How do you select the correct pitch?

[Anthony Billings]

Hi all,

when I get home from this working trip abroad, I am hopeful of starting my Ben Buckle Super 60 4ch build, after reading a post from Tim I intend to use a E Power BL 3526/05 685Kv motor and swing a 14" prop, but what pitch?

I want a really slow flyer, to my mind (maybe wrong) but the slower the flight, the more time I will have to correct if I make a mistake.

Is it a case of buying a number of props of varying pitch, fit them on a test bed with a wattmeter inline and see which gives the best readings? or is there a magic formula that will give you the thrust generated for a prop of given dia and pitch?

As a second question, what would be the best with this motor and 14" prop, a 3S or 4S Lipo? I have a few 3S already for my heli's.

 

Cheers

Tony

[Steve W-O]

You can't go too fine to slow it up, rather reduce the throttle!

 

There are magic formulas, quite accurate, but you need a bit more info.

 

A coarser pitch will give longer duration, as the throttle will be lower for the same airspeed.

 

That is assuming everything else is correct. ie correct choice for weight, stall speed, intended flight.

 

 

Also, if you want 3D it will be a different choice, you need a fine pitch to give the thrust.

 

But both must operate in the best efficiency range of the motor .

 

 

Best to look at other setups using the same motor, roughly the same plane, and the same type of flying

Edited By Steve W-O on 05/01/2012 18:41:07

[Seamus O'Leprosy]

And if you are planning to try more electric models buy yourself a Watt Meter this will help tremendously with setting up prop, motor, esc, combos.

[Frank Skilbeck]

Tony

 

download this you can then play with number of cells and prop size and it will calculate, speed, amps and thrust etc needs to be backed up with a watt meter though just to be certain but it's a good starting point.

 

I have a 700kv motor on a 4s battery and it gives 700w on a 13 x 6 so I think you'd need be better with a 3s for a 14" prop.

[Anthony Billings]

Hi all,

thanks for the replies, the wattmeter will be ordered when I get home.

Frank, had a quick look at that calc, and it looks just the job, will download it properly when I get home as the company is a bit funny when it comes to downloading stuff on the pc's.

Starting to get my head around the electric setup choices, as all my heli's came either rtf or with specific guidelines on motor, esc, lipo and rotor blade length, and my two other planes are rtf.

 

Cheers

Tony

[Reno Racer]

You will be amazed what a different pitch makes to Amps drawn.

 

When looking for the optimal prop for my 91 FS powered Corsair, I started a database of measurements for static thrust, thrust and airspeed for each type of prop at ste RPM intervals within the power range (8000-1000rpm roughly). Amazingly the difference in airspeed between a 14 x6 and 14 x8 was 20mph.

 

The electric database on this forum and the database on 4-Max's website give good indicators for similar motors. For pitch, you would probably be looking at a 4-6". AS far as prop diameter, it really depends on number of lipo cells. For that low KV or motor, you would be looking at quite a big prop on 3 or even 4S. Normally that low a Kv motor would be running on a 5 or 6S Lipo on a 14" prop.

Edited By Christian Ackroyd on 05/01/2012 22:18:30

[Reno Racer]

Here you go,

 

First recommended set up is a BB 60.

 

More detail here.

 

Regards

 

Chris 

 

Edited By Christian Ackroyd on 05/01/2012 22:21:35

[Ben B]

Pop all the information into

 

this

 

and see what your pitch speed is. Look at the plane. See if it sounds sensible.

[Anthony Billings]

Once again thanks to you all for pointing me in the right direction.

Ben, another good calc that I will download when I get home.

Chris, that set up sounds perfect, a 20 min flight on half throttle without anything getting hot and the guy managed to fit a bigger capacity lipo in so I will probably go with his set up.

 

Cheers to all, now itching to get home and start this build.

Tony

[Olly P]

That's a useful program, will have to instal when i get home....

[Martyn K]

Posted by Steve W-O on 05/01/2012 18:34:49:

<snip>

A coarser pitch will give longer duration, as the throttle will be lower for the same airspeed.
</snip>

I am not actually sure that is true. I would have thought that a propellor will work most efficiently (ie require the least amount of input power) when it has the least possible load for the task demanded of it. Getting the best match for performance/input power is complex and a full understanding of the local operational environment is required. Coarse pitched propellors work best at high speed - not necessarily high rpm. Try getting a Spitfire into the air with the propellor pitch set to coarse (see Douglas Bader - Reach for the Sky). It appears to me that a fine pitch propellor will work best with least load at lower speeds. (RPM as well as airspeed)

 

My 2.5p worth anyway.

 

M

[Swissflyer]

On this link you will find a calculator that will help you answer the pitch speed question rather well. The author is an advocate of larger props & lower kV motors in line with Tim's suggestions.

http://flbeagle.rchomepage.com/software/webocalc_1.05/screenshots/howto.html

There is a complete privately hosted thread on using the tool here. http://www.rcgroups.com/forums/showthread.php?t=1136470

Since you have already a motor in mind you may need the fine tuning tweaks suggested on the thread.

This tool has has helped me to get the 95% solution on more models than I can remember. Careful test flying does the rest, but at least the model is flying safely to start with!

Frank’s advice on Drivecalc is good, it is a great motor prop evaluation tool.

Happy flying

Mark

[Steve W-O]

Posted by Martyn K on 06/01/2012 13:49:05:

Posted by Steve W-O on 05/01/2012 18:34:49:

<snip>

A coarser pitch will give longer duration, as the throttle will be lower for the same airspeed.
</snip>

I am not actually sure that is true. I would have thought that a propellor will work most efficiently (ie require the least amount of input power) when it has the least possible load for the task demanded of it. Getting the best match for performance/input power is complex and a full understanding of the local operational environment is required. Coarse pitched propellors work best at high speed - not necessarily high rpm. Try getting a Spitfire into the air with the propellor pitch set to coarse (see Douglas Bader - Reach for the Sky). It appears to me that a fine pitch propellor will work best with least load at lower speeds. (RPM as well as airspeed)

 

My 2.5p worth anyway.

 

M

 

 

I did say that assuming all other things correct!

 

On a model, prop stalling isn't the same factor as it is on full size, the prop can be effectively stalled at low speed, and if it is a trainer type plane, it won't make much difference, as it will soon be above the prop stall speed on take off, and never be below it in flight. As I said, a 3D plane would be different.

 

My point was made in response to the OPs idea of having a fine pitch prop to have a lower flying speed, when moving the throttle back would also lower the flying speed, but making the pitch finer to achieve that would result in shorter flight times.

 

I suggested he look at other similar planes and their setups, and he has been given examples of that, which are the best advice.

 

The efficiency of the prop is dependent on several factors, but the flight time is mainly dependent on the motor operating within its most efficient parameters, and that would not normally be with a prop that has been selected to give a lower airspeed.

 

The pitch of the prop should be selected to give the correct pitch speed at the rpm of the selected motor(the correct pitch speed would be determined from the specs of the plane), the diameter of the prop should efficiently load that motor, and the combination should have the desired thrust to fly the plane in the manner intended.

 

There is too little info given to suggest any particular setup accurately, apart fr5om to refer to other successful setups, or to suggest a prop suitable for the stated motor.