Tiltrotor in the hover
Thread Starter
Join Date: Feb 2003
Location: Cheltenham, UK
Posts: 13
Likes: 0
Received 0 Likes
on
0 Posts
Tiltrotor in the hover
Can anyone here tell me whether the tiltrotor, when in the hover, can move in the horizontal plane (specifically sideways travel) or just vertical? 
Trying to settle a bet!
Trying to settle a bet!
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132
Likes: 0
Received 0 Likes
on
0 Posts
Omni directional
The V-22 is like a CH-47 turned sideways and is capable of maneuvering just like a helicopter. In fact it is controlled just like a CH-47 minus the complex control linkages.
Join Date: Apr 2003
Location: USA
Age: 75
Posts: 3,012
Likes: 0
Received 0 Likes
on
0 Posts
Tilt rotors have all the degrees of freedom of helicopters. In roll, they use differential collective pitch for roll control (increase the collective of one rotor, decrease it on the other).
They also have the advantage of allowing the rotors to tilt with the tilt mechanism independant of the fuselage, thus permitting the body to have a flatter angle than helicopters, which force the fuselage to rotate with the rotor.
They also have the advantage of allowing the rotors to tilt with the tilt mechanism independant of the fuselage, thus permitting the body to have a flatter angle than helicopters, which force the fuselage to rotate with the rotor.
Join Date: Nov 1999
Posts: 324
Likes: 0
Received 0 Likes
on
0 Posts
An Added Downside to Differential Collective
jstr4753
Think about this scenario (depicting an added downside to differential collective):
SCENARIO
During the night approach to the remote LZ the Osprey passed through an area of moderate
turbulence - just as they started their conversion to helo mode. Unbeknownst to them at the time, that
area of light turbulence had marked a distinct (but typical) nocturnal low-level wind shear and they
had now picked up a slight tail-wind.
The left-seat pilot noted that they were tending to arc-over in the approach, but hadn't related it to
having picked up a tailwind component and so he increased the conversion rate and went right through
to the full 95 degrees of nacelle-tilt in order to increase their rate of deceleration and decrease
their sink-rate.
Shortly thereafter he noted that their descent rate was a little high - so he added power in order to
reduce it. As the edges of the pad became clearer through his NVG's, he saw that they should correct to
the South in order to avoid a taller stand of pine-trees. Rolling 25 degrees of bank to the right he
corrected their flight-path.
Just as he started the roll left to once again point at the pad he felt the right wing drop and
introduced more left stick to pick up that wing. Something seriously wrong now as the a/c picked up a
rapid roll-rate to the right - despite full left stick. The nose dropped and the Osprey hit hard, short
of the pad - very nose down and right wing low. What had happened? There had been no failure
indications?
Well I've drawn the picture, the profile's already been flown. Osprey accident investigation for such
on-approach events should prove to be a doddle. I think I recall Nick saying that increased collective
shouldn't embed a helicopter that much more deeply into VR - but that was never my experience. And I'm
not sure that tilt-rotor blades have the same aerody as helo blades. But one thing is for sure,
AsymmVR is always guaranteed to be terminal. The instinctive corrective action for picking up a wing
just exacerbates any incipient condition by increasing the pitch on that downgoing wing - courtesy of
the differential collective. The 95 degrees of nacelle-tilt was originally provided for ground
manoeuvring but soon became regarded by Osprey pilots as an aid to deceleration on approach. Unfortunately it also
meant that their rotor vortices were projected slightly ahead and more likely to equate to their
approach flight path, particularly in tailwind conditions. And that is a VR qualifier.
If it gets into Fleet Service, I wonder how long before the grunts mutiny and refuse to board?
Think about this scenario (depicting an added downside to differential collective):
SCENARIO
During the night approach to the remote LZ the Osprey passed through an area of moderate
turbulence - just as they started their conversion to helo mode. Unbeknownst to them at the time, that
area of light turbulence had marked a distinct (but typical) nocturnal low-level wind shear and they
had now picked up a slight tail-wind.
The left-seat pilot noted that they were tending to arc-over in the approach, but hadn't related it to
having picked up a tailwind component and so he increased the conversion rate and went right through
to the full 95 degrees of nacelle-tilt in order to increase their rate of deceleration and decrease
their sink-rate.
Shortly thereafter he noted that their descent rate was a little high - so he added power in order to
reduce it. As the edges of the pad became clearer through his NVG's, he saw that they should correct to
the South in order to avoid a taller stand of pine-trees. Rolling 25 degrees of bank to the right he
corrected their flight-path.
Just as he started the roll left to once again point at the pad he felt the right wing drop and
introduced more left stick to pick up that wing. Something seriously wrong now as the a/c picked up a
rapid roll-rate to the right - despite full left stick. The nose dropped and the Osprey hit hard, short
of the pad - very nose down and right wing low. What had happened? There had been no failure
indications?
Well I've drawn the picture, the profile's already been flown. Osprey accident investigation for such
on-approach events should prove to be a doddle. I think I recall Nick saying that increased collective
shouldn't embed a helicopter that much more deeply into VR - but that was never my experience. And I'm
not sure that tilt-rotor blades have the same aerody as helo blades. But one thing is for sure,
AsymmVR is always guaranteed to be terminal. The instinctive corrective action for picking up a wing
just exacerbates any incipient condition by increasing the pitch on that downgoing wing - courtesy of
the differential collective. The 95 degrees of nacelle-tilt was originally provided for ground
manoeuvring but soon became regarded by Osprey pilots as an aid to deceleration on approach. Unfortunately it also
meant that their rotor vortices were projected slightly ahead and more likely to equate to their
approach flight path, particularly in tailwind conditions. And that is a VR qualifier.
If it gets into Fleet Service, I wonder how long before the grunts mutiny and refuse to board?
Join Date: Nov 2000
Location: Canada/around
Posts: 148
Likes: 0
Received 0 Likes
on
0 Posts
UNCTUOUS, hello again.
From what I've read of the V-22 incidents there was a lot more Herc experience in the cockpit than helicopter, and the "little high" descent rate was about 2500 fpm. These aren't airplanes any more than they are helicopters and they will need to be flown appropriately. VRS is not a new concept for rotary aircraft.
From what I've read of the V-22 incidents there was a lot more Herc experience in the cockpit than helicopter, and the "little high" descent rate was about 2500 fpm. These aren't airplanes any more than they are helicopters and they will need to be flown appropriately. VRS is not a new concept for rotary aircraft.
Join Date: Jul 2002
Location: Texas
Posts: 512
Likes: 0
Received 0 Likes
on
0 Posts
2500ft/min ROD at night is begging to die. I might allow 1000, during an enroute descent, but on a final approach the max is 500, & I try for 200-300. If it gets up to 500 I start thinking about a go-around, whether it's me or the copilot flying. 2500ft/min below about 5000 AGL is just suicidal, & nobody should be surprised when it prangs in, whether it's a tilt-rotor, a helicopter, or a fixed-wing. I'm still not convinced of the accidents were caused by asymmetric VRS, it could just as easily been a rate of descent too high to stop before hitting the ground, especially the one which involved formation flight. Lead just let things get out of hand, & his wingman paid the price.
It is curious that so many pilots have so many misconceptions about the V-22 crash in Marana (Yuma), Arizona in April 2002. Let's address just two of them.
1. Many pilots believe that Nighthawk 72 intentionally initiated a RoD of 2500+ fpm, and that this is what "caused" the aircraft to enter Asymm-VRS. This is what the Marines and Bell/Boeing would like you to believe without question. Trouble is, it's nonsense all around. If you look at the descent profile taken from the FDR, you'll see that as the formation came down, Nighthawk 72 (the second ship - the one that crashed and burned) did have a momentary RoD of 3945 fpm as they passed through 820 feet. Suicide! Right? Well...maybe not. A mere nine seconds later, as the ship passed through 560 feet, the RoD was down to 150 fpm, still under full control and with the nacelles at 90 degrees. During this time, the aircraft was decellerating from 101 knots to 52 knots at 3.3 kts/sec (i.e. they were putting on the brakes hard!).
At 500 feet, they were only coming down at 800 fpm and still doing a healthy 52 knots of airspeed - in other words, well within the U.S. NATOPS parameters for "safe" rotary-wing flight.
It was at this point that the PIC of the accident a/c pulled his nacelles back to 95 degrees and everything came apart. His RoD shot to 2083 fpm at 350 feet (yet his airspeed was still 41 knots), then 2247 fpm at 339 feet (40 knots). At that point they were only six seconds away from dying.
So. Do we really believe that Major Brow intentionally initiated a 2083 fpm RoD when he was only 350 feet above the ground? Personally, I do not. I believe that this "incredible" rate of descent was the result of the aircraft entering Asymm-VRS, rather than the cause of it. This Asymm-VRS was probably excited when the ship was around 500 feet AGL with the nose up and the nacelles tilted back to 95 degrees as Major Brow desperately tried to slow down, maintain position on Lead and salvage the approach.
In the end, remember, high RoD does not cause VRS. It is only when you try to recover from that high RoD by yanking in a bunch of power at very low airspeed (we're talking below ETL here) that VRS can be excited. From the crash stats, Nighthawk 72 never got below 40 knots of airspeed at any time during the approach.
2. Then GLSNightPilot weighs in with this bit:
See what I mean? Not to pick on him but there are probably many pilots who still don't even believe that Nighthawk 72 crashed because of Asymm-VRS. However it must be noted that the Osprey did not hit the ground in a level flight attitude. On the contrary, it crashed inverted. And if so, then the aircraft departed controlled flight at some point. It points out to me how little people really know about the accident in Marana. I could write a thesis on it. And I probably will.
The thing about VRS is that it is capricious. We have general guidelines, but one cannot say with certainty that you WILL get into VRS at "this" RoD and "this" power-setting (we will assume that A/S is zero because that is a requirement). Conversely, one cannot say with certainty that you will not.
All we know are the boundaries within which VRS has been demonstrated and/or experienced in the past. (And that's the key. Past peformance is no guarantee of future results.) For a helicopter, those parameters are:
a) low A/S (below ETL);
b) greater than 300fpm Rod; and
c) power applied (i.e. not in autorotation).
Unfortunately, Mother Nature does not like it when we try to make her conform to our "rules," no matter how logically and scientifically well thought-out they are. Or we think they are. Strangely, there are many pilots who've successfully flown deep in the area of the flight envelope defined by the three areas above. No crash! How 'bout dat? Why did they not get into VRS? Like I said, it's capricious.
The two pilots in Nighthawk 72 were highly experienced aviators - they just didn't have "much" helo time. I think about this and ponder the fate of the low-time crews that will be flying the operational tiltrotors if and when they are accepted into the fleet for unrestricted use.
God help them.
1. Many pilots believe that Nighthawk 72 intentionally initiated a RoD of 2500+ fpm, and that this is what "caused" the aircraft to enter Asymm-VRS. This is what the Marines and Bell/Boeing would like you to believe without question. Trouble is, it's nonsense all around. If you look at the descent profile taken from the FDR, you'll see that as the formation came down, Nighthawk 72 (the second ship - the one that crashed and burned) did have a momentary RoD of 3945 fpm as they passed through 820 feet. Suicide! Right? Well...maybe not. A mere nine seconds later, as the ship passed through 560 feet, the RoD was down to 150 fpm, still under full control and with the nacelles at 90 degrees. During this time, the aircraft was decellerating from 101 knots to 52 knots at 3.3 kts/sec (i.e. they were putting on the brakes hard!).
At 500 feet, they were only coming down at 800 fpm and still doing a healthy 52 knots of airspeed - in other words, well within the U.S. NATOPS parameters for "safe" rotary-wing flight.
It was at this point that the PIC of the accident a/c pulled his nacelles back to 95 degrees and everything came apart. His RoD shot to 2083 fpm at 350 feet (yet his airspeed was still 41 knots), then 2247 fpm at 339 feet (40 knots). At that point they were only six seconds away from dying.
So. Do we really believe that Major Brow intentionally initiated a 2083 fpm RoD when he was only 350 feet above the ground? Personally, I do not. I believe that this "incredible" rate of descent was the result of the aircraft entering Asymm-VRS, rather than the cause of it. This Asymm-VRS was probably excited when the ship was around 500 feet AGL with the nose up and the nacelles tilted back to 95 degrees as Major Brow desperately tried to slow down, maintain position on Lead and salvage the approach.
In the end, remember, high RoD does not cause VRS. It is only when you try to recover from that high RoD by yanking in a bunch of power at very low airspeed (we're talking below ETL here) that VRS can be excited. From the crash stats, Nighthawk 72 never got below 40 knots of airspeed at any time during the approach.
2. Then GLSNightPilot weighs in with this bit:
I'm still not convinced of the accidents were caused by asymmetric VRS, it could just as easily been a rate of descent too high to stop before hitting the ground, especially the one which involved formation flight. Lead just let things get out of hand, & his wingman paid the price.
The thing about VRS is that it is capricious. We have general guidelines, but one cannot say with certainty that you WILL get into VRS at "this" RoD and "this" power-setting (we will assume that A/S is zero because that is a requirement). Conversely, one cannot say with certainty that you will not.
All we know are the boundaries within which VRS has been demonstrated and/or experienced in the past. (And that's the key. Past peformance is no guarantee of future results.) For a helicopter, those parameters are:
a) low A/S (below ETL);
b) greater than 300fpm Rod; and
c) power applied (i.e. not in autorotation).
Unfortunately, Mother Nature does not like it when we try to make her conform to our "rules," no matter how logically and scientifically well thought-out they are. Or we think they are. Strangely, there are many pilots who've successfully flown deep in the area of the flight envelope defined by the three areas above. No crash! How 'bout dat? Why did they not get into VRS? Like I said, it's capricious.
The two pilots in Nighthawk 72 were highly experienced aviators - they just didn't have "much" helo time. I think about this and ponder the fate of the low-time crews that will be flying the operational tiltrotors if and when they are accepted into the fleet for unrestricted use.
God help them.
Join Date: Jan 2001
Posts: 84
Likes: 0
Received 0 Likes
on
0 Posts
PPRUNE Fan,
You are correct in saying that high ROD does not in itself cause VRS. It is however a contributing factor along with low airspeed and power applied. With the V-22 in question not getting below 40 KIAS and a low ROD was observed it seems that it was outside this criteria but the nacelles were back at 95 deg and the aircraft was decellerating. This would cause a large component of airspeed opposing the induced flow thus providing the required real ROD through the discs. The V-22 is always going to suffer from AsymVRS as VRS is an unpredictable thing and one disc will always reach it before the other causing the wing drop which when compensated for will increase the effect whilst reducing the effect on the other disc.
You are correct in saying that high ROD does not in itself cause VRS. It is however a contributing factor along with low airspeed and power applied. With the V-22 in question not getting below 40 KIAS and a low ROD was observed it seems that it was outside this criteria but the nacelles were back at 95 deg and the aircraft was decellerating. This would cause a large component of airspeed opposing the induced flow thus providing the required real ROD through the discs. The V-22 is always going to suffer from AsymVRS as VRS is an unpredictable thing and one disc will always reach it before the other causing the wing drop which when compensated for will increase the effect whilst reducing the effect on the other disc.
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132
Likes: 0
Received 0 Likes
on
0 Posts
How fast is fast?
In a conversation with Boeing Engineers I was told that the ROD in autorotation would be between 4000-6000 FPM. Granted the accident did not occur during autorotation but it is something to be considered in the design.
Lu Zuckerman asked:
Lu, it doesn't matter. To excite VRS, the rotorcraft has to be descending pretty much vertically, and its rotor has to be generating some downward thrust which is not being dispersed by wind and/or relative speed, therefore the need for "power applied" as opposed to autorotation in which the flow through the rotor would be "upward."
You are correct that an autorotating V-22 would be falling out of the sky like a greased lorry...and that would be in an auto with some forward airspeed. The RoD in a vertical auto would likely be heart-stopping. All the more reason to believe that V-22 pilots will never be making "blottle-on-the-bottom" approaches.
Further, I would go out on a limb and state that you cannot get into VRS of any sort with forward, translated speed...any speed above ETL in other words.
As CAC Runaway points out, VRS is unpredictable. The margins needed to avoid Asymm-VRS in the tiltrotor might be so large as to effectively limit the aircraft's usefulness - or at least guarantee that every future accident will be blamed on "pilot error"..."They violated NATOPS!" Asymm-VRS will always be a "problem" plaguing tiltrotor aircraft. Just how much importance we give or don't give this niggling, trifling little...little...flight characteristic peculiarity depends I guess on how much you believe in the inherent "goodness" of the tiltrotor design. Sorry, but I'm not sold.
How fast is fast?
In a conversation with Boeing Engineers I was told that the ROD in autorotation would be between 4000-6000 FPM. Granted the accident did not occur during autorotation but it is something to be considered in the design.
In a conversation with Boeing Engineers I was told that the ROD in autorotation would be between 4000-6000 FPM. Granted the accident did not occur during autorotation but it is something to be considered in the design.
You are correct that an autorotating V-22 would be falling out of the sky like a greased lorry...and that would be in an auto with some forward airspeed. The RoD in a vertical auto would likely be heart-stopping. All the more reason to believe that V-22 pilots will never be making "blottle-on-the-bottom" approaches.
Further, I would go out on a limb and state that you cannot get into VRS of any sort with forward, translated speed...any speed above ETL in other words.
As CAC Runaway points out, VRS is unpredictable. The margins needed to avoid Asymm-VRS in the tiltrotor might be so large as to effectively limit the aircraft's usefulness - or at least guarantee that every future accident will be blamed on "pilot error"..."They violated NATOPS!" Asymm-VRS will always be a "problem" plaguing tiltrotor aircraft. Just how much importance we give or don't give this niggling, trifling little...little...flight characteristic peculiarity depends I guess on how much you believe in the inherent "goodness" of the tiltrotor design. Sorry, but I'm not sold.
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
Posts: 2,132
Likes: 0
Received 0 Likes
on
0 Posts
A question for the experts.
If a CH-47 were to fly so the rotors were disposed to the left and right as opposed to fore and aft would it enter into AVRS if it followed the flight profile flown by the V-22 when the accident occurred?
Join Date: Jan 2001
Posts: 84
Likes: 0
Received 0 Likes
on
0 Posts
Lu,
You could get Asymmetric VRS in a CH-47 in straight flight at low speed. Say for example the front rotor went into VRS first the nose would drop which the pilot would correct with differential collective using an aft cyclic movement making the VRS worse and pitching nose down even more. So yes if you turned it sideways you could get it to look like the V-22 situation but for the CH-47 it will always cause a pitching problem.
You could get Asymmetric VRS in a CH-47 in straight flight at low speed. Say for example the front rotor went into VRS first the nose would drop which the pilot would correct with differential collective using an aft cyclic movement making the VRS worse and pitching nose down even more. So yes if you turned it sideways you could get it to look like the V-22 situation but for the CH-47 it will always cause a pitching problem.
LuZ asked:
Well Lu, it depends. Maybe IF the CH-47 rotors were spaced far apart like those of the tiltrotor.
But the CH-46 and CH-47 are nothing like tiltrotors. Can't compare 'em! For one thing, their rotors overlap by...what...nearly half? Because of this, the rotors tend to act in unison - as one big rotor. For another thing, the rotors are oriented fore-and-aft. *IF* the front rotor of a CH-47 happened to get into VRS, the nose of the aircraft would pitch down and it would begin to recover even without action on the part of the pilot. Asymm-VRS is simply not a problem.
A tiltrotor is more like two helicopters flying in formation. The proprotors do not overlap; they are spaced widely apart. In low-speed hovering flight, they interact and interfere with each other.
We do not hear much about VRS in CH-47's, primarily because it is no more of an issue than for a "regular" helicopter and therefore doesn't show up in the Fatal Accidents column. The problem with Asymm-VRS in a tiltrotor is that it will always lead to a fatal accident. It's already happened once.
And I wonder when the next one will be?
If a CH-47 were to fly so the rotors were disposed to the left and right as opposed to fore and aft would it enter into AVRS if it followed the flight profile flown by the V-22 when the accident occurred?
But the CH-46 and CH-47 are nothing like tiltrotors. Can't compare 'em! For one thing, their rotors overlap by...what...nearly half? Because of this, the rotors tend to act in unison - as one big rotor. For another thing, the rotors are oriented fore-and-aft. *IF* the front rotor of a CH-47 happened to get into VRS, the nose of the aircraft would pitch down and it would begin to recover even without action on the part of the pilot. Asymm-VRS is simply not a problem.
A tiltrotor is more like two helicopters flying in formation. The proprotors do not overlap; they are spaced widely apart. In low-speed hovering flight, they interact and interfere with each other.
We do not hear much about VRS in CH-47's, primarily because it is no more of an issue than for a "regular" helicopter and therefore doesn't show up in the Fatal Accidents column. The problem with Asymm-VRS in a tiltrotor is that it will always lead to a fatal accident. It's already happened once.
And I wonder when the next one will be?
