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Propeller Physics and VABs Program.

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Patrick

Mon May 04 2015, 03:45AM

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

the lead must also keep the rotational inertia up so it builds and fades slowly, so the motor and speed control can keep up.

Ash Small

Mon May 04 2015, 09:54AM

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

Do we know the 'design RPM' of these props, Patrick?

I'm still of the opinion that we want to be looking at solid carbon blades, with a much shorter chord and higher RPM.

The stiffness we need will come from the carbon, unless you can think of anything stiffer and lighter than carbon

Patrick

Thu May 07 2015, 04:07PM

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

yes for or purposes they would be solid. and if i remember the heli ones were 2,000 rpm.

Patrick

Sat May 09 2015, 05:12AM

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

I can confirm that the carbon fiber 45 and -45 weave is stiff when the strength is needed lengthwise, so thin solid propellers are entirely possible.

Ash Small

Sat May 09 2015, 10:28AM

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

Patrick wrote ...

I can confirm that the carbon fiber 45 and -45 weave is stiff when the strength is needed lengthwise, so thin solid propellers are entirely possible.

Yep

Patrick

Mon May 11 2015, 05:04AM

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

so with fixed pitch, were going for a common NACA profile right? Ill print a mold on my 3D printer, then layup some wet CF.

Ash Small

Mon May 11 2015, 11:31AM

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

Patrick wrote ...

so with fixed pitch, were going for a common NACA profile right? Ill print a mold on my 3D printer, then layup some wet CF.

I suggest we start with the NACA 0012 profile, purely because it's a very common profile and it's used by Bell.

The 'ideal situation' is where tip speed is as close to Mach 1 as possible, without any turbulence etc exceeding Mach 1

The rotor obviously needs to withstand the forces involved without deforming, and higher RPM helps here too.

Once we know how much carbon we need for stiffness we can calculate chord length and angle of attack for each 'station' along the rotor.

The short chord length and minimal amount of carbon help to reduce drag losses (of the rotor) to a minimum.

Obviously, we need to find a 'workable compromise' based on all the above. We just have to get as close to the optimum as practically possible. We can use XFOIL here to finalise and 'fine tune' things as far as possible.

EDIT: I suspect we can go thinner in section than NACA 0012. It's all about getting sufficient stiffness. Bell obviously felt that the 12% max thickness to chord ratio was required using the materials they used at the time. If we can go thinner, say NACA 0009 or even less, we'll get less rotor drag.

Patrick

Mon May 11 2015, 01:35PM

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

Since im operating near 5000 rpm how do we measure whether there flexing? I dont know if ill have to build a strobe camera or what?

Ash Small

Wed May 13 2015, 12:09AM

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

Patrick wrote ...

Since im operating near 5000 rpm how do we measure whether there flexing? I dont know if ill have to build a strobe camera or what?

We should be able to calculate it.....extrapolate from stationary measurements.

You said a couple of posts back that two layers of 45 degree cloth seemed stiff enough, or something.

Maybe we don't need a NACA profile, maybe a constant thickness blade will suffice?

That get's us into 'thin airfoil theory', but it should still work in XFOIL.

It doesn't matter if the rotor changes profile a bit, as long as it's predictable.

All we want to do is use the minimum amount of carbon required to achieve the required stiffness for the forces acting on it at 4000 to 5000 RPM. We can calculate the forces.

Patrick

Wed May 13 2015, 05:42AM

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

ok lets consider this a reduced complication scenario.

Each blade is a span, attached ridgedely at one end. and prgressively lifted at the tip, with maximum deflection and local force at the tip.

if considered as above, i should be able to predict "upward" propeller force. if i build a span on my work table, and hang lead weights i should find both breaking and deflection measurements. This of course ignores radial "throwing from the hub" force.

the phrase "exploding from the hub force" also works nicely.

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