The following notes apply only to three axis powered aeroplanes that have been tested and received type approval certification from a national regulatory authority. Owner designed and built aeroplanes do not go through the certification process and the builder would generally just rely on static load tests to prove the structure: nor would there be any flight testing program undertaken by a professional test pilot to determine the safe flight envelope and identify deficiencies.

Airframe strength and elasticity

Aeroplane structures are designed to have adequate strength and stiffness while still being as lightweight as possible. To receive type approval certification the design of a general aviation aircraft must conform with certain standards, among which are the in-flight structural load minimums for the category in which the aircraft may be operated.

In FAR part 23, the recognised world standard for light aircraft certification, the minimum load factors, which an aircraft at maximum take-off weight must be designed to withstand, are:

      +3.8g to –1.5g for the normal operational category;
      +4.4g to –1.8g for the utility category (which includes most training aircraft); and
      +6.0g to –3.0g for the aerobatic category.

At loads up to 50% greater – and imposed for 3 seconds – the structure may bend temporarily. At the ultimate structural load limitations (+6.6g and -2.7g for the utility category) permanent structural deformation is allowable but the aircraft should still be controllable. However there is an increasing risk of failure when exceeding the minimum load factors and each instance of excessive loading will compound the failure risk. (We use load factors in terms of g for convenience but what we are really considering is wing loading in terms of force per unit area; and remember that aerodynamic forces increase with the square of the velocity [lift = C_L × ½rV² × S ].)

Notes:
      1. It should not be thought that aircraft structures are significantly weaker in the negative g direction. The normal load is +1g so with a +4.4g limit then an additional positive 3.4g load can be applied while with a –1.8g limit an additional negative load of 2.8g can be applied.

      2. Many aircraft are type certificated in both normal and utility category in which case the MTOW when operating in the utility category would be about 15% less than that in the normal category – effectively the maximum wing loading, in terms of force per unit area, is the same in both categories.

Elasticity

All aircraft structures exhibit some degree of elasticity, that is they deflect a little – bending and/or twisting – under applied aerodynamic forces but normally spring back when the load is removed; this is particularly so with the wings and control surfaces. This aeroelasticity may lead to some problems at high speed but reducing elasticity means increasing rigidity, which usually involves an unwarranted increase in structural weight.

Wing structures are akin to a 'tuning fork' extending from the fuselage. When a tuning fork is tapped the fork vibrates at a particular frequency, the stiffer the structure the higher its 'natural' frequency. The natural frequency of a wing or control surface structure may apply another limiting airspeed to flight operations – related to structural instabilities: 'flutter' and 'wing divergence', which we will discuss below.

Weight and balance

There are fixed limits to the payload an individual aircraft may safely carry and that payload must be distributed so that the aircraft's balance – the position of the aircraft's centre of gravity – is maintained within calculated limits. In addition there is a maximum safe operating weight permitted by the aircraft designer, or by regulation.

The aircraft's weight and balance very much affect control and stability at high speeds: excess weight reduces the designed structural load limits while cg positions outside the designated fore and aft limits may enhance elasticity reactions to aerodynamic loads, or reduce controllability or delay (or prevent) recovery from unusual /high speed situations.

Standard airspeed limitations

If an aircraft is operated within its specified flight envelope, observing the limiting accelerations and control movements; and maintaining airspeeds commensurate with atmospheric conditions; then the only possibilities of inflight structural failure relate to:

• improper modification or repair of the structure

• excessive free play in control surface hinges, actuating rods or cables

• cumulative stress/strain in aging aircraft

• or just poor care and maintenance of the airframe.

Flight at airspeeds outside the envelope (or at inappropriate speeds in turbulent conditions or when applying inappropriate control loads in a high-speed descent) is risky and can lead to airframe failure. Vne is the IAS which should never be intentionally exceeded in a descent or other manoeuvre and is normally set at 90% of Vd, the 'design diving speed'. For a normal category aircraft, Vd is required to be 1.4 times Vno and, to receive certification, it must be demonstrated, possibly by analytical methods, that the propeller, engine, engine mount, and airframe will be free from overspeeding, severe vibration, buffeting, flutter, control reversal and divergence. To provide some safety margin, Vne is then set at 90% of the lower of Vd or Vdf. Vdf is a diving speed which has been demonstrated without problem in test flights and which must be lower than, or equal to, Vd.

Vne as a maximum airspeed applies only for smooth atmospheric conditions and for gentle control movements; even vertical gusts associated with mild turbulence or control movements greater than say 25% travel will lead to some nasty surprises, if operating close to but below Vne. At such high speed the controls are very effective with a high possibility for over-control applying extreme loads to the structures.

Be aware: deliberately exceeding Vne is the realm of the test pilot – who always wears a parachute!
 

Aerodynamic effects of flight at excessive speed

Flutter

When aerodynamic forces applied to the wing or a control surface alter the aoa, the dynamic pressure distribution changes. These changes plus the structure's elastic reactions may combine as an oscillation or vibration (probably initially noticed as a buzz in the airframe) which will either damp itself or, as the airspeed is increased, may begin to resonate at the natural frequency of the structure and thus rapidly increase in amplitude. This latter condition is flutter and, unless airspeed is very quickly reduced, will cause control surface separation within a few seconds.

Inertia has a role in flutter development requiring that control surfaces – ailerons, elevators, rudder – be mass balanced to limit the mass moment of inertia (and also to prevent them becoming heavier as airspeed increases). The critical flutter airspeed [or something akin to it] may eventuate well below Vd or Vdf if wear in control surface hinges, slop in actuating rods/cables/cranks/torque tubes, water inside control surfaces or other system weaknesses exist which alter the structure's reactions.

The following paragraph is an extract from an article by William P. Rodden appearing in the McGraw-Hill Dictionary of Science and Technology; it provides a succinct description of flutter:

"Flutter (aeronautics) – An aeroelastic self-excited vibration with a sustained or divergent amplitude, which occurs when a structure is placed in a flow of sufficiently high velocity. Flutter is an instability that can be extremely violent. At low speeds, in the presence of an airstream, the vibration modes of an aircraft are stable; that is, if the aircraft is disturbed, the ensuing motion will be damped. At higher speeds, the effect of the airstream is to couple two or more vibration modes such that the vibrating structure will extract energy from the airstream. The coupled vibration modes will remain stable as long as the extracted energy is dissipated by the internal damping or friction of the structure. However a critical speed is reached when the extracted energy equals the amount of energy that the structure is capable of dissipating, and a neutrally stable vibration will persist. This is called the flutter speed. At a higher speed, the vibration amplitude will diverge, and a structural failure will result."

Wing divergence

Wing divergence refers to a state where the aerodynamic twisting action on the wing structure, produced by the rearward position of the centre of pressure at very high speeds, further increases the moment, finally exceeding the capability of the wing/strut structure to resist it and causing the wing to separate from the airframe – with no warning! This could be brought about if a down gust is encountered at high speed.

Control reversal

As airspeed increases control surfaces become increasingly more effective, reaching a limiting airspeed where the aerodynamic force generated by the ailerons, for instance, is sufficient to twist the wing itself. At best this results in control nullification, at worst it results in control reversal. For example if the pilot initiates a roll to the left the downgoing right aileron will twist the right wing, reducing its aoa, resulting in loss of lift and a roll to the right, probably with asymmetric structural loads: all of which would make life difficult when attempting to roll the wings level during the recovery from a high speed dive.

Other effects

It is not just the preceding items that may be a problem at high speed. The maximum speed may be limited by the ability of the tail [and rear fuselage] to withstand the down load on the tailplane necessary to counter the rearward position of the centre of pressure at very low aoa. Some aircraft will 'tuck under' rapidly in a high speed descent which will certainly make the pilot wish she/he was somewhere else. Also the possibility of a runaway propeller in a high speed dive is always there for those aircraft with a constant speed propeller governor.

Recovery from flight at excessive speed

Generally excessive speed can only build up in a dive, though just a shallow dive can build speed – and rate of descent – quite quickly. The table is a calculation of the rate of descent after a few seconds at dive angles of 10°, 30° and 45° – for a moderately slippery light aircraft.

Recovery from an inadvertent venture into the realm of flight near, or even beyond, Vne is quite straight forward but requires pilot thought and restraint in the initiation of the recovery procedures, particularly so if the aircraft is turning whilst diving. Considerable height loss will occur during recovery so the restraint is required when terra firma is rapidly expanding in the windscreen.

• Halt the buildup in airspeed by closing the throttle.
   

• Unload the aircraft to some extent by moving the control column to just aft of the neutral position. Keep the slip ball centred – excess rudder at very high airspeed may strain the tailplane and rear fuselage.
   

• Then gently roll off any bank while using coordinated rudder: this will ensure the total lift vector is roughly vertically aligned. Maintain the aft of neutral control column position to avoid any asymmetric loading arising from simultaneous application of aileron and elevator at high speed.
   

• When the wings are level start easing back on the control column until you are pulling the maximum load factor for the aircraft – +3.8g or +4.4g. Do not pull back so harshly that the aircraft enters a high speed stall. Hold the wing loading near the maximum until the aircraft's nose nears the horizon then level off. The aircraft will have sufficient momentum to reach this position before opening the throttle.
   

• If you have ample height at the commencement of recovery then there is no need to pull such high g – particularly if the atmosphere is bumpy when gust loads, added to the high manoeuvring g, may prove excessive.

A problem with this procedure is that most light aircraft do not have an accelerometer fitted, so it is difficult to judge the g being pulled. However if properly executed 60° steep turns are practised then some idea of the 2g load on your own physiology can be gained. At the higher end the averagely fit person will probably start feeling the symptoms of greyout by 4g.

Recovery from a spiral dive

In a well developed steep spiral dive the lift being generated by the wings (and thus the wing loading) to provide the centripetal force for the high speed diving turn, is very high. The pilot must be very careful in the recovery from such a dive, or excessive structural loads will be imposed. If back elevator force is applied to pull the nose up while the aircraft is turning the result will be a tightening of the turn, thus further increasing the wing loading or possibly prompting a very high speed stall.

Power reduction and levelling of the wings must start first with the rudder and elevators held in the neutral position. As the wings become level with the aircraft still diving at high speed, all the lift that was providing the centripetal force may now be directed vertically (relative to the horizon) and if up elevator is applied the aircraft may start a rapid high g pitch up – even into a half loop. Thus to prevent this the pilot must hold the elevators in the neutral position while rolling level and even be ready to start applying FORWARD stick pressure even before the wings become level.