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Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Ok, so just looking at the graph, it looks like the intersection with the Y axis equates to 1Lb/foot^2 = 12Lbs/Hp, although the Wikipedia graph doesn't appear to support this, and suggests that 1Lb/foot^2 disc loading equates to significantly more than 12Lbs/Hp.
We definitely need more data to plot. I also think that if, as Udo suggested, higher disc loadings can by more efficient, this graph won't be a straight line, and will actually be a curve.
I will continue with the maths above, just to see where it goes, but I still think we need more data before performing any reliable extrapolation.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Well, I've got as far as y=mx+b with the graph, but when I try to put any numbers in it doesn't seem to make any sense. I'm probably making some fundamental mistakes with logs, it's been a long time.........
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
wrote ...
I will continue with the maths above, just to see where it goes, but I still think we need more data before performing any reliable extrapolation.
Extrapolation is, by definition, unreliable. You don't do new measurements, new science, with the intention of extrapolating, that way lies madness, unless you're a cosmologist of course. Dig the slope out of the log log graph to see if it is somewhere near a strght line, to see if you have a power law, and if you have, whether you can understand why. Then get new measrements, preferrably beyond the extremes of your existing data, and see if the line starts to bend, as another term, neglected in the middling region, starts to become significsnt (like I expect viscous drag to be in the region of very low disc loading - but, I'm spoiling the plot!)
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Dr. Slack wrote ...
Extrapolation is, by definition, unreliable. You don't do new measurements, new science, with the intention of extrapolating, that way lies madness, unless you're a cosmologist of course. Dig the slope out of the log log graph to see if it is somewhere near a strght line, to see if you have a power law, and if you have, whether you can understand why. Then get new measrements, preferrably beyond the extremes of your existing data, and see if the line starts to bend, as another term, neglected in the middling region, starts to become significsnt (like I expect viscous drag to be in the region of very low disc loading - but, I'm spoiling the plot!)
I agree that extrapolation is, by definition, unreliable. That's why I'm not prepared to try it without some more data
I also agree that viscous drag will become a factor with very low disc loadings, but I still don't think this graph is a straight line. It does seem to curve at both ends.
Any extrapolation won't be 'spot on', but the better the data, the more accurate it will be.
I'm still trying to get my head around logs again, it's slowly coming back......
EDIT: I think we'll lose all manouverability long before viscous drag becomes a dominant factor, resulting in a completely uncontrollable, useless aircraft.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ive got the 12 and 14 inch props on there way, but APC can be slow. They should be here soon. Then well be able to measure from 10 to 12 to 14 inch disc loading. graph that I think well see a concave up curve.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
ive got the 12 and 14 inch props on there way, but APC can be slow. They should be here soon. Then well be able to measure from 10 to 12 to 14 inch disc loading. graph that I think well see a concave up curve.
While I think those props are optimized for a higher disc loading than we need, they should provide some useful information. Props optimized for a lower disc loading will be more efficient.
The three plots should give us enough information to tell if it's a straight line or a curve.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
The confounding factor here is probably the fixed pitch of the blades; the manufacturer will have pitched them somehow, but you might want them differently, probably finer.
Probably the more or less ideal way to graph this would be to use a straight prop and then statically adjust its pitch to give the best efficiency for the same target thrust; and then do that over different lengths; keeping the target thrust the same.
Otherwise you're at the mercy of the manufacturers; whatever pitch they think their customers want.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
BigBad wrote ...
The confounding factor here is probably the fixed pitch of the blades; the manufacturer will have pitched them somehow, but you might want them differently, probably finer.
Probably the more or less ideal way to graph this would be to use a straight prop and then statically adjust its pitch to give the best efficiency for the same target thrust; and then do that over different lengths; keeping the target thrust the same.
Otherwise you're at the mercy of the manufacturers; whatever pitch they think their customers want.
I agree that we want a prop with less pitch than the 'all rounders' that the suppliers provide, but we should still learn something from three different diameter, mass produced props. We should be able to conclude whether or not it's a straight line or a curve.
An optimized prop for our application will be more efficient than the mass produced ones, however I don't think we want a prop with the same pitch all along the length (if that's what you mean by a 'straight prop').
Once we have these figures we should be able to design a prop with a bit of 'twist' using the relevant maths, though, although I'm still expecting a bit of 'trial and error' before the optimum design is reached.
We will, pretty much, be comparing 'like with like'.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
what about a prop hub that allows changing pitch with a set screw ? for multiple iterations in my thrust stand?
the problem I see either way, the heli prop or the MR prop. The inboard can produce a lot of thrust, but it will also be the first part to stall, with horrid separated flow.
if the intereior 30% of the blade, has a undercamber, but no variable pitch. The last 70% of the blade could be variable pitch if we are willing to go down that road. i dont like so many moving parts though, and variable pitch really and them on.
the advantage of variable pitch is that the head speed is held constant, and the variable pitch can take place quickly, therefore aiding the PIDs in stability. Where as in the current most popular process, the whole prop disc and motor most change inertial state, which takes time and hinders the PID stability.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Personaly I think we need to calculate diameter, RPM, pitch and slip, then design the prop accordingly.
We can calculate these once we have an idea of the disc loading we require.
The info from the props on order will be useful, however these props won't be optimized, so we won't need as big a prop as the data from these props will suggest.
I'd also suggest having less pitch than the calculations suggest at the root, due to the stall and drag that you refer to.
I assume, from looking at examples, that conventional 'swashplate' copters have 'straight' blades, with no twist, so that they can be accelerated up to speed before increasing pitch for lift. I think we can improve on conventional copter efficiency (assuming we don't use swashplates, and have 'fixed pitch') by adding some carefully calculated 'twist'.
Assuming we need to manufacture some blades, though, it may be simpler to use straight blades to start with, for prototypes, until we get close to where we want to be.
It may be worth considering using a hub with adjustable blades to get some initial data, but these needn't be conventional 'variable pitch' blades.
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