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Propeller free spinning RPM estimation
Dear all,
I am trying to use Qblade to estimate what speed would a given propeller connected to a BLDC motor spin at a given strictly due to windspeed (windmilling propeller)
So far I have created a 2-blade UAV propeller following the propeller data found in https://m-selig.ae.illinois.edu/props/volume-2/propDB-volume-2.html#NR640
I then scaled it according to my required measurement specification and ran BEM Analysis for ROTOR at a given wind speed(eg. 30m/s)
From the simulation I am able to obtained the Torque-RPM curve of the propeller and compared it to the motor’s Torque-RPM curve to obtain the equilibrium RPM the propeller will spin at.
May I ask if my approach is correct or is there a method to test this out — to find out the propeller/motor RPM at a given wind speed
My apologies for any inconvenience caused
Best regards,
QJ
I manage to browse upon this post which seems to be able to help me out.
https://qblade.org/qblade-forum/topic/how-to-predict-rotor-rotational-speed/
However, I am unsure on how to do that simulate as seen in his Fig.1 & 2 as well as including a generator for my case.
This is my first time using this and would appreciate any help
Hi QJ,
the approach that you have outlined, based on the rpm/torque curves for a specific windspeed, should work.
If you want to test this out in a time domain simulation you need to:
- setup the LLFVW aerodynamic modelling for the propeller
- define a structural model for the rotor so that mass/inertia information is available, you can set the tower height to zero and not use a tower file
- provide the generator torque, for examle by using an External Loading File (using HUB as the sensor name)
By carrying out the time domain simulation you can then observe at which equilibrium RPM the proepeller will operate for a given motor torque and how much thrust is generated.
BR,
David
Dear David,
Thank you for your response but I am still quite unsure what I need to do
For point 1, do you mean creating a model for my propeller? If so, I have done as seen in the first file.
For point 2, I have tried to come up with one but may I know if is there a simpler method or do I have to include all sections in the Structural Model Input File? I have included one in txt format in the second file but there a values I am not sure about. Also, must I include DATA OUTPUT TYPES,DATA OUTPUT LOCATIONS in it?
For point 3, if for example the generator torque is 2 Nm, will what I have done in the third file be correct?
Sorry for asking so many things, will really appreciate your help
Best regards,
QJ
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Hi QJ,
Regarding point 1, I was referring to the aerodynamic method (LLFVW) selected for the turbine design. A complete turbine design is necessary to perform time-domain simulations, as these cannot be conducted using just a rotor object (unlike steady BEM calculations).
For point 2, you need to generate the structural data table for your blades and also create the main structural file. Your example already looks quite close to completion, and many of the parameters are optional. Regarding the drivetrain, simply set the dynamics to ”false”. Once you load the main file into QBlade’s GUI, you will receive a notification if any required keywords are missing.
Concerning point 3, your file is not correctly defined. You need to apply the torque to the hub by setting the moment along the local x-axis, <mx1>. The first column represents time, and the format should be as follows:
<SensorName> <localflag> <time1> <fx1> <fy1> <fz1> <mx1> <my1> <mz1> <time2> <fx2> <fy2> <fz2> <mx2> <my2> <mz2>
BR,
David
Dear David,
I am not sure how to get the Blade.str data for my propeller so I downloaded NREL5MW_Main.str and adjusted the sections’ values accordingly.
I tried to run Turbine Simulation and set Rotational Speed Settings to ‘Free Rotation’ but the propeller is not spinning, could the issue be the propeller being just too poor to be able to spin or due to my settings having issues?
I have attached my settings in the files. Once again thanks for the quick response.
Best regards,
QJ
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Hi QJ,
you are effectively approximating the structural properties (stiffness & mass) of your small propeller with those of a 63m long wind turbine blade.
As a result, the mass and inertia of the propeller are significantly overestimated, preventing it from rotating when only 2Nm of torque is applied.
To resolve this, you need to obtain more realistic structural properties for your blade data table. A straightforward approach is to use the built-in QFEM tool in QBlade, which can generate structural properties for rotor blade designs.
Regarding the external loading file you attached, it is mostly correct. However, to apply torque properly, you must specify a second row to ensure that the interpolation function works correctly. If you intend to apply a constant torque, your file should be formatted as follows:
HUB true
0 0 0 0 2 0 0
1000 0 0 0 2 0 0
Additionally, I have changed the “false” flag to “true” so that the torque is always applied along the local x-axis of the hub, even when the propeller’s orientation changes.
BR,
David
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