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Post  ldehaven on Sat Jan 21, 2017 8:49 pm

I own a 1988 Cox Electric Malibu RC Airplane. The motor has failed and I'd like to get a replacement. The motor is brushed 7.2V, 30.8MM long, 24MM Dia with a pinion gear attached to the shaft. the propeller is geared with a single gear. The propeller shaft and the gear are in Ok condition.

Can anyone tell me where to get a replacement motor and what motor to purchase?

Thanks,
Leon DeHaven
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Post  pkrankow on Sat Jan 21, 2017 8:56 pm

Got pictures?  I am reasonably sure it is an off the shelf motor and not something airplane specific, due to the age of it.

I however, am not an expert.

Phil
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Post  ldehaven on Sat Jan 21, 2017 9:45 pm

Here are three photos. A pic of the airplane, two pics of the motor. I haven't been able to find any part numbers or model numbers on the motor, only "Made in Tiawan".

Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Img_1210
Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Img_1212
Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Img_1211
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Post  batjac on Sun Jan 22, 2017 12:51 am

Leon, the motor is a Mabuchi 380 motor, I believe. I don't know much more than that. Here's a weird trick you can try for now.  It may just be that the commutator is dirty or has a film built up.  Ideally you would bend up the tabs on the back of the motor can and pull the rotor out to clean the commutator, but since it has the pressed on pinion that probably wouldn't be possible.  Since you've already committed to replacing the motor, you could try what we did in the olden days to seat the brushes to the commutator.  If it doesn't work, you've not lost anything.  

If you have two D-cell batteries you can hook up in series, that's would be what I'd do and just let the batteries run for about 10 minutes.  If not, you can use the 7.2V battery and run the motor for a few minutes.  Place the motor in a glass of clean water and spin the motor up.  The water softens up the face of the brushes and allows them to conform to the commutator.  If you do this, make sure that you hook up the batteries so that the motor turns in the same direction that it would when flying the plane.  If you do this in the opposite direction than that used in normal flight, the brushes will seat incorrectly and you'll get no benefit from this.  When you're done, blow out the motor can and put a couple of drops of oil on the bushings.

Kind of like this video:





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Post  batjac on Sun Jan 22, 2017 1:25 am

Try comparing the measurements of your motor against the 380 specs on the Mabuchi website Do a search for "380".

https://product.mabuchi-motor.com/search.html


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Post  KariFS on Sun Jan 22, 2017 12:08 pm

If it is a 380 or 280 it should not be hard to find one. For example GWS Slow Stick, Beaver and just about all "pre-brushless" era slow flyers used the 280 motor. 380 was not uncommon either. So if you have a club or bunch of other hobbyists nearby, ask around and one should appear in no time Smile

You might want to replace the radio equipment too, with modern small servos and one of those really light speed controllers. And of course, LiPo battery.

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Post  fit90 on Sun Jan 22, 2017 1:01 pm

MAXX-PROD.com used to be a good source of electric motors and info.
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Post  rsv1cox on Sun Jan 22, 2017 3:09 pm

ldehaven wrote:Here are three photos. A pic of the airplane, two pics of the motor. I haven't been able to find any part numbers or model numbers on the motor, only "Made in Tiawan".

Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Img_1210

Like the airplane, love the cat.

Bob
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Post  batjac on Sun Jan 22, 2017 6:26 pm

Leon, can you take a picture of the motor mounting area in the plane? We might get a better idea of the motor size if we see the space it takes up in the nose.

The Nosey Mark
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Post  dckrsn on Sun Jan 22, 2017 9:26 pm

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Post  ldehaven on Wed Jan 25, 2017 4:28 pm

Thanks everyone. I found a MABUCHI MOTOR RC-280SA-2865 that looks like it will fit the bill. I can purchase it from Kysan Electronics. I liked the idea to replace the electronics also.

I am going to try the "Break-in" process posted by "batjac" to see if I can revive the old motor.

I've got some work to do. I'll let you know how it turns out.

Regards,

Leon
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Post  ldehaven on Mon Sep 24, 2018 1:08 pm

Its been a long while since I last posted.

I couldn't revive the old motor but I did get the new MABUCHI MOTOR RC-280SA-2865. I broke it in using the distilled water method added capacitors between the can and + and the can and - terminals. Runs fine. Except the prop is turning at 3500 RPM. The gear ration is 2.5 - 1 which results in a motor speed of 8750 RPM). That propeller speed isn't fast enough to power the plane. The propeller is a two bladed 7 X 4.

The specs for MABUCHI MOTOR RC-280SA-2865 says it should run at 14112 RPM at 7.2 V (Maximum Efficiency). There must be something I don't understand. If it did run at 14112 RPM, that would give me a propeller speed of 5645. Not really fast but better.

I was reading that a 7" propeller should run from 10,000 - 12,000 RPM.

Any ideas as to what I can do?

Thanks,

Leon
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Post  Levent Suberk on Tue Sep 25, 2018 5:35 am

It is better to mount a brushless motor instead of brushed one.
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Post  GallopingGhostler on Tue Sep 25, 2018 9:49 pm

ldehaven wrote:I couldn't revive the old motor but I did get the new MABUCHI MOTOR RC-280SA-2865. I broke it in using the distilled water method added capacitors between the can and + and the can and - terminals. Runs fine. Except the prop is turning at 3500 RPM. The gear ration is 2.5 - 1 which results in a motor speed of 8750 RPM). That propeller speed isn't fast enough to power the plane. The propeller is a two bladed 7 X 4.

The specs for  MABUCHI MOTOR RC-280SA-2865 says it should run at 14112 RPM at 7.2 V (Maximum Efficiency). There must be something I don't understand. If it did run at 14112 RPM, that would give me a propeller speed of 5645. Not really fast but better.

I was reading that a 7" propeller should run from 10,000 - 12,000 RPM.

Any ideas as to what I can do?

Leon, sorry, it took me a while to figure out the graph and its meaning, so here is my after the fact revision.

I'm a nitro engine guy versus an electric motor guy, but your question has sparked my curiosity. A good discussion may be found in: RC Groups Forums Thread 666541, Mabuchi 280 motor help!!!

General impression seems to be that the motor is rather weak in power. From there, I got this data from Mabuchi:
Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) 2018-081

Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) 2018-082

That 14,000 maximum no load RPM is free shaft without prop at I believe 6 Volts. Motor is rated at 5.56 Watts at its maximum efficiency. This is at roughly 58% maximum efficiency, which is why most nowadays go to higher efficiency brushless technology.

Looking at the graph, this is what I gather. Where RPM line intersects the Current line, there is a maximum torque of 175 G-cm or 0.01266 Ft-lbs x 5500 RPM / 5252 = 0.0133 HP x 745.699872 = 9.88 Watts, efficiency drops to 32% with it drawing 6 Amps at 6 Volts.

A Cox .020 Pee Wee puts out 0.036 HP at 18,000 RPM. (See Sceptre Flight Model Engine Tests, Cox Pee Wee 020.) This is 2.7 times the power of the electric motor. I don't know the exact size of your model (i.e., wingspan), but just by pictures on the Internet looked like it was Pee Wee sized (25" to 30" wingspan).

So, seems to me that the Mabuchi motor is undersized for the model, but then again, someone who knows their electric motors and who has a better working knowledge of these graphs can correct me and give you a better interpretation.

Again, I'm no expert and someone who works intimately with electric motors could chime in to what electric motor you need. Brushless nowadays are very common and brushed are getting much harder to find. They could chime in what motor / ESC / battery combination to use given the size of your model, plus may be what needs to be done to your radio. I don't know if your radio is standard or a specialized factory set up to reduce costs requiring further circuit modifications.


Last edited by GallopingGhostler on Tue Sep 25, 2018 10:47 pm; edited 1 time in total (Reason for editing : Correct line of thought.)
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Post  ldehaven on Tue Sep 25, 2018 10:31 pm

Thanks for the info on the motor size. I was coming to the same conclusion about the motor I bought being undersized.

I am Ok with replacing with a brushless motor, but I can't find one that fits the engine mount (24mm diameter) and has a 2mm shaft.

Can anyone make a recommendation?

Leon
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Post  GallopingGhostler on Tue Sep 25, 2018 10:49 pm

ldehaven wrote:Thanks for the info on the motor size. I was coming to the same conclusion about the motor I bought being undersized.
I am Ok with replacing with a brushless motor, but I can't find one that fits the engine mount (24mm diameter) and has a 2mm shaft. Can anyone make a recommendation? Leon
Leon, I corrected my thoughts, took a while to figure out just what that graph was saying. Yes, it does appear to be undersized, even if you put the most optimal prop to make best use of its torque and that at a heftier current drain. Hopefully someone else will be able to answer your question here.
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Post  Mark Boesen on Tue Sep 25, 2018 11:26 pm

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Post  Levent Suberk on Wed Sep 26, 2018 5:18 am

Try a Graupner Compact series motor. However you need to change motor mount. It is almost impossible to find a brushless motor in same size. Cox engine equivalent motors are 80-100 Watt.

https://www.graupnerusa.com/BRUSHLESS-MOTORS_c_237.html
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Post  GallopingGhostler on Wed Sep 26, 2018 6:19 am

Levent Suberk wrote:Try a Graupner Compact series motor. However you need to change motor mount. It is almost impossible to find a brushless motor in same size. Cox engine equivalent motors are 80-100 Watt.

https://www.graupnerusa.com/BRUSHLESS-MOTORS_c_237.html
The Cox .020 Pee Wee is 0.036 HP or 27 Watts. The Cox .049 Babe Bee is 0.056 HP or 42 Watts. The Cox .049 QZ (forerunner of the Sure Start) is 0.065 HP or 48 Watts. Are electric motors rated differently that their numbers would be higher?

ldehaven, what is the wingspan of your airplane? Regarding prop shaft diameters, you don't need to match it exactly. There are prop hubs available for a different diameter. This will also allow you to mount other props and experiment.
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Post  GallopingGhostler on Wed Sep 26, 2018 7:14 am

ldehaven wrote:The motor is brushed 7.2V, 30.8MM long, 24MM Dia with a pinion gear attached to the shaft. the propeller is geared with a single gear.

Here's a same sized motor, produces a greater free running no load RPM of 22,500 at 7.4 Volts, 19,000 RPM at 6 Volts versus your motor's 14,000 RPM at 6 Volts. Cost $2.00 shipping included from China.

E-Bay: DC 3V-5V-6V-7-4V 22500RPM High Speed Carbon Brush Mini 280 DC Motor DIY Toy Car, Item# 263241854814

Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Rp280c10

Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Rp280c11

I don't know if it would have sufficient power either, but is more powerful as it's no load Amps are 0.38 versus yours at 0.28 Amps. I found it doing a search on 280 electric motors, which is the physical size of your motor.
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Post  ldehaven on Wed Sep 26, 2018 2:18 pm

Thanks everyone for your interest and help.

GallopingGhostler,

The wingspan of my plane is 55". On my bathroom scale it weighs in at about 1.6 - 1.8 lbs (725 - 816 grams).

The motor you recommend looks promising.

Mark and Levent, I will keep looking for a brushless motor. I unfortunately don't have an option to change the motor mount but I'll keep putting some thought into it.

Regards,

Leon
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Post  GallopingGhostler on Wed Sep 26, 2018 3:14 pm

ldehaven wrote:Thanks everyone for your interest and help. GallopingGhostler, The wingspan of my plane is 55". On my bathroom scale it weighs in at about 1.6 - 1.8 lbs (725 - 816 grams). The motor you recommend looks promising.
Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Electr10

Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) Electr11

Leon, that wingspan is significant for a half-A sized airplane. With the original motor it must have flown very trainer like in slow flight, highly affected by wind, requiring flight in almost calm conditions. I truly can't say that the motor I pointed out would be equal to your original motor, but for $2 total and a small wait, might be worth a try to maintain your original set up especially since it is very cheap.

Your set up has a gear ratio 2.5:1 which would act as a torque amplifier for a larger prop. I'd have to go back to my physics classes some 40 years ago. With that torque amplification and perhaps repropping might put the replacement motor into the .049 power range.

About 30 years ago I built a House of Balsa 48" wingspan Nomad glider powered by .020 Tee Dee. Because I used Monokote to cover the plane made it too heavy for that power. I needed at least a good powered .049 reed valve engine. (It was originally designed to be clear doped balsa with clear doped silk or silkspan covered wing.)

About 20 years ago, I built a 52.5" (1335mm) wingspan Ace R/C Grasshopper kit. It has a foam wing similar to your plane. With a Cox .049 Golden Bee engine it performs fine. With a Norvel .061 Big Mig non R/C it performs even better. Box says 20 oz (1 lb 4 oz) compared with your 1.6 lb (1 lb 10 oz) Malibu.
Cox Electric Malibu RC Airplane - electric motor replacement (Circa 1988) A8352310

If direct drive instead of geared, I think this is why Mark Boesen was indicating to try a 380 brush size motor, as in general they are more powerful than the 280 brushed. I had a 40" wingspan Davey's Systems Baby Ace kit I gave to a friend. The brushed Hyperthrust 075 Cobalt motor was considerably larger than your motor.

There are 2 trains of thought, one to preserve the model in its original state and the other to do whatever modifications are required to fly better. I gather that your interests are to retain it basically in its original configuration.

Good luck on finding a suitable electric motor, would like to continue to hear your progress on the aircraft, what works and doesn't, and finally success.

- George
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