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Helicopter Stall


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You don't really stall a helicopter - if you do, you're pretty much history.

In helicopters, lift is generated by the rotary wings and as long as you maintain blade RPM, you maintain lift and stay aloft.  If you allow the RPM to degrade below the minimum % RPM then the blade will stall (just like a wing stalls - the air separates from the surface and lift is lost).  Unlike a plane, you can't push the nose down once you stall to recover - in fact you will be falling down like a rock since the lift is gone.

In helicopters, all emphasis is on keeping that blade RPM above the minimum.  So, if the RPM starts to degrade for whatever reason (primarily engine failure) the drill is to actually pull back on the cyclic and lower the collective which then transitions the helicopter into an autorotation.  If the RPM % is retained you glide it down and cushion the "landing" with some collective at the bottom.  

Very different flight regime and actually a bit counterintuitive when you're coming from the fixed wing world where we are taught to push nose down at the first indication of stall...

 

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10 minutes ago, elimansour said:

You don't really stall a helicopter - if you do, you're pretty much history.

In helicopters, lift is generated by the rotary wings and as long as you maintain blade RPM, you maintain lift and stay aloft.  If you allow the RPM to degrade below the minimum % RPM then the blade will stall (just like a wing stalls - the air separates from the surface and lift is lost).  Unlike a plane, you can't push the nose down once you stall to recover - in fact you will be falling down like a rock since the lift is gone.

In helicopters, all emphasis is on keeping that blade RPM above the minimum.  So, if the RPM starts to degrade for whatever reason (primarily engine failure) the drill is to actually pull back on the cyclic and lower the collective which then transitions the helicopter into an autorotation.  If the RPM % is retained you glide it down and cushion the "landing" with some collective at the bottom.  

Very different flight regime and actually a bit counterintuitive when you're coming from the fixed wing world where we are taught to push nose down at the first indication of stall...

 

Thank you for your response and information.

Could at rapid ascent cause the blade RPM to degrade to the stall perhaps?

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In a piston helicopter, putting a heavy load on the engine such as by applying full up collective (esp. in a high density altitude) may exceed the available power and start to degrade RPM but it is not so sudden and you would have other indications (such as loss of tail rotor effectiveness) which would cause the pilot to reduce collective and keep the RPM in the green.  I am not sure if that applies in the Sikorsky S-76 - it is a very powerful twin turbine helicopter and they were operating a low altitudes - but I am guessing here as I do not have any specific experience flying that particular helicopter.

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Just to add a couple of points: the reason why low rotor rpm stalls are unrecoverable is that once the helicopter starts to descend, the angle of attack far exceeds the stalling angle of attack and there's no way to change that. Also, in the stalled condition, drag on the blades is very  high. There's video on the Robinson safety course that shows an accident where the rotor came to a complete stop before the helicopter hit the ground. The rpm will decay if the engine fails and the drill is to bottom the collective immediately to reduce pitch and begin an autorotation. How quickly the rpm decays after a power loss depends on the rotor inertia. I've never flown a Huey, but I've heard that they had enough inertia to roll off the throttle on the ground, pick it up to a hover, do a 360 pedal turn and set it back down. On the R-22, they figure you have 1.1 seconds after the engine quits.

There is also a less serious phenomena called a retreating blade stall. As the helicopter moves through air, one blade is advancing and another is retreating. Since lift is proportional to the square of the airspeed, the advancing blade would have more lift and cause a rolling moment. The swash plate adjusts the angle of attack of the blades as they rotate around to compensate for this. But at some limiting forward speed, you reach a stalling angle and the helicopter will want to roll.

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Here are my arm chair quarterback thoughts, based on what I’m reading (which may not be accurate).

Kobe helicopter was traveling 150 mph, low, following Highway 101 towards Camarillo due to horizontal and vertical cloud obscuration.

Near Malibu canyon freeway interchange, sudden change in visibility, pilot lost all forward and ground visibility ( flew into a cloud), and slightly veered south of 101.  This places the helicopter over mountains just south of 101 as it winds through that narrow pass.

A few things come to mind here with the possibility of zero forward and ground visibility:

1) The terrain warning was already sounding off due to the low level flight near, and below surrounding mountain tops on either side of 101, so it was ignored.... or deactivated!

2)  The terrain warning did show mountain top directly in front of helicopter path and pilot attempted to climb rapidly..... obviously not enough altitude gained.

Or the pilot saw mountain top and attempted altitude gain.....too late.

With rapid altitude gain, the helicopter blades stalled and it fell off into the mountain side.

Consider the helicopter was traveling at approximately 150 mph so things happen very fast down low, just above the ground and within a very narrow flight path due to surrounding mountains there.

One eye witness stated the helicopter sounded funny...........of course that statement could mean all sorts of things [or not!], but could it have been the sound of the blades in a stall mode?

Again, my arm chair only .

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Modern Helis Fadec system are so good. There normally is so much power available that your first limitation is usually torque on the airframe. helicopters can get into settling with power, but it’s more of an aerodynamic wake stall than a departure stall. However, the critical angle of attack is still compromised. 

Really frustrating that operators and owners are so focused on costs, that they launch into nasty weather single pilot. In an S76 With 9 lives! It would be interesting to find a way to get owners to realize the dangers of single pilot operations, with out shooting the industry in the foot. I think it’ll be up to the insurance companies to mandate requirements.  At least have another guy there to keep the thing upright. Helicopters are amazing tools, but the environment they operate in is highly critical. 

Sad day!!
-Matt

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What happens to a helicopter when you raise the nose 25-30 degrees and enter a steep turning  climb? From watching the Red Bull airshow i would think it would behave like an airplane, lose forward speed and then enter a steep turning spiral.  

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I have made (ONE!) Special VFR departure in my Mooney, to cover the ~8 miles from the field to the edge of the 900' cloud base, where they cut off quite rapidly to clear skies.

Trying to go 80nm on a windy path through mountains, utilizing flight following and Special VFR? No thanks, I can wait . . . . and have done so. Getting the IFR rating has pretty much eliminated my interest in Special VFR anyway.

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Using special Vfr in a Heli has different rules than fixed wing...basically flight at a speed to see and avoid instead of 1 mile and clear of clouds.Obviously the S76 was traveling at near cruise speed and not at a speed to see and avoid.Too bad this flight didn’t take advantage of the Anaheim departure ,Ventura transition which than puts you over lower terrain to Camarillo right next door.I suppose Heli drivers hate all the departure delays.Also why the flight didn’t go direct over water under LAX TCA?

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1 hour ago, jetdriven said:

What happens to a helicopter when you raise the nose 25-30 degrees and enter a steep turning  climb? From watching the Red Bull airshow i would think it would behave like an airplane, lose forward speed and then enter a steep turning spiral.  

A lot of what you can do in a helicopter depends on the rotor system. Chuck Aaron uses a BO-105 for his Red Bull routine which has a rigid (hingeless) rotor system. One way to conceptualize a helicopter in forward flight is to think of the rotor disk as a big wing. The rotor does the flying and fuselage follows along. How the rotor attaches to the fuselage makes a difference. The rigid system is more like an airplane. At the other extreme is the teetering system originally used by Bell. On those, the fuselage only follows the rotor at positive g and if you unload it the rotor head can "bump" the mast and break it and then ... hey, maybe there's an untapped market for ballistic parachutes here. 

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Correct...mast bumping caused by pushing cyclic to negative G has taken out Robison r22 as well as Bells.I took a friend up in my Bell 47 and he wanted to try flying it.We are in cruise flight and I hand over to him saying your aircraft.He takes the stick flys for a while Than Releases the stick to test stability!Upon doing this the bell immediately pitches over (leaving slight witness marks from mast bumping)I grab the stick back and tell him never take your hand off the cyclic of a Helicopter.

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3 hours ago, MooneyMitch said:

Here are my arm chair quarterback thoughts, based on what I’m reading (which may not be accurate).

Kobe helicopter was traveling 150 mph, low, following Highway 101 towards Camarillo due to horizontal and vertical cloud obscuration.

Near Malibu canyon freeway interchange, sudden change in visibility, pilot lost all forward and ground visibility ( flew into a cloud), and slightly veered south of 101.  This places the helicopter over mountains just south of 101 as it winds through that narrow pass.

A few things come to mind here with the possibility of zero forward and ground visibility:

1) The terrain warning was already sounding off due to the low level flight near, and below surrounding mountain tops on either side of 101, so it was ignored.... or deactivated!

2)  The terrain warning did show mountain top directly in front of helicopter path and pilot attempted to climb rapidly..... obviously not enough altitude gained.

Or the pilot saw mountain top and attempted altitude gain.....too late.

With rapid altitude gain, the helicopter blades stalled and it fell off into the mountain side.

Consider the helicopter was traveling at approximately 150 mph so things happen very fast down low, just above the ground and within a very narrow flight path due to surrounding mountains there.

One eye witness stated the helicopter sounded funny...........of course that statement could mean all sorts of things [or not!], but could it have been the sound of the blades in a stall mode?

Again, my arm chair only .

Good analysis. What kind of terrain awareness equipment did this helicopter had?. I have terrain/obstacle awareness on my gns 530W and it is very accurate. It shows towers and buildings ahead very well.

See the source image

Edited by Gagarin
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2 hours ago, PT20J said:

A lot of what you can do in a helicopter depends on the rotor system. Chuck Aaron uses a BO-105 for his Red Bull routine which has a rigid (hingeless) rotor system. One way to conceptualize a helicopter in forward flight is to think of the rotor disk as a big wing. The rotor does the flying and fuselage follows along. How the rotor attaches to the fuselage makes a difference. The rigid system is more like an airplane. At the other extreme is the teetering system originally used by Bell. On those, the fuselage only follows the rotor at positive g and if you unload it the rotor head can "bump" the mast and break it and then ... hey, maybe there's an untapped market for ballistic parachutes here. 

Skip

Right but im talking about forward airspeed and flight path. His flight ended with a sharp left climbing turn followed by a rapid descent. 

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1 hour ago, jetdriven said:

Right but im talking about forward airspeed and flight path. His flight ended with a sharp left climbing turn followed by a rapid descent. 

Most common low viz heli accident is hitting wires. But the pilot reported climbing to avoid clouds. Guess is that it entered the clouds and the pilot lost control. I read that it made an impact crater, so it must have hit hard which would be consistent with LOC. But this is pure speculation. I don’t know how good the Stability Augmentation System is in the Sikorsky. I flew an R-44 with HeliSAS a couple of times and it helped a lot, but the helicopter was still not as stable as an airplane.

 

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5 hours ago, thinwing said:

Correct...mast bumping caused by pushing cyclic to negative G has taken out Robison r22 as well as Bells.I took a friend up in my Bell 47 and he wanted to try flying it.We are in cruise flight and I hand over to him saying your aircraft.He takes the stick flys for a while Than Releases the stick to test stability!Upon doing this the bell immediately pitches over (leaving slight witness marks from mast bumping)I grab the stick back and tell him never take your hand off the cyclic of a Helicopter.

Do you still have the Bell? I never missed Whirlybirds when I was a kid. I finally got to  fly a 47D once. Pretty stable with the stabilizer bar, the the correlator could stand improvement. :)

I met Arthur Young (creator of the Bell 47) once many, many years ago when he was living in Berkeley CA. He had long ago lost interest in helicopters and was into some sort of New Age cosmology I never understood. But I did read his early book on helicopter development. He built small tethered models and  flew them by remote control to study and develop the rotor system. When he built the full size prototype, scaling effects he didn't take into account made it too stable and difficult to control. The simplest fix was to put the dampers in the stabilizer bar linkages to slightly decouple the stabilizer bar from the rotor.

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8 hours ago, PT20J said:

I've never flown a Huey, but I've heard that they had enough inertia to roll off the throttle on the ground, pick it up to a hover, do a 360 pedal turn and set it back down. On the R-22, they figure you have 1.1 seconds after the engine quits.

There's apparently a LOT of rotational inertia and energy in a Huey rotor system.    A guy that used to race with us bought a Huey that had the mast fail after fatigue due to mast bumping.   The rotor separated and beat the airframe apart in flight pretty severely.  I spent a fair amount of time trying to figure out the dynamics of how that all happened from the accident pics and analysis, but it was extremely energetic and violent.   Witnesses said it essentially exploded in the air.

It was pretty much a story of how not to do things.   Those of us who knew him were not overly surprised.

https://www.ntsb.gov/_layouts/ntsb.aviation/brief2.aspx?ev_id=20130921X71327&ntsbno=WPR13FA417&akey=1

https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20130921X71327&AKey=1&RType=Final&IType=FA

I don't know what the potential failure modes on the Sikorsky S-76 flight system, but it's clear that things can go catastrophic in a short period of time with that much energy involved.   If things gets pushed outside the envelope a bit too much due to maneuvering or a failure or whatever it's easy to see that it could go very bad very quickly.

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This isn’t VFR flight into IMC is it?

Somebody posted video of 2EX as it was circling over their house.... matching the radar bread crumbs in the video above...

The crappy weather in the video was not predictable enough to know it would stay VMC.... for cruising at 150kias...

There are pics of the crash site as well... stuff spread over a distance... 

PP recollection of newspaper details...

Best regards,

-a-

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It appears it was VFR into IMC.  

My opinion, pilot lost sight of 101 tracking near Las Virgines (Malibu Canyon exit) and 101 interchange.

At low altitude, that pass area is surrounded by mountains either side of freeway.

With 101 visual lost, helicopter appears to have ventured South of 101 and into mountainous terrain.

Such a tragedy. 

 

 

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Maybe that but there is a misconception that CFIT happens because you are losing visual references in low VFR in IMC, the real conceptual reason is when flying at low altitudes (probably because of low IMC) you can't calculate a series of headings that takes you A to B without hitting terrain, I don't think flying on instruments, TWAS, SynthVision or even on naked MK1 eye can solve that problem (finding safe low level route) if you are flying at low altitudes


Did you ever end up in dead ends trying to take a direct short-cut on car GPS? The only way google map avoids you getting stuck in dead ends when finding a shortest route (called getting stuck in local minima) is by going on random headings and 180 degrees backtracks

Stick to Minimum Sector Altitude (even in sunny days) and you will do fine flying a single heading A to B as that involve more simpler science, if you end up bellow MSA with terrain ahead you may have to go for 180 degrees backtrack or bite the bullet and climb (even by going in IMC)

I was under the impression helis are more immune from CFIT than fixed wings as they can fly as slow as they want and can do easy 180 backtracks but this competitive advantage goes away given that they tend to operate complex paths much more closer to terrain in even lower visibility, so they could be actually prone to pilot errors

There is an easy way to easily think about possible routes in mountainous areas given a fixed AMSL altitude, take an electronic moving map, color in red any terrain higher than that altitude, zoom in/out, look for possible routes, change for lower altitude do the same exercise, this will give you a good awareness on what to do regarding your route when you are forced to change your planned altitude or route

Depending on terrain complexity, you will be surprised by 1/ how the number of safe routes shrinks when you lower altitude 2/ how many heading change you would need and 3/ how dramatically a safe route changes with -100ft change of altitude

Obviously 1-3) are not something you wanna do in a noisy cockpit :)

  

 

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18 hours ago, Gagarin said:

Good analysis. What kind of terrain awareness equipment did this helicopter had?. I have terrain/obstacle awareness on my gns 530W and it is very accurate. It shows towers and buildings ahead very well.

See the source image

I just read from the NTSB report that this helicopter didn't have any form of Terrain Awareness equipment.

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