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Propeller Physics and VABs Program.

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Ash Small

Tue Jan 13 2015, 12:39PM

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

Patrick wrote ...

lets focus of cross sections, ill test CF and figure out safety and make-ability.

I think we basically need to 'lay up' a 'blade section', then hang weights on the end until it breaks.

We can use the maths and stuff to get us in the ballpark.

We can work out the forces involved at hover, then extrapolate for max rpm.

Once we know how much CF we need, we can input the sections into XFOIL.

Patrick

Tue Jan 13 2015, 10:46PM

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

Ill start tonight.

Patrick

Thu Jan 15 2015, 08:08AM

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

Lay-up is slowed due to cold temp, so im waiting for the bend test.

Ash Small

Thu Jan 15 2015, 03:17PM

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

Patrick wrote ...

Lay-up is slowed due to cold temp, so im waiting for the bend test.

Based on the design we've discussed, each rotor exerts a force equivalent to 2.5kg @ hover, so maybe twice this (as a ballpark figure) at max RPM, so @ 2.5kg per blade in a two blade rotor, so I assume we have to be able to hang 2.5kg on each blade without too much deformation.

I do need to check the maths, though, this is very approximate. there are also 'safety factors' to consider.

Patrick

Thu Jan 15 2015, 09:51PM

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

does that 2.5kg speard out as a derivative, with increasing slope from hub to tip ? how much of the 2.5kg is at the itty-bitty tip?

Ash Small

Thu Jan 15 2015, 10:22PM

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

Patrick wrote ...

does that 2.5kg speard out as a derivative, with increasing slope from hub to tip ? how much of the 2.5kg is at the itty-bitty tip?

I can't remember offhand. More lift is generated at the tip. I need to look it up. I do remember it's pretty basic stuff, though. (I always check everything as a matter of course)

The other point to bear in mind is that the forces on the blade will try to deform it as far as angle of attack is concerned. If it deforms by much it won't have the same properties as before it deforms. I don't think this will be a major problem, but we need to keep it in mind.

We obviously want to use the minimum amount of CF we can 'safely' get away with, in order to keep drag down.

I will have a look at the maths soon, I think we're expecting snow next week, so I'll probably have some time to kill

Patrick

Sun Jan 25 2015, 07:36AM

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

Sorry ive been missing in action for so long. Had to help parents with some house repair. but im back ! ive got some parts and tests to get back to.

Im hoping to make point-load measurements along the span, then with math turn those coarse points into a fine derivative curve.

Patrick

Mon Feb 02 2015, 07:43AM

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

ok had some family health issues should be good now.

ive found this,

in which they state:

Im laying up another beam to be loaded from hub towards tip.

from the above math i see that " m-dot-sub-e " equals " Rho-sub-e x A-sub-e x V-sub-e " since the other componets are on both sides.

Im wanting to point-load the span to see how much carbo/glass is needed. As this constant pitch props are constantly accelerated and decelerated, that mass x acceleration eats battery power. So mass of propeller especially at the tip is important to keep down.

two counter requirements are: first, 80% of thrust occurs at the last 50% of radius. second, the propeller is generally weaker were the radial and bending forces are greatest, regardless of material used.

Ash Small

Wed Feb 04 2015, 06:16PM

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

I think the 'm.e' is 'mass of aircraft' in this case.

I'm assuming a constant cross sectional area (although not 'constant section') would be easier to lay up (certainly with unidirectional CF.

The mass of the whole prop needs to be kept to a minimum.

Patrick

Wed Feb 04 2015, 08:37PM

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

yeah but the mass of the tip integrated over radius matters for effciency of spool up and down, as we do that continuosly for stability.

my kick starter, drones for Other-Than-Military-Purposes.

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