Is this motor overkill?

[Dave Grennan]

Hi Guys,

Here's a link to a pic of my latest build. Its a scaled down (60%) version of the flitetest spitfire with some mods.

**LINK**

All in weight with the motor and electronics shown is 300g. Wingspan is 650mm.

However I think the motor might be overkill.

Its an NTM prop drive 28-26 1200kv with a 6x4 prop attached. I did some bench tests. It draws 2.93A from a 2S lipo. Given that the draw is quite low, I guess I could use a 6A turnigy plush ESC I have around. Instead of the 30A one shown in the pic.

So what do you think? Probably better with a 22XX sized motor? However a little voice inside me says the 28 size would go like a demon, so I'm tempted to give it a spin.

Opinions appreciated.

Regards,

Dave.

[adrian lawrence 1]

Hi Dave, from the figures you have quoted (given that the nominal voltage of a 2s pack is 7.4v) the wattage given by your combination is 21.6 watts which is extremely low for a model of this weight. On a 2212 100kv motor I use a 10x4.5 inch prop, and would expect to be using a bigger prop on the NTM motor you currently have fitted. Hope this is of help, cheers Adrian

P.S. The model looks nice, my friend has the original sized one and it flies extremely well.

 

 

Edited By adrian lawrence 1 on 13/11/2015 00:41:50

[Dave Hopkin]

300g is 0.6lb, as Adrian say 2.9A at 7.4V is 21.9W so you have around 35W per Lb - I would have expected something around the 150 W/lb mark myself - as Adrian says extremely low, If it were me I would be looking at going to a 3S with a bigger prop on that motor

[Dave Bran]

Adrian, I assume that 100kv you state is a typo and actually 1000kv. I agree if that so.

Dave,

6x4 is a prop size more usually found on 2000kv plus motors and I feel is totally unsuitable for that motor, esp on only 2S (less cells, more prop)

HOWEVER, effectively you are installing a motor far too large and heavy, then throttling it back by an over small prop.

For 650mm span and 300g I would be installing a 2208, probably my long used and favourite brand, Suppo (sold by a range of people with their own labels added) with probably an 8 or more likely 12 wind. 18A ESC but checked before flown.

If you want it to behave like a hooligan pylon racer then 2212-6 with a 6x4 APC Speed from a 20A ESC. However, I'd be using 3S, NOT 2S.

28xx size motors on that small an airframe will only give you pain.

[Dave Bran]

The following Video is a 300g plus plane on a 2212-6, 6x4, 3S....................just to give some idea..............

https://www.youtube.com/watch?v=2k6HHCc4RhY

[Dave Grennan]

Hi Guys,

Thanks a lot for the feedback. From reading your replies and thinking about it a bit, I think I finally get it.

According to the watt/lb rule I need between 80 - 120 watts for a small warbird. So lets call that 100 watts/lb to put it in the middle of that category. Now my plane is 0.6lb so I need 60watts to meet that criteria.

With 2S 7.4v I would be drawing 60/7.4 = 8.1A

With 3S 11.1v I would be drawing 60/11.1 = 5.4A

Therefore I need to size the prop so it draws close to this current on a test meter?

Dave Bran: I found one of those suppo 2208 motors local and ordered it.

I need to look closely at the battery, I think most 3S batteries will add too much mass to a plane of this size, At least with that part I can out a few different ideas and see what works.

Thanks again.

Dave.

[Frank Skilbeck]

Posted by Dave Grennan on 13/11/2015 13:43:20:

I need to look closely at the battery, I think most 3S batteries will add too much mass to a plane of this size, At least with that part I can out a few different ideas and see what works.

Thanks again.

Dave.

Dave, but on your estimates the 2S will need to pull 60% more amps, so for the same flight time the mah will need to be 60% larger, 2S - 800 mah batteryvs 3S - 500mah

[Dave Hopkin]

Generally better to up the volts rather than load the motor up overly high