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Rolling take offs in a Helicopter

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Rolling take offs in a Helicopter

Old 24th Jun 2021, 19:54
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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
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Old 24th Jun 2021, 20:09
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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.
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Old 24th Jun 2021, 20:50
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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.
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Old 24th Jun 2021, 21:03
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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.
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Old 24th Jun 2021, 23:33
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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.
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Old 25th Jun 2021, 10:34
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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.
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Old 25th Jun 2021, 10:48
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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 👍


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Old 25th Jun 2021, 13:26
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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.
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Old 25th Jun 2021, 13:33
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Originally Posted by high spirits View Post
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...
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Old 25th Jun 2021, 14:59
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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.
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Old 25th Jun 2021, 15:18
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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......


Hovering Downwind




Last edited by SASless; 25th Jun 2021 at 15:37.
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Old 25th Jun 2021, 16:25
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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....
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Old 25th Jun 2021, 16:44
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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.
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Old 25th Jun 2021, 16:57
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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.
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Old 25th Jun 2021, 17:58
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Originally Posted by [email protected] View Post
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.
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Old 25th Jun 2021, 19:20
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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.

Hovering Downwind
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Old 26th Jun 2021, 04:42
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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’ ?
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Old 26th Jun 2021, 09:17
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Originally Posted by stilton View Post
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:

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Old 26th Jun 2021, 09:17
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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?
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Old 26th Jun 2021, 12:02
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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.
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