Sikorsky S-97 hits 200kts

[Ravendriver2008]

The Sikorsky S-97 Raider hits the 200kts mark

https://www.youtube.com/watch?v=Elh0IM7Zp2U

[Snoop 95]

I wondered how that speed compared to the Lynx, which holds the world speed record for a helicopter. According to Wikipedia that still stands at 249.09 mph.

[turmo]

Conventional:
1970 S-67 191 knots
1978 Mi-24A-10 199.13 knots
1986 Lynx ( mod ) 216.31 knots

Hybrid:
1948 Gyrodyne 108 knots
1959 Rotodyne 166 knots
2013 Airbus X3 255 knots

Jet augmented:
1967 XH-51 263 knots
1979 S-72 300 knots

[Seahornet1]

I wondered how that speed compared to the Lynx, which holds the world speed record for a helicopter. According to Wikipedia that still stands at 249.09 mph.

The S-97 is a compound helicopter (with a pusher propeller), so the Lynx's record is not at risk. In fact, compound helicopters were hitting 250 knots (287 mph) back in the 1970s, so you'd hope that the S-97 is capable of much higher speeds than the 200 knots that it's demonstrated so far.

[C24]

There is a limiting factor due to the speed of the retreating blade tip. The lecture was more than fifty years ago so I have forgotten most of the details There must be someone out there who is more uptodate. Something about the tip stalling and loosing lift... I should have written it down

[Seahornet1]

There is a limiting factor due to the speed of the retreating blade tip. The lecture was more than fifty years ago so I have forgotten most of the details There must be someone out there who is more uptodate. Something about the tip stalling and loosing lift... I should have written it down

That's why the S-97 has a contra-rotating configuration. There are advancing blades on both sides of the rotor disc, so the behaviour of the retreating blades is less significant. The fact that the rotor only has to generate lift, and not forward thrust, also changes the game.

[Dan D'Air]

I do find it incredible that the Lynx still holds the record!
It is a great helicopter though.

[C24]

Sea hornet. I see what you are trying to say but all rotor discs have a retreating blade otherwise they would not be rotating. The two rotors are contra rotating which reduces the problem, perhaps. As I said, it has been a long time...

[wolfie138]

if you've got one blade advancing whilst the contra blade is retreating, would they not cancel each other out?

[Nighthawke]

No as there is more lift produced by the advancing blade.

[Agent K]

if you've got one blade advancing whilst the contra blade is retreating, would they not cancel each other out?

No. as the forward blade is going through the air faster than the retreating blade and also the retreating blade doesn't generate negative lift, just less to no lift depending on the actual relative speed.

[Seahornet1]

Sea hornet. I see what you are trying to say but all rotor discs have a retreating blade otherwise they would not be rotating. The two rotors are contra rotating which reduces the problem, perhaps. As I said, it has been a long time...

The reason that the retreating blade is a problem with conventional helicopters is that you still need to generate lift from it, to keep the roll forces equal on both sides. The faster you go, the more of the retreating blade has a reverse airflow, dramatically reducing its lift. To compensate, you have to increase the blade pitch as airspeed increases, but you can only do this up to the stalling angle of the blade aerofoil. Now, it's possible to overcome this issue by increasing the rotation speed of the rotor, allowing your retreating blade to cope with a higher forward airspeed, but at some point you will find your advancing blade tips approaching transonic speed, with which they are not going to cope (except for Airwolf, apparently... ). Hence, the theoretical maximum speed for a helicopter of roughly 250 -260 mph, which the BERP Lynx got pretty close to. (The BERP rotor tips were designed to cope with higher mach number than a conventional tip, allowing a bit more rotor speed.)

With a contra-rotating setup however, you only need the advancing blades to produce lift (because you have them on both sides of the aircraft), so you don't have to increase the pitch on the retreating ones, and therefore they don't stall. There's still the issue of transonic rotor tips, but you can use a slower rotor speed to delay this effect, and the result is a higher theoretical airspeed. The Devil is in the details, and there are a lot of practical issues in making it work, but I imagine that the 'Advancing Blade Concept' technology that the S-97 uses is largely aimed at overcoming these issues.

[C24]

FYI



https://en.m.wikipedia.org/wiki/Retreating_blade_stall

[Seahornet1]

FYI



https://en.m.wikipedia.org/wiki/Retreating_blade_stall

I'm glad to see that Wikipaedia agrees with me...

I do wish I'd used the phrase 'Disymmetry of Lift', though....

[C24]

It was my intention to use it too but as I originally had trouble spelling it, there was no chance now