Tied Down Helecopter

[mark.hudson6 0]

Oops.

http://www.liveleak.com/view?i=68b_1188677597

Cheers

Mark

[mutley 4,498]

Wow that is something!

Thanks for posting.

[ddavid 149]

Not as much fun as this, though:

http://en.wikipedia.org/wiki/Flying_bedstead

I remember seeing this on the TV - yes, we did have TV in those days (steam powered, though!!)

Cheers - Dai/David :wink:

[mutley 4,498]

I remember that Dai, it's something out of Monty Python I'm sure!

Cheers

[MartinW 0]

Any clever theories as to what this was all about.

My guess is a test to destruction; don't know why, testing the effects of rotor imbalance perhaps.

Any ideas?

[mutley 4,498]

I would like to think it was as you suggested Uncle Martin, but surely they could simulate anything like this on a powerful enough computer? The aircraft manufacturers put new aircraft into the sky solely designed on computer calculated stress calculations.

I reckon it was done for a series of "You've been Framed" :wink:

[mark.hudson6 0]

Hi Martin, this reminds me of that problem with the Millennium Bridge in London. Something about amplification of waves leading to destruction :wink:

Cheers

Mark.

[allardjd 1,853]

Actually, though it goes agains the grain, I have to side with Martin on this one. It may be an old video, and maybe computer assisted finite element analysis wasn't an option. Aircraft structures are pretty complex and more difficult to model than buildings and bridges anyway, all curvy with lots of small elements vice a few big ones in more conventional structures.

 

It does look like a case of rotor imbalance, and the aft rotor, up on that pylon seemed to be the one creating all the forces.

 

This further entrenches my prejudices about helicopters. If God wanted 'em to fly he'd have given them wings. They are amazing, but I'll decline any offered rides - maybe.

 

John

[mutley 4,498]

Amen John :wink:

[MartinW 0]

Hi Martin, this reminds me of that problem with the Millennium Bridge in London. Something about amplification of waves leading to destruction :wink:

Cheers

Mark.

Yes, I remember that Mark, the footfalls of the people using the bridge generated just the right resonance frequency to create the uncomfortable oscillation.

Have you seen the film of the bridge [in the US I think, John might know] where the wind set up the same sort of effect, exactly matching the natural resonance frequency of the bridge? The result was complete collapse.

In the case of the chopper though, I think it was just the stress caused by the imbalance, nothing to do with the frequency at which structures naturally resonate.

[MartinW 0]

This further entrenches my prejudices about helicopters. If God wanted 'em to fly he'd have given them wings. They are amazing, but I'll decline any offered rides - maybe.

:wink:

They do say that helicopters are actually safer John.

If you suffer an engine failure in a single engine aircraft your looking desperately for a long flat place to land. In a chopper you just disengage the clutch so the rotor continues to turn and rely on autorotation.

Don't like the effect much when the tail rotor goes bonkers though. :sad: It's like being in your washing machine on full spin. :yes:

[allardjd 1,853]

Some facts about autorotation...

 

1) Your choice of places to land is limited to what you can spit on when the failure occurs.

 

2) Autorotations sound easy, but the timing of pulling up on the collective is critical. Too early and you spend all that kinetic energy in the rotor and still fall; too late and you can't arrest the sink rate.

 

3) The nature of the failure can preclude an autorotation. You must have ALL of the following a) a main rotor (or rotors) that is able to free-wheel, :wink: a working collective - all linkages intact and functional (functional cyclic is a big help in achieving some lateral control as well) , c) sufficient rotor speed to arrest your descent or sufficient height to get that rotor speed, d) as you say, a working tail rotor and controls, and e) the presence of mind to do it all correctly in your one and only attempt at it.

 

It's all a matter of taste and prejucice, I suppose. Both fixed wing and rotor craft are reasonably safe. More people kill themselves in both types by oversight, arrogance or outright stupidity than by failures of the machine. It's just that helicopters are so easy to pick on...

 

John

 

PS: The bridge you refer to was the Tacoma Narrows bridge in the Seattle area, sometimes called Galloping Gertie. It was sometime in the 30s. They learned a lot about building bridges from that one. Expensive lesson, though.

[MartinW 0]

We're off again! :wink:

2) Autorotations sound easy, but the timing of pulling up on the collective is critical. Too early and you spend all that kinetic energy in the rotor and still fall; too late and you can't arrest the sink rate.

The pilot initiates the flare by using aft cyclic. No collective or pedal input is normally required. The height that the pilot should start to flare at depends on many factors, including the model of helicopter, the descent rate, the airspeed, the headwind component, and how rapidly the pilot is going to move the cyclic.

The only time the pilot would raise the collective is during the entry and only with a few types. [To adjust rotor speed]

Normally on entry you would lower the collective to maintain RPM and keep the rotor blade angle of attack at a normal value for the glide.

It's even possible and frequently recommended to visualize a standard traffic pattern imposed on the landing area and aligned with the wind. The pilot should figure out which leg he is currently on, and then fly the pattern so that he arrives on final approach at an altitude and airspeed, which will allow him to land in the selected area.

To be honest, I think most helicopter accidents are a result of tail rotor issues rather than main rotor failure. Helicopter pilots train diligently to perfect autorotation. Hence the excellant safety record of the Bell without a tail rotor.

You're just a wimp John. :sad:

P.S. Did you like the way I pretended I was some kind of an expert?

Actually it was all from Wikipedia. :yes:

http://en.wikipedia.org/wiki/Autorotation

Lets compromise and go for one of these:

[allardjd 1,853]

Martin,

 

Roger on the compromise! Land slow, fly fast, look cool. Can't beat it.

 

Maybe you or someone who is a real expert can explain to me why one would use aft cyclic at the termination of an autorotation, unless you also had a lot of forward speed. It seems to me that the vertical velocity is the killer and that using collective to convert all that rotor rotation to lift would be called for.

 

Raising the nose in a fixed wing AC before touchdown (with or without power) increases the angle of attack and thus, increases lift, reducing the descent rate. I don't understand that raising the nose of a helicopter increases lift, but maybe there are some forces at work there that I don't comprehend.

 

I don't know enough about rotorcraft to pretend to be an expert, but I just don't understand why raising the nose and not using a lot of collective would work. I need a layman's explanation here.

 

By the way, collective or cyclic, I think my original point stands - you'd better time it right because you only get one try.

 

John

[MartinW 0]

By the way, collective or cyclic, I think my original point stands - you'd better time it right because you only get one try.

Helicopter or fixed wing, it makes no difference, it's not dissimilar to the flare in a fixed wing, same timing. With a fixed wing aircraft with a failed engine you only get one try also.

Maybe you or someone who is a real expert can explain to me why one would use aft cyclic at the termination of an autorotation, unless you also had a lot of forward speed. It seems to me that the vertical velocity is the killer and that using collective to convert all that rotor rotation to lift would be called for.

You're forgetting that during the entry into autorotation the objective is to establish the proper fuselage pitch attitude, forward airspeed and main rotor speed by adjusting the collective pitch control to a degree, cyclic control and pedals in a coordinated manner. You have forward airspeed John that is why there is a limit to the altitude that an autorotation can be safely executed. It is possible to autorotate in the hover but with a greater vertical speed, use of collective then would be very critical indeed and not the normal autorotation procedure. This is why the technique you assumed was used isn’t, its not safe, forward airspeed is.

I believe around 700 feet per minute for a vertical autorotation.

In the autorotation you have forward airspeed, in most types the objective is to hold around 60-70 knots in the glide. Shortly before landing all of that forward momentum is converted to lift by pulling back on the cyclic, vertical speed reduces and a safe gentle touchdown ensues. Sometimes in autorotation a certain amount of forward airspeed is still present resulting in a slide, not a problem as long as it's not excessive.

Don’t forget the purpose of the collective is to change the pitch of the blades to increase thrust and therefore lift; whereas the cyclic tilts the rotor head forward providing rearward thrust and forward momentum. By pulling the cyclic rearwards you effectively change the angle of the rotor head and direct the thrust from the free spinning rotor down increasing lift.

I’m sure I’ve got something embarrassingly wrong so best to read the wikipedia link it explains the procedure well John.

How about this then, if you're still anxious, it has a parachute. :wink: