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Lead and Lag


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#1 Logan

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Posted 23 February 2008 - 22:37

Can someone explain what the "lead and lag" do on a fully articulated rotor system? I understand the meachanics of it, I am just confused on the purpose for which the blades are able to pivot from lead to lag. Can someone explain please?

#2 Wally

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Posted 24 February 2008 - 00:05

Can someone explain what the "lead and lag" do on a fully articulated rotor system? I understand the meachanics of it, I am just confused on the purpose for which the blades are able to pivot from lead to lag. Can someone explain please?


The blade flaps up, the blade radius(?) decreases but the blade is still moving as fast as it was when flat, perpendicular to the mast (the spinning skater analogy). Either the blade bends or pivots at a hinge, usually near the blade root, and advances in relation to the other blades, "leads". As it descends back into the plane of rotation, the process reverses, and the blade "lags". If the head has mechanical hinges, it's "articulated"; if not, then the blades bend, and it's a "rigid-rotor". Two-bladed systems are usually "underslung" so that there's very little change in radius as the blades flap.
The flapping, up and down, compensates for varying amounts of lift across the disc, due to differing relative winds.
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#3 apiaguy

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Posted 24 February 2008 - 01:30

to paraphrase:

as the blades flap there are lateral forces induced on the rotors as the blade center of mass moves up or down (the ice skater demo with their arms) ((conservation of angular momentum)) without the lead lag hinge the rotorhead would have tremendous stress on it and the rotor system could become unstable or break.

The lead lag hinge (and flapping hinge) were pioneered by Juan de la Cierva (1920ish) in his work on the autogyro.

#4 southernweyr

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Posted 24 February 2008 - 10:41

As the blade moves up it gets closer to the axis of rotation. Like apiaguy said, the law of conservation of angular momentum means that the rotational speed of the blade would increase as the cg of the blade moves inward. Then as the blade flaps back down it returns to being farther away from the axis and slows back down. This means that the blades are constantly speeding up and slowing down.

Another way to think of it is to get a three foot long string with a small weight on the end and swing it around. Then let the string start to wrap around your finger. As it gets woud up, it will rotate around your finger faster and faster.


Here is a link to the rotorcraft flying handbook that the faa puts out. It goes directly to coriolis effect aka the law of conservation of anular momentum.

http://books.google....uIupHI#PPT26,M1

#5 Logan

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Posted 24 February 2008 - 11:16

Correct me if I am wrong please. So when the blades flap up they tend to lead (decreasing the diameter of the circle) as the speed of the blades increases? This happens when you are applying throttle or foward thrust? Does this cause a "sling-shot" effect if you were to try to slow your head speed down quickly?

#6 NorCalHeliKid

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Posted 24 February 2008 - 13:31

Correct me if I am wrong please. So when the blades flap up they tend to lead (decreasing the diameter of the circle) as the speed of the blades increases? This happens when you are applying throttle or foward thrust? Does this cause a "sling-shot" effect if you were to try to slow your head speed down quickly?


Dampners dont allow a "sligshot effect", not that it would be possible even in a two bladed system, there are other compensating factors you have to consider which havent to be taken into account (explained above by Wally), the blades dont just turn freely in all stages of rotation and with power on-- decreasing head spead is controlled by correlator and rpm gov's.as well. You would have to ask a Aerospace Engineer the details on some of the more recent advancements in systems...any more details from me and it will start to sound confusing. Hope this made sense!

Edited by NorCalHeliKid, 24 February 2008 - 13:36.


#7 Linc

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Posted 24 February 2008 - 14:04

Correct me if I am wrong please. So when the blades flap up they tend to lead (decreasing the diameter of the circle) as the speed of the blades increases? This happens when you are applying throttle or foward thrust? Does this cause a "sling-shot" effect if you were to try to slow your head speed down quickly?

When the blade flaps up, that is when the diameter of the circle for the center of mass of the rotor blade is decreased. Because the blade speed hasn't changed, but the diameter of the circle is now smaller, the blade tries to get around the circle sooner (leading). When the blade flaps down, the diameter of the circle returns to a "normal" circumference and the blade now lags from the position it had while it was flapping up.

If the drag hinge wasn't there, over time the blade root would fatigue and eventually fail from the stress of the blade center of mass continually working in this back and forth motion as the blade flaps up (lead) and flaps down (lag). This is what happened with Cierva's autogyro ca.1927, and he created the drag hinge in response, with dampeners following shortly after.
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#8 BOATFIXERGUY

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Posted 24 February 2008 - 14:17

Can someone explain what the "lead and lag" do on a fully articulated rotor system? I understand the meachanics of it, I am just confused on the purpose for which the blades are able to pivot from lead to lag. Can someone explain please?


I'll try to answer your question on the LEAD / LAG...Flapping is totally different and has nothing to do with lead/lag. The blades FLAP up and Down...they Lead/Lag forward and back. The lead / lag hinges on a fully articulated system are there to compensate for the acceleration and deceleration of the individual blades caused by Coriolis Effect. Flapping is designed to compensate for dissymetry of lift.
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#9 Wally

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Posted 24 February 2008 - 16:39

Correct me if I am wrong please. So when the blades flap up they tend to lead (decreasing the diameter of the circle) as the speed of the blades increases? This happens when you are applying throttle or foward thrust? Does this cause a "sling-shot" effect if you were to try to slow your head speed down quickly?


Man, we're getting effects all crossed up, one reason why helicopter design isn't as simple as once thought. Blade hunting, "lead and lag", is, as earlier posted, a conservation of momentum effect, scratch that on a rock. Next, flapping induces hunting, and flapping is aerodynamic for pilot purposes. If you apply throttle, you're generally applying torque, which should increase aerodynamic loading, flapping the blades up, which would seem to be a blade "lead" situation. It's not, you're adding power to the whole, either accelerating one of the following: the unloaded rotor; or the loaded, and thus "decelerating" rotor. The blades will initially lag, but as a whole (generally) until it returns to equilibrium.
"Does this cause a "sling-shot" effect if you were to try to slow your head speed down quickly?" What speed are we trading for the proposed sling shot? I've never flown a helicopter that had air brakes, so in pilot speak a quick decel requires you to trade the energy of airspeed for altitude (quickest); NR (careful of rotor speed limits) as in the terminating flare of an autorotation; or lift (slowest) by "coasting", a so called "quick-stop". They're all the same process, effected at different rates.
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#10 helonorth

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Posted 03 March 2008 - 12:23

Logan,
As a former CFI, most of the posts confused more than anything else. I guess
i'll give it a try. Your question actually brings up a few other other things you need to know
before you can get a handle on lead/lag.

As the blades rotate in flight, you have a retreating and advancing side. The advancing side
has its rotational velocity PLUS the airspeed. The retreating side has the opposite. Its
rotational velocity MINUS the airspeed. You first need to understand this. Since we know
that a big part of lift comes from velocity, this creates a problem. It's called Disymetry of
Lift. The advancing side is creating more lift than the retreating side. Why does the helicopter
not pitch over and crash? It's through Blade Flapping. As the advancing blade moves forward
it creates more lift due to its increased velocity. This causes the blade to move up (or flap up).
The retreating side (due to it's decreased velocity) is creating less lift. It flaps down. When
the blade flaps upward (on the advancing side) it creates a slightly smaller angle of attack (AoA).
Since angle of attack also determines lift, the smaller AoA decreases the amount of lift.
Now the retreating side: The retreating blade, when it flaps down, creates a slightly larger
AoA (creating more lift). This equalizes the amount of lift across the disk and why the helicopter
doesn't tip over. With me so far? We're getting there.

When the blades flap, their center of mass changes. This is known as Conservation of Angular
Momentum. When the blade flaps up, its center of mass moves closer to the axis of rotation (the
mast or hub). When the center of mass moves closer, the blade speeds up. When the blade flaps down, its center of mass moves further outward, slowing down. Think of the
spinning ice skater. When the arms are pulled in, the skater spins faster. When the arms are
held out, the spin slows. The center of mass is moving inward and outward. Think of the blades as arms. As the blades speed up and slow down, they "hunt" as they move about on the lead/lag hinges. Movement is also absorbed by the lead/lag
dampeners. If you have the Rotorcraft Flying Handbook, look at the diagrams for disymetry
of lift, conservation of angular momentum and flapping. You need to understand all these
things for lead/lag to make any sense. I hope this helps.

Edited by helonorth, 06 March 2008 - 20:27.





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