Osprey tilt rotor question

[Quench]

Does anyone know much about the Osprey tilt rotor design. I saw a documentary on them.
Looks to me like the propellers don't clear the ground when in the down position (aeroplane mode.) see image
So I wonder what happens if there is a engine failure.

Imagine if a nacelle gets jammed in the down position.

Stuart.


http://www.navair.navy.mil/v22/img/content/schema1.gif

[flyboy2548m]

An engine failure wouldn't necessarily cause a nacelle to jam. Completely different things here.

[Dmmoore]

Does anyone know much about the Osprey tilt rotor design. I saw a documentary on them.
Looks to me like the propellers don't clear the ground when in the down position (aeroplane mode.) see image
So I wonder what happens if there is a engine failure.

Imagine if a nacelle gets jammed in the down position.

Stuart.


http://www.navair.navy.mil/v22/img/content/schema1.gif

There are three failure modes for the Osprey:
1. Engine failure. Each engine drives both propellers through an interconnecting drive shaft. A single engine failure "should" not present a problem.

2. Jammed nacelle - Vertical Position. The nacelles must rotate together. One jammed nacelle locks both nacelles in the jammed position. If both nacelles are locked in the vertical flight mode, no problem. Some tilt (about 20 degrees) is allowed and the aircraft landed in a roll on attitude without rotor damage.

3. Jammed nacelle - Horizontal Position, Double Engine Failure or Failure of the Interconnect System.
The aircraft can maintain horizontal flight on a single propeller but must be landed dead stick with both propellers in high (low rpm) pitch.

As I understand it, the Osprey does not have auto-rotational ability.

[Alessandro]

Yes, that´s the great weakness of this aircraft, no autorotation.

[Bok269]

THe way I understand it, if the Osprey has to land with the props/rotors down, the blades are designed to shred in such a way that it poses no danger. How that will work in actuality I don't know.

[Highkeas]

Yes, that´s the great weakness of this aircraft, no autorotation.

I assume it has glide capability.

[Flying Bear]

It could glide, but since its wing is not designed for takeoff and landing it is doubtful if it could glide at a speed that would make a forced landing survivable.

Its susceptibility to a fatal and non-recoverable vortex ring state is another big weakness...

[Dmmoore]

It could glide, but since its wing is not designed for takeoff and landing it is doubtful if it could glide at a speed that would make a forced landing survivable.

Its susceptibility to a fatal and non-recoverable vortex ring state is another big weakness...

It would depend on where the landing was being made. On an airport I would expect a fully recoverable aircraft. The landing would be made using engine power until the last moment when the engines are shut down prior to contact with the runway. The props are designed to disintegrate into small shards of material with very little mass. Touchdown speed would be in the 120 Knot range.

As with all rotor winged aircraft it is susceptible to vortex ring state but the limits of that condition are known. You avoid them.

[Bok269]

I assume it has glide capability.

But isn't glide only good if you have forward motion and lift on the wings? Meaning that if you are travelling vertically, gliding wouldn't be an option?

[Dmmoore]

The question was can the Osprey land if the engines would not rotate into the vertical position for a normal vertical landing.
It has a wing, it works just like a wing. when the engines are rotated forward the wing provides the lift, the engines the thrust. Just like an airplane. However, because the propellers are so large, they will be shredded upon landing.

[Flying Bear]

It would depend on where the landing was being made. On an airport I would expect a fully recoverable aircraft. The landing would be made using engine power until the last moment when the engines are shut down prior to contact with the runway. The props are designed to disintegrate into small shards of material with very little mass. Touchdown speed would be in the 120 Knot range.

As with all rotor winged aircraft it is susceptible to vortex ring state but the limits of that condition are known. You avoid them.

I was considering the dual engine failure scenario, meaning it won't reach an airport and it won't have engine power until the last moment. It will glide at high speed to the scene of the fatal crash.

Indeed! And as all rotor winged aircraft it will (and has) encounter such from time to time anyway; pilot errors happen. The difference is that other rotor winged aircraft can fly out of the state if they have enough altitude, and if it happens too low the result is sometimes only a very hard landing. The Osprey will promptly flip on its back and crash. Add the tiny rotors making it more susceptible in the first place, and it looks really nasty...

[Alessandro]

I assume it has glide capability.
Yes, but not very good. And I assume that the rotors can´t windmill either
(nor autorotate) so this will be a problem.

[flyboy2548m]

You guys trying to outparlortalk each other or what?

[Dmmoore]

I was considering the dual engine failure scenario, meaning it won't reach an airport and it won't have engine power until the last moment. It will glide at high speed to the scene of the fatal crash.

I don't think the dual engine failure scenario is a very good one in any twin engined aircraft.:smile:

Indeed! And as all rotor winged aircraft it will (and has) encounter such from time to time anyway; pilot errors happen. The difference is that other rotor winged aircraft can fly out of the state if they have enough altitude, and if it happens too low the result is sometimes only a very hard landing. The Osprey will promptly flip on its back and crash. Add the tiny rotors making it more susceptible in the first place, and it looks really nasty...

Again, you avoid the flight regime where the condition is likely to exist.
To quote from Wikipedia:
http://en.wikipedia.org/wiki/Vortex_ring

In testing of the V-22 Osprey (http://en.wikipedia.org/wiki/V-22_Osprey), the April 8 (http://en.wikipedia.org/wiki/April_8), 2000 (http://en.wikipedia.org/wiki/2000) crash which killed 19 was attributed to VRS. The specific cause was officially determined to be due to a rate of descent of over 2000 feet per minute (600 m/min) of the aircraft while at slow horizontal speeds of around 30 knots (56 km/h). In addition, there were two planes descending in tandem, a possible risk factor for VRS. The military claims that subsequent testing has shown that the Osprey, and the tiltrotor (http://en.wikipedia.org/wiki/Tiltrotor) in general, is less susceptible to VRS, that the conditions are easily recognized by and presented to the pilots, that recovery from VRS requires a more natural action by the pilot than for helicopters, and that the altitude loss is significantly less than for helicopters. They claim that with sufficient altitude (2000 feet or more), VRS recovery is relatively easy.[1] (http://www.wired.com/wired/archive/13.07/osprey.html?pg=3&topic=osprey&topic_set=) They also claim that it is easy to train new pilots in the recognition of and recovery from VRS.

[flyboy2548m]

I don't think the dual engine failure scenario is a very good one in any twin engined aircraft.:smile:



Why let such pesky little facts get in the way?

[Dmmoore]

Why let such pesky little facts get in the way?

Good point :grin:

[Flying Bear]

Flyboy, what facts you know will indeed be small and pesky.

I think you are all aware that helicopters crashing after entering vortex ring state is not a rare occurence... You try to avoid the condition; sometimes you are successful, occasionally you are not.

But what is special about the Osprey is that it has its rotors side by side. If one rotor enters VRS and the other does not, it flips on its back. That is what is particular to the Osprey in that respect.

That is not some imaginary made up scenario; it has already happened.

As you of course know.


But this part sure ranks among the biggest BS written in aviation: "They claim that with sufficient altitude (2000 feet or more), VRS recovery is relatively easy.". Duh... When does one usually enter VRS, if it happens at all? During the approach to land. You aren't going to be at 2000 ft when it happens; you will be at about 200 ft. What they are actually saying is that it is unrecoverable, for all practical purposes. As am I.

[flyboy2548m]

I think you are all aware that helicopters crashing after entering vortex ring state is not a rare occurence... You try to avoid the condition; sometimes you are successful, occasionally you are not.


Actually, I'm not aware of that. I have exactly 0 hours in rotorcraft and 0 hours in powered-lift aircraft. How many do you have of either one?

[Flying Bear]

Actually, I'm not aware of that.

Then why don't you just sit back, and leave the discussion to those who do know what they are talking about.

[Dmmoore]

Then why don't you just sit back, and leave the discussion to those who do know what they are talking about.

The condition becomes noticeable when you begin to pull collective at the point of flare. In a single rotor chopper, the tendency is to pull more collective but the rate of descent doesn't slow and now the engine is at max power, rotor rpm is critically low and you can not maneuver out of the condition.

The escape maneuver is simple, increase forward speed and fly out of it.

Twin rotor helicopters tend to have fewer events than singles. Part of the reason is the rotors tend to enter the state at slightly different times. Pilot awareness of un-commanded pitch movement can help identify the onset of the condition and provide an escape window.

The Osprey uses un-commanded roll as the signal. 2000 feet is not required to recover but it's nice to have. It isn't as though the aircraft fly's into an event and flips over. The event starts as a simple loss of some lift. You have a second or two where recovery can be (almost) instantaneous by increasing forward speed and reducing collective. If you try to save the landing by using roll control and collective, you could become a statistic.

In conclusion vortex ring state is a factor in safe operation of any helicopter. Osprey pilots need to be trained to deal with the condition but it should not be a significant factor in day to day operation. Most pilots will never experience one except in a simulator.

[Flying Bear]

I agree with what you are saying there Don. I would agree even more, if it wasn't because such an accident has already happened, killing a whole lot of people. Hopefully that was the last one for the Osprey, or at least one of very few!

[flyboy2548m]

Then why don't you just sit back, and leave the discussion to those who do know what they are talking about.

Gladly. When are they scheduled to show up?

[Spike]

Are you Charlie?

[flyboy2548m]

Are you Charlie?

Nope. BTW, who's Charlie?

[Dmmoore]

I agree with what you are saying there Don. I would agree even more, if it wasn't because such an accident has already happened, killing a whole lot of people. Hopefully that was the last one for the Osprey, or at least one of very few!

Then you must have the same fear for every helicopter in service. :-)

Follow the lint to an excellent site on VRS.
http://www.glue.umd.edu/~leishman/Aero/vring.html

[flyboy2548m]

Then you must have the same fear for every helicopter in service. :-)



He probably does, but that's just not very sexy. Having fears for the Osprey, on the other hand, makes one sound like he is one of those who "know what they're talking about".

[Flying Bear]

Then you must have the same fear for every helicopter in service. :-)

If you have a point, would you care to explain what it is?

[Quench]

Yes, that´s the great weakness of this aircraft, no autorotation.

Realy? Why is that?
I mean what if you run out of fuel? You are saying that you cant put the nacelles into the vertical position and yank the collective at the last minute helecopter style?

So if you runout of fuel your dead !?
That sucks!

[Bok269]

Realy? Why is that?
I mean what if you run out of fuel? You are saying that you cant put the nacelles into the vertical position and yank the collective at the last minute helecopter style?

So if you runout of fuel your dead !?
That sucks!

If you have a dual engine failure in airplane mode, you are (relatively fine). In rotor mode you can't autorotate, which could be a problem.

[Gabriel]

If you have a dual engine failure in airplane mode, you are (relatively fine). In rotor mode you can't autorotate, which could be a problem.
Why can't the Osprey autorotate?
Can it sustain stationary flight on one engine IGE and OGE?
If not, can it transition from stationary flight to forward flight after an engine failure, and how much altitude would be needed?

[Bok269]

Why can't the Osprey autorotate?


Due to the design of the rotors it can't autorotate. I'm not familiar with rotorcraft aerodynamics, so I can't give an exact explanation.


Can it sustain stationary flight on one engine IGE and OGE?

It can fly on one engine (and I believe is designed to have one power both, and land that way

If not, can it transition from stationary flight to forward flight after an engine failure, and how much altitude would be needed?
[/QUOTE]

You can't maintain forward flight without lift on the wings. In rotor mode, all the lift is on the rotors. There may be a way to push the nose over and get it to move forward, but I don't know how easy that would be.

[Quench]

If you have a dual engine failure in airplane mode, you are (relatively fine). In rotor mode you can't autorotate, which could be a problem.

I am trying to understand the mechanical/physical reason why it cant autorotate. If instability is the problem ISTM that it should be possible for the computer regulate the collective to ............ AH Does it have variable pitch propellers ?

Can a chinook autorotate ?

[flyboy2548m]

Why can't the Osprey autorotate?
Can it sustain stationary flight on one engine IGE and OGE?
If not, can it transition from stationary flight to forward flight after an engine failure, and how much altitude would be needed?

Do penguins have knees?

[einesellesenie]

Do penguins have knees?

"Penguins, in common with all birds, have a different arrangement of joints in the legs from those in mammals. However, they do have joints equivalent to our knees - the difference is that these joints are much closer to the hip than on mammals"

[FireLight]

IIRC, the Osprey has undertaken a single autorotation test at altitude. While it made the transition successfully, the conclusion was that it's vertical speed was such that contact with the ground would have resulted in a very high risk of fatal injuries to the crew and passengers. The relatively small diameter of the rotors reduce the Osprey's ability to maintain a slow vertical controlled descent. I don't think they have conducted a follow-up test since, and I think the requirement for autorotation for the Osprey has been dropped.

Apart from this, I understand that there is a concern that the Osprey would respond asymmetrically while transitioning into autorotation. If this occurred, there would be some tendency for the bird to roll with obvious resultant problems, and I don't believe the crew has ejection seats to leave the bird in the event of an emergency. Single rotor helicopters are more stable in autorotation with a single rotor approximately over the centre of mass.

I am curious about the autorotation capabilities of CH-46's and 53's. It would seem that they might be prone to some asymmetry as well.

[Gabriel]

Do penguins have knees?
You should ask that question in the OT forum. An Osprey thread in the Aviation Safety forum is not the appropriate place for asking about penguins and their knees. It is, however, the right place to ask about the Osprey's autorotation and single engine capabilities.

[Dmmoore]

If you have a point, would you care to explain what it is?

All helicopters are affected by Vortex Ring State. The larger the helicopter the more likely it it the event will be found however it is entirely predictable and avoidable. When flying a light helicopter in a descent at slow forward speeds if you feel a sudden vibration through the cyclic, you are entering VRS however the trained pilot response is to increase forward speed and decrease collective. At worst, you abort your approach.

[Dmmoore]

Realy? Why is that?
I mean what if you run out of fuel? You are saying that you cant put the nacelles into the vertical position and yank the collective at the last minute helecopter style?

So if you runout of fuel your dead !?
That sucks!

First you NEVER SHOULD run out of fuel!
However should you be dumb enough to run out of fuel, you leave the nacelles in the forward flight position and glide to a landing. The propellers / rotor's will be sacrificed but you and the aircraft will live to fly another day. The aircraft has many options available for emergencies.

[Dmmoore]

I am trying to understand the mechanical/physical reason why it cant autorotate. If instability is the problem ISTM that it should be possible for the computer regulate the collective to ............ AH Does it have variable pitch propellers ?

Can a chinook autorotate ?

You answered your own question. In the vertical mode the cyclic and collective operate the pitch change mechanisms in the rotor's. Therefore by definition in forward mode, the propellers are variable pitch.

In order to auto rotate, the rotors must be capable of moving to a negative pitch. I don't know that they can. The Osprey was never intended to have auto rotation capabilities.

[Dmmoore]

IIRC, the Osprey has undertaken a single autorotation test at altitude. While it made the transition successfully, the conclusion was that it's vertical speed was such that contact with the ground would have resulted in a very high risk of fatal injuries to the crew and passengers. The relatively small diameter of the rotors reduce the Osprey's ability to maintain a slow vertical controlled descent. I don't think they have conducted a follow-up test since, and I think the requirement for autorotation for the Osprey has been dropped.
I was unaware of that test, thanks.
I didn't think auto rotation was ever a requirement for the Osprey. With twin engines and forward flight capability, auto rotation seems a bit redundant.

Apart from this, I understand that there is a concern that the Osprey would respond asymmetrically while transitioning into auto rotation. If this occurred, there would be some tendency for the bird to roll with obvious resultant problems, and I don't believe the crew has ejection seats to leave the bird in the event of an emergency. Single rotor helicopters are more stable in autorotation with a single rotor approximately over the centre of mass.

I am curious about the autorotation capabilities of CH-46's and 53's. It would seem that they might be prone to some asymmetry as well.

Tandem rotor helicopters are required to have auto rotational capability although I'm not sure if the requirement is for an undamaged aircraft or a survivable event.

I know The Piasecki CH21 (Flying Banana) could and did auto rotate regularly due to it's single 450 HP engine design.

[flyboy2548m]

You should ask that question in the OT forum. An Osprey thread in the Aviation Safety forum is not the appropriate place for asking about penguins and their knees. It is, however, the right place to ask about the Osprey's autorotation and single engine capabilities.

Yeah but is a post commenting on my asking about a penguin's knees appropriate in the Aviation Safety forum?

[FireLight]

I was unaware of that test, thanks.
I didn't think auto rotation was ever a requirement for the Osprey. With twin engines and forward flight capability, auto rotation seems a bit redundant.

anytime. :)

Agreed about the auto rotation requirement. If they loaded it up with marines and gear, it will probably make the situation worse anyways. IIRC, I think there were also some questions about the glide slope of an Osprey. (noted as being worse than a 707, although I don't know whether that's a bad thing or not.)

I know The Piasecki CH21 (Flying Banana) could and did auto rotate regularly due to it's single 450 HP engine design.

Thanks in return. Good note about the FB. It would be interesting to know how the 46/53 respond in similar situations.