0
$\begingroup$

In other words, what happens to vertical speed and rpms in vertical autorotation if I change the pitch of the blades of the gyro I use to fly...?

$\endgroup$
  • $\begingroup$ Please forgive a perhaps stupid question, but doesn't autorotation imply zero thrust? $\endgroup$ – a CVn Dec 7 '17 at 11:42
  • 1
    $\begingroup$ No. You have a thrust exactly as the weight of the gyro. That's why it descends (in vertical autorotation) with a constant speed, and not accelerating... Thanks to your answer, I've now realized that the thrust is constant, so I'm now editing the question, changing 'thrust' with 'vertical speed' $\endgroup$ – xxavier Dec 7 '17 at 11:52
  • $\begingroup$ The fact that it descends with a constant speed does not mean it provides thrust. A falling feather descends at constant speed too. When a helicopter autorotates, it converts potential energy (height) into rotational kinetic energy (gaining rotor rpm). When it flares to land smoothly, it transforms kinetic energy of the blades into thrust or lift. (bleeding rotor rpm) $\endgroup$ – qq jkztd Dec 7 '17 at 12:30
  • 1
    $\begingroup$ @qq jkztd Wrong again... There are two annular zones in the autorotating rotor, the driven one (which overcomes most drag and produces most lift) and the driving one (that absorbs energy from the upward-flowing air, converting it to power to drive the rotor). Both zones produce thrust, if it's true that the driven one (the outward ring) is responsible for most of it. $\endgroup$ – xxavier Dec 7 '17 at 13:25
  • 1
    $\begingroup$ @xxavier Please also consider giving the same advice to the person who illustrated this ,since it doesn't fit to your understanding of elementary physics concepts. $\endgroup$ – qq jkztd Dec 7 '17 at 15:25
1
$\begingroup$

Unless I am misinterpreting your question, if you add pitch [pulling collective in a helicopter] your RRPM [rotor RPM] will decrease, as it is being used to accelerate air down decent, if in a vertical decent the aerodynamics are screwy. Now if you have some forward speed increase Pitch trades RRPM for lowering vertical speed. Decreasing Pitch, allows you to trade altitude for RRPM giving you higher VS and higher RRPM.

$\endgroup$
1
$\begingroup$

For what it's worth, when conducting routine maintenance test flying, Autorotation is performed in order to check the auto rpm. Helicopter manufacturers specify an rpm range for autorotation. If the rpm is too high the pitch change links (pcl) are adjusted to increase collective blade pitch. If the auto rpm is too slow, pcls are adjusted to decrease collective pitch. most pilots and flight departments prefer the rpm to be at the high end of the range, allowing additional rotor energy options for the pilot. This technique must not be overdone else collective authority in the flare/touchdown will be adversely limited.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.