Comment by Vince Hogg on December 23, 2012 at 12:19pm

Joe, can I ask which main bits you used for the cnc cutter. I almost started building one last year but got side tracked as usual.

I would like to ‘multi task’ the breakout board and stepper drivers so they also do a 3d printer and router.

I see there are many cheap ones that work from a printer port but think the USB ones would be more future proof.

How about this setup.

http://www.ebay.co.uk/itm/4-axis-CNC-USB-Card-Mach3-Breakout-Board-...

 

Vince

 

 

Comment by Martin on December 24, 2012 at 2:23am

Take a look at this program --> http://www.evolutiondesigns.eu/

Thread in RC Groups -- http://www.rcgroups.com/forums/showthread.php?t=1785953

The program is free to use, but if you want coordinates for one of the generated airfoils then you're going to have to pay for it.

It certainly won't find you an airfoil that has 2x lower drag than other comparable foils. But the performance gain might be worthwhile.

Comment by Gisela & Joe Noci on December 28, 2012 at 12:47am

Hi Vince,

Sorry, was out in the bush for a few days break..

Vince, the CNC Router I built from the ground up - so no real off the shelf bits I am afraid. Here are some pics if you want ideas - Bit off topic I suppose..

I used slides from a German company - the slides are called 'ROLLON'

Steppers were from an old (BIG) Calcomp drum pen plotter. The spindle is a 1kW Perske 45Krpm 3phase variable speed unit. Stepper drivers are GECKO DRIVE, very good. Table and base frame is aluminium, X and Y gantry are composite and Z axis all aluminium. 

Working Area is about 900X750mm - I would advise to try go for longer - 1600mm X 700 would be great...

Comment by Gisela & Joe Noci on December 28, 2012 at 1:05am

Martin,

I have been trying hard to get something useful out of the Evolution software....

I understand they have used it - or a derivative- for the Shark A/C and for a large span glider, but even if I use the setup as in their glider tutorial it gives a result which make little sense - And if you change anything even slightly, you invariably get strange error message , with no explanation, and cannot regress, ie, acknowledging the error message just dumps you out of the program, with nothing saved and you start all over again - sometime this happens after two hours of processing - not a very user friendly, or in fact usable, software...

They do not respond to any queries - this was 10days ago - maybe all on holiday..

If you have it and are playing - I gave two simple inputs point:

1st point - for takeoff and land-

Clmax = 1.5

Re=200K

Mach =0.033

Constraint = minimize Cd/Cl^1.5

2nd point for cruise

Cl = 0.4

Re=365K

Mach = 0.06

Constraint = minimize Cd

I also chose Very Light Aircraft under Geometry for run-1

For another attempt I chose Sailplane - RC-Model.

The results were not believable - you can save the co-ords of ay selected foil to an excel file - I did so , converted to .dat, and read into profilli Pro - the plots are nothing like what is shown in Evolution!

If you select a constraint such as - 

1st point - for takeoff and land-

Clmax = 1.5

Re=200K

Mach =0.033

Constraint = minimize Cd/Cl^1.5

2nd point for cruise

Cl = 0.4

Re=365K

Mach = 0.06

Constraint = maximize Cl/Cd - ( the only change from the previous runs)

Then it dies part way through and gives a useless error message, and you cannot restart/continue, etc.

Doing anything just exits to windows....

Seems like a tool that was used by the design engineers, who then thought is a great idea, and then pasted a GUI around it, warts and all..help files are useless...

No free lunches!

Joe

Comment by Martin on December 28, 2012 at 2:28am

Sorry to hear that, Joe. I haven't used the software myself.

Another thing. Thus far we've only looked at airfoil polars at certain Re numbers. The problem with a, let's say Re=300k polar, is that you only get one point of useful information off that polar. That's because that particular Re number occurs only at one certain speed. So, looking at polars where Re is a constant is a bit deceiving.

A model airplane is never a point design like an airliner. Luckily, there are type 2 polars in XFLR5 and XFOIL. In a type 2 polar the  Reynolds number is also a variable. So, in result you get a polar that takes Re changes into account. You could also construct a type 2 polar manually by picking points from a type 1 polars.

For example, here's a type 2 polar comparing SA7035 and S7075:

You can easily see that SA7035 has an edge at the lower Cl's you're flying at.

Comment by Martin on December 28, 2012 at 2:47am

I forgot to add a turbulator to S7075. With a turbulator (red curve) it looks a bit better than SA7035.

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