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.

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.

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

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!

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/

👍

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. 😉

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.

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.

https://cimg3.ibsrv.net/gimg/pprune....e7a697cab.jpeg

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

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

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....ba2220ae21.jpg

Your assumption would be correct Mr B!

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

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...

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

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

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.

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::)

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!

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.

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

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.

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.

Translational lift
 

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.

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.

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.

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....e228ce4a0.jpeg

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.

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

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.

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/15...ownwind-2.html

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

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.

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.

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.

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/15...ownwind-2.html

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:

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?

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.