Hovering Limitation - Time
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 ?
But maybe is my memory fainting ?
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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".
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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.
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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........
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........
AS365N3
Hovering in warm weather
Limited to 15 min with OAT above ISA + 25c
Limited to 10 min with OAT above ISA + 30c
Hovering in warm weather
Limited to 15 min with OAT above ISA + 25c
Limited to 10 min with OAT above ISA + 30c
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
I am quite prepared for someone to point out something obvious I have overlooked
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Wikipedia: Vortex Ring and Settling, Ground effect....
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.
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.
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Originally Posted by bellfest
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
I am quite prepared for someone to point out something obvious I have overlooked
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
I am quite prepared for someone to point out something obvious I have overlooked
Come on Dave...since you brought it up!
Originally Posted by 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!
Thread Starter
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Originally Posted by John Eacott
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
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Vortex Ring State
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.
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.
Originally Posted by SoundBarrier
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?
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!
That hovering experiment earned a visit from the local police!
