Tiltrotors

[relyon]

FLHooker mentioned tiltrotors in the Future Helcopters thread I started, so I looked up some references I'd seen before. Some good starting points are:

• http://history.nasa.gov/monograph17.pdf
  
• http://www.ffc.arc.nasa.gov/library_docs/t...or/ctr20th.html
  
• http://www.navair.navy.mil/v22/
  
• http://www.bellhelicopter.com/en/aircraft/...ry/bellV-22.cfm
  
• http://www.bellagusta.com/air_ba_main.cfm
  

The first reference is particularly good since in addition to XV-15 program it also covers efforts as far back as the 1920s & 1930s, developments through WWII, the XV-3 program, and touches on the V-22 Osprey as well. As I mentioned in the other thread, I like tiltrotors and hope to see them suceed but feel they need be the right tool for the job, so to speak. I think they may do well as regional air carriers (particularly in less than fully accessible areas) and executive transports. Of course, their future remains to be seen.

 

Bob

[FLHooker]

As I mentioned in the other post, I can see these things getting more popularity in the future. I have seen a V-22 flying around, incredibly fast for something that can hover. However, I do see a possibility of need to be cross trained in rotary wing and fixed wing since it does have the capability of both. We'll see! Hopefully this thread brings up some interesting debate about it all.

 

 

CHAD

[FLHooker]

You are telling me, nobody, with the exception of Bob and I have an opinion on this subject?! Get to posting!!

 

 

CHAD

[Linc]

An experienced helicopter pilot I know of recently rode in one and told my commander he'd rather fly in a Chinook. I've seen them approach into here and it seems there are certainly questions about both schools of thought within that community already. The lead Osprey had the nacelles at 45-60 degrees vertical in the downwind, while the trail Osprey maintained airplane mode until he was on final.

Edited March 20, 2008 by Linc

[Goldy]

My two cents. I checked out the civilian version tiltrotor they had at HAI in Houston. Sure is a lot of aircraft for a cramped 8 passengers or so. I just dont see the advantages of the ship for the money it costs. I would rather fly 12 people in one of the newer 180 knot helicopters hitting the market.

 

Goldy

[SquirrelFlight]

As I mentioned in the other post, I can see these things getting more popularity in the future. I have seen a V-22 flying around, incredibly fast for something that can hover. However, I do see a possibility of need to be cross trained in rotary wing and fixed wing since it does have the capability of both. We'll see! Hopefully this thread brings up some interesting debate about it all.

 

 

CHAD

 

I like the concept; I'm just waiting to see if the execution is as good as the apparent potential. I'm not sure exactly what niche it is being aimed at either: corporate transport (pick up CEO atop the office, fly cross country to the next meeting)? As someone else mentioned it looks like the cabin space may be a little small for that.... not that that matters so much to me: I like seeing the tech developed for its own sake.

 

Also, I believe the tiltrotors are officially designated as powered-lift category aircraft. So, niether a FW or RW certification would be enough to fly it - you'd need to get the PL add-ons.

[FLHooker]

While i have never flown in one, I can appreciate the thoughts here. I am just curious to see what is coming up in the future. I certainly agree that there is alot of aircraft for the space provided. I am also curious as to future larger (like the 4 MR'd concept on the drawing board) what they have to do about the good possibility of an engine loss. We'll just have to see.

 

CHAD

[relyon]

An experienced helicopter pilot I know of recently rode in one and told my commander he'd rather fly in a Chinook. ...

Did the experienced helicopter pilot mention why he'd rather fly in a Chinook? Similar comments were made by airplane pilots when they first saw helicopters and early tail rotor helicopter pilots when they first saw tandem helicopters. Just wondering.

 

My two cents. I checked out the civilian version tiltrotor they had at HAI in Houston. Sure is a lot of aircraft for a cramped 8 passengers or so. I just dont see the advantages of the ship for the money it costs. I would rather fly 12 people in one of the newer 180 knot helicopters hitting the market.

I think it depends on where you're coming from and/or going to and what you're trying to accompish. That's part of the reason I think small and medium tiltrotors are the niche the basic technology fits in. Assuming vertical lift capability is necessaru, I can think of many instances where moving the same twelve people could be done in far less overall time in two lifts by tiltrotor than one by helicopter.

 

The cabin space of the BA609 could be bigger at the expense of some weight/payload by increasing the fuselage diameter. It's not like the bizjets or medium helicopters it will compete against have loads of cabin space either. A series of compromises.

 

... I am also curious as to future larger (like the 4 MR'd concept on the drawing board) what they have to do about the good possibility of an engine loss. We'll just have to see.

Quad tiltrotors are farther along than the drawing board. Do some searches for "Joint Heavy Lift", "JHL", "Optimum Speed Tilt-Rotor", or "OSTR" and you'll get plenty of hits to start from. Wind tunnel research, design studies, and a competition similar to the JVX program that led to the V-22 are being done now. Much of it looks like a way for the DoD to spend money and private firms to capitalize on that.

 

Granted, the technology is pretty neat. FWIW, I don't see heavy lift tiltrotors being useful outside of perhaps a few specific military applications. In theory they could replace the C-130, the Chinook, and maybe several others but if that is really necessary is anyone's guess. Aside from the military proper, EMS and the Coast Guard are two areas I see tiltrotors coming into play. The vertical takeoff capability combined with high forward speed could really come into play for some missions.

 

Bob

[Linc]

Did the experienced helicopter pilot mention why he'd rather fly in a Chinook? Similar comments were made by airplane pilots when they first saw helicopters and early tail rotor helicopter pilots when they first saw tandem helicopters. Just wondering.

He didn't just see it, he rode on it. I'd imagine his comment was based on his experience riding in the back of it, versus riding in the back of a Chinook.

[Sundowner]

Nick Lappos summed everything pretty well:

 

http://www.madman.dk/documents/tiltrotorcomparison.pdf

 

Well the tilts are not for the payload, nor the range, it's all about the speed, though I like USAF approach of putting them into CSAR missions, where the V-22 belong.

 

The civilian market is a bit different, there is time pressure - so the fastest thing will win, the AB-609 should be successful there.

[gr8shandini]

I'll have to say I'm a bit biased as I used to work for a tiltrotor program, but I'd have to disagree with some of the assumptions made in that comparison. If you're going to compare aircraft, you need to look at models with a similar max gross or similar payload capability, not similar empty weight. The V-22 was intended to replace the CH-46, not the -53E which has a MTOW of 73K lbs vs. the V-22's 60K. Obviously, all the gear boxes, cross-shafting, and pivot mechanisims that a tiltrotor requires are going to add to the empty weight of the aircraft. Therefore, a tiltrotor is never going to be the optimal choice for a heavy lift platform and that's why the -53 kicks its arse in that mission.

 

Additionally, the payload to range comparison is only valid if the excess payload capacity of the heavy lift bird is at least partially converted to extra fuel. This may not make a difference to the USMC, but you can bet that civilian operators will care about burning the extra gas.

 

He also uses the same misguided approach with the -609. It's not intended to compete with the UH-60, but rather a combination of a taxi and a Lear. Nothing is going to get someone from two city centers less than 500 miles apart faster than a tiltrotor.

 

Now there are drawbacks. As relyon alluded to in his thread title, the tiltrotor is much closer to a hovering airplane that a fast helo. Due to the extremely high disc loading required to keep the rotor tips below mach in airplane mode, high density altitude performance is abysmal and the extremely high downwash velocity makes SAR ops very difficult. There are also the high maintenance costs to consider.

 

The problem here is that the tiltrotor fills a very specific need. For almost all missions, a helicopter or conventional airplane will do the job better, but if you need to carry a modest payload to a confined area quickly and efficiently, then a tiltrotor comes into its own.

[Sundowner]

Well as Nick was working for Sikorsky at the time of making this document, some things have to be taken with a grain of salt Although I find most of it true - not that the V-22 is rubbish, but the statements of pro-Osprey generals are wrong.

 

I'm wandering where the Sikorsky X2 will go, as it may achieve speeds of tilts, still offering higher payload and requiring less power... but how big, and how fast it can really grow ? The blades have to be very rigid, and they simply can't be with length greater than 30ft, so nothing bigger than S-92 can't be built... at least if we think about only one set of rotors...

[gr8shandini]

It depends on which generals you're talking about. The Marines really wanted another helicopter and seem to be justifying the decision to buy a tiltrotor. The Air Force, on the other hand, has a good handle on the fact that the V-22 isn't the go-to answer for all missions and they're having a much better time finding a niche for it. It also doesn't hurt that they have maintainers that are used to digital flight controls.

 

As for the X2, I'm also interested to see how that works out. As you mentioned, though, it's a very small airplane and it'd be tough to scale it up. For my dollar, I'd probably go with a tandem rotor approach, though. The -47 is already pretty quick at about 180kts (depending on payload) and it'd be interesting to develop a hybrid turboshaft motor that could bypass the power turbine to produce forward thrust instead of adding a pusher prop and all the associated gearboxes and shafting. Slap on a bit of a fixed wing to help unload the rotor in foward flight, and you'd have a blazing fast machine that would still do the helicopter role pretty well.

[Sundowner]

http://www.sikorsky.com/sik/Attachments/MI...007_Trifold.pdf

 

Well, now we see where Sikorsky is going with the X2.... Total World Domination ! muhahaha

 

Well, at least in the S-76 - EC-155 market region, plus that Attack version could be promising, although I would like them to bring more from the RAH-66 program... So it would not look like Cheyenne-Bis.

[FLHooker]

gr8shandini,

 

While this does not have any additional thrust, like you said, but they certainly tried to pin some wings on a few helos in the past, didn't go anywhere...

 

 

CHAD

[Jack Poller]

I'll have to say I'm a bit biased as I used to work for a tiltrotor program,

 

Maybe you can help me understand why, in all the tilt rotors, it seems necessary to swivel the entire engine housing?

 

Wouldn't it be easier and cheaper, and much lighter to have inboard engines, and have driveshafts on sponsoons that tilt?

 

By only moving the rotor/driveshaft sponsoon, rather than the entire engine, you're rotating a much smaller weight and can therefore reduce size/weight of the rotator components, etc.

 

About the only downside I can see with this is getting air to the engines, but that shouldn't be too hard to solve.

 

Or am I way off base?

 

Thanks

 

Jack

[relyon]

... If you're going to compare aircraft, you need to look at models with a similar max gross or similar payload capability, not similar empty weight. ...

Why? They're not being compared on mission or competiveness relative to one another, only empty mass and total horsepower. Do a maximum gross or total payload comparison and the helicopter will still come out significantly ahead compared to the tiltrotor. In turn, all vertical flight compares pretty poorly to airplanes in almost every aspect except takeoff and landing capability.

 

... Obviously, all the gear boxes, cross-shafting, and pivot mechanisims that a tiltrotor requires are going to add to the empty weight of the aircraft. ...

There's also the additional structural weight of suspending the entire aircraft from the wing tips. In the case of the V-22, add the mass associated with the fully automatic blade and wing fold and stow mechanism as well.

 

... Therefore, a tiltrotor is never going to be the optimal choice for a heavy lift platform and that's why the -53 kicks its arse in that mission. ...

Yet tiltrotors are exactly where the military is headed with the JHL program.

 

... Nothing is going to get someone from two city centers less than 500 miles apart faster than a tiltrotor.

Now. Adaptive rotor RPM alone will go along way toward alleviating the advance ratio and compressibility problems that all helicopters encounter at higher forward velocities. Fixed wings on helicopters have drawbacks too, downwash disturbance and additional structral mass being the main two.

 

... There are also the high maintenance costs to consider.

Like maybe the 450-ish hour (no typo) average V-22 engine life the Marines are discovering.

 

Don't get me wrong, I think tiltrotors are a very interesting concept and have a definite niche to fill. I just think that niche is nowhere near the size it's been marketed as being by the military or Bell, et al.

 

... and they simply can't be with length greater than 30ft, so nothing bigger than S-92 can't be built... at least if we think about only one set of rotors...

Well, an S-92 is pretty big as it is but I'm curious: Why do you think 30ft is the limit?

 

Maybe you can help me understand why, in all the tilt rotors, it seems necessary to swivel the entire engine housing?

 

Wouldn't it be easier and cheaper, and much lighter to have inboard engines, and have driveshafts on sponsoons that tilt?

 

By only moving the rotor/driveshaft sponsoon, rather than the entire engine, you're rotating a much smaller weight and can therefore reduce size/weight of the rotator components, etc.

That's exactly the approach the Bell XV-3 took. Besides having significantly heavier driveshafts and transmissions than having tip mounted engines would, it exhibited fairly severe vibrations especially in hover mode. These were eventually solved with a different proprotor configuration but obviously the outboard engine configuration was used in the V-22. All the same, I think engine placement would be worth reexamining in further designs.

 

Bob

[Sundowner]

Well, an S-92 is pretty big as it is but I'm curious: Why do you think 30ft is the limit?

The 30ft is my estimation, not calculated number. The biggest issue with ABC systems is the rigidness of the blades, as it have to be much higher than normal blade. And the longer we go, the forces on that blade grow exponentially. 30ft blade compared to 15ft one, will have to be 4 times stronger... or even more. And there we hit the wall which are the known materials, and design limitations.

[gr8shandini]

Maybe you can help me understand why, in all the tilt rotors, it seems necessary to swivel the entire engine housing?

 

Wouldn't it be easier and cheaper, and much lighter to have inboard engines, and have driveshafts on sponsoons that tilt?

 

By only moving the rotor/driveshaft sponsoon, rather than the entire engine, you're rotating a much smaller weight and can therefore reduce size/weight of the rotator components, etc.

 

About the only downside I can see with this is getting air to the engines, but that shouldn't be too hard to solve.

 

Or am I way off base?

 

Thanks

 

Jack

 

Nope, you're not off base at all. The V22 has wingtip mounted nacelles for a couple of reasons. First, it simplifies the blade fold / wing stow system (not that it can really be called all that simple) that's required for shipboard compatability. Then there's survivability and IR signature. On a military system, it's not a good idea to have both engines (and fuel lines, and FADECs, etc) so close together so that one bullet can take both out.

 

As for the XV-15 and -609, I have no idea why they went with that approach. My only guess is maintenance access and the fact that weight at the end of the wing helps relieve some of the lifting forces and allows a slightly smaller / lighter torque box and carry through structure.

 

And relyon,

 

I thought I was pretty clear that I don't believe that the tiltrotor is the be-all and end-all when it comes to a vertical takeoff aircraft. However, I still maintain that empty weight isn't a valid comparison factor for two aircraft. An F-4 also weighs about 30K empty, but it's designed for a completely different mission than a CH-53, so you can't really compare the two.

 

Likewise the V-22 is designed as a medium lift helicopter rather than the heavy lift machine that a CH-53 is. In other words, it's like an airline using a fleet of 747s on every route since they can carry the payload of an MD-80 to a greater range faster. However, in most cases the extra capacity of the 747 is going unused.

 

Also, it's important to think critically about statistics when it comes to controversial programs like V22, F22 or B2 (or maybe just programs with a bunch of 2's?). I'm not sure where that 450 hour number came from, but I'd hazard a guess that it includes time spent implementing design changes during the LRIP (Low Rate Initial Production) phase. We broke a lot of stuff on our flight test birds and our mx requirements were closer to 100 man-hours/ flight hour. I would imagine that folks flying within the normal envelope would do even better.

 

OK, that's about it for me trying to defend the tiltrotor concept. If it wasn't clear from my first post, my view is this: There's a mission for every aircraft and an aircraft for every mission. If that wasn't so, the DoD could just order up a bunch of KCAFBH-111 "Uniflyer"s and call it a day.

[gr8shandini]

gr8shandini,

 

While this does not have any additional thrust, like you said, but they certainly tried to pin some wings on a few helos in the past, didn't go anywhere...

 

 

CHAD

 

Yup, that's exactly the one I was thinking of. Without any additional forward thrust, a complex helicopter doesn't make a lot of sense as you're basically just changing the direction of the lift vector of the rotor. You'll pick up a little speed, but it's all but negated by the weight of the structure you added.

 

Anyway, I was mainly just wondering why the tandem rotor hasn't caught on outside of Boeing. I don't think it's patented. And it solves all of the same problems that they're gunning for with a coax system with less complexity as well as allowing a huge CG envelope compared to most helos.

[Sundowner]

To my understanding, tandem, like on Piasecki birds is for large transport helicopters only, because the rotors dimensions dictate overall fuselage size. For example, making an helicopter with two Bell 407 rotors would end up in a machine size of S-92... that couldn't really carry much load for its size. The B234 is probably somewhere the optimal size for that system. There is also problem of money. Two big gearboxes, and two big rotors with full sized blades is definitely more expensive than one big gearbox with rotor + some small gearboxes and small rotor/fan.

[gr8shandini]

To my understanding, tandem, like on Piasecki birds is for large transport helicopters only, because the rotors dimensions dictate overall fuselage size. For example, making an helicopter with two Bell 407 rotors would end up in a machine size of S-92... that couldn't really carry much load for its size. The B234 is probably somewhere the optimal size for that system. There is also problem of money. Two big gearboxes, and two big rotors with full sized blades is definitely more expensive than one big gearbox with rotor + some small gearboxes and small rotor/fan.

 

 

That's true, to a point. But this example is a little exaggerated. The CH-46 is pretty similar to the S-92 in terms of airframe dimensions and while it carries only 2/3 the payload, it only has 2/3 as much power.

 

Also, you can always increase the amount that the rotors intermesh (think sideways K-MAX as an extreme example) with various pros and cons. And I think that with a more rigid rotor system (but not necessarily as hyper rigid as the X2 requires), you could get a smaller aircraft without a ridiculously tall aft mast. It may not be the answer, but I do think it warrants more attention than it gets, especially from Boeing who should consider introducing a new model every 40 years or so.

 

By the way, if you're interested in the X2, check out the Boeing 360. It was basically an all composite CH-46 that did 200+ knots back in the late '80s.

[Linc]

The X-49 looks promising, too. 1960s technology reborn, and brought to you by the same guy who made tandems viable; Frank Piasecki.

Edited March 27, 2008 by Linc

[Sundowner]

The VTDP/X-49 is an interesting project, but have one major drawback. To achieve speeds similar to tilts and X2, it requires wings... and history proved it's not really a good idea to put them on helicopters, as they limit hover capabilities, and cause problems when flying backwards - that's why they were detachable on Mi-6, and why Mi-24 drivers look with envy at the Blackhawk jocks .

 

Yes, the B360 was an interesting project, somewhat a "SpeedKnight" although I wouldn't wait for Boeing to show something like that in near future. Not when the V-22 is made by them. The Osprey is their new B360.

[gr8shandini]

Yes, the B360 was an interesting project, somewhat a "SpeedKnight" although I wouldn't wait for Boeing to show something like that in near future. Not when the V-22 is made by them. The Osprey is their new B360.

 

I think there's quite a few folks that'd take the Boeing name off the V22 if they could. But you're right, that's the basket their eggs are in, so they gotta go with it. I just wish the folks in Seattle would cut loose some funding for a new helo model. Partly 'cuz it'd be cool, but mainly 'cuz I'd love to move back to Philly.