Blade CAD Tutorial.
Welcome to a quick tutorial on how I make the CAD models of my blades. These eventually produce the Gcode that my CNC router runs to machine the blades. I hope you find it helpfull.
For this tutorial I'm using a 1 metre blade radius and a TSR of 7.
Firstly I start up the blade calculator software, input the relevent numbers and select NACA2410 under "airfoil data", press the "update" and then the "Solve" button. This gives you the dimensions you'll need for Rhino. The blade calculator only supports the NACA2410 profile, which is low drag, high lift, so should be good for our purposes. Download the NACA-2410 profile here if you don't already have it. You may need to right click and choose "save As".
Next fire up Rhino and use (File>Import) to open the NACA2410.txt file. Select "Space" and "Create point cloud". You may have to zoom in to see much. The profile is only 1mm long by default. Make sure you are working in mm and that "Snap" and "Ortho" are turned on. (bottom of main screen). For now we'll be working in the "TOP" view only. To maximise this viewport, double click on the text saying "TOP" (blue/white, box/text). To return to all 4 viewports double click "TOP" again. For now we want just the TOP viewport.
Next choose (Curve>Free-Form>Fit To Points) from the menu. The command line asks you to select the points to build the curve through. Select any point as the first point then working in either a clockwise, or anticlockwise direction, select the rest of the points in order. You may have to zoom in qutie a lot to select all the points. There are two at the trailing edge, very close together. A scroll wheel on a mouse makes it easy to zoom in and out. A right "click and drag" action, will allow you to drag the screen around. Once all the points are selected press (Enter). Press (Enter) again to accept settings. You should now be looking at something like this:
The point cloud can now be deleted. Click on any of the points to select the cloud (A box may appear saying "point cloud" "curve" and "none", choose "point cloud") and press the "Delete" button on the keyboard.
Next we need to move the curve so that it intersects with 0,0. So select the curve by clicking on it. Turn "Snap" off, but leave "Ortho" on. Then drag the curve so that it passes through 0,0 as close as you can get it. You may have to drag it, zoom in a bit and drag it some more, zoom, drag etc. Once you're happy, turn "Snap" back on. We'll need it later.
Next we'll be needing the data from the blade calculator software. It may be an idea to jot down the chord numbers on a piece of paper to save a lot of screen swapping later on. I usually work from the tip end of the blade. The chord nearest the root is way too big to be practical, so I ignore the last section for now. We'll use the second to last profile twice later on. Ok. Select the curve by clicking on it. Choose (Transform>scale>scale2-D) from the menu. If you look at the command line it says "Origin point ( Copy=No ):". Click on the words "Copy=No" it should change to "Copy=Yes". Now choose the origin point 0,0 by clicking at 0,0
It should now ask for "scale factor or reference point". You may want to zoom out at this point, or during the next steps it may look as if nothing is happening.
Ok. It's still asking for "scale factor or reference point", so we type in the number from the bottom of the list in the blade calculator. For my 1 metre blade it says "0.045" metres. For Rhino we multiply by 1000 as we're working in mm, not metres. So type in 45 to the command line and press enter. Another profile should have appeared on the screen, only much larger. You can just see the original profile in yellow.
The command line is still asking for a "scale factor or reference point", so we type in the next number up the list, which for me is 0.05 Metres. So I type in 50 and press enter. This continues up the list from the bottom (just keep typing in the numbers and pressing enter between each one), untill you reach the second from last profile. For my one metre blade the last section is 448mm. This is way too big so I ignored the final section and finished at 224mm. We'll sort this out later. Once you have enterd the last number, press the "Escape" button to end the transform command. You should now be looking at 9 new profiles. Your screen, should look something like this:
If you wish, you can now delete the original profile. It's the tiny one near 0,0. You may need to zoom in to find it.
In the next part of the tutorial we'll rotate the profiles according to the angles given by the blade calculator and space them out to give some length to the blade. If you can't wait 'till then, save what we've done so far and have a play around. You may be able to figure the rest out on your own. It's really not that hard.
The next part will be written when I have some more time.............
Well. I'm back again....... You should be looking at 9 airfoil profiles, as in the picture above. The next steps are to rotate all of the profiles to the correct angles. Once again it may be a good idea to jot down the angles given in the blade calculator software, to save some screen swapping. I'll be starting with the smallest profile first, working upwards to the root profile.
I'll assume you can zoom in / out OK by now, and will leave it to you to decide if you need to zoom to perform the commands from here on in.
Select the smallest profile. It should turn yellow. Now choose (Transform>Rotate) from the menu. The command line will ask for "Center of rotation". Click on 0,0. It should now ask for "Angle or first reference point". Just drag a line, (any length), out to the right and click. This sets the zero degrees line. It should now be asking for a "Second reference point". Here you type in <-0.6 That is the angle given by blade calc for the tip airfoil angle. Now if you move your mouse up and down you'll see that the airfoil will rotate, snapping every 0.6 degrees. We need to move the mouse downwards so that the airfoil rotates to the first "snap position", and click. you should now see this:
One down, eight to go.
The next largest profile has an angle of zero. So we can leave that one as it is.
All the other profiles will rotate in the opposite direction. Only the tip profile has a negative angle.
Select the next profile upwards in size. Now right click anywhere in the workspace (viewport). This will run whatever the last command was, in our case, (Rotate), without having to go through the menus. The command line should now be asking for "Centre of rotation". Same as before. Click 0,0 and then drag a line out to the right and click. Next up is the second reference point. Type in the next angle from the list. <0.8 Move the mouse upwards to the first "Snap position" and click. Job done.
This process applies to all the remaining profiles. It's basically........Right click, to activate the Rotate command. Click 0,0 and drag a line out to the right. Type in the angle, preceeded by a < symbol. This tells Rhino that the data you entered is an angle. Then Rotate to first "Snap position" and click. Then onto the next profile. Easy eh?
You should be able to rotate all the profiles in under 30 seconds once you get the hang of the click, click click, click, type, move, click pattern.
You should now be looking at something similar to this:
So that's all the profile rotations done. Next we have to switch back to all 4 view ports. (Double click where it says TOP, in the blue box at the top left corner of the viewport.) Your screen should now look like this:
Next we'll rotate the entire set of profiles so that it's easier to see what we're doing. At the minute all the profiles are facing the wrong way, and are upside down. So. Working in the viewport named "Right". Zoom out so that you can see the entire length of the profiles. (Thin line). Then drag a box around them like so...
The profiles in all viewports should turn yellow.
Next. Choose (Transform>Rotate). Then, still working in the "Right" viewport click on 0,0 and drag a line out to the right, any length will do. Now move the mouse, so as to rotate the profiles 90 degrees anticlockwise and click. Should be somewhere near the green reference line. Now you should see this:
Next up we need to change to the "Front viewport", by clicking on the word "Front" in the blue box. If your profiles are not still yellow (selected), then you'll need to drag a box around them to select them. With the profiles selected, choose (Transform>Mirror) from the menu. Make sure that in the command line says "Copy=No". If it says "Copy=Yes". Click on it to change it to "No" . Ok. It should be asking for "Start of mirror plane". Click anywhere on the red reference line and drag a line out (left or right). You should see a mirror image of the profile appear. Left click to complete the command. You should now see this.
One more mirror command and we're done with this bit. Still in the "Front" viewport. Right click to activate the mirror command again. Check that the command line says "Copy=No". Now click anywhere on the green reference line and drag a line (up or down). The mirror image should niow appear. Left click to complete. You should see something similar to this, depending on your zoom levels for each viewport.
That's all for now. Next time we'll start building the actual blade. Most of the boring part is over. Save what we've done so far and we'll get back to it next time.
It may seem long winded so far, but once you're comfortable with Rhino, all of the above steps thus far, can be done in under 2 minutes.
Now we need to extend the size of the "grid" in Rhino, to allow us enough room to place the blade sections at a max length of 1 metre. To do this, right click on any of the "Viewport" labels. (Top, Perspective, Front, or Right). A menu should open. We need "Grid Options", second from the bottom. Where it says "Grid Extents", we need to type in 1000, and press enter.
OK. In the "Top" viewport, zoom out so that you can see both the top edge of the grid and also your profiles. Now working in the "Front" viewport, zoom in and select the smallest profile.
Back in the "Top" viewport, drag the selected (Yellow) profile up to the top of the grid (1000mm). Make sure you've got "Snap" and "Ortho" switched on.
Because we are zoomed out in the top viewport you may not be able to place the profile exactly at 1000mm, so get it roughly in the right place, then zoom in to get it's exact placement at 1000mm. The exact co-ordinate positions are show at the bottom left of the screen on the status bar, for X, Y, & Z.
Now pick the next largest profile and using the same process as above, position it at 900mm, zooming in / out,as needed to get the postion just right.
Continue this for the remaining profiles at 800mm, 700mm, 600mm and so on, untill they are all in position. Your screen should now look something like this:
Next up we need a couple of square root sections to give us somewhere to build the triangular mounting part of the blade. (The large solid part that forms the centre of the rotor). So, working in the "Front" viewport, select the rectangle tool.
Use your zooming in / out skills to draw a rectangle around the profiles, so that is as small as you can make it, but still encompassing all of the profiles. You may find it easier to maximise the "Front" viewport, by double clicking the label: "Front". To return to all 4 viewports, double click the label again. Remember, right click and drag, will scroll the display inside the viewport, even when a tool is active. You should now see this:
Press "Ctrl+ C" to copy the rectange. Now in the "Top" viewport drag the rectangle to a position of 80mm along the blade length, like we did for the airfoils.. Next up, press "Ctrl + V". A new rectangle should have appeared and be "selected", where the original one was. Drag this to the 60mm position along the blade. You should be seeing this:
Now for the exciting part.....Yay..... Make sure that all of the objects in the viewports are NOT selected, then from the top menu choose (Surface>Loft). The command line should ask you to "Select curves to loft". Starting with the smallest profile, select each profile in sequence along the blade,including the two rectangles we just made. Once all the curves are selected, press "Enter" on the keyboard.
You should get some black lines and white arrows appear along the blade like so:
The command line should be showing 3 options for adjusting the curve seams. Click on "Automatic", and press "Enter". A new dialogue box should have appeared for "Loft Options". From the dropdown list, choose "Tight". For "cross section curve options", choose "Do not simplify", and click preview. If your blade looks like this, then you can press "OK".
In the "Perspective" viewport, right click on the label "Perspective" and choose "Shaded Display" this will give the blade a more solid look with basic shading.
Well, that's the main part of the blade done. I'll add the 120 deg root triangle when I get more time.