Last Updated on 4 years by teboo
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081 02 01 00 Speeds
081 02 01 01 Speed of sound
High temperature, high speed of sound so speed records are done in warm places to minimise drag. You go faster before you reach M1.0
081 02 01 02 Mach number
(01) Define ‘Mach number’ as a function of TAS and speed of sound.Mach Number is the ratio between true airspeed and the local speed of sound.
081 02 01 03 Influence of temperature and altitude on Mach number
(01) Explain the absence of change of Mach number with varying temperature at constant flight level and calibrated airspeed.Flying into warmer air – The LSS increases but also causes an increase in CAS therefore the Mach Number or ratio, does not change.
Chicken Tikka Massala. On ye fingers..!
Going up relative to CAS – AoA and CL remain constant. Pitch and climb angle reduce.
Going up relative to Mach – CAS decrease, AoA and CL increase. Pitch and climb decrease.
1 - risk of exceeding the maximum operation speed (VMO) when descending at constant Mach number;
2 - risk of exceeding the maximum operating Mach number (MMO) when climbing at constant IAS;
3 - risk of a low-speed stall at high altitude when climbing at a too low Mach number.
1 – As density increases so does dynamic pressure and IAS.
2 – As density decreases IAS decreases relative to TAS and therefore Mach Number rises.
3 – Density decreases requiring a higher AoA to achieve enough lift with a risk of passing aoaCRIT
081 02 01 04 Compressibility
(01) State that compressibility means that density can change along a streamline, and that this occurs in the high subsonic, transonic, and supersonic flow.The density changes due to the bunching up or expanding of the molecules at all speeds.
It is…
Reducing effective AoA but increases stall speed and changing energy flow reduces aCRIT.
081 02 01 05 Subdivision of aerodynamic flow
(01) X List the subdivision of aerodynamic flow:— subsonic flow;
— transonic flow;
— supersonic flow.
Subsonic < M1.0
Transonic – some local mach numbers are >M1.0
Supersonic – All Mach Number are >M1.0
Compressibility happens >MFS 0.4 ish
081 02 02 00 Shock waves
081 02 02 01 Definition of shock wave
(01) X Define a ‘shock wave’.
081 02 02 02 Normal shock waves
(01) Describe a normal shock wave with respect to changes in:
1 - static temperature;
2 — static and total pressure;
3 — velocity;
4 — local speed of sound;
5 — Mach number;
6 — density.
1 – Increase
2 – Loss because of heat.
3 – Reduced behind the shockwave.
4 – In high transonic region ML Mach local, slows from about M1.2 to M0.83.
(02) Describe a normal shock wave with respect to orientation relative to the wing surface.
Normal means at 90deg to – the y-axis to the x-axis of the wing.
(03) Explain the influence of increasing Mach number on a normal shock wave, at positive lift, with respect to:
1 — strength;
2 — length;
3 — position relative to the wing;
4 — second shock wave at the lower surface.
1 – More intense.
2 – Gets longer.
3 – Moves aft with pitching effect.
4 – Develops with pitching effect.
(04) Explain the influence of α on shock-wave intensity and shock- wave location at constant Mach number.
081 02 03 00 Effects of exceeding the critical Mach number (MCRIT)
081 02 03 01 Critical Mach number (MCRIT)
(01) Define ‘MCRIT’.
α,
aeroplane weight, more lift needed and more suction, speeding up the air lowering MCRIT
manoeuvres, Higher load factor, more lift needed.
centre-of-gravity, Forward CofG needs a larger downforce from the tail and more overall lift from the wing. More suction again…
081 02 03 02 Effect on lift
(01) Describe the behaviour of CL versus Mach number at constant α.
A slightly greater upwash and so CL increases slightly.
CL drops drastically after MCRIT CLMAX also decrease
(03) Explain the change in stall indicated airspeed (IAS) with altitude.
— speed versus drag ratio;
— specific range;
— optimum altitude.
A good increase in speed for not much drag increase relatively.
Better range – why?
Optimum altitude for the engines – why.
081 02 03 03 Effect on drag
(01) Describe wave drag.As Mach Number increases beyond MCRIT, drag massively increases before settling down a bit lower but still much higher than below MCRIT.
High mach numbers give a lot less lift for the drag.
— drag rise;
— instability;
— Mach tuck;
— shock stall.
— drag rise; Massive
— instability;
— Mach tuck;
— shock stall.
MDRAG DIVERGENCE is faster than MCRIT.
081 02 03 04 Effect on pitching moment
(01) Discuss the effect of Mach number on the CP location.Because of the upper shockwave pressure there is an initial pitch up moment, when the lower shockwave develops further back, it becomes a pitch down moment.
It is not progressive, rate and direction change.
Overall is nose-down.
Mach Tuck, is sudden nose down moment from reduced AoA of the horizontal stabiliser.
(03) X State the requirement for a Mach trim system to compensate for the effect of the CP movement and ‘tuck under’ effect.
A certification requirement. Speed restricted if unserviceable.
Automatically counters pitch changes.
081 02 03 05 Effect on control effectiveness
(01) Discuss the effects on the functioning of control surfaces.
If the shockwave is immediately in front of the hinge of a control surface the lack of pressure may render it ineffective. The may need to be locked out and spoilers used for roll.
081 02 04 00 Intentionally left blank
081 02 05 00 Means to influence critical Mach number (MCRIT)
081 02 05 01 Wing sweep
— MCRIT;
— effective thickness/chord change or velocity component perpendicular to the quarter chord line.
Increases MCRIT because Component of the relative air flow perpendicular to the leading edge is less then that of the free stream flow.
The effective thickness reduces as the flow parallel to the leading edge sees no camber.
— CLMAX;
— efficiency of and requirement for high-lift devices;
— pitch-up stall behaviour.
Reduces CLMax, more efficient due less drag but require high lift devices to deal with take off and landing. Pitch up at the stall due to tips stalling first.
081 02 05 02 Aerofoil shape
(01) Explain the use of thin aerofoils with reduced camber.
Higher MCRIT due to slow airflow above,
Keep a high MCRIT whilst keeping a thicker wing which increases margin between MCRIT and MDrag Div.
Flatter top and a negative camber below and an extreme camber at the rear.
Adv. Increased margins as above. Stronger due to being thicker. Good lift characteristics due round leading edge.
Dis. Unpredictable separation point. For low speed handling. Nose down pitching (more lift at rear), Impractical shape. Trailing edge devices tricky to fit.
081 02 05 03 Vortex generators
(01) Explain the use of vortex generators as a means to avoid or restrict flow separation caused by the presence of a normal shock wave.Re energise flow separation caused by normal shockwave.