Propellers are planned as an essential tool for powering our blimp propulsion. A good understanding of the science behind those will make a difference between making the project’s numbers black or red. Blimp’s cruising speed needs to be as efficient as possible, while they also need to be powerful enough to provide necessary lift off power for emergency situations.

So let’s ask the Wikipedia first. Wikipedia, what actually is a propeller?

A propeller is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral, that, when rotated, performs an action which is similar to Archimedes’ screw. It transforms rotational power into linear thrust by acting upon a working fluid, such as water or air.

Wikipedia

Even Wright brother’s first air-plane had one!

Now, reading through all that Wiki, you’ll reach the part describing the propeller geometry characteristics:

- Pitch ratio
*PR*= propeller pitch/propeller diameter, or P/D - Disk area A
_{0}= πD^{2}/4 - Expanded area ratio = A
_{E}/A_{0}, where expanded area A_{E}= Expanded area of all blades outside of the hub. - Developed area ratio = A
_{D}/A_{0}, where developed area A_{D}= Developed area of all blades outside of the hub - Projected area ratio = A
_{P}/A_{0}, where projected area A_{P}= Projected area of all blades outside of the hub - Mean width ratio = (Area of one blade outside the hub/length of the blade outside the hub)/Diameter
- Blade width ratio = Maximum width of a blade/Diameter
- Blade thickness fraction = Thickness of a blade produced to shaft axis/Diameter

… all that reads very educated and is for me quite difficult to see which value will have an impact on our performance and how much it actually matter as there is a too many variables I can’t predict. So I took it a bit step sideways and prepared a tiny science project. Let’s use our code from OpenScad and generate some random-ish propellers and test how they operate in our intended EDF scenario!

I took our already-existing propeller code:

```
module edf_prop(leafs, twist, rot){
//Nose
Tx(-2)
Ry(90)
difference() {
Tx(0){
scale([1,1,1.3])
sphere(1.55);
cylinder(h = 2, r1 = 1.55, r2 = 1.55, center = false);
}
Tz(0.5)
cylinder(h = 2, r1 = 1.45, r2 = 1.45, center = false);
Tz(-1.2)
edf_lead_in_hole();
scale([1,1,0.4])
Tz(1.3)
sphere(1.45);
vent_holes();
Tz(-0.2)
cylinder(h = 0.3, r1 = 0.45, r2 = 0.45, center = false);
}
//Inner tube Wraps
nw = 12;
for(w=[0:360/nw:359])
Rx(w) {
T(-2,1.45,0)
Ry(90)
cylinder(h = 2, r1 = 0.07, r2 = 0.07, center = false);
}
*Tx(-0.1)
Ry(90)
TubeSimple(0.1, 1.5, 1.5, .1);
//Propeller
Tx(-1) {
difference () {
for(w=[0:360/leafs:359])
Rx(w)
proprep(tip_scale=1.48, L = 1.5, r = 4, twist = twist, rot = rot, naca = 1408);
//delimiter
Tx(-1)
Ry(90) {
TubeSimple(3, 4.4, 4.4, 0.6);
cylinder(h = 2, r1 = 1.5, r2 = 1.45, center = false);
}
}
}
}
```

And generated 14 scenarios:

```
T(0,-4,-4)
prop_to_print("1", "8/-30/-20", leafs = 8, twist = -30, rot = -20);
T(0,-4,-12)
prop_to_print("2", "12/-30/-20", leafs = 12, twist = -30, rot = -20);
T(0,-4,-20)
prop_to_print("3", "16/-30/-20", leafs = 16, twist = -30, rot = -20);
T(0,6,-4)
prop_to_print("4", "8/-40/-20", leafs = 8, twist = -40, rot = -20);
T(0,6,-12)
prop_to_print("5", "12/-40/-20", leafs = 12, twist = -40, rot = -20);
T(0,6,-20)
prop_to_print("6", "16/-40/-20", leafs = 16, twist = -40, rot = -20);
T(0,16,-4)
prop_to_print("7", "8/-50/-20", leafs = 8, twist = -50, rot = -20);
T(0,16,-12)
prop_to_print("8", "12/-50/-20", leafs = 12, twist = -50, rot = -20);
T(0,16,-20)
prop_to_print("9", "16/-50/-20", leafs = 16, twist = -50, rot = -20);
T(0,26,-4)
prop_to_print("10", "8/-60/-20", leafs = 8, twist = -60, rot = -20);
T(0,26,-12)
prop_to_print("11", "12/-60/-20", leafs = 12, twist = -60, rot = -20);
T(0,26,-20)
prop_to_print("12", "8/-50/-10", leafs = 8, twist = -50, rot = -10);
T(0,36,-4)
prop_to_print("13", "12/-50/-10", leafs = 12, twist = -50, rot = -10);
T(0,36,-12)
prop_to_print("14", "16/-50/-10", leafs = 16, twist = -50, rot = -10);
```

Code above generates 3D models like this:

Separated per picture, you can list through the gallery below. Each model has its ID, number of blades, blade angle and twist angle value.

Rendering and transforming it into STL model and consequently into a 3D printer code (GCODE) took a while, while actual printing took … days, but result was worth it!

Next stage is to collect some data on how each propeller behaves. We’ve designed a see-saw test-bed.

I prepared following table for kids to have some fun.

# | 10V/5A | 15V/5A | 17V/5A | 10V/10A | 15V/10A | 17V/10A | 17V/Max A |

1 | N/A | N/A | N/A | N/A | N/A | N/A | N/A |

2 | 1200 | 1650 | 1800 | N/A (4.40A max) | 2480 (8.8A max) | 3100 | 3070 |

3 | 1380 | 1600 | 1780 | N/A (6.33A max) | 2900 | 3100 | 3300 |

4 | 1360 (4.79A max) | 1720 | 1690 | 1360 (4.79A max) | 2300 (8.89A max) | 3300 | 3450 |

5 | 1300 (4.61A max) | 1700 | 1900 | 1300 (4.61A max) | 2700 (8.31A max) | 3300 | 3400 |

6 | 1150 (4.23A max) | 1550 | 1670 | 1150 (4.23A max) | 2250 (8.25 max) | 2710 (4.5A max) | 2710 (4.5A max) |

7 | 1200 (3.96A max) | 1900 | 2000 | 1200 (3.96A max) | 2480 (3.96A max) | 3050 (8.36A max) | 3050 |

8 | N/A | N/A | N/A | N/A | N/A | N/A | N/A |

9 | N/A | N/A | N/A | N/A | N/A | N/A | N/A |

10 | N/A | N/A | N/A | N/A | N/A | N/A | N/A |

11 | N/A | N/A | N/A | N/A | N/A | N/A | N/A |

12 | 1410 | 1730 | 1820 | 1420 (5.3A max) | 2920 (9.6A max) | 3240 | 3100 |

13 | 1400 | 1780 | 1890 | 1580 (5.8A max) | 3000 | 3380 | 3300 |

14 | 1370 | 1670 | 1800 | 1400 (5.1A max) | 2600 (9.22A max) | 2900 | 3000 |

X1 | N/A | N/A | N/A | N/A | N/A | N/A | 2200 |

.. it sort of took weeks to get this done, but we’ve finished it! 🙂 As you can see there, prints 1,8,9,10,11 didn’t print properly, so they were not test. Still I think it all gave us some indication on what we are dealing with.

Having a fun, we’ve also added the X1 model – which is just a simple 2-blade propeller.

Putting it all in a graph – turbines 3,4,5 and 13 are clearly standing out.

Long story short – our winner is number 4! 8 blades, 40′ blade twist and 20′ blade rotation.

Oh, and I almost forgot – we had a bit of fun while doing that as well. Sebi asked what would it be like to throw in a paper napkin … 😉

Very interesting. Would be even better if I understood it all, but my conclusion is that if I need a new propeller for a boat, I will call you! Cheers, Serge

……………………………….. -Sent by “either” Robin or Serge. -Mail may be from; acrohc@me.com, acrohc@iCloud.com or acrohc@yahoo.com They are all valid. -Also, any bad grammar or spelling is due to self correcting!!!

>

Remember that a prop sucks in air, which means the air going past the blades is the sum of the vehicle speed, plus the incoming air speed, and that the angle of the blades needs to be sensible relative to that flow throughout the blade span. My free Fusion360 add-in “Airfoil Tools” has all the math to get that right (plus auto-selecting the correct blade profiles as well): See http://www.foils.pro

s/incoming/net accelerated/

All your props are actually the same, and you’ve got 2 mistakes in your data (and your chart makes no sense – you got the axes mixed up and the data doesn’t match the scale). Here is a fixed chart of your experiments: https://chrisdrake.com/ductchart.png

* not sure what your numbers are eg “1200” and “3050” etc – I assumed grams of thrust on some scale, but that’s maybe not the case?

So I just tested my Airfoil-Tools version (first print of https://a360.co/3Hynpn9 ) – it’s getting 340grams at 8.23 amps @ 17volts (and doing 14400rpm) on my test rig https://a360.co/3Sihibz – I bought your exact motor from aliexpress for testing 🙂

My scale has an extra digit after the decimal place, so it said 340.0 on the screen, which makes me think that your mystery numbers like “3300” are the reading from your scales, forgetting to copy the decimal point down?

So I think that’s a pretty big “win” for my maths? I got 10grams more thrust than your 10amp prop, using only 8.23amps, which is exactly 25% performance improvement.

My math assumed a target thrust of 2.45newtons, but the prop was actually pushing 3.33newtons – 36% more than design – so the pitch of all my fan blades in my test was wrong by that amount (which was 6.6 degrees at the tip).

Fun fact – if you forget to bolt your fan onto your motor, it stays tightly attached until 12000RPM, at which point it comes screaming off the motor and puts a pretty circular pattern of little holes in your leg 🙂