PPRuNe Forums - View Single Post - VNE of light helicopter, altitude reduction ?
Old 26th Jun 2017, 19:34
  #24 (permalink)  
NickLappos
 
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Vne is set during the design and test phase, and is the result of a mix of possible things. There is no reason why a manufacturer has to go to a limit to make a Vne, it could be just where they stopped testing and therefore where the public cannot exceed.


The FAA requires many points to be tested at 1.11 times Vne, so if a particular trait is going bad (vibration, stick stability, strong stresses on the rotors) the manufacturer will stop at a value of .9 times the trouble spot and declare that Vne.


It is not possible to tell what makes the Vne of any aircraft, unless you talk to the test team who did the work at the time. In fact, stopping at a given speed is sometimes done because the fight time and cost of diving to higher speeds, and then climbing back up to the next point, is so high that there isn't a reason to make a big hairy chested maximum speed when the customer can get everywhere he needs to go at a lower speed.
One thing is that once Vne is set, many calculations are made using it, and the stresses at that Vne help determine the component lives and overhaul times, so it is almost always true that Vne drives component lives. It is obvious, if you use Vne stresses for the calculated life, then Vne sets the life.


The Vne chart is what the pilot uses, so it uses Calibrated Airspeed and thus will drop as altitude increases. Of course, the density drops and true airspeed achieved uses less CAS. I say CAS and not IAS because the CAS uses no instrument error. Use the error placard to adjust your IAS to CAS if you decide to bounce near Vne regularly. The typical drop in Vne with altitude is also a bit more that just density because the manufacturer wants the lives calculated to be based on more or less constant stresses, so as retreating blade stall is peeking out at higher altitudes, the Vne drops a bit more than just CAS to keep the stresses and lives where they were at lower altitude.


For some helicopters, Vne is set by handling where the "longitudinal stick stability" becomes a problem, for others it is vibration, for others it is rotor speed performance, and for some it is rotor stall.
Basic rule: Don't exceed Vne, unless you are drawing Test Pilot paychecks.
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