Rolling take offs in a Helicopter
 

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

"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:)

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.

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? :(

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 !

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.

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!

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.

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.

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. !!

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.

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.

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.

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

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.

That's correct

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?

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.

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.