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Blade pitch question

Blade pitch question

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#1 Mobile1

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Posted 13 May 2018 - 13:09

As the main rotating propeller spins, do the blades change pitch or angle?



#2 chris pochari

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Posted 14 May 2018 - 16:07

Not unless cyclic or collective is applied. Once the rotors start spinning beyond a certain RPM they will flex a little bit but not change in pitch. I'm interpreting your question as "will the rotor blades change in pitch when spinning"? The rotor pitch is hydraulically actuated, unless the controls are moved the rotor blades will not change pitch on their own, otherwise, the aircraft would start moving on its own.


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#3 Eric Hunt

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Posted 14 May 2018 - 16:33

Terminology, Mr Mobile: a propellor, as its name suggests, propels the aircraft forward through the air.

 

On a helicopter, it is known as a rotor, and does the combined job of providing lift to get into the sky, and thrust to move in a particular direction.

 

If cyclic input has been applied, to tilt the disc in some direction, the blades continually change pitch. The swash plate, connected to the cyclic, will use pitch links to make the blades increase pitch on one side of the disc and decrease it on the other side. The difference in lift provided will tilt the disc in the desired direction, the thrust will start to move the aircraft in that direction. 

 

There is an angular difference between where on the rotational path the pitch input is made and the blade reaches its highest or lowest point - it is called the phase angle, and allows for Newton's laws to take place. When the pitch is increased, more lift is generated, a force acts on the blade, and it starts to accelerate upwards, but it takes time to reach it highest point. The blade is spinning, so the angle between where the input is made and where the blade reaches its highest point is fairly close to 90 degrees. To make it easier for people to understand how this happens, the phenomenon has been likened to a gyroscope, but calling it the Gyroscopic Effect is wrong on so many levels. 

 

Gurgle it up or look on Yoochoob, there are lots of good films taken along a rotor blade in flight, it looks pretty scary.


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#4 mike0331

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Posted 14 May 2018 - 19:21

So I have a "Gyroscopic precession" question that's bugging me and it sounds like you answered it. I 100% understand the physics behind gyroscopic precession. I understand the reason you'd have a 90* off-set to that end. My question is as follows:
 

Is that what is actually happening with the rotor-disk, or is it a delayed response from the lift generated by the increase in AOA of the blades? IE, if you put the thing in a vacuum, would you still see the rotor disk move, or just see the pitch angle change with the swash-plate? I certainly think it could be the reason, afterall the pitch horns I've seen attach to the rotor blade after the hinge, so some upward pressure will be exerted on the blade relative to the hinge, but most, or perhaps nearly all of that pressure should go towards rotating the blade at it's root and changing AOA. 

Do any helicopters with fully articulated rotor systems have the pitch control on the side of the hinge closest to the hub, with the entire hinge/root/blade articulating? In that scenario, the only force applied to the disk would be from the lift of the blades, in which case I'd still expect to see the disk move, and still at an offset from when that input is given the laws of inertia and such. 

 

ETA: Read your whole post and it sounds like my theory is/was right. Just bugs me cause I've talked to pilots who look at me like I'm crazy when I ask this. For someone even a little versed in aviation and helicopter physics, it seems pretty simple -- I dunno why they go down the gyroscopic precession train when they teach it. 

 

Mike



#5 Eric Hunt

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Posted 14 May 2018 - 20:00

 

 

 if you put the thing in a vacuum, would you still see the rotor disk move, or just see the pitch angle change with the swash-plate?

In a vacuum, there isn't any air, so no lift, so no movement, other than the pitch change. No gyroscopic precession, because the blades, hinges, mast etc are not a gyroscope. Precession is purely a way of explaining phase lag to the unwashed masses. If it was really precession, it would always occur exactly 90 degrees out of phase from the input , but for example the R22 has 72 degrees of lag.

 

 

 

Do any helicopters with fully articulated rotor systems have the pitch control on the side of the hinge closest to the hub, with the entire hinge/root/blade articulating?

 

Wow, I don't really understand what you are getting at here. The pitch change horn, the part of the blade that sticks out at front or at rear, is attached to the blade, so it will flap and lead/lag along with it, but the swash plate below it, from which the pitch change link comes, does not.

 

Similarly, the lead/lag dampers have one end on the moving blade, and the other end is fixed to the hub, so these ones will move up and down with the blade to some extent.

 

Have a look at the head of a BK117, in which there are no flapping hinges or lead/lag hinges - it is a rigid head, and all the forces are absorbed by the flexible blades. It does have feathering hinges, with the pitch change horns.

 

Then look at an A109, to see the separate flapping hinges and lead/lag dampers and feathering hinges. With 4 blades, each at roughly 90 degrees, the pitch inputs can be seen to be about 90 degrees in advance.

 

Blow your mind with a Jolly Green rotor head, a triumph of science and technology over common sense. Working out the phase lag on this one must have been fun.


Edited by Eric Hunt, 15 May 2018 - 16:43.

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#6 mike0331

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Posted 14 May 2018 - 22:17

So that's what I figured... Not really gyroscopic precession. It blows my mind that the industry has opted to explain it like that... heck even the guy from the SmarterEveryDay, who seems to always get it right, did a video on his youtube channel on it. How you are describing it actually works is always what I figured, when I heard gyroscopic precession I figured there was no way. 

What I was getting at with the pitch change horns was that if they were connected before the flapping hinge it would be clear that they weren't imparting anything other than rotational force to the blade. Since they attach after the hinge. Just wasn't sure if there were designs out there that did it, not presuming any benefit. 

Interesting stuff. I'll take a look at some of the setups you listed.

 

Mike



#7 Eric Hunt

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Posted 15 May 2018 - 00:07

Mike, the pitch change horns are precisely as described - they change the pitch. It might be from cyclic input, where each blade gets a different pitch change, or as collective input where all blades get the same amount.

 

What happens to each blade after the input depends on the lift each one generates, the inertia in the blade when responding to the changed force, and centripetal force (from the RPM).


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#8 nightsta1ker

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Posted 15 May 2018 - 12:11

Interesting discussion. Meanwhile the op is nowhere to be found.
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#9 mike0331

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Posted 15 May 2018 - 19:32

Oh I get the function of the pitch change horns. I'm just saying from a physics perspective because there is force exerted on the side of the hinge that flaps, it's possible some of that "pushes" the blade up. That is the only reason I never wrote off gyrsocopic precession entirely... I assumed everyone else was right and I was wrong, and that teeny-tiny amount of force was for some physics reason I didn't understand enough to deflect the blade along with changing the AOA. 

 

Now that I know gyroscopic precession is not in fact at all what's going on, that "upward" force is likely for all practical purposes nothing. The centrifugal force of the blade increases the amount of upward force needed to deflect them along the flapping hinge to probably some astronomical amount, and given the pitch change horns impart that force offset from the blade root which rotates, it obviously has no issue changing the AOA on the blades. Part of me just thought that maybe that itty-bit of force was enough to deflect the blade enough to cause this so-called gyroscopic precession -- given everyone said that's what was happening. 

 

So on to the new stuff...

Here's a follow-up question (assuming CCW rotor system): So the AOA changes ~90* offset to the direction you want the disk to move. So let's say we want to move forward. We push the cyclic forward, roughly the rear of the swash plate moves up, peak AOA is achieved at the 9-o'clock position given the position of the pitch horns on the swash plate vs the rotor. So peak lift is generated by the blades as they pass over the 9 o'clock position. This results in the blades reaching a peak "height" at the 6 o'clock position, in effect tilting the rotor "disk" forward. So the blades relative position in the "disk" is what moves the helicopter forward, the fact that peak lift is generated in the 9 o'clock position doesn't matter? So the main objective of the AOA change in blades is relative to cyclic input is to get the disk to move, not to generate the most "lift" through blade AOA at a certain position? 

Here's a follow-up to the follow-up. If we had a 100% rigid rotor system where the blades were made of some magical inflexible material such that the disk was always "flat" relative to the fuselage, would our design principles need to change? Would we then went peak AOA to be opposite the direction of intended movement?

 

Mike



#10 Eric Hunt

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Posted 15 May 2018 - 20:12

I think the first question comes down to this part:

 

 

 So the main objective of the AOA change in blades is relative to cyclic input is to get the disk to move, not to generate the most "lift" through blade AOA at a certain position? 

Once the disc is already tilted, and we are in a steady state, the lift generated on any part of the disc is the same as the part on the opposite side - otherwise it is not steady and the disc would tilt again. So, the fact that the retreating blade is being moved to the position of greatest pitch and greatest lifting force as it approaches 9 0'clock, just means that it will rise up on the retreating side and be at the position of maximum displacement from the level position at the 6 o'clock spot. The overall disc is tilted forward, the Total Rotor Thrust (considered to be at 90 degrees to the Tip Path Plane)is tilted forward, and the vertical component of TRT is equal to the weight, and the forward component is equal to the drag.

 

(The phase lag can be plotted on graph paper as a pretty sine curve, with the cyclic input curve being about 90 degrees ahead of the blade position curve.)

 

A completely rigid system is

(1) unable to be built, and

(  2) something I have never considered, because of that.

Theoretically, then, I suppose you might be correct, with max pitch applied at 6 and min at 12. The "virtual disc" would be tilted forward, and it would go that way, even though the actual disc was still flat.

You would still need a big elevator at the rear to provide the downforce needed to keep the fuselage at an acceptable angle for the passengers, because the difference in lift front-to-back will create a couple to rotate the fuselage nose-down.

 

Many moons ago, the early experimenters with helicopters flew successful models, so they then tried to make scaled-up versions - but they were unstable because the blades were too rigid. When they wondered why the models flew but the full-scale ones didn't, they found that the models had bamboo blades, which had some flex in them. So, they put flex into the real blades, and at some later stage, flapping hinges, and the big machines flew successfully.

 

Some helicopters, like the Sikorsky Retreating Blade Concept (RBC) Comanche, have very rigid blades, to avoid clashing with the disc above it, but there is still some flex in the blades.


Edited by Eric Hunt, 15 May 2018 - 20:13.

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#11 mike0331

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Posted 15 May 2018 - 20:51

Awesome thanks. Fascinating stuff. I still wonder why almost everyone seems to teach and believe the gyroscopic precession stuff. The actual concept is more simple in my opinion... probably because that's what is actually going on.

Mike

#12 Eric Hunt

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Posted 15 May 2018 - 21:16

Simplicity, Mike. Easier for them to grasp the concept, but that's all it is - a concept.



#13 Thedude

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Posted 15 May 2018 - 22:54

Awesome thanks. Fascinating stuff. I still wonder why almost everyone seems to teach and believe the gyroscopic precession stuff. The actual concept is more simple in my opinion... probably because that's what is actually going on.

Mike


Don’t get too wrapped up in it, you’ll have to learn and memorize the concept to make it through flight school.
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#14 overtorque

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Posted 16 May 2018 - 02:32

Interesting discussion. Meanwhile the op is nowhere to be found.

 

On this slow moving forum, even this trivial topic is now worth talking about.



#15 Eric Hunt

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Posted 16 May 2018 - 05:05

SLOW MOVING?? 

 

If it wasn't for Butters throwing in a few starters, it would be as dry as a dead dingo's donger.


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#16 mike0331

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Posted 16 May 2018 - 17:44

Simplicity, Mike. Easier for them to grasp the concept, but that's all it is - a concept.

 

I see where they are going with it... but it's wrong. And frankly it seems more complex than what's actually happening, that's my main hang-up. What's actually going on is super straight-forward. The only reason I was ever confused in the first place was the GP crap. 

Mike






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