Props and Current

[Nigel Heather]

When looking at prop diameter and pitch independently is it a valid generalisation to say

 

Reducing diameter >> reduced current

Reducing pitch >> reduced current

 

My thinking, is that in each case the volume of air that the prop cuts though in a revolution is reduced.

 

Also, I can imagine how reducing the pitch reduces the speed but what characteristic is related to the diameter.

 

Cheers,

 

Nigel

Edited March 7, 2023 by Nigel Heather

[Dickw]

In general. yes.

Dick

[Frank Skilbeck]

Reducing the prop diameter will reduce the thrust, due to the reduced area of air being moved.

 

As a rough rule of thumb, if you increase the pitch the power required will go up by the same percentage, i.e. 5 to 6" pitch will be roughly 20% more power, but diameter is the square, so going from a 9 to a 10" diameter will be 100/81, so roughly 20%.

[GrumpyGnome]

If you reduce the pitch, you increase the acceleration of the airframe but lower top speed, and reduce current.

 

If you reduce the diameter, you reduce the current and potential top speed, as well as reducing acceleration.

 

Don't ask about number of blades 🙂

[Don Fry]

4 hours ago, Frank Skilbeck said:

Reducing the prop diameter will reduce the thrust, due to the reduced area of air being moved.

 

As a rough rule of thumb, if you increase the pitch the power required will go up by the same percentage, i.e. 5 to 6" pitch will be roughly 20% more power, but diameter is the square, so going from a 9 to a 10" diameter will be 100/81, so roughly 20%.

Frank, thank you, I have often wondered why increasing diameter has such an obvious effect.

[Geoff S]

Some years ago I calculated the nominal volume of air moved by propellers/rev as a means of comparing the loads they presented (in my case to electric motors but presumably the same applies to liquid fuelled engines).  ( pi 0.5 dia ^2 x pitch) Obviously it's quite crude as it doesn't take account of the propeller design but I find it helps. 

 

Low pitch propellers are really like low gears in a car or motorcycle.  If you want performance at low speed as in (say) funfly type model then low pitch is the way to go (eg I used 11x3 props with ST34 engine on my Limbo dancer).  Most of my models are low speed types like 1930s style biplanes and I tend to use a lot of 12x6 or 13x6.5. 

Edited March 7, 2023 by Geoff S

[Allan Bennett]

Getting the right pitch and diameter for an electric model is quite an art -- or more likely, trial and error.  With one of my models I achieved the motor's rated amps using an 11 x 7 prop, but take-off was a bit leisurely.  Changing to 12 x 6 achieved roughly the same amps but a much more rapid take-off, and lower theoretical top speed was not an issue.

[Nigel Heather]

12 hours ago, GrumpyGnome said:

If you reduce the pitch, you increase the acceleration of the airframe but lower top speed, and reduce current.

 

If you reduce the diameter, you reduce the current and potential top speed, as well as reducing acceleration.

 

Don't ask about number of blades 🙂

 

That is really useful.  Out of interest why does reducing the diameter reduce the top speed?

[Peter Wedlake]

I use Ecalc a lot to calculate motor loads with different props, takes out the guess work when trying to find the best prop size required. Its not free but you can try out the program with reduced capability for free.

 

Not only does it do prop sizing but so many other things as well, C of G, performance calc even calculations for EV charging. 

https://www.ecalc.ch/index.htm

 

[Nigel Heather]

11 hours ago, Geoff S said:

Some years ago I calculated the nominal volume of air moved by propellers/rev as a means of comparing the loads they presented (in my case to electric motors but presumably the same applies to liquid fuelled engines).  ( pi 0.5 dia ^2 x pitch) Obviously it's quite crude as it doesn't take account of the propeller design but I find it helps. 

 

Low pitch propellers are really like low gears in a car or motorcycle.  If you want performance at low speed as in (say) funfly type model then low pitch is the way to go (eg I used 11x3 props with ST34 engine on my Limbo dancer).  Most of my models are low speed types like 1930s style biplanes and I tend to use a lot of 12x6 or 13x6.5. 

 

I understand the calculation for the volume of air in a revolution but what does that equate to

 

Does volume of air relate to thrust?

 

Does volume of air to work the motor has to do and therefore, amps.

[GrumpyGnome]

29 minutes ago, Nigel Heather said:

 

That is really useful.  Out of interest why does reducing the diameter reduce the top speed?

It's always a trade off, and a balancing act, but simplistically (and assuming no motor limitations etc.) a 10x6 propeller would be slower than an 11x6, simply due to less air being moved.  

[Nigel Heather]

1 hour ago, GrumpyGnome said:

It's always a trade off, and a balancing act, but simplistically (and assuming no motor limitations etc.) a 10x6 propeller would be slower than an 11x6, simply due to less air being moved.  

But surely a prop with a 6" prop moves the plane 6" for each revolution regardless of diameter.

 

So for the same RPM they should fly at the same speed.

 

BUT I would have though that as the prop diameter is increased the motor might struggle to achieve the RPM so would be slower.

[Trevor]

Reducing the prop diameter will, as has been said, substantially reduce the load on the motor and hence reduce the power generated. As a result, thrust and drag will equalise at a lower speed.

[GrumpyGnome]

As I said, ignoring motor limitations...

 

Yes, theoretically, the propeller will move 6" if it's 6" diameter. But air isn't solid 🙂

 

My comments are from real life, rather than theory I'm afraid.

[Dickw]

Airframe drag increases as the speed increases, so a fast plane needs more thrust than a similar plane flying slower.

Thrust is produced by the volume of air being moved by the prop (related to diameter and pitch) and also the speed at which it is being moved (related to pitch and rpm).

As GG said, “air isn’t solid” so if the thrust being produced by your prop is not equal to the thrust required by your plane at ‘pitch speed’ (prop pitch x rpm) then the prop will “slip” i.e. it will not move forward as much as the pitch and will be operating less efficiently.

 

Reducing diameter or pitch will reduce the load on the motor, so it will also speed up a bit as well as the current dropping.

 

It is all one big balancing act. Experience and performance calculating software are both very useful.

Dick