Resonance

[Geoffersincornwall]

At a recent safety briefing on the subject of Ground Resonance I saw a video on the technicalities of soft and hard in-plane resonance. The piece about soft in-plane resonance was accompanied by a picture of a machine with a conventional hinged rotor. The piece about hard in-plane resonance was, however, accompanied by a picture of the Osprey.

I have never heard of this 'hard in-plane resonance' before and am intrigued by the association with the Osprey. Can any of the technical boffins enlighten us. Is this problem known to be a feature of the Osprey? What exactly is it?

Thanks

G.

[212man]

I would imagine tiltrotors will behave in a unique manner because a) they don't have lag dampers (or, if they do then they are very close to the hub) and b) the wing is not totally rigid and will have a natural frequency too.

My off the top of my head guess, but I await an expert!

[Harry the Hun]

Why didn't you ask the dude who did the briefing? He might know best what he meant.
Ciao

[Geoffersincornwall]

The dude who gave the briefing was doing so via a video otherwise we may have had an interesting conversation.

Has an Osprey ever had ground resonance? Can an Osprey experience resonance in flight? if so what are the likely consequences? (silly question?)

You're test pilot Harry so you can enlighten us I am sure.

Aye

G.

[Harry the Hun]

I would, if I only could!

Have never heard of soft and hard resonance before.

The only thing I can imagine is a resonance in one plane caused by an imbalance in the other. Both rotors should have the same harmonic frequency and they are mounted rather stiff and quite high over the connecting wing, when in helicopter mode. I do not know if there is any dampening of the individual nacelles for tumbling arround the rotor-mast-axis, but I can imagine, that when one rotor is in imbalance and the rotor starts some "pressesion" arround it's axis, then the other one might get excited by this, leading to some funny behaviour.
I am a simple man, as you know, so my ideas might be utter bollocks as well😜

[Geoffersincornwall]

Apparently the Chinook suffers from ground resonance so maybe there's something going on between the two rotors. The traditional thinking is that you can't get resonance in flight - hence 'ground' resonance. I'm no structural designer but surely if you have multiple sources of excitation then if you are in ground contact or not you may encounter a destructive resonance.

The conventional explanation is that if the damping provided via the landing gear is faulty (damper u/s for example) then the fuselage/rotor critical vibration frequency can develop, then, hey presto, resonance then bye bye airframe.

Is it just possible that even when the structures are tuned away from their critical frequency a multi rotor system can get itself in a situation whereby the critical frequency can be encountered in flight?

G.

[MightyGem]

Apparently the Chinook suffers from ground resonance
Oh yes. Just do a search on YouTube for Chinook Ground Resonance.
https://m.youtube.com/watch?v=-LFLV47VAbI

[GoodGrief]

The conventional explanation is that if the damping provided via the landing gear is faulty (damper u/s for example) then the fuselage/rotor critical vibration frequency can develop, then, hey presto, resonance then bye bye airframe.
That's not entirely correct.Has nothing to do with a critical vibe freq.
It's the rotor that goes out of balance in the plane of rotation due to shocks sent in via the skid/wheel.
Try a Hughes 300...On a crap running landing...

http://www.helistart.com/Figures/GroundRes.jpg

[RVDT]

The traditional thinking is that you can't get resonance in flight - hence 'ground' resonance

Should read - The traditional thinking is that you are unlikely to get resonance in flight - hence 'ground' resonance.

"Hard in plane" is the opposite of "soft in plane".

To quote pprune of 2013, Prouty R.W. Leishman et al.

Soft-in-plane refers to a rotor head system where the lead-lag natural frequency is less than or equal to the shaft rotational frequency. For a fully articulated head (soft-in-plane) the lead-lag frequency is typically 0.2 to 0.3 the shaft rotational frequency (these numbers are from "Principle of Helicopter Aerodynamics", Leishman).

Stiff-in-plane is where the lead-lag frequency is greater than the shaft rotational frequency.

A general discussion of the term "soft-in-plane" can be found in"More Helicopter Aerodynamics", R.W. Prouty.

[rjsquirrel]

The diction is the relationship of the in plane natural lag frequency to the rotor frequency. Soft in plane means that the lag frequency is below 1/rev, typically about 1/3 per rev. A stiff (hard) in plane frequency is above 1/rev. The newest rotors with limited lag motion are often tuned to be stiff.

[Geoffersincornwall]

Thanks guys - all very interesting but can you get resonance in flight? - maybe I should say has it ever occurred in flight? If so I can't imagine any other outcome other than the sudden destruction of the whole aircraft. I've never heard of such an event but maybe someone out there has?

G.

[The Sultan]

Geof

Helicopter type ground or air resonance is not an issue on the V-22 or 609.

Airplane type flutter is checked to speeds well beyond the never exceed velocity to verify it will never be an issue.

The Sultan

[Devil 49]

Geoffersincornwall:
I am told that Aerospatiale admitted that dynamic divergent excitation (a $100 engineer phrase?) can occur in flight in the AS350. I can't imagine what would set that in motion. I've had 2 separate events of main transmission suspension bar bearings fail (metallic thunk and nose dropped an inch) and I talked to a pilot who survived a starflex arm failure- he said it shook pretty hard but held together long enough for a successful forced landing.

[Geoffersincornwall]

I guess all these checks would have been made with a structurally sound airframe and rotor system. Can you get a destructive resonance in a tilt rotor if something bends, breaks or cracks and changes the harmonics of the beast.

A long time ago (1973??) the Navy sent a few of us QHI's on a five day helicopter design course at Bristol University. The lectures and demos were fascinating. I remember the structures guy giving us a demo of resonance. He had a wooden model of a building with (one or two - can't remember) vibrators attached to it. The vibrator was also attached to a digital readout of the vibration frequency. He showed us low and high amplitude vibrations and they had no effect but at one particular frequency the structure flew apart in a hundred pieces. This he said was the effects of resonance.

Putting this past lesson together with the safety blurb delivered on Tuesday (remember the video associated the Osprey with 'hard in-plane resonance') I am keen to understand it better. Maybe you can help.

G.

[RVDT]

I have seen 3 H269's come apart from ground resonance and have experienced the onset personally a few times.

The eye opener is what breaks when this happens!! Things that you would never imagine.

First action is to push the cyclic forward and bump the droop stops which wakes the MR dampers up as they are possibly sticking and out of phase.

I have heard of resonance in flight with an SA315B with spray gear. But possibly not "in plane".

Of course the AS350 was prone to the onset stage back in the days of DU bush wear in the Starflex prior to "carbide" stars.

[paco]

Just for the sake of clarity, here is our take on the subject, from a distillation of Prouty, et al. Comments invited! :)

In flight, the assorted rotors, engine(s) and drive shafts transmit their own vibrations to the structure. They are generally stabilised because there is no real focal point for them to attach to. Sometimes, though, a couple can occur between the lead/lag tendency of a blade against fore and aft or sideways movement of the rotor shaft, resulting in the rotor head (or the top of the mast) whirling in a circle, usually inwards, taking the centre of rotation of the disk away from the centre of gravity of its total mass. This can focus through the landing gear when it is just touching the ground, with the damping components fully extended and unable to do their job properly.

Although they are there to counteract vibration caused by movement of the centres of mass of the blades, dragging hinges can also allow those centres to get closer and unbalance the disc as a whole, where its C of G moves slightly to one side of the mast, when it should be over the centre. Thus, because of the lead/lag tendency of the blades, shocks from the landing gear can be transmitted to the rotors, where the blades across the contact point with the ground will be forced closer together. In other words, the blades hunt back and forth and take up abnormal positions, so that the C of G of the rotor disk moves off-centre, to cause an imbalance that starts an oscillation outwards from the centre which can be aggravated when the wheels or skids touch the ground. They are not properly damped as they lead/lag, so the problem is blade motion that resonates with the fuselage rocking motion.

Peculiar to helicopters with dragging hinges*, Ground Resonance is indicated by an uncontrollable lateral oscillation (roll inertia is lower than for pitch) increasing rapidly in sympathy with rotor RPM.

*This is why helicopters with teetering heads don’t have dragging hinges, because they are most susceptible to ground resonance. It is because articulated rotors have dragging hinges that they become a problem.

[[email protected]]

A colleague had an alarming experience of ground resonance in a Wessex many years ago caused by a drag damper lock being left in with the warning flag neatly wrapped around it and tucked away - not so easy to spot on an early morning walkround.

I had a similar event, again on a Wessex, on a check ride in Cyprus - the planned aircraft went u/s so the spare was wheeled out; I started the rotor and the slight padding very quickly increased in severity so that by the time I pulled back the SSL, each mainwheel wheel was lifting clear of the ground alternately and the start crew was heading at high speed away from the aircraft.

Fortunately rapid application of the rotor brake resolved the situation and we went to the bar for a beer instead of going flying. The cause was simply uneven tyre pressures!

Geoffers - I did a similar short university course at Southampton where the lecturer explained how the undercarriage has to be 'de-tuned' to avoid the natural resonant frequencies of the rotor system.

A chap called Lofty Marshall used to teach P of F at Shawbury and had an excellent demonstration of resonance - he used a long wooden ruler, held one end on a desk and prodded the free end at a particular frequency with a finger - when he got it just right (both the length of the ruler able to vibrate and the frequency of tapping) the ruler would snap. He was unpopular with the stationery dept as he kept going back to ask for new rulers:)

It would be interesting to know if they have to match the 2 rotors/props on a V22 so they don't aggravate each other or the airframe/undercarriage.

[Thracian]

I have seen 3 H269's come apart from ground resonance and have experienced the onset personally a few times.

Never seen it happening to it's end, but experienced it a few times also in a H269 during my initial training. Sometimes with FI, sometimes solo. Everytime, my landing technique has been "with some chances to improve". Feels very shaky, but all the time I managed to get away by just pulling her slightly up again in the air and improving my next landing ;-)

Thracian

[[email protected]]

That's because you didn't offer a prayer or sacrifice to the 'Concrete Monster' who lurks beneath the landing surface waiting for unsuspecting learners to try and land their helicopters;)

If they just made the skids/wheels asymmetrical so that the fuselage is already in the hover attitude when on the ground - all take-offs and landings would be much easier:)

[Gray 14]

Geoff,

The All 2 was prone to ground resonance. If the surface was not very level, it was difficult to sync the rotor/eng needles during start-up without resonance setting in. Sometimes simply aborting and re-engaging the clutch with the cyclic set in a different position would solve the problem.

Sometimes it was necessary to place a small bit of wood under a forward crosstube mount to give more stability.

This was also a problem when landing so an immediate liftoff and reposition a foot or so till a more balanced skid load was found.

Mind, these events were on remote jungle log pads. At camp we put boards on the pad for skid placement to prevent this from happening.

All in a days work!

Cheers,