Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

How to chose prop?

Old 03-24-2009, 11:24 AM
  #1  
Petrik59
Member
Thread Starter
 
Join Date: Sep 2008
Posts: 39
Default How to chose prop?

Hi,
I am very confused about how to chose a prop for my deprone plane (p-40 Dart by Gene Bond).
I was out flying on Sunday and first flight went well, I could fly very fast or wery slow. But wen starting the next flight the wind had increased and the plane nearly could not move forward at times.

I do not need high speed but I want power to move through wind, is this possible??

I read somewhere that small prop gives horsepower and large prop gives speed, is that correct.
Should I use a prop with different pitch?

Then I read this at Amp Aviator:
"By changing the diameter and the pitch of the propeller, we can have an effect similar to changing the gears in your car or a bicycle. It will be harder for your motor to turn a 9x7 propeller than an 8x7 propeller. And it would be harder to turn a 9x7 propeller than a 9x6 propeller.
The larger or steeper pitched propellers will require more energy, more watts, more horsepower, to turn them. Therefore we need to balance the diameter and pitch with the power or wattage of the motor/battery system"

Please help
Petrik59 is offline  
Old 03-24-2009, 12:24 PM
  #2  
slipstick
Super Contributor
 
Join Date: Oct 2005
Location: N.Staffs, UK
Posts: 2,350
Default

Originally Posted by Petrik59 View Post
Hi,
I do not need high speed but I want power to move through wind, is this possible??

I read somewhere that small prop gives horsepower and large prop gives speed, is that correct.
Should I use a prop with different pitch?
Because you're standing on the ground watching the plane you're getting confused by ground speed (what you're seeing) and airspeed (the speed the plane thinks it's doing). If the plane is flying into a 30mph wind and hardly moving (as you see it) then from its point of view it's doing at least 30mph. The only way you'll get it to move across the ground faster is by making it fly EVEN faster (or by turning downwind, then it will really move ). So if speed upwind is what you want then you DO need high speed.

And you have the props the wrong way round. A large diameter low pitch gives you more thrust (e.g. for take-off acceleration) whereas a small diameter higher pitch generally gives you more top speed.

It could be worth trying a couple of different props to see how you like them but unless you are measuring the current to check that the motor is o.k. you need to be careful. If you go down 1" in diameter and up 1" in pitch that might give you more speed but there can also be quite a difference between different types of prop of the same theoretical size.

If you say exactly what battery, motor and prop you're using now it might be easier to give detailed advice.

Steve
slipstick is offline  
Old 03-25-2009, 01:13 AM
  #3  
Petrik59
Member
Thread Starter
 
Join Date: Sep 2008
Posts: 39
Default

I have no data on the motor more than it is a Avionic Breese 33 gram.
Speedcontroller is an Avionic Captain 8 and the LiPo is a DesirePower 0450mAh 3S 25C/50C.
Prop was 8x3,8 i think (hard to see the small letters).

Does 0450mAh and 25C mean that I get 11,25 Amp?
If that is the case then my speedcontroller is a bit small, do I risk burning it up?

And another thing, I just noticed that the LiPo I used during my second flight only was a two cell Lipo (DesirePower 0360mAh 2S 25C/50C).

Maybe that was what made me feel that there was not enough power.
Petrik59 is offline  
Old 03-25-2009, 12:43 PM
  #4  
slipstick
Super Contributor
 
Join Date: Oct 2005
Location: N.Staffs, UK
Posts: 2,350
Default

As everything else stayed the same then replacing the 3S battery with a smaller 2S battery gave you less than HALF the maximum power you had for the first flight.

Though if that motor is the one I think it is then it should only be run on 2S so you may be overpowering it on 3S.

Steve
slipstick is offline  
Old 03-25-2009, 04:36 PM
  #5  
Buck Rogers
Member
 
Join Date: Apr 2007
Location: Bristol, UK
Posts: 511
Default

Originally Posted by Petrik59 View Post
I have no data on the motor more than it is a Avionic Breese 33 gram.
Speedcontroller is an Avionic Captain 8 and the LiPo is a DesirePower 0450mAh 3S 25C/50C.
Prop was 8x3,8 i think (hard to see the small letters).

Does 0450mAh and 25C mean that I get 11,25 Amp?
If that is the case then my speedcontroller is a bit small, do I risk burning it up?

And another thing, I just noticed that the LiPo I used during my second flight only was a two cell Lipo (DesirePower 0360mAh 2S 25C/50C).

Maybe that was what made me feel that there was not enough power.

I think you are getting confused about the battery ratings.

450 mAh is the capacity of the battery and means you should be able to draw 450 mA for a period of 1 hour before the battery is flat.

25C is the maximum rated current for your battery (25 x The capacity) - in this case the maximum you can safely draw is 11.25A

The actual amount you are drawing depends on a number of factors including the motor, prop and battery voltage. The only way to tell for cartain how much current you are drawing is to hook up a watt meter and measure the your setup is taking.

In going from a 3s to 2s you will have reduced the amount of power the setup is taking - you could counter this by changing the prop to greater diameter, greater pitch or both. In my experiance a prop that works with a 3s battery isn't usually suitable for use with a 2s battery and vice versa.
Buck Rogers is offline  
Old 03-27-2009, 12:54 AM
  #6  
Petrik59
Member
Thread Starter
 
Join Date: Sep 2008
Posts: 39
Default

I have one more 3S 450 mAmp LiPo but that one is only 10c-12c.
Does that mean that I get much less power or is it better than th 2s 450 20C?
Petrik59 is offline  
Old 03-27-2009, 05:14 PM
  #7  
Buck Rogers
Member
 
Join Date: Apr 2007
Location: Bristol, UK
Posts: 511
Default

Originally Posted by Petrik59 View Post
I have one more 3S 450 mAmp LiPo but that one is only 10c-12c.
Does that mean that I get much less power or is it better than th 2s 450 20C?
Ok, for the 3S battery, lets assume it will hold 11V under load. Its 10C so the max amps you can get out of the battery is 4500 mA (4.5A) so the max watts you can push is 11 x 4.5 = approximately 50 watts.


For the 2s battery lets assume 7.33 V under load and max current of 9A at 20C. The max watts you can get is 7.33 x 9 = 65 watts approx.

So in theory, if you pick the right prop and your motor and ESC can handle 9A of current, then you will get more power from the 2S battery.

All you need to do is select the correct prop that will cause your motor to draw around 9A with the 2S battery.

Be careful though, its not usually a good idea to run batteries (or anything else) at the limit.


Also,if you were to use your other 3s battery rated at 25C you could potentially draw upto 125 watts (again if the motor and ESC can handle it).


I cant get any data on the motor you have so you'll probably have to measure the current being drawn directly. Since we are talking low current here (under 10A) most multi-meters should be able to handle it - its either that or you need to buy a watt meter.
Buck Rogers is offline  
Old 03-27-2009, 10:43 PM
  #8  
Petrik59
Member
Thread Starter
 
Join Date: Sep 2008
Posts: 39
Lightbulb

Thank you all for explaining everything so good to a newbe like me, but I think I finally understand this. It seams to be impossible to say that this is good but that is bad becouse it all depend on what prop to what motor and what battery and speedcontroller.
So I`m going to build myself a simple test bench where I can mount different motors, batteries, speedcontrollers and propps and then by me one of this e-STATION Two-Meter units.
http://www.rcgroups.com/forums/showthread.php?t=888230
Then I am going to do some real testing.

It is going to be "learning by trying".

But as usual, nothing is fully tested until the setup is in use in a plane.
Petrik59 is offline  
Old 03-28-2009, 04:42 PM
  #9  
Dr Kiwi
Super Contributor
 
Join Date: Aug 2005
Posts: 1,726
Default

In case you don't want to re-invent the wheel, here is my test stand set-up: http://www.rcgroups.com/forums/showthread.php?t=645168
Attached Thumbnails Click image for larger version

Name:	Eagle Tree Speed sensor 005 640.jpg
Views:	120
Size:	209.7 KB
ID:	97451   Click image for larger version

Name:	EP Model 2814 1080 640.jpg
Views:	107
Size:	218.1 KB
ID:	97452  
Dr Kiwi is offline  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
madpatter
Warbird Electrics
8
02-08-2009 11:58 PM
Idaho
Power Systems
28
01-19-2009 03:54 AM
ELEMODEL
RC Radios, Transmitters, Receivers, Servos, gyros
2
04-17-2008 02:55 PM
fletchrrr
Beginners
9
01-09-2006 05:38 AM

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Quick Reply: How to chose prop?


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.10501 seconds with 16 queries