another terrible ATPL PoF question
 

sideways drift may be corrected by:

a) putting the tail rotor below a line which is normal to axis of the main rotor
b) desiging the control linkage so that the rotor disc will tilt when collective pitch is applied, the stick position not being affected
c) fitting delta-three hinges to the tail rotor
d) a tail stabiliser

answer?!

well, it isn't 'b', 'c', or 'd'. so it must be 'a' - ?

but 'a' is gibberish?!

help

Why not 'b'?
In Bell helicopters for example, sideways drift is corrected by installing the main rotor shaft with a slight side inclination (2 degrees).

B seems to be the right answer. "A" won't solve the drift problem as the tail rotor thrust is still causing the drift. If you have the main rotor tilt as you raise the collective, you offset the drift in the opposite direction. As someone said though, I believe this is a bit cost prohibitive in most helicopters. It's easier just to have the pilot apply opposite cyclic.

b is the right answer. You can either tilt the MRGBox and shaft to the side (many have it tilted forwards as well), or you can rig the length of the lateral control runs in the mixing unit so that one lateral jack produces a greater input than the other for a given collective movement. On the Wessex and Sea King it is known as Starboard Lateral lead.

Pofman
 

As Crab says answer B.
Straight forward tailrotor drift (translating tendency) question. Was a common correction to use control mixing for helicopters which spent a long time in the hover(logging, pinging etc) so you did not have to hold the cyclic offset for long periods.

The tail rotor helicopter is using a moment (T/R force x length of tailboom) to counter a torque (engine twisting the gearbox) and you have an unbalanced resultant force, tail rotor drift.

Tilt the main rotor left (or right, s'il vous etes Francais) to generate a counteracting force.:8

UH-60 helicopters have a mechanical mixing unit to "reduce pilot workloads". One of the mixes is a collective-to-roll to help deal with translating tendency.

It is complicated, though...

I have a vauge memory from my UH-1 instructor days of "a" being responsible for the left-skid-low attitude.

Come on Controller......surely this is "Jack and Jill" stuff to you....

Just keep memorising those bar codes on the baked bean tins :E


Jones the bond :ok:

Hey Varangian,

The left-skid low in the Huey was the chubby instructor in the left seat!:sad:

Hueys have a high-set tail rotor to minimise tail rotor roll, which causes the left-low. Low-set helos are easier to build, but the left one hangs down a bit.:uhoh:

lifting, height of the tail rotor with respect to the main rotor hub does affect the roll - I agree with AC that the Huey T/R was set high to reduce the effect.
Nose wants to yaw right, and tail left, because of rotor torque.
Tail rotor forces tail right to counter this.
Now you have a side force making helicopter want to move right (tail rotor drift), so we introduce left cyclic to counter it.
If the tail rotor is lower than the main rotor hub, this creates a force couple (left from M/R hub, right from T/R when viewed from the rear) which rolls the fuselage left until an equal and opposite vertical couple formed by lift (up from the M/R hub) and weight (down through the CofG) is reached.
The closer the tail rotor is to the M/R hub, vertically speaking, the less the rolling couple.
The tail rotor on the Huey is high-set, but not as high as the main rotor hub, particularly with an aft CofG, so there is a roll because of it - slight disagreement with AC there - but it's certainly lessened by the higher mounted tail rotor than if it was just on the side of the boom.

The point of raising the TR is to give it a better moment arm about the C of G. The horizontal component of the MR thrust is trying to roll the fuselage left about the C of G so mounting the TR high gives it an equivalent lever in the opposite direction.

If you are building a helicopter that will spend most of its working life in the hover, it makes sense raise the TR to produce a more level (laterally) hover attitude which keeps the cabin floor level and stops things like winch cables fouling the fuselage (see the Merlin/Cormorant for this problem).

If you are building a helicopter for high speed cruising then you probably won't bother to raise the TR as it adds weight and another gearbox at the back end.

Battlefield helicopters do benefit from having a higher TR as it is safer both from ground clearance and troop clearance poits of view.

I think we're pretty much in agreeance, lifting.
The part of your post that got me thinking was this:

"Nope, disagree. Height of the t/r has nothing to do with left low.", when all that couple stuff I was blathering on about convinces me that it does.

Anyhow, as you say, I just lift to the hover and hang whichever way the machine wants to.

Thank heaven for Crab. The myth that the Tail Rotor's position relative to the Main Rotor Head is a factor in the hover attitude has somehow evolved, much to the surprise of those who actually build these things.

For the record:

the tail rotor makes the left wheel hang low in a hover, due to the sum of two competing factors:
1) the translating tendency (TR thrust tries to move the helo to the right, it takes left bank to stop it)
http://webpages.charter.net/nlappos/attitude1.jpg
2)TR thrust acts to rotate the fuselage, depending on how far above or below the tail rotor is relative to the CG. The higher the tail rotor, the flatter the hover attitude, because the TR roll component cancels the translating tendency roll attitude. the lower the TR, the less cancellation, the more pronounced the roll. (this factor probably lead the aerodynamically challenged to deduce the "theory of rotor heights" explanation.
http://webpages.charter.net/nlappos/attitude2.jpg

Nice picture Nick but a pity you made a fundamental mistake - rotors on proper helicopters go clockwise!

HC

Yea, if viewed from the bottom.....;)

Aw shucks - you didn't bite very hard - must be getting soft!

HC

Nick, You said: "The myth that the Tail Rotor's position relative to the Main Rotor Head is a factor in the hover attitude has somehow evolved, much to the surprise of those who actually build these things."
I know you're a vastly experienced test pilot, and maybe I'm not thinking straight about the subject, but can you please shed some light on the following:

In your picture above, the translating and anti-translating forces, as you've labelled them, are vertically displaced. This creates a force couple, making the aircraft roll.
How far does it roll?
Until the opposing couple formed by the lateral displacement between the CofG (from which weight acts straight down) and the rotor hub (from which lift acts straight up) becomes large enough to be equal and opposite to it.
Obviously the greater the roll angle, the greater the displacement between the lateral positions of lift and weight, and so the more the opposing force.

Likewise, the pro-roll couple formed by the translating and anti-translating forces gets stronger the more vertically displaced they are from eachother, so it seems to me that the vertical position of the tail rotor with respect to the main hub is a major factor in how much roll you'll get.

You said: "The myth that the Tail Rotor's position relative to the Main Rotor Head is a factor in the hover attitude has somehow evolved, much to the surprise of those who actually build these things."
To my mind, the vertical displacement between these two determines the strength of the pro-roll couple, and is therefore far from mythical.

To put it another way, say you had a helicopter with a standard main rotor, central CofG, and two antitorque rotors, one on the nose pushing left and one on the tail pushing right.
If the two antitorque rotors were at the same vertical level, there wouldn't be any rolling.
If they were significantly displaced vertically, there'd be lots of roll, increasing with the displacement.
How is this any different to the main rotor / single tail rotor situation?

I rest my case, your honour.:)

Arm,
It is unfortunate that we cant just get to a chalk board, it would be so much easier.

Your picture is quite selective, you see the translating component of the rotor thrust, but you don't see the lift portion, which precisely cancels your argument.

One must resolve the forces to the CG to understand their effect of the motions of the body. For the main rotor thrust, for some convenience I show the vector summed at the head. In fact, the main rotor thrust is a single force vector that passes through the aircraft and directly through the CG. Otherwise, the rotor would be rotating the helo (which it does when we maneuver.)

The idea that the height of the rotor is a driver in this roll angle discussion is simply not correct. The rotorhead height DOES help the cyclic control the helo, because the higher the rotorhead, the more lever arm the thrust has when the pilot purposely tilts the lift away from the CG. For teetering rotors, high rotorheads are needed to make the cyclic more powerful. That is why Hueys and Robbies have high masts and Boelkows and Sikorskys have heads tucked down close to the fuselage.

With your dual TR helo (nice idea and a great "thought experiment, Arm!) there would be no translating tendency, because the two would cancel out the lateral force imbalance. Therefore there would be no need for the pilot to tilt the lift to stop the slide, and the helo would hover level. It would hover level no matter how high or low those pesky tail rotors were, as long as they were on the same height. It would hover level no matter how high or low those tail rotors were relative to the main rotor head, as well.

http://i109.photobucket.com/albums/n...lla/TRRoll.jpg
Thanks for the discussion, guys.

Sorry, I'm starting to feel a bit thick, but the force diagram as I see it is like this.
The vertical couple between translating force and anti-translating force rolls you.
As the CofG moves laterally with respect to the head, the horizontal couple formed between the two becomes a more powerful restoring force until the two couples balance.
If there was more vertical distance between the tail rotor and the main rotor hub, there'd be a greater rolling force.
Why is this wrong?

Arm,
The picture you show is of a helicopter during a very rapid right rolling maneuver. Note thet the lift vector does not pass thru the CG, so there is one hell of a right rolling moment on the helo.