UK supplier of 14x8.5 APC-E CW + CCW props

roycruse

roycruse

Member
Anyone know of a UK supplier of 14x8.5 APC-E CW + CCW props

Does anyone know of a supplier of 14 X 8.5 APC-E Normal and Pusher Props - I can find the normal ones but finding a UK supplier of the Pushers is proving tough.

Ive ordered one pair from the US but im too impatient to wait for them to arrive so looking for a UK supplier too.
 
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J

jes1111

Active Member
Do you want these for a multirotor? I hope not - they are way too high on the pitch. A pitch/diameter ratio of 50% is about your maximum, i.e. 14" props should be maximum 7" pitch. Nevertheless, you really want less than that - like 14x4.7 or 14x4.
 



roycruse

roycruse

Member
jes1111, Any chance of filling me in on the science behind that ?

I am buying in some props to do some experimenting with brushless setups both in side by side config as in an Octocopter and above and below config as in an X8

I raised the question on here a few days ago as my theory is that when 2 props are trying to move the same column of air (ie as in a X8 or Y6 config) you need to have much steeper pitch to move the same volume of air as in a side by side situation (as in a straight X or octocopter)

Nobody has been forthcoming with any info so I am planning to do some experiments my self.

If my theory is correct if you take a motor with propeller of certain size and pitch that draws an efficient and powerful ampage through the motor and therefore making the most of its available power when its in a single config - having two identical motors one above the other with the same size props moving the same column of air they will both be under considerably less load than on their own. The result will be an under utilized motor that will draw less amps than is ideal. - the solution (steeper pitched props) If however there is a reason to avoid steeper pitched props on multirotor aircraft then i would love to hear it as i really am starting literally from the ground up.

I was really hoping for a bofin on this forum to confirm or deny my theories - but in the absence of one - i will carry on with my experiments and post my findings on here. (I LOVE DOING EXPERIMENTS)
 

J

jes1111

Active Member
:) Certainly you are right that that you need to consider pitch on coax arrangements. The optimum setup is with a higher pitch prop on the bottom, but nevertheless they should both still be "low pitch" props. According to some boffins in the Power Systems forum on RCG, the difference should be about 20%. So (off the top of my head), a reasonable starting point would be 12x3.8 top and 12x4.5 bottom. The top prop is operating in still air but the bottom one is seeing an already-accelerated airflow and therefore needs higher pitch to get a good "bite" on the air and accelerate it further.

The reason not to go above 50% pitch/diameter ratio is the aerodynamic phenomenon of "blade stall" - high pitch props are designed to achieve very high speeds on fixed wing aircraft but perform poorly at low speed. Multirotors are operating in an effective forward airspeed of zero (hover), so a high pitch prop would stall under such conditions whereas a low pitch prop will be operating in its intended domain (more or less).
 

roycruse

roycruse

Member
thanks. the blade stall thing sounds logical. is there a way of calculating at what rpm it will occur for a given prop.

if i plot a thrust vs ampage graph for a given prop / motor combination i presume i will see a sharp drop in the curve at the stall point?
 

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