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I would love to see a diagram which depicts the various angles at which air is moving through the rotors, and away from them, during maximum forward thrust. Normally, with a chopper which has its nose pointed downward during forward movement, there must be a speed which it would be inadvisable to exceed, because there would be more air moving past the rotor, than being moved BY the rotor, robbing it of purchase on the air around it, and therefore robbing it of lift as well.
originally posted by: Zaphod58
a reply to: TrueBrit
The UH-60 has a never exceed speed of 222 mph, and a top speed of 183 mph. At 222 mph, both the airflow around the rotors, and the airframe stresses would reach that point.
Other than the added complexity to the gear box, this is an amazing design.
originally posted by: tomra
I would assume the co-axial design also helps with retreating blade stall issue? No benefit in having a pusher prop if the helo is gonna roll over on its side..
originally posted by: buddah6
originally posted by: Zaphod58
a reply to: TrueBrit
The UH-60 has a never exceed speed of 222 mph, and a top speed of 183 mph. At 222 mph, both the airflow around the rotors, and the airframe stresses would reach that point.
Other than the added complexity to the gear box, this is an amazing design.
The limiting factor on helicopter is an effect called retreating blade stall (RBS) which happens at higher speed. This is usually the major factor in establishing a never exceed speed. A helicopter has an advancing blade and a retreating blade while in forward flight. Let's say the helicopter is flying at 100 MPH and the rotor is turning at 100 MPH. The advancing blade will be advancing at 200 MPH and 200 MPH worth of lift. The retreating blade while turning at 100 MPH minus 100 MPH forward speed equals 0 MPH worth of lift. The helicopter will roll hard left and pitches nose up which can ruin your day.
A coaxial rotor systems with rotors turning in opposite direction will always have an advancing blade on each side. This allows the helicopter to fly faster than a conventional rotor helicopter because RBS is no longer an issue.
Chinooks and SeaKnights are faster than Hueys due to NO RBS issues.
Helicopter cruise speed is usually .30 to.40% of rotor RPM in FPS.