Hovering Limitation - Time

[SoundBarrier]

Hi Guys,

Being a non-rotorhead I am not sure about this. I have a work mate who was talking to me about some hovering limitation where a chopper cannot hover for too long as it creates a vacuum "or something" which causes said machine to fall out the sky.

Immediately I thought, no way. And even now I wonder if he got the wrong end of the stick. Is there any truth to the matter?

[thecontroller]

this is one of the funniest things i have ever heard

[SoundBarrier]

So to clarify your in-depth statement thecontroller :E are there any limitations to hovering besides fuel, MAUW and weather? Oh and skill of driver doing such.

[Nigel Osborn]

There are no limitations to the time you can hover other than fuel & boredom.

[Arm out the window]

Not unknown in some types for the engine oil temp to get too high with prolonged OGE hovering - easy fix though, do a quick 60 kt circuit and come back.

[[email protected]]

Engine, gearbox or hydraulic oil temperatures can be limiting factors when hovering but I like the 'vacuum or something' excuse for not going flying.

[22clipper]

I wouldn't be too hasty to dispell the vacuum notion. I know for a fact that, when I first learned to fly helos, concrete helipads had an inverse square law repulsive effect on the chopper, the closer I got to the pad the harder it was to put the machine down.

As the years have passed this localised gravitational anomaly seems to have have removed itself from the physical laws of the universe.

However we all know that choppers don't really fly, they simply beat the air into submission. So it is not beyond the bounds of possibility that enough beating could trigger a quantum mechanical implosion therby generating a localised vacuum?

[soupisgoodfood]

Perhaps he thought that hovering in the same place could create a vortex ring? It may be wrong, but at least it makes a bit of sense.

22clipper: I thought it was because they vibrated so much, that gravity no longer had an effect of them, like lightwaves or radiowaves or something.

[albyskoons]

Is Soundbarrier referring to confined area recirculation?

[oldbeefer]

22Clipper - every heard of Ground Effect?

[vpaw pilot]

Well,

Don't know about a vacuum, but the AS365N3 has a tailboom limitation on hovering at certain OATs...the exhaust heat/composite tailboom combination etc..

Oldbeefer, I think 22clipper is referring to 'concreteitis' :(

[2000hrsinVN]

Out of ground effect settling when holding position (hovering) will bring you downward in still air when you create your own verticle vortex of a downward moving localized air column. Effect goes away as you approach ground effect at the bottom of the air column. Not desirable to experience, since rate of descent is not controlable, and ability to recover requires sufficient altitude.

Not the same as holding relative horizontal ground position in a headwind.

This effect has not been personally verified -- but I scared myself once -- is supposed to be a situation whose rapidity developes porportional to the gross weight, and increases if additional collective is used to maintain height or stop rate of descent. Instead this increases the rate of descent -- a visual nearby reference is necessary to determine if holding altitude, or already descending, since IVSI would probably not be accurate.

Controls get real mushy and response to input negligible.

Solution is to decrease collective, increase forward speed and fly out of the downward moving column.

In ground effect, no wind ground-shyness is caused by increased air pressure under the rotor swept plane and greater lift produced by the denser air on the blades when they have relatively the same lift on the advancing and retreating blades. Watch the dust with a hovering aircraft in still air, you'll see the vortex pattern, and can imagine the drag slowing the air escaping from under the aircraft as it comes into contact with the ground, this drag is what increases the pressure, adding lift -- hence "ground effect".... ;)

[ShyTorque]

:confused:

After reading that, I'm begining to think that English isn't my first language....... :uhoh:

[GoodGrief]

@2000hrs

What a load of bull...

[212man]

In turbine engine aircraft their may be a limitation based on engine limits if you are not MGB torque limited.

So, apart from oil temperature, engine limits, hydraulic temperature and tail boom temperature, there are no time limits on hovering!:ok:

[ZH844]

Young SoundBarrier has obviously never met a pinger has he! I have sat for many of an hour in the dip with the 'body' in the water! The main limitation is fuel and trying not to fall asleep in the right hand seat!:ok:

[Bertie Thruster]

There is a vertical cliff face in Snowdonia where with the correct wind speed (20kt+) and wind direction (up the valley) it is not possible to hover (at least in a Wessex or Seaking) for more than about a minute.

As you stop along side the cliff face in the vertical updraughting air you can hover on around half power for a few seconds before the lever has to be gently raised to maintain position. If you hold position you will start to run out of power and must fly out. Happens every time.

[SASless]

Nigel,

You left out the most important factor for ageing pilots....bladder limits. Chug down a couple of cuppas of Earl Gray and that time limit shrinks quickly.

That is why we older guys prefer shuttling to long transits.

[CyclicRick]

If you sling a nice big slab of concrete underneath....hey presto HIGE..I knew I should have become a rocket scientist;)

[HELOFAN]

Helicopters fly because they vibrate so badly the earth rejects them.

Quoted from a Phantom Jet pilot.

[cpt]

I remember having read on a Mi 25 thread, in "pprune" that there was a time limitation on hover ( 6 mins....?) but I cannot imagine what could be the reason (overheating mechanical component maybe ?....)
But maybe is my memory fainting ? :hmm:

[soupisgoodfood]

Given that the Mi-25 can barely hover to begin with, I'd say it's probably something like the gearbox. From wikipedia: "the Hind is certified to hover for only about 200 hours during its entire lifetime".

[OWAP]

AS365N3

Hovering in warm weather

Limited to 15 min with OAT above ISA + 25c
Limited to 10 min with OAT above ISA + 30c

:8

[Flingwing207]

I s'pose you could get a vacuum of sorts in the fuel tank after awhile...

[McGowan]

There was a limitation on the time in the hover for the Bell 407. Can't remember the whole thing but it was due to the down draft forcing warm/hot exhaust gases into the engine/transmission oil cooler, you had to limit the time in the hover with the wind from certain directions. The limitation was only in effect for a short time, until they bought out the scoop things added onto the engine cowls. This sheilded the oil cooler intake area, problem solved.

[Whirlybird]

Going back to the original statement, it sounds like someone has misunderstood an explanation of Vortex Ring. You might tell a non-pilot that hovering at height for any length of time is difficult, because if you start to descend, the helicopter will get into the turbulent air underneath it, and could fall out of the sky. A simple, short explanation for the non-expert, isn't it?

But, not understanding too well in the first place, your listener firstly forgets the "at height" bit. He then remembers vaguely about air under the helicopter not doing what it should because of the rotors doing something to it....and that becomes "creates a vacuum or something".

And so a rumour is borne........

[bellfest]

AS365N3

Hovering in warm weather

Limited to 15 min with OAT above ISA + 25c
Limited to 10 min with OAT above ISA + 30c

That's quite interesting how they have done that. Am I missing something here?

10'000' ISA is -5 +25=+20 degrees.

12'500'ISA is -10 +30=+20 degrees.

Both have an OAT of +20 yet one is 15min and the other is 10min:confused:

I am quite prepared for someone to point out something obvious I have overlooked:*

[800]

maybe check to see if you can actually hover at all at those pressure heights

[bellfest]

2000' ISA is 11c+25 = 36 degrees
4500' ISA is 6c + 30 = 36 degrees:ok:

[2000hrsinVN]

Vortex ring effect in helicopters
The curved arrows indicate airflow circulation about the rotor disc. The helicopter shown is the RAH-66 Comanche. (Graphic)
In typical flight, the rotor disc directs the airflow downwards, creating lift. A vortex ring state though involves a toroid-shaped path of airflow circumscribing the blade disc, as the airflow moves down through the disc, then outward, and then down through the top again. This circulation can negate much of the lifting force and cause a catastrophic loss of altitude.
A helicopter typically induces a vortex ring state by descending into its own downwash. This requires low airspeed and a moderate rate of descent with power applied, and can lead to an undesirable phase of flight known as settling with power. This condition can be corrected by lowering the collective, which controls the pitch angle of the rotor blade, slightly pitching nose down, and establishing forward flight. The aircraft will fly into "clean air", and will be able to regain lift.
And:
Settling with power
From Wikipedia, the free encyclopedia
Settling with power is a hazardous helicopter flight condition in which the aircraft's available power is not sufficient to overcome gravity and inertia, and may not have enough power to stop the descent. Settling with power usually occurs under conditions of high all-up-weight and high altitude.
The effects and of settling with power are similar to vortex ring state. A standard procedure exists to correct both conditions.
Moral of the story:
Don't let airspeed get to 0 at altitude/high DA, esp. when heavy....
The physics which describe ground effect are still very much under debate. A common belief is that ground effect is caused by a "cushion" of compressed air between the wing and the ground. However, wind tunnel testing and experiments have indicated that while a "cushion" effect is present, ground effect is almost solely due to the ground interrupting the formation of wingtip vortices.
:p

[vpaw pilot]

That's quite interesting how they have done that. Am I missing something here?
10'000' ISA is -5 +25=+20 degrees.
12'500'ISA is -10 +30=+20 degrees.
Both have an OAT of +20 yet one is 15min and the other is 10min:confused:
I am quite prepared for someone to point out something obvious I have overlooked:*


Come on Dave...since you brought it up! :ooh:

[John Eacott]

Young SoundBarrier has obviously never met a pinger has he! I have sat for many of an hour in the dip with the 'body' in the water! The main limitation is fuel and trying not to fall asleep in the right hand seat!:ok:

Because the left hand seat is already asleep, the U/C is reading a book, and the looker has got a fishing line out of the door ;)

[SoundBarrier]

Because the left hand seat is already asleep, the U/C is reading a book, and the looker has got a fishing line out of the door ;)

If you catch a fish can you whip it out of the water into the blades so as to have it scaled and filleted ready for cooking? :}

[ConwayB]

It sounds like he was trying to explain Vortex Ring State where one can imagine it being like a vaccuum (as an explanation for the novice) and sucking you out of the sky.

As we all know, it requires (amongst other things) some downwards motion of the aircraft so technically it's not in the hovering state... but I would suggest that's the state he was trying to explain.

As far as hovering limitations are concerned, TGT or TQ time limited operations spring to mind first.

Hope this helps.

[John Eacott]

If you catch a fish can you whip it out of the water into the blades so as to have it scaled and filleted ready for cooking? :}

Dunno, but I do know an observer who didn't have a great deal of success trying to hook a Manta Ray with the rescue hoist :eek:

[Bertie Thruster]

Or you can hover (for as long as you want) with the winchman, on the end of 300ft of rescue hoist, shooting rabbits with a shotgun.

That hovering experiment earned a visit from the local police!