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Rolling take offs in a Helicopter

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Rolling take offs in a Helicopter

Old 23rd Jun 2021, 06:18
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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.
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Old 23rd Jun 2021, 06:30
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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.

Last edited by paco; 23rd Jun 2021 at 06:42.
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Old 23rd Jun 2021, 07:55
  #43 (permalink)  
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Originally Posted by [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.

Interesting

I was referring to operations from a carrier or other unobstructed deck but I didnít think to specify that !


Hadnít 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
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Old 23rd Jun 2021, 08:55
  #44 (permalink)  
 
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Originally Posted by stilton
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!
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Old 23rd Jun 2021, 09:41
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Originally Posted by stilton
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_poli...ying_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_poli...uals/aviation/

👍

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Old 23rd Jun 2021, 09:49
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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. 😉


Originally Posted by Greek God

Most of our departures required the rolling technique. God Bless the Alouette 3 and the abuse she took without complaint!
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Old 23rd Jun 2021, 12:21
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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.
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Old 23rd Jun 2021, 12:43
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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.

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Old 23rd Jun 2021, 12:45
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Transational lift that barsteward.
In fact we werenít going anywhere as we flamed out.
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Old 23rd Jun 2021, 13:05
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Someone out there must have a picture of an Army CH-21C doing a running takeoff on the nosewheel?
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Old 23rd Jun 2021, 13:31
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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.




Last edited by SASless; 23rd Jun 2021 at 18:31.
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Old 23rd Jun 2021, 21:22
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Your assumption would be correct Mr B!
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Old 23rd Jun 2021, 23:52
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SAS, what is with the guy in the right seat wearing shorts and white sox?? Great shot anyway
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Old 24th Jun 2021, 02:31
  #54 (permalink)  
 
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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...

Last edited by tartare; 24th Jun 2021 at 04:10.
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Old 24th Jun 2021, 04:10
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Army Hertitage Flight....privately owned aircraft.....thus no uniform requirement.


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Old 24th Jun 2021, 06:33
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Originally Posted by 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...
You are correct though. That burble you could hear is usually the onset of translational lift. And vv when you come out of it.
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Old 24th Jun 2021, 07:26
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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.
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Old 24th Jun 2021, 07:37
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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
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Old 24th Jun 2021, 13:58
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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!
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Old 24th Jun 2021, 18:30
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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.
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