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Hovering question

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Hovering question

Old 1st Oct 2020, 14:03
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Hovering question

Iíve noticed certain helicopters while hovering do so with a small but noticeable and steady angle of bank


What is the reason for this ?
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Old 1st Oct 2020, 14:05
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resultant couple between the thrust from main rotor and tail rotor
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Old 1st Oct 2020, 14:11
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Which is a long-winded way of saying tailrotor drift.
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Old 1st Oct 2020, 15:14
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The tail rotor pushes sideways so you have to tilt the main rotor in the opposite direction to maintain ground position.

Depending which side of the pond you are on it is either called tail rotor drift and tail rotor roll or translating tendency.
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Old 1st Oct 2020, 15:38
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Thank God for that! I thought it was just my crappy hovering...Ö..
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Old 1st Oct 2020, 15:45
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I think both answers are correct. Any difference in vertical height between the centre of the disc and the tail rotor centre creates couple and an element of roll. In addition the tail rotor is pushing one way, so need to fly slightly into that thrust. I think both happen at the same time, and the answer to remaining at a fixed place (flying slightly into the thrust) is to compensate for both at the same time.

Wrong?
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Old 1st Oct 2020, 16:11
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Tail rotor drift happens if you keep the wings/rotor disc level - it is producing thrust laterally - therefore you have to tilt the rotor so that some of the MR thrust is directed laterally in the opposite direction.

That sorts out the drift - the roll attitude is the result of the forces balancing - the TR thrust and horizontal component of MR thrust create a rolling couple and the vertical component of MR thrust and the mass of the aircraft create an opposing couple.

DB - no, it was your crappy hovering
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Old 1st Oct 2020, 17:57
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I always thought it was because you are hovering above a rotating planet. If you donít fly sideways to keep up with it, the ground will rotate away from you. 😉
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Old 1st Oct 2020, 18:41
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Originally Posted by [email protected] View Post
Tail rotor drift happens if you keep the wings/rotor disc level - it is producing thrust laterally - therefore you have to tilt the rotor so that some of the MR thrust is directed laterally in the opposite direction.

That sorts out the drift - the roll attitude is the result of the forces balancing - the TR thrust and horizontal component of MR thrust create a rolling couple and the vertical component of MR thrust and the mass of the aircraft create an opposing couple.
Excellent use of physics there.

Tilting the the rotor happens, but didn't we establish in the earlier thread that there is also a main rotor head roll couple owing to offset blade hinges and that this could be very significant.

Some helicopters hover with roll even when the tail rotor is at the same height as the main rotor.

It is complicated in ways that I did not understand until coming here.
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Old 1st Oct 2020, 19:37
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Regardless of the tail rotor height, there is tail rotor lateral thrust that must be compensated for with the main rotor. Even the NOTAR hovers left skid low.
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Old 1st Oct 2020, 20:13
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I understood what it was whilst learning to fly, In R22, R44, B206 I didn't notice it too much but when i got rated on the Gazelle, even though I knew opposite rotation It took me a while to get used to it hovering right skid low.
Been a while now, and I'm only PPLH so no doubt the very experienced mil pilots on here could have helped me with that one.
R

Last edited by RINKER; 1st Oct 2020 at 20:14. Reason: better word
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Old 1st Oct 2020, 20:56
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Tilting the the rotor happens, but didn't we establish in the earlier thread that there is also a main rotor head roll couple owing to offset blade hinges and that this could be very significant.
that would be the effect of control power - it just means you have to move the cyclic less to achieve the required lateral MR thrust and the hinge offset helps roll the fuselage.

Regardless of MR hinge offset, from nil in 206, a tiny bit in R22, more in articulated heads like Gazelle to 17% effective hinge offset in the Lynx - they all still hover one skid lower than the other.
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Old 1st Oct 2020, 21:40
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Also depends on gearbox rigging and Cof G,fore/aft and lateral....
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Old 1st Oct 2020, 21:57
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Originally Posted by [email protected] View Post
they all still hover one skid lower than the other.
Not all of them. ;-)

Originally Posted by sycamore View Post
Also depends on gearbox rigging and Cof G,fore/aft and lateral....
Originally Posted by HissingSyd View Post
It is complicated in ways that I did not understand until coming here.
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Old 2nd Oct 2020, 02:48
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Just another cases of the multiple control interaction that exist in the flight dynamics of the helicopter, that translating tendency can couple with your action on the lateral cyclic when you really try to nail a perfect touch down.
on touch-down its very easy to overfocus on the cyclic control, while all the time its the sloppy work on the pedal that create varying translating tendency and the bulk of your poor lateral control.

For example the AS350 has this typical wobbliness sneaking up on you as you try to land it,
due to ground effect air flow around the tail gets lousy --> tail thrust get a little less predictable --> pedal work increase --> translating tendency fluctuates --> lateral cyclic gets sloppy playing catch-up --> fail to impress with masterful touch down

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Old 2nd Oct 2020, 08:08
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When you eventually got the Bristol Sycamore into the hover you had to look at the controls and memorise their position because if you wanted to hover it again that's where you had to put them.
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Old 2nd Oct 2020, 09:15
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You can engineer some of it out - contra rotating twin rotors is one way. The Wessex and Sea King had something called starboard lateral lead where the output from the mixing unit was proportionally longer to the starboard lateral jack than the port one - as you raise the lever, it tilts the disc slightly to the left for you helping counteract the TR drift.

Various gearbox tilts are another option but ISTR they were mostly forward to reduce mastbending stress in the cruise.

Which conventional helos don't hover one skid low Syd? And the answer isn't 'ALL of them if you leave the ground power connected'
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Old 2nd Oct 2020, 09:27
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Originally Posted by [email protected] View Post
You can engineer some of it out - contra rotating twin rotors is one way. The Wessex and Sea King had something called starboard lateral lead where the output from the mixing unit was proportionally longer to the starboard lateral jack than the port one - as you raise the lever, it tilts the disc slightly to the left for you helping counteract the TR drift.

Various gearbox tilts are another option but ISTR they were mostly forward to reduce mastbending stress in the cruise.

Which conventional helos don't hover one skid low Syd? And the answer isn't 'ALL of them if you leave the ground power connected'
Donít the HH-53 and the Blackhawk get round it partially by having an offset tailrotor. ? Given that tail rotor roll on the jolly green would be significant, if only because of the sheer bloody size of the thing.
From memory even with the starboard lateral lead the Wessex and the Sea King still hovered one wheel low.
We need 50 words now on translational lift.
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Old 2nd Oct 2020, 10:27
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From memory even with the starboard lateral lead the Wessex and the Sea King still hovered one wheel low.
yes they did. As for the canted TR , I'm sure it makes a difference but don't know by how much.

I don't think there is anything cut and dried in helicopter design - Ray Prouty always said that what you gain in the hover you lose in forward flight and vice versa.
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Old 2nd Oct 2020, 11:42
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Globo, no the same phycics apply and they hover left wheel low.

Only aircraft that addressed the issue to my knowledge are the Sikorsky S-64 Skycrane and the Kaman UH-2 ( recall being told that anyway ), and the solution there was to design in ( 3 degrees in the case of the 64 ) lateral main rotor shaft tilt.
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