# How to design propeller, with respect to its lifting(Thrust) Capabilities?

what is the best Starting point, Software not recommended.

I love to get the information regarding the propeller design and the weight to lift ratio calculation. And also the engine types which is already in use.

### 6 Answers

Hi Sourbh:

Victor is right, your question is not simple.

From the link provided by Victor, the essentials of the Blade Element Momentum Theory can be got. This theory allows for the calculation of a propeller which properties are known. Or for the calculation of a set of propellers, which parameters change through a preset interval(Parametric design). Anyway those calculations are a bit too theoretical and do not have all effects into account, but they allow for approaching the final propeller design.

In the case of Naval propellers, the way to go is starting from the boat total Drag and looking for a propeller which thust could overcome that drag and able to be moved with the less torque, avoiding cavitation effects at the same time.

What is the same: Suitable thrust + less energy-fuel wasting.

The practical procedure is calculating some parameters and entering statistical datas for known propellers series, obtained from real tests in hydrodynamic channels.

Check Wagenningen and Gawn-Burril series.

Once a prop is choosen all dimensions and shapes can be obtained from the same data sets.

Regards

Finally after a long time, i got someone after our dear victor who understand this question.

I really loved to see your answer, I studied the lots of model approaching the Air-foil design and the propeller's lifting capabilities. But unable to got exact thing as you said.

It's totally practical to get the actual thrust and the speed of lifting with respect to it's RPM and size.

Is you having some theory paper to make my vision clear on this??

If yes than it will great for me and also for the enthusiast of Aerodynamics.

In Heli, and the similar flyer need the propeller to make impact in all direction but in Ship needs it for two movements only. I got lots of about the Air-foil but i am still unable to get the heli thruster.

Regards.

Hi,

I am having good knowledge in fluid dynamics. But its enough to understand the working criteria of propeller.

I am still studying more about the Aero-dynamics shape and CFD analysis.

Will you elaborate little more enough about the CFD analysis.

And about the thing which i wanna build is almost completed, I will send the detail file CAD through mail.

Just simply stuck on the propeller design, and with your answer i am totally cleared my remain doubts.

Your words are helpful for me.

Like this:

1- The more propeller diameter-the less pitch, the better performance.

2- The less Area ratio Ae/Ao the better.

if R is the propeller radius: Ae/ao= Propeller_area/PI R²)

3- The less number of blades the better.

4- The less RPM the better.

Thanks and regards to get more about CFD Analysis.

Answered with a tutorial: https://grabcad.com/tutorials/how-to-design-model-of-propeller-with-respect-to-its-lifting-capabilities-in-solid-works--1

Glad to be helpfull Sourbh:

Nice drawing ¡¡ I see you know what you are speaking about :-)

A few more things:

In order to be able for comparing different propellers, a parameter "J" is defined, and named "Advance coefficient".

Its value is: J= Va/(N D^2).

Where Va is the advance velocity (Real flow velocity through the blade disk).

As a very rough first approach, this speed can be taken as:

Va= RPS * 2 * PI * R * sin(pitch)cos(pitch)*(1-a).

Where "a"(induced velocity factor) can be taken between 0.1 and 0.2 and RPS stands for Revolutions Per Second.

Tunnels or CFD allow for a more accurate computation of "a" by making a comparison between predicted and obtained values of Va.

As pitch changes along the blade, pitch and radius are taken, in the general practice, at 70% of the blade radius.

With Kt,Kq and J, the dynamic Efficiency of the propeller comes from:

Eff= J * Kt/(2 PI Kq)

The practical work goes for looking for the highest Eff.

A very important parameter is the propeller linear speed at the blades sections. If that speed that comes from Vs= SQRT(Va^2 + (2 RPS PI R)^2), results to be supersonic, the lifting capacity of the blade from that section to the tip, is almost lost. At the same time, a shock wave developes along the involved blade lenght, that will eventually seriously damage the whole device due to vibrations. This effect limits RPM and blade radius.

The problem could be solved by a drastical modification of the blade profiles and plan shape, but it results a lot complex.

Im working on a blade design software, more intended for ships propellers(My work in real life) than for choppers or planes. Anyway it is able for making reasonable predictions.

I can send you a copy. In return, I would like to have your feed back with your results, in order to refine the program.

As I told you above, our general practice is using real propellers test parameters for the design proccess, unless the case of very special propellers that require a whole calculation from scratch.

Im sure there must be equivalent test data for aero props. Look for those datas which are quite useful for the preliminary design stages. Even with full scale datas, the scale factors can be calculated for addapting the device to the required size.

Dont forget that propeller will be coupled to a motor. Hence, both and the "coupled assambly" efficiencies must match as close to the best point as possible.

Regards.

PD:

Here you have a good Excel file by Michael Duffy for axial rotors calculation.

Hi Soubh:

Im not sure if I understand what you whant to mean with "make inpact". I think it's related to the thrust force direction.

A propeller is a propeller Soubh. On some ships, the thrust axis(direction) can be also changed, as it is on a helicopter.

Do you have any knowledge about fluid dynamics ? If yes I could try to explain you how to approach the analysis. If not, I think it could take us quite a long time.

Anyway. There are many approaches to this subject. Here is only one of them.

Lets suppose the case of a single propeller. The required propeller has to be able to overcome the device weight, and to make it move along the three directions(X,Y,Z). Everything is achieved by tilting the propeller rotation plane.

For horizontal translation(X,Y) at the "design speed" the horizontal component of the tilted propeller thrust, must be able to overcome the drag force.

For vertical movement(Z), the vertical component of the tilted propeller thrust must overcome the device weight plus its drag force at the vertical desired speed.

In the pic: Thrust is the total thrust developed by the prop. Tz is the vertical component and T(x,y) the horizontal one.

Hence: Weight, drag, tilting limits of the rotation plane and design speeds must be stated for the first approach.

Once the required thrust is known a few things must be taken into account.

Thrust is achieved by means of RPM, Diameter, surface, shape and pitch of the propeller.

1- The more propeller diameter-the less pitch, the better performance.

2- The less Area ratio Ae/Ao the better.

if R is the propeller radius: Ae/ao= Propeller_area/PI R²)

3- The less number of blades the better.

4- The less RPM the better.

Now everything goes to fit a motor or engine power and torque, with the best propeller, taking into account all above.

May I know what kind of device are you trying to build ? What kind of motor/engine ?

For a CFD analysis, once having a starting idea of the propeller to use ...

1-] Build an accurate model.

2-] Perform a CFD study:

SURFACE GOALS: Axial Force and Torque (Z axis in the drawing).

EQUATION GOALS:

Surfaces: Both goals for the lower blades surfaces.

Both goals for the upper blades surfaces.

Equation goals: -1- Total force: Add both Forces.

- 2- Total torque: Add both Torques.

With d=fluid density; N= Revs per SECOND; and D=Prop diameter:

-3- Kt: Total force/(d N^2 D^4)

-4- Kq: Total torque/(d N^2 D^5)

-5- Kt/Kq

2-] Perform a parametric study through a RPM range around the engine/motor service RPM. Fix axial speed for the required total service speed.

3-] Get the graph and table for Kt/Kq and choose the bigger value. Best thrust with less required torque.

Repeat all above for the the best RPM point. Now the parametric study is done with fixed rpm along a speeds range around the desired service speed.

More in next chapter if it is being useful.

Regards