A tiny 4s multi?

Over the last couple of months I have been slowly collecting bits that were of interest - and were also extremely cheap!
As I have no practical space left to store any more 'normal' planes whatever next has to be small - very small.
This is what I accumulated so far.

Five tiny 12V brushed motors.
Four 3x3 props (CW & CCW)
Four 1200mAh Li-Ion batteries
Total cost so far £9.67 including postage! :woohoo:

Obviously brushed motors are pretty feeble but what I have in mind is a something along the lines of this simple 24" brushed twin (it uses a 500mAh 2s) but with four motors.

With the batteries in a 'tube' fuselage loaded from the front.
I already have a micro 4 channel Rx and two 2.5g servo.
The most expensive 'new' item will be a 10A brushed ESC.

If it does all work it I wonder if the bits could go into something scale - Armstrong Whitworth Atlanta perhaps?

I built a simple test rig to find how much (or little!) thrust was produced.


The results are disappointing.
0.39 oz thrust drawing 0.54A at 15.3V or 8.4W.
Which gives a specific thrust of 22W/oz

By comparison the twin above.
2.29 oz thrust drawing 3.5A at 7.0V or 24W
Which gives 10.5W/oz
Or just over twice as efficient despite using smaller diameter props! :woohoo:
When this is coupled with the anticipated all up weight of the multi at 7 oz compared to the twins 3.8 oz to quote from the chorus of song "Reviewing the situation" - I think I'd better think it out again!

ps
Nevertheless I do rather like the Armstrong Whitworth Atalanta!

Looking at your thrust-test rig, I suspect that your measurements are faulty.
Why - the circular base of the rig is almost the same diameter as the little prop, thus will be masking/deflecting what little there is.
If you had 'free air' both front and behind the prop, you may find you get a higher reading. Think of the ring vortex effect that applies to a hovering heli in still air - instead of providing lift, the same bit of air goes round and round, and the heli descends uncontrollably at high power.
Then again, I'm probably wrong, as usual! [badass]

Eric
It did occur to me that the rig was a short. I suspect 2D under the prop would be required to avoid noticeable airflow restriction however the readings are likely not far out it really does produce very little thrust even when I was holding the motor directly in my fingers.
The next try was with a 5x3 prop.
This gave a dramatic improvement.
0.81 oz thrust drawing 0.58 A at 15.2 V giving 8.9 W
That is a specific thrust of 10.9 W/oz.

The only problem is that with four 5" props the plane is going to get quite big - an Atalanta would be 42" span!

Simon, I don't see anywhere the size of the original test prop, was it smaller diameter than the 5x3? If so, couldn't you up the battery count to give more rpm?

Can you not find a dead one of these and just use the innards?

Klippy
The first prop I used was a 3x3.
Robmoff
What I am trying to do is make a very small 'multi' ideally no more than 20" span so the props have to be really small, say no more than 2.5" diameter as indeed they are in my brushed 'twin'.

The problem is those 12V motors. I suspect it would need at least 18V before the revs were high enough for such a small prop. Such a high voltage would make a miniature ESC expensive and hard to find.
I am coming round to a plan "B" and use the motors and props from one of these .
And stick to a 2s LiPo!

What about coreless mini motors?

Going back to your test rig Q, the distance on front and behind the prop is not really relevant as it's the lift of the airfoil section of the prop that produces the 'thrust, not the displacement of the air. So, when you apply the same analysis to the problem with the whole affair moving forwards, as in flight, the static 'thrust' doesn't bare much relevance to the thrust you are going to get in the case when the model is flying.

I know the above doesn't help you too much with the problem you have , but nevertheless, it's how it is!

Agreed.
But surely it you redirect a major part of the airflow side ways by a surface actually on the scales the reaction it causes will reduce the effective thrust reading.

Quorneng wrote: Agreed.
But surely it you redirect a major part of the airflow side ways by a surface actually on the scales the reaction it causes will reduce the effective thrust reading.

Agreed, too, Q. So, in the end, what relevance will you be giving to the test results?

Ha!
Well it rather confirmed what my fingers initially felt - not enough thrust!
With a larger prop, even a 6x3, it would likely work but that would not met the primary objective of building a "micro multi".
So its definitely plan "B" and use 4 direct drive 'nano' quad motors and props.
Depending on how well they work I might go straight to a 'stand off' scale AW Atalanta!

Quorneng wrote: Robmoff
What I am trying to do is make a very small 'multi' ideally no more than 20" span so the props have to be really small, say no more than 2.5" diameter as indeed they are in my brushed 'twin'.

The problem is those 12V motors. I suspect it would need at least 18V before the revs were high enough for such a small prop. Such a high voltage would make a miniature ESC expensive and hard to find.
I am coming round to a plan "B" and use the motors and props from one of these .
And stick to a 2s LiPo!

So a 26" span B17 donor plane running 4 motors off a 1S lipo and full 4channel control weighing in at 78 grams won't cut it for you? If I ever wreck mine would you like to buy the electronics; or should I just build the Short Sunderland or VC10 I have drawn up? It's the unsuitable motors and massive battery pack that is causing your problems so just save them for a more suitable project. IMVHO

one tenor wrote: What about coreless mini motors?

Like these www.google.co.uk/webhp?sourceid=chrome-i...oreless%20dc%20motor Some have gears as well
Also these Many cheaper here www.google.co.uk/webhp?sourceid=chrome-i...arbox+from+Bang+Good

one tenor
I am pretty certain that is the type of motor I was looking at and now have on order.
I suspect their internal electronics are limited to 1s so the Rx and servos will have to be also.