31 Limitations Flashcards
What is the Limit Load
Maximum load expected during normal service
What is the Ultimate Load?
The failing load of the structure
What is the Factor of Safety and what is it for commercial AC?
- The ratio between the ultimate load and the limit load
- For commercial AC this is 1.5
What is the maximum and minimum Load Factor for a normal AC?
2.5 g and -1 g
What is the maximum and Load Factor for a Utility AC?
4.4 g
What is the maximum and minimum Load Factor for an aerobatic AC?
6.0 g and -3.0 g
What is the definition of Va?
‘The maximum speed at which a sudden, full-up elevator pitch can be made without exceeding the Limit Load Factor’
I.e. The max speed which can be flown where with increasing Load Factor the the AC will stall before it reaches the Limit Load Factor
Why is Va slower than the intersection of CLmax and Limit Load Factor?
This is due to when the AC pitches up rapidly, the tail plane has a downward velocity. RAF now comes from below creating a large upward load and load factor on the horizontal stabiliser.
To allow for this Va is slightly slower then the intersection of the stall speed and the limit load factor
How does Va vary with weight?
As an AC becomes heavier, Va increases
What is the effect of altitude on Va?
- As altitude increases, Va increases
- This is due to compressibility effects increasing the stall speed at altitude
What happens to the max load factor and the manoeuvre envelope with flaps extended?
- Flaps are only designed to work up to s certain speed
- The max load factor reduces to 2.0 g
- The manoeuvre envelope reduces and is limited by Vfe
Define Vb
- Design speed for the maximum gust intensity
- Speed at which the AC can withstand the greatest expected vertical gusts
- or - 66 fps at 20,000ft
Define Vc/Mc
- Design speed for maximum cruising speed
- Used to determine the structural strength of the AC
- If a shallow dive was caused from Vc/Mc then the AC would not exceed Vd
Define Vd
- Design dive speed
- Selected such that if an upset caused the AC to enter a shallow dive from Vc/Mc then Vd would not be exceeded
What is the definition of Operating Speeds?
The maximum speeds that an AC can intentionally be flown under CS25 and CS23
Define Vmo
- Maximum operating speed that must not be exceeded at any phase of the flight
- EAS which must be less than Vc
- At high altitude Vmo is replaced by Mmo which becomes the limiting factor
Define Mmo
- Maximum operating Mach Number
- At high altitudes this replaces Vmo as the limiting factor
Define Vne
- Velocity never exceed
- Applicable to smaller AC
- Indicated by the red radial line at the fast end of the yellow sector on the ASI
- 0.9 x Vd
Define Vno
- Maximum structural cruise speed
- Applicable to smaller AC
- Must not be greater than the lesser of Vc and 0.89 x Vne
- At the slow end of the yellow section and the fast end of the green section on the ASI
Define Vle
- Maximum landing gear extended speed
- Maximum speed the AC can be flow with the landing gear down and locked
Define Vlo
- Maximum landing gear operating speed
- Maximum speed the AC can be flow with the landing gear deploying
- Sometimes slower than Vle as the doors are not as strong when not locked
Define Vfe
- Maximum flap extension speed
- Maximum speed an AC can be flown with the flaps extended in prescribed position
What happens to the AOA on an aerofoil when it is hit with a sudden:
- Up Gust
- Down Gust
- Up gusts cause the AOA to increase
- Down gusts cause the AOA to decrease
What is the effect of altitude and the density ratio on the Gust Load Factor?
- Gust load factor decreases with increasing altitude
- Climbing at constant IAS, TAS increases with altitude
- The same size vertical gust will therefore increase the AOA by a greater amount at a lower altitude
- Therefore at a lower altitude, gust load factor is greater
What is the effect of Aspect Ratio and Sweep back on Gust Load Factor?
- High aspect ratio and straight wings have a much steeper CL to AOA curve compared to low aspect and swept wings
- Therefore the change in AOA caused by a vertical gust is much more significant on straight, high aspect ratio wings
- The gust load factor decreases with:
- Increasing sweep back
- Decreasing aspect ratio
What is Wing Loading?
Wing Loading is the weight per unit area of wing
What is the effect of Wing Loading on Gust Load Factor?
- Increasing Wing Loading, decreases Gust Load Factor
- I.e. A light AC with large wing has a higher Gust Load Factor
What is the effect of Airspeed on Gust Load Factor?
- An increase in airspeed, increases the Gust Load Factor
- An AC flying at a higher IAS has a lower initial CL value
- A vertical gust which increases CL by a specific amount, will have a greater percentage increase in CL for the high IAS AC
- E.g. A high IAS AC has an initial CL of 0.4 experiences a gust increasing CL by 0.2. This is a greater percentage increase than a low IAS AC with an initial CL of 0.9.
AC with a higher Gust Load Factor are more or less susceptible to turbulence?
More susceptible
The factors which tend to increase Gust Load Factor are:
- Low altitude
- High speed
- High aspect ratio
- Straight wings
- Low wing loading (i.e. Lighter with bigger wings)
What speed should be flown during turbulence?
Vra / Mra
What causes Flutter and how can it be delayed?
- Flutter occurs when the frequency of the aerodynamic-elastic couple matches the natural frequency of the structure
Delayed by:
- increasing stiffness (weight penalty)
- moving the wing mass (and therefore the flexural axis) forward
What is Control Surface Flutter?
Result of the inertia of the control surface interacting with the torsional twisting of its associated main surface
How can Control Flutter be prevented?
- Mass Balancing
- Moving the CG of the control surface onto or forward of the hing line
What should be done if control flutter is experienced?
Reduce speed immediately
