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Is there a difference between Helicopter and Airplane SAS?

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Is there a difference between Helicopter and Airplane SAS?

Old 15th Sep 2017, 12:43
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Is there a difference between Helicopter and Airplane SAS?

I don't know if this is the proper place to post this but are there any specific features that stability augmentation systems have in a helicopter compared to an airplanes? Thanks.
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Old 15th Sep 2017, 15:45
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Depends on the breadth of the question and whether you truly mean a SAS that just addresses short-term rate damping? Probably the first point is that most aeroplanes without swept wings do not need a SAS as they rely on natural stability. Swept wing aircraft will typically have a yaw damper to counteract Dutch Roll, but nothing for pitch and roll axis, whereas a helicopter SAS will have stabilisation in pitch, roll and yaw as there is no natural dynamic stability. More advanced ones may also include the collective and have gust response modes to enhance hovering.
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Old 15th Sep 2017, 22:36
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A basic plane will fly without any SAS and also without a pilot, because it is dynamically stable in all axes.

A helicopter will NOT fly without a pilot because it is dynamically unstable in pitch and roll. The pilot has to fly it all the time, so if any SAS is fitted it would be working like a cut snake all the time. And for a chopper, they are expensive, which means you usually won't see one in anything that isn't IFR or the size of an AW109 or better.

"An airplane will fly, but a helicopter must be flown."
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Old 16th Sep 2017, 06:48
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you usually won't see one in anything that isn't IFR or the size of an AW109 or better.
or a British military Gazelle (not the AAC ones)
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Old 16th Sep 2017, 11:06
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are there any specific features that stability augmentation systems have in a helicopter compared to an airplanes?
To answer the specific question:

A panel, with a SAS ON/OFF button. A Force Trim switch, so that the SAS and autopilot are permitted to control the cyclic, pedals, and collective.

If joined to an autopilot, an ALT button, HDG button, YAW button, VS (vertical speed)button and some more things to play with if it a 4-axis setup, connected to FMS and so on.
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Old 16th Sep 2017, 12:56
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Cybetech - a SAS will need sensors (usually gyros) and linear actuators in the control runs to do its job.

The linear actuators are fast acting but with limited authority (typically 10%) so that a runaway can't produce a 'hard-over' condition.

The actuators are just a motor and a screw arrangement that can either shorten or lengthen the control runs between the flying controls and the main jacks.

A rapid control movement or disturbance (a gust of wind for example) will alter the aircraft attitude, be sensed in rate by the gyro which sends a signal to the SAS computer which opposes the movement using the linear actuator in the appropriate control run.
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Old 16th Sep 2017, 20:33
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or a British military Gazelle (not the AAC ones)
Apart from the ones that used to deploy as part of AMF(L), I believe. They belonged to 2 Flt at Netheravon.
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