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Hanging one side low

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Hanging one side low

Old 31st Aug 2002, 17:19
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Flygunz, I think you may have me wrongly ID'd. I was a QHI in my
former life but never had aspirations to be an A2. As for planking,
the last plank I flew was a Chippy in '83

Regards Dave B
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Old 31st Aug 2002, 17:40
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What a wally, sorry MG we do know each other but I had you clocked as someone else, the fun part of PPRuNe
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Old 1st Sep 2002, 06:01
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GOSH !!! I love it when Nick starts with the explanation of all the angles of the dangles....and the thrusts of the hinges....but really appreciate the concept of the Brand X rotor system being simple...must be why I keep finding myself herding them around the sky.....matches my intellectual abilities to the rotor system ! That or the 0330 get up this morning with only Decaf in the rig chowhall got me ! Did you guys ever update that lovely little blue book that Sikorsky used to produce on aerodyamics for helicopters ? Wish I could find mine...but it got lost in one of the 157 moves I made while following my helicopter career.
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Old 1st Sep 2002, 12:42
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Nick Lappos
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SASless,
sorry about how I start to get off into angles and stuff, but it is math that serves as the best smokescreen when real understanding is in short supply!

That Blue Book is still around, and does need an update! Perhaps a nice retirement task for me, as my sailbaot glides across the Pacific, the Satdish locked on a commsat and me cruisin the net as that nice bikini-clad shipmate makes another cuppa coffee. I could distill a few years of PPRuNe into a nice set of Q and A's!
 
Old 1st Sep 2002, 20:29
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Maybe I'm confused about all this, but it has always been my understanding that the forces acting on an aircraft in flight (and I consider a hover to be flight, it's off the ground) act around the plane(s) of the center of gravity, not the plane of the rotor, or the skids, or any other plane. It might be possible to make the opposite skid hang low if the tail rotor were below the CG, but I still don't see what having the tail rotor above or below the main rotor has to do with it. ISTM that the tail rotor imparts thrust, countering torque, & we counter that by putting in a combination of pedal & cyclic, to keep the nose straight, & the cyclic input rolls the fuselage around the CG, & we end up with a balance of forces, more or less, changing by the microsecond, resulting in a tilt into the t/r thrust. Does this sound right, Nick, or anyone?
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Old 1st Sep 2002, 21:01
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Nick Lappos
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GLSNightPilot,
You are exactly right, the relationship to the CG is what counts. If the Tail rotor were much higher than the CG, the roll attitude in a hover is less. That is why a centerline tail rotor helo has about 4.5 to 5 degrees degrees left roll, while a high tail rotor helo (S-76) has about 3.5 degrees.

Somehow, the idea that the height above or below the main rotor is important has been surfaced from time to time. It is not correct.

Nick
 
Old 2nd Sep 2002, 14:17
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Nick Lappos

you said to GLSNightPilot,
You are exactly right, the relationship to the CG is what counts. If the Tail rotor were much higher than the CG, the roll attitude in a hover is less. That is why a centerline tail rotor helo has about 4.5 to 5 degrees degrees left roll, while a high tail rotor helo (S-76) has about 3.5 degrees.

so the roll degrees reduces as the tail rotor moves towards the main rotor (this is what your saying) doesnt this reinforce what im saying? if the tail rotor is level the roll will be '0'.
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Old 2nd Sep 2002, 14:37
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Hey Nick,

How about spending your pre retirement time on writing a book for us low hours types? Something slightly less basic the the PPL guides and including usefull stuff to practice while building hours?

Some of us need all the help we can get
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Old 3rd Sep 2002, 09:37
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Basic physics a bit off

Nick GLS and vorticey (but especially Nick)

Respectfully I say: you are wrong (close but wrong)

For simplicity first consider only a pendulous type rotor head:

In the horizontal plane the disc must create a force equal and opposite to the t/r force - or you'll translate (right, right?)
(NO fuselage roll required yet)

In the lateral vertical plane an additional couple is created if the t/r is not in the same plane as the disc - the couple which opposes this the cg/vertical thrust couple. This is created by the cg 'stepping out' from under the point of suspension. I.E.:Fuselage is rolled by an ammount to the left.

(consequenses of this are odd: ie disc will be 'tilted to the left' but fuselage will tilt either left if t/r is below plane of rotation or right if t/r is above plane of rotation (no-one's made one like that tho') , zero FUSELAGE tilt if t/r in plane of rotation)

It is absoloutely not a function of t/r thrust height w.r.t. cg UNLESS you look at the INERTIAL scenario (ie accelerations - for instance if 'weightless' when t/r thrust above/below cg WOULD definately created a roll acceleration). This does not apply in the steady state hover. Cg 'height' does have a bearing on degree of roll since the 'restoring force' will be greater per degree of tilt for a cg which is further away from the disc.

So to answer the question: Yes you can make the fuselage level by messing around with the cg position - but the disc has still "gotta do what a disc's gotta do" ie tilt left.

Now consider the other rotor heads (breifly).
They all make a direct rolling moment on the fuselage (if there's a differance between disc plane and head plane) - so if the disc is tilted there is the additional consideration of how much that will tilt the fuselage BUT but a disc has still "gotta do what a disc's gotta do" ie tilt left.



(left/right when viewed from behind, terminology watered down for readability, some simplifications)
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Old 3rd Sep 2002, 11:59
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Q MAX

GOOD STUFF, I TOTALLY AGREE
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Old 3rd Sep 2002, 15:09
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Thank you

vorticey - thanks - really nice to have a comment like that
(... positive agreement so rare around here) - you've made my day
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Old 3rd Sep 2002, 16:26
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Relative and Absolute co-ordinates

Nick and Q,

I have just read both of your posts a couple of times and with the exception of a couple of points in Q's last post I think you are both arguing the same point. Nick is taking the standard mathematical approach in flight mechanics of referring the forces and moments back to the aircraft CG while Q is using the hub.

When balancing forces you balance in the three directions and the line of action of forces is very important (i.e its point of application). The point of application of a moment on a body is irrelevant as long as the axis about which is occurs is known. Therefore, whether you choose to take moments and do the force balance about the hub or the CG the answer will be the same. Provided you stick to your convention and consider all of the forces.

Taking moments about the CG (as Nick has) means that weight cannot provide a moment as there is no moment arm. MR side-force and TR thrust both produce moments about the CG and hence the perpendicular distance is important.

Taking moments about the hub (as Q has) means that the MR side force cannot provide a moment as again their is no moment arm. However, the tail rotor thrust provides a moment as does the weight, so in this case it is the perpendicular distance of the TR thrust from the hub and the CG from the hub that is important.

If you adopt either system and follow either convention through thoroughly then they will both give the right answer. It is common to take moments through the CG since is simplifies the maths significantly as we are fundamentally dealing with inertial forces in aircraft. The acceleration due to gravity (9.81 m/s/s) and any additional maneuver accelerations.

Fundamentally, Q, the height of the MR and TR from the CG are important even in steady state hover because the weight and inertial forces can be idealised as acting at the CG, but your view of taking moments about the hub is correct providing you follow the same convention all the way through your analysis.

I also agree that in a teeter-rotor helicopter with the TR hub at the same height as the MR in a perfectly trimmed helicopter with no other external forces acting on it (CG directly below hub centre) then the helicopter would hover skids level. However, this is because both FORCES, (TR Thrust & MR Side force) are equal and opposite and have the same moment arm about the CG.

Hope this helps
CRAN



Last edited by CRAN; 3rd Sep 2002 at 20:04.
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Old 3rd Sep 2002, 16:34
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Nick,

Forget the Q & A....and Bikini.......my goal is Bora Bora....42' pilothouse cutter.....T&A....and no "kini".....surrounds. I intend to carry a tail rotor blade and shovel with me.....walk around every port of call with them over my shoulder....until the third person in a row asks me what they are.....then I will dig a hole...throw both of them in....kick the sand over them...and settle right there! The third question would confirm the locals did not know what a helicopter is nor what hard work is....and that is the kind of place I wish to retire to.
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Old 3rd Sep 2002, 19:23
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CRAN - Yup - I agree

Sure it works with either (any) origin.

Yes. It is more conventional to take cg as the referance.

I think (unimportant) that in this case it is more instructive to take the hub center as the referance - as it's not really an inertial problem - maybe you can see the benefit of it?*

Incidentally do you know the other useful unit for describing gravitational strength: gravity; 9.81 Newtons/kilogram


* you wrote:
"Fundamentally, Q, the height of the MR and TR from the CG are important even in steady state hover because the forces are actually** created about the CG, but your view of taking moments about the hub is correct providing you follow the same convention all the way through your analysis."
- My 'zero at the hub is 'nicer' for this bit - since 'cause' and 'cure' become two independant functions. Whereas in 'yours' if you knew how high the TR was from the CG you still wouldn't know if it were going to tilt left or right yet.


**I'm not sure I like the referance to "actually created about the CG" - since they are 'actually created' about anywhere you define.

- Slavery to convention
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Old 3rd Sep 2002, 19:32
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For all who attempt to answer the basic question, it does not matter where you sum the forces and moments at, as long as you are consistent with that reference point. If you do, you will realize that the height of the tail rotor above or below the main rotor hub has nothing to do with the hover angle. There is a misconception that the aircraft "hangs" by the rotor hub, so all forces are summed there. This is fundamentally in error, and will lead you to determine some bizarre findings.
 
Old 4th Sep 2002, 02:22
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Cool

Jeep, since you started this discussion you've been conspicuous by your absense. So what's your theory??
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Old 4th Sep 2002, 03:04
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Head above the parapet, shields up!

MG:

The maths, physics and aerodynamics are way above my pay scale, but I have benefited from the answers given. I asked the question originally because since becoming a student again, my long held belief that an aircraft hung low on one side because I was sitting on that side no longer holds true, as I sit behind or in front of my other crewmember. On asking an old sweat IP he tells me that the AH64, hangs left wheel low, not because of tail rotor drift and roll, but the balanced centrifugal forces acting through the fully articulated head.

So firstly I posted the question, secondly I watched the aircraft in the hover and though even though it sits 5 deg nose-up the TR is about level with the mast, which is higher (more level in the hover) than Ive seen on my previous types because they were designed to be level in forward flight; my reasoning being that with the moveable stabilator on the AH64 the attitude can be controlled more throughout the speed ranges, so the hover attitude has been optimised for missile launch constraints/sensor sweeps etc.

So with a TR almost level with the MR, yet still hanging left wheel low, it must be to do with the TR drift, requiring a tilted disc all normal. Because of the fully articulated mast, with offset hinges and a static mast, I reasoned the MR Head is canted off, so the body of the aircraft will tilt.

I enjoyed the discussion about c of g and disc relationships, as I had not thought of it anything other than the height of the discs before, but all are important. But when someone asks me why it hangs low, I shall tell them tail rotor drift (or the US equivalent), fully articulated head, static mast and balanced forces of c of g, wind, rotor heights and the way it was designed for maximum effectiveness to get hardware down range.

Now if only I could get a board plan, diagrams and some simple maths.

Nick:
Do hurry with that blue book, it may well be a best seller.

MG - PS after 4 weeks of flying and academics, it is calming down a bit. I start the bag on Friday, so Im in the calm before the next storm. I shall let you know.
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Old 4th Sep 2002, 08:55
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Someone (sound) referee please....

Nick sez:
"you will realize that the height of the tail rotor above or below the main rotor hub has nothing to do with the hover angle"

That has got to be wrong.

... sure, for rotor heads with flapping hinges at a distance from the hub center there is a direct couple which will try to force the fuselage (hub) to adopt a similar attitude. The disc attitude is determined by the need to combat translating tendancy. So, yes rotor head type has a bearing on the subject - no dispute there.

If you eliminate the rotor head influence by choosing a pendulous type rotor head eg B206 teetering head (semi-articulated, semi-rigid) then you will see more clearly that TR height above or below the rotor hub is EXACTLY what does influence whether the fuselage is forced to roll right or left by the TR thrust.

I know it sounds counter intuative - but just try and look at it freshly again:
If the TR is above the hub it will attempt to roll the fuselage 'the wrong way' ie to the right.

It follows that with the TR in the plane of MR (ie at 'the same height') there IS NO rolling tendancy due directly to TR thrust (or MR's need to complete the couple).

Yes because the disc must 'tilt' to counter translating tendancy there is a force, at the head, which will attempt to tilt the fuselage to comply....


Rushed response...


I hope you can just say : "yes on that point you are right."

......... Otherwise I might have to say it!
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Old 4th Sep 2002, 21:38
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Something that comes to mind: do designs with no tail rotor hover with one or the other side low? I've never flown one, & didn't pay that much attention to that when I had to ride in them - I was too scared to have time to think about it.

I still don't believe you can treat a 206 as freely hanging from the head - there's a lot of coupling present, as you can see when you put the rotor disk forward to take off, or do a turn with forward motion.
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Old 4th Sep 2002, 21:50
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Yes NOTAR helicopters will hang one skid low, as the NOTAR system still produces a side force on the boom. They will hang left skid low.

Co-axials, intermeshers and tandems won't.

CRAN
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