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-   -   Sikorsky SB-1 flies for first time (https://www.pprune.org/rotorheads/619699-sikorsky-sb-1-flies-first-time.html)

The Sultan 26th Jun 2021 15:05

Noneo wrote:


I have to believe Sikorsky has already told the Army that their demonstrators can't pull high g maneuvers. But that they will increase rotor spacing for production. Which will make a slow, tall Defiant even slower and taller.
You are correct in identifying one of the few performance points the 97/SB-1 programs demonstrated which was the ability of the rotors to collide in moderate roll reversals. You are, however, naive in thinking the SB team will admit this concept flaw. They will instead hide behind a claim that a simple tweak to the fly-by-wire control laws has eliminated the chance of this flaw reoccurring. Only after the loss of a couple more aircraft will they propose changing rotor separation.

noneofyourbusiness 26th Jun 2021 16:20


Originally Posted by The Sultan (Post 11068717)
Noneo wrote:



You are correct in identifying one of the few performance points the 97/SB-1 programs demonstrated which was the ability of the rotors to collide in moderate roll reversals. You are, however, naive in thinking the SB team will admit this concept flaw. They will instead hide behind a claim that a simple tweak to the fly-by-wire control laws has eliminated the chance of this flaw reoccurring. Only after the loss of a couple more aircraft will they propose changing rotor separation.

It's going to suck to be one of their test pilots.

SansAnhedral 29th Jun 2021 15:35


Originally Posted by The Sultan (Post 11068717)
You are correct in identifying one of the few performance points the 97/SB-1 programs demonstrated which was the ability of the rotors to collide in moderate roll reversals.

Lets not omit the old XH-59. One wonders what the public perception of the X2 concept would be if the videos of both the XH-59 and S-97 prangs had made their way online like some of the old V-22 mishaps managed to.

https://i.imgur.com/s5fWKok.png

https://i.imgur.com/RwDNhDZ.png

https://i.imgur.com/y2zOH6W.png

https://i.imgur.com/150dfsr.png


The Sultan 29th Jun 2021 20:33

Both programs were lucky the rotor collision events occurred on, or near, the ground or this "feature" of the ABC would not be so amusing.

The Sultan 13th Jul 2021 15:31

FLRAA RFP
 
Article in Defense News:

https://www.defensenews.com/land/202...t-competition/

Finally new stuff to talk about.

Recently read an article on the Bradley replacement where they had to cancel the original program because only one vendor responded. They then dumbed down the requirements and went out for new responses. Apparently it is unfair to have requirements that only the best can meet. This explains the FLRAA changes.

As to the issue discussed most: \ the speed requirement is as follows:


The Army has set its threshold speed requirement for the aircraft at 230 knots, with an objective requirement of 280 knots, according to the RFP. The contract would set up an incentive to reach the objective speed requirement as well.
So the min acceptable speed was reduced from 250 kts to 230 kts to keep SB in the competition to avoid the Bradley replacement problem. Much to SB's despair the objective speed was defined as 280 kts which the V-280 easily meets and the ABC concept can never meet. Additionally, the 280 kt objective matches nicely with the Marine new medium lift minimum speed requirement.

I think everyone knows how this plays out. No vendor team will get both the FLRAA and FARA. Even if Bell's Invictus equals the Sikorsky Raider in every way, Sikorsky will be given the program. FARA will only go to Bell if the ABC's concept deficiencies are overwhelming so Bell gets the FLRAA without years of protest from the losers.

Once the two programs are awarded, the emphasis on Pacific operations will keep FLRAA safe while the FARA can be terminated for cause when Sikorsky stumbles (face it what program have they recently executed flawlessly?) with minimum impact to the US's warfighting capabilities.

noneofyourbusiness 20th Jul 2021 20:16

What maneuverability?
 
From "Spyclip":

https://www.secretprojects.co.uk/thr....13812/page-46

"I can confidently say that there have been a total of zero published flight demonstrations where level 1 handling qualities were demonstrated with requisite rates in pitch, roll, or (particularly) yaw on any ABC aircraft historically.

If any of these had been accomplished over the Jupiter swamps with Raider I would eat my shoe if Sikorsky would decline to publicize such footage.

The fact that the V-280 was able to perform these maneuvers at double the gross weight of S-97 makes the ongoing ad nauseum Sikorsky marketing of mythical extreme rigid rotor agility that much more egregious.

All of this is to say absolutely nothing of SB1's lack of demonstrating anything of note whatsoever."
____________________________________________________________ _________

A "feature" of the Kamov is that the rotors occasionally hit each other:
https://www.bbc.com/russian/russia/2...licopter_crash

The Raider and Defiant are more complex than the Kamov Alligator. They have a very large pusher prop, some say the largest prop ever produced in the US for Defiant. In a fast maneuver, there will be large 1-P moments input to the airframe. The control system must account for this, which may be ?impossible?. Note the AVX proposal had two ducted fans, so would not have this problem.

noneofyourbusiness 20th Jul 2021 23:27

The massive diameter prop on Defiant will have a massive roll torque input to the airframe. When a conventional helicopter loses tail rotor authority, the helicopter spins about its axis. Large gear reduction, large torque, transmitted through the ring gear to the airframe. Normally the only roll torque in a conventional helicopter is from the much lower torque from the engines. The Defiant has to have a substantial gear reduction from the engine to the prop. Large gear reduction, large torque. The AVX design was better.

IFMU 21st Jul 2021 13:08


Originally Posted by noneofyourbusiness (Post 11082254)
The massive diameter prop on Defiant will have a massive roll torque input to the airframe. When a conventional helicopter loses tail rotor authority, the helicopter spins about its axis. Large gear reduction, large torque, transmitted through the ring gear to the airframe. Normally the only roll torque in a conventional helicopter is from the much lower torque from the engines. The Defiant has to have a substantial gear reduction from the engine to the prop. Large gear reduction, large torque. The AVX design was better.

Now your rambling is just getting silly. Why would you think the prop torque would be a problem? Do you think there is not enough cyclic trim to balance it?

noneofyourbusiness 21st Jul 2021 13:43


Originally Posted by IFMU (Post 11082556)
Now your rambling is just getting silly. Why would you think the prop torque would be a problem? Do you think there is not enough cyclic trim to balance it?

Your last sentence is a straw man argument.

Did I say Raider/Defiant couldn't fly because of it? No. Is it better not to have a large reaction torque? Yes.

To create a roll moment with the cyclics, the rotors are tilted with respect to each other, reducing clearance between the rotors, compared to the AVX design. There is a structural weight penalty. Flying with the rotors tilted with respect to each other reduces efficiency.

Edit -If cyclic control is lost, the helicopter could flip. AVX is just more proof, the better design does not always win.


Care to discuss Raider/Defiant lack of demonstrated maneuverability?

SansAnhedral 21st Jul 2021 14:37


Originally Posted by noneofyourbusiness (Post 11082142)
From "Spyclip":

https://www.secretprojects.co.uk/thr....13812/page-46

"I can confidently say that there have been a total of zero published flight demonstrations where level 1 handling qualities were demonstrated with requisite rates in pitch, roll, or (particularly) yaw on any ABC aircraft historically.

If any of these had been accomplished over the Jupiter swamps with Raider I would eat my shoe if Sikorsky would decline to publicize such footage.

The fact that the V-280 was able to perform these maneuvers at double the gross weight of S-97 makes the ongoing ad nauseum Sikorsky marketing of mythical extreme rigid rotor agility that much more egregious.

All of this is to say absolutely nothing of SB1's lack of demonstrating anything of note whatsoever."
____________________________________________________________ _________

A "feature" of the Kamov is that the rotors occasionally hit each other:
https://www.bbc.com/russian/russia/2...licopter_crash

The Raider and Defiant are more complex than the Kamov Alligator. They have a very large pusher prop, some say the largest prop ever produced in the US for Defiant. In a fast maneuver, there will be large 1-P moments input to the airframe. The control system must account for this, which may be ?impossible?. Note the AVX proposal had two ducted fans, so would not have this problem.

The comment made in that post about yaw is interesting.

I think a historical knock on tiltrotor maneuverability was probably based on yaw rate, which is primarily affected by rotor flapping limits. The V-22 has around 10 degrees of flapping cooked into the hub and I do not believe meets level 1 HQ yaw rate (probably exacerbated in crosswinds). The V-280 on the other hand showed some very impressive ADS-33 level 1 yaw maneuvers for its gross weight, thanks wholly to the massive increase to 15 degrees of flapping. When ground taxi turning in their demo footage, the flapping can look pretty wild!

Inherent to tiltrotors, the control power from flapping is applied at the wingtips, creating a solidly effective yaw moment arm.

Now with the compound coaxial pusher prop design, you have the large and heavy static pusher prop assembly, gearbox/bearings, massive stabilizer, and tailwheel assembly - a huge amount of dead weight at the end of a very long moment arm from the mast. The only way to achieve yaw control is by differential torque between the rotors, which have been designed to be as stiff and lightweight as possible. This is a massively different situation from a conventional helicopter where the yaw moment is applied by the tail rotor at that same location perpendicular to the longitudinal axis.

I wonder, at the same medium to large gross weights, is a high flapping tiltrotor going to have substantially better yaw control than a rigid rotor coaxial design with a pusher assembly? I would say almost certainly. It's a real shame that Defiant hasn't even attempted (that we know of) high yaw rate maneuvers that could be compared to the myriad V-280 demos. Perhaps they have and the results were disappointing.

noneofyourbusiness 21st Jul 2021 15:16

I am going to Ramble on. Even with a clutch, the prop placement of Raider/Defiant is an accident waiting to happen. Unlike a Black Hawk, with a high mounted tail rotor.

Igor Sikorsky, truly an aviation genius, used two jet engines on opposite sides of the helicopter, to provide forward thrust on the ABC demonstrator. Then someone at Sikorsky had a wet dream that they would use a single large pusher prop instead.

MeddlMoe 21st Jul 2021 15:19


Originally Posted by noneofyourbusiness (Post 11082573)

To create a roll moment with the cyclics, the rotors are tilted with respect to each other, reducing clearance between the rotors, compared to the AVX design. There is a structural weight penalty. Flying with the rotors tilted with respect to each other reduces efficiency.

A cyclic input will tilt both rotors in the same direction. Therefore the clearance will stay approximately the same.

That being said, I think the coaxial rotor idea is not very good. A lot more complexity. a large minimum level of rigidity in order to avoid collision between the rotors (which will lead to a rought ride, even worse than the BO105).
The rotor head on conventional helicopters creates about 30% of the parasitic drag. Parasitic drag is the main reason for power consumption at high speed. The SB-1 has two rotor heads. These may be encased in fairings, but many protoypes with rotor head fairings in the past showed only a small reduction of parasitic drag.

The effect of "advancing blade theory" could have been created just as well with a comparatively small asymmetric wing. This means a conventional configuration with an asymmetric wing and a thrust vectored tail rotor, or tail rotor at an fixed intermediate angle and compensating fin would have much less drag, less complexity, less risk of rotor blade collision, and easier to inspect and mantain main rotor head.

SansAnhedral 21st Jul 2021 18:13


Originally Posted by noneofyourbusiness (Post 11082625)
I am going to Ramble on. Even with a clutch, the prop placement of Raider/Defiant is an accident waiting to happen. Unlike a Black Hawk, with a high mounted tail rotor.

Igor Sikorsky, truly an aviation genius, used two jet engines on opposite sides of the helicopter, to provide forward thrust on the ABC demonstrator. Then someone at Sikorsky had a wet dream that they would use a single large pusher prop instead.

The XH-59B was the intended follow on to the XH-59A, and it had a ducted pusher prop design going all the way back to ca 1980.

https://www.sikorskyarchives.com/ima...A)/S-69-11.jpg


Interestingly, Igor died a year before the XH-59A first flew. He was quoted posthumously by Ben Kocivar in popular science's Sept 1982 piece on the "ABC copter" as having once said:


Having two overhead rotors on a helicopter is like having two cooks in the kitchen. They will not get along.

Genius indeed ;)


noneofyourbusiness 22nd Jul 2021 20:32


Originally Posted by MeddlMoe (Post 11082628)
A cyclic input will tilt both rotors in the same direction. Therefore the clearance will stay approximately the same.

That being said, I think the coaxial rotor idea is not very good. A lot more complexity. a large minimum level of rigidity in order to avoid collision between the rotors (which will lead to a rought ride, even worse than the BO105).
The rotor head on conventional helicopters creates about 30% of the parasitic drag. Parasitic drag is the main reason for power consumption at high speed. The SB-1 has two rotor heads. These may be encased in fairings, but many protoypes with rotor head fairings in the past showed only a small reduction of parasitic drag.

The effect of "advancing blade theory" could have been created just as well with a comparatively small asymmetric wing. This means a conventional configuration with an asymmetric wing and a thrust vectored tail rotor, or tail rotor at an fixed intermediate angle and compensating fin would have much less drag, less complexity, less risk of rotor blade collision, and easier to inspect and mantain main rotor head.

There are two cyclics. Tilting both rotors in the same direction creates a sideways force, the same as a conventional helicopter, but with no tail rotor. So we end up with a sideways push as well as roll. We need to counteract the roll torque from the prop. Tilting the rotors in opposite directions creates a pure couple, force times the distance separating the rotors.

You make very good points. Interesting, thank-you. Wouldn't the design you propose still have retreating blade stall at high forward speed?

noneofyourbusiness 22nd Jul 2021 20:38


Originally Posted by SansAnhedral (Post 11082693)
The XH-59B was the intended follow on to the XH-59A, and it had a ducted pusher prop design going all the way back to ca 1980.

https://www.sikorskyarchives.com/ima...A)/S-69-11.jpg


Interestingly, Igor died a year before the XH-59A first flew. He was quoted posthumously by Ben Kocivar in popular science's Sept 1982 piece on the "ABC copter" as having once said:




Genius indeed ;)

Thanks for posting, very interesting! I was not aware of the XH-59B. From the picture, the pusher is counter-rotating and shrouded. So better than Raider/Defiant!
I can't blame Igor for that, or for being frustrated with coaxial designs.

noneofyourbusiness 22nd Jul 2021 20:45


Originally Posted by SansAnhedral (Post 11082601)
The comment made in that post about yaw is interesting.

I think a historical knock on tiltrotor maneuverability was probably based on yaw rate, which is primarily affected by rotor flapping limits. The V-22 has around 10 degrees of flapping cooked into the hub and I do not believe meets level 1 HQ yaw rate (probably exacerbated in crosswinds). The V-280 on the other hand showed some very impressive ADS-33 level 1 yaw maneuvers for its gross weight, thanks wholly to the massive increase to 15 degrees of flapping. When ground taxi turning in their demo footage, the flapping can look pretty wild!

Inherent to tiltrotors, the control power from flapping is applied at the wingtips, creating a solidly effective yaw moment arm.

Now with the compound coaxial pusher prop design, you have the large and heavy static pusher prop assembly, gearbox/bearings, massive stabilizer, and tailwheel assembly - a huge amount of dead weight at the end of a very long moment arm from the mast. The only way to achieve yaw control is by differential torque between the rotors, which have been designed to be as stiff and lightweight as possible. This is a massively different situation from a conventional helicopter where the yaw moment is applied by the tail rotor at that same location perpendicular to the longitudinal axis.

I wonder, at the same medium to large gross weights, is a high flapping tiltrotor going to have substantially better yaw control than a rigid rotor coaxial design with a pusher assembly? I would say almost certainly. It's a real shame that Defiant hasn't even attempted (that we know of) high yaw rate maneuvers that could be compared to the myriad V-280 demos. Perhaps they have and the results were disappointing.

A coaxial design will be limited in yaw when operating at maximum lift. Reducing collective on one of the rotors to change the reaction torque, reduces lift. If Sikorsky could have demonstrated a high yaw rate, they would have.

noneofyourbusiness 22nd Jul 2021 21:50

A good summary of the Raider crash.
https://verticalmag.com/news/ntsb-re...ider-accident/

IFMU 22nd Jul 2021 22:37


Originally Posted by noneofyourbusiness (Post 11083342)
A coaxial design will be limited in yaw when operating at maximum lift. Reducing collective on one of the rotors to change the reaction torque, reduces lift. If Sikorsky could have demonstrated a high yaw rate, they would have.

Totally wrong. The coax limitation for yaw is in autorotation. They leave room for differential collective on top of collective.

IFMU 22nd Jul 2021 22:41


Originally Posted by noneofyourbusiness (Post 11083333)
There are two cyclics. Tilting both rotors in the same direction creates a sideways force, the same as a conventional helicopter, but with no tail rotor. So we end up with a sideways push as well as roll. We need to counteract the roll torque from the prop. Tilting the rotors in opposite directions creates a pure couple, force times the distance separating the rotors.

You make very good points. Interesting, thank-you.

A conventional helicopter has to react the thrust from the tail rotor with lateral cyclic. It's no big deal.

An X2 rotor system would use lateral cyclic, same direction for both rotors, to react any torque from the prop. Also no big deal.

In the absence of education or experience in these things, you might find it helpful to learn to fly helicopters, or anything.

noneofyourbusiness 22nd Jul 2021 23:37


Originally Posted by IFMU (Post 11083397)
A conventional helicopter has to react the thrust from the tail rotor with lateral cyclic. It's no big deal.

An X2 rotor system would use lateral cyclic, same direction for both rotors, to react any torque from the prop. Also no big deal.

In the absence of education or experience in these things, you might find it helpful to learn to fly helicopters, or anything.

There is a major difference between a prop and a tail rotor. Answer how the side force is counter-acted when both rotors are tilted in the same direction. Don't just give an arm wave. The reaction torque from a pusher prop is a pure torque. The torque from a tail rotor provides side thrust and moments (above the cg). Conventional helicopters have a much smaller roll torque from the engines. Did you ever take a physics class?

When you get an answer from your bosses at Sikorsky, I am willing to learn and be educated.


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