Author Topic: Prop Size..  (Read 6143 times)

skyhawk newbie

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Prop Size..
« on: January 17, 2011, 18:54:20 PM »
Just wondering.......


I'm useing a 8x4 folding Prop with 1750Kv Brushless Motor a 20a ESC and 1500mAh Li-Po's....

What I'm thinking of doing as I have 2 sets of 9x5 Folding Props (ordered by mistake)  was fitting the 9x5 to the above set up...

Will I burn the motor / ESC out  ?
if not , will I be able to fly longer ?? or just Faster


Thanks

 Sean
Too many to name
Gliders/Planes/EDF Jets.16 total...
cars.
hpi Baja

Happy Days

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Prop Size..
« Reply #1 on: January 17, 2011, 22:28:50 PM »
Assuming that the larger prop will fit on the motor's shaft, it’s very unlikely that either the motor or ESC will burn out Sean, because you’re only fitting a slightly larger prop.

What will happen are three things.........

1/ The plane will fly a little bit faster. 8)   (Not Much though)

2/ The plane will accelerate, and be able to climb a little bit quicker. :)  

3/ The battery will run down a bit quicker. :(

There, nice and simple. :D

Now, just to make life complicated, if you fitted the bigger prop you could simply reduce the throttle setting on your transmitter so the plane didn’t fly any faster or climb any more than it did with the smaller prop. In which case the battery would last for about the same amount of time.

K.
Try not to run out of airspeed, altitude and ideas....... all at the same time.

skyhawk newbie

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Prop Size..
« Reply #2 on: January 18, 2011, 00:08:23 AM »
Quote from: "Happy Days"
Assuming that the larger prop will fit on the motor's shaft, it’s very unlikely that either the motor or ESC will burn out Sean, because you’re only fitting a slightly larger prop.

What will happen are three things.........

1/ The plane will fly a little bit faster. 8)   (Not Much though)

2/ The plane will accelerate, and be able to climb a little bit quicker. :)  

3/ The battery will run down a bit quicker. :(

There, nice and simple. :D

Now, just to make life complicated, if you fitted the bigger prop you could simply reduce the throttle setting on your transmitter so the plane didn’t fly any faster or climb any more than it did with the smaller prop. In which case the battery would last for about the same amount of time.

K.
   Thanks Kieth.........  
 I only use full throttle to get hight and then it's down to 3/4 to 1/2 when flying....  But I want to climb  quicker and higher :)  and my 1500mAh Li-Po's give's me about 15 mins if I don't gun it all the time...

Yeah the Prop fits the spinner and "if" the worst does happen and I do burn out my motor and ESC I've another on the way from HobbyKing...only $20 (€15) plus the postage.....

Sean
Too many to name
Gliders/Planes/EDF Jets.16 total...
cars.
hpi Baja

GliderIreland - Ireland RC Forum - Flying Model forum in Ireland

Prop Size..
« Reply #2 on: January 18, 2011, 00:08:23 AM »

Aidan

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Re: Prop Size..
« Reply #3 on: January 18, 2011, 11:22:23 AM »
Quote from: "skyhawk newbie"
Just wondering.......


I'm useing a 8x4 folding Prop with 1750Kv Brushless Motor a 20a ESC and 1500mAh Li-Po's....

What I'm thinking of doing as I have 2 sets of 9x5 Folding Props (ordered by mistake)  was fitting the 9x5 to the above set up...

Will I burn the motor / ESC out  ?
if not , will I be able to fly longer ?? or just Faster


Thanks

 Sean
Hi Sean,

It's impossible to make a good guess as to whether this will cause you problems or give you better/worse performance without knowing a few more details.
i.e. What size is the motor? What are it's other specs and it's name? Is there any test data available from the manufacturer/distributor?
Also how many cells do you have in the 1500mAh pack. The voltage supplied to the ESC is one of the most important factors. I expect you're probably using either a 2s or 3s pack (2s = 2 cells in series, 3s = 3 cells in series) but it makes a big difference which.

There's 3 ways to know in advance how your power system is likely to perform:
1. Calculation using MotoCalc or similar software - can be very inaccurate but a good starting point if the manufacturer doesn't publish test data
2. Test data from manufacturer or other owners/testers - this is the best way to know what to expect before buying or doing your own tests
3. If there's no test data going, you can compare your motor to other similar motors (similar size stator and kV) that do have available data. This will only give you a very rough estimate. You could be a long way off.

If you don't have good data from the manufacturer and ideally even if you do, I suggest you purchase a current & voltage meter so you can see what your power system is actually doing. This is really the only way to know if you're pushing anything too hard (or not hard enough) and is essential if you need to do any trouble shooting.

The basic relationships for motor kV, prop size to electrical figures for a given power system are as follows:
Kv x supply voltage = theoretical RPM for no load (i.e. no prop)
The bigger the prop you put on a motor the lower the actual RPM will be.
As the difference between the theoretical no-load RPM and the actual RPM increases the current will also increase. So a bigger prop or a "draggier" prop will cause the current to rise. How much depends on lots of variables, some specific to the motor, some to the props, and the typical in-flight current will also depend on the airframe itself.
Lost power in the form of heat generated in the motor is down to power and efficiency. If the motor is, say, 75% efficient in an application where it's running at 16A on 7V then the electrical power input is 16x7 = 112W. Shaft power (what goes to the prop) is 112Wx0.75 = 84W and heat is 112W-84W=28W.
This 28W of heat will be made up of electrical losses from the resistance of the wires, etc and also mechanical losses due to bearings etc.

As you put more and more load on the motor the current will rise and in most of our applications the efficiency will drop. Both changes increase the heat generated. If the motor can't dissipate this heat quickly enough the motor temperature will rise high enough to damage the magnets or the burn/melt the wire insulation. That's how a motor usually dies.

...I seem to be rambling on a bit. Apologies I'll get back to the point now!

I'd guess a 9x5 prop of similar type to the 8x4 will probably cause a current increase of something between 20% and 40%.

more current = more power. If the batteries are well in spec then voltage won't drop much with the higher current and you'll get say 15% to 30% more power. The larger prop will be more efficient which will improve performance. Speed may not increase much unless the plane was originally very low on power because some of the extra pitch (5" instead of 4") will just compensate for the drop in RPM. However you should have significantly more thrust so steeper and faster climbs should be possible.
That's assuming you're still in the efficient operating range of the motor.

Aidan

skyhawk newbie

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Re: Prop Size..
« Reply #4 on: January 18, 2011, 12:26:50 PM »
Quote from: "Aidan"
Quote from: "skyhawk newbie"
Just wondering.......


I'm useing a 8x4 folding Prop with 1750Kv Brushless Motor a 20a ESC and 1500mAh Li-Po's....

What I'm thinking of doing as I have 2 sets of 9x5 Folding Props (ordered by mistake)  was fitting the 9x5 to the above set up...

Will I burn the motor / ESC out  ?
if not , will I be able to fly longer ?? or just Faster


Thanks

 Sean
Hi Sean,

It's impossible to make a good guess as to whether this will cause you problems or give you better/worse performance without knowing a few more details.
i.e. What size is the motor? What are it's other specs and it's name? Is there any test data available from the manufacturer/distributor?
Also how many cells do you have in the 1500mAh pack. The voltage supplied to the ESC is one of the most important factors. I expect you're probably using either a 2s or 3s pack (2s = 2 cells in series, 3s = 3 cells in series) but it makes a big difference which.

There's 3 ways to know in advance how your power system is likely to perform:
1. Calculation using MotoCalc or similar software - can be very inaccurate but a good starting point if the manufacturer doesn't publish test data
2. Test data from manufacturer or other owners/testers - this is the best way to know what to expect before buying or doing your own tests
3. If there's no test data going, you can compare your motor to other similar motors (similar size stator and kV) that do have available data. This will only give you a very rough estimate. You could be a long way off.

If you don't have good data from the manufacturer and ideally even if you do, I suggest you purchase a current & voltage meter so you can see what your power system is actually doing. This is really the only way to know if you're pushing anything too hard (or not hard enough) and is essential if you need to do any trouble shooting.

The basic relationships for motor kV, prop size to electrical figures for a given power system are as follows:
Kv x supply voltage = theoretical RPM for no load (i.e. no prop)
The bigger the prop you put on a motor the lower the actual RPM will be.
As the difference between the theoretical no-load RPM and the actual RPM increases the current will also increase. So a bigger prop or a "draggier" prop will cause the current to rise. How much depends on lots of variables, some specific to the motor, some to the props, and the typical in-flight current will also depend on the airframe itself.
Lost power in the form of heat generated in the motor is down to power and efficiency. If the motor is, say, 75% efficient in an application where it's running at 16A on 7V then the electrical power input is 16x7 = 112W. Shaft power (what goes to the prop) is 112Wx0.75 = 84W and heat is 112W-84W=28W.
This 28W of heat will be made up of electrical losses from the resistance of the wires, etc and also mechanical losses due to bearings etc.

As you put more and more load on the motor the current will rise and in most of our applications the efficiency will drop. Both changes increase the heat generated. If the motor can't dissipate this heat quickly enough the motor temperature will rise high enough to damage the magnets or the burn/melt the wire insulation. That's how a motor usually dies.

...I seem to be rambling on a bit. Apologies I'll get back to the point now!

I'd guess a 9x5 prop of similar type to the 8x4 will probably cause a current increase of something between 20% and 40%.

more current = more power. If the batteries are well in spec then voltage won't drop much with the higher current and you'll get say 15% to 30% more power. The larger prop will be more efficient which will improve performance. Speed may not increase much unless the plane was originally very low on power because some of the extra pitch (5" instead of 4") will just compensate for the drop in RPM. However you should have significantly more thrust so steeper and faster climbs should be possible.
That's assuming you're still in the efficient operating range of the motor.

Aidan


This is a vertical learning curve.. :lol:

I'm using this motor..  http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=4321

With this ESC.. http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=14871

on these Li-Po's..  http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=6307

When using the 8x4 Prop the only thing to get warm is the Li-Po's , but that's only it I'm gunning it ...

Sean
Too many to name
Gliders/Planes/EDF Jets.16 total...
cars.
hpi Baja

Aidan

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Prop Size..
« Reply #5 on: January 18, 2011, 13:24:57 PM »
Looking at the specs for those components, the ESC is fine up to 20A and the cells are rated for 30A or bursts of up to 45A. The motor however has a max current rating of 13A so it should be the first to go - by a long way.
There's some test data given for the motor on an 8x4 prop at various voltages. Unfortunately they don't say which 8x4 they're using. They should as all props have different characteristics.

Configuration:      Propeller:      Volts:      Amps:      Thrust:    
  Direct Drive      8040      7.4V      8.2A      12.0 oz / 342.86 grams    
  Direct Drive      8040      8.4V      8.6A      13.8 oz / 394.29 grams    
  Direct Drive      8040      9.6V      11.1A      17.0 oz / 485.71 grams    
  Direct Drive      8040      11.1V      13.1A      21.0 oz / 600 grams    


Assuming they used a non-slowfly 8x4 similar to say an APC-E prop there's probably not a huge difference between these figures and what you should expect with your folder.
For a 20C battery I usually estimate a supply voltage of 3.5V per cell under moderate load for the purposes of working out a power system. So your 3s pack should supply about 10.5V. From the data above you can see that should produce a current draw of somewhere around 12A.
If their test data is accurate and unless the 13A maximum current figure is very conservative you might well have problems if you go to a 9x5. If you had an instrument for measuring the current I'd say give it a quick try but otherwise you'll be taking a chance with the motor. Not the end of the world at that price but you might be without a plane until you get a replacement. I think if you want a significant improvement in performance the ESC and cells are fine but you'll need to change motors.

The larger prop sizes mentioned in the motor spec are presumably intended for 2s applications but the info is poor and that's not made clear.

Aidan

Happy Days

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« Reply #6 on: January 18, 2011, 14:13:06 PM »
“If forty volts and forty amps produced but half a watt,
I wonder,” asked the student, “would Ohms Law still work, or not?” :?:
“I doubt it,” said the tutor, “but it would get bloody hot!” :lol:
Try not to run out of airspeed, altitude and ideas....... all at the same time.

skyhawk newbie

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« Reply #7 on: January 18, 2011, 14:21:10 PM »
Cheers Aidan

Think I'll just stick with the 8x4 then...

Here's where I got the info for the
1750 kv motor/20a ESC / 8x4Prop Set up...
http://www.dmitrynizh.com/tw742.htm

Sean
Too many to name
Gliders/Planes/EDF Jets.16 total...
cars.
hpi Baja