Ground Resonance.

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Cacophonix
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Ground Resonance.

#1 Post by Cacophonix » Sun Oct 21, 2018 2:51 am

Been looking at the phenomenon of ground resonance and came across this flying disaster area.

PA-97-header.jpg
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The Helistat concept was to augment the helicopters' dynamic lift with the static lift of an air buoyancy envelope. This would give greater maximum lift capability for heavy-lift work. At low weights (i.e. traveling to site without a payload) it would also free up the helicopters' rotor thrust for forward thrust, requiring less dynamic lift and lower fuel burn.

To maintain coincidence of the dynamic and static lifts (otherwise the envelope would pitch as helicopter power increased), it is impractical to use a single helicopter rotor, so multiple rotors are arranged around the center of buoyancy of the envelope.

Differential changes to the collective pitch (i.e. thrust) of the rotors gives powerful control forces. Propulsion and retardation are obtained from the cyclic tilt of the rotors, as for a normal helicopter. Yaw moments are produced by the differential cyclic tilting of the rotors (i.e. one side forward, the other back). In forward flight, the ruddervators at the tail of the blimp also add their pitch and yaw control moments.
A gust of wind from the rear of the aircraft induced some movement across the tarmac. The undercarriage responded badly to this, the bogies shimmying uncontrollably. Vibration in the framework then coupled with a helicopter phenomenon known as ground resonance. The vibration was sufficient to cause a structural failure as the starboard rear helicopter broke off its mounting, its rotors cutting into the gasbag.The unbalanced lift then made the vibrations worse and the remaining three helicopters broke free.
https://en.m.wikipedia.org/wiki/Piasecki_PA-97



What were they thinking in this Heath Robinson or rather Piasecki design?

THE HELISTAT, A HYBRID A/C WITH 4 H-34 MAIN FUSELAGES ATTACHED TO A FRAME ALONG WITH A ZPG-2 HELIUM FILLED ENVELOPE HAD JUST COMPLETED IT FIRST HOVER TEST FLT SUCCESSFULLY AND LANDED. A PWR LOSS WAS NOTED ON THE NO. 3 HELICOPTER AND THE TEST WAS TERMINATED AND THE MOORING MAST CALLED FOR. PRIOR TO RE-MOORING A WIND SHIFT CAUSED AN UNCOMMANDED LEFT TURN WHICH THE PILOT COULD NOT CONTROL WITH THE FLT CONTROLS. WITH A TAILWIND, NO WHEEL BRAKES OR GND STEERING A TAKEOFF WAS ATTEMPTED. THE 4 MAIN LANDING GEAR WHICH HAD NO SHIMMY DAMPNERS STARTED TO SHIMMY. THE FOUR HELICOPTERS STARTED TO REACT TO THE SHIMMY WITH GROUND RESONANCE. AS THE HELISTAT FINALLY LIFTED OFF, THE FOUR INDIVIDUAL HELICOPTERS BROKE OFF AND FELL TO THE GROUND. ONE PILOT RECEIVED FATAL INJURIES, 3 RECEIVED SERIOUS INJURIES AND ONE MINOR INJURIES. THE HELISTAT WAS DESTROYED. THE PRW LOSS ON THE NO. 3 HELICOPTER WAS TRACED TO A MISSING THROTTLE LINKAGE CORRELATION PIN. WHY THE PIN WAS MISSING WAS NOT DETERMINED.
From NTSB report.

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Re: Ground Resonance.

#2 Post by CharlieOneSix » Sun Oct 21, 2018 8:24 am

I'm surprised that accident can be attributed to ground resonance as the abomination was already airborne before the first helicopter fell off. Ground resonance in a helicopter ceases immediately once there is clear air between the ground and the helicopter - maybe it's different with a structure like this. This looks more like a sympathetic resonance of the structural frame induced by the four helicopters themselves.

How on earth did they persuade four presumably sane pilots to get into that thing! Removing the tail rotors can't have done anything to help the stability of the airship. It must have always been wanting to go into a turn with the torque reaction of four H-34 helicopters (piston engined predecessor of the turbine powered Wessex).

Here's a better video of the accident - you can see the machine is well clear of the ground. Look from 1:27 onwards,
.
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Re: Ground Resonance.

#3 Post by Cacophonix » Sun Oct 21, 2018 8:38 am

CharlieOneSix wrote:
Sun Oct 21, 2018 8:24 am
I'm surprised that accident can be attributed to ground resonance as the abomination was already airborne before the first helicopter fell off. Ground resonance in a helicopter ceases immediately once there is clear air between the ground and the helicopter - maybe it's different with a structure like this. This looks more like a sympathetic resonance of the structural frame induced by the four helicopters themselves.

How on earth did they persuade four presumably sane pilots to get into that thing! Removing the tail rotors can't have done anything to help the stability of the airship. It must have always been wanting to go into a turn with the torque reaction of four H-34 helicopters (piston engined predecessor of the turbine powered Wessex).

Here's a better video of the accident - you can see the machine is well clear of the ground. Look from 1:27 onwards,
.
Thanks for that video C16, which gives a much clearer view of the predictable "accident". Your questions are all totally apposite. I note the pilot(s) lift(s) off in response to the initial resonance but by that stage the whole shoddy edifice was shaking itself to pieces due to the sympathetic resonance between the 4 machines as you say. How did the pilots co-ordinate control inputs? So many questions are begged by this thing.

How could a well known, and reputable helicopter design team such as Piasecki get involved in such an "abomination" (perfect description for it) get it so badly wrong? It really does boggle the mind!

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Re: Ground Resonance.

#4 Post by Boac » Sun Oct 21, 2018 9:01 am

"Even as it is wheeled out, some people are sceptical of the design..."

Yup - count me in there.... in fact it could have been earlier :))

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Re: Ground Resonance.

#5 Post by Cacophonix » Sun Oct 21, 2018 9:20 am

The branch of physics covering ground resonance and other resonance modes in helicopters (and other structures) was an established and well understood science by the end of the 50's and by the time this helistat took off in the 1986, it should have been clear, due to the mathematical modelling (clearly never undertaken), that it would never work and should never have been built!

Some more on ground resonance here..

https://www.helis.com/howflies/groures.php

A pilot recovers from a ground resonance incident during the filming of an episode of McGyver.



And talking about Heath Robinson...

The Piasecki helistat was a death trap, just, like these abominations (one or two of which might have actually flown in the hands of a better pilot). Not really about ground resonance but mostly rollovers (dynamic and static) but interesting nonetheless.




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Rollovers - Dynamic and Static...

#6 Post by Cacophonix » Sun Oct 21, 2018 9:22 am


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Re: Ground Resonance.

#7 Post by CharlieOneSix » Sun Oct 21, 2018 10:53 am

I've had incipient ground resonance twice - both time in a Wessex and both times sorted by taking off, although it can also be sorted by shutting down quickly and applying the rotor brake.

There are times when you can't take off. A Squadron mate was doing a tie-down ground run in a Wessex. A clamp goes around the rear fuselage and is chained to rings in the tie down base. The main oleos are secured in a similar way. Something went wrong - I think a chain broke when the Wessex was at full power, resonance occurred and the helicopter destroyed itself. Even though he was wearing a helmet D suffered head injuries and he never flew again.

Same incident but lifted from an unmentionable place:
Then there was the 706NAS Wessex scheduled for a tie-down run: chained to the ground run bay, pilot relaxed and comfortable, no need for shoulder harness, right?

Wrong: when the ground resonance finished ripping the chains from the ground, and the stubs of the blades were grinding away into the concrete, with an unconscious pilot in the cockpit, the only way they finally stopped the engine was to pour firefighting foam into the intake. Then they were able to get in and rescue the pilot, who spent 6 months (IIRC) recovering from his injuries
There are similarities to this accident to a Chinook:
.

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Re: Ground Resonance.

#8 Post by ian16th » Sun Oct 21, 2018 11:43 am

Remember, as I've said before, this is coming from someone totally ignorant about helicopters, but I do find them intriguing.

Re: the PA-97 beast, Caco asked: "How on earth did they persuade four presumably sane pilots to get into that thing?"

I have to ask, were these H-34 'main fuselages', with no tail rotors, and fastened to the gas bag, technically and legally 'helicopters'?

If not did the need and indeed have licensed, and sane, helicopter pilots at the controls?

Are there different rules for 'prototypes'?
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Re: Ground Resonance.

#9 Post by CharlieOneSix » Sun Oct 21, 2018 1:09 pm

I don't know what the rules are in the US but I suppose the PA-97 was defined as a prototype. I don't see how those H-34's could be defined as helicopters as basically they were just power plants so goodness knows what qualifications the pilots required!

The rules have probably changed in the UK since 68 but that year after my RN time I was at the Oxford Air Training School doing an 8 week CPL groundschool crammer. Also on the course was the late David Lockspeiser. He was ex-RAF but after that he joined Hawkers as a production and development test pilot. He didn't need a civvie licence in that job but he was about to join the Civil Aviation Division of BAC as a communications pilot so that was why he was getting his CPL.

Presumably the same applied in the US for test flying - one wonders if any licence was required - was it an aeroplane, an airship or what!?

David Lockspeiser obituary

That quandary is much the same as what licence will be necessary to fly the AW609 tilt rotor once it has civil certification. Will it be helicopter or will it be fixed wing or will be it be a tilt rotor licence.
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Re: Ground Resonance.

#10 Post by CremeEgg » Sun Oct 21, 2018 3:38 pm

Fascinating stuff - never been an egg beater fan but I'm amazed with the footage of the Chinook; particularly that even with the rear rotor breaking up it still managed (somehow) to miss the front rotor.

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Re: Ground Resonance.

#11 Post by Cacophonix » Sun Oct 21, 2018 4:41 pm

The tilt rotor seems to be hostage to the possibility of engine failure. What happens when one of the donks goes quiet? Clearly both engines need to be able to autorotate or feather to allow typical fixed wing flight, but what happens if the failure occurs in transitioning from vertical powered flight mode to horizontal flight, or when in horizontal mode at low speed? The possibility of immediate loss of control seems so much higher with tilt rotors, but what do I know?

Versatile when it is working, potentially bad news when all is not working as one would want? The aircraft type also seems to harbour the potential for so many hitherto unexpected oscillation couples etc.

AW609-nacelle.jpg
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AW609 Final Report
Severe latero-directional oscillations during a high-speed dive caused the fatal crash of a Leonardo AW609 during flight-testing in 2015, Italian investigators have found.

The aircraft’s excessive yaw angle forced its proprotors to hit its wings multiple times, damaging the hydraulic and fuel lines, and causing an in-flight breakup and fire. The resulting crash fatally injured test pilots Herb Moran and Pietro Venanzi.

The findings are detailed in the final report on the accident from Italy’s national agency for flight safety (Agenzia Nazionale per la Sicurezza del Volo, or ANSV), in which it also points to two other causes: the AW609’s flight control system (FCS) controls laws, and a project simulator (SIMRX) that “did not foresee the event in any way.”

The ANSV also noted that the accident flight was the first in which such speeds had been reached in the new configuration of a streamlined fuselage in the tail and a reduced tail fin surface.

https://www.verticalmag.com/news/aw609- ... trol-laws/

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Re: Ground Resonance.

#12 Post by ian16th » Sun Oct 21, 2018 4:53 pm

My 'what if' about tilt rotor machines is what if, when in horizontal flight the tilt mechanism doesn't work, how do you land with them big props spinning?
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Re: Ground Resonance.

#13 Post by Cacophonix » Sun Oct 21, 2018 5:06 pm

ian16th wrote:
Sun Oct 21, 2018 4:53 pm
My 'what if' about tilt rotor machines is what if, when in horizontal flight the tilt mechanism doesn't work, how do you land with them big props spinning?

Who knows ian16th, it certainly smacks of having to put one's head into "kiss my arse goodbye mode"! I would also imagine that Vmca is very high for a tilt rotor in horizontal flight even with a feathered prop and I fail to see how a safe transition from one mode to another could be completed without loss of control even when the tilt mechanism is working.

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Re: Ground Resonance.

#14 Post by Cacophonix » Sun Oct 21, 2018 5:16 pm

Returning to Ground Resonance, here is a good, simple intro to the subject...

http://www.vibrationdata.com/tutorials2/helicop.pdf


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Re: Ground Resonance.

#15 Post by CharlieOneSix » Sun Oct 21, 2018 10:42 pm

Cacophonix wrote:
Sun Oct 21, 2018 4:41 pm
......What happens when one of the donks goes quiet?
That's all explained in the video in my last post. There is an interconnecting shaft between the two engines. Much the same as what happens in a twin engined helicopter, if one engine fails the other one will automatically compensate by increasing its output. In the case of the AW609 the video says a hover can be maintained on one engine at "slightly below MAUW". At MAUW in the hover the aircraft will sink "gently to the ground". The test pilot in the video, Dan Wells, didn't mention at what OAT or altitude these were true statements!
Cacophonix wrote:
Sun Oct 21, 2018 5:06 pm
......I would also imagine that Vmca is very high for a tilt rotor in horizontal flight even with a feathered prop .....
I cannot see how there can be a feathered prop on a tilt rotor in horizontal flight due to the interconnecting shaft maintaining power to both props.
ian16th wrote:
Sun Oct 21, 2018 4:53 pm
My 'what if' about tilt rotor machines is what if, when in horizontal flight the tilt mechanism doesn't work, how do you land with them big props spinning?
The big props are composite and designed to disintegrate on ground contact and the debris is thrown away from the fuselage. The effective of this has unfortunately been proved in more than one accident to the V22 Osprey tilt rotor.
.


Unlike a helicopter, one problem peculiar to tilt rotors moving sideways to land on an aircraft carrier was that it was possible for one rotor to be in ground effect over the deck whilst the other was in out of ground effect over the sea. I presume operating procedures overcame this one!

Autorotations with the AW609 have been demonstrated by the test pilots. However it has always been done to a powered recovery at altitude. It has never been done in an autorotation to the ground. I read somewhere - can't find it at the moment - that AW are requesting that for certification purposes the auto to the ground is done in the simulator rather than in the actual aircraft to avoid damage to a prototype. I recall that the rate of descent in autorotation is 5000ft/min. Also the ability of the props/rotors to store energy during a flare is poor. I have no idea how autorotation is achieved in the event of a double engine failure in the cruise!! At least in the military Wasp/Scout helicopter, which had not quite as bad ROD if their single engine failed, a very pronounced flare would make the touchdown reasonably comfortable.

I think we have to get away from thinking the AW609 is a helicopter. It's not - it's a new breed of machine. Why should an autorotation to the ground be physically demonstrated when effectively this is a twin turboprop fixed wing, albeit with an appalling ROD in helicopter mode if both engines fail. Was a power out landing required to be demonstrated when the King Air was certified? I very much doubt it.
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Re: Ground Resonance.

#16 Post by Slasher » Sun Oct 21, 2018 10:51 pm

Jeez Caco - first you introduced us to mast bumping...now this! Are you sure these helicoptic machines are safe?

I'm terrified now to go anywhere near the bloody things! :-s

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Re: Ground Resonance.

#17 Post by Cacophonix » Mon Oct 22, 2018 6:37 am

To be fair to the helicopter Slash we are looking at one helistat and the (very interesting) tilt-rotor concept which is, as C16 says, a totally different aircraft to the helicopter. At least the concept of a tilt-rotor has been proven militarily but they have had a gruesome accident record to date whereas the helistat under consideration was a total failure.

I think the tilt-rotor deserves its own thread given the very many interesting technical questions and discussions that one can have about it and the likelihood that this unique aircraft will emerge soon in the civil commercial world.

Helicopters, for their part, are, proven, very useful machines and their accident rates world wide, in all sectors, have improved over the years and are currently not much worse than most fixed wing aircraft in most comparable areas of operation...
"Prior to 2006, the number of worldwide civil helicopter accidents was rising at a rate of 2.5 percent per year," says the FAA. "Since 2006, the worldwide civil helicopter fleet has grown by 30 per cent but the number of accidents has decreased in key global regions by 30 to 50 per cent."

The European Helicopter Safety Team says that 311 accidents were reported in Europe between 2000 and 2005; this fell to 162 between 2006 and 2010.

Are helicopters more dangerous than planes?

On the face of it, yes. The FAA says the fatal accident rate across all aviation types is 0.84 per 100,000 flight hours, significantly less than the 1.02 reported for helicopters. But the fact that helicopters are used in risky operations, such as search and rescue missions, in war zones, and sometimes in bad weather, skews this figure.

https://www.faa.gov/news/updates/?newsId=87406

Of course, getting into a helicopter with a heli newbie is going to be more risky than doing so with a +-10,000 commercial helicopter pilot who is current on his/her machine and whose recency has been proven. You make your choices and you take your chances! ;)))

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Re: Ground Resonance.

#18 Post by ian16th » Mon Oct 22, 2018 9:33 am

Slasher wrote:
Sun Oct 21, 2018 10:51 pm
I'm terrified now to go anywhere near the bloody things! :-s
+1
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