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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Shrad wrote ...
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 ;)
Yes, you are correct, Shrad, and that does become a factor to be taken into consideration when working out the peak forces on the blades, but for now, we're just considering the 'hovering' scenario, as the maths is much simpler.
The point I was making, I think, is that you want to keep the mass of the blades low, as this minimises the centripetal/centrifugal forces.
I suspect that we'll end up with pure CF blades, with no foam, etc. as carbon has huge tensile strength to combat the centripetal/centrifugal forces and will therefore allow the thinnest section, etc. and also facilitate high RPM.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
hovering in a controlled environment?
in my point of view hovering is the result of constant acceleration/deceleration in reaction to the environment (wind, turbulence, etc...)
if you account that it's part of the deal, then I think you still have to account these stresses even if they are reduced, as different materials will have different behaviors facing such small and perpetual variations of forces (flexing, vibrating, etc...)
maybe error calculation can help take that into account? I recall from my courses back in the past that it's not that hard to calculate
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Shrad wrote ...
hovering in a controlled environment?
in my point of view hovering is the result of constant acceleration/deceleration in reaction to the environment (wind, turbulence, etc...)
if you account that it's part of the deal, then I think you still have to account these stresses even if they are reduced, as different materials will have different behaviors facing such small and perpetual variations of forces (flexing, vibrating, etc...)
maybe error calculation can help take that into account? I recall from my courses back in the past that it's not that hard to calculate
That's correct, all that 'bending moments in a beam' stuff is pretty straightforward, but the maths I provided above only applies to hovering, and is for the 'ideal' case. Obviously the blade has to withstand greater forces than when hovering, as, RPM will be higher when ascending, manouvering, etc. The 'hovering in ideal conditions' scenario gives us a good starting point, though.
Manouverability will also require lightweight blades (manouverability will suffer as disc area increases, but as we've already seen from the maths above and from the graph that BigBad posted in the 'other' thread, large disc area is essential for efficiency), in order to accelerate/decelerate the rotors quickly they will have to be as lightweight as possible.
Drag is the other factor that requires a thin blade section.
High RPM does facilitate a small 'angle of attack', but does increase the centripetal/centrifugal forces in the blade. From what I've read about full size 'copters, it's these forces that ultimately determine maximum RPM, as strength is proportional to mass. As the mass of the blade increases, the strength increases, but so do the centripetal/centrifugal forces within it, due to the incresed mass.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
how about a really lightweight body, even with foamy material, which would then be dipped in a carbon fiber and resin mix in a very light mix, many times like a lacker?
it would ensure minimal physical weight, maximum surface resistance, and the outer layer would maintain the cohesion of the innards...
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Shrad wrote ...
how about a really lightweight body, even with foamy material, which would then be dipped in a carbon fiber and resin mix in a very light mix, many times like a lacker?
it would ensure minimal physical weight, maximum surface resistance, and the outer layer would maintain the cohesion of the innards...
The problem is, (as ive layed up many fiberglass, CF and aramid fiber structures), they are already so thin at the mid and outer radius theres' no space for shear web or lightening pockets / foam.
with my 3D printer, printing nearly hollow structures, with minimal shear web, ive blown out my tail at 15+ meters above ground as Andrew predicted
These are about 18 inch props, note the root area. its also steep there. (about 80 to 120 us$, for two)
these are 26 inch props, they go all the way up to 29 inch and from 5 to 8 pitch. (about 300 us$ for two)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Well, the props you posted above certainly look more like the rotors on a V-22 Osprey than conventional 'copter blades.
The V-22 does have high disc loading (even higher than originally planned, as rotor diameter was reduced from 43 feet to 38 feet in order to facilitate below deck storage on carriers).
The kind of prop used on conventional aircraft can be wider with plenty of camber/angle of attack, as it's designed to travel forwards at hundreds of miles per hour, a completely different scenario to hovering, where that design will produce loads of drag.
If we want low disc loading, I think the best approach would be to start with total estimated mass of the 'copter, and number of rotors, then decide on the disc loading we're after, and then see what the disc area of each prop comes out at.
The disc loading on the V-22 is around 100kg/m^2, and power to mass ratio is 427W/kg, compared to the disc loading of Atlas, the human powered helicopter, which is less than 0.1kg/m^2.
It may be worth working out the disc loadind on your existing tri-copter first,
The point I'm making, I suppose, is that the blade design is also dependant on disc loading. It may also be worth looking at the disc loading that the rotors posted above are designed for.
Conventional copters get down to under 20kg/M^2, we should be looking to do better than this, if we're after greater efficiency and longer flight times. (I think we should be aiming for less than 5kg/M^2, maybe even less )
EDIT: The Robinson R-22 has a disc loading of 14kg/m^2.
You can see the cross section etc in the photo. Admittedly, this is designed for use with a swashplate, so we may want to include some twist and maybe taper it a bit towards the tip
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ok so lets start this way.
1,430 g AUW. 3 rotors, all 12in diameter, 3.8 pitch, called a "slow-fly" prop.
while moderately maneuvering, a drew 31 amps at about 10.7 volts. from A 3 Ccell lipo (12.6 fully charged, 9.9v discharged) im also having a battery problem, but we'll deal with that later.
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Propeller Physics and VABs Program.
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