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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Ive got permission to use VABs but the link expired since Purdue university hosting service went into the trash can. the AnalSwift company has told me how to get going again, and ive foun the PDF manual.
in a few days theyll release a link and ill start loading prop sections in. There is a problem however, i dont know how to use matrix math.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
just had a quick skim through of the VABs manual and I don't see how it will help your uav design much; . it will not help at all with propellor shape/speed/size design . the static stresses on the airframe are negligible compared to landing/crashing (to simulate all variables is impractical) . even if you could work out optimal profiles for beams etc. you are stuck with commercially available shapes (uniform rods, tubes, slabs etc.)
I guess you will have to rely on; . experience of others . intuition . field trials
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I've just looked at the VABS manual too, and it does look a bit disappointing. It would appear from first glance that it is only useful for working out the stresses in a 'pre-defined' propeller blade, as opposed to helping to arrive at a blade design.
I'm not sure yet whether we can input different designs and then compare lift generated in various scenarios, as there is a relationship between the lift generated and stresses within the blade.
To elaborate, if it predicts the 'bending moment' at the blade root, then this is proportional (if not equal to) the lift generated by the blade. and this could be very useful when comparing different blade designs under different conditions
I don't think it tells us anything about losses associated with disc area, etc., however I'm not sure about this, as the 'downward force' exerted by the propeller is equal to the 'upward force' of the accelerated column of air.
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874
If you don't mind waiting 2-3months, I could writre a program to do the simulations, but then back of the note book cacultions, should be enought.
pressure/velocity = volume velocity^2 pressure = volume energy in energy out at 100%
In the past did research on wind turbine, which pretty much work in the opposite theory, but a TSR of greater than 1> is more efficiency but, the torgure is low, a high torgue but low efficiency is a there low tip to speed ratio, in other wards,
if you could make high velocity flow, but without the power of two then it would be the most force, but high velocity and air volume, formulas workout to the same value, "1", so the other equation come into play, if you wan't high moverabilty, make the velocity high to make the power change quick, which will make a unstable aircraft, make the velocity slow, will make a stable craft, the power in and out is the same. the one thing above that isn't in the theory is pressure, that can be reality to weight, in which the gradient of the atmosphere changes with alternative, the pressure changes, which relates to a drop in pressure, and there fore weight, but multi formula maths, says that there is more advances in high altitude, as velocity=P/V is easy and P = V2/V is lower so less pressure more volume gets turned into speed, all in all you can't change the energy input, much, but at high altitude, with a good Carma you will get better effect, forgot the back flow from ground hover, which might confuse you if you add it to statical models.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Andy wrote ...
If you don't mind waiting 2-3months, I could writre a program to do the simulations, but then back of the note book cacultions, should be enought.
pressure/velocity = volume velocity^2 pressure = volume energy in energy out at 100%
In the past did research on wind turbine, which pretty much work in the opposite theory, but a TSR of greater than 1> is more efficiency but, the torgure is low, a high torgue but low efficiency is a there low tip to speed ratio, in other wards,
if you could make high velocity flow, but without the power of two then it would be the most force, but high velocity and air volume, formulas workout to the same value, "1", so the other equation come into play, if you wan't high moverabilty, make the velocity high to make the power change quick, which will make a unstable aircraft, make the velocity slow, will make a stable craft, the power in and out is the same. the one thing above that isn't in the theory is pressure, that can be reality to weight, in which the gradient of the atmosphere changes with alternative, the pressure changes, which relates to a drop in pressure, and there fore weight, but multi formula maths, says that there is more advances in high altitude, as velocity=P/V is easy and P = V2/V is lower so less pressure more volume gets turned into speed, all in all you can't change the energy input, much, but at high altitude, with a good Carma you will get better effect, forgot the back flow from ground hover, which might confuse you if you add it to statical models.
Hope it helps
You've not taken losses into account here, Andy, which increase as prop size decreases and as air speed increases.
The most efficient is always large diameter and low RPM, and it's pretty much exponential, very similar to the 'drag equation'.
Patrick wrote ...
Is there any fluid flow or prop Sim anyone knows about? I see random things on our great all knowing Google overlord.
EDIT, found this :
Certainly looks worth downloading, unfortunately, the website appears to have crashed at the moment.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Just been doing some maths, Patrick. Can you check my transposition?
I came up with this:
Power required = square root (Force^3 / (4.9 x Area))
Force remains constant while hovering for a fixed mass multicopter, the 4.9 is 4 times density of air, also a constant, so, at a glance you can see that, as area increases, required power decreases.
I transposed it from here:
"Note that these calculations are only valid for when the incoming air is accelerated from a standstill - for example when hovering."
EDIT: Now, if we know the mass of the 'copter, we know the force required for hovering (9.8 Newtons per kg?), so then we just plot a graph of power required against disc area and we have a graph that 'should' tell us something
EDIT2: We also need to know number of rotors (n), and divide mass by n, then multiply power by n, to get total power in Watts, etc.
EDIT3: As Sulaiman pointed out (and as I pointed out in the 'other' thread for a 'heptacopter') for a hexacopter, if the rotors are close enough together, the losses are even less, but I'd like to see a graph for tri-copter against hexacopter of the same mass. The tricopter will presumably have less overall mass, though.
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