Brown trouser moment.......

[CharlieOneSix]

A Los Angeles County Fire Department Sikorsky S70 Firehawk was on a training flight when........


Very, very lucky guys. They landed safely shortly after the video finishes. That blade slap noise reminds me of a Wessex when you lose a blade pocket!!

[Boac]

What had happened?

[CharlieOneSix]

It was a training exercise and one of the main rotor blades clouted the rock in the foreground. The slapping noise would be due to the damaged parts of the blade protruding into the airflow. I suspect, but don't know, that the blades are of a modern carbon fibre and fibreglass construction which is very forgiving of battle damage compared to those blades of around 50 years ago. Somewhere online a while back I saw a modern main rotor blade of a helicopter with battle damage - I think a 50mm round had gone right through the blade but the blade otherwise remained intact....probably making the same kind of noise as that S70 which itself is designed for the military as the Blackhawk.

[CharlieOneSix]

This photo shows the amount of damage a Blackhawk can sustain to its main rotor and still land safely.

The article that accompanied the photo did not give the cause except to say there was a loud bang and the helicopter went down in autorotation, losing 6000ft in one minute.

Lt. Col. Andrew Batten, the state aviation officer for the Guard, said "we anticipate it was a mechanical issue" that caused the malfunction.

No *****, Sherlock!

[Ex-Ascot]

That is incredible. Was the rock OK?

[Rwy in Sight]

C16, does it change anything the damage was not on the leading edge of the wing - if I am correct on how I interpret the picture?

[CharlieOneSix]

C16, does it change anything the damage was not on the leading edge of the wing - if I am correct on how I interpret the picture?

Yes, the main strength of the blade is towards the leading edge where the spar is on a metal blade, or where the strength of the blade is on a composite blade. The blade area behind the leading edge is either a metal covering with a metal honeycomb filler or on later blades a composite covering, again with a honeycomb filler. This website explains it better than I can.......

From wood to composite materials - the evolution of the rotor blade

My first civvie type was a Bell 47D-1 G-ASJW with wooden main rotor blades - a shortlived experience thankfully. If we left the helicopter outside overnight and it rained then, with the blades tied down fore and aft to the tail, the water ran down towards the root of the higher blade and towards the tip of the lower blade and the wood absorbed the moisture. It made for an interesting first start of the day as the blades were out of balance but, after a while ground running, the water was thrown outboard and you could go flying. Photo below of Bell47D-1 G-ASJW.

LA00072-1.jpg (190.6 KiB) Viewed 787 times

[Rwy in Sight]

It the blades were not tied the problem was less pronounced? And another question: is there any reason not to tie the blades?

[CharlieOneSix]

Yes is the answer to your first question above. It was absoutely required to tie the blades down when the helicopter was unattended. On the Bell 47 - and any other two bladed type with a teetering head main rotor - if you didn't tie them down and the wind was gusty then the blades would flap up and down. This could then cause "mast bumping", that is the head assembly could hit the main rotor mast. That can also happen in flight in a teetering head helicopter if you put in cyclic inputs having allowed a negative or low g situation to arise. There have been many accidents like that where, with negative or low g, the teetering head has struck the mast in flight and the mast fractured allowing the main rotor blades to depart from the helicopter - Bell 47, Bell 206 and Robinson R22 and R44 helicopters often feature in accident reports when this occurs.

EDIT: Here is an accident report on the R22,G-CHZN, fatal accident in 2012 which was due to mast bumping: AAIB Bulletin 2/2013

[FD2]

C16 - that Blackhawk blade gives me the shivers. It's a wonder it stayed in one piece long enough to get down.

I'll try and get the Facebook photo of the Firehawk past my adblocker.

[TheGreenGoblin]

A rock! It looked, and sounded, like the rotor blade was attacked by a particularly angry buzzard (or a buzz saw)! Extraordinary that the rotor didn't come totally unglued. Carbon composites are made to take a licking and keep on ticking. Amazing video. Looked like the initial contact was close to the hub and then spread out as the pilot attempted to ease up away from the rock and a very hard place indeed.

[CharlieOneSix]

Ah, confusion reigns. It was the yellow and white LA County Firehawk in the video which hit the rock. The photo of the military Blackhawk relates to a separate incident that happened at about 6000ft and was posted to show the amount of damage that modern blades can endure - the cause of the latter incident not known by me.

[FD2]

I managed to watch the Firehawk incident on my phone C16. It sounded like they were very lucky indeed.

[TheGreenGoblin]

Ah I knew I was confused. Not your fault C16 but mine. The water content in my cerebellum had been boiled away by the heat and it only took eyes to see that the aircraft weren't the same. I wonder what caused the damage to the Blackhawk which admirably served to make your point about rotors?

[CharlieOneSix]

TGG - re my post #9 about tying down the blades on an unattended helicopter with two main rotor blades and a teetering head, I cannot find a single picture online which shows a static R44 with its blades tied down to the tail. Isn't that done on Robinsons? What happens when they are parked on gusty days? I know nothing about Robinsons but surely it does have a teetering head......?

[G-CPTN]

Semi-Rigid Main Rotors.

[G-CPTN]

I believe that aircraft of the BofB Memorial Flight are unwilling to fly in heavy rain (said to be due to possible damage to the airscrews).
Do helicopters have similar concerns? A heavy hailstorm could be quite pernicious.

[TheGreenGoblin]

TGG - re my post #9 about tying down the blades on an unattended helicopter with two main rotor blades and a teetering head, I cannot find a single picture online which shows a static R44 with its blades tied down to the tail. Isn't that done on Robinsons? What happens when they are parked on gusty days? I know nothing about Robinsons but surely it does have a teetering head......?

Thurston's usually hangar their aircraft at base C16 but woe betide those who leave the rotors untied out on the pad on a windy day.

Ties are carried in the aircraft.

You are quite right about the Robinson's teetering head.

Not a Thurston aircraft.

[CharlieOneSix]

I believe that aircraft of the BofB Memorial Flight are unwilling to fly in heavy rain (said to be due to possible damage to the airscrews).
Do helicopters have similar concerns? A heavy hailstorm could be quite pernicious.

Yes, similar problems with helicopters. The leading edge of blades usually have a metal abrasion strip which in time becomes rough and pitted. I never thought to ask when I was a driver airframes but I doubt whether the abrasion strip was repairable in house - a case of back to the manufacturer for repair.

Going back to the days of the Wessex, we had clear polyurethane tape stuck to the leading edge of the main blades. In the Far East in particular, if you flew through a heavy rain storm then sometimes the tape was perforated by the rain, the airflow got in underneath and removed a length of tape. It was quite disconcerting as the rotor assembly became unbalanced and vibration set in - plus both in the cockpit and for those on the ground the change in rotor note was obvious. No doubt FD2 had that happen to him in his Wessex days as well. I never flew the Sea King so don't know if they still used the tape on those, the civvie version, the S61 (my avatar) didn't have tape, just the abrasion strip..

[TheGreenGoblin]

Yes is the answer to your first question above. It was absoutely required to tie the blades down when the helicopter was unattended. On the Bell 47 - and any other two bladed type with a teetering head main rotor - if you didn't tie them down and the wind was gusty then the blades would flap up and down. This could then cause "mast bumping", that is the head assembly could hit the main rotor mast. That can also happen in flight in a teetering head helicopter if you put in cyclic inputs having allowed a negative or low g situation to arise. There have been many accidents like that where, with negative or low g, the teetering head has struck the mast in flight and the mast fractured allowing the main rotor blades to depart from the helicopter - Bell 47, Bell 206 and Robinson R22 and R44 helicopters often feature in accident reports when this occurs.

There is a good article here covering mast bumping and the types that C16 mentions above.

https://www.verticalmag.com/features/un ... d-reasons/

The R22 rotor has very little rotor inertia and it is possible to get into a low RRPM situation very quickly, the imperative being get the collective down quickly lest the RRPM drops below 80%, with the type and this situation may also induce mast bumping. Talk about a double whammy.

The R44 is much kinder in this regard.