Ch 31 - Limitations Flashcards
Limit Load
The maximum load expected in surface
Ultimate Load
The failing (breaking) load of the surface
Based on a factor of safety
Safety factor is usually 1.5 which is the expected maximum load it can take before failing
Factor of Safety
The ratio of the ultimate load to the limit load
Usually 1.5 for AC
AC Load Factors - Limit Load Factors and Ultimate Load
Limit Load. 1.5 Ultimate Load
+VE. -VE. +VE. -VE
Normal. 2.5. -1.0. 3.25. -1.5
Utility. 4.4. -1.76. 6.6. -2.64
Aerobatic 6.0. -3.0. 9.0. -4.5
The Manoeuvre Envelope
The performance boundaries necessary for safe flight
Vs1g
Stall speed at 1g
Va
The design manoeuvre speed - where you will stall before you bend the AC
Won’t bend the AC no matter how hard you pull, you will just stall
Va = Vs1g x the square root of the limiting load factor
Effect of Weight on Va
A 10% heavier AC will have a ~5% increase in Va
Va - the design manoeuvre speed
Use the 3 step process
Compressibility on the Manoeuvre Envelope
Altitude simply rounds off the edges of the V-n graph
High altitudes, the reduction in density causes compressibility effects which increase the stall speed and therefore Va.
Altitude poses a problem for the envelope
The Effect of Flaps on the Manoeuvre Envelope
Greatly restricted when the flaps are extended. The flaps are only designed for slow speed flight therefore the maximum load limit is 2g for transport AC.
Stall speed increases though
CP moves back towards the flaps (camber changing)
Vb - Maximum Gust Intensity
The design speed for maximum gust intensity
The speed at which the AC can withstand the greatest expected vertical gusts ±66fps at FL200
Normally found between Va and Vc
Can withstand 50ft/s gusts
Vra
The speed for normal turbulence conditions
Found between Vb and Vmo/Vc
Vc
The design cruise speed
After this speed you will start to over stress you AC which we wont fly near
Vertical gust of 50ft/s
Vmo
The normal operating speed which we will never exceed (even in an emergency descent)
Vd
Design Die Speed
The speed at which the AC will fall apart
FL200 with gust Requirements of 22fps
Vmo
The maximum operating speed which must not be intentionally exceeded
Mmo
The maximum operating speed at altitude. Must remember to change to Vmo on the descent as your Mach speed will remain constant whilst you IAS will increase (you may go over your Vmo)
Vne
The never exceed speed for Light AC
Vno
The normal operating speed for light AC (cruise speed) the slow end of the yellow or the fast end of the green band
Vle
The maximum speed at which you can operate with the landing gear extended (down and locked)
Vlo
The maximum landing gear operation speed which is usually lower than Vle
Vfe
The maximum speed that the AC can fly with flaps extended in each prescribed conditions
Gust Loads
Gust loads are the sudden extra loads felt when an aeroplane flies through a vertical or horizontal velocity gradient (I.e turbulence)
Gust Load Factor
Load Factor = Lift / Weight
Lift is proportional to the coefficient of lift so a change in AoA has a large effect on the Load factor
Factors that affect the gust load factor:
Density ratio and altitude
Cl relationship to AoA
Wing Loading
Airspeed
Density Ratio and Altitude on Gust Load Factors
When climbing at a given IAS the TAS increases as the altitude increases. This is because the density ratio decreases with alt.
As the altitude increases, the gust load factor decreases. This means that the AC is subjected to smaller loads at higher altitudes.
Cl relationship to AoA on Gust Loads
Depends on aspect ratio and sweep back
Straight wings experience greater gust loads than swept wings
Higher aspect ratio Wings experience greater gust loads
A low aspect ratio, swept wing will experience a significantly smoother right than a straight wing high aspect ratio aeroplane
Wing Loading
The weight per unit of wing area.
An AC with a higher wing loading has a greater Cl for that uni of Aerofoil.
Gust load factor is greater in light aeroplanes with relatively lower wing loading.
Airspeed on Gust load Factors
Gust load factor increases as airspeed increases.
Gust Load Factor Summary
AC with higher gust load factors are more sensitive to turbulence. Factors that increase the gust load factors are;
Flying at low altitude
Flying at high speed
Straight wing
High aspect ratio wing
Low wing loading (large wing area with lighter weight)
Aerolasticity
If aerodynamic forces acting on a structure become large enough, they can cause the structure to twist about its torsional axis. The happens because all structures are elastic to a greater or lesser degree.
As airspeed, and thus aerodynamic force increases, so too will the opposing elastic forces. Integration known as aero-elastic coupling
Can cause the structure to fail if pushed too far - torsional divergence speed
Must be faster than Vd/Md
Flutter and Resonance
Natural frequency is the frequency a structure will oscillate at if disturbed (can have several)
Subjecting a structure to a particular frequency of vibration is known as the forcing frequency - if this is substantially different form the natural frequency, it will only cause a small vibration.
As it approaches the natural frequency, the vibration will increase. When equal, the vibration will be very large - known as resonsant frequency.
When the wing/tailplane resonate, the behaviour is known as flutter.
Could lead to failure
Flutter Prevention
Greater stiffness
Or moving the mass distribution - the flexural axis forward - underslung engines positioned forward which damps the potential oscillations
Control Surface Flutter
Occurs at high speed, the only remedy is to reduce the speed immediately
Caused by the inertia of the control surface interacting with the torsional twisting of its associated main surface,
Factors Influencing Flutter
Elasticity
Backlash
Aero-elastic Coupling
Mass distribution
Mass distribution
Structural Properties
IAS
Aileron Reversal
Low speed aileron reversal - when you are travelling close to the stall speed, downgoing aileron may stall the wing which causes opposite roll
High speed aileron reversal - torsional twisting when you deflect the aileron downwards, it actually causes the upgoing lift to go down
