Does prop diameter affect speed and how does prop pitch work?

[GreaterAviation]

Does prop diameter affect speed and how does prop pitch work? I know that pitch is how far the prop moves in one rotation so in a flight does the prop just drag the plane? Or is pitch the speed of the thrust that the prop diameter creates? Is prop diameter like torque in a car motor and pitch wheel diameter?

 

[Paracodespoder]

Larger diameter at the same pitch and rpm will in theory have the same speed, but greater thrust. Increased pitch will increase speed.

 

[GreaterAviation]

Larger diameter at the same pitch and rpm will in theory have the same speed, but greater thrust. Increased pitch will increase speed.

What is the difference between thrust and speed? Does thrust affect speed though?

 

[Ketchup]

What is the difference between thrust and speed? Does thrust affect speed though?

Thrust is like the amount of weight that the prop can pull. Speed is how fast the prop pushes the air. If you have more thrust you would have better acceleration, and possibly a little more speed. One example is when I switched from a 9x4.7 to a 10x4.7 prop on one of my planes. Everything was the same except for diameter, but there was a definite increase in speed and performance. If you want to design a plane for only speed though, you would want higher pitch.

 

[Ketchup]

Also, I wanted to say that you would probably only get a speed increase because there is more force counteracting the drag. It's not like the air is actually going faster or anything.

 

[GreaterAviation]

Alright thank you!

 

[quorneng]

GreaterAviation
In principle the plane will speed up until the drag of it being pulled through the air exactly matches the thrust generated by the prop.
The problem is without some very sophisticated wind tunnel equipment you have no way of measuring the planes drag or the actual thrust of the prop at speed. All you know for certain is that at the planes maximum speed they are equal. Even accurately measuring a planes speed is not that easy.
You can measure the static thrust (the pull) when the plane is stationary easily enough but unfortunately the thrust varies depending on the speed the prop is travelling through the air so static thrust is only give a rough guide. If you can achieve a static thrust equl tot he planes weight it is likely to be able to fly but it may be a long way form achieving the best performance.
From this you can see there are many variables at work. There is no simple 'black and white' answer.
Ultimately it is the power that the motor can produce and its RPM that dictates the maximum product of prop diameter and pitch.
If you are using a proven plane design there will be a recommended motor, prop and battery recommendation. This will have been tested by the designer to give the best overall performance, in his opinion.
By your own trial and error testing you may be able to improve a particular characteristic, like top speed, but probably only by a relatively small margin but in doing so something else may be degraded.

I hope this helps answers a some questions but 'plane performance' is a very complex topic.

 

[Hondo76251]

The motor you are using is a big factor in selecting a prop. It only has so much power.

As was mentioned most motors have a recommended prop. A general, but by no means scientific way, of playing with prop sizes is to start with the standard recommended prop size and go from there. If you want to go bigger, lower the pitch. IE: you have a 9 x 6, to go bigger look for a roughly 10 x 5 prop and vice versa. If you want to go to a 3 blade either lower the pitch or the dia. IE: You have a 10 x 5 two blade, look for a 9 x 5 or a 10 x 4 three blade. This is of course a generalization but it gives you a place to start.

If you want to go faster increase pitch, if you want more low speed power go with a longer but lower pitched prop...

 

[TEAJR66]

A watt meter is an invaluable tool in these endeavors.

As you accumulate the basics and have bench stock of common parts, start to expand your tool repertoire.

Watt meter
Prop reamer
Servo tester

Just a few that can be useful early on.

 

[Merv]

Does prop diameter affect speed

A larger diameter prop will produce more speed ONLY if the first prop was under sized, that is, not fully loading the motor. The motor was capable of producing more power but the prop was not large enough to draw the power out of the motor.

how does prop pitch work?

It takes the same amount of power to move a large amount of air slowly OR a small amount of air faster.
Prop diameter is area, Pi x R^2. Prop pitch is air speed. Bonus, higher rpm’s (Kv) also increases the air speed.

If you have a prop that full loads a motor and you want to increase the pitch, you must decrease the diameter to prevent overloading the motor. If you can increase the pitch without decreasing the diameter, the first prop did not fully load the motor.

Take 2 props that both pull the same amps. One prop is a large diameter shallow pitch, the other smaller diameter high pitch. On a test stand the large diameter prop will always produce more thrust. Say the air speed it produces is 30 mph, the top speed of a plane will be something less, maybe 25 mph. The small diameter prop will produce less thrust on the test stand but will produce a higher air speed, say 60 mph, flying a plane maybe 50 mph.

Think about a transmission. In low gear you can move a heavy load but the top speed will be slow. In high gear you have much less lugging power but a higher top speed.

If you want to do 3D, hang a plane from the prop, use a large diameter shallow pitch prop. If you want to fly fast, use a smaller diameter high pitch prop.