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Propeller Physics and VABs Program.

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Andy

Mon Dec 29 2014, 09:23PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Just been working on a design for a project, thought it might of use to you, the larger numbers are the R axis, with the numbers after the commer the Y axis, cos(Y)/R should equal the X axis, the blue line is the bottom of the blade, and the yellow the top.

Patrick

Mon Dec 29 2014, 10:17PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

is the yellow line in the drag bucket towards the origin? else i fail to understand . . .

Andy

Mon Dec 29 2014, 10:42PM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Don't know what you mean by drag bucket?
The right side is the front of the blade, the left the back, the top part is 20degrees of mid, the bottom blue line is 5 degrees of mid, with rought values that want mean anything apart from a ratio, 400(say Neutons) to 297 for drag the top side.

Ash Small

Mon Dec 29 2014, 11:14PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

ok we should start with getting and keeping the drag bucket in the first quadrant and the tip below M0.7 ?

Well, this is the way I see it. Several examples I've seen seem to have a drag bucket from Mach 0.5 to Mach 0.7. It seems standard practice and seems easily feasible to get the drag bucket between these values.

Now these figures are for a 'wing section'. In our case this is 'tip speed', or there abouts.

We need to stay within the drag bucket for efficiency, so the 'operating region' has to be from the bottom of the drag bucket to the top, which implies (I assume) that hover equals Mach 0.5 tip speed and max thrust equals Mach 0.7, or there abouts.

With these parameters fixed (approximately), I suggest we design for hover at Mach 0.5, then 'simulate' at 0.7 to test that no part of the flow goes super sonic, and that everything else is ok, but we primarily design for hover at Mach 0.5.

We then, for each blade section, work out the Mach speed at hover, and use a section with suitable Cl and Cd, then simulate for max speed at that section.

We know disc loading (~5kg/m^2) and disc area (0.5m^2 per rotor). We can work out thrust required and hence Cl. Cd just needs to be kept to a minimum.

We may even still be able to use VABS to work out the 'forces on a beam' thing

EDIT: I'm using Mach numbers for velocity as that is apparently what the Pro's do

(and that is what XFOIL uses)

EDIT: I'm assuming it will spend most of it's time at, or around, hover, so we design for max efficiency at around that point, or slightly above, to allow for manouvering, stability, etc.

EDIT: If we use the 'forces on a beam' thing to work out how much CF we need to withstand the thrust, we have our CSA. We then design for the Cl we need using that CSA, while ensuring that Cd remains at a minimum. We then end up with long, thin, lightly loaded, low drag blades.

Patrick

Mon Dec 29 2014, 11:31PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ok so lets start getting some pulled together dimensions so i can start a mold. then we'll compare to existing props on my dynomometer.

Ash Small

Mon Dec 29 2014, 11:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

ok so lets start getting some pulled together dimensions so i can start a mold. then we'll compare to existing props on my dynomometer.

I just edited the above post while you posted this.

I added:

Ash Small wrote ...

.
EDIT: If we use the 'forces on a beam' thing to work out how much CF we need to withstand the thrust, we have our CSA. We then design for the Cl we need using that CSA, while ensuring that Cd remains at a minimum. We then end up with long, thin, lightly loaded, low drag blades.

I think, knowing the max thrust required (or thereabouts, we may need to 'tweak' later), we can work out how much CF we need to use, so, therefore, have our CSA.

This, I think, is the first dimension we need

Ash Small

Mon Dec 29 2014, 11:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Andy wrote ...

Just been working on a design for a project, thought it might of use to you, the larger numbers are the R axis, with the numbers after the commer the Y axis, cos(Y)/R should equal the X axis, the blue line is the bottom of the blade, and the yellow the top.
1419888237 4266 FT167327 Blade

There are no units on your graph, Andy. This makes it difficult to even begin to interpret

Andy

Tue Dec 30 2014, 05:11AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

Its based onn 11 values from 10_110, with a x,y value base it on 10mm ,

8.5090 y
5.554 x

17.01
13.56

25.52
16.55

Should be sin(.5540)*34.0361 =
34.0361
17.91

Replace or subtract 10,20,30,40, from each number from right to left
Will fix or update it later

Andy

Tue Dec 30 2014, 05:16AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

The top view should be like the apache helo shape with the first the front the second the back

Andy

Tue Dec 30 2014, 05:18AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874

From this

1419916690 4266 FT167327 Img 20141230 181739

mm from point						
	20	20			20	20	
Back Dims	x	y		Frount Dims	x	y	
1	8	-0.03			8	14.75	
2	16	13.11			16	7.63	
3	24	16.24			24	3.14	
4	32	14.51			32	1.13	
5	40	10.56			40	0.25	
6	48	6.74			48	-0.01	
7	56	4.66			56	0.06	
8	64	4.8			64	0.21	
9	72	6.5			72	0.31	
10	80	8.5			80	0.31	
11	88	9.64			88	0.22	
12	96	9.42			96	0.11	
13	104	8.17			104	0.03	
14	112	6.69			112	0	
15	120	5.83			120	0.02	
16	128	5.97			128	0.06	
17	136	6.9			136	0.09	
18	144	7.98			144	0.1

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