Presentation on theme: "Aircraft Stability and Control AE 1350 Lecture Notes #11"— Presentation transcript:
1 Aircraft Stability and Control AE 1350 Lecture Notes #11
2 We will study What do we mean by aircraft stability and control? Static and Dynamic StabilityLongitudinal, lateral and roll stabilityNecessary Conditions for Longitudinal stabilityStability MarginRelaxed Stability Margin
3 A system is said to be stable if it can recover from small disturbances that affect its operation. A cone restingon its base isstable.UnstableNeutrally stable.Assumes new positioncaused by the disturbance.
4 An aircraft is subjected to some disturbance, say a gust, a cross wind or turbulence FreestreamUnexpected GustWill it recover automatically, without pilot’s intervention,and resume its original direction of flight?If so, the aircraft is longitudinally stable.
5 Longitudinal Static Stability The initial tendency ofthe vehicle is to bringthe nose up. If so,The aircraft is staticallyunstable.AlphaAlphaTimeTimeThe initial tendency of the vehicleis to bring the nose down. If so,The aircraft is statically stable.Aircraft is insteady level flightGust pitches the nose up
6 Aircraft may be statically unstable, but dynamically stable Over a longperiod,vehiclerecovers.Dynamicallystable.Initial tendency may be to pitch the nose upStatically unstable.
7 Aircraft may be statically and dynamically stable Gust pitchesnose upInitial tendency and long-term tendency both are to recoverfrom a gust or disturbance
9 Condition for Static Stability + dLAircraft c.g. (center of Gravity)The gust generates a small clockwise Moment about c.g. dM, and asmall positive additional lift dL.For static stability, if dL is positive (upward gust),dM must be negative, causing the nose to drop.Otherwise the wing will pitch up further increasing lift.dM/dL must be negative for static stability.
10 Nondimensionalization Lift and pitching moment M are usually non-dimensionalized.L is divided by [1/2 r V2 S] to yield CLM is divided by [1/2 r V 2 S c] to yield CMHere c is a reference length, e.g. average chord.From the previous slide, dM/dL must be negative for static stability.In nondimensional form, dCM/dCL must be negative forstatic stability.The quantity -dCM/dCL is called the static stability margin.Notice the negative sign.The more positive it is, the more longitudinally stable the aircraft.
11 How can a Designer Ensure Longitudinal Static stability? LiftAircraft c.g.Rule #1 : Place the c.g. as far forward as possible. This willcause the nose to drop, if lift increases due to a gust, reducinga, and lift. The opposite will occur if there is downward gust.
12 How can a Designer Ensure Longitudinal Static stability? Tail LiftAircraft c.g.Rule #2 : Place the horizontal tail as far aft as possible. This willcause the nose to drop, if there is a vertical gust, reducinga, and lift. The opposite will occur if there is downward gust.A canard is a tail upstream of the c.g., statically unstable!
13 The price paid for a large static stability margin The aircraft may become sluggish, hard to maneuver. The tail will resist the pilot’s attempt to change the aircraft “angle of attack.”A large tail adds to aircraft weight, and cost.A smaller tail will require a long fuselage(“ a long enough crowbar!”) to generate enough of a pitching moment to bring the nose up or down.Tail generates drag, including wave drag!
14 Horizontal Tail in Steady Level Flight needs to produce a download to balance all moments. Tail LiftAircraft c.g.The wing produces a counterclockwise moment about the c.g.The tail will have to produce a clockwise moment about the c.g.These two moments (I.e. force times distance) must roughly balance.The wing has to generate enough lift to overcome the weight + Tail lift
15 Relaxed Static Stability For improved maneuverability, some fighter aircraft sacrifice the static stability margin.Some fighter aircraft are statically unstable.Their nose will continue to pitch up, the lift will continue to go up when a upward gust is encountered. Result: A/C will stall, flip over.These aircraft must be actively controlled by the pilot, or an onboard computer.Redundant computer systems are present in case a computer based flight control fails.
16 Directional Stability Freestream comes from pilot’sright side, due to cross wind.It causes nose to rotate to leftviewed from the top.The force on the tailcauses the aircraftto rotate back tooriginal direction.A cross wind may cause the nose to rotate about the vertical axis,changing the flight direction.The vertical tail behaves like a wing at an angle of attack,producing a side force, rotates the aircraft to its original direction.All of this occurs without pilot action or intervention.
17 Why twin tail? Some fighter aircraft have twin tails. Each of the tails may be small, reducing radar cross section.Alternatively, twice the surface means twice the amount of side force that can be generated, giving good directional control.Disadvantage: Cost of manufacturing, weight go up.
18 Lateral StabilityIt is the ability of the aircraft to recover from a roll without pilot’s intervention.If the wing is tilted upwardsfrom root to tip, it has adihedral.Dihedral is good forlateral stability.
19 Anhedral If the wing dips down from root to tip, it has an anhedral. Anhedral is bad forlateral stability.
20 What happens when the aircraft undergo a roll? LiftLiftA portion of the liftis pointed sideways.The vehicle moveslaterally.This is called sideslip.
21 During sideslip, a relative wind flows from right to left This wind hasa component normalto the wing onthe right, viewingfrom the front.This is an upwash.The upwash increaseslift on the right wing.A downwash occurson the left wing,reducing lift.As a result, the aircraft rights itself,and recovers from the roll.