flyingturtle
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Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Mon Jan 09, 2012 1:05 pm

Hello everybody,


over the weekend I did some aviation-related reading in the good ol'e Wikipedia.

I remember that when I was 14 years or so, I saw the first TV documentary about the V-22 Osprey development. Since then, I thought about a VTOL aircraft like the V-22 – but with high-bypass turbofans instead of turboprops. Like this, but just fatter...


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Photo © Daniele Faccioli



...fat like a 320. 

Now I discover more or less obscure aircraft – namely, I've read about the Canadair CL-84 that was tested between 1957 and 1972 – and now I'm asking: Why was the V-22 designed to be a tilt-rotor aircraft when the tilt-wing CL-84 ended her flight testing so favorably?

I can imagine that a tilt-wing VTOL aircraft is technologically much simpler (especially the engine-wing joint), but would have some problems in head- or crosswind. And the aircrafts center of gravity would shift when tilting the wing, especially when the tanks are still full.


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Kind regards,

David

[Edited 2012-01-09 05:16:13]
Keeping calm is terrorism against those who want to live in fear.
 
rwessel
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Mon Jan 09, 2012 11:23 pm

As a general rule, getting more stuff to move is harder than getting less stuff to move. You'd basically be replacing two big, heavy, complex nacelles tilting mechanism, with one bigger, heavier, and even more complex wing tilting mechanism. And as you mentioned, you introduce other problems, like tilting the fuel tanks, changing the CG (depending on where the wing pivot is, of course), and for a military aircraft, you might have the issue of under-wing stores.

And just to add complexity to the situation, the V-22's wing twists (and the rotors fold) to reduce the space required to store one of the things below deck.

But probably the biggest disadvantage of the tilt-wing design is that you'd have more issues on the transition to horizontal flight - the V-22's wing can start producing lift as soon as it starts moving forward, a tilt-wing is going to be stalled until the wing's alpha drops to about 14 degrees. So tilt-rotors can do rolling (STOL) takeoffs with the wing contributing considerable lift, which may significantly increase their maximum takeoff weight. And a tilt wing is going to be stuck doing lift-offs with just the available vertical thrust from the rotors.

Edit: I should mention that translation lift would be available in a rolling takeoff in either case.

But neither is impossible, and both involve tradeoffs. And frankly neither makes sense for anything other than a cost-is-no-object military aircraft.

[Edited 2012-01-09 15:26:14]
 
flyingturtle
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Tue Jan 10, 2012 6:16 pm

Hello rwessel,

your answer is surprising, somehow. I always thought a tilt-wing aircraft would be much simpler, given that the engines don't need a delicate tilting mechanism; and when the engines and the wing form a unit, a driveshaft connecting the engines would also be easier to build.

Then, the engine tilting mechanism has to fit into the already room-restricted wing, while the crown area of our tilt-wing aircraft would offer ample space for a tilting mechanism.

Quoting rwessel (Reply 1):
But probably the biggest disadvantage of the tilt-wing design is that you'd have more issues on the transition to horizontal flight - the V-22's wing can start producing lift as soon as it starts moving forward, a tilt-wing is going to be stalled until the wing's alpha drops to about 14 degrees.

Well, at first I had a completely impractical thought about how to solve this...

Helicopters drop their nose in order to gain forward speed. The same could be done with a tilt-wing aircraft - the wing tilt relative to the fuselage is still 90°, but the nose drops, the wing AoA is decreased, and the engines begin to produce a partial forward thrust in addition to the lift.

The "impractical" part comes in when dropping the nose of the aircraft: I first thought of moving weights inside of the fuselage crown because Zeppelin airships had the same method of controlling their attitude...    Well, now I've read about the various tilt-wing models and most (if not all of them) feature a tail rotor for that purpose.

I'm not sure, but I think a tilt-wing flying thingamajic would be better at STOL takeoffs. I'd firewall the throttles and gradually tilt the wing from initally 20° to about 45° - the wings will be stalled but they still produce lots of lift (at a terrible efficiency, though). But with a thrust-weight ratio > 1, this is a rather academic problem.


I remain amazed about the variety of contraptions designed and tested during the cold war and I'm sure a more economical tilt-wing or a tilt-rotor aircraft would have been great for SAR duty, for example...   


David

[Edited 2012-01-10 11:01:21]
Keeping calm is terrorism against those who want to live in fear.
 
rwessel
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Tue Jan 10, 2012 8:26 pm

Quoting flyingturtle (Reply 2):
your answer is surprising, somehow. I always thought a tilt-wing aircraft would be much simpler, given that the engines don't need a delicate tilting mechanism; and when the engines and the wing form a unit, a driveshaft connecting the engines would also be easier to build.

Then, the engine tilting mechanism has to fit into the already room-restricted wing, while the crown area of our tilt-wing aircraft would offer ample space for a tilting mechanism.

The drive shaft just has to pass through the center of the nacelle pivots. Not really a big deal. Actually you don't even have to do that, but it's usually the simplest and lightest solution.

But as I said, there are tradeoffs either way. As I mentioned, the mechanisms for tilting the nacelles will be smaller, and for a low speed aircraft, it may be easier to increase the size of the nacelles that consuming a lot of room in the middle of the cargo area of the fuselage.

Quoting flyingturtle (Reply 2):
Helicopters drop their nose in order to gain forward speed. The same could be done with a tilt-wing aircraft - the wing tilt relative to the fuselage is still 90°, but the nose drops, the wing AoA is decreased, and the engines begin to produce a partial forward thrust in addition to the lift.

The "impractical" part comes in when dropping the nose of the aircraft: I first thought of moving weights inside of the fuselage crown because Zeppelin airships had the same method of controlling their attitude... Well, now I've read about the various tilt-wing models and most (if not all of them) feature a tail rotor for that purpose.

No, you'd drop the nose just like a helicopter does, by apply the correct amount of cyclic. IOW, you increase the pitch of the rotor blades when they're in back (thus increasing the lift there), and decrease them in front (reducing the lift there). More lift in the back, less in the front, the nose goes down. The tail rotor is not involved. You'd only need a (vertical thrusting) tail rotor if you decided to not make the rotor blades cyclically adjustable.

Besides, you'd have to tilt the nose pretty far down to get the wing unstalled.

Quoting flyingturtle (Reply 2):
I'm not sure, but I think a tilt-wing flying thingamajic would be better at STOL takeoffs. I'd firewall the throttles and gradually tilt the wing from initally 20° to about 45° - the wings will be stalled but they still produce lots of lift (at a terrible efficiency, though). But with a thrust-weight ratio > 1, this is a rather academic problem.

While a stalled wing will produce lift, it produces less than an unstalled wing (and huge of amounts of drag too, of course). And the power to overcome that drag has to come from someplace - if you're doing a STOL takeoff for weight reasons, you *don't* have a thrust-to-weight ration above unity. You'd generally want as short a roll as possible in most cases anyway.
 
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Starlionblue
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Wed Jan 11, 2012 3:34 am

Quoting flyingturtle (Reply 2):
I remain amazed about the variety of contraptions designed and tested during the cold war

Dude:

"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
flyingturtle
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Thu Jan 12, 2012 4:48 pm

Hello rwessel,

Quoting rwessel (Reply 3):
The drive shaft just has to pass through the center of the nacelle pivots. Not really a big deal. Actually you don't even have to do that, but it's usually the simplest and lightest solution.

I suspected that - but then I wonder how the fuel line is built like in the V-22. Do you or somebody else here some pictures explaining this? Or for that matter, is there a good book on the V-22? All I find using the keyword "Osprey" are battles from 500 to 100 B.C...   

Quoting rwessel (Reply 3):
No, you'd drop the nose just like a helicopter does, by apply the correct amount of cyclic. IOW, you increase the pitch of the rotor blades when they're in back (thus increasing the lift there), and decrease them in front (reducing the lift there). More lift in the back, less in the front, the nose goes down. The tail rotor is not involved. You'd only need a (vertical thrusting) tail rotor if you decided to not make the rotor blades cyclically adjustable.

Thank you for your explanation, but I already unterstood how helicopters change their attitude. I got you on the wrong track by comparing my imaginary VTOL aircraft with a helicopter during vertical-horizontal flight transition. Tilt-wing aircraft often had a tail rotor, albeit not one to counter main rotor torque, but one improve to make attitude change.

To your other points.... later. Dinner time here.  


Thank you for your postings!

David
Keeping calm is terrorism against those who want to live in fear.
 
OldAeroGuy
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Thu Jan 12, 2012 10:42 pm

For more information on Tilt Wings, you might want to read up on the XC-142A

Here's a start

http://en.wikipedia.org/wiki/XC-142A
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
Geezer
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Fri Jan 13, 2012 2:44 am

Quoting Starlionblue (Reply 4):
Quoting flyingturtle (Reply 2):
I remain amazed about the variety of contraptions designed and tested during the cold war

David............That "contraption" in Styarlionblue's reply...........I was "around" while that boondoggle was going on; read quite a few articles about it. The thing actually had no practicality to it; yes, it did manage to get airborne, but could do absolutely nothing while airborne, then the pilot was faced with the daunting task of getting back on the ground, hopefully while still in one piece; the thing actually had a big downward facing hook mounted on the "belly", and there was a big "ring" mounted on a vertical upright; the problem then became one of the pilot getting this downward pointing "hook" into the "ring", all without being able to see what he was trying to do ! "Guesswork" if you will.........I'm not sure, but I seem to recall that it was Ryan who came up with this scheme. During the 50's and 60's there were boondoggles like this going on almost all of the time; one of my favorites, and probably even more bizarre than the VTOL pictured, was the so-called "Flying Saucer" ( or Flying Pan Cake ).............the "thing" was essentially a big round disc shaped body, with two huge propeller shafts sticking WAY OUT the front, with propellers that were almost as big in diameter as the A/C was, with two gigantic, long main LG legs, ( which held the whole thing about 15 feet from the ground !) Lol ! What a monstrosity that was !

I'll no doubt be yelled at for saying this...........but during this period, to a civilian it would have seemed that the AAF and the USN were being run to produce circus acts, rather that weapons systems.

I hope someone can come up with a photo of the Fying Pancake !

Charley
Stupidity: Doing the same thing over and over again and expecting a different result; Albert Einstein
 
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Moose135
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Fri Jan 13, 2012 3:03 am

Quoting geezer (Reply 7):
I hope someone can come up with a photo of the Fying Pancake !

Here you go - Vought V-173:

http://afghanistansquid.blogspot.com...09/weird-weapons-vought-v-173.html
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HaveBlue
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Fri Jan 13, 2012 3:25 am

Quoting geezer (Reply 7):
That "contraption" in Styarlionblue's reply...........
Quoting geezer (Reply 7):
the thing actually had a big downward facing hook mounted on the "belly", and there was a big "ring" mounted on a vertical upright; the problem then became one of the pilot getting this downward pointing "hook" into the "ring", all without being able to see what he was trying to do !

Actually I think you are mixing up 2 of our eclectic (to be polite) vehicles of the day my friend. The aircraft pictured in Starlionblues post is the Convair XFY-1 Pogo, and that along with Lockheeds XFV-1 were both VTOL aircraft that had to transition from horizontal flight to a vertical zoom, then 'back down' vertically to land on their castering wheels.

The hook/ring concept you speak of was used on the Ryan X-13 Vertijet, and as you describe it the pilot had to somewhat blindly try to hook the trap without seeing it.

The problem with all 3 of the above aircraft along with others like the XF-85 Goblin (B-29/B-36 parasite fighter) is that the average line pilot didn't stand a chance of flying them safely. The test pilots had their hands full and that was under ideal and expected conditions, so alas the dream was abandoned for that and other reasons.


http://img6.imageshack.us/img6/8378/vtolfig127.jpg


XFY-1





XFV-1




Ryan X-13 Vertijet

http://img822.imageshack.us/img822/4103/ryanx13.jpg
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rwessel
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Fri Jan 13, 2012 6:36 am

Quoting flyingturtle (Reply 5):
I suspected that - but then I wonder how the fuel line is built like in the V-22. Do you or somebody else here some pictures explaining this?

While I don't know the specifics, I'm not sure why it would require anything beyond a simple flex line. After all, the nacelle only rotates 110 degrees or so. You'd just need a version of the same thing done for any swing-wing aircraft. Same for the hydraulics and electrical. Don't know if there are any pneumatic systems on the V-22 that need bleed air.

Quoting HaveBlue (Reply 9):
The problem with all 3 of the above aircraft along with others like the XF-85 Goblin (B-29/B-36 parasite fighter) is that the average line pilot didn't stand a chance of flying them safely. The test pilots had their hands full and that was under ideal and expected conditions, so alas the dream was abandoned for that and other reasons.

I'd say even the Harrier and Yak-38 were marginal, at best, for average line pilots. The F-35B (assuming it gets built) will probably be the first VSTOL aircraft really within the capabilities of most pilots. Of course it does that by beating the problem into submission with a level of FBW and stability augmentation impossible 30 years ago.
 
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Starlionblue
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Fri Jan 13, 2012 7:27 am

Quoting rwessel (Reply 10):
The F-35B (assuming it gets built) will probably be the first VSTOL aircraft really within the capabilities of most pilots. Of course it does that by beating the problem into submission with a level of FBW and stability augmentation impossible 30 years ago.

I am getting this image in my head of white coated engineers using heavy spanners to beat the crap out of an avionics box in an F-35B 
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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Moose135
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RE: Tilt Wing Vs. Tilt Rotor Vtol Aircraft

Fri Jan 13, 2012 1:12 pm

Quoting Starlionblue (Reply 11):
I am getting this image in my head of white coated engineers using heavy spanners to beat the crap out of an avionics box in an F-35B

So you've figured out how they make things work in the aerospace industry...   
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