Rolling take offs in a Helicopter

[stilton]

I understand that in limiting conditions of density altitude / heavy weight a helicopter can use a rolling take off rather than a vertical one


My question is how this works, particularly in reference to control inputs


Do you start the roll with a little forward cyclic and full up collective, lifting into the air when enough lift has been achieved or start with a little forward cyclic, full down collective and slowly bring the collective up as you gain speed and lift off ?


Additional questions, will you have achieved translational lift before lifting off ? Is this necessary?


Do helicopter manufacturers endorse and or certify their aircraft for this generally and do they provide recommended flight manual techniques ?



Do I have this correct in when you might use this type of take off ?


Finally would you ever perform a run on landing for similar reasons ?



Fixed wing pilot here, trying to understand this ( rare ?) aspect of helicopter flying

[jimjim1]

trying to understand this ( rare ?) aspect of helicopter flying

"Rolling" takeoffs are also used where the helicopter is equipped only with skids. The idea is to have sufficient energy at all times to make a safe descent to the ground in the event of say an engine failure.

Hope that helps:)

[JimEli]

I understand that in limiting conditions of density altitude / heavy weight a helicopter can use a rolling take off rather than a vertical one

My question is how this works, particularly in reference to control inputs

Do you start the roll with a little forward cyclic and full up collective, lifting into the air when enough lift has been achieved or start with a little forward cyclic, full down collective and slowly bring the collective up as you gain speed and lift off ?

Additional questions, will you have achieved translational lift before lifting off ? Is this necessary?

Do helicopter manufacturers endorse and or certify their aircraft for this generally and do they provide recommended flight manual techniques ?

Do I have this correct in when you might use this type of take off ?

Finally would you ever perform a run on landing for similar reasons ?

Fixed wing pilot here, trying to understand this ( rare ?) aspect of helicopter flying
We performed rolling takeoffs in the UH-60. They were conducted when the aircraft was incapable of hovering due to the combination of GWt and density altitude.

Generally speaking, the takeoff was performed by coordinating collective increase with forward cyclic to achieve the quickest possible acceleration. Upon reaching a preplanned airspeed (above ETL, respecting GS limits), maximum collective was applied for the climb out. Many additional techniques were employed to enhance safety margins.

Since there were no flight manual procedures or performance charts supporting the maneuver, methods were developed to determine a takeoff speed and predict when OEI capability would be achieved. Until then, a powered-descent and run-on landing was the only option in the event of an engine failure.

A run-on landing is also the accepted procedure for OEI conditions and other emergencies.

[Robbiee]

I've always been under the impression that if you cannot hover you shouldn't takeoff. However, if the zombies are approaching, and you must, well then, just enough up collective to get light on the skids, then push forward until you get through ETL, and up you go. Thing is, you won't know just how far you need to drag along the ground to get to ETL,...and you just may run out before you do? :(

[Sir Korsky]

We would always get moving with a rolling take off in the 76 with a low IFR departure from a runway. We'd take of like an airplane. You want to get as close to Vmini as possible before lifting. Somebody will have to remind me, but I think max ground speed was 60 knots on the gear and 50 or 60 knots vmini depending on 76 variant. Our ops specs permitted 600RVR departures. That was handy for lifting in fog. You'd certainly need to be proficient on the machine to attempt it. Pull a little collective, push the cyclic a little forward, keep it straight on the center line lights with pedal and she'd come away easy. As soon as you were in a stable climb you'd hit the modes and let George fly with your thumb hovering on the G/A button. Fun times !

[gulliBell]

...but I think max ground speed was 60 knots on the gear...

Ummmm, 54 knots? And if you're doing 50 knots on the wheels in a 76 that does seem awfully fast, even though it's within limits. I certainly wouldn't want to be taking the taxiway exit at that speed, nor even 40 knots for that matter.

[gulliBell]

Oh yeah, I seem to recall a Bristow S76A doing a running landing at 80 knots at Barrow Island with jammed controls and he pulled that off OK, apart from cooking the brakes. Now that would have been exciting!

[Ascend Charlie]

Usually when departing in the 76 from KSA at Sydney with a full load (or even any other time), we would be rolling along the taxiway from the GA area, and would get the takeoff clearance before reaching the holding point - just increase the collective a little, a small amount of forward cyclic, keep rolling along and it just lifted off around 20 kt. Then add power as required. I expect it was just translational lift that caused the takeoff, as there was not sufficient power applied to hover. Taking off from our private heliport at the other end was similar, I had about a 50m runup along the taxiway to the pad, and launch into the blue. Private operation, no requirement for single-engine accountability, but usually we had power to satisfy the requirements anyway.

I would practice running landings at other uncontrolled airports when empty, just using less than 45% dual Tq, touch down less than 45kt, the brake limit was (I think) 38kt or thereabouts, but the machine was slowing anyway and I would need to add power to get to the taxiway exit.

[MENELAUS]

Whilst I can’t say it was a regular occurrence it was certainly practised on Wessex III, V and Sea King. And running landings for that matter as well, as well as practising OEI, tail rotor issues etc. In fact a certain RN Wessex III squadron had a contest going for a while for maximum run on speed until someone ( nameless ) over cooked it and ran on… fortunately on to the grass and no major damage done. That, however, was the end of that.
As for technique, it is as the foregoing experts have described. Certainly less torque required than a hover IGE.

[megan]

Our 76's had glass windscreens rather than the acrylic, a running take off or landing was desirable if loose stones were a problem, as we found out, shattered windscreen, big bucks.

The charlie model gunship in Vietnam was unable to hover when loaded for bear, not until the -11 engine was swapped for the -13, making it a mike model. The chaps were well practiced in hovering autos, hover taxi from the revetment to the runway until rotor bleed caused you to run out of pedal, chop throttle, land, get RPM back, repeat until runway made for running take off. Backseat crew might run alongside during take off to ease the load and jump on when a bit of speed was gained.

[MENELAUS]

Our 76's had glass windscreens rather than the acrylic, a running take off or landing was desirable if loose stones were a problem, as we found out, shattered windscreen, big bucks.

The charlie model gunship in Vietnam was unable to hover when loaded for bear, not until the -11 engine was swapped for the -13, making it a mike model. The chaps were well practiced in hovering autos, hover taxi from the revetment to the runway until rotor bleed caused you to run out of pedal, chop throttle, land, get RPM back, repeat until runway made for running take off. Backseat crew might run alongside during take off to ease the load and jump on when a bit of speed was gained.

Brilliant. Well and truly black cats everything else. !!

[paco]

To answer the OP, you remain within ground effect up to about 12 knots, where translational lift takes over. With a fixed wing aircraft, ground effect only occurs because the wingtip vortices are reduced. On a helicopter, you also reduce the induced flow coming in from above the disk (it bounces back from the ground), so the angle of attack increases for two reasons (this is propeller theory, only sideways).

And to anticipate another question, translational lift occurs because the air is flowing into the disc by itself, so you don't need to use so much power to suck it in, which also reduces the induced flow (and increases the angle of attack of the blades).

To answer another poster:

"I've always been under the impression that if you cannot hover you shouldn't takeoff."

Indeed, that is the EASA rule.

[tu154]

The S92 has a rolling takeoff technique for use in low visibility, or departing from (snow or possibly sand) contaminated runways. Normally requires more distance than a class 1 takeoff profile, 1.5 the todr, from memory, half a coffee in.

[[email protected]]

Stilton - please understand that whilst a 'rolling take off' can be from the ground (especially with wheeled undercarriage) the same profile can be carried out from a very low hover when there is insufficient power to either hover higher or make a normal transition.

This technique s known as a 'cushion creep' and relies on gaining translational lift shortly after losing the benefits of ground effect, at which point the aircraft will want to climb away.

The venerable Wessex at high DA had a VMin limit so a running take-off in that case would be held on the ground until that speed was reached and then lift off - all to do with TR power.

[212man]

"I've always been under the impression that if you cannot hover you shouldn't takeoff."

Indeed, that is the EASA rule.
Doesn't/didn't the AW189 have only a rolling PC1 runway profile?

[MENELAUS]

Stilton - please understand that whilst a 'rolling take off' can be from the ground (especially with wheeled undercarriage) the same profile can be carried out from a very low hover when there is insufficient power to either hover higher or make a normal transition.

This technique s known as a 'cushion creep' and relies on gaining translational lift shortly after losing the benefits of ground effect, at which point the aircraft will want to climb away.

The venerable Wessex at high DA had a VMin limit so a running take-off in that case would be held on the ground until that speed was reached and then lift off - all to do with TR power.

And they were great fun. Tried it once at Hollywood barracks when our friendly SLF had loaded on too much kit. Stiff bollocking from the adj of the Green Howards ensued.

[finalchecksplease]

Doesn't/didn't the AW189 have only a rolling PC1 runway profile?

That's correct

[SASless]

Megan....will a Huey fly at 5800 RPM instead of the normal 6600 RPM with a resulting decrease in TR authority due to the reduced RPM (per the B/C/D/H models used in Vietnam prior to the arrival of the -13 engines?

[Ascend Charlie]

On a helicopter, you also reduce the induced flow coming in from above the disk (it bounces back from the ground)

Oh, Paco, Nick Lappos will be so sad that you said that.

Same as an aeroplane wing, the rotor has a reduction in induced flow with the airflow changing direction because of the proximity of the ground. It doesn't bounce back, it doesn't increase the pressure under the disc, there technically is no "pressure bubble", just a reduction in induced flow. So for the same collective position, you get more lovely lift.

Sassy, sounds like you speak from experience, but being at 5800 N2 with the horn bleating away (I suppose you could cancel it?) would scare me a bit... I recall a flight with BO'L where the RPM light was on, and he took out his black chinagraph pencil and coloured it in.

[SASless]

Megan will probably chime in again and confirm that of which I speak.....as that was a very common occurrence back in our youthful days.

[Fareastdriver]

One could try the Towering Takeoff Method used on the Bristol Sycamore; notorious for lack of power and manoeuvrability.

On the ground accelerate the rotor to max permissible. IIRC 287 Rrpm.
Pull the collective up at the same time opening the throttle to keep as the power at max whilst you overpitch to min Rrpm.
Pitch forward with the cyclic at the same time milking the collective to maintain Rrpm as you transition forward and down.
When you achieve level flight continue take off as normal.

Worked every time.

Still used in a way. On the S76A+ we would use a similar technique so that one could throw it over the side of a platform and allow gravity to achieve VTOSS.

[SASless]

The Bell 47G2 at Redhill on a Summer Morning with two of us aboard made for an interesting takeoff.

A certain large framed fellow with an Aussie accent and I were to do some training in the G2 in anticipation of my doing a Check Flight for my UK ATPL.

Being full of Gas (the aircraft) and the two of us onboard.....I hit the Up Stop on the Collective and Max Throttle and other than some vibration change....there was no discernible movement by the skids.

When asked by the CFI...."What you going to do now Shag?".....I uttered those famous last words of some helicopter pilots....."Watch this!".

The grass was still wet with Dew.....and with a bit of wiggling of the cyclic we were off like a herd of Turtles until we finally gained enough speed to achieve ETL....and leave the ground.

In time...we were at Pattern height and went on to doing Confined Areas....with Towering Takeoffs being done in ultra slow motion.

There are real risks in doing limited power takeoffs as there can be less than adequate margins should you goof it up.

We used to pick up to a three foot hover....check handling and power available....then land back and proceed to do a takeoff from the ground barely touching the toes of the skids to the ground to use the least amount of power possible when we were at max power at the three foot hover.


As seen in the video.....as a Gun Ship does a takeoff but in the video it does have an excess of power required looking at the way it maneuvers.


https://www.youtube.com/watch?v=rcQ5guV4JMM

[212man]

When asked by the CFI...."What you going to do now Shag?".....I uttered those famous last words of some helicopter pilots....."Watch this!".

The grass was still wet with Dew.....and with a bit of wiggling of the cyclic we were off like a herd of Turtles until we finally gained enough speed to achieve ETL....and leave the ground..

I hope that didn't knock the ash off the end of his parked fag/cigarette but?

[[email protected]]

FED - sounds like a similar technique to the Single Engine Water Take Off (SEWTO) for the Sea King which some of us were able to practice on the Waterbirds Course in Nova Scotia.

[ShyTorque]

I once had to ferry a Puma "single engine" from an oiled sand strip on a Caribbean island. The other engine refused to start despite the efforts of some of our ground crew/engine mechs on scene.

We used the normal running takeoff departure technique, basically as described above. That was uneventful because we regularly practiced them. I was actually rather more concerned about the over water flight that followed because we were the only available local SAR cover!.

[[email protected]]

Ah, the sunshine and snorkelling det to Belize Shy?

[212man]

I once had to ferry a Puma "single engine" from an oiled sand strip on a Caribbean island. The other engine refused to start despite the efforts of some of our ground crew/engine mechs on scene.

We used the normal running takeoff departure technique, basically as described above. That was uneventful because we regularly practiced them. I was actually rather more concerned about the over water flight that followed because we were the only available local SAR cover!.
One of our 332s did the same while on contract to UN in Liberia around 1994. They had shutdown, but were then alerted by lots of shouting on the hand held radios, as their group of frantic Pakistan (?) peace keepers came running back being pursued by unfriendlies. I can't remember if the second engine failed to start, or they just thought it prudent to go with what they had!

[ShyTorque]

Ah, the sunshine and snorkelling det to Belize Shy?

It was tough, but someone had to do it.

[I have to admit, I really didn't like the place much at all. With very high OAT and over 95% humidity during the summer I was usually on the point of melting, especially in the cockpit. Then, being based in Germany, having to go from from operating plus 28C to -28C in the winter, or vice versa a couple of days later was "interesting". It was so stressful that those poor delicate Harrier pilots in the same block as us had to have air conditioned rooms. Helicopter pilots not so privileged].

There's a bit more to the above story, btw. We needed the AOC's personal permission to carry out a single engine transit. He willingly gave it, he was supposed to be on the aircraft and had no other way of getting back to the mainland! Not to mention the Air Commander and every RAF Harrier pilot in Belize at that time, some of whom had gone out to the island by a private speedboat. It had also broken down with steering failure and could only go round in tight circles!

[Fareastdriver]

The other engine refused to start despite the efforts of some of our ground crew/engine mechs on scene.

As I had to shout up to my crewman in Belize a couple of times. "BELT THAT BOX WITH THE SHINY COVER"

It usually got it to start.

[malabo]

Running takeoffs are pretty common where I've flown, both wheels and skids, when heavily loaded or in reduced RVR. Never had to in the 139 or 407, all that power I guess. Watch any fire-fighting machine slide and skip its way through translation when heading out for the season - stuffed with nets, bucket, long lines, engineer, tools, and a jag of fuel to minimize fuel stops to contract location.

[SASless]

212man......nope...that cigarette long ash never quivered or fell from that takeoff.....but the next morning it fell to the floor...Butt and Ash....along with his Jaw but that is story best told in private!


I hope that didn't knock the ash off the end of his parked fag/cigarette but?

[stilton]

Very interesting, thanks for the informative replies


I’m thinking a helicopter taking off from a ship underway is always doing the equivalent of a rolling take off ?

Does this require a particular technique ?

[casper64]

Oh yeah, I seem to recall a Bristow S76A doing a running landing at 80 knots at Barrow Island with jammed controls and he pulled that off OK, apart from cooking the brakes. Now that would have been exciting!

There was an Israeli Apache that lost tail rotor drive and landed with more than 120 kts… Just keep it nicely level and aligned and easy on the brakes!

[Greek God]

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1986x1504/9a0d60a8_a68a_4fb4_9f47_8179867d7622_c9e972c08fc212f344c1628 c0a8590bc05635fad.jpeg
Most of our departures required the rolling technique. God Bless the Alouette 3 and the abuse she took without complaint!

[Sir Korsky]

Ummmm, 54 knots? And if you're doing 50 knots on the wheels in a 76 that does seem awfully fast, even though it's within limits. I certainly wouldn't want to be taking the taxiway exit at that speed, nor even 40 knots for that matter.

Never broke nothing and never had a complaint from mx....brilliant solid aircraft.

[John Eacott]

I’m thinking a helicopter taking off from a ship underway is always doing the equivalent of a rolling take off ?

Does this require a particular technique ?

Not at all: you're lifting to a hover over a particular spot, sometimes in a strong wind. From the hover you move sideways off the ship and then transition into forward flight, climbing to a nosebleed 200ft.

[John Eacott]

https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1986x1504/9a0d60a8_a68a_4fb4_9f47_8179867d7622_c9e972c08fc212f344c1628 c0a8590bc05635fad.jpeg
Most of our departures required the rolling technique. God Bless the Alouette 3 and the abuse she took without complaint!

And there lies a particularly brave photographer! :cool:

[SASless]

When you bleed RPM on an Alouette III....you have accomplished something!

[megan]

I’m thinking a helicopter taking off from a ship underway is always doing the equivalent of a rolling take off ?

Does this require a particular techniqueThe only difference operating on a ship is the ship heaving, rolling and pitching. In order to land once had to get the ship to run down wind in big seas, to get a steady deck, and then land facing the stern and into the 40 knot wind.

SAS, 5800? One I can't answer, never noted the RPM when running out of pedal and having to chop the throttle, only time I ever saw that figure was on start up, shut down, or pulling pitch at the bottom of an auto. My Vietnam time was in the H, though went for rides in the left seat with the C chaps on my days off to see how they went about their business.

[stilton]

Not at all: you're lifting to a hover over a particular spot, sometimes in a strong wind. From the hover you move sideways off the ship and then transition into forward flight, climbing to a nosebleed 200ft.


Understood

I made the comparison as when lifting off from a ship underway in a helicopter you already have the relative wind that would be generated by a rolling take off on land

[[email protected]]

Stilton - on pretty much any warship other than a carrier, the landing area is behind the superstructure and in turbulent air if the relative wind is from dead ahead so the vessel would normally take up a course with the wind off to one side so the relative wind comes from about 30 degrees off the head.

Yes, airflow across the rotor disc will give you benefit and if above 12-15 knots will give you translational lift.

Bear in mind that rolling take offs are often used when very limited on performance which is not what you want when operating from a deck.

[paco]

Ascend Charlie - I was trying to make it easier for the OP :). But Nick should speak to the people in EASA - this is from their skybrary:

The direct effect on lift arises because a reduction in both upwash and downwash, as the air beneath a wing is compressed by ground proximity, creates a cushion effect.

[stilton]

Stilton - on pretty much any warship other than a carrier, the landing area is behind the superstructure and in turbulent air if the relative wind is from dead ahead so the vessel would normally take up a course with the wind off to one side so the relative wind comes from about 30 degrees off the head.

Yes, airflow across the rotor disc will give you benefit and if above 12-15 knots will give you translational lift.

Bear in mind that rolling take offs are often used when very limited on performance which is not what you want when operating from a deck.


Interesting

I was referring to operations from a carrier or other unobstructed deck but I didn’t think to specify that !


Hadn’t thought about landing a helicopter on the aft helipad of a destroyer for example


I spent a week on the USS Milius a few years ago and it was apparent how turbulent it was back there on a good day


Thanks for the interesting information

[212man]

Understood

I made the comparison as when lifting off from a ship underway in a helicopter you already have the relative wind that would be generated by a rolling take off on land
That's true, but also true of any wind, even on a stationary deck/location. Offshore operations typically limit windspeed to 60 kts, as a function of passenger safety on the deck and also sea state for rescue, but it certainly improves the power margins!

[MrBernoulli]

Fixed wing pilot here, trying to understand this ( rare ?) aspect of helicopter flying
stilton, could I suggest you start with a look at the US FAA 'Helicopter Flying Handbook', one of many very useful documents available for download on the faa.gov website?

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/

Download Chapter 10 and have a look at the section titled 'Running/Rolling Takeoff', which starts near the bottom of page 10-3.

There is a corresponding section in the Helicopter Instructor's Handbook, page 11-4, available on the same website:

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/

👍

[MrBernoulli]

Those air intake filters on the Alouette III look very familiar, as does the bush scenery. Might I hazard a guess at the photo having been taken in a country that lies between the Zambezi and Limpopo rivers? Or just possibly the country on the south side of the Limpopo? But I am reasonably sure it is the former. 😉


https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/1986x1504/9a0d60a8_a68a_4fb4_9f47_8179867d7622_c9e972c08fc212f344c1628 c0a8590bc05635fad.jpeg
Most of our departures required the rolling technique. God Bless the Alouette 3 and the abuse she took without complaint!

[meleagertoo]

In the '80s we practiced limited power (running take-offs and landings) extensively in the Gazelle, down to where you had to wiggle the cyclic and pedals to induce any movement at all and sometimes run for tens of metres before gaining TL.
This became even more important a technique on the Bell 47 - a normal situation.
Subsequently I was grateful for a solid grounding in limited power when operating 206s in hot and high environments. I'm astonished that running take offs and landings are considered odd, or that advice is not to fly if a vertical take off isn't achieveable. How pampered are todays pilots with such surpluses of power! And presumably how hampered when the surplus runs out.
I'm pretty sure we practiced routine rolling take-offs in the S61 too - and of course single-engined landings. The Chinook ran on in an auto at about 60Kts afair, nose high in the air, needed hundreds and hundreds of metres to stop.

[SASless]

Chinooks can do a near zero groundspeed EOL....it takes a bit of practice to get it down to that.

Our standard EOL was a 180 degree although we did do a lot of 360 overhead EOL Landings,....which started about 2,000 feet AGL.

One should recall that the vast majority of our descents to field sites were with the Thrust Lever (Collective) full or. nearly full down and included steep turns until reaching 250-300 feet AGL then regaining power and a standard steep approach angle.

That limited the exposure time to ground fire and made the aircraft a harder target to hit and allowed you time enough to deal with a Single Engine failure of any kind.

In a one year tour where you flew a thousand hours or more you had plenty of practice to get it right.....and that translated to being able to do better touch down EOL's.

But you are right....run on at 60 Knots....and you better be at London Heathrow sized runways as aerodynamic braking only works to slow you down and upon getting all four gear onto the ground you are still moving at a fair ol' pace.

[MENELAUS]

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/300x210/1fe044d3_d369_47c1_95d7_d7a5e249844b_68005831d55901657ceb6ba d104c787e7a697cab.jpeg

Transational lift that barsteward.
In fact we weren’t going anywhere as we flamed out.

[JohnDixson]

Someone out there must have a picture of an Army CH-21C doing a running takeoff on the nosewheel?

[SASless]

Brother Dixson,

That reminds me of a story told by a friend who flew H-21's out of the Washington DC area during his Army days.

The short version is the H-21 Unit he was assigned to at Fort Belvoir was tasked with the emergency evacuation of key personnel from the Pentagon to a Navy Ship that was positioned not far from the Pentagon....should the balloon go up and nuclear war was imminent.

He told of taking off from the Pentagon on a practice flight as part of a Training Op.....and just after takeoff having experienced an engine failure....performing an autorotation into a rather large grassy field....and as the nose wheel dropped to the ground, hidden in the tall grass was an open Man Hole, into which the nose gear quite unfortunately dropped right inside.....resulting in the tall spindly front gear snapping right off and causing the aircraft to flop down on its nose. He said it remained up right and not a lot of damage was done. He seemed miffed that despite doing a text book response to the engine failure fate intervened to place that open man hole exactly where the nose gear came to earth and thus undo all of his good airmanship.


https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/900x627/h_21b_6086e5dedb3eaa1d353a07bc9136b6ba2220ae21.jpg

[Greek God]

Your assumption would be correct Mr B!

[JohnDixson]

SAS, what is with the guy in the right seat wearing shorts and white sox?? Great shot anyway

[tartare]

Fixed wing pilot question.
I had understood translational lift as being when your forward speed gets to the point that the airflow `sees' the main rotor as a solid disc, and you get lift from the disc in the same way that you'd get lift from a fixed wing (i.e. air flowing over the disc).
I've noticed when taking off as pax in a helicopter that there is a `woosh' feeling above certain forward airspeeds - the machine kind of swoops upwards, and had assumed that was the onset of translational lift.
Yet some of you are talking about obtaining translational lift at quite low forward speeds... how so?
Am I visualizing the whole concept incorrectly?

EDIT: OK, Googled ETL and now have a slightly better idea I think. Vector of main rotor thrust points rearward due to forward motion as well...

[SASless]

Army Hertitage Flight....privately owned aircraft.....thus no uniform requirement.

[MENELAUS]

Fixed wing pilot question.
I had understood translational lift as being when your forward speed gets to the point that the airflow `sees' the main rotor as a solid disc, and you get lift from the disc in the same way that you'd get lift from a fixed wing (i.e. air flowing over the disc).
I've noticed when taking off as pax in a helicopter that there is a `woosh' feeling above certain forward airspeeds - the machine kind of swoops upwards, and had assumed that was the onset of translational lift.
Yet some of you are talking about obtaining translational lift at quite low forward speeds... how so?
Am I visualizing the whole concept incorrectly?

EDIT: OK, Googled ETL and now have a slightly better idea I think. Vector of main rotor thrust points rearward due to forward motion as well...

You are correct though. That burble you could hear is usually the onset of translational lift. And vv when you come out of it.

[Ascend Charlie]

In the still-air hover, all the airflow is being sucked from above the rotor and blown downwards. The column of descending air is called Induced Flow, and it reduces the angle of attack, making the pilot use more collective pitch to compensate.

With forward movement, or with a breeze, the air at the front of the disc is seeing airflow coming at it more horizontally instead of all vertically, that is, not induced flow, so the front gets a higher angle of attack and more lift. (This causes other effects such as flapback and further complications like inflow roll, but that's another topic.) As the forward speed increases, more of the disc sees the airflow coming more horizontally than vertically, though at the back there is still a large vertical component - you can't suck the air down without the air above it coming down to fill the gap.

Overall, though, somewhere between 12 and 18 knots is the "leap into the air" point, the sound of the rotors changes with the change in AoA, and the pilot smiles and gets hard.

[[email protected]]

Tartare - ETL is broadly in the range 12-20 kts - it is difficult to know exactly when flying since the airspeed indicators are poor at low speeds.

As someone mentioned before, the rotor without ETL has to work hard to suck all the air through it and accelerate it to produce thrust = high power requirement.

Once moving forward (or in any direction) at sufficient speed, the air approaching the rotor has a horizontal component that changes the inflow angle, increasing the AoA and giving more thrust for the same collective setting. The aircraft wants to climb so we normally let it - the whoosh!

A number of other aerodynamic effects are also noticeable just before ETL (flapback/blowback and inflow roll/transverse flow) but these are easily corrected for by the pilot. You often get pitch disturbances above ETL on helicopters with large horizontal stabilisers as the airflow changes during transition to forward flight - the AS365 had a marked pitch nose down at about 30 kts.

Managing all of this whilst still looking good and talking cool on the radio is clearly why helicopter pilots are so superior:ok::)

[SASless]

Crab.....as usual you also overlook important parts to the equation.....not only superior.....but also handsome and debonair as confirmed by our wear of self tinting Ray Bans, Big Watches, and always looking for a Skirt to chase!

Well...at least for the un-woke among us!

[Two's in]

A good demonstration technique is to pull power and settle to a very low hover (skids) or gently descend so the wheels are lightly on the ground. Note the power setting, hold that torque steady at that value, and just gently nudge in some forward cyclic. As you start to accelerate through ETL, holding the same power setting, you will start to climb away, conclusively proving that something you learnt about PoF may (or may not) be true.

[Tinribs]

Sat in the back of a Russian M8 doing a rolling take off at Murmansk. Went on for ever, After an hours cruise Vlad was able to do a normal landing. It seems Red Boys do rollers much more than we

[[email protected]]

Two's in - what is often misunderstood is that the maximum power required for that type of takeoff is not what you pull to initiate it but what you need just before the onset of ETL as you have to power through the roll-up vortices of the rotor.

So you won't get away with the same Tq you have in the very low hover.

[WASALOADIE]

Back in the days of the Wessex. We had a Max All Up Weight of 13600lbs for hover and a Torque of 3200lb/ft
However we could go to 14000lbs with a running take-off. Simply start the aircraft rolling with forward cyclic and some collective, gently increasing collective to a torque between 2800-3000lbs/ft then as speed increased to over 20Kts as transational lift gently increase torque further to 3200lbs/ft and the aircraft became airborne.

[high spirits]

I’m not so sure that I believe that the ‘burble’ is TL. That’s just the helicopter breaking into clean air not affected by the ground. Want proof? Take a helicopter to 200ft, ie way outside ground effect. See if you get the ‘burble’ like you do on transition close to the ground……
Translational lift occurs even with slight airflow across the disc, not 12-15kts.

[Torquetalk]

Even if you argue that TL is experienced from 1kt to 150kts, there is a big gain in rotor thrust as the helicopter flies clear of its vortices. That happens at 4ft and at 200ft. What is missing at 200ft is ground effect, which gives a further performance advantage by allowing you to hover near the ground with a lower collective setting. If you set up for an OGE hover in nil wind (a lot higher than 200ft AGL if SE) you will fly back into the vortices and experience the same burble. Then if you muck up applying enough power to maintain the hover, you will experience another burble as you accelerate into incipient VRS.

[[email protected]]

The burble is there whether you are at 2 ft or 200 ft - it is just more noticeable at low level because of the way the vortices 'roll up' due to the proximity of the ground.

TL doesn't occur at 'very slight' airflows across the disc unless your definition of very slight is more than about 8 -10 kts.

Try a downwind transition in 10 to 15 kts of wind and you will clearly see the loss of ETL, the 'smooth phase' of zero airspeed and then the pronounced burble of ETL - that works in a downwind OGE transition as well.

[John Eacott]

This power demand curve (from Eurocopter) may help some here understand the effect of translational lift, as shown by the increase and then reduction in power required from an IGE departure when passing TL 👍


https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1891x1478/b872b346_435b_4066_b467_147843484265_0fefcd23bc90a40050c5dd5 ca672d7fe228ce4a0.jpeg

[SASless]

Crab,

Does the Main Rotor have a way of knowing it is in a "Down Wind"?

The aircraft is designed for a certain direction of airflow that exists in forward flight thus is more efficient and offers less drag when the relative wind aligns with that aircraft orientation.

Of course when we talk about "down wind" we are referencing a certain and definable ground reference.

Is that what is causing the difference in power demand?

Drag alone...which is increased due to the airframe is what produces the demand for extra power as compared to being "into" wind with the most efficient orientation of the aircraft....or is it?

I seem to recall Mr. Lappos held forth on this topic at a time. period he was doing the flight testing on the Commanche....where they were doing 75 MPH sideways hovering and related testing.

If we do some searching...maybe we can resurrect that thread from right here at Rotor Heads.

[ShyTorque]

I’m not so sure that I believe that the ‘burble’ is TL. That’s just the helicopter breaking into clean air not affected by the ground. Want proof? Take a helicopter to 200ft, ie way outside ground effect. See if you get the ‘burble’ like you do on transition close to the ground……
Translational lift occurs even with slight airflow across the disc, not 12-15kts.

Weyup, here we go...;).
I'm just going to pull up a comfy chair and a cuppa...

[[email protected]]

Crab,

Does the Main Rotor have a way of knowing it is in a "Down Wind"? Sas - that is my point - it doesn't, as you know from the Chinook you can transition sideways which gives ETL to both rotors together.

My reference to the downwind transition was purely to highlight the sequence of events such that you can have ETL facing downwind, lose it as you move forward and then gain it again when your airspeed becomes positive.

Losing the burble as you move forwards coincides with an increase in power required to maintain height and the 'smooth patch' in the low to zero airspeed condition is a marked difference from ETL at the beginning and at the positive airspeed position.

The last couple of knots before gaining ETL again is where the maximum power required is - something that often catches people out when transitioning downwind with limited power.

[SASless]

I understand what you are sayiing and mostly agree......the one question I would pose to that is if that "demand for more power" is a product of attempting to accelerate too quickly which would not matter if you were into wind or "down wind"....and as I recall Doctor Lappos correctly what he opined is the takeoff distance downwind is far greater than when done into wind and that if the control inputs are done properly there is no difference in the power required for the takeoff but rather just a much longer distance. Note....I am working off my memory of that discussion from several years back and may be mis-stating what he had to say.

The trick with the Chinook was not going sideways....as there is just a wee bit of drag that you encounter doing that.

The trick was to kick some pedal into the old girl so as to rotate the aft head into clean undisturbed air just as you reached ETL with the Forward head which was in clean air all the time.

The alternative method was to have a quartering head wind component so as to accomplish much the same thing.

Chinooks, particularly the early models, could be very limited on power when loaded to or over max allowable takeoff weight.

There were times we measured the weight of our loads by use of the Torque meter alone....if we could pick it up without bleeding RPM with the load at a Ten Foot Hover.....we went....so getting all the advantages were key in getting the job done.

There were times we debated about how to get over a three foot high berm.....so it matters not what kind of helicopter you are flying....you have to be on top of your skills when operating without any excess of power.

The early A and B Models of the Chinook were interesting to fly on a single engine....and in both we had to burn off fuel to be able to get them light enough to do a single engine rolling takeoff and climb away.

The Vy for both was from about 58-61 Knots IAS.....get behind the curve on that and you only went down until you regained that speed again.....and you better have the height to do so or you would be. making an unplanned landing somewhere.


A bit of searching using google found this......


https://www.pprune.org/rotorheads/157371-hovering-downwind-2.html

[212man]

I understand what you are sayiing and mostly agree......the one question I would pose to that is if that "demand for more power" is a product of attempting to accelerate too quickly which would not matter if you were into wind or "down wind"....and as I recall Doctor Lappos correctly what he opined is the takeoff distance downwind is far greater than when done into wind and that if the control inputs are done properly there is no difference in the power required for the takeoff but rather just a much longer distance. Note....I am working off my memory of that discussion from several years back and may be mis-stating what he had to say.

I don't recall what Nick said, but I don't think he said that....

[ShyTorque]

If you are downwind in the hover in a single rotor helicopter you may need slightly more power to remain there to keep it from weathercocking round, in that using yaw inputs requires some tail rotor power.

[Torquetalk]

If you are sitting at 4ft in the hover with 15kts tailwind, the rotor vortices are ahead of you. Move forward and you fly into them, and thereby increase the power requirement (assuming you want to stay at 4ft). A downwind take-off requires more power. That’s why we usually take-off the other way.

[Two's in]

Two's in - what is often misunderstood is that the maximum power required for that type of takeoff is not what you pull to initiate it but what you need just before the onset of ETL as you have to power through the roll-up vortices of the rotor.

So you won't get away with the same Tq you have in the very low hover.

Crab, I was referencing AAC limitations: plus or minus 50% torque, HLS in the approximate 100km grid square, TOT plus or minus 3 days and crew are fully SQEP - Sometimes Qualified and Eventually Proficient.

[SASless]

For what Nick said....go to the linked thread that I posted and read down through the thread an read what Nick did say.

Then let's discuss "what" Nick did say....and not debate whether one reference itself is right or wrong or different than or in concurrence with.....old prune thread with his input is that Link...but you have to scroll down through it to find all of his posts.

https://www.pprune.org/rotorheads/157371-hovering-downwind-2.html

[stilton]

This has become highly technical and I don’t follow all of the finer points !


Just wanted to reiterate on my original question regarding rolling take offs


If you did initiate this by starting with full up collective then apply some forward cyclic to initiate movement, lifting into the air when adequate lift is achieved would that be a ‘valid technique’ ?

[Torquetalk]

This has become highly technical and I don’t follow all of the finer points !


Just wanted to reiterate on my original question regarding rolling take offs


If you did initiate this by starting with full up collective then apply some forward cyclic to initiate movement, lifting into the air when adequate lift is achieved would that be a ‘valid technique’ ?

Ah, THAT kind of rolling take-off. Gotta love them:

https://youtu.be/VJeBtHjqJz0

[Marly Lite]

Stilton, this is a suggested technique:

Apply 95% power (not max pwr).
Apply forward cyclic.
allow heli to accellerate.
top up power to maintain 95% (because TL is reducing tq for your applied pitch setting)
more forward cyclic to maintain disk attitude (overcome flapback)
allow heli to acellerate through 'the burble' (you are flying through your tip vorticies at this point)
heli will now want to leap airborne...
... apply 100% tq (for a clean break with the ground)
apply enough cyclic to maintain disk attitude (this might be fore or aft depending on heli type)
fly away....

​​​​​​​Any further questions?

[Ascend Charlie]

starting with full up collective then apply some forward cyclic to initiate movement,

If it won't even get off the ground with full up collective, you should be staying ON the ground. There are some exceptions, however, and it usually involves high speed pieces of lead proceeding in your specific direction.

[Winnie]

We use these because of incredible dust. Much easier with wheels! Also prevents pelting the fixed wings with rocks when we lift into hover!

[megan]

If it won't even get off the ground with full up collective, you should be staying ON the ground.There are some exceptions, however, and it usually involves high speed pieces of lead proceeding in your specific directionAs I described earlier it was also a procedure used in order to get to the scene of high speed lead coming in your direction. One C model gunship landed to pick up the crew of an aircraft shot down which then made them too heavy to take off, by pulling some power they were able to yaw the aircraft in order to aim the mini guns at the opposition and give them a dose of high speed lead. Once the opposition was placated a slick landed to pick up the crew.

[SASless]

Winnie,

What prevents you from segregating helicopter and fixed wing operations to provide a safe distance that would eliminate the throwing of rocks?

As dust is a shallow and short radius issue at takeoff....why not do a Towering Takeoff as done for ages in the past with no hovering....do a quick very low hover to ascertain all is right with the aircraft....land back....then take off from the ground and climb out of the dust cloud on Instruments?

If there is no question of the aircraft being ready to fly.....skip the low hover and just do the ITO.

[[email protected]]

.the one question I would pose to that is if that "demand for more power" is a product of attempting to accelerate too quickly which would not matter if you were into wind or "down wind"....and as I recall Doctor Lappos correctly what he opined is the takeoff distance downwind is far greater than when done into wind and that if the control inputs are done properly there is no difference in the power required for the takeoff but rather just a much longer distance. I'm sure he is correct - the amount of power required seems the same as long as you use the same technique both ways and are reasonably gentle on the controls.

What I have seen catch pilots out is being in the downwind hover with ETL from behind at close to max Tq then convincing themselves they can transition downwind without further thought.

Then they get very close to max Tq in the zero airspeed condition having lost ETL and tilted the disc and actually overTq as they hit the rotor vortices just before gaining ETL

[Wirbelsturm]

I've used the 'running take off' technique a few times in the AEW MKII Seakings of 849 to get off the deck of the (now old) carriers. We would wait for a clear deck, line up right at the back, perform a 'running' take off and flop off the left hand side of the deck just prior to the ramp, around '2' spot. The idea was to slightly over pitch the blades for the initial 'flop' as to clear the deck then use the height to accelerate into the ground cushion and translational lift.

The 'Bag' often required a fair bit of help off the deck in hot and humid conditions when the fuel load necessitated a long fighter controller sortie.

Fun times!

[Fareastdriver]

The idea was to slightly over pitch the blades for the initial 'flop' as to clear the deck then use the height to accelerate into the ground cushion and translational lift

Ground cushion? You must have only practised this dockside..

[SASless]

Ground Cushion over open water?

Sounds like ground breaking news to me!

Darn tooting does not work for OH-58A's over Lake water!

[Wirbelsturm]

Sorry to say that you still achieved 'ground effect' over water.
Thankfully as it was pretty useful when conducting HIFR and Advanced single engine flight continued technique training.

:ok:

[[email protected]]

If you were 'overpitching the blades' how did you not exceed Tq and PTIT limits?

[Wirbelsturm]

Hi crab,

You would pull to the torque limit, whilst observing the PTIT limits as well, overpitching was probably the wrong terminology. That was enough to clear you off the deck with forward speed but would not have been adequate for a 'normal' lift into the hover and transition as, when you moved off the deck, you would not have enough lift as you would lose the ground effect of the deck.

Was a fair while ago but worked well.

[[email protected]]

That makes more sense to me - a 'flop and drop' to gain speed :ok:

[Fareastdriver]

Similar to the S76A+ in the South China Sea. Beep the rotor up to 107%, pull hard up to 100%(+) on both and as it leapt into its tortured path into the air push forward steeply enough so that the boom wouldn't collect the deck as you hurtled over the side. You then plunged towards the sea hoovering up the 200 ft. or so available. 30 knots and the judder would follow and you pulled out of the dive at about 50 ft..

American passengers in the back would go "Yee Haw". The Chinese passengers didn't worry. They had fallen asleep as soon as they had fastened their seat belts.

[albatross]

AKA as “The Dance of Death” in the 76A.
Similar to the S76A+ in the South China Sea. Beep the rotor up to 107%, pull hard up to 100%(+) on both and as it leapt into its tortured path into the air push forward steeply enough so that the boom wouldn't collect the deck as you hurtled over the side. You then plunged towards the sea hoovering up the 200 ft. or so available. 30 knots and the judder would follow and you pulled out of the dive at about 50 ft..

American passengers in the back would go "Yee Haw". The Chinese passengers didn't worry. They had fallen asleep as soon as they had fastened their seat belts.

[stilton]

Fascinating stuff, this is a facet of helicopter operations that’s probably little known to most people and us fixed wing aviators

[Wirbelsturm]

Similar to the S76A+ in the South China Sea. Beep the rotor up to 107%, pull hard up to 100%(+) on both and as it leapt into its tortured path into the air push forward steeply enough so that the boom wouldn't collect the deck as you hurtled over the side. You then plunged towards the sea hoovering up the 200 ft. or so available. 30 knots and the judder would follow and you pulled out of the dive at about 50 ft..


That's the puppy!

Exactly what we used for rig work as well. Unfortunately the Seaking was 'computer' (not recognisable as such by the yoof of today:}) controlled at 104% NR as set by the speed select levers. The collective pitch anticipators would help in maintaining rotor speed but you did still have a very small amount of transient droop at 100% twin torque in hot temps. Over speeding the NR wasn't an option sadly. (I can't remember the exact figure allowed in auto but I seem to recall 110%) Although, thinking back on it, you could have selected more than 104% but it wasn't the 'done' thing I assume.

Distant memories of the windscreen filling with vision of sea wondering when the ground effect and translational lift would kick in! :eek:

[212man]

Similar to the S76A+ in the South China Sea. Beep the rotor up to 107%, pull hard up to 100%(+) on both and as it leapt into its tortured path into the air push forward steeply enough so that the boom wouldn't collect the deck as you hurtled over the side. You then plunged towards the sea hoovering up the 200 ft. or so available. 30 knots and the judder would follow and you pulled out of the dive at about 50 ft..

American passengers in the back would go "Yee Haw". The Chinese passengers didn't worry. They had fallen asleep as soon as they had fastened their seat belts.
Used it in 'anger' a few time sin Nigeria too, aided by a boot of right pedal as you lifted so you could pull some more Tq. Not helped by having engines that hit the N1 limits more or less the same time as the Tq limit, just to make the scan more interesting, and your left thumb on the N2 beeps busy...

[ShyTorque]

212man, yes flying the C, with the “B” gearbox was easier as the same engines topped out before the gearbox limits were reached so the “full power” scan was easier.

It didn’t really fly any better, though…especially as the MAUW was increased.