Understanding propeller size

Selecting the correct propeller size for your gas powered rc airplane is very important, if you want to get the optimum performance from your glow engine. The subject of prop selection can be a bit of a minefield, but there are some common recommendations of prop size for each and every engine.

The first and foremost plan of attack is to follow the airplane manufacturer's recommendations for the plane (and engine) that you have. Failing that, the chart further down the page should help you out.

Understanding propellers

You'd be forgiven for just seeing your airplane's prop as the thing at the front of the plane that spins very fast, but understanding a bit about how it works is no bad thing.

Propellers for rc airplanes are nothing more than vertically mounted rotating wings. Their job is to convert the engine power in to thrust, to pull/push the plane through the air. Thrust is generated in exactly the same way as lift is by the wing, and that's why props have a profiled airfoil section.

Propellers are designated two measurements, both given in inches...

Propeller size shown on a prop

The first number is the diameter of the imaginary disc ('arc') created by the spinning prop ie the length of prop from tip to tip. The second number is thepitch, and this is the harder of the two to understand - but we'll give it a go...

The pitch measurement of a prop indicates how far that propeller will move through the air per every single revolution of the engine (ie every complete turn of the prop). However, the pitch measurement of your prop must only be taken as a guideline because real-life factors come in to play to influence this distance eg the material of the prop, its condition, air density etc.etc... 
So, pitch measurement is really only a theoretical value but is good enough to let you choose the right size propeller for your airplane.

Essentially, the higher the pitch, the faster your plane will go. One way to see it is to imagine the gauge of two different screw threads, coarse and fine, and picture both screws turning at the same speed. The coarse thread will cut in to the material a lot faster than the fine one will. It's the same for propellers.

In the illustration below, the two arrow lines represent the path of each propeller tip. You can see that the higher pitch prop (eg?x8) takes only one and a half turns to cover the same distance that the lower pitch prop (eg?x4) takes 3 turns to. So, conversely, with both engines and props spinning at identical RPM, the higher pitch prop will travel further in the same amount of time. 
Again, remember that this is only true for a theoretically perfect world!

Propeller size and pitch

So, you can see that selecting a different pitch prop is going to significantly change your airplane's performance, speed being the biggest factor.

The diameter of the propeller (10" in the example above) will also effect how the airplane flies, but also how the engine runs and, again, following your airplane manufacturer's recommendations is the place to start. Roughly speaking, diameter influences the amount of torque generated, but an ever-increasingly and non-performance related issue these days, linked to prop diameter, is that of noise.

A faster turning prop (and most props spin in excess of 10,000 RPM) generates a lot of noise as the tips cut through the air. In fact, when you hear a gas rc airplane flying, it's more than likely the propeller that you're hearing more than the engine! 
So it's important to note that a larger diameter prop reduces the engine's RPM at any given power setting, because there is more for the engine to turn over and hence more work to do. And slower turning props generate less noise - therefore, larger diameter props run quieter than smaller diameter props, all else being equal.

Prop size recommendations

As already mentioned, following the prop size recommendations made by your airplane's/engine's manufacturer should always be your first point of reference. But there are generally recognized prop size ranges for each engine please refer to chart below

RC Prop Charts

Prop Chart For Two - Stroke Engines

 

 

 

Alternate Propellors

Starting Prop

Engine Size

5.25x4, 5.5x4, 6x3.5, 6x4, 7x3

6x3

.049

7x3,7x4.5,7x5

7x4

.09

8x5,8x6,9x4

8x4

.15

8x5,8x6,9x5

9x4

.19 - .25

9x7,9.5x6,10x5

9x6

.20 - .30

9x7,10x5,11x4

10x6

.35 - .36

9x8, 11x5

10x6

.40

10x6,11x5,11x6,12x4

10x7

.45

10x8,11x7,12x4,12x5

11x6

.50

11x7.5, 11x7.75, 11x8,12x6

11x7

.60 - .61

11x8,12x8,13x6,14x4

12x6

.70

12x8,14x4,14x5

13x6

.78 - .80

13x8,15x6,16x5

14x6

.90 - .91

15x8,18x5

16x6

1.08

16x10,18x5,18x6

16x8

1.20

18x8,20x6

18x6

1.50

18x10,20x6,20x8,22x6

18x8

1.80

18x10,20x6,20x10,22x6

20x8

2.00

 

 

 

Prop Chart For Four - Stroke Engines

 

 

 

Alternate Propellers

Starting Prop

Engine Size

9x5,10x5

9x6

.20 - .21

10x6,10x7,11x4,11x5.11x7,11x7.5,12x4,12x5

11x6

.40

10x6,10x7,10x8,11x7,11x7.5,12x4,12x5,12x6

11x6

.45 - .48

11x7.5,11x7.75,11x8,12x8,13x5,13x6,14x5,14x6

12x6

.60 - .65

12x8,13x8,14x4,14x6

13x6

.80

13x6,14x8,15x6,16x6

14x6

.90

14x8,15x6,15x8,16x8,17x6,18x5,18x6

16x6

1.20

15x6,15x8,16x8,18x6,18x8,20x6

18x6

1.60

18x12,20x8,20x10

18x10

2.40

18x10,18x12,20x10

20x8

2.70

18x12,20x10

20x10

3.00


 

 

 

 

 

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