# Swept Rotor Design?



## Sh0rtbUs (14 Oct 2005)

This is just a quick question for those who have an idea...

i was watching a show last night on helicopters, and the Lynx Helicopter was rated pretty high for its shere speed and maneuverability (they were pulling loops with it).

They mocked this up to its Swept Rotor Design, which has allowed a modified version of the ZB500 G-Lynx to reach speeds around 400km/h. my question is, why is this so, but other aircraft with the same feature are only reaching speeds around 200 km/h (such as the Cormorant for example)?

Is it the design of the actual aircrafts structure, its weight, or engine capabilities?
I did some searching around, but didnt really come up with any real information that answered my exact inquiry.


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## Zoomie (14 Oct 2005)

Sh0rtbUs said:
			
		

> ...but other aircraft with the same feature are only reaching speeds around 200 km/h (such as the Cormorant for example)?



The Cormorant can fly more in the region of 150 knots = 280 km/hr...  The CH-149 is a huge helicopter when compared to the Lynx and has a maximum takeoff weight that is 10,000 kg more.


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## Sh0rtbUs (14 Oct 2005)

Zoomie said:
			
		

> The Cormorant can fly more in the region of 150 knots = 280 km/hr...  The CH-149 is a huge helicopter when compared to the Lynx and has a maximum takeoff weight that is 10,000 kg more.



So it would be the weight, thanks. That was my suspicion, but was also thinking that it would be probable to include an Engine proportionate to the Aircrafts size...


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## Sf2 (14 Oct 2005)

There are two major aerodynamic factors acting on the helicopter that will limit its top speed:

1)   Advancing Blade compressibility stall - while the rotors turn, the blade that is advancing (picture looking at a chopper from overhead, looking down - the blade that is moving forward is the advancing blade) is approaching the speed of sound as the helicopter approaches a certain speed.   As this happens, air flow begins to separate from the blade and it stalls out, which would cause the chopper to pitch up.

2)   Retreating blade stall - the retreating blade is the opposite of the advancing blade.   Its similar to trying to land an airplane with a crazy tail wind, because there isn't enough air going over the wings right??   Same with the retreating blade - as its moving backwards in the airflow, it stalls out, causing the chopper to pitch down.

There are alot of other factors, but those two are major players in rotor design.   Also, rotor designs will comprimise between lifting power and aircraft speed.


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## Good2Golf (16 Oct 2005)

ShOrtbUs, it's not weight the affects max speed, but as Shortfinal noted, the need to balance various aerodynamic and inertial forces.

The Lynx flys as fast as it does (for a single main rotor helo) because of its BERP blade (or more importantly the tip of the blade). http://www.enae.umd.edu/AGRC/Aero/berp.html  Only the tip's outer edge is swept, the remainder of the main blade is straight.  Aside from the biggest benefit of delaying the onset of compressibility effect (which therefore allows higher tip speeds and overall higher aircraft speeds), what also makes the BERP blade tip different is that it has minimal twisting force coupled from the tip-shape inwards along the blade to the swashplate and controls..i.e. much smoother vibration levels.  Other helos had to some degree or another, some type of twisting force (due to non-symmetry about the aerodynamic center of the blade).  The Blackhawk and Apache actually have a swept tip the maintains contant width but sweeps rearwards behind the blade's extended centerline. This means at higher speeds, tip pressures will actually warp or twist the blade's leading edge forward and cause the leading blade to tuck slightly, a known quantity but one that must be compensated for via the swashplate and control actuators.  The BERP tips have a neutral moment as speed increases and make for a smoother ride at high speeds.

Interestingly, although there are also advancing compressibility and retreating stall effects on the blades of a mutli-rotor helo, the rotors can run at a lower rotational speed and the counter-rotational configuration (no matter co-ax [as in Hokum or Werewolfe], tandem [Chinook] or lateral [Kaman K-Max lifter]) generally balances any rolling effects between the two rotors.

Highest speed I've flown in a helo was 171 KIAS (knots indicated air speed) on a test flight of a CH-147.  171 kts works out to about 318 km/h..not the 400 of the Lynx, but it was a stock machine...not a special one prepared for a few flights.

Cheers,
Duey


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