RunRyder - elevated tail rotor design

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Main Discussion > elevated tail rotor design

trunkmunki
Senior Heliman
Location: Bangor

If a helicopter is hovering (or flying, for that matter) with the tail rotor below the horizontal plane of the rotor head, it will fly with a greater tilt to the fuselage. The tail rotor, being lower, has an "arm" (think CG and moments) and is pushing out-of-plane with the main rotor. This causes the tail rotor to push the right (or left for a CCW main rotor) side low while in a tail-low hover/flight.

This is more pronounced on semi-rigid rotor systems (Huey, JetRanger) thus no so much on a model, most of which are more like a cross between fully articulated (or fully rediculous) and rigid rotor systems. The reason for this is the fuselage MUST always be perpendicular to the mast on these systems, therfore in a hover, since the main and tail rotors must produce the same amount of side thrust to cancel eachother out, the gyroscopic forces of the main prevent the tail from "rolling" the fuselage. On a semi-rigid system, the fuselage "hangs" below the rotor, therefor it is free to pitch and roll underneath it.

I have been thinking about doing a thread and running through some basic helicopter dynamic discussions. If anyone thinks that is a good idea, let me know. Otherwise, I will keep quiet.

07-20-2008 01:02 AM

trunkmunki
Senior Heliman
Location: Bangor

One more thing, any helicopter with a rigid or articulated rotor system (not counter-rotating, and not tandem rotors) will hover with one side lower than the other, dependant on the direction of main rotor rotation. This is also because of translating tendancy, the main rotor has to produce the same amount of side thrust as the tail in the opposite direction. As stated in my previous reply, the fuselage attached to these rotor systems will be perpendicular to the mast, and therefore will be "tilted" to one side as the head is tilted slightly to produce side thrust.

The UH-60 tail is indeed tilted to produce additional lift.

07-20-2008 01:07 AM

Yug
rrProfessor
Location: UK. Herts

Unless its coaxial which in itself has it's own flight behaviour whether the 2 sets of blades are ontop of each other or configured in tandem like the chinook, then you will allways have some kind of corrective main blade thrust vector to counteract the TR vector regardless of how rigid the head is. Every machine has certain behavioral characteristics which are underlying in its topology, even fixed wing aircraft require compensation for the torque of the propellor and the way the wash moves over the airframe and wings.

Vegetable rights and Peace

07-20-2008 01:16 AM

kangarooster
Senior Heliman
Location: Orlando Fl-USA

trunkmunki,
I would be interested in any insight you might want to share on helicopter dynamics.

07-20-2008 02:30 AM

Roscoe
Senior Heliman
Location: northfield,ohio - USA

Here's a close-up of the tail mod:

Here's a vid; notice it has " The Lean "; seems to fly nice.
[youtube.com/watch?v=jwPLjOygyuI&NR=1/]

" GO BUCKEYES !!! "

07-20-2008 03:46 AM

trunkmunki
Senior Heliman
Location: Bangor

As I have time, I will try to start a thread to discuss some of the basics. Most of this information is also available through FAA and military manuals. When I put stuff up on here, I will try to relate it to the models and simplify some of it so it is a little bit clearer on how this affects us as RC flyers.

Maybe some of this will help somebody understand why things happen and how different design and control aspects relate. I won't try to make any engineers, and I am certainly not one. I hope the info I give helps, and I will try to be as accurate as I can be.

07-20-2008 06:44 AM

TMoore
rrProfessor
Location: Cookeville, TN

Horizon 60 had an upswept tail as did the SX81 and there were upswept tail boom options for the Heli-boy and Mini-boy. All of these were using a tachometer cable drive. Hit the tail on the ground for any reason and watch the cable unwind. Been there, done it.

TM

"If you can do it, it ain’t bragging." - Will Rogers

07-20-2008 05:20 PM

HOMEPAGE

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

There are two forces created by the tail rotor.

The main force is to counteract the torque of the main rotor by producing a side thrust back on the tail. A side effect of this is a net side thrust that causes the heli to translate sideways. To counter act this to hover in place, the main rotor needs to be tilted and you end up with a side tilted heli.

The other less evident force is if the tail rotor is below the main rotor disk. This causes an additional roll force (not tilt) in the rotor system which is countered by a slight roll cyclic input and does NOT result in a tilted heli.

The tilt is only to counter the side thrust of the tail rotor, high mounted or low.

If you want to negate the rolling effect and side thrust, put a second "tail" rotor in front.

07-20-2008 06:13 PM

HOMEPAGE

trunkmunki
Senior Heliman
Location: Bangor

The above statement is true, for a SEMI-RIGID ROTOR system.

If you want the main rotor to provide a side thrust to counter translating tendency, it MUST be tilted to the side slightly, which is simple vector-math. On anything but a semi-rigid system (not what most models have) the mast MUST be 90 degrees to the disk, and since it is rigidly mounted to the fuselage, you will have a tilted helicopter.

On any single-rotor system that has a rigid or fully articulated system, the tail rotor has less of a tilting effect on the fuselage because the fuselage is still 90 to the mast, and the mast is 90 to the disk, and the disk is only tilted enough to counter the translating tendency. The gyroscopic forces counter the direct rolling tendancy of the out of plane tail rotor. On a semi-rigid (teetering) system, the fuselage "hangs" below the head and is free to swing in all directions, so the tail rotor can push the fuselage to the side (in a rolling motion) if it has an arm to do so. If the tail rotor is below the head in hovering flight, it will roll the fuse.

On models, we do not have semi-rigid rotors (unless you are a scale modeler that just loves abuse) and therefore the tail rotor has a negligable effect on the roll attitude of the fuselage except for the tilt necessary to counter translating tendency.

07-20-2008 08:49 PM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

Are you saying that our typical model helis are NOT semi rigid ?

07-20-2008 10:42 PM

HOMEPAGE

trunkmunki
Senior Heliman
Location: Bangor

Yes, our typical model is NOT semi-rigid. A Huey, or jet ranger has a semi-rigid rotor. It is free to flap (teeter) and the blades are rigidly connected to eachother so they lead/lag together.

Our models are closer to a rigid rotor system, they have limited flapping capabilities outside of blade flexing, and are free to lead/lag independantly of eachother. And, more relevant to this discussion, the mast is ALWAYS perpendicular to the head and plane of rotation, whereas a semi-rigid system is free to swing in all directions underneath the head.

Model helicopter rotor systems are more characteristic of a combination of rigid (becuase of the RELATIVE lack of flapping capability) and fully articulated heads (because of the ability of each blade to lead/lag independantly of eachother).
In a semi-rigid system, if one blade flaps up, the other must flap down the same amount. If one leads, the other also leads the same amount. The most important point here is this: even though the spindle "teeters" and allows some limited flapping like a semi-rigid, the motion is extremely restricted by the dampers and most importantly, the helicopter is not free to "hang" beneath the rotor system.

That is the defining element of a semi-rigid rotor system. By definition, a semi-rigid rotor is a system where the blades are rigidly attached to the hub (thats the rigid part) and the hub is free to pivot in any direction about the top of the mast. Our model rotor heads can only change pitch.

07-20-2008 11:49 PM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

Can't help it, your comments are confusing and verbose.

My earlier comments apply to our "rigid" rotor heads completely.

07-21-2008 12:01 AM

HOMEPAGE

trunkmunki
Senior Heliman
Location: Bangor

I realize it got a little complicated in my last, but I respectfully disagree that the tail rotor does not have appreciable effects on the roll angle of the fuselage other than the roll required for the head to compensate for the side-thrust. Any noticeable roll you are talking about is specific to semi-rigid systems.

07-21-2008 12:10 AM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

I said you got verbose . . . . not complicated.

You say "does not have appreciable effects"

I say "which is countered by a slight roll cyclic input"

Are we not on the same page ?

07-21-2008 12:22 AM

HOMEPAGE

Bad Karma
Veteran
Location: UK

trunkmunki, let me introduce you to AirwolfRC......

If you are the one dispensing information, you are a fool, you talk out of your ass, over complicate things and are generally inferior to the almighty AirwolfRC.

If you are not the one dispensing information, then prepare to behold the infalable all knowing AirwolfRC

07-21-2008 12:36 AM

trunkmunki
Senior Heliman
Location: Bangor

Sorry, I was paraphrasing confusing + verbose = complicated, and I was agreeing with you on that.

As for what we are talking about, no. We are describing two separate causes for fuselage roll attitude during hover, one which does not really affect our rotor systems.

I feel we are starting to get slightly argumentative here, and I am not trying to do that. I apologize if I am coming across in a negative way. I was just trying to explain an aspect of the dynamics of flight.

07-21-2008 12:37 AM

AirWolfRC
rrProfessor
Location: 42½ N, 83½ W

My point was, and still is, that to counter the tail rotor side force, the rotor disk will tilt. Whether the main shaft and heli go along for the ride is a matter of rotor head design and does not affect the rotor disk tilt.

And there is a SLIGHT roll torque from a low mounted tail rotor that does NOT contribute to tilt.

It looks like Bad Karma has been spreading Bad Karma again

07-21-2008 01:27 AM

HOMEPAGE

steve9534
Key Veteran
Location: yakima, wa.

Rotor tilt

Sounds like another good thread that got away from it's owner. I have thought also that having the t/r on the same plane as the main rotor blades would eliminate at least one of the confounding factors in making the heli fly more accurately. Just as a practical matter, having a bend in the back of the tail boom is going to add more weight and complexity than it's worth, IMHO. A more practical compromise that would get the t/r close to on plane with the main blades would be to have the t/b attached near the bottom of the heli at the front and angle back and upwards. Unless the t/b were longer than needed, the t/r still wouldn't be on plane with the main blades, but you could get it close while allowing for adequate clearance between the boom and blades. No extra pieces or extra weight would be needed, just a minor rearrangement of the existing hardware.
As for the amount of tilt of the main rotor disc at hover, it will be dependent on the amount of translational force generated by the t/r. This really is a vector problem and I fail to see how rigid vs semi-rigid vs articulating affects the situation except as the heli body may or may not be as tilted as the main disc. The t/r, if out of plane with the main rotor disc will try to tilt the heli, but the amount of main rotor tilt is dependent on the vector needed and will require some cyclic input to maintain. The t/r in so far as it tilts the helicopter will just influence the amount of cyclic needed to maintain the correct degree of main rotor tilt. I'm particularly perplexed by the thought that gyroscopic forces might somehow enter into the equation??? Could you please explain that statement? Thanks. steve.

07-21-2008 01:53 AM

kangarooster
Senior Heliman
Location: Orlando Fl-USA

This is a question, not an argument.
If the tail rotor being low mounted creates a roll torque,It should act 90 degrees later and be a nose up force.
I can't detect any elevator change.
Is it so small I am missing it or is something else happening?

07-21-2008 03:07 AM

Roscoe
Senior Heliman
Location: northfield,ohio - USA

Kangarooster- I think the gyro effect of the flybar/rotorhead would cancel out any nose pitch up that the tail rotor(small diameter)could muster. Totally different beast than the guys are talking about.
I know, there I go thinking again.

" GO BUCKEYES !!! "

07-21-2008 04:23 AM

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Main Discussion > elevated tail rotor design

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