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Increase prop pitch - what is the consequence?

#1
I've been enjoying flying my FT Spitfire with a 1000kV and a 8x4 prop. I seems to be going great, apart for a shortage of trust right after launching. Usually I'm able to gain height and build up speed, but yesterday I didn't and "landed" with full throttle, breaking my propeller. Now I only have a 9x6 prop left. How would this work? Will the engine struggle to run such a high pitch?
 

quorneng

Well-known member
#2
maxboo
What motor are you using?
The kV on its own is not an indicator of power only the speed it will run at for a given voltage.
At the same rpm a 9x6 will require as much as 40% more power than an 8x4.
Without knowing the motor details it is not possible to say whether it will be able to handle it or even if your 8x4 was the correct prop to get the maximum from the motor.
 
#3
It came in kit with a 10x45 prop, which seems a little too much.

Motor Specification:
Brand: XXD
Item NO.: XXD A2212
KV:KV1400 , KV2200(optional)
Dimensions: 27.8 x 27mm
Shaft diameter: 3.17mm
Battery: 2-3 cells lithium battery
Efficiency: 80%
Current: 4-10A (efficiency >75%)
No-load Current: 10V/0.4A
Max. Instantaneous Current: 12A/60S
Propeller: 2200KV with 6035 CW propellers,1400KV with 8060 CW propellers,930KV and 1000KV with 1045 CW & CCW propeller.
 

Merv

Well-known member
#4
It came in kit with a 10x45 prop, which seems a little too much.
If the kit came with a 10x45, you will be fine with a 9x6

To be safe, test it. Run it for 10 seconds then stop & feel the temperature of the motor. Warm is OK, but if it is so hot you can hold it, then it’s too much prop. If it passes the 10 second test, try 30 seconds, then 60 seconds.

It always a good practice to test the temperature or the motor, battery & ESC after each flight. On a few of my planes, I need to provide extra cooling when it gets above 95F.
 
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quorneng

Well-known member
#6
The 10 x 4.5 is quite acceptable as far as the motor goes see here and delivers 770 g of thrust which will almost certainly eliminate your initial thrust problem.:D
I doubt if the 8 x 4 gave any more than 500 g thrust. The additional pitch of the bigger prop will actually help getting up to speed as well.
Just be aware that using full power with the bigger prop will drain the battery a bit faster but I expect you will find full power is simply not required all the time so the actual flight duration will be similar but with quite a bit more available as and when you need it.
Have fun.
 
#7
I have played with pitch a lot. My findings are thrust (low end) is by and far more effected by prop diameter. Pitch affects speed more so.

So a 9x6, vs a 8x4 will give you more bottom end thrust, and more speed at the cost of higher battery draw.

I currently fly my HZ Super Cub with a 950 kv Park 450, a 9x7.5 slow fly prop and 1300 mah 3s battery and get 15 min of flight time still coming down at storage level.
 
#9
The 10 x 4.5 is quite acceptable as far as the motor goes see here and delivers 770 g of thrust which will almost certainly eliminate your initial thrust problem.:D
I doubt if the 8 x 4 gave any more than 500 g thrust. The additional pitch of the bigger prop will actually help getting up to speed as well.
Just be aware that using full power with the bigger prop will drain the battery a bit faster but I expect you will find full power is simply not required all the time so the actual flight duration will be similar but with quite a bit more available as and when you need it.
Have fun.
Wow, that's an interesting way of measuring.
Turned out the spare prop was 8x6, not 9x6 as I wrote in the first post. Sorry about that.
I would still think that a lower diameter (going from 10" to 8") will be compensated with increasing the pitch (going from 4,5 to 6)
Is that sort of right?
 

quorneng

Well-known member
#11
maxboo
If as you suggest you are slow 'getting away' then you need more static thrust and than comes from diameter. The 10" will generate a lot more.
Adding pitch does very little for the slow speed thrust but does use more power doing it.
A brushless motor has a maximum 'no load' rpm (kV x battery volts). Any prop has a maximum forward speed that is set by its pitch and rpm. As a plane speeds up the rpm rises. As the rpm approaches the motor no load speed then both the power and thrust fall away to a point where it simply will not go any faster.
Finding the right prop for a particular plane is always a compromise between diameter and pitch. If a 10 x 4.5 was suggested then there is a fair chance somebody has already found it was a 'good' compromise.
If you want to experiment (there is no reason why not) it would be sensible to start with the recommended prop so you can tell if any change you make is actually giving characteristics better suited to what you are after.
 

Tench745

Well-known member
#12
To reiterate and add on to what has already been said:
-Upping pitch will up the max speed the airplane could achieve with 100% efficiency.
It's possible to put so much pitch in that the prop will stall and not develop good thrust until the aircraft gains enough forward motion for the blades to fully bite.
-The only way to increase thrust is to increase the area of the prop disc acting on the air.
There are three ways of increasing prop disc area.
1) Go for a bigger diameter
2) Make the blades bigger, like on a slow-fly prop
3) Add more blades.


Increasing the pitch will require more power from the motor. Likewise; increasing diameter will require more power, more blades require more power, and larger blades require more power.

If you are already drawing at the motor's maximum amperage and you want to increase any of these factors you'll need to decrease another to stay within operating limits. Typically to keep the load on the motor the same when you increase pitch by one you'll need to decrease diameter by one and vice-versa.
HOWEVER, in your case you've been running a prop with less pitch and a smaller diameter than recommended, meaning you haven't been using the full potential of the power system. So you could increase pitch and diameter as necessary up to the recommended prop size adjusted accordingly.
Naturally, you should test everything to make sure you're not going to overheat anything. A watt meter is very helpful here.
 
#13
Thank you very much guys, this is so interesting and I'm learning a lot! (y)
So, I'm starting to understand the combination of engine and prop, but what about the plane and prop? The FT Spitfire is recommended with "8x6 - 9x4.7 Slow Fly" (from build plans) How will an increase in diameter (8" to 10") influence the plane?
 

Hai-Lee

Old and Bold RC PILOT
#14
Thank you very much guys, this is so interesting and I'm learning a lot! (y)
So, I'm starting to understand the combination of engine and prop, but what about the plane and prop? The FT Spitfire is recommended with "8x6 - 9x4.7 Slow Fly" (from build plans) How will an increase in diameter (8" to 10") influence the plane?
Simply put the Pitch is how far the plane wants to move forward per revolution of the prop whereas the diameter is how much power it will use to do so. Greater the pitch the greater the speed BUT the greater the diameter the more air it pushes and therefore the more power the motor needs to supply.

An 8x6 prop would be great for speed assuming that the motor can deliver its maximum power to the 8 inch prop. The 9x4.7 will fly slower but push more air to do so, (the acceleration will be greater but the speed will be lower).

You need to plug into the motor specs and determine what is the maximum prop diameter you can turn for the battery voltage that you are going to use. As a rule of thumb if you increase the diameter you will need to reduce the pitch and of course if you increase the pitch you will need to reduce the prop diameter.

Go back to the motor specifications and then select the prop you wish to use to either fly fast or slower!

Have fun!