MatthewDB
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Power of control surface feel feedback

Mon Apr 08, 2019 7:17 am

This is a question I've long wondered about, but the talk about the events around the to 737 MAX crashes has brought it back up. From what I understand, with mechanical (cable) / hydraulic controls, the force feedback is entirely from a feel computer applying backforce on the controls. Is that correct, there is no force from the surface itself? If that's the case, why would it be so powerful that a pilot could get fatigued under some of the abnormal conditions like the MCAS caused out of trim? Either that, or some way to turn it down or off in those conditions where abnormal control surface positions are going to have to be maintained for the remainder of the flight?
 
thepinkmachine
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Re: Power of control surface feel feedback

Mon Apr 08, 2019 1:04 pm

The high force is there to prevent pilot from breaking up the airplane at high speed due to excessive control deflections - so you can’t just switch it off.

Airbus airplanes have no force feedback, but they have load factor protection in their FBW system instead.
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Starlionblue
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Re: Power of control surface feel feedback

Mon Apr 08, 2019 10:47 pm

The high force if out of trim is for exceptional situations only. You wouldn't normally be very far out of trim for very long. The assumption is that the situation will be resolved before the arms of the pilots fall off from fatigue.
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GalaxyFlyer
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Re: Power of control surface feel feedback

Tue Apr 09, 2019 12:05 am

The cables only transmit the pilot’s inputs to the hydraulic valves on the jacks to port 3000psi in the desired direction. If the pilot wants nose up elevator, the cable opens the valve to extend the hydraulic piston pushing the elevator in the trailing edge up position. Without feel systems, often on bungee cords, the pilot would have only position of the yoke and view out the windows to nudge the effect of the input. Feel Systems articially mimic the pressures that a pilot in s Cessna 172 gets directly from wind on elevator. Hydraulics can not duplicate the feedback from the controls.

GF
 
WPvsMW
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Re: Power of control surface feel feedback

Tue Apr 09, 2019 12:55 am

IIRC, (i) the Airbus sidestick is spring-loaded to neutral... not sure how much "feel" there is, and (ii) Boeing "feel systems" are hydraulic... not the high pressure hydraulics that actuate a control surface, but a low pressure feedback system on, e.g., elevator (pitch).
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Starlionblue
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Re: Power of control surface feel feedback

Tue Apr 09, 2019 2:28 am

WPvsMW wrote:
IIRC, (i) the Airbus sidestick is spring-loaded to neutral... not sure how much "feel" there is, and (ii) Boeing "feel systems" are hydraulic... not the high pressure hydraulics that actuate a control surface, but a low pressure feedback system on, e.g., elevator (pitch).
https://www.coursehero.com/file/p21eo6h ... peed-from/


The only feel is increasing force with increasing stick deflection, which makes sense if you think about it. Moving it a couple of millimeters takes hardly any effort. Holding it fully deflected takes a bit of muscle. However there is no feedback from aerodynamic forces on the surfaces themselves.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
spacecadet
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Re: Power of control surface feel feedback

Tue Apr 09, 2019 6:01 am

Starlionblue wrote:
The only feel is increasing force with increasing stick deflection, which makes sense if you think about it. Moving it a couple of millimeters takes hardly any effort. Holding it fully deflected takes a bit of muscle. However there is no feedback from aerodynamic forces on the surfaces themselves.


I haven't flown an actual Airbus but I have flown an A320 Level D simulator. I'd call the side stick feel very similar to a PC-based flight stick, maybe with a little less resistance.
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Starlionblue
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Re: Power of control surface feel feedback

Tue Apr 09, 2019 6:27 am

spacecadet wrote:
Starlionblue wrote:
The only feel is increasing force with increasing stick deflection, which makes sense if you think about it. Moving it a couple of millimeters takes hardly any effort. Holding it fully deflected takes a bit of muscle. However there is no feedback from aerodynamic forces on the surfaces themselves.


I haven't flown an actual Airbus but I have flown an A320 Level D simulator. I'd call the side stick feel very similar to a PC-based flight stick, maybe with a little less resistance.


Like the more expensive PC sticks, yes. The cheap ones have barely any resistance. It probably costs a bit more as well. :)

The armrest settings and arm position are very important. The entire armrest should be resting on it, so you only need your wrist to fly. If your arm position is off fine control becomes difficult.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WIederling
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Re: Power of control surface feel feedback

Thu Apr 11, 2019 8:54 am

Starlionblue wrote:
Like the more expensive PC sticks, yes. The cheap ones have barely any resistance. It probably costs a bit more as well. :)

Easy.
you get issues with transferring the reaction forces ( to counter higher stick forces ).
either the base of the joystick has to have more mass or it needs attachment to the table it sits on.
( you see these scution cup thingies on some.)
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spacecadet
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Re: Power of control surface feel feedback

Thu Apr 11, 2019 9:47 pm

I can tell you that airlines who fly Airbus planes often use Logitech Extreme 3D Pro joysticks in their PC-based testing setups and low-level simulators. I was pretty surprised when I first saw that - no specialized stuff built just for them and nothing all that high end, just an off the shelf Logitech stick. And I've seen that at multiple airlines and being used for multiple different purposes. That also happens to be what I have at home so that's what I was comparing the feel of the Level D sim to. The Extreme 3D Pro definitely has a bit more resistance than the side stick in the Level D sim, which I assume would be the same as the stick in the real plane.
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