Timeline for Why is *vertical* autorotation in a helicopter not recommended?
Current License: CC BY-SA 3.0
12 events
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| Aug 16, 2017 at 21:58 | comment | added | supercat | @Koyovis: To create lift, the rotor must create a pressure differential between the air above and the air below. If the pressure of the air immediately below the rotor is greater than the pressure of air below that, such difference will cause the air below the rotor to accelerate downward. Can an unpowered rotor reduce the pressure of air above without increasing the pressure of air below? | |
| Aug 16, 2017 at 5:41 | comment | added | Koyovis | @DavidSchwartz If the air flows through the rotor fast enough, the lift is obtained by decelerating the air - outflow is in the same direction as inflow. | |
| Aug 16, 2017 at 5:09 | comment | added | David Schwartz | @Koyovis Any lift the helicopter is getting comes from pushing air down. If that lift is coming from the rotors, then the rotors are pushing air down. The helicopter will be embedded in a block of downward moving air, carrying it down with the air. | |
| Aug 16, 2017 at 4:26 | comment | added | Koyovis | @DavidSchwartz To be in a pure windmilling state, the helicopter has to descend downwards with quite a high velocity. Obviously that is not desired - VRS is the region between hover and windmilling, and in order to reduce the rate of descent as much as possible the pilot will stay as close as he can to the VRS region. There is usually a little induced down flow through portions of the rotor disk, although most of the flow will be upwards. ref | |
| Aug 15, 2017 at 22:11 | comment | added | TomMcW | My reasoning might be flawed, but the upward moving air toward the center of the disc is the very thing creating the rotation. If you increase pitch enough to create a significant downwash then the rotors will slow. AFAIK, vrs occurs when attempting to hover right at the performance limits, therefore at a high aoa. | |
| Aug 15, 2017 at 22:10 | comment | added | TomMcW | @supercat I'm not saying it's best to go straight down. Translational motion makes rotors more efficient in all situations. I'm just saying a vrs is not possible in ar. | |
| Aug 15, 2017 at 21:20 | comment | added | supercat | @TomMcW: Even though air is moving upward through the rotors during auto-rotation, the helicopter is still going to be disturbing the air underneath it. Efficient lift generation requires entering undisturbed air. | |
| Aug 15, 2017 at 19:30 | comment | added | TomMcW |
Here. I found a source for that: It is not possible to enter the vortex ring state whilst the helicopter is in autorotation. skybrary.aero/index.php/Vortex_Ring
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| Aug 15, 2017 at 19:29 | comment | added | TomMcW | I don't think it's physically possible to enter a VRS in autorotation | |
| Aug 15, 2017 at 17:11 | comment | added | David Schwartz | A simple way to understand the problem is this: To get lift, the rotor has to push air down. So there will be descending air just below the rotor. If you descend straight down, you're staying with that descending air, and it will carry you down with it. Flying forward gets you out of the descending air the rotor created so it can't push down on you as much. | |
| Aug 15, 2017 at 14:32 | review | First posts | |||
| Aug 15, 2017 at 15:03 | |||||
| Aug 15, 2017 at 14:29 | history | answered | user24860 | CC BY-SA 3.0 |