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Problem with Sport Cub S motor speed

#1
So, I have been flying for about a week now and today my motor would not get up to full speed. I took the plane apart and noticed when I accelerate the rudder servo moves a little bit. The rudder servo is mounted on the receiver and I'm wondering if the receiver may be faulty. Im pretty handy but this stuff is new to me. I'm getting about 3.8 volts to the motor but the pins are so small I have a hard time keeping the probes on to monitor the voltage to see if it is fluctuating. Also, the motor won't turn unless I spin the prop. After the initial spin I can move the toggle and it responds ok up and down until I take it to zero throttle again. Any suggestions?
 
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#3
I'm beginning to think that may be part of the problem. I just tried the motor connected directly to the battery and the speed was much better. It still wouldn't start without spinning it by hand first though. Possibly a problem with the receiver also because I don't think the servo should move when I throttle up. Great little plane but I have been hard on it.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#5
Just be warned that a bent motor shaft or a damaged bearing/bushing can cause the motor to draw far more current and possibly be so hard to start that the motor just sits there, "Stalled", and dragging all the voltage supplies down to where other things are not getting enough power to operate, (unless you hand start it of course)..

I would suggest that you replace the motor before judging the operation of other components!

Have fun!
 

FDS

Well-known member
#6
I had the exact same problem, I replaced the motor, it’s been fine ever since. My first one lasted two weeks. The vendors claimed I couldn’t get one for free because my plane showed signs of being crashed so they never got any more business from me! I had to buy the motor full price as a spare part.
Watch out for getting the mounting stuck down properly when you re fit everything.
 
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#7
Yeah I'd check the motor. I'm assuming too that the motor appears seated good with the gearbox? Those are brushed motors and that can wear out.

As for rudder movement, it's really hard to tell without really seeing it move and how much it moves. I know the plane does have it programmed that as you apply the throttle the elevator will automatically move to pitch the nose of the plane up. I haven't seen all of them but I would say most of the SAFE planes (Sport Cub S, Apprentice, etc) have that setup.
 

FDS

Well-known member
#8
They just die, it’s a common problem apparently.
I used the U bend in the control rods to set a bit less up elevator so there was room to trim it out a little. Always do your rough trim on the bench with Safe OFF.
Another trick with the sport Cub is to put some nose weight on it. It should sit slightly nose down with the battery in using the spar as the balance point. I put two M4 washers under the nose then covered them with foam safe cyano and a layer of blu tack. Mine flew a lot better in the wind with that set up and stalled less easily.
They are great fun in small spaces but I found I learnt more from my Tiny Trainer flying with low rates on 4channel than I did flying the Cub in intermediate/advanced. I now fly mine in silly sized flying areas for fun, it’s well under 250g loaded so sticks it to the man at the same time.
 

Arcfyre

Well-known member
#10
Was this a brushed motor? I don't see how else you could have hooked it directly to a battery as you said and gotten anything useful out of it.

If it is a brushed motor, I would look into replacing it with a brushless system. The brushless motors have come a long way: no need for a gearbox, fast and powerful, readily available and cheap. They can also handle much higher voltages and make more power. Just my $.02
 

FDS

Well-known member
#11
There’s no structure in the Sport Cub S to mount a motor to, it’s a UMX size, it also has no battery space available.
The SAFE system (main reason people buy it) is all on the same board as the ESC, two servos and receiver.
You can fit Carbon Cub internals in the Sport Cub but at that point you might as well build a new plane.
It’s a small brushed motor, it’s under $15, the replacement is easy to find.
Unless the gears are damaged you won’t need the gearbox. There’s several YouTube videos on how to replace the motor.
 
#12
I ordered the motor, gear box and a new body. I beat this one up pretty bad and have only tried intermediate mode a couple of times. Hopefully I can keep the new body in better condition, lol. I have a new P51 Mustang that I know I'm not ready for. Looking forward to it though. I also have my first balsa kit on order. Let the fun begin.:)
 

FDS

Well-known member
#13
Build a Tiny Trainer or Simple Cub, they fly really well and are a good next step up from the Sport Cub.
I found Intermediate harder to fly than advanced mode, the limited throws and pronounced upward pitching on acceleration were really hard to counter, it felt like the gyro was fighting corrections. I tend to take off and land with full safe on then fly acro once up. Setting the CG forward and trimming out in advanced mode made it more fun to fly.
The body of my Cub is in 6 pieces, the tail has been broken off amongst other damage. I use light weight tape and foam safe cyano or super phatic to stick it back together.
By the time you are buying a new body you might as well move the electronics wholesale into something like a Micro Aces model.
 
#14
Thanks for the tips FDS. Those Micro Aces models look interesting but I have ordered the new body already. Maybe I'll try one of them next. Pretty big learning curve to this stuff but its lots of fun.