View Full Version : Help me with my AXI Power KMP P-38

03-24-2006, 05:00 PM
I fly all electric, I have a Hanger 9 P-51 setup with a 4130/16, a stuka and some smaller planes with brushless.

My latest is a KMP P-38 Lightning (Twin boom, twin engine WWII Fighter - I am sure you know the plane)

Weight - 19lbs
Motors - 2 AXI 4130/16s
Controllers - Jeti 70amps
Batteries - I have and can fit: 6 11.1 3000 12C Lipos (from Dymond Modelsports) - I have these set up as 3 pairs in series, those 3 pairs if packs joined in parallel so I have 22.2 volts at 9000mah total.
Props at the moment: APC 15/10s

OK - I run my P-51 with 4 of the above packs, 2 pairs in series, then those two pairs of 2 packs in parallel - similar to above but 6000 mah. I run a 16/10 APC prop with the 51 and get great performance and duration, average flight time is around 15 min or more with mixed throttle. Stellar.

On to the P-38 - I ran this on the bench with 15/10 props - 3.5 min at full throttle gave me a steady 55-58 amps (settled to this after an initial 60-62 amp draw I throttled down just a click or two), 6200 rpm. After the 3.5 min, the packs started to get warm, so I dropped down to a cruise like setting of 3/4 throttle - amps dropped to 36 amps and I continued the run at that setting for another 5 min then started to see a drop in rpm, batteries getting hot and in my opinion it was time to stop.

QUESTION: 1. With the batteries - Should I prop down to a 14x8 - I feel like the batteries are stressed just beyond at true full throttle, if I remember I did throttle down just a click or two to get a sustained 55 amps.

After the run of 3.5 min at 96% throttle - I went to cruise of 36, during that time I went to full 100% throttle and one of the two motors dropped RPM and was shutting down until I reduced throttle a bit and it came back up - voltage deppression I would guess and since one controller started to spool down the other kept going? - I have two controllers feeding off one set of power leads (sharing the packs) - is this the best way to do it?

I need your thoughts - In short, I think that the batteries are maxed and I should prop down. Is my setup the "right" way to go? What can I improve upon using this gear and batteries? I know I could place another 2 packs to double what I have in my P-51 but space would be tight and that would add another 1lb to the plane. I plane to maiden this weekend.

e power
03-24-2006, 05:56 PM
can I ask what props are you using.APCEor normal glow props,
I run Axi 4130/16s in several models and your amps seem high for the prop sizes,for example on 6s 5400 mah lipo using a 17x10 I get 52 amps at full power,are your props three bladers,or two,best of luck curing your problem.
Euan Galloway

03-24-2006, 06:56 PM
APC Electric Props - 2 blade only. Remember I am talking about 2 axi motors and I am getting the amp reading off of one of the main power leads before they split to each motor.

03-24-2006, 08:20 PM
.................I am getting the amp reading off of one of the main power leads before they split to each motor.

You would be better off getting power consumption readings between ESC and battery. Brushless motors are in effect 3 phase motors, so taking a reading off of 1 leg is not going to give you correct power consumption.

I thought long and hard about getting one of these P-38 also, but decided that I shouldn't have to take out a loan to get the plane (and all the extras). My thoughts were using the AXI 4130/20s, as they are able to swing bigger props.

With those motors, I figured I could use Master Airscrew 3 blade 16 x 10 props, which incidently: they make in tractor AND pusher, so I could have the motors contra-rotating just like the full scales!

With 6S Li-Po, current draws (each motor) would be in 30 ~ 35 amp range, while delivering over 7 lbs static thrust (each).

I really wanted to get one (KMP P-38), but can't afford it at this time.

19Lbs? I thought these were supposed to weigh 14 ~ 16 lbs. My electric conversions weigh less than the fuel powered ones, since I switched to Li-Po batteries

02-18-2008, 07:43 PM
Hi guys,
I have a Kmp P38 that I have gone electric on...its my 3rd one (yes I AM nuts)

the 2 previous ones were 4 stroke and after several flights spun in each...w/flame outs....

anyway I have this third one on 8s each nacell w/ cc HV45s and 4 4s packs (2 3700ma and 2 4000 ma tied in series then in parallel)

I am using a "cheap" chinese motor from united hobbies the hxt 50-65B 35o kv/rpm

w/this set up and Robart retracts the plane weighs in at 19.5 lbs

I am using 13x8 props for the moment and have had 3 successfull flights of 7+ min w/ a 20% reserve when the bateries were recharged.

All good so far except the performance is modest,scale warbird like.

I had much better performance w/ ys63's and the saito 72's (which performed about =)

So here is my problem(s)....
#1 I would like to consider going to 6s by purchasing 2s ,4000 packs so I will hve more than one 7 min flight after charging 4 packs....only have one charger...

this may require a new motor and speed controler,would be great if I could use the CC hv 45s...

to get a sence of my prop options I tested a few props on hand w/ my hyperion "e-mitter" these were the results....

13x8 8s (1 4000ma and 1 3700 ma in series)
608 Watts (this may be why it felt underpowered)
7860 Rpm

then I changed props w/out toping off the packs

27.37 V
23.5 Amps
644 Watts
7545 Rpm

then 16X10 w/out recharging
25.46 V
41.1 Amps
1047 Watts (now thats more like it!!!)
6450 Rpm

Yes I realise the 16x10 looks like the obvious winner but I think I will have less than 5 min run time and ?? reserve....

another consideration is that I am trying to stay w/ 13x8 or 16x10 as Apc has them in pusher so I can go counterrotating...not a huge thing but nice.

I love the reliability of electric but if I can't figure a way to make this more practical I may need to consider going <Gulp> back to glow....

Really apreciate any thoughts


Glacier Girl
02-19-2008, 03:49 PM
topher, I'm surprised no one mentioned this. Running the motors that long static, I'm surprised something didn't smoke on you.

15 seconds or so should really be all you need to safely test out draw.

Anywho, I'm building the VQ version,
HTX 63-64A motors, 280 kv.
Spinning 15.75 x 13 x 3 APC's.
CC 60's for escs, and Ultimate ubec's.
6S4P 3700, 20C pack.

Pulls approx 100 amps total, produces over 22 pounds of thrust, and speed will be around 70 mph.

02-19-2008, 05:57 PM
As Glacier Girl points out - you are VERY lucky. NEVER run on the bench for more than 30 seconds. It is dangerous and very hard on equipment.

Run it in the air - after you have verified that the ground run is within the capabilities of the Batteries/ESC's and motors.

That way the equipment gets full cooling air and you can judge after a short flight.

For a 19lb warbird like the P-38 you are going to want power. I would prop for no less than 125w/lb static and then go from there.

The propeller you settle on will have to do with flight characteristics (speed and thrust) and I never decide what prop to use on the ground (just what one to start with!). For my large warbirds I sometimes check 10+ propellers before settling.

Check the packs temps after flight see where you are at - do you have direct cooling for the ESC's and batteries?


02-19-2008, 09:10 PM
Glacier girl,
how long a run time do you antisipate w/ the 6s 3700's your planning on running?