Sikorsky + Boeing pitch ‘X-2’-based design for US Army JMR TD effort
Join Date: Aug 2014
Location: New Braunfels, TX
Age: 70
Posts: 1,954
Likes: 0
Received 0 Likes
on
0 Posts
horlick said: the only way to still employ the counter rotating rotors (so as to negate the need of a counter torque tail rotor) is to use the tandem configuration with the rotors interleave as in the Chinook.
Actually, no. Sikorsky's coaxial rotor is an ABC (Advancing Blade Concept) rotor system. That means only the advancing blade makes lift. The retreating blade makes no lift. The problem with high speed in any conventional rotor system is that the retreating blade will stall at high airframe airspeed. By unloading the retreating blade it cannot stall. But that means you have to have two advancing blades on a common center, hence a coaxial configuration. If the two advancing blades are not coaxial (as in a tandem configuration) the lift will not be symmetrical about one axis making control impossible.
Picture a Chinook with the forward rotor generating lift only on its left side and the aft rotor generating lift only on its right side. To bank right the forward rotor would generate more lift (more lift on left side) and the aft rotor less lift (less lift on right side). But this would not only generate a roll moment to the right, but also a nose up pitch moment. Rolling left would generate a nose down pitch moment. And changing pitch would result in a roll. And because the rotor centers are nowhere near the CG, rolling would also generate a yaw moment. So all three control axes would be coupled to each other. It would be uncontrollable.
Actually, no. Sikorsky's coaxial rotor is an ABC (Advancing Blade Concept) rotor system. That means only the advancing blade makes lift. The retreating blade makes no lift. The problem with high speed in any conventional rotor system is that the retreating blade will stall at high airframe airspeed. By unloading the retreating blade it cannot stall. But that means you have to have two advancing blades on a common center, hence a coaxial configuration. If the two advancing blades are not coaxial (as in a tandem configuration) the lift will not be symmetrical about one axis making control impossible.
Picture a Chinook with the forward rotor generating lift only on its left side and the aft rotor generating lift only on its right side. To bank right the forward rotor would generate more lift (more lift on left side) and the aft rotor less lift (less lift on right side). But this would not only generate a roll moment to the right, but also a nose up pitch moment. Rolling left would generate a nose down pitch moment. And changing pitch would result in a roll. And because the rotor centers are nowhere near the CG, rolling would also generate a yaw moment. So all three control axes would be coupled to each other. It would be uncontrollable.
Join Date: Aug 2014
Location: New Braunfels, TX
Age: 70
Posts: 1,954
Likes: 0
Received 0 Likes
on
0 Posts
KenV, not chasing the drogue has nothing to do with rotor blade flex. It's an issue of PIO, you're trying to chase the end of a whip instead of watching the hand that controls the whip.
By itself, this is no big deal, but in aviation, nothing happens by itself. Unloading the rotor does two things: it reduces drag (which increases airspeed) and reduces the rotor downwash. The increased airspeed results in the overshooting getting worse, which means the drogue is well inside the rotor disk, but below it. Reduced downwash means the drogue starts moving back up. Since the drogue is inside and below the rotor disk, the HAC must chase the drogue up or risk having the drogue move up into the rotor disk. That would be very bad. When the HAC commands nose up, the rotor loads up again and you can actually see the rotor flex down in the video. In this case the blades flexed downward enough to impact the probe tip.
Note that the helo in the video never went into PIO. The HAC immediately leveled the helo and stopped its climb right at rotor/probe impact and then moved the helo aft and outward, away from the KC-130 tanker.
Defiant?s delay due to blade manufacturing challenges
Well, clearly they already knew this was an issue when they announced that everything was on schedule earlier this year.
Well, clearly they already knew this was an issue when they announced that everything was on schedule earlier this year.
Join Date: Oct 2002
Location: 1 Dunghill Mansions, Putney
Posts: 1,797
Likes: 0
Received 1 Like
on
1 Post
Especially interesting given the previous discussion on the S-97's rumored blade supplier issues (through, ironically, the supplier in question is now on the competing V-280 program).
I/C
I/C
Last edited by Ian Corrigible; 11th Sep 2017 at 19:56.
The Dog Ate My Homework
This must mean the Defiant PDR, CDR, and Fuselage delivery milestones were also caused by manufacturing difficulties with the rotor blades.
If just having acceptable for flight blades was the cause for the program delay there would be Defiant program press release photos of the aircraft rolled out of the hangar with not for flight blades installed.
Even system checks and limited ground runs with not for flight blades should be possible.
Maybe a dog ate the Defiant program master schedule is a more believable excuse.
