Prop selection for glow to electric conversion

[Alan Davidson 1]

I have converted my Derek Woodward Amelia to electric and chose a motor esc combination based on model size and a prop 10x4 9x6 basis it flew ell on with an ST G20-23 glow (did I say I was old). When I ran the combo (Graupner 7738 30A motor with 40A Overlander esc the max throttle reading climbed through 70A. Didn’t realise how fast I could cut the throttle!

The 9x6 prop still exceeded 55A on full power. As a total newby to electric can anyone help please or should I wait for November when none of us maybe flying

Al

[Dad_flyer]

It sounds as though the motor is trying to spin the prop very quickly. What kv rating is the motor, and how many cells are you using?

[PatMc]

I think it's this 1500 kv motor 7738 is the Graupner part number.

It seems to be rated at a max of 30A but it's not stated if this is continuous running. OTOH max output at nominal voltage (7.4v) is 148W, which suggests that the max continuous current is actually 20A .
I think an 8x4 on 3s would probably take it to around 20A giving about 220W which is about the same power as a ST20-23 with silencer, albeit on a smaller prop at higher revs.

[Dad_flyer]

That would make sense. The specs are for a 10x6 on 2S (7.4V), giving 1kg thrust. That is about right for 25-30A.

[Former Member]

[This posting has been removed]

[Levanter]

Posted by Percy Verance on 29/06/2019 07:39:43:

Hi Alan

None of us flying in November? I'm out flying all through the Winter if the weather plays ball.....

Hi Percy

I think the comment relates to your point where you said you might be boating from November

Providing of course you are not iced over!

Levanter

[paul coleman 1]

I have gone all electric now,as a rule of thumb,is it the more cells the smaller the prop ,the less cells the bigger the prop,or do i have it wrong haha.

[Former Member]

[This posting has been removed]

[Alan Davidson 1]

Thanks for all the help folks.

The details are

flying weight 1.4kg

46” span, 8.5” cord

the motor is a graupner compact 345z #7738

1500Kv, 7.2 - 11.1 v, 20 - 35A, spinning an 11x6 on 2s and 10x6 on 3s

for the 10x6 the blurb quotes 27A and 11x6 - 32 A. and states the motor is compatible with a model between.75 and 1.5 Kg auw

As I said in my original post, even using the higher volts available from the 3s lipo, the current draw is well outside these guidelines. Hence my uneducated confusion.

The replies trend toward trying a lot smaller prop and try and ascertain if the power train is a: incompatible or as I think b: the power train is incompatible given the prop size / efficiency..

Where did I put my glow fuel....🤔🤪

[Trevor]

I don’t know if it helps but, to understand the differences in behaviour of electric and I.c. motors, particularly when it comes to propellers, you might like to think of it this way:

Power out of a motor is torque x rpm.

An i.c. engine approximates to a constant torque device, so to get more power out of it you need more rpm, hence reduce the prop size.

However, an electric motor (for a given battery voltage) is essentially a constant rpm device, so to get more power out of it you need more torque, hence the need for a larger prop.

This effect arises from the way that input power (volts times amps) translates to output power (torque x rpm) in an electric motor. Rather counter-intuitively, voltage determines rpm (which is why the kv figure for motors is so significant), and the current (amps) relates to torque. So, when you fit a larger prop, the motor still tries to turn it at the same rpm but needs to generate more torque to do so, and this results in the higher current draw.

Does that help at all - or just confuse the issue further?!

Trevor

[Denis Watkins]

More confusion Alan, but I fly a 48" wingspan foamboard midwing own 3lb auw concoction

Fly's on unknown make 12v motors, 3s 45A ESC on an 8x4 ???? Prop

Will measure A when I get it out again

[Dad_flyer]

My reading of the spec was 10x6 on 2s, rather than 3s, and 27A sounds likely, and 32A for 11x6, also on 2s. This will be about 10,000rpm.

On 3s, the motor will be trying to turn at 15,000 rpm. That takes a lot of power and therefore draws a lot of current.

For 3lbs, the 2s specs will fly, but not 3D.

 

You have three options from here, all of which should end up at about 200W:

Keep the same motor and 3s, and use a smaller prop, probably 8x4

Keep the motor and prop, and run on 2s.

Keep the 3s and prop, and use a 900kv motor. I use that for a cartoon scale Storch at about 3lbs with an 11x4.5 prop

 

Edited By Dad_flyer on 29/06/2019 21:12:17

[Alan Davidson 1]

Thanks guys

will try the smaller prop range for experiment and gently see what results, and then the 2s with the 10x6 prop with a view to using this configuration

Many, many thanks for all the wonderful assistance. I will post again if it all goes well. I will also post pictures of the smoking wreck if it doesn’t.

Thanks again

Al

[PatMc]

If the 10x6 draws 27A on 7.4v then that limits the power to 200W. A smaller prop that draws the same current on 11,1V will increase the power by 50%. Even though the smaller prop is less efficient at converting this power into thrust it will not be 50% less efficient so you could reasonably expect a significantly better performance by using 3s rather than 2s.

OTOH the motor you have has a weedy 3.17mm shaft which means that it will be prone to bending or breaking easily in any sort of prop strike [such as a nose over on landing].
Also IMO it's too light to convert the battery power to thrust at the power level that would do a 1.4 Kg model justice.

The motor needs to be about 30% - 40% heavier, have a 4mm (min) or 5mm (preferred) shaft & preferably lower KV (900kv - 1100kv) for instance this Turnigy from HK. There are plenty of other similar specified & reasonably priced motors out there.

[Trevor Crook]

Yes, my rule of thumb is, for a 3s setup to use a motor of around 900kV. That normally allows fitment of 10-11 inch props, which will, as said above, be more effecient than smaller ones.