With advancements in technology why there isn't much increment in helicopter top speeds from what they were in the 1980s? We have newer designs like the Eurocopter X3 which is a hybrid, but the speed is still limited to 300+mph.

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    $\begingroup$ Neither have airliners', nor even fighters'... $\endgroup$ – Zeus Jul 16 '18 at 13:51
  • $\begingroup$ It's because of damned environmentalists. $\endgroup$ – Fattie Jul 17 '18 at 18:05
  • $\begingroup$ environmentalists ? Because of noise ? $\endgroup$ – Huntkil Jul 18 '18 at 4:20
  • $\begingroup$ @Zeus we have but its not feasible for fighters whereas airliners have increased their avg speed. Mach .7 to .8 is optimum for them to operate. $\endgroup$ – Huntkil Jul 18 '18 at 4:22
  • $\begingroup$ @Huntkil not really. The current long-range airliners have max cruise around M0.85, which is slightly less that the airplanes from decades ago could offer (VC10 had 0.92 or so, even 747 can 0.88 I think). Similarly, nearly all Gen 2-3 fighters were Mach 2+ aircraft (some even M3), whereas later designs are commonly <M2 (F-35 is M1.6, for example). $\endgroup$ – Zeus Jul 18 '18 at 5:39

The underlying limit is that the forward-moving blade tip must remain subsonic, and the receding blade must not stall. Hybrids do better than conventional or compound helicopters because the rotor can be slowed when the fixed wing picks up some of the lift load -- but this only solves the forward blade tip speed issue; unless the rotor can be operated at zero lift, receding blade stall is still a problem.

Some futuristic designs have proposed completely halting the rotor in forward flight (see the aircraft in the movie The Sixth Day with Arnold Schwarzenegger -- cheesy movie, but the aircraft were based on a real concept), using an airfoil shape for the rotor blades that can develop lift with either edge forward. Others have proposed completely folding the rotor once forward speed is high enough for the fixed wing to support the craft -- but as far as I know, no flying example of either of these concepts has been built. Until they are, the same speed limitations apply to helicopters and autogyros that were discovered in the 1930s, only partially mitigated by hybrid configurations.

Bottom line is, a helicopter's niche is in going slow, not going fast. Other VTOL solutions exist for high speed (thrust-hover jets like the Super Harrier and F35 are supersonic). Attack and high-rate maneuverability in hover are the special domain of choppers; high forward speed is not.

  • $\begingroup$ retreating blade problem could be (has been?) solved with coaxial stiff rotors where the retreating side no longer needs to generate equal amount of lift as the proceeding side, e.g. Sikorsky S-97. $\endgroup$ – user3528438 Jul 16 '18 at 12:46
  • $\begingroup$ @user3528438 These rotors, however, still need to keep the proceeding blade subsonic. Various designs have been proposed to allow slowing the rotor enough to keep the proceeding blade subsonic (retractable blades in a disk/dome structure, halting/swept blades like in Sixth Day, etc. None have yet been built. $\endgroup$ – Zeiss Ikon Jul 16 '18 at 12:49
  • $\begingroup$ @ZeissIkon - to be fair, at least 2 stopped rotors designs have been built...they either just did not fly for programmatic reasons (Sikorsky S-72 X-Wing) or did not successfully transition between forward hovering and forward flight during flight tests (Boeing X-50 Dragonfly). $\endgroup$ – Marius Jul 16 '18 at 13:22
  • $\begingroup$ @user3528438 Kamov has been doing that for years (coaxial) but have not materially increased the speed of the aircraft. (For a variety of reasons). When S-97 is a reality, rather than still under development (yes, prospects are good) your use of that example will carry more weight. X-2 would have been a better example, given that it broke some records. $\endgroup$ – KorvinStarmast Jul 16 '18 at 18:52

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