Difference between these two motors

[Stephen Belshaw]

I'm thinking of buying this 1.7M hotliner from TopmodelCZ Link

Topmodel recommend this motor which is rated at 1130Kv max power 570W. However at €90 I think it's expensive!

I have this 4-Max motor recently acquired from George, this is rated at 1105Kv and 450W max power and which was half the price of the above motor, obviously I would prefer to use what I already have.

I asked Topmodel if the 4-Max would suit the airframe and they replied that "whilst it would work it the flight performance would not be very dynamic due to the low power". I'm slightly puzzled because on paper (at least to my untrained eyes) they look similar specs.

However having run the 4-Max in a not dissimilar airframe I was underwhelmed with the performance, running on a 3S 2200 LiPo and a 9x6 prop I got around 300W on the watt meter.

Running the figures for the Topmodel airframe and the 4-Max motor through e-calc the best I can achieve is 320W with a 12x8 prop and a 3s 3000mAh LiPo, Topmodel recommend a 13x8 prop but e-calc doesn't like that combination. Either way this is hugely below the theoretical maximum of 450W.

What am I missing here?

[Frank Skilbeck]

Reading the 4-max motor specs it's rated for 30 amps, which is why Ecalc doesn't recommend the 13 x 8, the recommended Topmodel motor is rated at 57amps. To get more power out of the 4max motor you'd need to up the volts (4s battery) and reduce the prop size to keep the amps below 30 amps.

[Stephen Belshaw]

Thanks Frank, when I put the figures in for the Topmodel recommended motor/ESC/prop it comes out at a staggering 700W but e-calc saying everything is overloaded:

This would give power to weight ratio of around 200W per pound!

[Don Fry]

Don't know anything about hot liners, but don't they go straight up, and fast. And don't they overload everything, but for a few seconds only.

200 watts a pound doesn't sound excessive for that ability.

Edited By Don Fry on 26/02/2020 09:48:34

[Dickw]

Many glider type hotliners are indeed intended to have motor runs of a few seconds to gain height and speed followed by a period of "motor off" while you throw it about. This on/off type operation allows the motor to cool a bit between runs allowing it to be operated at much higher ratings than normal.

To take one excessive example - I use motors rated at 1400 watts but at over 5kW with a cycle of 2 secs on and 18 secs off. They have been fine over 100s of flights.

Bigger props are generally best for acceleration and climb rate, and 200w per pound shoud be fine for a vertical climb,

Dick

[Shaun Walsh]

or there is this.

**LINK**

[Stephen Belshaw]

Posted by Shaun Walsh on 26/02/2020 13:36:47:

or there is this.

**LINK**

Yes, very good value for money, I have one in another airframe and rather than buy another I was trying to use my newish but redundant 4-Max motor.

With a 4S battery and 9x5 prop I can get close to max watts from the motor, some vertical climb and a half decent performance, albeit probably not rocket ship. A sort of hot warmliner, which will do for me!

Edited By Stephen Belshaw on 26/02/2020 14:11:08