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GMAX: Realistic curvature for propeller blades (Read 1554 times)
5th Sep, 2005 at 7:53am

Paco Sanchez   Offline
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Hi all, here's a method I tought of for building a propeller so that its geometry will be exactly like in real life. Until now I just applied a twist modifier to my propellers blades, recently talking in a post with Bill "Lionheart" I realised that this method is no good, as gives the blade a constant rate of curvature change, unlike in real life, where the curvature abruptly changes near the propeller axis, and smootly at the propeller tips. So I got the dust off my propeller hydrodinamics book; I hope it's not so different in air than in water...

This tutorial will not tell you how to build the geometry of your propeller blades (I assume you already know if you want to go for this extra bit of realism), it will just tell you how to get the proper curvature of it.

Please forgive my English (and my lack of aerodynamics knowledge, only hydro- here Grin).

Step 1: Create one blade matching the shape of the real blade, but make it flat. I normally do this by a box (with an even number of segments from the root to the tip) with several EditMesh and Taper modifiers. The blade’s back face (pressure face) should be nearly flat, while the front face (suction face) should be convex. Create also a spinner, and position your blade in the correct place relative to the spinner.

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Step 2: Create a quad patch perpendicular to the blade's plane. I suggest working in right view, otherwise the patch may become invisible later on because of looking at the wrong side. Place it so that its center is on the back face of the blade. The length of the patch should be equal to the legth of the blade, and its width should be calculated with one the following formulas:

W = 2* A; being A the propeller advance at cruise optimum speed (if known)

W = 4.71 * R * TAN (alpha); being R the radius of the propeller and alpha the propeller pitch at cruise optimum speed (if known)

W = 2.72 * R as a rough guess if the data above are not known.

Make the quad patch so that the segments of the propeller blade will be coincident with segments on the quad patch. Give it an even number of segments (That's why the even number of segments in the blade, otherwise it can be difficult). I used 2.

The quad patch should be given an even number of width segments. I suggest 8.

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Step 3: Add a twist modifier to the quad patch. Its axis should be same as the propeller's axis. In the modifier stack, select the twist axis and move it to the propeller's center.

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Step 4: Give the twist angle a value of 360. You've created a helix that follows the movement of your blade in the air.

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Step 5: Hide your propeller hub, and make the propeller blade see-through, and de- activate backface cull (right click on it, properties). Do the same to the helix. This will make the next steps easier.

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Step 6: Now add an Edit Mesh modifier to your blade, and go into vertex sub-object selection. Select all vertices at the root of the blade. Rotate them around the z axis so that the back face of the propeller (the pressure face) will be parallel to the helix.

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Step 7: Do the same with all sections of the propeller blade

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Step 8: Make your blade solid again, unhide the spinner and delete the helix. Your prop is close to completion! The blade is now seen as it would be at cruise flight. Select it, and rotate it around the z axis until the tip has almost zero pitch. (In fact, it should be so that the pitch at 0.75 R is equal to the propeller’s minimum pitch – 6 degrees in my connie)

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If you propeller has a skew (e.g., tips curved backwards), is should be done now, by bending them perpendicularly to its pressure face.

Step 9: Make an array with the blade with the appropriate no. of blades, name them, animate them and link them as necessary. Propeller finished! Now its curvature is exactly (well, almost) as a real one.

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Reply #1 - 5th Sep, 2005 at 8:00am

Paco Sanchez   Offline
Inspecteur des Polygonnes
I want axis-driven animations
back NOW!!!
Vigo, Spain

Gender: male
Posts: 1686
*****
 
Please note that for changing the direction of rotation, the twist applied to the quad patch should be negative, an then it's better to work in left view instead. I just realised that the sample propeller I made for the tutorial is roatating just opposite as most propellers do  Embarrassed  Embarrassed  Embarrassed   Embarrassed  Embarrassed You should look at the images through a mirror!

Paco
 

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Reply #2 - 5th Sep, 2005 at 10:26am

lionheart   Offline
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Phoenix Arizona

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Wow Paco.

Very nice tutorial...   8)

Bill
 

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Reply #3 - 5th Sep, 2005 at 11:48am

Tyme   Offline
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Canada

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Very cool. 8)
 

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Reply #4 - 10th Sep, 2005 at 8:49am

pilot2242   Offline
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nice guide there but it dosent help me heh.im new to this gmax and i need help to shape the nose and tail and to make engines im not sure how to curve and make engines there is so many tools i dont know what tool to use could anyone help me please.i will be happy to give credits to anyone that helps me on every plane i make. Smiley
 
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Reply #5 - 10th Sep, 2005 at 10:17am

Felix/FFDS   Offline
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Pilot - this is not the only tutorial available.  Have you gone through the tutorial selections on this Forum and the FFDS tutorials pages?

Also, all any tutorial will (can) do is give you hints and methods to apply to meshes, etc.  The final design and creativity is up to you...
 

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