bomber996
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MV-22 Rotor Rotation, Why Not Centerline?

Sun Feb 12, 2012 7:14 am

Simple question. Why does the right engine on the MV-22 spin counter clock-wise and the left engine spin clock-wise? Shouldn't the engines spin opposite that so that the downward turn of the rotor is closer to the centerline of the aircraft therefore reducing the effects of P-factor? I know the two rotors are connected so both rotors spin at the same time. Could this be the reason? Any insight would be interesting.


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rwessel
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Sun Feb 12, 2012 8:17 am

P-factor is not an issue on the Osprey since both proprotors are connected, and will continue to turn together even if one engine quits. Or even if both engines quit (the V-22 can autorotate, although not all that well).

The V-22 needs counter-rotating proprotors for obvious reasons

Presumably the choice between inboard-blade-down and inboard-blade-up rotation had do to with flight performance. On the P-38, for example, it was found that the inboard-blade-up rotation (like on the V-22) improved stability, leading to better gunnery, presumably caused by a more advantageous interaction of the propeller wake and the empennage. They went with that despite the impact on engine-out performance.

In either event, reversing the direction would be fairly trivial, so I'd assume that there was at least a slight advantage to the direction chosen.
 
HaveBlue
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Sun Feb 12, 2012 10:32 am

Quoting rwessel (Reply 1):
On the P-38, for example, it was found that the inboard-blade-up rotation (like on the V-22) improved stability, leading to better gunnery, presumably caused by a more advantageous interaction of the propeller wake and the empennage. They went with that despite the impact on engine-out performance.

Actually, it's way more than that. With counter rotating engines the P-38 couldn't even get airborne. The weird vortex of lift that was disrupted by the meeting of the airflows cancelled out any lift provided by the center wing, which was a significant part of the total lift. As soon as they changed it to both props going the same way.. well, history has shown how great an aircraft the P-38 became.

But in summary, the P-38 could not fly with counter/contra rotating props.

If needed I will provide references tomorrow.. just got home, its late and not gonna happen atm.

[Edited 2012-02-12 02:34:03]

[Edited 2012-02-12 02:34:37]
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rwessel
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Sun Feb 12, 2012 11:09 am

Quoting HaveBlue (Reply 2):
Actually, it's way more than that. With counter rotating engines the P-38 couldn't even get airborne. The weird vortex of lift that was disrupted by the meeting of the airflows cancelled out any lift provided by the center wing, which was a significant part of the total lift. As soon as they changed it to both props going the same way.. well, history has shown how great an aircraft the P-38 became.

But in summary, the P-38 could not fly with counter/contra rotating props.

Errr... Given that they were all delivered with counter-rotating propellers, that would have limited their usefulness a bit.

The first prototype, the XP-38, had the conventional counter-rotation (inboard-blade-down), but that was reversed with the YPs and the production airframes, because that increase the stability of the aircraft as a gun platform.

I don't know if any were ever delivered from Lockheed without counter-rotating props, but a few had such, at least temporarily, apparently for maintenance reasons.
 
HaveBlue
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Sun Feb 12, 2012 8:04 pm

Quoting rwessel (Reply 3):
The first prototype, the XP-38, had the conventional counter-rotation (inboard-blade-down), but that was reversed with the YPs and the production airframes, because that increase the stability of the aircraft as a gun platform.

Okay I should not post at 6am after drinking all night lol. You are correct, they still had counter rotating props. What I should have said is when they had the props rotating towards the cockpit at the top of their arc, it cancelled out lift from the center wing to the point that the P-38 could not lift off of the ground. They reversed the rotation of both props, and all was well. My apologies.
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rwessel
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Sun Feb 12, 2012 11:12 pm

Quoting HaveBlue (Reply 4):
Okay I should not post at 6am after drinking all night lol. You are correct, they still had counter rotating props. What I should have said is when they had the props rotating towards the cockpit at the top of their arc, it cancelled out lift from the center wing to the point that the P-38 could not lift off of the ground. They reversed the rotation of both props, and all was well. My apologies.

The first several flights of the XP-38 (the prototype), were made with the props rotating in the "conventional" manner, inboard-blade-down. The buffeting of the tail feathers caused by the wake led to them swapping the engines.
 
dragon6172
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Mon Feb 13, 2012 2:36 am

Quoting rwessel (Reply 1):
In either event, reversing the direction would be fairly trivial, so I'd assume that there was at least a slight advantage to the direction chosen.

You need to think of the prop rotation when the Osprey is in helicopter mode. They spin so that the leading edge is coming forward on the outboard side of the engines vice coming forward over the wing. This keeps the forward flying blade (which in theory generates more lift with forward motion) in less turbulent air.
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Max Q
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Mon Feb 13, 2012 3:15 am

Ah, the P38, what an Aircraft.



Anyone know another Fighter that had a Control yoke instead of a joystick ?
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rwessel
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Mon Feb 13, 2012 4:36 am

Quoting dragon6172 (Reply 6):
You need to think of the prop rotation when the Osprey is in helicopter mode. They spin so that the leading edge is coming forward on the outboard side of the engines vice coming forward over the wing. This keeps the forward flying blade (which in theory generates more lift with forward motion) in less turbulent air.

An interesting point.

It strikes me that you could use that technique to deal with retreating blade stall and beat the normal helicopter speed limits.

IOW, at higher speeds just set the retreating blade pitch to zero, and generate all of your lift from the advancing blades. Given the counter-rotation, that would be balanced. Interestingly as speed increases, you'd have to slow your rotors.

Of course that's approximately what the Sikorsky X2 does, although with coaxial rotors.
 
timz
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Mon Feb 13, 2012 6:09 pm

Suppose the V-22's rotors did turn the same way-- say, clockwise when viewed from above when it's taking off vertically, with the rotors turning around a vertical axis. Each engine would be torquing the body of the V-22 counterclockwise-- wouldn't it need a tail rotor to stay straight?
 
redflyer
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Mon Feb 13, 2012 9:52 pm

Quoting Max Q (Reply 7):
Anyone know another Fighter that had a Control yoke instead of a joystick ?

The de Havilland Mosquito had a control yoke, although, it wasn't a pure fighter, either. But she was a bitchin' machine every bit as fascinating as the P-38.
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rwessel
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Tue Feb 14, 2012 7:17 am

Quoting timz (Reply 9):
Suppose the V-22's rotors did turn the same way-- say, clockwise when viewed from above when it's taking off vertically, with the rotors turning around a vertical axis. Each engine would be torquing the body of the V-22 counterclockwise-- wouldn't it need a tail rotor to stay straight?

Yes it would (or some other anti-torque mechanism). Elimination of the dangerous, fragile and power hungry tail rotor is a prime attraction of a dual rotor design. AFAIK, no one has ever thrown that advantage away by making the rotors rotate in the same direction. Of course someone will now point out some oddball one-off someplace where they did just that...    

[Edited 2012-02-13 23:18:13]
 
Max Q
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RE: MV-22 Rotor Rotation, Why Not Centerline?

Tue Feb 14, 2012 7:24 am

Quoting redflyer (Reply 10):

The de Havilland Mosquito had a control yoke, although, it wasn't a pure fighter, either. But she was a bitchin' machine every bit as fascinating as the P-38.

Yes the Mosquito was a hot rod, but not a fighter really.
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