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As preparation for my CFI checkride I have been trying to understand aerodynamics very well. One of the things I’m struggling with is stability

If I have a stable aircraft, this means is less controllable and less maneuverable, right?

Do this means an F-16 is poorly stable but very controllable and maneuverable? And a Cessna or a B737 are stable with less controllability and maneuverability?

Thanks for your help guys

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  • $\begingroup$ Several answers to this question on here, but I don't know how to link them for you. Basically yes, stability and control are n opposition. $\endgroup$ – Mike Brass Jan 30 '18 at 6:06
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a control system is "stable" if, upon a perturbation (like a gust or up/downdraft), the system will naturally return to its unperturbed state with your hands off the controls.

a plane exhibits "neutral" stability if, upon a control input from the pilot or a gust perturbation, the plane will not right itself if the pilot takes his or her hands off the controls, but neither will it diverge and fly itself into a steeper turn, roll, or pitch attitude hands-off.

"negative" stability means the moment you take your hands off the controls, the plane will by itself pitch up or down, roll left or right, or skid one way or the other. it cannot be flown hands-off unless it has an artificial stability augmentation system built into its control hardware.

A control system is "controllable" if pilot input is successful in recovering from a perturbation. But without pilot inputs, a "controllable" system will not necessarily by itself recover from a perturbation.

Maneuverability refers to the airplane's overall sensitivity to control inputs from the pilot and the effectiveness of the control surfaces over their control range.

An airplane that is highly maneuverable MIGHT be unstable- that is, it MIGHT not fly hands-off because of how sensitive it is to control inputs or gusts. In general, a high degree of maneuverability usually is associated with marginal stability.

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This might help

Stability - Imagine a stick hold it at the top and then displace it. It swings and then hangs straight. This is a dynamically stable system. Now take the same stick and stand it on your hand, this is a dynamically unstable system and you have to make constant corrections to keep it standing. (aeroplanes - Dynamically stable / Helicopters - Dynamically unstable)

Control-ability - Capability of an aircraft to respond to the pilots control inputs. If an aircraft fails to respond to a pilots inputs (or responds n the opposite manner), then it lacks control-ability. Some examples are high speed control reversal, mach tuck, control lag through inertia, and control authority to dampen oscillations eg Dutch roll or spiral dive. The ability to recover from a spin (A/B ratio) or a stall.

Manourverability - the ability for the aircraft to commence and sustain maneuvers, its responsiveness (eg control effectiveness) and its performance eg rate of roll, or turn and pitch rates(G load)

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If I have a stable aircraft, this means is less controllable and less maneuverable, right?

enter image description hereImage source

Controllability: Above depicts a ball in a cup: the centre situation is a statically stable equilibrium, you need to actively deflect the ball from equilibrium and it will want to return to the centre if released. That is how you want the aircraft to be as well, statically stable. When the ball is released it will eventually return to rest in the equilibrium state after some number of overshoots: it is dynamically stable as well. If there was no friction and/or aerodynamic damping, the motion would continue forever: the system would be statically stable and dynamically neutral.

You definitely want a stable aircraft in order to be able to control it, an unstable aircraft is harder to control: the ball wants to roll away all the time, and you need to actively roll it back up the hill. How hard to control, that is a function of the time period of the unstable motion. In helicopters for instance, this is in order of 10 seconds: it is unstable in the hover, but the unstable motion is slow enough for humans to learn how to compensate for it. An F-16 at cruise speed has artificial stability, it is aerodynamically unstable but an active control system provides stabilising inputs without the pilot noticing them. So from the pilot's perspective, the aircraft is stable and relatively easy to control.

Manoeuvrability: Responses to flight control inputs are indeed a function of how aerodynamically stable the aeroplane is, but also of the control volume: surface area times distance from Centre of Gravity. Install a big enough elevator and a stable aeroplane will make lovely loopings. Aerobatic aircraft are aerodynamically stable and very manoeuvrable.

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