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Old Jan 08, 2007, 12:59 PM
edi
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Originally Posted by mnowell129
Here I'm tempted to ask the paraphrased question "If a tree falls in the cyberwoods did it make a sound?".
If it is a balsa tree, the answer is yes.
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Old Jan 09, 2007, 09:07 AM
Mickey from Orlando. Really.
Joined Nov 2004
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It must be a light sound.
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Old Jan 09, 2007, 10:35 AM
Mickey from Orlando. Really.
Joined Nov 2004
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Don't lag behind...

Let's tackle an easy one for now. Lead Lag hinges. The lead lag hinge is the hinge that lets the blade move backwards and forwards in the plane of rotation. If the blade is running exactly straight out from the hub (viewed from the top) it is not leading or lagging at all. If the blade is running slightly ahead of the rotation it is leading. If the blade is running behind it is lagging.
This doesn't quite make sense because the blade is where it is, so what's to compare to? But consider a 4 bladed head with the blades rigidly fixed to the hub. All the blades are 90 apart. Suppose that one of the blades experiences some mysterious force (to be explained later) that pushes this blade of exact 90 alignment. That is, the other three blades are 90 apart but this last blade is 85 from one blade and 95 from the other. This blade is leading or lagging depending on the direction it is off.
But because the head is rigid, the blade/blade grip is being bent to make the angle. A lead lag hinge is a hinge that allows the blade to lead or lag without producing stress. The pin axis of the led lag hinge is parallel to the shaft so that the blade can swing forward and backwards without stressing the hub. Rotors with more than two blades have lead/lag hinges. In some cases the lead/lag hinge is not an actual hinge but is some material designed to bend, this being called an elastomeric head.
So the question is why do we need this hinge, that is, what mysterious force causes the blade to move forward or backward around the circle.
The answer is called conservation of angular momentum. A mouthfull (handfull when typing?) of words that simply mean that when something is spinning at a certain energy level, inertia tries to maintain the energy level. Linear inertia most people have a feel for, when you shove a heavy object and quit shoving it tends to keep moving. Angular inertia is a bit different in that the inertia of a spinning object depends on how far the mass is from the center of rotation. This is the old ice skater example. An ice skater starts spinning with arms out, as the arms are drawn in the rotation speeds up. But in both cases the momentum is the same. A higher speed of the mass at a shorter radius is equal to a lower speed of the mass at a higher radius.
If you've ever played on a kids merry-go-round (don't know what the international term for this is, but its the big spinning wheel in the kiddie park), you've experienced conservation of angular momentum. If you spin the wheel and stand at the edge you get one speed, but as you move to the center the wheel speeds up. But the angular momentum is constant.
Rotor blades do the same thing. As a blade flaps, say for example up in the front to cancel asymmetric velocity, because it flaps up with respect to the shaft it's radius goes down. As the radius goes down it speeds up. The blade in the back slows down for the opposite reason. So what happens is that the blade in front leads and the one over the tail lags. At the sides they neither lead nor lag. So once per revolution the blades lead and lag around the circle. If you didn't have lead lag hinges the hub would break from the constant bending. Cierva ignored this for a long time and had chronic hub failures until he relented and put lead lag hinges on.
On models we usually don't have lead lag hinges for a couple reasons. One is that our hubs are so much stronger compared to the full sized ones, they can take the stress. Second is that most models, especially helicopters, have the blades attached with one bolt. This allows the blade to lead/lag slightly when necessary.
Finally, two blades systems don't usually have lead lag hinges either. The hub is strong enough generally and the hub simply transfers the lead or lag to the other blade across the hub.

Some side thoughts.

The more the blade flaps, the more it is going to lead lag.

Pinning your blades to prevent lead lag may give you a pretty good shake if your blades are flapping. Besides centrifigul force does a much better job on keeping the blades aligned.

In two bladers, the fore blade is leading, the aft one lagging. So both blades are angled off to the retreating side at the same time and the center of mass of the rotor system isn't in the center of the shaft anymore and you get a 2/rev shake. (The teetering, Underslung head is used to mitigate this and another shake that occurs, more when we get to underslung head).

mick


More topics I though of: Underslung heads and blade balancing.
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Old Jan 09, 2007, 03:32 PM
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Mickey,

post #88: it's a) and b) for me. And c) it's interesting to read.

Let's make this thread sticky. And I suggest the same for 'the Challenge of Gyrocopter Design'. I always have to search for that one.

Jochen
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Old Jan 09, 2007, 04:02 PM
Mickey from Orlando. Really.
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Originally Posted by JochenK
Mickey,
And I suggest the same for 'the Challenge of Gyrocopter Design'. I always have to search for that one.

Jochen
Thanks.
I didn't know that the other thread still had any interest.
Glad that's its interesting. I'm enjoying the writing.
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Old Jan 09, 2007, 04:43 PM
iPhly R/C with iPhone
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Mick, thanks for the explanation. The way I had it explained to me was that Cierva came up with lead-lag hinges (which had shafts offset from vertical) to cyclically change blade pitch to offset asymmetric lift, and that even with a vertical shaft, the LLH helped offset AL because a lagging blade saw a reduced airspeed and consequently reduced lift. I thought it was probably the wrong explanation, now I know. Thank you!

It's interesting to me how asymmetric lift is a common explanation for so many things in rotor aerodynamics, and how many of them turn out to be stress-related, like delta-3 and lead-lag hinges. Thanks for giving us a coherent explanation.

I've been looking at different coaxial helicopters lately, and I noticed that Blade CX and a few similar designs have no LL hinges, but others, like the Lama, do. Do you think that's related to the original Lama's light and weak foam blades? Lama's weaker, more flexible foam blades can't take the bending loads, but BCX's nylon blades can? (I think Lama's blades were designed to shatter on impact)

Quote:
Originally Posted by mnowell129
More topics I though of: Underslung heads and blade balancing.
Yes, blade balancing! There is something that works for me and anyone who flies a Slow-G or a Penni, blades that have no tip weights, but they seem to be important, and I'd like to have a better understanding of why it works even though it shouldn't. Many smaller helicopters, especially the coaxial ones, seem to have no tip weights, but the larger ones all do. Is that related to higher torsional rigidity in smaller blades?

Ari.
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Old Jan 09, 2007, 04:45 PM
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Quote:
Originally Posted by mnowell129
Thanks.
I didn't know that the other thread still had any interest.
Glad that's its interesting. I'm enjoying the writing.
That thread is very important. It would be best if you could meger it into this one, or at least to add a link from your first post in this thread to that one. I don't know if we want to have 4 stickies in this forum, but it would be good to have the "challenege..." more accessible.

Ari.
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Old Jan 09, 2007, 06:27 PM
Mickey from Orlando. Really.
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Quote:
Originally Posted by iter
Mick, thanks for the explanation. The way I had it explained to me was that Cierva came up with lead-lag hinges (which had shafts offset from vertical) to cyclically change blade pitch to offset asymmetric lift, and that even with a vertical shaft, the LLH helped offset AL because a lagging blade saw a reduced airspeed and consequently reduced lift. I thought it was probably the wrong explanation, now I know. Thank you!

It's interesting to me how asymmetric lift is a common explanation for so many things in rotor aerodynamics, and how many of them turn out to be stress-related, like delta-3 and lead-lag hinges. Thanks for giving us a coherent explanation.
My belief is there are a lot of "explanations" that come from pilots and mechanics who were told something by the engineers they didn't quite follow or the explanation was abbreviated to avoid a lengthy mathematical explanation or who made a good educated guess at the way something worked and people believed them because obviously they could fly the thing.....
As an example the seemingly most hallowed autogiro book there is, "Autogiro, The story of the Windmill Plane" by George Townson. The author is/was a pilot, not an engineer. Here are some gems from the theory section of his book:
"Cierva also put a vertical hinge on the blade to permit it to move fore and aft to relieve the bending as the drag increased on the "advancing" side and decreased on the "retreating" side."
(mickey: this completely misses the conservation of angular momentum discussion of blade in-plane lead-lag.)
"As the blades advance, the increased air speed causes the blade to climb or "flap.".. This action effectively equalizes the lift on each side of the rotor disc and permits the autogiro to fly level in forward flight instead of rolling because of the "unbalance of lift" across the rotor disc."
(mickey: this ignores the basic physics of flapping that have been covered here.)
"On the autorgiros produced by Pitcairn and Kellet, the rotating mast of the rotor was inclined to the retreating side...in that way "encouraging" the blades to flap..."
(mickey: this is of course fully explained as cyclic to the retreating side to compensate for coning induced roll. It's also silly to think that the blades need "encouragement" to flap.)
I can't tell you how many times I've seen this used as reference. The rest of the book is a very nice coverage of the history and characteristics of various autogiros, but the theory section is just plain wrong in several places and it gets referred to over and over.
Quote:
Originally Posted by iter
I've been looking at different coaxial helicopters lately, and I noticed that Blade CX and a few similar designs have no LL hinges, but others, like the Lama, do. Do you think that's related to the original Lama's light and weak foam blades? Lama's weaker, more flexible foam blades can't take the bending loads, but BCX's nylon blades can? (I think Lama's blades were designed to shatter on impact)

Ari.
My educated guess is that the lead lag hinges on the lama are strictly to reduce blade failures when you hit something and make them easy to replace, which apparently you have to do fairly often. The lead/lag forces don't happen in a hover either, which is where lama's/BCX's spend most of their time. Keep in mind that coning is not flapping so a rotor in hover, even with coning has all the blades running at the same radius, therefore without varying lead/lag. It's when you start forward movement that the rotor flaps up in response and the lead/lag variations happen.


Quote:
Originally Posted by iter
Yes, blade balancing! There is something that works for me and anyone who flies a Slow-G or a Penni, blades that have no tip weights, but they seem to be important, and I'd like to have a better understanding of why it works even though it shouldn't. Many smaller helicopters, especially the coaxial ones, seem to have no tip weights, but the larger ones all do. Is that related to higher torsional rigidity in smaller blades?

Ari.
The short answer is yes. It's related to torsional rigidity. When I visit blade balancing I'll elaborate.

Thanks for the questions!
mick
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Old Jan 09, 2007, 06:50 PM
Mickey from Orlando. Really.
Joined Nov 2004
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Quote:
Originally Posted by iter
I don't know if we want to have 4 stickies in this forum, but it would be good to have the "challenege..." more accessible.

Ari.
I think I would be too embarassed to have 3 stickies at the top of the forum.......
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Old Jan 09, 2007, 08:20 PM
iPhly R/C with iPhone
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Mick, face it, you /are/ the forum.

Ari.
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Old Jan 09, 2007, 08:46 PM
Mickey from Orlando. Really.
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Quote:
Originally Posted by iter
Mick, face it, you /are/ the forum.

Ari.
Flattering. However there's lots of cool stuff here and great ideas from everywhere. I'm just trying to help all the great experimenters by supplying the best applicable knowledge.
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Old Jan 09, 2007, 09:18 PM
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Quote:
Originally Posted by iter
Mick, face it, you /are/ the forum.

Ari.
I second this. Mickey's on going contributions, advice, lessons and answers to questions transform this forum from "interesting" to "intensely interesting and valuable".

Dan
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Old Jan 10, 2007, 05:40 PM
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Quote:
Originally Posted by mnowell129

In two bladers, the fore blade is leading, the aft one lagging. So both blades are angled off to the retreating side at the same time and the center of mass of the rotor system isn't in the center of the shaft anymore and you get a 2/rev shake. (The teetering, Underslung head is used to mitigate this and another shake that occurs, more when we get to underslung head).

mick


More topics I though of: Underslung heads and blade balancing.
Would a 3 bladed rotor have less shake than a 4 blade design, all else being equal?

Dan


ps: not only do I read this stuff, I reread it many times!
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Old Jan 12, 2007, 07:56 AM
Mickey from Orlando. Really.
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Quote:
Originally Posted by leadfeather
Would a 3 bladed rotor have less shake than a 4 blade design, all else being equal?
I think that it's true of any odd number of blades.
It occurred to me that we should discuss teetering heads. They have some different properties from flapping heads.
Gonna be out for a few days, but will get back to this when I return.
Everybody go fly for the weekend!
mick
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Old Jan 12, 2007, 10:58 AM
edi
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Quote:
Originally Posted by mnowell129
I think that it's true of any odd number of blades.
Really? Also for a one-blade rotor vs. a two-blade one?

Quote:
Originally Posted by mnowell129
Everybody go fly for the weekend!
Stormy outside ...
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