Replacing a 600 motor with a brushless outrunner

[John Miller 4]

I am contemplating building a twin engine model which will weigh about 6lb to 6 1/2 lb The design calls for 600 size motors. I would wish to replace this with a modern brushless outrunner. I have been unable to find out what the power output of a 600 size motor is. Can anyone help.

Edited By John Miller 4 on 06/05/2013 15:41:42

[Swissflyer]

John,

Please post wingspan, total wing area and sort of performance you want (fast sport aerobatics, gentle scale etc).

KR Mark

[Chris Bott - Moderator]

I'm struggling to remember John but something in the back of my mind is saying up to about 20A from an 8 Cell NiCad. The NiCad would have dropped to about 8V in use, so maybe 160W each. I doubt they would have reached 200W.

I think for a twin of that sort of weight, I'd be looking for 300W per side. That will have far more punch than the Speed 600's.
Somehow the sum of two motors/engines on a twin always seems to give more "umph" than a single motor of equivalent power. Not sure why, or if there is any theory to back that up.

So a total of 600W ought to be plenty.

[Swissflyer]

Well Chris, yes, sort of. The key point is that the model is going to have a massive weight reduction so it is usually better to recalculate from square one.

Yes, we used to fly 600 motors on 8 x 2’400mAh Cells with a weight of 540g per pack. The Speed 600 motors weighed about 195g each.

So the total weight of the propulsion system for a twin could have been 540+540+195+195 = 1’470g = 3.2lbs

A modern brushless motor equivalent to a Speed 600 will weigh around 80g with a 3S x 2’200mAh LiPo weighing 120 g. That means our modern power system could weigh around 400g = 0.88 lbs

So the model may lose as much as 2.3lbs out of 6.5lbs, about 1/3^rd.

Yes, putting 300W per side does seem a good start for the original mass but with the new mass you may be looking at 600W on a new AUW of closer to 4lbs. So what seems like 100W per lb may become 150W / lb.

I don’t know John’s project but many models from that era were designed to stooge around doing the best they could to carry 1-2 lbs of batteries and had airframes designed for much lower power levels.

I hope it is OK for me to take you through this, but we often fail to realize the huge power boost we have benefited from in recent years and often over power our models.

Below is a calculation I did for a Speed 600 to brushless conversion a few years back.

KR Mark

[crispin church]

and dont forget prop sizes you 2 motors that take 12" props and got to fit them on end of the wings to fit them in and 2 esc can be a nightmare to get going right

[John Miller 4]

Okay guys This is the project Traplet plan BN Islander 72" span 715sq in area. So a nice gentle scale flight is what I will be after. I had originally thought 300watts a side which Chris confirmed but Mark has now thrown another variable into the mix that I had not thought about regarding the reduction in weight. If the original 600 motors were knocking out 160watts or so, a couple of 200watt motors along with the reduction in weight may do the trick. Waiting on the drawings, will peruse them and make a decision. Thanks for your help so far, I may well be back.

[Trevor Crook]

John, all the above advice is excellent. All I would add is if you stick with your 300W per motor scheme, and find it is over-powered (probable!) you can always prop the motors to pull less amps, which will keep everything running nice and cool.

[Swissflyer]

Hi John,

Nice model choice, I like twins.

I think with 200W per side you will be in the right ball park.

If you go for 3S LiPo’s, a well chosen 900kV motor on an APC 11x7 is going to give you a pitch speed close to 50mph & 35-40oz of thrust per prop so about 1:1 thrust ratio if the model comes out at 5lbs.

KR Mark

PS Here is a DriveCalc estimate for a Speed 600

[crispin church]

the reduction in weight mite not be such a problem COG wise as its all on COG any way

you just end up with a nice light one

[Olly P]

I would go with 300 per side - you can always throttle back!