Hovering question

[stilton]

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


What is the reason for this ?

[Hughes500]

resultant couple between the thrust from main rotor and tail rotor

[Bell_ringer]

Which is a long-winded way of saying tailrotor drift.
:E

[[email protected]]

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.

[DOUBLE BOGEY]

Thank God for that! I thought it was just my crappy hovering...…..

[John R81]

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?

[[email protected]]

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;)

[GS-Alpha]

I always thought it was because you are hovering above a rotating planet. If you don’t fly sideways to keep up with it, the ground will rotate away from you. 😉

[HissingSyd]

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.:ok:

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. :ugh:

[Three Lima Charlie]

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.

[RINKER]

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

[[email protected]]

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.

[sycamore]

Also depends on gearbox rigging and Cof G,fore/aft and lateral....

[HissingSyd]

they all still hover one skid lower than the other.

Not all of them. ;-)

Also depends on gearbox rigging and Cof G,fore/aft and lateral....
It is complicated in ways that I did not understand until coming here.

[Agile]

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

[Fareastdriver]

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.

[[email protected]]

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':)

[MENELAUS]

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':)

Don’t 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.

[[email protected]]

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.

[JohnDixson]

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.

[MENELAUS]

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.

Yes that’s why I said partially. Kaman aircraft, anything hormone or helix, sea knight and chinook. No. For obvious reasons

[Sloppy Link]

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


What is the reason for this ?
Here us a basic explanation...
https://youtu.be/0JKjOA-i5CY

[kintyred]

It’s not confined to tail tutor aircraft. The Chinook hovers a couple of degrees left wing low.

[MENELAUS]

It’s not confined to tail tutor aircraft. The Chinook hovers a couple of degrees left wing low.

Ok. Thanks you live and learn. ! Aren’t the mast heads on the chinook and sea knight etc slightly offset. ? We’re talking probably microns here.
Back to the OP. It is one of the joys of rotary wing flying ( there are many ) which no one gets unless you’ve tried it.

[MightyGem]

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.
Thought that was the case with all helicopters to be honest. ;)

[MightyGem]

It is one of the joys of rotary wing flying ( there are many )
Another is when asking a technical(especially a POF one) question, you will get many answers, some contradictory, as everyone thinks they know how they work. The Rotary Test Pilot at Boscombe Down once told me that nobody truly understands how, or why, some things happen with helicopters.

[stilton]

Thanks for the very informative replies, this characteristic does seem more pronounced on some types than others


One poster indicated this can be ‘engineered out’ to some extent ?


I would think that would be desirable and something all helicopter manufacturers would strive for ?


One other thing, does this slight angle of bank while hovering require a small cyclic input or is it ‘built in’ with a centered cyclic maintaining the bank ?

[LRP]

Thanks for the very informative replies, this characteristic does seem more pronounced on some types than others


One poster indicated this can be ‘engineered out’ to some extent ?


I would think that would be desirable and something all helicopter manufacturers would strive for ?


One other thing, does this slight angle of bank while hovering require a small cyclic input or is it ‘built in’ with a centered cyclic maintaining the bank ?

All of the above. Some designs have a mast tilt, some have rigging solutions. The Bell 407 has a mast tilt and an interconnected linkage that couple fore and aft cyclic with lateral input. These designs, like other design solutions, have to compromise between hover performance vs. cruise flight performance.

[Ascend Charlie]

One other thing, does this slight angle of bank while hovering require a small cyclic input or is it ‘built in’ with a centered cyclic maintaining the bank ?
It also depends on your CG - a chubby pilot or a Trump-esque pax will require some cyclic adjustment to stay in one spot. But helicopter pilots, being the sky gods that we are, automatically make adjustments without needing to activate too many brain cells. Which is lucky, in some cases.

[MENELAUS]

In some respects there is no merit in engineering this out, as this leads to mechanical overcomplication of what is an already complicated device. And of course added cost.
As stated, aside from the aerodynamic interactions of the tail rotor and main rotor(s) the BMI and indeed distribution of your punters/ stores/ arrays and indeed ordinance can affect all this. The Gazelle could on occasion hover one skid low but also with an aft inclination of the skid. Which required a gentle touch; in fact the thing is still technically in the hover until she’s finally on the deck, which can be witnessed on occasion by ab initiates (and it has to be said even experienced old f@rts) ‘cyclic hopping’ around the tarmac. Aircraft that were designed to deck land on small ships for example tend to have highly absorbent gear to cater for the fact that you couldn’t really demonstrate too much finesse in the final foot or so of landing due to deck pitch and roll, and just had to put it down, irrespective of whether you had one skid or wheel lower than the other.
Once you’ve mastered hovering it becomes second nature, which as another poster has alluded to doesn’t involve too much in the way of grey matter, and which is why in a subjective way is very hard to explain. You just have to observe it and get on with it. All great fun, as is downwind work and transitions, and I for one still miss it.

[kintyred]

Ok. Thanks you live and learn. ! Aren’t the mast heads on the chinook and sea knight etc slightly offset. ? We’re talking probably microns here.
Back to the OP. It is one of the joys of rotary wing flying ( there are many ) which no one gets unless you’ve tried it.
it’s the same principle as described for tail rotor aircraft. The heads are a foot or two displaced vertically when hovering and thus generate a couple. Because the aft head is doing more of the lifting (ie the c of g is closer to the aft head than the front) the difference in force means that lateral cyclic (and pedals) are needed to maintain the hover.

[Cornish Jack]

I assumed that the Kaman Husky would be an exception, not having a tail rotor. A one hour 'famil' wet winching offered no confirmation - the various bits of the airframe were in constant, and contradictory, motion !

[[email protected]]

Back in the pre-DHFS days at Shawbury, it was always entertaining to see the students transfer from the Gazelle (right skid low) to the Wessex (left wheel low) as in their first take off, they inevitably adopted the Gazelle hover attitude and set off to the right fairly sharply!

All the instructors knew this would happen and were prepared to intervene when required but it was a good teaching point and gave most onlookers a giggle.:)

For the first hover checks on SAR, the hover attitude was noted as it varied with AUM, C of G and wind conditions and, if conditions outside worsened, the AI could be used - very useful especially over the water.

[MENELAUS]

And when the trusty old Gnome’s on the Wessex were slow to start on a cold day, you could always stamp your feet get them going. !
What a great bit of kit that was. One of the few airframes we took off the Yanks and made better.

[JohnDixson]

Globo, not just the airframe-the Gnomes were GE T-58’s built under license too, as I recall being told.

[MENELAUS]

Yes believe you’re right. In much the same
way as the mighty Bristol Nimbus was developed, I think, from Turbomeca and the Artouste ?
I’ll stand corrected however I think the only home grown engine for helos of that period was probably the Napier Gazelle ? Wx III

[JohnDixson]

Globo, you wrote: “In some respects there is no merit in engineering this out, as this leads to mechanical overcomplication of what is an already complicated device. “
Amen. Fly the Crane from say 15 to 60 KIAS trim it out and see where the nose is pointing. Suffice to say, SA never repeated the lateral shaft tilt idea.
Also, re my somewhat snarky comment re the Gnomes, here is some reality: when I joined SA in 1966, I was shortly joined in a small cubical by a former USMC pilot back from flying S-58’s in Vietnam. He told me that they were changing engines at 300 hours and did not fly single ship missions with the aircraft due to the engine situation. Strong case for the Gnome addition.

[[email protected]]

So was the RR Gnome the same as a GE T-58, just built better or was the design changed by RR? Did the GE have the fuel computer like the Gnome?

[[email protected]]

Wikepedia answered my question - yes, same engine - no, fuel computer was De Havilland/Lucas so introduced on the Gnome

[JohnDixson]

Crab, sorry for the thread drift: The first GE electronic supervisory control for the T-58 came to the USN with the T-58-10, around 1966-67, and was called , ( appropriately according to more than a few USN SH-3D pilots ) the PMS. Its in-flight reliability was poor enough to merit SA having to install a PMS cut-off button on the 3D cyclic grips. Thus, when GE announced that the T-700 engine ( for UTTAS -UH-60/61) would arrive with a hybrid hydromechanical and electronic engine control we and I think Boeing were expecting trouble. Turned out that GE had done their homework well and the reliability of the electronic control surpassed that of the hydromechanical unit, and neither were an issue.

[sycamore]

The Gnome is/was a good engine....when it worked properly...economical,light,good power....but it needs setting up properly with the computer...
It did not take kindly to operating in tropical conditions,where everything gets wet/damp,dusty/sandy,etc.,and being in the nose of the Whirlwind,with only a gauze screen intake,poor protective treatment of the 10-stage compressor blades and IGVs,one would get power losses after 25-50 hrs,and no torquemeter .The computer electrics likewise were a problem,so one could end up doing a sortie in `manual` throttle.Similarly,the `engine surge margin` became degraded,so it could lead to a surge,,HP COCK OFF,lever down ,forced landing.....Had 2 of those...A helo engine ,in my view,should have at least a couple of centrifugal stages at the front ,as they are more resistant to wear,and give better surge protection. Best engines for rapid response..Astazou in Gazelle,and Nimbus in Scout/Wasp....you can pull from joined needles to max in less than 1 second..they may surge(very audible),but no flames out of the jetpipe...Yes you can do that in WX and SK,but the HP cocks are electric(2 seconds) and if it surges,there`ll be metal in the jetpipe......
Globo...Wessex 1 and 3 Napier Gazelle,different Mks,and Belvedere.....another good engine...maybe the Navy might not agree....but a lot depends on the operating environment.....

thats my 50 p worth....

[[email protected]]

In nearly 6000 hours of flying with the Gnomes in Wessex and Sea King, I had one partial rundown and one real computer freeze so I'm pretty happy with their reliability.

I flew with the FAU (Uruguayan Air Force) a few years ago and they had a dispersal full of old Wessex we had sold them many years before. They had had such problems with them because they didn't know how to set up the fuel computers that they became impossible to use (no manual override on a Wessex).

[ShyTorque]

I enjoyed the challenge of wet winching in manual throttle in the Whirlwind 10......but not a lot.

Those winchman deserved a medal.

[ShyTorque]

I flew with the FAU (Uruguayan Air Force) a few years ago and they had a dispersal full of old Wessex we had sold them many years before. They had had such problems with them because they didn't know how to set up the fuel computers that they became impossible to use (no manual override on a Wessex).

Those particular Wessex (ex 28 Sqn if I’m thinking of the same five airframes) suffered very poor serviceability in the latter days of their RAF service; it’s hardly surprising that engineers with far less experience had trouble with them.

[Cornish Jack]

Whirlwind computer control was, obviously a great advantage. At Valley, we had a Master Pilot returning to S&R after previous tours on the piston WW and his hovering was 'variable' ... until he went 'computer out'. Instant perfection and rock-solid rotor speed! How long does 'muscle-memory' last?

[ShyTorque]

CJ, in the late 1970s the French exchange QHI at RAF Shawbury had apparently never flown an aircraft with a computer controlled engine before he began teaching on the Whirlwind 10. His students hardly ever got to fly computer in, he obviously thought it was too easy (or maybe out of his personal comfort zone).