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Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
model your prop, split in two parts along the most practical axis, apply in negative to a filled box, and obtain a 3D printable mold which you can scale at will
im not sure what the Vx and Vt numbers mean, but the graph and gradient are amazing.
Vx is the horizontal air velocity. I wonder how accurate this simulation is, since it doesn't seem to include the air viscosity. A sudden jump in air velocity e.g. at the boundary of the slip streams tail to the surrounding air looks unlikely.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
Ash Small wrote ...
The other option would be to put a series of 'theoretical' dimensions into a 3D CAD package, and then produce a 'net'. From this you could program a 3D printer to produce a mold.
i dont get this... elaborate please.
Well, theoretically, if we know the disc area and the RPM when hovering, we should be able to work out theoretical airspeed (of the accelerated column).
Theoretically, all the air wants to be at the same velocity (we can discuss this in detail later)
so, at various points from the nub to the tip we can calculated the required 'angle of attack' (I've not taken 'slip' into consideration yet)
once we have a series of angles at various distances from the hub we can plot the shape, and then adjust for NACA profile, or whatever.
Starting from the 'theoretical optimum' should give useful data to enable us to work out slip, and other losses by comparing thrust, RPM and power consumed.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
for a hovering prop, wouldnt the slip be 100%?
also, the outer radius produces most of the thrust. if this is true, and we go to the tip, then the tip captures the most air mass and accelerates it to the constant screw speed,
Or, does each x posistion of radius capture the same air mass, making up for velocity change with increase picth towrads the root?
EDIT: im really trying to figure out a 0 to x axis (radius) with a 0 to y axis (acceleration) function, im not sure if this is linear or not? i guess i would have to give dimensions for the prop...
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
for a hovering prop, wouldnt the slip be 100%?
No. The column of air is being accelerated with a force equal to 9.81 newtons per KG of 'copter mass.
(Actually, now you mention it, you may be correct, I'm used to marine props where lift itself isn't a factor, just the boat's velocity. but the same principle applies, I think, because a force is still being generated. (hovering requires a force of 9.81 newtons per kG of 'copter mass)
I think what I actually mean here is the propeller efficiency. I may need to consider this some more.
Patrick wrote ...
also, the outer radius produces most of the thrust. if this is true, and we go to the tip, then the tip captures the most air mass and accelerates it to the constant screw speed,
Or, does each x posistion of radius capture the same air mass, making up for velocity change with increase picth towrads the root?
EDIT: im really trying to figure out a 0 to x axis (radius) with a 0 to y axis (acceleration) function, im not sure if this is linear or not? i guess i would have to give dimensions for the prop...
You still want the accelerated air velocity to be 'theoretically' constant throughout the column, which means that the angle of attack of the propeller has to be greater towards the centre as the propeller velocity is less than at the tip, which travels at a greater velocity. (In practice this isn't the case as losses close to the hub increase with increased angle of attack, but as this produces very little of the 'overall' thrust, we can reduce drag here without unduly affecting overall lift.)
Obviously, a prop with less pitch needs to rotate faster to generate the same force, but we already know you want a thin section rotating fast for efficiency, so you want a fairly shallow pitch (all the 'copter design stuff tells us this)
If the angle at the tip(x) is theta, then the angle at 1/2x has to be 2 x theta, the angle at 3/4x needs to be 1.5 x theta, and the angle at 1/4x needs to be 4 x theta. In order to keep drag low, we need high RPM, so that 4 times theta is still small. (this assumes a constant blede width)
The ratio of radius to circumferance is constant (2 x pi x r, or pi x d),
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Ash Small wrote ...
You still want the accelerated air velocity to be 'theoretically' constant throughout the column, which means that the angle of attack of the propeller has to be greater towards the centre as the propeller velocity is less than at the tip, which travels at a greater velocity. (In practice this isn't the case as losses close to the hub increase with increased angle of attack, but as this produces very little of the 'overall' thrust, we can reduce drag here without unduly affecting overall lift.)
Obviously, a prop with less pitch needs to rotate faster to generate the same force, but we already know you want a thin section rotating fast for efficiency, so you want a fairly shallow pitch (all the 'copter design stuff tells us this)
If the angle at the tip(x) is theta, then the angle at 1/2x has to be 2 x theta, the angle at 3/4x needs to be 1.5 x theta, and the angle at 1/4x needs to be 4 x theta. In order to keep drag low, we need high RPM, so that 4 times theta is still small. (this assumes a constant blede width)
The ratio of radius to circumferance is constant (2 x pi x r, or pi x d),
this is what I needed to know, sometimes I get it, then lose it all in a blur.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I have read that the limiting factor should be centripetal (or centrifugal, I can't remember the difference offhand, might be the drink ) force acting on the prop, although drag is also a factor, as is the 'bending moment' in the blade itself, but the limiting factor in most 'copter designs is centripetal (or centrifugal) forces.
Also, you want a a thin, lightweight section, and high RPM.
Hope this helps.
EDIT: Centrifugal, I think or is it both, as they act against each other?
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
I was mentioning the centripetal/centrifugal vectors which are of opposite direction, but combines with the acceleration force vector
as I see it, at constant speed acceleration is null, and centripetal/centrifugal forces being of opposite direction and at equilibrium, a propeller designed for those parameters will hold without problem
now accelerate or decelerate, and you combine a third force vector which will induce stress in another direction
I was simply thinking about the system being incomplete without the acceleration force vector, and was actually wondering if I was right or not ;)
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Propeller Physics and VABs Program.
) force acting on the prop, although drag is also a factor, as is the 'bending moment' in the blade itself, but the limiting factor in most 'copter designs is centripetal (or centrifugal) forces.