Originally posted by Dynamite Dean
2.What are the effects of compressibilty - how do I answer this one?
Compressibility is basically the effect of Mach number. The following are the main aerodynamic effects of Mach number.
In the low to mid Mach number range - up to say M0.6-M0.7, depending on the design of the aircraft - there are few effects and these can be safely ignored for practical considerations. One would consider an aircraft where this applied to be purely 'subsonic'.
As Mach number increases further we enter the 'transonic' region. This is where the local flow on various parts of the aircraft is approaching sonic velocity - typical areas where this will happen are at the point of maximum wing thickness on the upper surface, as the flow is accelerated over the wing, and also near the first area of increased fuselage cross section (near the cockpit for most designs) again due to flow acceleration.
As the flow in various places approaches then becomes sonic some or all of the following will be happening:
Increased drag - the so called 'drag rise'.
The drag for a given lift or angle of attack will increase pretty dramatically as the flow becomes sonic over the wings, in the case of most transport aircraft this drag rise will impose a performance-based maximum level speed limit.
Changes in handling At transonic speeds the formation of shocks on the flying surfaces will drastically change the effectiveness of the control surfaces; trailing edge surfaces become rather less effective when located behind a shockwave. Marked reduction in roll control power from ailerons would be one example, and spoilerons or differential tail deflection (on fighters, typically) are used to provide a compensating increase in control power.
The shock system also changes somewhat the underlying aerodynamic stability characteristics of the aircraft; the static margin will increase massively as the aerodynamic centre moves aft, classically moving from about 25% chord to 50% chord or so. This makes the aircraft far more longitudinally stiff, and coupled with loss of elevator power can cause significant control problems, especially if significant pitching moment changes occur. Many early fighters were lost in transonic dives due to loss of pitch control effectiveness.
The aircraft will also become markedly less directionally stable in most cases - the fuselage destabilising effect becomes far greater and the ability of the fin to compensate does not increase correspondingly. That's why you see the following design features on some supersonic aircraft: very large fins (sized for the transonic/supersonic case), multiple fins (which are proportionally more effective than a single fin transonically/supersonically) and deployable fin surfaces (XB-70 wing tips, MiG-23/27 ventral fin)
5.What effects AoA at the stall - I didn't think that AOA could be effected at the stall only the speed
The stalling angle of attack is affected by many things, including the folllowing:
Aircraft configuration i.e. deployment of flaps and/or slats. All other things being equal, increasing the camber of an aerofoil by deploying flaps will reduce the stalling angle of attack. Likewise, deploying slats will increase the stalling angle of attack. If you mentally picture the classical "fish-hook" CL-alpha plot, flaps cause the line to move upwards and slightly left (less alpha) while slats extend the line upwards and to the right.
Control activity Use of wing-mounted control surfaces will also modify the angle at which the flow stalls on the local area of the wing. So trailing edge down aileron causes that section of the wing to stall at a lower angle of attack, while t/e delays the stall slightly. Spoiler deployement, by unloading that section of the wing, will slightly delay the stall AOA in a like manner. This is why it is not possible to raise the heavy wing in the 'normal' fashion close to the stall; you're actually making things worse, not better, with control activity.
Mach number The stalling characteristics of an aerofoil are influenced by the Mach number. At higher mach the wing will stall at a lower angle of attack.
Reynolds number The flow behaviour is very dependent upon Reynolds number, which is a measure of how easily the flow will trip from laminar to turbulent flow (crudely). This dependence cannot be simply expressed, because it can result in very dramatic changes to the type of stall experienced, depending on the actual Reynolds number.
The
practical effect for an aircraft of the Mach and reynolds number variations show up in the variation of stalling behaviour with altitude, load factor, weight, etc.
Altitude affects the behaviour because at higher altitudes a (constant) stalling speed would result in a higher Mach at higher altitudes; the Mach effect would then reduce the stall AOA. The increase in altitude will also reduce the Reynolds number for a given speed. Combined, this makes the efect of increased altitude to be a reduced stall AOA.
Weight and
Load Factor ('g') both work in a similar fashion. Increase in either requires the generation of more lift, and so a higher speed is required. This speed increase will result in a higher Mach and higher Reynolds number. Generally the higher Mach number effect will dominate, and a lower stalling AOA will result.
In all of the above when talking of stalling angle of attack we are referring to the actual aerodynamic stall of the wing. To further muddy the waters this may not be what is seen by the pilot to be stalling AOA.
On aircraft with stall protection systems, the aircraft may have a stall which is defined by operation of e.g. a stick pusher. The operation of this is determined by the logic built into the system, and this may not incorporate all of the above effects. For example, while use of configuration, MAch and/or altitude for SPS inputs is common, few if any manufacturers will use control wheel (aileron). The pusher firing angle is instead set to allow for variation in control activity and demonstrated to be safe for these. It will therefore appear to the pilot that stall AOA is not dependent on control activity - because the artificial system is masking that variation.