flight at excessive speed
(Copyright John Brandon)
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
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
+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 = CL
× ½rV² × S ].)
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.
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
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
improper modification or
repair of the structure
excessive free play in control
surface hinges, actuating rods or cables
cumulative stress/strain in
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
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
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 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
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.
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.
||Rate of descent [fpm]
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.
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
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.