PDA

View Full Version : O.T. Prop wash...?



A.K. Boomer
05-14-2011, 07:15 AM
I tried to find some info about why the high perf. helicopter lost lift at the recent event in Pakistan - all I came up with was prop wash due to it being so close to an 18' wall favoring one side and that's why it also rolled --- ?


If anybody knows more or can shed some details on the subject I think it would be an interesting O.T. discussion, If it is something as simple as this then im still a little perplexed as to why one of the the best pilots in the biz would get into a situation like this in the first place? don't get me wrong - I don't mean any disrespect to anyone - just trying to understand what happened.
could be one simple answer called "chaos" and that kinda fits but chaos is those guys middle name ------- ?

maybe special blade design (stealth) could make it more susceptible to this effect?

please lets keep this discussion purely technical so everyone can learn something and not turn it into another permatex threadlocker...

laddy
05-14-2011, 01:19 PM
news here said tail rotor hit the wall as they were coming in for a landing

SteveF
05-14-2011, 01:59 PM
From what I read on an aviation web site the problem was VRS.

From wikipedia ------

Vortex ring state (VRS), also known as settling with power, is a hazardous condition encountered in helicopter flight. It occurs when three things occur during flight: A high rate of descent, an airspeed lower than effective translational lift, and the helicopter is using a large portion of its available power. A helicopter's main rotor typically directs airflow downwards to create lift, but with low horizontal airspeed, it induces a vortex ring. A toroid-shaped path of airflow circumscribes the blade disc, as the airflow moves down through the disc, then outward, up, inward, and then down through the top again. This re-circulation of flow can negate much of the lifting force and cause a catastrophic loss of altitude. Specific to vortex ring state is that the helicopter, operating in its own downwash, is descending through descending air. Applying more power (increasing collective pitch) serves to further accelerate the downwash through which the main-rotor is descending, exacerbating the condition.

--------------

The air was pushed down by the blades, up the wall and sucked back down by the blades, and repeat, creating the vortex and subsequent loss of lift.

Steve

Evan
05-14-2011, 02:12 PM
The air was pushed down by the blades, up the wall and sucked back down by the blades, and repeat, creating the vortex and subsequent loss of lift.



That's what I thought the moment I read the news. The wall isn't necessary at all. It can and will happen in an open field. Once you get into that situation you can't fly out of it unless you have altitude to burn. If you have ever seen a helicopter land on a runway like an airplane that is one of the reasons they do it that way, especially in still air. Still air makes it especially likely to occur. Helicopter pilots much prefer a light breeze to fly in as it blow the vortex away while taking off and landing.

Evan
05-14-2011, 02:23 PM
On the subject of "prop wash" this is probably the best picture I have seen that actually shows it.

http://ixian.ca/pics8/proptrails1.jpg

Mad Scientist
05-14-2011, 03:04 PM
Considering how all the facts in this story appear to be continually changing on an almost daily bases I would not rule out the possibility that the crafts "mechanical failure" might have been caused by hostile ground fire.

The Artful Bodger
05-14-2011, 03:33 PM
On the subject of "prop wash" this is probably the best picture I have seen that actually shows it.

http://ixian.ca/pics8/proptrails1.jpg


Not really the same thing.

Evan
05-14-2011, 04:50 PM
The thread is titled "Prop wash".

The Artful Bodger
05-14-2011, 04:56 PM
The thread is titled "Prop wash".


Your picture shows a good example of prop tip condensation but does not show the most significant characteristics of "prop wash", your example would have been much better if the camera had been behind and above the aircraft showing the condensation 'bouncing' off the top side of the wings thereby demonstrating that the air flow was rotating.

As it stands you picture shows condensation in a nice helix and from it one could assume the air is moving in a nice horizontal stream towards the rear of the aircraft, which is not the case.

The Artful Bodger
05-14-2011, 04:57 PM
Considering how all the facts in this story appear to be continually changing on an almost daily bases I would not rule out the possibility that the crafts "mechanical failure" might have been caused by hostile ground fire.


One report even said that the helicopter 'stalled' and the pilot was unable to restart it!!:rolleyes:

Evan
05-14-2011, 05:53 PM
I hate the news media. Well, except for my daughter's paper. She knows science and also hates the news media in general for their complete and total ignorance of how things work.


Landing a helicopter straight down in a confined space is a very risky business. While I haven't flown a helicopter I fixed plenty of them and know a number of pilots. In that situation you have no way out and the chance of sinking under power is very real. It usually results in a hard landing which in itself isn't all that bad. But, the energy stored in the rotors has to go somewhere and that can get really messy, especially if the rotors contact the tail.

Thruthefence
05-14-2011, 09:15 PM
"settling with power" as noted, is probably what happened.

"prop wash" (a quart or so.....) is what you send the "new guy" to get over at the tool room.

"50 ft. of "flight line" is also commonly requested.

A.K. Boomer
05-14-2011, 09:19 PM
This VRS that Steve brings up sounds like it can be amplified close to a wall and if so then that side of the air current can go into this recirculation phenomenon even more so - possibly same if it was a disturbance due to prop wash on that side except not in a recirculation phenomenon as much as a turbulent stagnation one.

This would fit the early reports I heard that while the helicopter lost lift it pitched to one side, wonder if it pitched into the side that had the wall?

It may have actually had enough lift - just not balanced lift.


Edit; all things considered I bet these guys wanted to "drop" out of the sky as soon as possible to avoid RPG's as I think this was probably fear #1 and the VRS theory makes perfect sense, But would not a 15 degree angle of drastic decent and then a quick counter measure just before hover cure this situation? From what Iv read now it sounds like it takes a little time for VRS to get established and that's with giving it it's ideal conditions. although maybe trying to get all those choppers to come swooping down at the same rate and angle of descent would be too hard to sync.

This is that hindsight 20/20 thing - that pilot had his hands full and you could practice this to possible perfection and then have any one of the other 1,000 things go wrong...