What is oscillatory stability?
Joined: Mar 2005
Posts: 175
Likes: 0
From: UK
Oscillatory Instability -
The tendancy for an aircraft which is laterally but not sufficiently directionally stable, to recover from a sideslip with an increasing wallowing motion.
From my JAR66 Module 8 (Aerodynamics) notes, hope that helps
The tendancy for an aircraft which is laterally but not sufficiently directionally stable, to recover from a sideslip with an increasing wallowing motion.
From my JAR66 Module 8 (Aerodynamics) notes, hope that helps
Joined: Mar 2006
Aviation Qualifications: PPL
Posts: 365
Likes: 144
From: Vance, Belgium
This is a generic Physics concept:
A system with oscillatory stability is an oscillatory system whose interaction with the outer world tends to decrease the oscillation amplitude (or energy), and for a system with oscillatory instability the interaction with the outer world tends to increase the oscillation amplitude.
I think that PhilM refer to the "Deutch Roll".
The wings diedral increases the lateral stability of a plane, but it creates an oscillatory system on the rolling axis.
When the diedral angle is higher than some threshold, the energy given to this system by the sideslip air stream
is sufficient for making the oscillator instable, and thus the plane tends to have increasing rolling oscillations.
The rudder fin surface increases the directional stability of the plane and has a stabilizing effect on the rolling oscillator
by reducing the lift of the wing on the slip side and increasing the lift of the other wing.
Luc
A system with oscillatory stability is an oscillatory system whose interaction with the outer world tends to decrease the oscillation amplitude (or energy), and for a system with oscillatory instability the interaction with the outer world tends to increase the oscillation amplitude.
I think that PhilM refer to the "Deutch Roll".
The wings diedral increases the lateral stability of a plane, but it creates an oscillatory system on the rolling axis.
When the diedral angle is higher than some threshold, the energy given to this system by the sideslip air stream
is sufficient for making the oscillator instable, and thus the plane tends to have increasing rolling oscillations.
The rudder fin surface increases the directional stability of the plane and has a stabilizing effect on the rolling oscillator
by reducing the lift of the wing on the slip side and increasing the lift of the other wing.
Luc
Joined: Mar 2000
Posts: 8,571
Likes: 3
From: Arizona USA
Generally correct, Luc Lion, and you have perfectly described the unstable tendencies of early model long body Boeing 707 aircraft.
Now, the 'fix' was to first fit a 39 inch ventral fin, but then the height of the vertical fin was increased, and the ventral fin was eliminated on later aircraft, along with improvements of the yaw damper system.
But therein lies a slight problem.
IF you increase the height (area) of the vertical fin too much, the aircraft may then not meet the certification requirements concerning spiral stability.
Aerodynamics...may times a large tradeoff is made so certification can be completed sucessfully.
I really would like to take a few new generation jet transport pilots (both Commanders and First Officers) along on a flight in one of these really old long body 707's, and watch the astonished looks on their faces, when the yaw damper was switched off, during takeoff and landing, as it was required to be, in conformity with the checklist.
Throw in gusty crosswinds during the landing, and it could well become quite a wild ride.
It surely would be worth the price of admission.
Now, the 'fix' was to first fit a 39 inch ventral fin, but then the height of the vertical fin was increased, and the ventral fin was eliminated on later aircraft, along with improvements of the yaw damper system.
But therein lies a slight problem.
IF you increase the height (area) of the vertical fin too much, the aircraft may then not meet the certification requirements concerning spiral stability.
Aerodynamics...may times a large tradeoff is made so certification can be completed sucessfully.
I really would like to take a few new generation jet transport pilots (both Commanders and First Officers) along on a flight in one of these really old long body 707's, and watch the astonished looks on their faces, when the yaw damper was switched off, during takeoff and landing, as it was required to be, in conformity with the checklist.
Throw in gusty crosswinds during the landing, and it could well become quite a wild ride.
It surely would be worth the price of admission.
