Let's talk about propellers

[Simon Chaddock]

Interesting!

 

I think what these programmes give you is a good "ball park" figure so you can size everything with a margin of safety.

 

As Ed says ultimately there is no substitute for actually measuring the full load amps (to check you are not 'cooking' the motor or ESC, volts under load (to avoid damaging the battery), the static RPM and thrust will also give you some idea of what might happen in the air but trial and error testing is still the order of the day.

[Tim Mackey]

I gave up using them years ago - I much prefer to use my own wattmeter / eagle tree logger.  Providing the supplier gives an indication of the suggested propellor ( most do with one noteable inexcusabel exception - and for that reason I rarely recommend them these days - rant over )  - then most peeps can usually get in the ball park - having a few props either side of the recommendation then allows one to play around with the wattmeter to find the one thats best for your model.

Its nigh on impossible to state an ideal prop when simply "selling" a motor as the seller at that stage has no indication of the model involved!

Being slightly on the anorak side, I actually enjoy this trial and error testing method

Edited By Timbo - Administrator on 02/04/2010 18:45:40

[Ed Anderson]

I use the tools for planning, so I know what to look for when I am shopping.

 

I can say that I want 100 watts/pound as a target but that does not tell me what size prop, what kV motor I will need, or what voltage battery pack I need to hit the kind of performance I want.

 

I can set targets for battery size, thrust targets, etc.   Modeling those I can decide that I want to look for a motor in this kV range that will handle this amp/watt range and turn this size prop. 

 

Now I can go shopping.

 

In some respects this is like what I did in high school when I helped my friend build racing cars.  You can put in a 400 hp V8, but are you going to run a close ratio or wide ratio gear box.   What differential ratio will you run?  What size tires.   If you are building high performance cars, this is what model.  

 

For road racing you may want one set-up with that engine.  For drag racing, a different set-up.

 

For e-glider I might set-up one way. For pylon racing I would set up differently.   Both might be 100 watts/pound.

 

If you are going to design glow or electric power systems, you can model the options till you get the type of performance model you want.  Then you know what to buy.

[Stephen Grigg]

Hi Ed,on my last electric, I was having problems with the motor cutting out at top throttle.I was using items I bought over a year ago and the detaols had been filed!!!!!!!!!.I cured the problem by reducing the prop size and the model is a perfect set up.It was then I checked the set uyp on my watt meter and was pulling way above watts than before/Interesting how some people test there set up by how warm the battery is after a flight

[Danny Fenton]

I must agree with Timbo here, the calculators are not much use for working out electric flight set-ups IMO. none seem to take into account the unloading in the air, and I have just found them totally innacurate. So like Timbo, calculate the rpm a given motor will give and use a wattmeter to see what is going on. I start with a scale size prop and work from there. Remember a large slow turning prop will be more efficient.

 

Just my 2p......

[Ed Anderson]

Posted by Danny Fenton on 03/04/2010 10:50:08:

I must agree with Timbo here, the calculators are not much use for working out electric flight set-ups IMO. none seem to take into account the unloading in the air, and I have just found them totally innacurate. So like Timbo, calculate the rpm a given motor will give and use a wattmeter to see what is going on. I start with a scale size prop and work from there. Remember a large slow turning prop will be more efficient.

 

Just my 2p......

 

 

Danny,

 

I am struggling with your answer.  You calculate the rpm for a given motor. But there are hundreds to choose from.  How do you narrow down the field?  

  

 

Timbo,

 

You outlined what you do in an earlier post.  You rely on the manufacturer's documentation.  And if all your planes are of similar type and similar performance profile, that will work fine.  

 

However, the thing that escapes me is, if you are woring with a completely different type model, or if you are helping someone else, how do you find a starting point?  How do you know where to start, or to focus in on a small, select group of motor/prop combinations and all that go with that?

 

Here's the situation, and if you feel this is unreasonable, just say so.

 

You go to a swap meet or a show and you purchase an airplane. It is a high wing trainer type plane.     You are going to set it up electric ( or glow )

 

Wingspan:      69.5"         (1765mm)
Weight:        7.5lb         (3400kg)

 

How do you select a glow or an electric power set-up?   Just want you to walk me through the process.

 

 

Or perhaps a friend has built a kit, a  Bird of Time  3 Meter glider.  He wants to set it up for electric launch ( or glow if you prefer ). He wants your help to pick a power system.  How do you advise him?

 

Wingspan:   118" (3000mm)
          Wing Area: 1070" (69 sq dm)
          Wing Loading: 5.5 oz/sq ft (17g/sq dm)
          Weight: 41oz (1160g)
          Fuselage Length: 49 in (1245g)
          Airfoil: Flat-bottom Polyhedral-Wing

 

You want to set up a 3D Biplane .  How do you pick the power system?

 

Top and Bottom Wingspan: 48" (1220mm)
          Wing Area: 1145 in² (73.9dm²
          Weight: 7-7.5 lb (3170-3400 g)

 

This is not a challenge about these specific planes, it is the process I am after.  When you have a good starting point, that's fine.  If the MFG says buy "this" motor, there is no need for calculators.  But I have club members or forum members come to me and ask these questions.  Often times it is a used plane, we don't even know the name.  I use the calculators as my starting point.  How would you do it?

 

 

Edited By Ed Anderson on 03/04/2010 11:47:56

Edited By Timbo - Administrator on 03/04/2010 12:24:03

[Tim Mackey]

Ed. As you say, this thread is really about propellors, not powertrains per se. I am sure Danny will chime in here soon with his methods, as he is a very experienced electrric flyer also. First off I should stress that it is a personal thing about the motorcalc type progs - I can see that for some people they may well be useful, but for me, having been involved with electric flight for a good few years, I approcah things differently.

I have an article due in the mag shortly - the subject of which is converting IC models to leccy power, so in line with our normal policy, cant really quote too much from that as it has not yet been published. I am sure I /others have made many postings over the past few years about how we go about the process, unfortunately with over 13000 posts on here alone, I simply cant pinpoint them now

However heres a brief synopsis of one of my methods ( I use different methods in some situations )

 

I like to try to establish which prop I want to use on any given model based on a few criteria.

1) Model type ( hotliner, scale WW1 bipe etc )

2) Scale size to get closer to the "real deal" - generally larger than if it were an IC.

3) Pitch speed required to match full size

4) I like large slower props in most cases to keep down noise.

 

If I arrive at say a 15" prop, then I will calculate pitch speed required as in 3 above.

Once I have pitch speed, I know the approximate WOT RPM required, and based on the proposed battery size I intend to use, can now calculate approx Kv required of my motor. Motor wattage will already have been calcualted based on power to rate ratio of the model.

 

So as you can see, I often ( but not always ) start with the most desirable prop and work backwards as it were. 

Hope that makes some sense

[Ed Anderson]

Gents,

 

After submitting the post above I realized that I had taken this discussion outside the topic.  The topic is choosing a propeller which reasonably assumes you already have a motor. I was thinking in terms of full power systems. 

 

Sorry for going off topic.  I wanted to edit the post, but the system won't allow it.  

 

Timbo, if you want to delete that post and this one, please do so. thanks.

 

Edited By Ed Anderson on 03/04/2010 12:05:58

Edited By Timbo - Administrator on 03/04/2010 12:21:51

[Tim Mackey]

An instance where I may use a different method could be for where I already have a motor "laying about spare" which may  not be of the ideal Kv, but with a bit of juggling with cell counts, it could be made to work reasonably well on proposed model, with a prop that is close but  not spot on to that required.  I appreciate that efficiency will not always be at its greatest, but I really enjoy the flexibility that electric power brings to modelling by experimenting with different cell counts ( in other words voltages ) on a motor to produce a wide range of props that then be used - resulting in equally wide range of models that said motor can be utilised for.

[Tim Mackey]

Posted by Ed Anderson on 03/04/2010 11:59:47:

Gents,

 

After submitting this post above I realized that I had taken this discussion outside the topic.  The topic is choosing a propeller which reasonably assumes you already have a motor. I was thinking in terms of full power systems. 

 

Sorry for going off topic.  I wanted to edit the post, but the system won't allow it.  

 

Timbro, if you want to delete that post and this one, please do so. thanks.

Edited By Ed Anderson on 03/04/2010 12:00:27

 

Thats OK Ed....  hopefully my little piece above will help a few to understand all the elements involved, and further demonstarte why its very difficult to isolate prop selection as single topic......especially in leccy stuff

[Tim Mackey]

Staying firmly within topic it may be good to remind people of the formula used for pitch speed.

As many will know pitch is the theoretical distance ( in inches) that the prop moves through the air in a single turn.  Pitch speed is then the speed at which that prop moves through the air, with the model attached to it ! 

High pitch speed means lower thrust and theoretically  you can calculate the approxiamate speed of the model by the following method

RPM X Pitch ( in inches ) divided by 1056. This gives speed in MPH.

Example. 10000 X 7 = 70000.

70000 /  1056 = 66 MPH

 

All theory to great extent as other factors apply such as "slippage", drag, and so on.

 

[Danny Fenton]

Sorry guys I am probably just as guilty pushing this off topic....

 

Ed I try to limit my advice to the type of models I have explored. I don't mess with gliders, not because they don't appeal but because I love my scale warbirds. I have dabbled in WWI and post WWII biplanes so know a bit about the power solutions for those that work.

The method I use is almost identical to Timbo. Hardly surprising as much of my early electric flight knowledge was amassed with some advice a few years ago now from Timbo. We have both moved along since then and our methods have developed. But interestingly along the same lines and to a similar end point. Large slow props are quieter and more efficient. And we both start with a prop size befitting the model. Even Timbo has seen the light on A123 cells

 Lately I have been using much lighter motors and lower power levels than most would think appropriate. 80 watts to the Lb for a Chipmunk for example.

 For my scale flying its fine, but for those that want to fly like an ic at WOT for the full flight they will not be appropriate.

I look at the motor KV and set the cell count to give between 6 and 7,000 rpm. I pick a motor size/diameter appropriate to the amount of power/heat that needs dissapating.

Hope this helps?

 

[Ed Anderson]

That does help. So you have an RPM target.  That makes sense.

[Roberto Gava]

I don't think that talking about the whole propulsion system is really off-topic.

Because the right propeller is for one particular motor, plane, flying style, etc.

The fact is that for many years we were happy using far from optimal props in our glow engines.

Today with electric we have the need to optimize as far as we could to get the most of the weight of the battery-motor combination.

The problem is far complex to have a unique solution.

So I think that talking about strategies to solve this problem is really a good thing.

[Danny Fenton]

Hi Roberto, I think you are correct. We have in the past just stuck any old prop on IC and hoped for the best it seems. We all know that a carbon Biela prop will work well, but how many of us could afford one of those?

 

The wooden propellers we tend to use on IC are similar to wooden props of the 30's and 40's in shape and profile. They were only really efficient at low revs. We stick em on glow engines doing upwards of 10,000 rpm So the prop is very important indeed, to all of us.

 

 

[Dusty]

Can you do some testing on different prop brands? I have found same size props (ballanaced) testing on a given motor/setup can give a significant increase/decrease in performance/specs. APC clone electric props are nasty (green things) avoid!

 

EMP electric props seem to be VFM and come in 3 blade varients too! I have been playing around with the Turnigy wooden electric props, GVFM

 

However on a said setup a genuine APC electric prop would be the most efficient and powerful compared to other brands in some testing I have done on new models?

[Xander Caswell]

ok im still confused

 what prop would i use for a moki 135

[Ed Anderson]

Posted by Xander Caswell on 05/05/2010 20:10:38:

ok im still confused

 what prop would i use for a moki 135

 

OK, I am going to expose my ignorance of European products.  What is a moki 135.  Is that an airplane or a motor?

 

What motor?

What is the kV rating?

What voltage are you going to use ( battery Pack)

 

Do you have a link to the site for the motor so we can see the specs?

 

If you want to know if it is a good fit to a given plane:

 

What plane?

What is the weight of the plane?

What kind of performance/flying style are you trying to acheve?

 

Most motor MFG have recommended propellers for their motors based on what battery pack you are using.

 

 

You may also find this discussion helpful as an introduction to propellers:

http://www.modelflying.co.uk/forums/postings.asp?th=31200

Edited By Ed Anderson on 05/05/2010 20:51:30

[Stephen Grigg]

Can I use a wooden prop on an Electric motor?

[Ed Anderson]

Yes, and this might be appropriate on a large plane.  however for small planes wood props are typically heavier than the typical plastic or composite blades used on electric planes.

[Stephen Grigg]

Thanks Ed

[Danny Fenton]

Hi Stephen, I have used wooden props on larger electric models and they work fine, but are not as efficient in my opinion, they are also noticeably noisier. Often the IC props don't have a very coarse pitch range.

 

But wooden props can be more scale-like in appearance, so if you have power to spare then go for it.

 

Cheers

Danny

[Yorkman]

What would be the effect of going from a 2-blade 9x6 (electric) prop to a 3-blade 8x6 i.c one?

I understand a reduction in thrust due to smaller diameter, but what else? Just a loss of efficiency, or will the motor require significantly more current?

Or, more simply, going from a two blade to an identical sized three blade on an electric?

[Simon Chaddock]

Yorkman

IC props are typically stronger than electric as they have to survive the huge torque reversals between compression and power strokes. This means the root section of the blade is thicker and the hub is larger. The result is they are slightly less efficient than an electric one and a bit heavier as well.

[Yorkman]

So slightly less efficient, slightly heavier...but no significant change re current draw etc. I will give the 3-bladers a try on my next venture out.