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Author Topic: Help with indoor duration helicopter  (Read 10776 times)
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JetPlaneFlyer
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« Reply #25 on: November 25, 2010, 02:19:15 AM »

John,

For a prop/rotor blade it would be almost impossible to achieve elliptical lift distribution because the outer part of the blades will always tend to lift more due to their much higher airspeed. Unlike a wing where airspeed is constant along the span elliptical lift distribution, if it could be achieved, would not the most efficient on a prop or rotor.

The varying inflow argument is interesting. On a heli I'd expect to see the greater inflow out quite near to the tips where most lift is produced. That would mean to achieve the same AoA the blade would have to have the twist reversed i.e. the blade angle would increase as you moved away from the hub. In practice based on the reality that real helis have constant angle I suspect the effect is insignificant.

Kody.. The other challenge in addition to efficiency is achieving stability. The idea of putting the rotors on top with the weight of the rubber motor etc hanging below sounds intuitively right but I'm not sure it would in itself actually produce stability. In fact counter intuitive though it sounds.. having the rotors at the bottom and the weight up high should be more stable. This is something that's really hard to get one's head around but this is the principal that the Hiller: http://www.aviastar.org/helicopters_eng/hiller_platform.php and De Lackner: http://www.aviastar.org/helicopters_eng/lockner_helicovector.php helicopters operated on.. Neither had any form of auto stability whatsoever (no gyros or anything similar), the stability came from the position of the CG being higher than the blades. Also anyone who has flown a normal RC or full size helicopter that intuitively you would think 'should' be stable due to the pendulum effect of the fuselage would tell you that they are anything but stable.

Steve
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Greg Langelius
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« Reply #26 on: November 25, 2010, 08:30:22 AM »

Jim Walker's Ceiling Walker.

...And..

Greg
« Last Edit: November 25, 2010, 08:45:42 AM by Greg Langelius » Logged

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« Reply #27 on: November 25, 2010, 01:12:12 PM »

In my first posting I tried to avoid being accused of confusing a simple subject with theory – it never works, people always want to know more! I will try to explain the fundamentals as simply as I can.

 A propeller is formed of (usually) two wings traveling along a helical path. The design problem is to ensure that each part of the wing is approaching the airflow at a suitable angle of attack. Please look at figure 1. Imagine looking down on the aeroplane and seeing a section part way along the nearest propeller blade. The propeller in moving forward goes up the page and in rotating goes across the page from left to right. If the aeroplane is flying forward at a speed V then it will appear to be meeting a wind of speed V coming down the page as indicated by the line V+aV which is drawn to scale to represent the forward speed. In rotating from left to right the propeller blade appears to meet a wind coming from right to left as indicated by the line 2πrn – b, which is drawn to scale to represent the rotational speed. Now if you remember a bit about vectors from school you will know that the line Vr, completing the triangle, gives the strength and direction of the combined airflow approaching the propeller blade at that point. The propeller blade is then set at an angle of attack to the combined airflow Vr.

Now I must explain the labels, V+aV and 2πrn - b, which I put on the forward and rotational speed vectors. The air is accelerated as it passes through the propeller disk by a small percentage called ‘a’ (known as the inflow) which is usually about 5% for an average rubber model so the speed that the propeller blade sees is a little more than the forward speed of the aeroplane. The 2πr is the circumference of the circle traveled by the piece of blade at radius r and multiplying that by n, the revs/second of the propeller, gives the circumferential speed of the blade at that radius. The symbol ‘b’ I have put in just for completeness, it is a swirl given to the air as it passes through the propeller disk which gives a slower circumferential speed but is usually so small as to be ignored.

That one simple triangular diagram tells you pretty well all you need to know about propellers! If the speeds are considered constant then the only variable is the radius where you are looking at the propeller. If the radius is large, near the tip, then the bottom line of the triangle is long and the angle A gets less whereas if the radius is small then the bottom line of the triangle is short and angle A is high so the propeller blade twists from root to tip which you knew already. But now think of a hovering helicopter, there is no forward speed so the triangle just changes to a single line for the rotational speed, there is no angle A and you just need an angle of attack.

I think there is a limit on the length of posts so I will stop this one here and give a few more comments in another post.

John
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« Reply #28 on: November 25, 2010, 01:18:36 PM »

It will be apparent that if the forward and rotational speeds are known then a very simple bit of trigonometry will allow the angle A to be calculated for any radius r along the propeller blade in a few seconds with a hand calculator. Ignoring the small factors a and b the equation is:

A=arctan(V/2 pi rn)

To shew how this equation could be used. Although ideally the helicopter would hover during all the flight we know, because it is rubber driven, that it will climb during the early parts of the flight; assume it climbs 30 feet in the first minute then V=0.5 ft/s. The largest change in the angle A will be at the root of the blade; assume this is 2 inches from the centre then r=2/12= 0.1667ft. Now for the rotational speed I am guessing 4 rev/s but that certainly needs some experimental verification. Anyway putting those figures in the equation, A=arctan(0.5/2 x 3.14 x 0.1667 x 4) which gives 6.8 degrees. So you would mount the blades at least at 6.8 degrees to ensure the centre part was not thrusting downwards.

Something else to bear in mind. As you move out along the blade the rotational speed increases, as you double the radius you double the speed, but remember that aerodynamic forces increase as the square of the speed. So if I have a certain lift force at my nominal root at 2 inches from the centre then my three quarter radius point, at 6 inches from the centre, is going to give 9 times the lift. I mention the three quarter radius point because this is the area on a propeller where most of the thrust is generated, after this point the thrust falls off because of tip losses – a good reason to round off the tips. If you do decide to do this I would recommend keeping the trailing edge straight and curving the leading edge back to meet it.

You may remember in the last post I mentioned the inflow factor ‘a’ which adds to the forward speed. This would be a reason to increase the blade angle towards the tips: however other reasoning says we should try to even out the lift forces by washing out (reducing) the faster moving outer parts. Perhaps this is why full size and radio helicopters appear to use flat blades. To me there seems to be an argument for tapering the blades, perhaps using Larrabee shape on these indoor ones with restricted diameter. I guess the full sized people don’t worry because of their high aspect ratio.

Well, that has stuck my neck well and truly out. I shall be interested to see the outcome and good luck to the entrants. (I suppose I could make one myself – I never thought of that before!)

John (I have just noticed whilst posting that I have overlapped some of the stuff from Steve and OZPAF. I am sorry about that but it seemed too late to change it all.)
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« Reply #29 on: November 25, 2010, 06:35:45 PM »

I'm sure this will help Kody with his project, Hepcat . I found it a clear coverage and I agree with your comments re Larrabee Planforms. What I am beginning to see is just how complex the whole subject of Helicopter rotor behaviour is. It was interesting to read all the comments - theory and practical.

The theory of hovering duration Helicopters probably needs more discussion perhaps on the Aerodynamics section.

John
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« Reply #30 on: November 25, 2010, 07:27:23 PM »

I don't understand why one set of rotors needs to be set at a different pitch from the other unless you need their speed to match. Why would that be necessary?

If the blades spin at a different speed, the torque would be uneven and that would throw the balance off, would it not?

I like the idea about weighting the tips for stability. I'll try without the weight first and gradually increase it as necessary to see its effectiveness.

I went and looked at some helicopter blade tips and it looks as if they are designed primarily for high speed applications, of which this is not. However, I still believe that there are actual tip designs that would be most efficient. I'll have to test some out (maybe a sweptback design?), but that will come after I get a decent design up and flying. EDIT: Later on in this post I find that a sweptback rotor tip is beneficial and most likely ideal. Cool

I've been thinking a bit about the small vs. large chord. If I were to choose the former option, I would clearly need more blades to slow the rotor rotation and increase lift. It puzzles me as to why they would be more efficient. Again, something to look into...

Hmmm...interesting thoughts on CG and stability. So the first helicopter will be made with the powered rotor on the bottom. I apologize for changing my mind so much. I can guarantee that it will not be the last time though. Saying that the learning curve is steep right now is an understatement and I will make decisions (and change them) as I continue to learn.

Now in response to John's (Hepcat's) post (The Biggie)- I followed the math to the best of my ability. I'm in pre-calc, so I know it shouldn't be that difficult (being only trig really), but it becomes more complicated when you throw in the theories and ideas that we are discussing. In addition, the only schooling that I've had is ground school, and I don't think that's the school you were referring to in your post. Wink On second thought, I doubt your intended audience was actually me! Roll Eyes Am I the only one without an engineering degree in aerodynamics of some sort here?

Okay. In all seriousness, I do have a couple of questions: In part II of your post, you said that I would want to "mount the blades at least at 6.8 degrees to ensure the centre part was not thrusting downwards." I assume that you are talking about the root of the blade when you say "centre part". I'm confused as to why the root would thrust downwards if the blade angle is at all positive. Would you please tell me what I am misunderstanding here?

You also talked about tip losses. I tried to find some information on the subject, but wasn't very successful. Could you explain the cause(s) of tip losses or guide me to some write-ups on it? So now I'm imagining rotors with straight leading edges and trailing edges, but the last 25% of the LE will curve back to meet TE, just as you described. This is the Larrabee prop design?

John (OZPAF)- I read your post as I wrote this reply. It does help me very much. The topic is challenging, which I can see because I've found myself in the middle of aerodynamic discussions and debates before, and this is probably the most complex that I've experienced. It should be even more fun testing and working out the kinks! Shocked


Here are some links that I found thanks to Yahoo! Groups:
http://freedomflightmodels.com/paypal.htm
http://freedomflightmodels.com/images/catalog/Full%20Helicopter%20Kit%20Large.jpg
You think that's a fixed blade angle? Tongue

-Kody
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« Reply #31 on: November 25, 2010, 07:49:04 PM »

John B.
Thanks for the education! This discussion and some of its offshoots have caused me to rethink a lot of what I do - that is if you can call my earlier efforts thinking.
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« Reply #32 on: November 26, 2010, 10:30:09 AM »

Tip losses...

Swept and tapered tips: http://www.b-domke.de/AviationImages/Rotorhead.html

The Westland Lynx held the world speed record for helicopters. While speed is not your goal, speed records imply very efficient rotors.

I suspect a very simplified Hoerner Tip http://www.zenithair.com/stolch801/design/design.html could be a simple way to help with your rotors' efficiency.

Greg
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« Reply #33 on: November 26, 2010, 12:14:13 PM »

Tip losses...

Swept and tapered tips: http://www.b-domke.de/AviationImages/Rotorhead.html

The Westland Lynx held the world speed record for helicopters. While speed is not your goal, speed records imply very efficient rotors.

I suspect a very simplified Hoerner Tip http://www.zenithair.com/stolch801/design/design.html could be a simple way to help with your rotors' efficiency.

Greg

Swept tips are likely used on the Lynx because on all fast helicopters the limitation on speed is when tips begin to go sonic. A swept tip raises the critical mach number, so allows a little more forward speed. On an indoor rubber power duration model supersonic tip speed is not likely to be a big issue Wink

A Hoerner tip would be as good as anything if it can be done without adding weight.

Steve
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« Reply #34 on: November 26, 2010, 01:20:46 PM »

Greg,

Thanks for the links. There is a lot of good information there, including the information on tip losses that I couldn't find. The Hoerner tip design is so brilliant to me. It shouldn't be too hard to apply it to this design, especially considering that it just comes up to meet the top of the airfoil...unlike the raised or drooped wingtips.

I drew up some plans last night for the rotor and prop hub. Nothing fancy here, just to get some ideas on paper. What to you guys think? I'm sure there can be much improvement. For now, I will just go with hand drawn plans, but I might try computer drawn later on. My rounded tip just looks too big. Plus, it sounds like a Hoerner tip would be a design in itself, not to be combined with a sweptback tip. Since the blade tips aren't going fast enough to raise the critical mach number, the only thing they would do over a square wingtip is lower weight, I would imagine. The Hoener wingtip, on the other hand, would be aerodynamically favorable, at the cost of a bit of weight. I think that the additional weight is worth the efficiency in this case though. So it looks like I'll be redrawing my plans for Hoerner tips.

-Kody
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« Reply #35 on: November 26, 2010, 03:56:18 PM »

I just built one, just to see what would happen...all sheet construction.

Rotor span 11", 2 flat rectangular blades, 4.75 x .5 on bass spar stubs
Bearing to hook 7.9"
10.5" loop of 1/16
Weight 2.25 gms with rubber

Initial pitch settings were by the TLAR method and on 500 turns she seemed to want to climb but instead wandered all over the room.
TLAR wasn't exactly R, so I added some pitch to the bottom rotor and that seems, for some reason unkown to me, to have tamed it a bit. Now it stays in one general area and goes up a little. It seemed to me that the lower rotor was trying to outclimb the upper one, and that pushed it out of vertical, setting it off in horizontal paths. The lower rotor had substantially higher pitch but evidently needed more. Didn't one of the Penaud machines have a rotor only at the top and some sort of a stabilization empennage at the bottom end?

Art.
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« Reply #36 on: November 26, 2010, 04:15:58 PM »

Art,

I didn't think that I was that far behind! Cheesy

Possibly the top rotor was forcing more air than expected on the bottom rotor, so the bottom rotor was spinning much faster than the top rotor. Once you twisted in more pitch, the speed of the rotors was close, so the stability was better.

I believe one of the Penaud helicopters did have a stabilizing bottom section; either that or I thought of the idea. It reminds me of a rocket really, with stabilizing fins. I can't remember why I dropped that idea. I don't see a reason why it wouldn't work.

I've heard of TLAR before, but I've never figured out what it stands for. Would you care to enlighten me?

After hearing about your results, maybe I should just quite asking questions and go build one for myself!

-Kody
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« Reply #37 on: November 26, 2010, 04:29:30 PM »

Nope, turns out it's way more stable the other way around. First, I made up a quick protractor so I could have repeatable numbers (not to mention analyzable, which I'll have to leave to better minds than mine).

Now she's running 25° on top and 15° at the bottom, and she's a whole different animal. Right up to the ceiling on 672 turns and stays there 'til she's out of steam.

TLAR = "That Looks About Right". It evidently doesn't work on 'copters as well as it does on normal planes.

a.
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« Reply #38 on: November 26, 2010, 04:39:37 PM »

Interesting results! Now I'm really confused. It really gets fun when things on paper don't work out in real life!
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« Reply #39 on: November 26, 2010, 05:29:11 PM »

Great stuff Art. I always admire the way you get on with the job - and produce useful results. It is nearly bedtime so I can't discuss much now but can you tell me where the CG is and could you let us know what happens if you reduce the two blade angle settings to, say, a half (12 and 7)?

John
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« Reply #40 on: November 26, 2010, 05:59:18 PM »

Cub, I didn't intentionally run out in front of you, but this was just knocked out quick... no drawings... the specs I listed were measured off the finished machine as there were no plans.

John, hub to hub is 7.95 and she balances at 4.15, so that's 52.2%. Running 12 & 7 she looks like she wants to go off the vertical but never gets past what looks like 25-30°. Things happen faster at the low pitch, and the flight attitude looks cone-shaped. Steady, though, without any tipping all the way over to 90° and darting around the room, the way the first attempts at flight did. I think it's stabler at the higher pitches. I'll take it over to my daughter's house. She has an 18' ceiling; mine's only 13.

a.
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« Reply #41 on: November 26, 2010, 06:57:30 PM »

Nope, turns out it's way more stable the other way around. First, I made up a quick protractor so I could have repeatable numbers (not to mention analyzable, which I'll have to leave to better minds than mine).

Now she's running 25° on top and 15° at the bottom, and she's a whole different animal. Right up to the ceiling on 672 turns and stays there 'til she's out of steam.

That's very interesting - appears you have moved the centre of lift of the combined rotors above the vertical position of the CG?

The relative speeds of the rotors would be interesting as well.

John
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« Reply #42 on: November 26, 2010, 07:55:55 PM »

I think the only way I could figure out the relative speeds of the rotors is to check how long it takes the motor to run out at various pitch angles. We also have to take into account that the motor weakens with each test run, and there's the factor of the rotor that's carrying the motor stick might be running a bit slower because of it.

But here's what I did in the interest of scientific research. Leaving the machine intact, I wound it 600 turns backwards, launched it upside down, and ran. She started to tip over as expected, but never exceeded about 45°, and flew around in big circles without much altitude gain. As the power wore down, it got more vertical and climbed to maybe 11 feet, cruised there for a bit, and descended.
I'll fool with this toy for another couple of days, and then try to engage in some built-up rotors and a little less stout construction.

A. (The more I observe, the less I know.)
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« Reply #43 on: November 26, 2010, 09:02:14 PM »

John,

For a prop/rotor blade it would be almost impossible to achieve elliptical lift distribution because the outer part of the blades will always tend to lift more due to their much higher airspeed. Unlike a wing where airspeed is constant along the span elliptical lift distribution, if it could be achieved, would not the most efficient on a prop or rotor.

The varying inflow argument is interesting. On a heli I'd expect to see the greater inflow out quite near to the tips where most lift is produced. That would mean to achieve the same AoA the blade would have to have the twist reversed i.e. the blade angle would increase as you moved away from the hub. In practice based on the reality that real helis have constant angle I suspect the effect is insignificant.

Steve,
I missed this post, but I still feel that maximum efficiency - that is max thrust power for given input power will come from an elliptical lift distribution along the wing/blade. To achieve this blade needs to be working at a local CL related to the local chord width rather than a constant AOA. Thus I’m inclined to favour Larabee Planforms as the ideal as they were designed for elliptical distribution..

I’ve included an old Aeromodeller article from Reg Boor which gives a fair description of Larrabee Props and the theory

I'm beginning to feel the same way Art. Interesting to see that it finds equilibrium with forward motion - why doesn't it tumble? The centre of combined lift would be slightly behind the CG (as it would be below for the inclination to happen) you would imagine but the forward movement while inclined must alter the balance between the top and bottom rotors.

John

Whoops Here is the link to the complete Larrabee article by Reg Boor.
www.tpbweb.com/media/catalog/1001.p
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« Reply #44 on: November 26, 2010, 10:15:43 PM »

I threw this together tonight to get an idea of building this type of wing. I'm getting used to making split ribs (my first try at them) and I am getting better consistency as I go. I used about 50-50 or 60-40 Duco to acetone respectively. My cheapo scale measured it as .2g, but that's as accurate as it is. When I actually get to building, I'll use the precise scale in my chemistry class.

If you look closely, you can already see it starting to warp. I used 6# B-grain. Would anybody choose something else? I don't have any 6# C-grain. Sad A spar should help it out. I can't even guess as to if I'll need it or not for flight loads. Anyway, I'll have to find a way to prevent warps. Maybe soaking it and leaving it clamped down overnight will help solve things...

And on another note, maybe I shouldn't be worried about building so light. I will have to meet the minimum 4g "dry" weight, and maybe I could strengthen it up a bit. Hmmm...
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« Reply #45 on: November 26, 2010, 11:13:19 PM »

We have to wait for Hepcat to rejoin us in the morning because, other than Larabee himself, John is the leading authority on how and why they work. That said, I've built a couple of them, and as I remember, their main thing is the blade shape, and they are to be applied to a helical blade only (no washin/washout). This kinda lets the helicopter crowd out. Besides our using paddle blades, the only 'copter blades I ever saw that aren't rectangular, or close to it, were the ones on the Cierva autogiros. What's that, the 1920's??

I just tried shifting the CG south (will that make sense to OZPAF?) by driving a plastic headed pin (.46 gm) into the bottom of the stick. Aside from the expected degradation of climb performance, it made the stick flail around, at about 30° off vertical, pivoting about the nose bearing. It doesn't do that with the high CG.

a. the clueless
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« Reply #46 on: November 27, 2010, 12:01:14 AM »

I was thinking about using the Hoerner tip, but it's effectiveness will actually be little to none because the bottom of the rotor is uncovered. So I guess that means laminating a Larabee tip?
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« Reply #47 on: November 27, 2010, 12:24:11 AM »

A Hoerner tip can't be built onto a wing with no thickness and shallow camber, so that's out.

A Larrabee blade has a rather straight LE from the hub to the max chord, about 50%, then turns a sharp bend and runs straight out to the tip. The TE has a full curve to the 50% point and then a very shallow curve out to the pointy tip. The effect is to minimize induced drag in the power phase, and to reduce parasite drag in the freewheel. It's not just the tip, it's the whole integrated shape. It works great on a Senator; I don't know why, but Hepcat does. I can't see any application here.

Maybe try tip plates?

a.
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« Reply #48 on: November 27, 2010, 02:40:37 AM »

I just tried shifting the CG south (will that make sense to OZPAF?) by driving a plastic headed pin (.46 gm) into the bottom of the stick. Aside from the expected degradation of climb performance, it made the stick flail around, at about 30° off vertical, pivoting about the nose bearing. It doesn't do that with the high CG.

a. the clueless

CG South - towards me - that's Ok but I'm puzzled by the results. More thinking required. Thanks for your experimentation Art.

John
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« Reply #49 on: November 27, 2010, 04:57:37 AM »

I just tried shifting the CG south (will that make sense to OZPAF?) by driving a plastic headed pin (.46 gm) into the bottom of the stick. Aside from the expected degradation of climb performance, it made the stick flail around, at about 30° off vertical, pivoting about the nose bearing. It doesn't do that with the high CG.

a. the clueless

I know i shouldn't say 'told you so'.. but can't help myself:

Quote from: Steve
The idea of putting the rotors on top with the weight of the rubber motor etc hanging below sounds intuitively right but I'm not sure it would in itself actually produce stability. In fact counter intuitive though it sounds.. having the rotors at the bottom and the weight up high should be more stable. This is something that's really hard to get one's head around but this is the principal that the Hiller: http://www.aviastar.org/helicopters_eng/hiller_platform.php and De Lackner: http://www.aviastar.org/helicopters_eng/lockner_helicovector.php helicopters operated on. Neither had any form of auto stability whatsoever (no gyros or anything similar), the stability came from the position of the CG being higher than the blades. Also anyone who has flown a normal RC or full size helicopter that intuitively you would think 'should' be stable due to the pendulum effect of the fuselage would tell you that they are anything but stable.

Can you try sticking the pin in the top and see what happens? if the weight up top does prove to enhance stability then maybe extending the motor stick above the top rotor (or moving the top rotor down the existing stick) will work well?
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