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Propellers...for beginners - please keep it simple

Choose and match a prop to your electric model

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Tim Mackey23/08/2009 12:36:52
20920 forum posts
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15 articles
Electric models are more critical of propeller selection than IC engines.
If you stick an oversized prop on an IC engine, it will reduce the revs a fair bit, probably prevent the engine from achieving its stated output, maybe make the engine  labour a bit if you go well oversize, but apart from that not much else happens.
Stick a prop that is just one 1" bigger than it should be on your electric motor and potentially you release the magic smoke ! This is because the electric motor will continue trying to reach its designed speed ( Kv ) for the battery you are using, and in the process consume more current ( Amps ) through the battery, ESC, and itself.
Electric motors are also catered for in the prop department with nice lightweight props - usually designated "E". These do not have the mass, or strength ( especially around the hub ) that a "normal" prop needs in order to absorb the more stressful "bang bang" type of power output of the IC engine. You should NEVER use an "E" style prop on an IC engine, and preferably dont use IC props on electric motors either ( although there is no safety risk if you do ) as they can be a bit heavy going for the motor. Several programmes exist for estimating the likely results ( in terms of RPM, Current consumption, Voltage drop, and thrust ) odf fitting different props to various motrs with varying battery voltages. Amongst these are Motorcalc, and Drivecalc.
A quick google should reveal all. However, IMO there is nothing to beat using a Whattmeter" or similar when experimenting with props, as these devices will immediately show if your chosen prop is just asking too much from your drivetrain, and use of these gizmos - together with a rev counter should enable you to select just the right size. Any serious electric flyer should always do initial static bench testing of his intended rig using these tools, before committing to flight.
As with all props, IC or electric - the general rule to remember is that...
Increasing diameter will increase thrust / pull -  but not speed ( like a car in low gear )
Increasing pitch will increase speed but not thrust ( car in top gear )
Increasing either or both will have an effect on the curent drawn as mentioned above.
For more info on using the "Whattmeter" style device, see HERE.

Edited By Timbo - Moderator on 27/08/2009 23:40:40

Lee Bentley10/09/2009 16:45:10
132 forum posts
8 photos
Hi Timbo,
Interesting piece about props. I don't have a wattmeter (not got around to buying one yet) however I have been trying different combinations with confusing results.
I bought a Tucano from a mail order place in Austria, a GRP fus model weighing in at 9.25lbs designed for a 120 4stroke. I wanted to convert this to electric so fitted a Pro Flight 60 400kv with a 6s, 100A ESC and 14x8 prop. With this setup I get 8880 RPM which is the correct max rpm. Similar output can be obtained with a three blade 12x7. Model flies nicely with this combo as long as I keep the power up, reduce power and model wallows a bit. I then thought a larger motor would be better suited so purchased a 380kv motor recommended for models up to 15lbs. With the same prop I can get no more than around 6000 rpm although specs would suggest just over 8000. Now this is where things can get a little confusing. Now I always believed that the more rpm, the better for both thrust and speed irrepective of the pitch so why would motor manufacturers etc recommend motor with lower kv for heavier models?
The reason I prefer higher speed is that the Tucano has shorter wings therefore more airspeed necessary for stable flight, much the same as a jet! 
Also I have noticed that even with recommended combos, on all my electric stuff, after a flight the battery is very hot to touch, is this normal?

Edited By Lee Bentley on 10/09/2009 16:45:38

Tim Mackey10/09/2009 17:02:10
20920 forum posts
304 photos
15 articles
last things first.... no it should not be very hot ...very warm to fairly hot maybe, but never VERY hot. It sounds like you are pulling too much current from it - what is the C rating, and capacity of the pack.?
As for your figures...I am a little puzzled - and a bit sceptical of the RPM figures - is your rev counter accurate - are you sure?
The reason is, that even with no prop, the 400Kv motor on a 6s Lipo would be spinning around 8800, but with any load, these revs will drop, and also the battery will be very unlikely to be holding the full 3.7V per cell, I find usually 3.5 is more realistic.
6 X 3.5 = 21V. 
21 X 400 = 8400 and in probability a fair bit lower is what I expect..
The 380Kv motor RPM sounds more likely TBH - either the 400Kv motor is off spec and wound slightly lower, or the revcounter is suspect.
The reason larger and heavier models normally need lower Kv motors is because the likely power required ( often over 1000 Watts ) needs high voltages in order to keep the currents reasonable.... using these high voltages on a high Kv motor would result in far too much revs....and very high currents. Also, due to large propellers, as these need low Kv and high torque motors to spin 'em.
Your 9.5lb aerobatic model needs around 150 Watts per pound I would suggest, in order to perform well - thats 1425 Watts. On a 6s lipo ( approx 21.5V under load ) that a whopping 66 Amps of current... no wonder your batteries are hot

Edited By Timbo - Moderator on 10/09/2009 17:04:47

Steve Hargreaves - Moderator10/09/2009 17:31:44
6765 forum posts
197 photos
Well Lee I'm not sure where to start but for sure anything that is getting "very hot to touch" is going to have a much shorter life than anything that is staying cool.
I'm amazed that you appear to have spent lots of money on a large electric model, batteries, ESC etc but could lose the lot for want of a £25 wattmeter to say nothing of who or what a crashing model might injure!!!!
My immediate suggestetion is to buy a wattmeter & see whats going on before something (possibly) fails....looking at the revs is no indication of what it actually going on with the current......remember that a propeller is not a linear takes ever more power to drive it as the revs go double the revs you may need 4 times the power....
Now, kv values.....unlike ic motors which will drive a prop at certain revs & no more an electric motor has a certain speed of rotation designed into it...the kv value expressed as rpm per volt....the motor always wants to turn at this speed & will simply draw more current from its power source until either it gets there or it melts. Hence if you apply 10 volts to a 1000kv motor it wants to turn at 10,000 rpm. With no prop on thats exactly what it will do & consume only enough power to overcome the friction of the bearings. Add a prop...apply 10 volts & again our motor will try & reach10,000 rpm only this time it has to turn the prop...a get does it cope with this load? It draws more current.
If we consider for example a 10x6 prop...lets say it takes 300watts (equals 30A from our 10 volt battery) to turn it at 10,000 rpm......provided the motor that trying to turn the prop can take 30A then we're fine...if however its only rated to 25A then we have a problem....we need more power than our motor can supply & it will fail.
Lets go the other way & fit say a 6x4 prop...a much lower load than the 10x6...again our motor will try & spin it at 10,000 rpm but at its a smaller prop the load isn't as great, therefore it takes less than 30A to get it to 10,000 rpm...maybe it only pulls 15A so now our set up is only developing 150watts.
This is something often overlooked by leccy flyers. Electric motors don't "over-rev" as it were....I've often heard people at my club talking about fitting a smaller prop "to get the revs up"....this doesn't happen....all you find is that your model doesn't fly very well.
(please note there are some "over-simplifications" in the above to protect the innocent but the gereral explanation is sound)
Lee Bentley15/09/2009 20:33:58
132 forum posts
8 photos
Hi Timbo

Sorry for delay getting back to you. Just got back from a weekend of flying down south.
I may have over exaggerated the temperatures, should have said the batteries are hot but not that hot that they are untouchable.
The rev counter is a McGregor and I don't have another to compare the readings so must therefore assume the figures to be fairly accurate. As regards the voltages you mention, I have just measured the voltage of both 6s lipos and fully charged, off load they measure:

6s 4000 15c 24.7v
6s 2200 35c 24.9v

I then ran these for 15secs full power with following results:

6s 4000 15c 23.1v
6s 2200 35c 22.8v

Two different multimeters were used, a Fluke model 75 and a Metrotest MIC 3500.

I'd be interested in your observations.

Incidentally these are the specs for the motor:

Pro-Flight 60

Rpm/V: 400Kv
Watts: 1100
IC: Eq 60
Input Voltage: 11.1V-22.2V (3-6S Li-po)
Max Current: 55A
Constant Current 40A
Weight: 300g
Shaft: 8mm
For models: 5.5lb - 9lb
Example of use: 13x8 - 15x8 Prop 6 cell lipo pack
Hi-Model 4130
Type: A4130
No. of Cells: Ni-Mh 16-24, Lipo 5-8 cells
Max. efficiency:88%
Max. efficiency current:18 - 40 A (>84%)
No load current / 10 V: 1.5A
Current capacity:60 A/60 s
Internal Resistance:63 mΩ
Dimensions:50x65 mm
Shaft diameter:6 mm

Recomended model weight:3000 - 6000 g

Both motors supplied  from

I have ordered a wattmeter Steve, just to avoid further admonishments from you but it costs a bit more than £25, Incidentally, the cost of large motor, 100A ESC and two 6s lipos costs less than an OS 120 which is what the model was designed for.

(quote....remember that a propeller is not a linear takes ever more power to drive it as the revs go double the revs you may need 4 times the power....quote

Remembering my physics.....surely, quite the opposite, it would require more load on initial startup than when running! 

Edited By Lee Bentley on 15/09/2009 20:35:28

Tim Mackey15/09/2009 22:05:02
20920 forum posts
304 photos
15 articles
The batteries seem fine - OK at best they would show 25.2 fresh off the cooker - but frankly this will drop a tad almost straight away. Under load, it seems they both hold up extremely well....I usually reckon under load each cell to be at about 3.5V or so, and yours are way better than that.
However what is not clear from your figures is whether in fact the voltage readings are on or off load. I suggest we wait until your meter arrives and see what is happening under load - as i said earlier, if the watts are as high as they need to be for that model, then the current is very high too on a 6s.
66A on the 2200mahr battery is 30C and the battery is only rated at 35C - and thats probably both optimistic, and  short burst rated only.
Steve Hargreaves - Moderator18/09/2009 10:01:12
6765 forum posts
197 photos
Sorry if you thought I was having a go at you...its a free country & you must do as you see fit but I was just a bit surprised that you appeared to be risking the loss of your aircraft & possible damage to people & property on the ground simply for the lack of few pounds spent on a must remember that an OS120 will not destroy itself simply because you put too big a prop on it......a leccy system will & often take the airframe with it!!!
I think you are only remembering part of your physics...yes a load has inertia that initially needs to be overcome which will take more power than keeping it just moving but a prop is most definitely not a linear load....when you have your watt meter connected spin the motor at say 2,000 rpm & note the current.....take it up to 4,000 rpm & note it again....I guarantee it will have more than doubled.....take it up to 8,000 rpm & note it again......big innit???
Lee Bentley18/09/2009 19:53:26
132 forum posts
8 photos
Hi Steve,
No offence taken,  Its just that having flown for over 30 odd years and having more than my fair share of incidents, even at shows, I am aware of what a rogue avion can do. Also don't purposefully take unnecessary risks. I am fairly new to electric and gas turbine flight and find its a pleasant learning curve. Keeps my old grey cells active.
Thanks for the links incidentally, they are quite helpful and I will certainly pass them on to my fellow members here in France, Unfortunately I have already ordered a wattmeter from my supplier in Austria who doesn't require a mortgage for postage.
Will let you all know what results are when meter arrives.
Lee Bentley03/10/2009 13:39:37
132 forum posts
8 photos
Hi All,
Me Watt meter arrived today so have taken some readings, would be interested in your observations. Measurements were taken twice. The Lipos used were both 6s, figures in brackets refer to a 2200 35c lipo:

Prop size
RPM Watts Amps
Lipo Lipo ()
2000 45.4 (29.1) 2 (1.5)
4000: 15c 2200 35c

4000 171 (136) 7.8 (5.8)

6000 479 (447) 25.9 (19.6)

8000 750 (1150) 38.5 (55)

2000 42.5 (20.4) 1.7 (.9)

4000 250.4 (120.8) 11.9 (5.5)

6000 480 (350) 39 (16)

Andy Gates03/10/2009 15:17:15
643 forum posts
20 photos
Are you sure the figures in your last post are correct?
Using the values in amps and watts, the figure for 8000 rpm on the 14x7 prop look like they are the wrong way round!
First the lipos.
Assuming the packs are good the 4000mah 15C should be good for 60A max.
The 2200 35C should be good for 77A max.
So both packs should be within their respective limits.
If the packs are getting so hot that you can't hold them against your cheek then something is wrong.
It could be the packs are not really capable of delivering the C ratings shown on the label. This looks to be the case with the 2200mAH one.
Your figures show the 4000mAH pack is holding up better under load than the 2200 pack.
I am going to hazard a guess that you normally fly IC, hence the focusing on prop rpm. With electric, providing you are not running the prop faster than its designed speed, then you can almost forget rpm in the performance equasion.
You are looking to match the battery pack supply to the motor and prop combination to supply the performance you require for your plane.
More useful would be tests run on both those props at full throttle, measuring wattage, current and voltage. The voltage will help us see what is happening to the voltage stabillity of the packs under load.
Lee Bentley03/10/2009 16:55:58
132 forum posts
8 photos
Hi Andy,
Yes you are correct, in my haste I transposed the figures in the 8000 rpm reading. Should read:
750watts 55A  and 1150 Watts  55A respectively, also made same mistake on 14x8 prop figures, should read: 480 Watts 39A and 350 Watts 16A.
From your observation both packs are within limits although the 4A pack doesn't reach 8000RPM with a 14x8 prop hence the missing figure.
My early post regarding the the RPM attaining over 8000RPM with a 14x8 prop was due to a faulty Lipo hence the heating query.
You are also correct in that I used to fly IC. Most of my aircraft are electric now as the technology has certainly improved over the years in this field. they are also cleaner and quieter. I cannot see how one can forget the RPM as surely this is necessary for thrust etc. The voltage remained at around 22 V with both packs and the full throttle RPM are as the figures supplied.
 It might be worth noting the recommended prop pitch for the Pro Flight 60 is 8 yet the RPM of the 7inch pitch seems a better match for this motor, or am I missing something?
Andy Gates03/10/2009 20:21:28
643 forum posts
20 photos
As explained earlier by another poster, the prop/motor/battery combination defines the performance. RPM is far less critical.
A larger diameter prop will develop more thrust than a smaller one.
A higher pitch prop will develop more pitch speed than a lower pitched prop.
I kind of guess you already know the above.
With glow engines the motor has a fixed power band where the motor works best and so you normally prop to make best use of the power band.
With electric motors there is no power band as such.
What you are trying to do is to make the best use of the power available from the battery with the motor you have to suit the way the plane flies.
Using an example of a Cub type plane.
With electic you are looking for plenty of thrust at a low speed range, so a large diameter  broad bladed lower pitched prop is best. In this example a "slow fly" prop would be used.
A prop for a pylon race would be of no use since it does not need to generate quite so much thrust due to the sleeker design of the plane but it does need to be producing thrust at quite a high air speed. In this instance a "E" prop would be used with a smallish diameter and a high pitch.
The above is the reason that your plane wallows with the 3 blade 12x7" prop. It has insufficient thrust available at speed due to the finer pitch. A 12x8 would improve this a little, a 12x9 would be better still but you would have to check the current draw stays within the limits of the drive train - motor, ESC and battery pack.
Timbo did most of the calculations for you in an earlier post.
9.5lb plane.
100-150 watts per lb power loading required.
That means you need between 950 and 1425 watts to be developed for your plane to fly the way you want it to.
Using the data in your post for voltage and inserting the values into the following formula 
Watts = Amps x Volts
 This means your battery pack needs to be providing between 43 and 65A.
Both of your motors are rated for continuous current fo 40A so you already have a bit of a restriction. I guess you are going to stick within the recomendation of maximum current for no longer that 60 seconds.
So assuming a maximum current of 55A, you would be within the 950 - 1425 watt needed.
Now you need to prop your plane to make best use of that power.
That is why we use full throttle measurements with electric flight.
I will assume the data posted earlier is full throttle (which it may not be).
The 14x7 on the 4000mAH pack looks to be the best option.
But just because the 14x8 prop does not reach 8000 rpm does not mean it is of no use.
The 7" prop will stop providing useful thrust at a lower speed than the 8" prop which is more suited to your application - providing the current stays within the component limits.
With the plane in the air, the motors will not use so much power as the propeller loading on the motor will ease as forward speed increases.

Edited By Andy Gates on 03/10/2009 20:22:36

Lee Bentley04/10/2009 00:15:07
132 forum posts
8 photos
Thanks for your very informative post, I am now somewhat wiser thanks to your explanation regarding motor, battery, prop combination and matching. The Tucano will be flight ready sometime next week so will take some notes on different combos although I intend to steer away from 3 blade props for now. Incidentally, just noticed, the 14x8 prop is an IC prop, somewhat heavier than an E prop so will be ordering the correct one this weekend.

Edited By Lee Bentley on 04/10/2009 00:18:43

Andy Gates04/10/2009 11:32:10
643 forum posts
20 photos
I hope it helped, good luck with the testing.
Let us know how you get on.
Steve Hargreaves - Moderator04/10/2009 15:54:52
6765 forum posts
197 photos
Interesting stuff Lee....amazing how much extra power you need to increase the revs once you get the prop spining isn't it??
3 bladed props are good if you're struggling for ground clearance but tend to be more inefficient since the blades are closer together & are hence more susceptable to the turbulance caused by the blade in front.....
Always worth noting the voltage your Wattmeter shows as well......much below 3.5volts per cell (21volts on yer 6S pack) & it means that your battery is struggling to cope with the demands placed on it......
Probably best we close this issue now......I think Timbos idea of these specific "How to" threads was to explain the basics & any specific issues should be dealt with on a different thread!!!
Tim Mackey04/10/2009 18:40:33
20920 forum posts
304 photos
15 articles
Lee Bentley04/10/2009 19:13:43
132 forum posts
8 photos
Oops!   Sorry....Got a bit carried away......Must this hot weather here.

Edited By Lee Bentley on 04/10/2009 19:14:13

Edited By Lee Bentley on 04/10/2009 19:14:30

Ed Anderson30/03/2010 21:52:49
247 forum posts
10 photos
Nice discussion on props.  I really liked the opening post.
It might be good to further explore the relationship of span vs pitch in prop selection as this is one of the things we tune or target when we buy or set up a motor.  We can target a particular type of performance then select the motor and battery that will allow us to use the right prop to provide that type of performance.   This will influnce your kV targets for the motor.
Andy touched on it a bit but it might be interesting to go a bit deeper into the subject.
Just as an opening remark, I fly mostly gliders.  I am not all that interested in top speed or stall speed as my planes are made to fly with the motor off.  I am much more interested in climb angle, or how quickly I can get to 600 feet, for example. Some of my gliders will climb straight up.   When I hit my desired altitude, I power off and hopefully finish a long flight having never turned on the motor again.  I don't measure my battery life in minutes, but in how many climbs I get.
Therefore I tend to favor wider props that will produce a lot of thrust rather than a lot of speed.  This would certainly apply to 3D pilots as well, and I imagine pattern pilots would also favor thrust over speed.
Since my gliders are hand launched and have folding props I don't have to worry about ground clearence.  But ground clearence will be a key design factor for a plane that takes off from the ground.  In fact one might choose their landing gear set-up in order to accomodate the propeller that will give them the performance they wish.
I have one glider that I use with both 2 cell lipos and 3 cell lipos.   When I switch battery packs I also switch props.   The 2 cell gets an 11X8 prop and I am about to try a 12X8.   The 3 cell gets a 10X7 prop.  If I leave the 11X8 on with the 3 cell I will over work the motor in the way that Timbo discusses in the first post.
Us "wide prop" guys tend to favor lower kV motors or gearboxes.  Where the speed pilots tend to favor high kV motors and direct drive propeller set-ups.
This is one of the reasons that the "calculators", like motocalc, are so valuable to electric pilots who like to design their own power systems.  There are many variables that you can change in order to get the performance you want.

Edited By Ed Anderson on 30/03/2010 21:56:47

Tim Mackey30/03/2010 23:46:01
20920 forum posts
304 photos
15 articles
Hi Ed, thanks for the comments .  I actually dont use the "Motorcalc" style programmes as I find there are too many discrepancies, and prefer instread to just experiment with a few props either side of my expected best one, using my trusty wattmeter or Eagle tree logger. I created this particular thread ( in fact this whole forum section ) for the basics, to try and help the raw beginner, so we try to keep it real simple. However, I agree that firther discussion on prop selection and choice for different models and flying would be a good idea - please feel free to startone up...maybe in "all things flying" as the principles are the same for IC or leccy
Ed Anderson31/03/2010 01:21:56
247 forum posts
10 photos
II had not thought of starting a discussion on props that would address both glow and electric.   I have no experience with wet power systems.  But I will start the thread if you think it would be of interest.
As for tools, I have been having fun with Webocalc. 
It is a free, web based tool.  Compered to motorcalc it is very limited, but once you get to understand its limitations, it can provide some good guidance. 
However I still prefer to work with well documented motors.

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