84" KMP P-38 F Build - RC Groups
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May 12, 2007, 12:32 PM
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tom bacsanyi's Avatar

84" KMP P-38 F Build

I have been flying a 65" World Models P-51 for 3 years now, using a Phasor 45 with a 2.4:1 belt drive on 7s lipo and emoli. Other than the usual retract problems it has been reliable and has around 200 flights. See http://www.ezonemag.com/archive/200608.html What I want is a large P-38 that is reasonably scale that I can campaign like the P-51. There are two major problem areas as I see it.
1. Motor failure. Try flying a P-38 on a simulator like the G3 on one engine How many twins have I seen go in on one engine/motor.
2. Retracts, need I say more?
A couple years ago I started to build a 75" P-38 from a Wing (name of company) partial kit. It would need extensive mods to be decent looking and I abandoned it. CMP has an 84 inch P-38 that I am sure would fly well and is a bargain but is not as scale as I Would like it. KMP came out with two mostly fiberglass P-38's, F and J models! The J model moved the intercoolers from the outer wing panels to the distinctive large chin scoops to make room for fuel tanks in the outer wing panels. The earlier models had a sleek nose and "Phoebe" flown by Tom Lamphier shot down Admiral Yamamoto, commander in chief of the combined fleet in the earlier version. Payback for Pearl Harbor! I went with the F version and it will be Phoebe. After seeing what is available and pondering the special requirements of the 38 I decided to solve the gear issue by building my own.
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May 12, 2007, 05:06 PM
MattZC's Avatar
Yay for Custom Retracts!

May 13, 2007, 03:23 PM
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tom bacsanyi's Avatar
The engine/motor issue:
I found a device made by Allegra that consists of a copper buss bar with a ferrite ring around it. The ring has a Hall effect device embedded in it combined with a signal conditioning module. The output is a calibrated 25 Amps per volt with a range of .6 to 4.6 volts and the output is completely isolated fro the input. I put one of these in each esc in series with the negative inut power lead. The current signals are sent to the comparator input of a processor. Aso input is the throttle pulse. there are two output pulses whose width is always limited to be equal to or less than the input pulse.
The state of the comparator is used to decrease the pulse of whichever motor is drawing more current. The short version is that both motors will consume equal power
since they run off the same voltage. The main reason for all this falderall is so that if one motor dies or esc shuts off etc., then the other motor will also throttle down. For me this is the preferred scenario as I have no confidence in my ability to fly on one motor or to realize what is happening soon enough to sort out a solution.
May 14, 2007, 04:07 PM
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Art Newland's Avatar
Nice work! Any chance of a video of the retracts in operation?
May 14, 2007, 04:22 PM
Done it all

In the next few days I am starting a build thread on the VQWarbirds P-38. The build will consist of, off the shelf air retracts with oleo type struts, and outrunner direct drive motors. As with the many other ARF twin warbirds that I have built, my mission is not to re-invent the wheel but to provide some humor and drama in the build which might help others that follow.

It might be fun to compare the process and the results. The only problem that I invision encountering is modification to the nose wheel steering design.

Keep up the good work and innovative engineering of the retracts.

May 14, 2007, 05:17 PM
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tom bacsanyi's Avatar
Al aka scalefan,
I followed your threads with great interest and appreciated your no nonsene, tell it like it is style. I also felt your pain and with persistance, your sweet sucess. The p38 had monster props that in scale size would be a problem in the model especially on the less than perfect grass fields that I fly on. I think the VQ is about the same size and my 14 inch props are a good fit. I have fitted out two friends planes with Aeronuts 4130 outrunners using their 4100 3s lipos seriesed for 6s. also using the aeronuts 85 amp HV controller. This setup turns a 15-10 at around 50 amps That makes a slick relatively inexpensive package. I am using Jeti 77's in the 38 but if i were doing it now, I would go Aeronuts all the way. My p38 is using 6s 2p 4100 Aeronuts lipos. CG required the batteries to protrude thru the firewall about a third of th battery length. I have been sandbagging with this thread as I now have 10 flights and so far all aspects of flying the plane have been outstanding in my humble opinion. I am using a HS81mg servo for steering that is just behind the retracted nose wheel with two light coil springs pulling the cables apart during retraction. that also gives some springiness to the steering similar to some full scae planes and may keep from stripping gears. The nose wheel was 2.75" (about scale) and I buried it in soft ground so now it is 3.75" and the mains are 4". I will be lurking on your new thread!
Last edited by tom bacsanyi; May 14, 2007 at 07:08 PM.
May 14, 2007, 05:24 PM
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tom bacsanyi's Avatar
I dont have video stuff at the moment but will eventually get some of the plane. The gear takes 10 seconds to transition.
May 14, 2007, 06:38 PM
It will work....I know it will
Bravo Tom!

I love innovative designs like this......

That bird is a beauty to behold....bet it flies like the clappers. Who sez we need "stock" carriage systems? Excellent work....

Happy flying

May 14, 2007, 07:17 PM
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tom bacsanyi's Avatar
Thanks Mark
The steering servo is a HS81mg hitech. The servo is on a subplate to make it easy to remove. Also see spring arrangement
May 14, 2007, 08:36 PM
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After the first couple flights and gear collapse, I rebuilt the retract mounting structures which were origionally fabricated from ply similar to light ply. most of the structure was replaced with birch ply with added reinforcement. The last flights to date required nothing more than battery charging. Now for cosmetics. The 38 rolled out in 1939 as a high altitude fighter capable of 40,000 feet with its turbocharged intercooled Allison engines. This was long before the P51 got the Merlin engines to give it high altitude capability. So I tried to inhance the appearance of the "new secret GE turbo supechargers". Also fascinated by the gun installation, four 50's and a cannon in staggered configuration so the magazines could fit one behind the other. The guns are scale including the bores. The turbo needs some grey exhaust deposits airbrushed on. I need to do the other turbo and then a pilot and markings and............
May 14, 2007, 10:33 PM
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Nice Tom!
Where did you find those details? Our KMP P-38 (aluminum) is finished except for dialing in the radio. It lacks guns and a pilot and that is no way to maiden a big bird like this. How did you find the maiden? trim? CG from spec worked OK?
May 14, 2007, 11:13 PM
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tom bacsanyi's Avatar
Laxlife, The guns are aluminum tubing with a turned insert for the barrel. The turbo rotor is thin aluminum used for flashing available at any hardware. I made a cad drawing with 70 radial lines and rubber cemented it on to the aluminum disc then made 70 cuts. Tweak with needle nose. Any ideas for a 7.5 scale pilot? The cg is very slightly nose heavy from the instructions with the wheels down. The plane is very stable and neutral trim is too much up. the elevator is down about 1/8" with no flaps, 1/4" with 1/2 flaps and 3/8" with full flaps. Set full up trim at neutral so you get a lot of down trim. This airplane is a piece of cake to fly compared to the Comet. It is reluctant to stall and doesn't drop a wing. The only caution I have is to not run out your batteries and if you do get an engine/motor failure, chop the throttle and make a dead stick as best you can 'cause flying on one engine while possible, is not going to work. So far I have done loops, rolls, snaprolls, and hammer heads. The split flaps are very effective and I have been using 1/2 flaps for T.O. and landings.
May 14, 2007, 11:46 PM
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Here is a picture of my ongoing build, our KMP P-38 (hangar queen? heavens no!)
Last edited by LaxLife; May 15, 2007 at 12:04 AM.
May 15, 2007, 02:44 AM
Done it all

I thought you were just starting this project, had no idea you were already flying and adding more scale modifications. I couldn't help but post when I saw the belt drive propeller system. I instantly remembered the Cessna 321 with geared motors, also the Aero Commanders, nothing but trouble and the motors and gear boxes could not handle jerking the throttle. I imagine belt drive would be the same, and a rough throttle might actually cause a belt to break.

I'm not into scale as deeply as others and my planes don't have pilots. The truth of the matter is I can't get a pilot to volunteer, not even a dummy pilot. My attitude at the moment regarding RC construction is to finish the plane to flying condition, test fly the plane and see if it survives to see the next round of additional time investment. If it crashes, it is not worthy (or I am not worthy). The survivors then could be retrofitted and updated. The line is forming for updates....

I suspect the P-38 will be a slam dunk project, just like the second P-61 and it will be pwered by 50-52b HXT motors (sometimes called Tower Pro) and 6S Aero-Nuts 3 cell 2100mah battery packs. Counter rotating props are in the plan, as I believe the P-38 did have counter rotating props. (not sure)

I'd love to see your plane fly, it looks very realistic.

Laxlife, I was wondering what happened to you, you've been very quiet. It seems that P-38s are the in airplane right now, so, get to it.

May 15, 2007, 08:16 AM
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tom bacsanyi's Avatar
Al, 2100 packs sound small to me, how many are you using? Assuming they can handle the current, flight times will be short. With a total of 8200 mah and 6s I get 8 minutes of mixed flight with safe reserves. Belt drives have been real durable for me. My p51 cowl is really beat up from nose overs in the grass and aside from broken props, the drive has been fine. Inexpensive outrunners is the way to go now but an inrunner belt drive has these advantages:
1. a 30 size inrunner 3:1 becomes a 90 size. my little Gunther 30's are running 1kw a piece flying a 21 pound airplane with gusto.
2. lighter weight.
3. 3/8 hardened prop shaft, no bend no break.
4. silent, very efficient, long life.
5. easy to match any motor/battery perfectly to any prop with pulley changes
6. motor is protected from crash damage.
The p38 had counter rotating props with the top blades rotating away from center which is unconventional in twins. I am using 14-7 three blade Master Airscrew props counter rotating. Yes they have a pusher version. Also included rx battery installation pix One battery/regulator for gear/flaps, the other for flight controls. The motor battery charging is done here also. The end taps use a Deans and the other taps on small connector. FMA balance pro charger. The nose cone, cowls, and canopy are held on with magnets.

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