Motor for Bristol M1C

[Tony Read 2]

I bought a very nice Balsa USA Bristol M1C from Reno Racer the other day (what a beautiful part of the country he lives in). It had been IC powered and he had later converted it to electric by fitting a 5055 motor (which he thought the KV was a bit low if I remember rightly, maybe 400KV).

The model is 60" wingspan and is just about the same size and weight as a Flair Magnatilla.

I've been looking at the various options for 4 cell lipos and here are my thoughts:

5050 - 600KV

4260 - 600KV

4250 - 800KV

3548 - 900KV

Obviously the smaller the motor the more nose weight I will have to add. I would prefer longer flight times rather than out and out speed, although could always throttle back of course.

Any thoughts on the above, and have I overlooked a motor option?

[eflightray]

What size prop and what size Lipo pack are you considering ?

Start with the prop, then choose a motor to drive it on the required battery.

[Tony Read 2]

I've got quite a few 4 cell batteries, so that's what I intend to use.

I must admit that I thought that I would select the prop based on the amps/watts required.

[PatMc]

I use a early version Turnigy 3548/900, 4s 4000 lipo, a 60A ESC & a 12x6 APC prop in my Magnatilla. Model is overweight at just aver 6 lbs but flies well drawing 31A for 460 Watts. Slightly better performance than when powered by an old type ST 40 driving an 11x6.
Average current is 21A in a mixed aerobatic, T&G etc flight of around 10 minutes, less aeros extends the flight time to around 15 mins without reaching LVC. I'm about to try it with a 4s 2650 as I think it's a tad nose heavy & I'm curious to see if this will reduce the average current draw.

[will -0]

Posted by Tony Read 2 on 29/09/2014 12:43:18:

I bought a very nice Balsa USA Bristol M1C from Reno Racer the other day (what a beautiful part of the country he lives in). It had been IC powered and he had later converted it to electric by fitting a 5055 motor (which he thought the KV was a bit low if I remember rightly, maybe 400KV).

Any thoughts on the above, and have I overlooked a motor option?

Keep the 4 cell battery..............................to power the servos

I reckon a Laser 70 would be perfect.

[Tony Read]

Hi Pat

It was your set-up that I spotted on the electric data base. I like the flight times you are getting from your power system. Would there be any disadvantage of going for one of the larger motors (I need the nose weight)? I'm thinking that the amp draw might go up and flight times go down? I can live with adding a bit of lead if it means a couple of more minutes in the air (the CG is close to where the battery fits so increasing the size of the battery might not make too much difference, but I can play around with that).

Thanks for the suggestion Will - I was asking for that one I suppose.

[Tony Read]

One other thing. The Bristol has a large cowl, and I'm probably going to fit the large spinner that comes from Traplet. Would a 12x6 be large enough if a good proportion is hidden by the spinner, or is most of the thrust derived from the outer half of the prop anyway?

[eflightray]

If it helps, here's a thread on the Bristol that talks about quite a few motors/prop/cell combinations --

**LINK**

The reason I mentioned 'start with the prop', is that's the prop is the thing that will fly the model. The amps/watts is just there to drive it.

Picking what you feel is the right diameter and pitch range for the expected flight style is a good starting point.

Pick the 'wrong' motor that then restricts what props you can use, (e.g. too small for the cowl size), and the plane wont fly how you want.

Ray.

[eflightray]

Another view, I don't necessarily choose a motor for the number of cells either.

I have a 67" span Dauntless that I wanted a fairly large diameter prop on, (big radial cowl, plus I didn't need speed).

I chose a 17" x 8" prop, and then selected a motor that was actually 'classed' as a 5-6s Lipo. I run it on 3s, I have low amps, bags of thrust, and a plane that flies just how I want it.

Ray.

[PatMc]

The Magnatilla has a 7" dia cowl & the 12" prop provides plenty of thrust. (TBH when it was ic powered the 11" prop was fine)
A 60" M1C should have a slightly smaller dia cowl plus it has a the spinner which I think should improve the prop slipstream.

A larger/heavier motor need not take more current, just needs the prop size to provide the right balance of thrust & current consumption.

[Tony Read 2]

Thanks guys,

Looks like to maintain a reasonable ground clearance the prop should be a maximum of 14", so something between 12-14" seems to be ideal. If I'm right this would appear to rule out the 5055 which, from Overlanders website would drive a 14" on a 3 cell with a 4 cell requiring a larger prop???

Not all motors list all the 'stats' so selecting the right one is proving a bit of a curfuffle for me, therefore any advice (apart from fitting an IC engine!) would be most welcome.

[Tony Read 2]

Would this be a good contender?

Tornado Thumper V2 MK2 4250/07 710KV
700 watt motor
suitable for 10 - 15 Nimh cells
3 - 5 Lipoly cells
Prop size from 12in (larger prop with less cells)
Shaft Diameter 5mm Shaft
motor weight 195gm
Speed Controller Recommended 60A ESC

Maybe driving a 13" prop?

[Tony Read 2]

I spoke to george (I think that's his name) at 4-Max, who was very helpful. His suggested set up was a 5055 580KV running on 4 cells with a 14x7 prop. Any thoughts?