Retreating Blade Stall before VNE - loss of control

[Randy Daytona]

Great article on an Australian BK117 hitting retreating blade stall before VNE due to a variety of factors. American helicopters will pitch up and roll left, most European & Russian helos will pitch up and roll right - and the Boeing -uh, I will leave that one for resident test pilot Ike. Remember to get the collective down!

Retreating blade stall started at 5,000. Aircraft pitched 70 deg nose up and then rolled 120 deg over to the left before the pilot recovered at 1,000 ft (800 AGL).

http://aerossurance.com/helicopters/retreating-blade-stall/

On reaching the cruise altitude of 5,000 ft, light to moderate turbulence and a tailwind of about 15 kts were noted, while at a cruise speed of 115 kts (5 kts below the never exceed speed). Shortly after, the helicopter suffered a violent, uncommand, nose-up pitch and rolled left before descending.

 

[IKE]

...-uh, I will leave that one for resident test pilot Ike.

Only because you asked. (I don't want anyone thinking I earned my callsign by providing unsolicited information.)

If the rotor turns CCW when viewed from above (American, German, Airbus/EC of German descent), severe blade stall = pitch-up, roll-left.
CW (French, Russian and derivatives: China, India, Poland) = pitch-up, roll-right.

If you really want to know why, further nerdery is a dual-concurrence item.

 

[Randy Daytona]

Only because you asked. (I don't want anyone thinking I earned my callsign by providing unsolicited information.)

If the rotor turns CCW when viewed from above (American, German, Airbus/EC of German descent), severe blade stall = pitch-up, roll-left.
CW (French, Russian and derivatives: China, India, Poland) = pitch-up, roll-right.

If you really want to know why, further nerdery is a dual-concurrence item.

The single rotor stuff is pretty clear. My question was if the Boeing forward rotor stalls first will it pitch up and roll left or if the aft rotor stalls first will it pitch down (rear end pitching up) and roll right - or do they end up cancelling out?

As for dual concurrence, that requires 3 people - concurrence by itself indicates 2 people (one of our check airman's favorite sayings...)

 

[DocT]

Purely anecdotal but I've stalled the aft head of a phrog and it manifested as a good shudder and about a rapid 20 degree nose up attitude. Very uncomfortable but we recovered in a few seconds by dropping collective and rolling out of the turn.

As for which way it will roll I'm not sure. We were in a 45 AOB turn to the right when it stalled. It didn't exacerbate the roll so I would guess that it's may be close to neutral.

 

[IKE]

Oh, I completely forgot about the H-46.

It looks, from the picture on Wikipedia, that the front rotor turns CCW and the rear turns CW. So, the stall would occur on the left side of the front rotor and the right side of the rear rotor. Both rotors would induce a nose-up pitch rate. The pitch rate would then cause the front rotor to want to roll left and the rear rotor to roll right, and you'd have some serious torsion on the fuselage.

Maybe this is what led to the rumored mishap where the pilots survived an autorotation in just the front half of a 46.

 

[IKE]

On second thought:

The adverse roll in single main rotor birds is because the c.g. is usually very near the main mast and a pitch-up moment leads to the front blade climbing (less lift) and the aft blade diving (more lift).

Where's the c.g. in an H-46? If it's near the middle of the bird, I'd expect a pitch-up moment to reduce lift on the front rotor (all blades) and increase lift on the rear rotor, so it's not that the adverse roll cancels, but that it isn't generated (or barely). It would also self-correct the pitch rate. Was the 46 really stable in pitch?

 

[BleedGreen]

Oh, I completely forgot about the H-46.

It looks, from the picture on Wikipedia, that the front rotor turns CCW and the rear turns CW. So, the stall would occur on the left side of the front rotor and the right side of the rear rotor. Both rotors would induce a nose-up pitch rate. The pitch rate would then cause the front rotor to want to roll left and the rear rotor to roll right, and you'd have some serious torsion on the fuselage.

Maybe this is what led to the rumored mishap where the pilots survived an autorotation in just the front half of a 46.

Do you have more info on this story??

 

[DocT]

There is a story where the aircraft separated at the 410 section (where the ramp meets the fuselage) in flight and the pilots survived. I forget which book it I read about it in, maybe Bonnie Sue. The Phrog had a history of breaking apart at the 410 in the 60's.

 

[ChuckMK23]

Do you have more info on this story??

The surviving pilot was leaving HT's when I showed up and gave a blow by blow account to the wardroom. I remember it was HC-5 or 11 and late 80's timeframe.

If memory serves, sync shaft failure and the forward section of the aircraft separated. There was sufficient hydraulic pressure to somehow cushion impact to the point that he survived (only survivor).


This was not a station 410 type incident that the Marines had with CH-46A's.

 

[ChuckMK23]

@Randy Daytona says a lot of the ruggedness and shear strength of the BK-117 rotor system, and transmission/airframe interface. There is a lot of metal in the BK - full monocoque tailboom, solid aluminum 7075 alloy spars - and support beams - big hunks of metal. Oh and a rotor head milled from a solid block of titanium.

It was a f*cking beast and I loved flying it. The only damage I ever saw to a BK was the paper machete vertical stabilizers getting wacked by a flapping blade sitting on the heliport.

 

[rotorhead1871]

very interesting stuff, i suppose you could hit blade stall before vne if you were fast and really went into a high g turn....but why ??? you got a death wish?

 

[Gatordev]

very interesting stuff, i suppose you could hit blade stall before vne if you were fast and really went into a high g turn....but why ??? you got a death wish?

I've hit the beginnings of blade stall while straight and level on an airway at normal cruise. It's not an abnormal condition to encounter. We even have charts and stuff to let us know about it.

 

[Jim123]

Purely anecdotal but I've stalled the aft head of a phrog and it manifested as a good shudder and about a rapid 20 degree nose up attitude. Very uncomfortable but we recovered in a few seconds by dropping collective and rolling out of the turn.

Did pretty much that in the mighty TH-57 danger ranger. The other IP had just come from flying Hueys, and, well, the two of us quickly agreed to not try to maneuver like that again.

The nose actually did two quick pitch ups, about that same amount, in the time it took him/us to lower the collective. With regard to, "very uncomfortable," I might instead use the phrase, "scared the shit out of both of us."

 

[insanebikerboy]

very interesting stuff, i suppose you could hit blade stall before vne if you were fast and really went into a high g turn....but why ??? you got a death wish?

I've also seen it happen to different aircraft flying together in the same formation, some felt it, some didn't. Lots of factors exacerbate it.

 

[rotorhead1871]

Did pretty much that in the mighty TH-57 danger ranger. The other IP had just come from flying Hueys, and, well, the two of us quickly agreed to not try to maneuver like that again.

The nose actually did two quick pitch ups, about that same amount, in the time it took him/us to lower the collective. With regard to, "very uncomfortable," I might instead use the phrase, "scared the shit out of both of us."

well said!!...it would be very un-nerving!. the more blades you have the less violent the entry and departure would be...I am guessing! but with only 2 blades I'd imagine its pretty sudden in onset. but you are here to talk about it , so good recovery.