Newbie motor issues

[Arkstedt]

Hi!

I have gone and bought myself the Volantex Super Cub 4ch RTF from hobbyking and it's been a great trainer airplane with a few minor issues and a recent quite big one.

What happens with this plane when the battery starts to run low is that if you go full throttle and then back to zero, the plane doesn't react and the servos stop responding for a second. I encountered this while trying to fly straight up, resulting in the plane stalling and crashing nose down into the dirt.

The only damage however was some minor foam destruction and a bent prop shaft. That however turned out to be a bigger deal than expected as I can't find replacement shafts for this plane anywhere and now I'm struggling to find either a new shaft or a new engine with a shaft.

The motor is specified as VOLANTEX RC LY2808CD-KV1800
Prop shaft: either 2,9mm or 3mm
Diameter for motor mount: 8,5mm

Battery: 7,2V lipo 500mAh
ESC: 10A Brushed/Brushless
Flight weight: 210g

What I'd like is some kind of package (preferably from hobbyking) where I get a new motor, motor mount, prop shaft and some suggestions on propellers that will work for this new setup. I have been looking at these items but I am unsure if they will work;

Motor
Prop holder & motor mount?
Prop shaft
Props

Will this work or is there anything i ought to change? What should I get if I want to use prop savers?

 

[stay-fun]

Hey man,

That looks like a good plane for a beginner!

I'd like to give you a piece of advice: never, ever fly until your battery dies. Especially if you feel that the battery is getting low, don't give it full throttle. This not only results in crashes, but you're being hard on the battery, if you do this a couple of times you will need to replace your battery.

A simple solution for this is to fly with a timer.

As for fixing your plane, I'm not sure... Can you tell me what the original propeller is? A 7x4? Or 8x6? or.... Because that will deterimine which motor you'll need.

 

[Arkstedt]

Hi!

The prop size according to the manufacturers website is 7040 and look alot like the ones I've suggested for myself.

The plan has never been to run the battery dry, but the time between full performance and near no performance is about 7 seconds so it's really hard to tell when it's about to run out. I might just get a timer to make sure it doesn't happen again.

 

[stay-fun]

Definitely the timer .

I think the motor on that thing is pretty small, you may want to consider upgrading to a larger motor. But that's just me, if you're happy with how it flies right now, there is no need to upgrade .

I see that the newer link you gave me says the wingspan is 750mm, while the HK website says 800mm. Are you sure it's the same product?

I think this motor comes pretty close to the original one. I can't find what motor it came with (I would like to know the weight, and especially the kV rating) but this motor I linked to is designed to run the same (or at least similar) prop, so it should work just fine.
I hope the ESC can handle it. According to the HK website, this motor will draw 8-8.6A at full throttle, which is just over the 80%-rule (the motor should require at most 80% of the ESC's amp rating - so a motor for a 10A ESC should be rated to draw 8A or less).

 

[Arkstedt]

Yes, it is indeed pretty small and I'm sorry - I thought I'd specified the original motor.

It is described as a 2808 brushless outrunner and on the motor itself it says kv1800 so i'm thinking that's the KV rating. The only thing speaking against a larger motor is the lousy battery time - I have larger models about to be assembled with more powerful setups, so I think I'll just keep this one as close to newbie strength as possible.

Another thing worth keeping in mind is that the prop shaft on the original motor is about 4cm long and threaded, so my improvised prop saver is attached with a couple of nuts and some pressure. It needs to have this rather long prop shaft to go through the plastic motor cover on the front of the plane.

It could probably be shortened down a cm or two, but at the risk of the prop getting to close to the motor cover

 

[stay-fun]

In that case, I'd go for this motor. The kV rating is the same, the current rating is higher than your ESC can handle, BUT that is based on a 3s battery. The current draw with a 2S battery will be lower. Also it has a longer shaft, you'll have to find out yourself if the shaft is long enough for your application. HK does show the exact dimensions in their specs, which is really convenient, so you can find out if it'll fit beforehand.

Hope this helps!

 

[quorneng]

Arkstedt
I know it is part of the original design and there is little you can do about it but a long prop shaft with the prop some distance from the front of the motor is simply asking for it to be bent and/or making the effect of any prop imbalance much worse.

Hauling a 210g plane around with just a 500mAh 2S is bound to have limited endurance. Customer reviews of the plane on the HK web site suggest 5 minutes is about the limit.
They also mention the same complete cut out issue that you have found. This suggest that the speed controller is 'crude' and does not have a LVC function that cuts power to the motor when the battery voltage drops below a set figure but keeps power to the radio and servo to allow a controlled landing.

 

[Arkstedt]

I completely agree regarding the retardedness of this solution for the prop, but unfortunately for me it is very hard to find what motors will possibly fit that has other methods of attaching the props as swedish stores charge around 5x the price they do on hobbyking, including the shipping(!!)

I ended up placing an order for this motor for the moment - hopefully it'll work despite having almost half the ammount of kv's.

Unfortunately the one you linked stay-fun wont work as it has a "stick mount" which sounds like it will require some serious modifying to get it to fit properly and that plane simply isn't worth it.

The review with the 5m flight time is mine actually, but you might be right about the crap ESC.

I have an extra Turnigy Plush 25amp ESC since my last order - would it be overkill for me to install that one or are there any arguments why I shouldn't?

 

[ktmrocks]

If the original motor is a Turnigy, you might try Grayson Hobby. He has some shafts that might work. Send him an email and he'll help if he can. There's always the shipping problem, but...

 

[stay-fun]

With that motor, go with a 500-800 mAh 3s battery instead of 2s. The higher voltage will (partially) compensate for the lower kV.

Good luck, and let us know how it turns out!