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Ash Small

Thu Jul 03 2014, 08:01PM

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

Patrick wrote ...

EDIT: on the graph, we see the harrier like implementation on the lower right. and the traditional heli of the upper left. but the effciency is best in the upper right side right?

Disc loading is along the bottom, and efficiency is up the side. Efficiency requires low disc loading, is basically what the graph is saying.

Our disc loading is low (close to the left hand side) but you'd expect better efficiency from a full sized copter. This could be due to scale, or due to poor prop design, or maybe a combination involving other, unforseen factors.

We are in the area of the graph where a reduction of disc loading results in a big improvement in efficiency, if I'm reading it right, but I want to draw a log/log graph before I make any 'predictions'.

I am interested in this stuff, I had marine props going round and round my head 20-25 years ago. I also had an interest in hovercraft design as a kid, but in that case the improvements in efficiency are due to the 'plenum chamber'.

EDIT: I may need to 'tap' you for a loan oneday, when you've made your first million

Patrick

Thu Jul 03 2014, 08:43PM

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

I'll be cashing a 25 million check soon... you want some?

i see the same trend here, no coincidence i think.

Ash Small

Fri Jul 04 2014, 11:59AM

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

Well, the graph is the same shape for the same reasons, although it's a different graph.

It does depict that the space shuttle flys faster than a 747, but that the 747 is more efficient, I think, but I'm not familiar with the 'specific impulse' term. Sounds like 'rocket science' to me

Ash Small

Fri Jul 04 2014, 03:01PM

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

Going back to the disc loading graph, I've done some quick calculations regarding disc area divided by circumferance for two props with the same disc loading but one of 1 metre diameter, and one of 20 metres diameter.

Disc area divided by circumferance for the 1 metre prop works out at 0.25

Disc area sivided by circumferance for the 20 metre prop works out at 5

I guess this illustrates how scale affects these things. These numbers will be proportional to efficiency, with 5 being more efficient than 0.25.

I guess it also implies that we can reduce the disc loading still further on a smaller prop without unduly affecting manouverability, though.

EDIT: The smaller prop has more losses per unit area of disc, even when the disc loading is the same. This will account for the shaded ares on the graph and also accounts for us being outside of the shaded areas, although it may still be possible to improve on prop design, etc., the main reason we are outside of the shaded area is due to 'scaling', as we suspected. I'll repeat it again, for a 'first order approximation', all the losses are at the periphery of the accelerated column of air. Losses increase with V^2 (this may be slightly oversimplified, but it's 'in the ballpark')

I still need to look at the propeller equation again, which I hope to do over the weekend.

Ash Small

Fri Jul 04 2014, 07:12PM

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

Sory about the multiple post, but new info:

Here's my first attemp at the log/log graph. Should I go for the best straight line, or try and plot a curve?

I think there probably were some discrepancies on my part, but it almost looks as if it should be a curve.

Any comments welcome.

EDIT: I'm currently thinking I should try 'best straight line', I don't think the points accurately represent a curve, but I could try both. Any suggestions?

Maybe someone else should check, or plot their own, for comparison?

I can provide a .pdf of the log/log paper if required.

Patrick

Fri Jul 04 2014, 09:15PM

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

so what about prop shape?

Ash Small

Fri Jul 04 2014, 09:48PM

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

Patrick wrote ...

so what about prop shape?

As I said before, more like conventional heli. Right now I'd prefer a second opinion on this graph.

Neil suggested it should be a straight line, no-one else has given an opinion as yet.

I'm planning to re-do the plot, and see if I get the same thing, or something that looks like a straight line or a curve.

But I'd appreciate any other input

EDIT: It does look fairly straight, but maybe I don't have enough plots to tell if it turns upwards at the left hand end?

Maybe we need some more thrust data from 14" props, Patrick?

Patrick

Fri Jul 04 2014, 11:12PM

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

Ash Small wrote ...

Maybe we need some more thrust data from 14" props, Patrick?

ill get some data on the 14's...

until i get the props in the mail heres the link. its listed in 1000's of RPM...

lnik:

Uspring

Sat Jul 05 2014, 10:47AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Ash Small wrote:

Here's my first attemp at the log/log graph. Should I go for the best straight line, or try and plot a curve?

Have a look at the equation for thrust:

F = (2 * rho * A * (Î¶*P)^2)^(1/3)

Raise to the third power on both sides:

FÂ³ = 2 * rho * A * (Î¶*P)Â²

Divide both sides by F * PÂ²:

(F/P)Â² = 2 * rho * Î¶Â² * (A/F)

This is the equation, which relates "efficiency", i.e. F/P to disk loading, i.e. F/A. In a log-log plot it should be a straight line. The graph on helis doesn't exactly follow this description, but gets close. Prop efficiency Î¶ seems to be somewhat larger for heavily loaded props.

Note that the usage of the term "efficiency" is ambiguous in this discussion. I prefer using it for Î¶, which is solely a property of the prop. The "efficiency" F/P also depends on the disk load or the props rpm.

According to the tables Patrick posted, the Î¶ for the 10" prop is 0.41, for the 14" it is 0.40. Both are quite less than sensational values. Note that the lower value for the 14" prop does not imply, that it gives less lift per power than the 10" one.

Ash Small

Sat Jul 05 2014, 11:24AM

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

Thanks for the input, Udo. I'll study this later.

Patrick, do you have any smaller props that we can plot as well?

As Udo points out, I also don't think the prop design you're currently using is optimal for this application. These are, after all, mass produced, relatively cheap propellers.

EDIT:

Uspring wrote ...

Note that the usage of the term "efficiency" is ambiguous in this discussion. I prefer using it for Î¶, which is solely a property of the prop. The "efficiency" F/P also depends on the disk load or the props rpm.

Yep. I was lumping them all in together, but treating the 'prop losses' as a 'secondary factor' which doesn't dominate at this point, and I was assuming an efficient prop.

I agree these need to be considered before finalizing a design, but I was initially concentrating on the 'disc loading' thing.

I don't think Patrick will find the improvements in flight time that he is after by just looking at prop efficiency

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