 22, 2016, 11:21 AM
Registered User
I was giving some consideration to buying this plane but my main hesitation is that it is designed to use a 4S battery. I don't want to invest in 4S batteries since I already have 18 3S batteries that I regularly use. Could you do a test of the plane with a much lighter 3S 2200 and see how it performs? I spoke with a tech at Motion who said that his "official" position is that I should use the recommended 4S battery, but "unofficially" the plane should fly slower but quite fine with a 3S.
Jul 22, 2016, 11:59 AM
Registered User
Excellent write up cmdl. Great detail that's very helpful to those who have and yet to acquire the FMS/RocHobby P-39.

I'm flying mine on both 3s 2200 and 4s 1800 batteries. Both fly the P-39 very well, I'm hoping you will fly yours with a 3s 2200 to give your impressions.
Jul 22, 2016, 03:30 PM
http://www.sgvhumane.org/
cmdl's Avatar
Thread OP
Thanks, Donodo, Leo.
Unfortunately, I don't own any 3s lipos, so I cannot deliver on a 3s flight. However, using Ron's current estimate, the max you should be able to draw on a 3s would be 47A/1.8 = 26.1A. So, power = 11.1x26.1A = 289.83W and then Watts/lb = 96.7 Watts/lb. She should fly fine but Donodo will have to provide the input here since he has actually flown it on 3s.
Jul 22, 2016, 06:43 PM
Registered User
Quote:
Originally Posted by cmdl
Thanks, Donodo, Leo.
Unfortunately, I don't own any 3s lipos, so I cannot deliver on a 3s flight. However, using [URL="https://www.rcgroups.com/forums/showthread.php?t=2475738"]Ron's current estimate. So, power = 11.1x26.1A = 289.83W and then Watts/lb = 96.7 Watts/lb. She should fly fine but Donodo will have to provide the input here since he has actually flown it on 3s.
Well back in the day (10-15 years ago) when electric motor planes were really becoming reasonably decent to fly we had a rule that if you have 100 watts per pound set up, it was good for most maneuvers a plane could do.

My impression is that the FMS P-39 is still quite a fast flying plane when using a 3s battery. Mine flew at half throttle and had a hard time landing with flaps deployed at 1/4 throttle. Any slower and the wing begins to show big signs of a tip stall.

In summary, all the 980mm size warbirds fly exceptionally well, but they do fly faster than most all other planes in this size (1000-1100mm). 3s is just fine, 4s is fine and crazy speed.
Jul 22, 2016, 07:12 PM
Team JR/DFA, KBDD Team Captain
Ah Clem's Avatar
cmdl,

Welcome to the dark (P-39) side my friend!

Extremely nice write up and video (long, sustained inverted flight!)!


Leo L,

I have only flown mine on 3S batteries. No problem with power at all. Not as fast as on 4S, but plenty of power for normal flying.
Last edited by Ah Clem; Jul 22, 2016 at 07:59 PM.
Jul 23, 2016, 07:16 PM
http://www.sgvhumane.org/
cmdl's Avatar
Thread OP
Thanks, Ah Clem.

Quote:
Originally Posted by Donodo
Well back in the day (10-15 years ago) when electric motor planes were really becoming reasonably decent to fly we had a rule that if you have 100 watts per pound set up, it was good for most maneuvers a plane could do.
And given the constraints on power, I will wager that pilots were better than pilots today, specially compared to pilots like me. You can see my uncertainty with lower power levels in the inverted flight portion of my video - I'm always wary of the higher stall speed.
On that note, Ah Clem flies on the same field as me - it is a pleasure watching him do slow flying and touch-and-go on the P-39 - he demonstrates absolute command of the plane.
Jul 23, 2016, 09:01 PM
Registered User
You are by far the better pilot than I am. And we have our own way of flying these beautiful warbirds including the P-39.

The point is, this P-39 is a great flying plane that requires a little extra speed than a lighter wing-loaded plane. Anyone who wants to use 3s batteries in it is going to find it's plenty fast enough for spirited flying. Using a 4s in it turns the P-39 into a plane that will shoot out of sight in a few seconds.
Aug 18, 2016, 05:16 PM
Team JR/DFA, KBDD Team Captain
Ah Clem's Avatar
Donodo,

"The point is, this P-39 is a great flying plane that requires a little extra speed than a lighter wing-loaded plane. Anyone who wants to use 3s batteries in it is going to find it's plenty fast enough for spirited flying. Using a 4s in it turns the P-39 into a plane that will shoot out of sight in a few seconds."

Well stated sir! One thing that may help a bit on the landing-a friend of mine raised his ailerons very slightly on his P-39. This has the effect of increasing the washout and lets the wingtips stall slightly later (i.e. at a slightly lower speed). His landings improved greatly after doing so.

cmdl,

The P-39 is an excellent aircraft and a pleasure to fly. I am still amazed at your ability to fly it inverted indefinitely! OUTSTANDING!

Great thread, sir!
Last edited by Ah Clem; Aug 19, 2016 at 05:25 PM.
Aug 23, 2016, 03:08 PM
http://www.sgvhumane.org/
cmdl's Avatar
Thread OP
Thanks for the kind words, Donodo and Ah Clem.

Ah Clem: I enjoyed watching your 3D heli flying at the field today. I know we disagree on this but once again I realize why inverted on a plane is so much easier than inverted on a heli.
Aug 23, 2016, 04:06 PM
Registered User
Just re-read your excellent tip Ah Clem on raising the ailerons while deploying flaps for landing. I'll have to attempt this as the wing wagging on landing approach gets my nerves jumping a little. Maybe a 2 degree aileron up deflection should be enough to begin with and no more than 4 degrees is my guess. Thanks for the tip!
Aug 23, 2016, 07:06 PM
Team JR/DFA, KBDD Team Captain
Ah Clem's Avatar
Donodo,

I never measured it in degrees, but it should be very subtle (about a turn or two on the clevis at the most).

If the wings are starting to wag, you are a bit too slow! Keep the flaps down, the nose down, and power on (fairly low but on)! Feather the throttle, as needed to control the descent, as you are coming in.

cmdl,

My turn to thank you for the kind words!

I think it has to do with what you have more time and practice on-you fly fixed wing most of the time-with me it is helicopters. I still don't know how you manage to fly the P-47, Spitfire, or P-39 for such long times inverted (even though I see you do it every time I go to Whittier). [email protected] good flying young man!
Aug 23, 2016, 08:05 PM
Registered User
Quote:
Originally Posted by Ah Clem
Donodo,
I never measured it in degrees, but it should be very subtle (about a turn or two on the clevis at the most).

If the wings are starting to wag, you are a bit too slow! Keep the flaps down, the nose down, and power on (fairly low but on)! Feather the throttle, as needed to control the descent, as you are coming in.

cmdl,

My turn to thank you for the kind words!
I think it has to do with what you have more time and practice on-you fly fixed wing most of the time-with me it is helicopters. I still don't know how you manage to fly the P-47, Spitfire, or P-39 for such long times inverted (even though I see you do it every time I go to Whittier). [email protected] good flying young man!
Yeah the problem we have at the little dirt airstrip is the wind-sheer on approach so the angle of decent is a bit higher than one would normally do which also means the speed will be higher. Even though I still have enough control with the wings wagging most touch downs with my P-39 are about half way on the airstrip so there's about 50 feet left. Needless to say if I come in any slower I'll definitely have in crash, if I come in faster the touch down point is even further down the dirt strip. Lots of fun in every flight!


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