WMAC Group build - Impressivo

David Tappin wrote: As I'm in Germany I'm thinking big black crosses would be quite appropriate.

David

Just remember that some German crosses are less appropriate than others!!

Mike

Mike, the outrunner is supposedly good (HK specs)for 650 watts so 550 watts should be okay. The ESC though really need to be generous and get adequate cooling somehow. I would suggest that the outrunner would more capable of self cooling than the inrunner.

Tests were completed successfully(ish) today. I'd converted an old MPX twinjet by cutting off the two old brushed speed 400 motors and adding a mount for the new motor. I used some of the old wiring partly because it was already in place and partly because I don't think I had enough silicone wire for the job.

First flight was with the inrunner which clocked 23,500 rpm at 50A on a 5.5x4.5 prop. Performance was good but the model was all over the place - too twitchy - because it used to fly a lot slower with only about 150W compared to over 500W. I landed and introduced some expo and it flew a lot better. Then the ESC cut and I was forced to land. Inspection revealed that the old wiring (PVC covered) had melted and created a short circuit. I fixed that with some heat resistant tape and put the outrunner in. This showed 65A and 24,500 rpm on the same 5.5x4.5 prop. It was not clear if this extra power actually resulted in any better performance because I was having to run at part throttle most of the time anyway. A second flight with the outrunner on a 6x4 prop ended with the wires melting together again. Outrunner also sounded a little rough when you turned the prop by hand indicating that a bearing might have gone.

So I put the inrunner back in and insulated the wires again for a couple more flights using the 6x4 prop. Current was up to 58A and rpm down to 23,000. The extra thrust made the launch and climb out a little better and with throttle management I'm not sure I could tell the difference in flight.

My preference is still the inrunner after these tests. Kevin wondered if a less powerful motor would be better if you hardly ever needed/wanted to use full power. My thoughts are that the power is nice to have and that putting on a smaller prop is the way forward. I will try the following next weekend if weather permits and if hobbystores in Southampton have them in stock:
4.1x4.1
4.5x4.1
4.7x4.2
4.75x4.75

These will give less thrust but hopefully still enough. They will also give more rpm and less amps.

I suppose the next question is when do we want to place an order...before Christmas or after?

Mike

PhilF51 wrote: Mike, the outrunner is supposedly good (HK specs)for 650 watts so 550 watts should be okay. The ESC though really need to be generous and get adequate cooling somehow. I would suggest that the outrunner would more capable of self cooling than the inrunner.

On a freshly charged 3s 2200 battery I was seeing 65A (measured), so assuming 11V from the battery this is 715W. So the outrunner is not running within spec. The inrunner wasn't inside spec either on the 6x4 but it was (just) on the 5.5x4.5. If I get one of the smaller props it will be fine.

As for self-cooling, I'd be happy to hear the logic but I always assumed inrunners cooled better. This is because the copper is in direct contact with the can, the the heat can conduct out easily. The inrunner has the copper in the middle and some of the heat has to pass through the magnets before it gets out.

Mike

Re - required power levels

My rational for querying the power requirements are as follows:-

- What do we want to achieve ?
I guess the favourite will be to get several in the air at once and whiz around the sky like maniacs !
- However, after viewing Mikes test aircraft today, it appears impractical to fly these consistently at full power because:-
- They are incredibly fast.
- At constant 55 amps, a 2200mA battery will deplete in about 2 mins
- Maintaining cooling of motor & ESC could be challenging
- For me, avoiding the other "Any colour you like as long as its black" deltas will require all my concentration without having to constantly manage power levels. - If we all fly at the same speed, it might help to minimise mid airs.

So Mikes proposal to to prop down for group flying is more appealing to me.

I am happy to order them before xmas, but obviously will go with the majority vote
I say make them as quick as poss :woohoo: