CGS Ground School Principles Of Flight Stability Crown

CGS Ground School Principles Of Flight Stability © Crown Copyright 2012 No Part of this presentation may be reproduced without the permission of the issuing authority. The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.

Stability A body will remain at rest or in uniform motion, unless acted upon by an external force. If a force is applied to a body, then stability is concerned with what happens to the body after the force has been removed.

Stability can be looked at under two headings: Static stability: The immediate reaction of the body after the disturbance. Dynamic stability: The subsequent reaction of the body.

Static Stability

Static Stability There are 3 possible states of static stability: 3. Positive static stability - body returns towards origin. Neutral static stability - body remains in new position. Negative static stability - body continues away from origin. Negative Displacement 1. 2. Neutral Disturbance removed Positive Disturbance applied Time

Static Stability These three states can be demonstrated as follows: Positive Static Stability: Force (disturbance) applied. On removing the force, the object returns towards its origin.

Static Stability These three states can be demonstrated as follows: Neutral Static Stability: Force (disturbance) applied. On removing the force, the object remains in the new position.

Static Stability These three states can be demonstrated as follows: Negative Static Stability: Force (disturbance) applied. On removing the force, the object continues away from the original position.

Dynamic Stability There are three possible states of dynamic stability: Displacement Positive dynamic stability: Force applied. Amplitude reduces Force removed: Object displays positive static stability initially then positive dynamic stability. Time

Dynamic Stability There are three possible states of dynamic stability: Displacement Positive dynamic stability: Amplitude reduces Or Oscillations cease (dead beat convergence) Time

Dynamic Stability There are three possible states of dynamic stability: Displacement Neutral dynamic stability: Force applied. Amplitude remains constant Force removed: Object displays positive static stability initially then neutral dynamic stability. Time

Dynamic Stability There are three possible states of dynamic stability: Displacement Negative dynamic stability: Force applied. Amplitude increases Force removed: Object displays positive static stability initially then negative dynamic stability. Time

Dynamic Stability There are three possible states of dynamic stability: Negative dynamic stability: Displacement Amplitude increases Or Motion diverges (divergence) Time

Dynamic Stability All of the previous examples showed positive static stability - that is the body initially returned towards its original position. However the dynamic stability was different in each case.

Aircraft Stability A designer will incorporate a number of design features to ensure that an aircraft has the correct level of stability: too little stability and the aircraft is difficult to fly; too much stability and it is difficult to manoeuvre.

Aircraft Stability - Directional The directional stability of an aircraft is dependent on the amount of keel surface behind the C of G as compared to that in front of it. Consider an aircraft fuselage: If the aircraft is disturbed from balanced flight, and the keel surface ahead of the C of G is greater than that behind, then the airflow striking the keel surfaces ahead of the C of G will be greater than that behind. This will cause the aircraft to continue to yaw away from the original heading (Negative Static Stability).

Aircraft Stability - Directional The directional stability of an aircraft is dependent on the amount of keel surface behind the C of G as compared to that in front of it. Consider an aircraft fuselage: However if the keel surface ahead of the C of G is less than that behind, and the aircraft is once again disturbed from balanced flight, the airflow striking the keel surfaces ahead of the C of G will be less than that behind. This will cause the aircraft to yaw back towards the original heading (Positive Static Stability).

Aircraft Stability - Directional. To provide sufficient keel surface behind the C of G most aircraft are fitted with a tail fin. The tail fin increases the keel surface and also acts as an aerofoil to increase stability. In balanced flight the tail fin has zero angle of attack. Being symmetrical, the "lift" from either side is balanced. When the aircraft is disturbed, the tail fin gives an angle of attack, which alters the balance of "lift“, returning the aircraft to its original heading. 0° A o A 8° A o A

Aircraft Stability - Directional To increase an aircraft's directional stability the designer can either: or or 1. Increase the size of the tail fin. 2. Increase the distance of the tail fin from the C of G. 3. A combination of both.

Aircraft Stability - Longitudinal stability is very similar to directional stability, except that it is the tailplane that provides the required stability. If the aircraft is disturbed in pitch, the tailplane angle of attack is altered, and a correcting "lift" force is produced, which returns the aircraft to its original attitude.

Aircraft Stability - Longitudinal Aircraft are normally flown at a small positive angle of attack. To ensure that they return to this attitude when disturbed, most aircraft have the angle of incidence of the tailplane set slightly lower than the angle of incidence of the mainplane. This is called longitudinal dihedral. 0° A o A 5° A o A

Aircraft Stability - Longitudinal At 0° angle of attack, the tailplane produces a down load, returning the aircraft to a small positive angle of attack. At small positive angles of attack the tailplane produces no load and is therefore stable. At larger positive angles of attack the tailplane produces an upload, returning the aircraft to a small positive angle of attack. 0° -5° 8° A A oo A A 14° 0° Ao o. A A 5° A

Aircraft Stability - Longitudinal To increase an aircraft's longitudinal stability the designer can either: or or 1. Increase the size of the tailplane. 2. Increase the distance of the tailplane from the C of G. 3. A combination of both.

Effect of C of G position We have seen that both longitudinal and directional stability are affected by the distance of the tail fin / tailplane from the C of G. The position of the C of G for an aircraft is not fixed, but alters with changes in crew weight, fuel etc. Aircraft designers give a range of C of G that is permissible for flight (Vigilant 271 - 427 mm aft of datum). Flying outside of these limits will cause stability problems. C of G inside limits - aircraft stable. C of G forward of limits - aircraft too stable. C of G rear of limits - aircraft unstable. 271 mm 427 mm Aft of datum

Aircraft stability - lateral There are four main considerations regarding lateral stability: 1. 2. 3. 4. Wing dihedral effect. Swept wings. Size of keel area above C of G. Wing position.

Lateral stability - dihedral The different angles ofthe attack and If Now thewings compare wingsare aretherefore the inclined airflowat upwards over each from wing theoflateral sideaxis they producing different amounts of lift. are slipping referred aircraft. to as having dihedral. The lowerof wing has more lift, the wingthe has lesstip lift, Consider Because an the aircraft dihedral in straight angle, flight: a higher point near wing this causes the aircraft to in roll to a wings level. If will the be aircraft higher isthan disturbed a point near roll the side fuselage. slip will (A develop. is higher This of positive staticpath stability. This than is causes B, an D example higher a change than C). in the flight of the aircraft, and therefore Because of a change the sideinslip, thethe relative airflow crosses the wing at an angle, A to B (high to low) and C to D (low to path high). t h g i l F Sideslip A Dihedral angle C A B D B C D

Lateral stability - sweepback A swept wing has a lower CL than an unswept wing. The greater the sweep back the lower the CL. If the aircraft is disturbed in roll, a sideslip will develop as before. The effective wing sweep angle for the lowered wing is reduced whilst that for the raised wing is increased. The lowered wing therefore has a higher CL than the raised Side wing and produces more lift. slip The aircraft therefore returns to wings level, (Positive Static Stability).

Lateral stability – keel area If the aircraft is disturbed in roll and a side slip develops the relative airflow will strike the side of the aircraft. If the keel area above the C of G is greater than that below it, then the aircraft will return to wings level (positive static stability). The reverse would be true for a larger keel area below the C of G than above. A large tail fin will help to prevent this situation from occurring.

Lateral stability – wing position If the aircraft is disturbed in roll and a side slip develops, then the airflow striking the side of the fuselage will either flow over or under it. For a mid-fuselage wing this has no effect. However on a high winged aircraft the lower wing has a different relative airflow than the upper wing, increasing its angle of attack, and therefore its lift, returning it back to wings level. A high wing therefore gives positive static stability. The reverse is true of a low wing, which will show negative static stability, rolling further away from wings level.

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