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Discussion on Motors and Props

Updating data for a magazine article

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Colin Bishop17/09/2020 12:51:21
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This topic is intended to support my request for firther data on selecting motors and prop to revisit an earlier magazine article on the subject.. See:

**LINK**

The above topic should be used for data, this topic can be used for general discussions and points that might inform the updated article for the magazine.

Feel free!

Colin

Edited By Colin Bishop on 18/09/2020 11:19:25

Chris E18/09/2020 10:46:26
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Colin

Firstly let me say that I regard a wattmeter as essential so that you know what is going on as soon as you deviate from a few well trusted combinations.

Propellers

To be useful you need rather more propeller data that you have asked for. The make and model of propeller makes a large difference to its performance and loading. With model aircraft props the data is available to compare any 2 propellers and be very precise about what they will do.. With model boats it is alchemy.

Take the simple example of a 40mm 3 blade propeller. The Raboesch list above lists pitches of 41mm, Proteus Design (Propshop) are 45mm, Graupner Plastic are 21mm, etc. These differences are enormous and haven't started to include the effects of the different blade designs that are out there.

Even with basic 2Bl plastic props there are large differences between P and X types.

Without good propeller identification I would find the results of very little value.

Edited By Chris E on 18/09/2020 11:08:47

Moved from data topic. Colin

Edited By Colin Bishop on 18/09/2020 11:18:26

Chris E18/09/2020 11:03:13
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Colin

Brushless Motors

My experience of brushless motors in aircraft is that specifying them is simple. There is very little difference in the performance of different brands of motor that have the same specification although the more you pay the better the marginal performance. Indeed the same motor can often be purchased in many colours with many different labels and for many different prices. Better quality expensive motors do have advantages but they are beyond the scope of what I am saying here.

What matters is the kv of the motor and its ability to dissipate heat. Test data is available for brushless motors that tells you exactly how many watts of power they use if loaded to any particular rpm ,With model aircraft I know for any propeller exactly how many watts is required to turn it at any given rpm and how much thrust will be achieved. It is thus easy to match motor and prop.

With model boat props it is guesswork but see my previous post above. For model aircraft I work on around 3W per gramme of motor weight but that is with a nice big cooling fan. For model boat use I would go less - particularly if I didn't use a cooled motor mount.

Moved from Data topic. Colin

Edited By Colin Bishop on 18/09/2020 11:18:05

Colin Bishop18/09/2020 11:30:27
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Chris - thanks for your comments. I have moved them from the other topic which, as I said, should be used for data only otherwise things get confusing.

I don't know if you read the original article but people did find it useful I think. It was mainly aimed at the less to medium experienced modeller rather than someone like yourself willing to do the maths.

I have amended the data for props to include more detail as you suggest but I'm afraid that will lilkely only apply to new information unless previous contributors update their original data.

I agree 100% that a wattmeter is invaluable. These were not so readily available when the article was originally published which relied on ammeters and voltmeters for measuring purposes. Obviously this will all be updated.

Again, thanks for your useful input.

Colin

Mark Beard18/09/2020 13:12:19
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I fully agree with Chris E’s comments (which I received as email notifications and for some reason are yet to appear here), that specific data are needed for motor and propeller combinations to enable good mathematical modelling of boat performance. The kv parameter and motor count are key for motors, and drive voltage for the battery. Gear or belt speed reduction ratio is key for the drive system. Diameter, blade count, blade pitch and prop count are key for the prop(s). The most difficult to enumerate is the hull shape, as there are enormous performance differences between, say a full displacement hull, such as a tug, and a fast planing hull as in an MTB, with everything in between. The aim should be a mathematical model which accurately predicts scale performance for a model boat, as easily as is achieved for model aircraft. Notwithstanding the fluid dynamic difficulties of hull characteristics, our field should not be inferior!

Edited By Colin Bishop on 18/09/2020 14:03:56

Chris E18/09/2020 13:58:38
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Mark

Nice as it would be I wasn't really proposing developing an all inclusive modelling programme for model boats but rather observing that some real data for props would make drive examples that were posted much more useful in a wider sense. The motor data is already available.

Edited By Colin Bishop on 18/09/2020 14:07:36

Colin Bishop18/09/2020 14:18:17
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Mark,

As you will see from my earlier post on this topic, we are wanting to keep things really simple for the magazine article using an empirical approach. Chris' suggestion of more precise information on props being used ties in with this but I'm afraid that mathematical modelling techniques will fly right over the head of the average kit builder, for example, who simply wants a proven setup to install in their boat.

It is unfortunately the case that many people struggle with even basic concepts like watts, volts and amps so fluid dynamics characteristics would not be something they would readily engage with!

Colin

Gareth Jones18/09/2020 16:58:19
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Hi Mark,

Glad to see you are still around and still got the anorak.

Colin,

I will post some data when I work out the best way of displaying a table. I have used a Springer tug as a test bed for a number of brushless motor/prop/battery combinations but it would probably be easiest to use as a comparison if they were all listed side by side.

Gareth

Colin Bishop18/09/2020 17:14:51
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Gareth, thanks, info will be welcome but it might not be printed that way as it will depend on how the designer needs to display it so don't spend a lot of time trying to format it specially.

Did you come up with one or two combinations which are the best ones to use? If so use those as space in the article will be limited to some extent.

If you were using the same model all the time I assume you have a favourite?

Colin

Chris E18/09/2020 17:32:02
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Lots of pictures and very litte information seems to be the rule these days.

What I would like to see is a simple comparison between different props but all the same size. Take any size of prop (lets say 40mm) and connect it to a motor with plenty of power then give us the watts consumed and the thrust delivered for a range of props - 2 blade plastic "S" & "X" and 3 blade plastic and, 2, 3, 4 & 5 blade brass (all 4 being one make & model).

I have never seen that done but it would be so useful and increase the value of any examples enormously as we could see that if the example worked well with say a 40mm 3 blade brass it would/wouldn't work well with a much cheaper "X" plastic .

I realize that this makes a whole load of assumptions but I am convinced it would be useful and might debunk some of the odd statements that are made here & elsewhere.

 

 

Edited By Chris E on 18/09/2020 17:53:34

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