Principles Of Flight Stability, Both Static And Dynamic And Report Your Findings
Principles Of Flight Stability, Both Static And Dynamic And Report Your Findings
Aeroplanes must be designed to overcome undesirable forces while in flight through inbuilt stability systems. Lack of stability systems in aeroplanes would not only expose the aeroplane to damage but also to a number of other serious flight risks such as crashes. In modernised aeroplane safety and flight stability systems, automation and refined technology have been implemented to ensure that flights are secure. Sensitive control systems have been designed in aeroplanes to facilitate tight motion sensor systems that aid detection and correction of instabilities during flights. The most important flight management and control information is based on stability principles that are mainly referred to as static and dynamic principles. Considerations of the stability elements as highlighted in the section below are useful in the formulation of appropriate stability systems in aeroplanes.
Static Flight Stability
A balanced position of an aeroplane in flight is termed as a trimmed position in terms of stability, which is affected by introduction of static instability forces. Static stability is the capacity of an aeroplane experiencing static disturbance to revert to its trimmed state. An aeroplane is deemed to be in stability if there are no changes of acceleration towards a particular axis when in flight. When the aeroplane is exposed to static forces of disturbance, instability is experienced and the ability of the aeroplane to return to the static condition is determined by its capacity to overcome the disturbance and achieve the earlier stability (Ly 1997, p23). In the trimmed state, the aeroplane is generally in a stable condition that does not require extra input in order for it to be sustained in the condition.
The aeroplane must be installed with systems to assist it revert to the steady state after a disturbance, which creates a disorientation of the stable axis conditions. Along the longitudinal axis, the appropriate conditions are achieved by introducing elevator trim tabs which are engaged until the previous static condition is achieved (Pamadi, 1998). This is achieved by the aeroplane system where the input of the pilot is not needed because it is on a hands-free operation installed in the design of the aeroplane. To assist in the detection of the instability of the aeroplane after a static disturbance, there are two types of analyses systems that can be used in such systems. On one hand, there is the stick-fixed manoeuvre which detects disturbance and maintains the same position in response while stick-free system is designed to enable the elevator to position itself in the natural position. Moments around the centre of gravity introduce the disturbance due to the displacement from the steady state position achieved by static stability during flight. In attaining the static stability required in flight, the pilot determines the appropriate attitude and maintains the aeroplane in aerodynamic moments that achieve the steady state (Allerton 2009, p111). The moments are engaged by the use of three controls from which the stability of the aeroplane in flight is achieved.
Dynamic Stability
Dynamic stability involves achieving some form of aerodynamic equilibrium while the aeroplane is in flight. Dynamic stability system in an aeroplane is dependent on a number of factors among them how the steady state condition is maintained in the flight. According to Swatton (2011, p276), it is not necessarily possible for a statically stable aeroplane to be sequentially dynamically stable. Dynamic stability is the inbuilt capacity that an aeroplane has to overcome disturbances usually derived from its physical design. In terms of duration of the stability achieved under dynamic aspects, it is a long term equilibrium characterisation feature that enables flight capacity to overcome disturbance. Temporal aspects of aeroplane stability when exposed to various forces of disturbance are taken care of by the dynamic stability aspects. In order for dynamic stability to be achieved, the aeroplane must have achieved static stability as a prerequisite requirement for long term stability. Dynamic stability is therefore achieved upon the acquisition of static stability whose compromise causes instability of any other level.
Besides the design and make features that the aeroplane has towards tackling dynamic stability issues, the aeroplane must be equipped to meet stability across various speed levels as well as the altitude. It therefore implies that both longitudinal and lateral dynamic stability issues must be considered in an efficient stability system of an aeroplane (Swatton 2011, p276). In order to achieve these stability elements in terms of flight dynamic consideration, the following must therefore be deliberated on; static stability, momentum, angular velocity and damping moments. Momentum features that a dynamically stable system aims to achieve are mainly in line with linear velocity as well as mass needs while the plane is in flight against various disturbance forces Etkin and Reid, 1996, p46). Alternatively, angular momentum issues must be taken care of in the system for instance by handling angular velocity as well as moments as exposed to various planes of axes. In addition, dynamic stability would involve various static aspects of stability against a backdrop of various planes of movement while in flight to respond to trimmed state disturbances. Additionally, damping moments that the stability system must achieve are targeted at forces such as rolls, pitching and yawing.
As an illustration, a disturbance of dynamic stability of an aeroplane in flight occasions an oscillation which must be overcome through the dynamic stability system. This happens after a certain period of time has lapsed during which the disturbance is removed. The removal of the disturbance is usually in form of motion realignment in the period of time consumed. During this period of time, the aeroplane induces dynamic stability during which five states must be achieved alongside a positive static stability value. Dynamic stability could be negative, neutral, positive, dead beat positive or divergent negative values during which various oscillation conditions determine the stability of the aeroplane in flight (Swatton, 2011, p277).
Q2. Investigate and evaluate how this stability is achieved in modern commercial aircraft
Aeroplanes are equipped to overcome various instability situations experienced during flight through a number of means. The modern aeroplane stability systems are enabled to overcome disturbance that are caused by a number of causes which include; pilot control initiatives which cause material motion changes, power setting changes of the aeroplane power system, changes in airframe positions as well as external forces for instance gusts and air waves commotion (Cook, 2007, p183). To this end, according to the author, the aeroplane is equipped with stability mode systems for various longitudinal and lateral disturbances. The author highlights three longitudinal stability systems variously referred to as modes which include; roll subsidence mode, spiral mode as well as dutch roll mode.
Firstly role subsidence mode of dynamic stability that applies non-oscillatory response since it counters disturbance force that is generally in a rolling version. In order for this dynamic mode to be achieved, static stability is a condition that must be met. The operation of this mode is in the form of an exponential lag that is usually associated with rolling motion. Angular acceleration is usually experienced by the aeroplane when exposed to rolling motion type of disturbance. To overcome such disturbance, both the port wing and starboard wing attempt to resist the rolling motion and restores the steady state after a short period of time. The physical phenomena can be explained in a paddle damping response that results in a stabilising effect which makes the roll subsidence stabilising response to be referred to as damping in roll (Cook, 2007, p184).
Secondly, the spiral mode of dynamic stability of aeroplanes that are in flight is achieved in a non-oscillatory approach. Exponential convergence is involved in stabilising the aeroplane after a disturbance which takes a relatively longer period of time when compared to other modes. Attitude changes are associated with disturbance which is corrected by regaining the level attitude that sustains the stable position. Slow response to disturbance characterises this mode when in stable state while regaining stability is also slowly experienced when in a neutral or unstable conditions. Both roll and yaw impacts are overcome in a slow response when the mode is activated in the aeroplane in flight. Unstable conditions are characterised by a spiral descent which should be corrected to avoid crashing, which is activated by the slow response (Cook, 2007, p186). Due to the large period of time that the instability experienced is allowed before the regaining of stability, it is usually easy for pilots to position themselves for flight response.
Thirdly, the dutch roll mode employs an oscillation response mechanism that reacts to the disturbance exposed to the os axis of the plane. According to Cook (2007, p186), damped oscillation is corrected by the mode which tackles yaw and results in roll sideslip. This mode involves complex interaction for all axis movements. Disturbance that triggers this mode is usually overcome through the tailplane which is used to dampen the impact alongside the fin. The synergy of these applications is due to the fact that the fin is not in a good opposition to overcome the yaw along the axis. A torsional spring is placed along the os axis to respond to disturbance about the axis as determined by the aeroplane fin system. A yawing moment is generated by the mode through the torsional spring as detected by the fin when a disturbance is experienced by the aeroplane. The yawing moment is important in the restoration of stability in an aeroplane in flight when oscillatory forces of disturbance are launched from various angles against the aeroplane (Seckel, n.d, p103).
Static stability is achieved in different ways among which include designing the aeroplane in such a way that the centre of gravity is furthest rearward as possible. This has been achieved through relocation of the fuel to the rear parts of the airplane. The double delta wing has also been involved to develop stability in aeroplane configuration. Alternatively, canards are also placed on the front part of the aeroplane which achieves nose-up moments beneficial in supersonic plane models (USCFC n.d, para.4).
In order to develop a balance between directional and lateral stability elements of an aeroplane, it is important that the various stability aspects are engaged in the aeroplane design. As an illustration, the use of ventral fins in the plane design is very vital in the creation of the expected balance between directional and lateral stability. In such a case, the aeroplane is able to maintain stable conditions since disturbances overcome by dutch roll are also overcome (USCFC n.d, para.15). static stability is also enhanced through introduction of compressibility concepts in aeroplanes which achieve further movement of the aerodynamic centre towards the rear end thereby achieving high stability capacity (USCFC n.d, para.3).
References
Allerton, D. (2009) Principles of flight simulation. West Sussex, UK: John Wiley and Sons
Cook, V. (2007) Flight dynamics principles, Burlington, MA: Butterworth-Heinemann
Etkin, B. & Reid, L. D., (1996) Dynamics of flight: stability and control, 3rd Ed., West Sussex, UK: John Wiley & Sons
Ly, U. (1997) Stability and control of flight vehicle, Seattle, WA: University of Washington Press
Pamadi, B. N., (1998) Performance, stability, dynamics, and control of airplanes, New York, NY: AIAA Education Series
Seckel (n.d) Lateral-directional dynamic stability of the unaugmented blended-wing-body aircraft, [online] Available from <https://dspace.lib.cranfield.ac.uk/bitstream/1826/119/8/Chapter%206%20.pdf> [accessed 29 September 2011]
Swatton, P. (2011) Principles of flight for pilots, West Sussex, UK: John Wiley and Sons
USCFC (n.d) Dynamic longitudinal, directional and lateral stability, [online] U.S Centennial of Flight Commission, Available from <http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability_II/TH27.htm> [accessed 29 September 2011]
