Elevator hands off movement

[XPMorten]

Hi all,

For simulation purposes I have a question regarding elevators.

If you enter a longitudal sinus pattern in pitch and let go of the yoke, the
aircraft will stabilize after some time (periods).

- I'm wondering, will the elevator (hands off yoke) MOVE somewhat with
this motion due to the forces acting upon it?
- Or, will the elevator stay dead centered throughout the period.

I guess this might vary from acf to acf, so I'd be interested in
information on any types.

Cheers,

XPM

[Intruder]

The elevators will tend to move, but their movement will depend on the specific airplane and its bobweights, force-feel system, dampers, and other components of the flight control system.

[grob103]

I believe this depends on whether the controls are reversible or not.

Certainly on the gliders I fly with simple push-rod connections to the elevator I get direct feedback through the stick on buffeting and the like, and I would expect to see the stick move if it weren't in my hand. It certainly springs back to a different position if I let go of it when I'm out of trim.

However, I've never actually seen it oscillate, as when I'm hands-off I'm usually in stable flight trying to pee into a bag. :eek:

[Mad (Flt) Scientist]

Irreversible controls can still move in the circumstances described.

irreversible refers to the fact that the controls do not move in response to changed aerodynamic loads on the surface - so if I put the elevator in position A it stays there, until I decide to move it again, even if the aerodynamic hinge moment chnges, say due to a speed change.

But - unless the flight controls themselves are perfectly balanced, the changes in loads in the control system due to, for example, 'g' oscillations can cause an input to the controls.

Oh - and not all aircraft will stabilise hands-off, of course. Neutral stability in the Phugoid mode isn't that unusual, and isn't really a bad thing (low drag tends to reduce Phugoid stability, and low drag is generally a good thing). The Phugoid mode is sufficiently slow that it can be easily controlled even if mildly unstable.

For specific examples: I know of one case with an "irreversible" pitch control curcuit where, due to an unbalanced control system (a design oversight, it turned out to be), there was 'g' feedback to the controls and the aircraft was PIO-susceptible as a result. We had to redesign elements of the control system as a result.

[lefthanddownabit]

Elevator movement is due to two effects.

1. Changes in elevator aerodynamic hinge moment due to changes in local airflow as the aircraft pitches.

2. Mass unbalance effects in the control circuit, which can cause the controls to move under varying g loads. For example if the control yoke's neutral position is not vertical, or if there is a bobweight. The elevator itself might not be mass balanced and so move under g loads.

Hinge moment changes cannot effect irreversible powered controls. Mass unbalance effects can if they are "upstream" of the servo valve.

If you are going to account for these forces in the elevator, you might also want to account for similar effects in the rudder during yaw oscillations or sideslip for example.

[Mad (Flt) Scientist]

An important caveat about irreversible controls that applies both to my post, and lefthanddownabit's ...

No control is irreversible past the hinge moment limit of the PCUs, and even below that you'll see effects on control rates.

[XPMorten]

Thanks guy's :)

Followup question;

- What category's of aircraft typically have irreversable controls?
(GA, gliders, airliners, fighters, choppers etc)

Any airliner etc pilots that can confirm "force feedback" on the yoke?

XPM

[grob103]

Thanks for the useful corrections, guys. As is usual with engineering, the closer you look, the more complicated it gets. :)

Also very nice to learn about Phugoid stability. Reduction in drag would explain why pitch stability reduction is a desirable tradeoff in some gliders (Cirrus specificially comes to mind).

I love this forum, and I just bumble around in class D/G looking at the lovely fluffy clouds. :}

[lefthanddownabit]

Generally speaking:

Up until the 1950s everything had reversible manual controls. As aircraft got heavier and flew faster this had to change.

Large and medium sized modern airliners have irreversible controls. The 707 was an exception, being fully manual with a boosted rudder (all reversible).

All FBW aircraft controls are irreversible.

Small airliners with hydraulic power controls can have a mix of both types, depending on the design.

Fast military jets are probably all fitted with irreversible power controls. Lower speed military aircraft might vary.

Manual controls are all reversible, so that would include GA types and even some airliners (DC-9, MD-80, BAe 146 for example).

If an aircraft or helicopter has irreversible controls there is always some kind of artificial feel system, either springs or more complex hydraulic systems. Even the FBW Airbus has springs and dampers in it's sidestick.

To be certain you need to do a bit of research on the aircraft concerned.