Groundschool – Theory of Flight
Safety: flight at excessive speed
Revision 13a — page content was last changed February 20, 2009; consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au
| | Home | Contact us | About us | Learning to fly | Flight training facilities | Site map & search | |
|
Groundschool – Theory of Flight |
Safety: flight at excessive speedRevision 13a — page content was last changed February 20, 2009; consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au |
|
The following notes apply chiefly to three-axis powered aircraft that have been tested and received type approval certification from a national regulatory authority. Owner-designed and built aircraft do not go through the certification process; the builder would generally just rely on static load tests to prove the structure, and no flight testing program would be undertaken by a professional test pilot to determine the safe flight envelope and to identify deficiencies. However, the principles apply even if there is no properly established flight envelope data.
The theme common to all problems encountered when moving at very high speed is that there is no warning, and no time to do anything about it! The term 'excessive speed' is entirely relative. In a low-momentum aircraft it might be 70 knots. In an aircraft that cruises at 100 knots, excessive speed might be 140 knots. The only safe procedure is not to push the high-speed end of the envelope at any height. There is a companion document 'Don't fly real fast' in the guide 'Decreasing your exposure to risk which includes and expands some of the following material and should be read. 13.1 Airframe strength and elasticityAircraft structures are designed to have adequate strength and stiffness, while still being as light as possible. To receive type approval certification, the design of a general aviation or ultralight 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 that an aircraft at maximum take-off weight [MTOW] must be designed to withstand are: +3.8g to –1.5g (or –1.9g) for the normal operational category (which would include most factory-built recreational aircraft); +4.4g to –1.8g (or –2.2g) for the utility category (which includes most GA, and perhaps some RA, training aircraft); and +6.0g to –3.0g for the acrobatic (i.e. aerobatic) category. For more information see 'Limiting loads and ultimate loads'. 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 also considering is total aerodynamic loading — remember that dynamic pressure increases with the square of the velocity; i.e. dynamic pressure = ½rV². Notes: 1. Uncertificated minimum ultralight aircraft, even with their low wing loading of perhaps 12 kg/m², can be overstressed readily just by flying at maximum level speed and increasing g in a pull-up (positive g) or a push-over (negative g). 2. Many non-ultralight aircraft are type certificated in both normal and utility category, and some are certificated in those plus the acrobatic category. In this case, the MTOW and cg limits are not fixed values, but vary according to the flight operating category. See the table in weight/cg position limitations. ElasticityAll aircraft structures exhibit some degree of elasticity; that is, they deflect a little, changing shape — flexing, bending and twisting — under applied aerodynamic loads. Those structural distortions also contribute to a change in the aerodynamic forces, so the distortions and forces are mutually dependent. This is particularly so with the wings, tailplane and control surfaces. However, structures usually spring back to the normal position when the load is removed. This aeroelasticity may lead to some problems at high speed, but reducing elasticity means increasing rigidity, which perhaps involves an unwarranted increase in structural weight. So, aircraft structural engineering must be a compromise between rigidity and elasticity. See the notes on 'stress and strain' in the 'Builders guide to aircraft materials'.Weight and balanceThere are fixed limits to the payload an individual aircraft may carry safely. The 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. However, for many recreational aircraft, the MTOW will be limited by national legislation, which has nothing to do with aeronautical engineering.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 unfavourable reactions to aerodynamic loads, affect stability, reduce controllability, or delay (or prevent) recovery from unusual or high-speed situations. [ The next section in the airmanship and safety sequence is the following section Aerodynamic reactions to flight at excessive speed ] |
13.2 Aerodynamic reactions to flight at excessive speedFlutterWing 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 tailplane structure may apply another limiting airspeed to flight operations, related to interaction between aerodynamic and inertial loads — 'flutter'. Consider a flag fluttering in a light breeze.When the airflow around a wing or control surface is disturbed (by aerodynamic reactions, turbulence or pilot inputs) the structure's elastic reactions may combine as an oscillation or vibration of the structure — possibly felt in the controls as a low-frequency buzz — that will quickly damp itself out at normal cruise speeds. At some higher speed — the critical flutter speed, where the oscillations are in phase with the natural frequency of the structure — the oscillations will not damp out but will resonate and rapidly increase in amplitude. (Pushing a child on a swing is an example of phase relationships and amplification.) This condition is flutter and, unless airspeed is very quickly reduced, the increasing dynamic loads will cause control surface (or other) separation within a very few seconds. The following is an extract from an article by William P. Rodden which appears 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 [author's emphasis]. 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." Torsional stiffness has a role in the onset of flutter development. Providing high torsional stiffness in an airframe structure — particularly a high aspect ratio, sailplane-type wing — may incur weight penalties that are unacceptable for those aircraft whose MTOW is limited by national legislation, rather than normal design parameters. Mass inertia is also involved in flutter development. Consequently, control surfaces — ailerons, elevators, rudder — must be mass-balanced (i.e. the centre of gravity of the control surface as close as possible to the hinge line) to limit the mass moment of inertia. It may be acceptable for the control surface to be over-balanced; i.e. its cg is slightly forward of the hinge line. Mass-balancing of the control surfaces should prevent flutter of the surface, but the possibility of, for example, wing flexing/twisting flutter may still exist. Even if mass-balanced, a critical flutter airspeed (or something akin to it) may occur well below Vne if any of the following conditions exist: wear in control surface hinges; lack of tension or slop in actuating rods/cables/cranks/torque tubes; water or ice inside control surfaces or absorbed within a foam core; mud outside; faulty trim tabs; additional surface coatings applied after balancing; or other system anomalies that alter structural reactions. So, flutter is a vibrational instability that (if the structure is not sufficiently stiff) may be related to both the aerodynamic pressure and the true airspeed. Flutter problems and their solutions are very complex; analysis is further complicated by the existence of many flutter modes. This is an extract from an RA-Aus accident investigation report: "(Witnesses) observed the aircraft in a steep dive at what appeared to be full power. The port wing appeared to detach from the aircraft ... The wing that tore away from the fuselage had the attach points intact but had pulled the mountings out of the top of the cockpit. This action would have released the door, which landed close to the wing. The wings were intact but the ailerons were detached. There was no delamination of the fibreglass structure. The ailerons were not mass balanced. The (prototype) aircraft was a conventional design being a high wing, monoplane of composite construction. While the fuselage was a proven design the pilot /builder had designed his own wing including the aerofoil section. The workmanship was excellent and there is no evidence of any lack of structural integrity. The eyewitnesses reported seeing a sort of 'shimmying' from the aircraft. It is believed that this shimmying was aileron flutter which led to the detaching of both ailerons. This same flutter condition would account for the massive forces required to detach the wing from the aircraft in the manner that occurred. Flutter could have been triggered by the wing aerofoil design combined with the manoeuvre the pilot was conducting or from the aileron control design ... The aircraft suffered a massive inflight structural failure almost certainly caused by severe aileron flutter and the aircraft speed in the dive. Any flutter would have been exacerbated by the lack of mass balancing." Wing divergenceWing divergence refers to a state where — at very low angles of attack and high speed (when the nose-down pitching moment is already very high) — pressure centres develop, which push the front portion of the wing downward and the rear portion upward. This aerodynamic twisting action on the wing structure — while the rest of the aircraft is following the flight path — further decreases the aoa and compounds the problem. The action finally exceeds the capability of the wing/strut structure to resist the torsional stress, and causes the wing to separate from the airframe with no warning. This could be induced if a downdraft is encountered at high speed.Control reversalAs airspeed increases, control surfaces become increasingly more effective. They reach a limiting airspeed where the aerodynamic force generated by the ailerons, for example, may be 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 and 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 recovery from a high-speed dive.Many of the uncertified minimum ultralights, and perhaps some of the certificated aircraft, have low torsional wing rigidity. This will not only make the ailerons increasingly ineffective with speed (and prone to flutter), but will also place very low limits on Vne and g loads. Vne may be so low that it can be achieved readily in a shallow descent at 75% power. Effect of wing washoutWings incorporating geometric washout have a significantly lower aoa towards the wing tips. At high speed when the wing is flying at low aoa, there are high aerodynamic loads over the wings. However, the outer sections could well be flying at a negative aoa and the reversed load in that area will bend the wingtips down, possibly leading to outer spar fracture. See the accident technical report below.Vertical gust shear and gust loadsThe effective aoa of an aircraft encountering an atmospheric gust with a significant vertical component (updrafts, thermals, downdrafts, microbursts, macrobursts and lee waves) will be increased momentarily if the air movement is upward relative to the aircraft's flight path, or decreased momentarily if the air movement is downward. Thus, an updraft will increase CL and lift, increasing the aerodynamic loading and lead to an upwards acceleration of the aircraft. The magnitude of the acceleration is determined largely by the change in aoa, the aircraft speed (the higher the speed, the greater is the g load), the design wing loading and the aspect ratio. The lower the design wing loading and/or the higher the aspect ratio, the greater is the change in load factor for a given increase in aoa and the easier it is to overstress the wings at high speed. The effects of shear and gust loads are expanded in the section on wind shear and turbulence.Other effectsIt 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 fuselage to withstand the bending moments caused by the loads on the tailplane necessary to counter the wing's substantial nose-down pitching moment at very low aoa, or the aoa changes due to vertical gust shear, or the extreme loads caused by a high speed pull-up. Applying rudder in a high speed pull-up applies twisting loads to the rear fuselage. Even a very small bird can cause severe damage in a high-speed bird-strike.When nearing the zero-lift angle of attack in a high-speed descent, many cambered wings suddenly experience a strong nose-down pitching moment and the aircraft will 'tuck under' rapidly; this will certainly make the pilot wish she/he was somewhere else. The symmetrical aerofoil wings often used in aerobatic aircraft don't have this problem. Also, the possibility of a runaway propeller in a high-speed dive is always there for those aircraft with a constant-speed propeller governor or perhaps an in-flight adjustable system. The following is a condensed version of an Australian Transport Safety Bureau Technical Analysis Occurrence Report. Note: the Coroner's findings in relation to the fatal accident near Atherton does not support any view that the accident was caused by pilot mishandling; rather, the Coroner's "preference is towards port side wing tip separation as a consequence of the un-airworthy state of the aircraft ..." "An Airborne Edge microlight aircraft impacted terrain during a 2005 flight to Atherton, in Far North Queensland. The pilot, the sole occupant of the aircraft, was fatally injured. In 2006 a similar Airborne Edge aircraft impacted terrain at Cessnock, New South Wales, also fatally injuring the pilot, the sole occupant of the aircraft. In both instances, RA-Aus initiated safety investigations to determine contributing factors to these accidents. During the course of these investigations, similarities in the structural failures of both aircraft were observed. In addition, a third accident involving an Airborne aircraft registered with HGFA with similar structural failure was identified. This accident had occurred in 1996 in Hexham, NSW. In order to determine possible connections between all three accidents, ATSB was asked to conduct technical examination and analysis on recovered parts from the Atherton and Cessnock accidents, to assist the RA-Aus investigation. Information regarding the 1996 accident was taken from coronial findings. In all three accidents, the failure of the main wingspars had occurred near the wingtip. Qualitative analysis of the structural design and loading of the part during this safety investigation and the examination of the coronial findings from the Hexham accident, revealed that all main wingspars had failed under negative G loading. Such loading was likely if the aircraft entered or encountered flight conditions outside the manufacturer's specified flight envelope. Examination of material characteristics of the failed wingspars did not show evidence of material deficiencies that could have contributed to these accidents. The manufacturer's operating handbook prohibited all aerobatic manoeuvres including whipstalls, stalled spiral descents and negative G manoeuvres. The manual specified that the nose of the aircraft should not be pitched up or down more than 45 degrees, that the front support tube of the microlight and the pilot's chest limit the fore and aft movement of the control bar, and that the aircraft should not exceed a bank angle of 60 degrees. Review of photographs of the Airborne Edge, indicate that the wing adopts a degree of twist while in flight. Twist will effect the load distribution by shifting some of the lift from the tips inboard (i.e. more lift is generated in the middle of the wing). Given the structural restraint of the tip struts and battens located at the tip of the trailing edge of the wing, the aerofoil at the wing tip must adjust and try to align with the relative airflow. This results in a smaller amount of lift generated near the wing tips due to a reduced angle of attack to the relative airflow." (Or an aoa reduced below the zero lift aoa, i.e. reversed lift ... JB) [The next section in the airmanship and safety sequence is the section below 'Standard airspeed limitations'] |
13.3 Vne — the standard limiting airspeedIf an aircraft is operated within its specified flight envelope, observing the limiting accelerations and control movements, and maintaining airspeed commensurate with atmospheric conditions, then the only possibilities of in-flight structural failure relate to:
To provide some safety margin, Vne is then set at 90% of the lower of Vd or Vdf. Vdf is a diving speed that has been demonstrated without problem in test flights and which must be lower than, or equal to, Vd. Vne is often limited by the critical flutter speed. Many ultralights would not achieve the flight envelope extent specified in FAR Part 23; for example, Vd may be nowhere near 1.4 times the design cruise speed, so the flight safety margin is lower. Vne for light aircraft is always specified as an indicated airspeed and marked as such on the ASI (the red line). For most ultralight aircraft, only one Vne is specified in the Pilot's Operating Handbook or aircraft flight manual and that value is probably conservative and applicable for operations below 10 000 feet amsl. However, some ultralight aircraft have a capability for achieving considerable altitude and the designers have felt it wise to limit maximum speed to a particular true air speed. See Does Vne stay the same no matter how high you fly?. Vne as a maximum airspeed applies only for smooth atmospheric conditions and for gentle control movements; even vertical gusts associated with mild turbulence or relatively small control movements will lead to some nasty surprises, if operating close to, but below, Vne. At such high speed, the controls are very effective. However, there is a high possibility for over-control to apply extreme loads to the structures; some aircraft control systems don't provide an adequate feedback of the load being exerted, i.e. a high load can be applied with a relatively low stick force. See 'I like flying my aircraft fast. If I stay below Vne, I won't have to worry about structural failure, right?' Be aware: deliberately exceeding Vne is the realm of the test pilot — who always wears a parachute! [ The next section in the airmanship and safety sequence is the following section Recovery from flight at excessive speed ] |
13.4 Recovery from flight at excessive speedGenerally, excessive speed can only build up in a dive, although just a shallow dive can build speed — and rate of descent — quite quickly. The table below 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 initiating 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.
A problem with this procedure is that most light aircraft do not have an accelerometer [g-meter] 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 of acceleration the average fit person will probably start feeling the symptoms of greyout by 4g. [ The next section in the airmanship and safety sequence is the following section Recovery from a spiral dive ] 13.5 Recovery from a spiral diveIn a well-developed spiral dive, the lift being generated by the wings (and thus the aerodynamic loading) to provide the centripetal force for the high-speed diving turn, is very high, and practically all of it is directed inward. The aircraft is at the extremes of its flight envelope, with very high aerodynamic loading and very high speed well past Va. The pilot must be very careful in the recovery from such a dive, or damaging structural loads will be imposed. If rearward stick force is applied to pull the nose up while the aircraft is turning, the result will be a tightening of the turn and further lowering of the nose, thus dramatically increasing the applied loading or possibly prompting a very punishing high-speed stall.The recommended procedure is:
The theme common to all problems encountered when moving at very high speed is that there is no warning and no time to do anything about it! The only safe procedure is not to push the high-speed end of the envelope at any height: make gentle, smooth control movements and avoid asymmetric flight loads. [ The next section in the airmanship and safety sequence describes 'Risk management' ] |
13.6 Notes: compressibility of airflow and Mach numberThese notes have little value for the recreational aviator, but are included for interest.
For interest, the following table is the maximum permissable speed/altitude for a late 1940s/early 1950s piston-engined naval fighter — the Seafire 47:
Subsonic jet transport aircraft are designed to cruise close to their maximum allowable speed — Vmo/Mmo. Vmo is the limiting indicated airspeed and Mmo is the limiting Mach number. Mmo is probably between Mach 0.80 and Mach 0.85. In normal operations the limiting airspeed is Vmo, up to a change-over pressure altitude (perhaps around 25 000 feet). Above this altitude Mmo becomes the limiting speed value because of compressibility problem restraints. Vmo could be shown as a fixed red line on the ASI (or 'Mach/Airspeed Indicator') but, because the speed of sound decreases as altitude increases, Mmo can't be represented by a fixed marking on the indicator. So, a moving red-and-white striped pointer, the 'barber pole', shows the limiting Vmo/Mmo varying with altitude. It shows the IAS corresponding to the lower of Vmo or Mmo for the current altitude. For further explanation read this Boeing flight operations review document. The next module in this Flight Theory Guide discusses loss of control in low level turns. |
Groundschool – Flight Theory Guide modules
| Flight theory contents | 1. Basic forces | 1a. Manoeuvring forces | 2. Airspeed & air properties |
| 3. Altitude & altimeters | 4. Aerofoils & wings | 5. Engine & propeller | 6. Tailplane surfaces |
| 7. Stability | 8. Control | 9. Weight & balance | 10. Weight shift control | 11. Take-off considerations |
| 12. Circuit & landing | [13. Safety: flight at excessive speed] | 14. Safety: control loss in turns |
Supplementary documents
| Operations at non-controlled airfields | Safety during take-off & landing |
Copyright © 2004 — 2009 John Brandon [contact information]