35-30 motor for the Cub

[Dillzio]

Hi gang,

I was wondering if anyone is using a 35-30 motor in their cub? I've got my eye on the Turnigy one at HobbyKing www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=3883


I know the 28-30 motor is popular for the super cub, but I wanted something that can climb vertically with a bit of extra weight (such as a GPS) on board. It also seems to me that putting such a light motor in is a bit of a waste since even with the stock motor and gearbox, you still have to add weight to the nose to balance it out. I'd rather have a heavier motor and have to add less weight.

I was hoping for info from ppl that have installed and used this motor in the cub about how well the cub flies with it, how easy it is to mount, and if you need to change the thrust angle at all or if you can just mount it on a cut down gearbox?

thanks :-)

[flydiver]

Has anyone weighed the motor/gb assembly?
Seems that would be a useful bit of info for these mods since the lipo placement doesn't have a whole lot of latitude.
Better useful weight than dead weight. A lot of the older systems designed for brushed/gb and NiXX suffer from this issue.

[Dillzio]

I just weighed the stock motor and gearbox, with wires connected, without a prop or prop saver. 145 grams

[ginginho]

Hmm, the figures listed by posters on HK for that motor seem to contradict each other a fair bit. Max Amps are not listed which is not helpful, as many have stuck big props on it to get decent wattage readings. They probably are only shortening the life of the motor. This one HERE is the Turnigy equivalent to mine which would give you the performance you require. As advised elsewhere, stick the ESC in the cowl and you save on having to add weights.

Either motor (35-30 or 2217/16) = 70grams roughly.
40A Turnigy Plush ESC (3A switched BEC inc) = 33grams.

This only leaves you around 40grams for the motor mount, screws etc to get the same weight as the stock setup. I don't see the need for weighs, I never needed em, nor my flying mates after they'd converted.

P.S. Be aware that both those motors are reverse shaft, so you'll either have to use a cage mount (something like THIS (32 grams without any nuts/bolts))or use the star shaped mount that attaches to the opposite end of the bell to the shaft (accessory in right hand side of picture with the thread and nut showing), and then mount the motor as per the US norm Park 450/480's.

[Dillzio]

Ging, could you clarify what you mean by "reverse shaft"?

Do you mean that the motor is made to be mounted from the front (Eg behind a firewall, or cage with the prop shaft sticking through) and that to mount it from behind you need the cross-shaped adapter?

Thanks for the tip on the other Turnigy outrunner too, it looks like a tough little motor.

[ginginho]

Nov 19, 2009 7:40:03 GMT 1 Dillzio said:

Ging, could you clarify what you mean by "reverse shaft"?

Do you mean that the motor is made to be mounted from the front (Eg behind a firewall, or cage with the prop shaft sticking through) and that to mount it from behind you need the cross-shaped adapter?

Thanks for the tip on the other Turnigy outrunner too, it looks like a tough little motor.

Dillz,
If you take a look at the pictures you'll see that the shaft sticks out of the motor at the end where the wires are. This is the end that doesn't rotate, and hence is mounted to the aircraft. Now take a look at a Park 450 or 480 and you'll see that the shaft sticks out at the opposite end to the where the wires exit. These sort of motors are mounted outside of a firewall, whereas normally the Turnigy ones will be mounted behind the firewall in a wooden model, or inside a cage mount to use the shaft as it is. It's not possible with the SC to mount a motor behind the firewall (all that foam in the way) so you have two choices:

a) Use a cage mount and put the motor insde the cage.
b) Mount in a similar fashion as a Park 450/480 with the mounting plate allowing the shaft to stick through it, and use the other mount (within the accessory pack) which bolts onto the opposite end to where the shaft is, that being the spinning bit.

If you look closely at this picture of mine when I first did it, you can just see the shaft sticking out towards the rear, and the star shaped mount thing bolted onto the other end for the prop to connect to.

Mine is blue, the Turnigy ones are gold. Made in the same factory, just different distributors.

I had a look at the two motors in drivecalc. The one you posted is slightly more efficient (although based on theorectical figures) and has a wider (again theoretic) power range, and hence suitable prop sizes but a little less powerful. It seems to be best suited to a bigger but finer coarse prop than will fit the SC. Either motor should give you the performance you seek. Don't forget, stick your ESC in the cowl, saves adding dead weight. I just cable-tied mine to the motor stand-off bolts.

I hope this helps.

[Dillzio]

Ah, I think I'm getting it now, thanks for the info.

I was thinking that I could use that cross-bracket to attach to the back of the outrunner, and then attach the bracket to the firewall. Then the motor and the shaft is facing forwards and everything looks good.

I think I was overlooking that in an outrunner, the entire outer case of the motor rotates (all the blue section in your picture) rather than just the shaft. Is this correct, or am I way off track again?

Thanks

[Dillzio]

I think this might be a much cheaper alternative than using a cage mount, i'm not sure if you can adjust the thrust angles with this one though: www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=4689

[ginginho]

Nov 19, 2009 15:53:19 GMT 1 Dillzio said:

Ah, I think I'm getting it now, thanks for the info.

I was thinking that I could use that cross-bracket to attach to the back of the outrunner, and then attach the bracket to the firewall. Then the motor and the shaft is facing forwards and everything looks good.

I think I was overlooking that in an outrunner, the entire outer case of the motor rotates (all the blue section in your picture) rather than just the shaft. Is this correct, or am I way off track again?

Thanks

Yep that's it, it's all spinning, but because the shaft sticks out the other end on the Turnigy's, you effectively have two spinning parts either side of where it attaches to anything. That mount may work, as for thrust angle, you may be able to shim between the cross parts and the horizontal rails. It would be down to how long the screws are that connect these together as to whether you would get a sufficient angle without compromising the integrity of the structure. Either of the motors discussed would sit within the assembly with the shaft sticking through the hole in the middle of the X, whereas a Eflite type would be fitted to the end of one of the cross members.

Have you looked at alternative brands? To mount in the same way as a a Park 4XX there are such makes as E-max and the Keda Thumrun range mount in this manner. Something like an E-Max BL2215-25 would be a little less powerful but would work OK at a fraction of the cost of a E-flite.

[Dillzio]

Thanks for the info Ging.

I just looked up the Eflight 480 outrunner aka the Park 480 www.horizonhobby.com/Products/Default.aspx?ProdID=EFLM1505

and I was suprised to see that it's rated to 275W, which is about 20 amps on a 3S. The Turnigy 35-30 is only 15 amps, so the stock parkzone brushless actually seems to be more powerful. The odd thing though is that under comments for the 35-30 many people recommend it as an upgrade for the T-28, the T-28 comes with the Park 480 doesn't it?

What is the general relationship between amps and thrust output anyway? Both the 35-30 and the 28-30 are 15 amps, yet the 35-30 apparently has more power. Is one more powerful than the other because it's a heavier motor and hence has more torque when it's spinning, or is there more to it than that? One clue I've come across is that with the APC 8x6 prop for example, the 35-30 draws less amps than the 28-30.

I think I actually want a 1200-1400kv motor, I like the idea of being able to use an 8" prop. Thanks for the tip on the Emax motors, the EMAX BL2215/20 looks like a nice 1200kv motor, and they're only $20 on ebay. There are lots of outrunners on headsuprc.com that don't have the reverse shaft too, their 35-30s look like a good alternative to Turnigy.

Thanks for your help with selecting a good outrunner. Hopefully by the time I finally decide which one I want it won't be a choice i regret.

[ginginho]

I think you'll find a Park 480 will be pulling more like 27A for 275W. Remember there will be a voltage drop under load. Most use 10v for a 3S as basis for balllpark power calculations. I don't know what the T-28 comes with. The 35-30 isn't strictly a 15A motor, only that this is the upper limit listed as "Efficient" current. Any B/L motor will take any amount of current (dependant on prop size) however the balance is to get theright prop/motor combo for the power to be useable thrust rather than creating heat.
Someone has posted their findings on HK following testing which would be more believable:

Good performance motor for 500g - 1000g weight plane.
I checked Eagletree data logger & Hyperion Emeter.
Propeller Volt Amps rpm Epwr Speed Thrust ETA.
APC E 9x4.5 10.98V 15.50A 9652rpm 170.19W 66km/h 910.8g 74
APC E 9x6 10.86V 18.87A 8928rpm 204.93W 82km/h 914.6g 67
APC E 10x5 10.86V 19.18A 8928rpm 208.29W 68km/h 1128.2g 79
APC E 11x5.5 10.83V 18.45A 7763rpm 199.81W 65km/h 1153.2g 79
APC SF 11x4.7 10.56V 27.48A 7190rpm 289.91W 51km/h 1469.8g 65

So, depending on prop (and quality of lipo) you could potentionally put nearly 290W into the motor. How much of this is wasted power as heat is unknown, but it will account for some of it, quite a lot I'd suspect.

Download drivecalc and have a play with that, you'll get a good feel of various motors and what is suitable for the SC.

[flydiver]

eFlite - Weight : 77g / 2.72oz; RPM/Volt (Kv): 1020
Turnigy - Weight: 87 g (3.1 oz); KV : 1100 rpm/V

In general bigger motors are more powerful than smaller motors (make sense?)

Motors with higher KV will handle more amps than lower KV, IF EVERYTHING ELSE IS THE SAME.
But with higher KV you end up spinning the prop faster so must use a smaller prop-therefore possibly less thrust. Depending on your application one scenario may be more desirable than the other.
Extreme example - 2 different 20A motors, one with a 2200 KV and the other with a 900KV. The high KV would be great on a jet, the low KV on a 3D plane. Swap motors and both planes would be terrible but the output is the same.
As the KV goes up generally the pitch needs to come down but the plane still is faster as the prop is turning faster. My Funjet only is a 6x4 but it probably goes 60-70. The Cub with a 10x8 is probably pushed to ever go over 40.

Emax - Weight: 59 g/2.08oz; RPM/V: 1200
It's a much smaller motor with even higher KV so is not even in the same class. You are almost comparing apples to oranges. If if fits your needs though it's usable.
Max. efficiency: 82% - THAT is absolutely flagrant BS so ALL the other specs are suspect IMO. I'd almost be willing to bet a plane that the KV spec is low so would need even smaller props than indicated.

At least HeadsUp does his own prop testing so his results are believable.

Also, be skeptical of many of the postings in HC. A lot of those folks have a wattmeter but really don't know how to interpret the results. All it reports is Amps/Watts. If you keep propping a motor up you get more Amp/Watt draw, BUT, after a point it's not thrust, it's merely heat which is also Watts, just like your toaster. Ideally to truely test a motor you have a tach, an infrared thermometer, a thrust stand, and a wattmeter. Then you can get real data.

[Dillzio]

Thanks heaps for the info guys, I'm starting to feel like I can make an informed decision as to what motor I want now. This thread has turned into an excellent reference for anyone wanting to choose a brushless motor for the cub.

I've downloaded and tried out drivecalc, thanks for the tip Ging. It's a great little program for playing around with, but alas the database is still a bit lacking. Still though, by playing around with different combinations you can get a pretty good idea of how different factors interact.

I can see how that HC info can be pretty useless without indicating thrust or rpm. Apparently most outrunners work best when they're run at 2/3 their no-load speed. If you chuck on a prop that runs at 1/2 the no load speed it's going to be absolutely chewing through the power, probably cooking the motor with the excess heat, the battery life would be terrible and the thrust would be less than a smaller prop anyway. The wattmeter would indicate that you've got a very powerful prop but as you say, that power is going to heat, not thrust.

One more question I have. I've read that the impedance of a motor indicates how efficient it is. A motor with less impedance (less resistance) will allow electricity to flow through it at a greater rate than a motor with more impedance. This suggests that a motor with an impedance of 140 ohms would be more efficient that a motor with an impedance of 220 ohms, but is that really the case? Wouldn't a bigger/more powerful motor be expected to have a higher impedance anyway, and if so, does that mean that you can only compare impedances for motors of the same size?

[flydiver]

Impedance is largely a function of wire size (length and diameter) and quality. Ignoring quality, most motors are wound with multiple strands of small wire-smaller wire is cheaper and easier to use. You want to fill the stators.
Rewinders will use fatter wire to accomplish the same fill, impedance goes down and power handling goes up with the fatter wire and the same # of winds.

If you take the same motor, use even fatter wire, and fewer winds then you get even lower impedance, more power handling, BUT fewer winds makes the KV go UP. That's one of the reasons you'll see the higher handling power of higher KV but otherwise identical motors.

A larger motor is just bigger. It'll take a longer wire to go around the stator so the impedance will go up. But, as it is a bigger motor it'll generate more power. So you can't compare the little motor with lower impedance (shorter wire) to a large motor with a higher impedance.

A general rule of thumb is that you can't get much more out of a motor than 3w/gm of motor weight. You should be skeptical of outrageous claims to more.

There are a myriad of other factors making a good motor: magnet strength and placement, air gap between the magnet and stator, stator material, stator plate thickness, bell material, weight, and trueness, and bearing quality. Mostly, as price goes down, all those suffer. There are occasional gems that avoid this but not a lot.

Case in point. The little 'blue wonder' is a very good motor but the winding is poor.
A new motor just coming to market is Hoffman Magnetics. I'm not familiar with the whole line but I looked at 2 costing as much as eFlite and they are in the same performance deptartment as cheap TowerPro-a real poor showing.

You are right. If possible put a motor on your plane that operates ~3/4 no prop RPM at WOT and performs the way you want at no more than 3/4 throttle. This puts it at good efficiency and does not overtax your motor.

If you bench test your motor for 30 seconds and it's too hot to hold, it's over propped most likely. Since air cooling is compromised on the bench longer testing is not advisable.

[vegatron75]

Dill, I'd use HeadsUprc, The guy has done all the work for different props for you. Check out his page. Each motor is shown w/ a variety of props so all you do is pick.

My buddy had a small turnigy on his sc, the one from ebay that has the wooden mount, he bent the shafts all the time, and eventually the motor quit on him. It did make the sc scoot much faster than stock.

Same friend ordered this one, www.headsuprc.com/servlet/the-1425/2830-dsh-11-Outrunner-Brushless-Electric/Detail , I havent seen it fly but he says it's stronger than the turnigy.

I'm going to make a mount like the one pictured above, the stock base and extenders, and use either the headsup motor I linked to or the 35-30 from headsup.