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Gremline Flight Safety Report: Ultralight Stall Speeds & Handling / Inherent Instability of Gyrocopter Design

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the gremline digest —  ultralight stall speeds / gyrocopter instability

Ultralight Stall Speeds and Handling
The pilot of an ultralight aircraft submitted a confidential report to CHIRP after he experienced loss on control on the approach in an incident that could have resulted in a very serious accident. This report is taken from the General Aviation CHIRP Feedback Winter 2009 with the kind permission of CHIRP and with the intention of bringing it to the attention of all ultralight pilots who log on to this site.

 


The pilot was approaching to land in his ultralight in benign weather and was flying at about 55mph with full flap and half throttle. When about 40 feet agl the aircraft rolled rapidly through 90 degrees and did not respond to the application of full corrective aileron. The pilot found himself passing low over a field adjoining the landing strip with the wingtip perhaps a foot above the ground. The pilot does not remember applying opposite rudder or of applying full power but after about 10 seconds the wings levelled after the aircraft had turned through 180 degrees. The pilot reports that the aircraft flopped into ground effect, nose high at full power. He was able to accelerate in ground affect and climb away, before returning for a successful landing.
      The pilot reported that the Pilot Operating Handbook for his aircraft recommends 68mph for the approach but his approach was slower, as it had been on many previous occasions without problems. He noted that the stall speed with full flap and power on is 29mph and is 36mph without power. He stated that the approach speed on the incident flight was 90% above the power-on stall speed and 53% above the power-off stall speed. He said a line on the ASI shows the stall speed.
      The pilot believed that the sudden roll was caused by hot air rising from a hangar roof and that the ailerons were too small to control the roll. He presumed that this is the reason for the manufacturer’s recommendation for an approach speed so far above the stall speed. He said he was trained on Cessna 152s and had considered a reasonable margin above the stall to be 15%. He now believes it to be dangerous to be anywhere near this speed in this aircraft and that the line on the ASI to be quite misleading.
      The pilot said that this incident had taught him to remain at or above the manufacturer’s recommended speed of 68mph until in the flare. He commented that other aircraft with limited aileron authority may be vulnerable to similar effects.
      The CHIRP response to this very valuable report contains information that may not be understood by all pilots of ultralight aircraft. This response is repeated
verbatim below.


The certification requirements for Very Light Aircraft (VLA) include the ability to roll from 30 degrees to 30 degrees in less than 5 seconds in both directions at an airspeed equivalent to 1.3xVs (the power-off stalling speed). If not achievable at that speed, a higher speed may be used but this will result in an increased approach speed; this was probably the basis for the higher approach speed recommended in the POH for this type.
      The reporter had converted from a conventional C152 to a much lighter low inertia ultralight type; ultralight aircraft similar to the type in this report can suffer a much more pronounced loss of speed during the flare than heavier conventional GA types; this in turn can significantly reduce the roll response. Proper conversion training would have covered these points and probably avoided what was a very serious incident. The BMAA and the LAA strongly recommend familiarisation training.
      One final point; the reporter’s reference to the approach speed margin being 15% is not correct; the correct margin between the approach speed and the stalling speed is 30% (1.3Vs) unless further increased by another factor, as in this case.”

We thank the pilot for his detailed and confidential report to CHIRP that allows this potentially serious situation to be brought to the attention of other ultralight pilots. We also thank CHIRP for permission to publish the report and the CHIRP comments on this site. Our UK Links page includes one to CHIRP. Your confidential report to CHIRP could enhance safety for everyone.

 

 

Inherent Instability of Gyrocopter Design

The following is based on an Air Accidents Investigation Branch (AAIB) Field Investigation (EW/C2003/06/05) into a fatal accident involving a Ponsford Bensen B8MR (Modified) gyrocopter. The accident happened on the pilot’s first unsupervised flight following his PPL (Gyroplane) conversion course. The AAIB subsequently added a number of recommendations to the existing Safety Recommendations 2003-01 and 2003-02. Safety Recommendations 2004-42 to 2004-45 were accepted by the Civil Aviation Authority (CAA). Necessary amendments were made to the Private Pilot’s Licence (Gyroplanes) Requirements and the specified number of supervised flying hours for training on type. The CAA also accepted the recommendations that test pilots evaluating the handling properties of gyroplanes against BCAR Section T should appropriately trained, and that flight test schedules should record all of the required data. The PFA has endorsed the recommendations, and see the re-evaluation of gyroplane types as a positive step towards addressing a very high accident rate. The process that was put in train by this single accident is a fine example of the quality of work undertaken by AAIB and their contribution to Flight Safety.


The accident resulted from the rotor blades striking the rudder while the aircraft was in level flight at about 250-300 feet agl on the downwind leg of a visual circuit. Witnesses heard a single ‘bang’ and the engine noise stopped. The aircraft then pitched nose down and fell vertically to the ground with the rotor apparently stopped. The aircraft impact was near vertical with very little forward speed. The pilot was killed instantly.
     An examination of the aircraft, and subsequent computer modelling by the University of Glasgow indicated that the aircraft could have had poor longitudinal stability characteristics. The AAIB investigation also highlighted the poor safety record of gyroplanes in general when compared to other types of recreational aircraft.

 


The Accident Aircraft
The aircraft was a single seat gyroplane with a pusher engine and an open cockpit. It had 22-foot diameter ‘Dragon Wing’ rotor blades and a Rotax 532 engine driving a three-bladed composite propeller. The control stick was of the ‘pump-action’ type that pivots below the seat and moves vertically during fore and aft stick movements. This differs from a keel-mounted stick found in other gyroplane designs and in conventional fixed wing aircraft.
      Non-standard modifications included the fitting of a nose cone fairing from the Air Command gyroplane design, the addition of side pod fuel tanks and a seat incorporating a fuel tank, also from the Air Command design. The cockpit was fitted with a short, Air Command-style, throttle lever. The Popular Flying Association (PFA) had approved the nose cone and seat tank modifications but the side tank modification had not yet been approved because of its potential adverse effect on vertical centre of gravity. A weight and balance study by the University of Glasgow determined that the side tanks had little effect on the vertical centre of gravity. The vertical position of the CG was calculated to be 4.8±1.2inches below the thrust line. The aircraft mass was calculated to have been 252 kg with the maximum total authorised weight of the aircraft being 280 kg.
      The pilot had attempted some unsupervised wheel balancing (running along the ground while balancing the gyroplane on its main wheels) when the aircraft suffered a ‘blade flap’ resulting in a roll over and damage to the rotor and propeller. Accelerating the gyroplane too rapidly for the rotor rotational speed causes this form of blade flap. The pilot purchased a new rotor blade and a new propeller eight months before the fatal accident.

The Accident Site Examination
There was no indication of any appreciable rotor rotation on impact. One rotor blade had buckled on impact. A large section of the upper part of the red rudder was missing from the impact area and was found two months later in two pieces 60 and 120 feet from the main wreckage. The rotor blades had red marks along their leading edge and underside between 4.6 feet and 6.2 feet from the rotor hub. These marks and the location of the rudder pieces indicated that the rotor struck the rudder while in flight. The rotor blades also had marks indicating contact with the propeller. The condition of the propeller blades suggested that the propeller shaft was rotating at low speed on impact.

Detailed Wreckage Examination
The teeter stop plate was bent downwards on both sides consistent with a hard impact between the rotor blades and the teeter stops, suggesting a violent vertical motion of the rotor blades causing them to strike the rudder. The engine started and operated normally after minor repairs to rectify impact damage.

Aircraft Approval Process
Most gyroplanes are now built from kits but the accident aircraft was built from plans for a Bensen B8MR with additional modifications. The PFA was delegated by the CAA to investigate and make recommendations for approval of this gyroplane type. After build the accident aircraft was inspected and then test flown by a pilot accepted by the PFA for this task. Seven test flight were completed over a three day period before the pilot submitted a declaration to the PFA that he considered the aircraft met the British Civil Airworthiness Requirements (BCAR) Section T. The PFA then recommended that the CAA issue a Permit to Fly. The Permit to Fly was issued concurrent with a Certificate of Validity that was still valid on the date of the accident. The builder sold the aircraft to the accident pilot before the Permit was issued.

 


Stability Characteristics of Gyroplanes
A fixed wing aircraft has longitudinal static stability when the CG is forward of the aircraft’s lift vector. A gyroplane has longitudinal static stability when the CG is forward of the Rotor Thrust Vector (RTV). In this configuration, if a gust causes the gyroplane to pitch up the rotor thrust will increase giving a restoring nose down pitching moment. The vertical location of the propeller thrust line relative to the position of the vertical CG is a major factor in determining the balance of moments that affect the location of the RTV in steady flight.
      It is assumed that the aerodynamic drag acts closely in line with the vertical CG. If we look at the two possible configurations of the propeller thrust line relative to the CG we can understand how these different propeller thrust lines affect the inherent pitch stability of the gyroplane. To establish equilibrium in flight when the propeller thrust line is
below the CG the RTV lines up aft of the CG and balances the nose up pitching moment of the propeller thrust line. If an upward gust causes the aircraft to pitch up the RTV will increase and tilt aft (‘flap back’) and pitch the aircraft nose down – a restoring moment.

      However, if the propeller thrust line is above the CG the RTV lines up ahead of the CG. When a disturbance causes an aircraft in this configuration to pitch up the RTV will still increase and tilt aft but because the RTV is ahead of the CG the effect will be to increase the aircraft pitch up even further – an unstable configuration.
      It is desirable that a gyroplane possesses dynamic longitudinal stability as well as static longitudinal stability. A gyroplane that has static stability does not necessarily have dynamic stability. A gyroplane with positive longitudinal static stability but negative longitudinal dynamic stability would pitch down in response to an upward gust but the restoring moment would be excessive and the pitch divergence would increase with each overshoot unless the pilot was able to counteract this increasing pitch divergence.
      The University of Glasgow conducted a study of the stability characteristics of gyroplanes using a simulation model based on both wind tunnel data and flight test data. This verified that aligning the propeller thrust line close to the vertical CG had a favourable effect on both static and dynamic longitudinal stability characteristics. The study recommended that the CAA revise BCAR Section T to include a limit for vertical CG position to within ± 2 inches of the propeller thrust line. A small amount of instability with the thrust line slightly above the CG was deemed acceptable but a thrust at or below the CG was deemed desirable. The CAA plans to require a more rigorous demonstration of acceptable handling characteristics if the ± 2 inches thrust line to CG relationship is not met. It should be noted that aligning the thrust line close to the vertical CG does not in itself guarantee that a gyroplane will have good longitudinal stability characteristics.
      The aerodynamic drag vector will also affect the stability of a gyroplane if it is not closely aligned to the vertical CG. Changes in speed will cause changes in drag and result in pitch changes when the drag vector is displaced from the vertical CG. A drag vector below the vertical CG will be a speed-unstable configuration because an increase in speed (and drag) will pitch the aircraft nose down causing the speed to increase further.
      The addition of a properly sized and located horizontal tail surface can theoretically improve both speed stability and pitch stability by providing a restoring pitching moment and by reducing the number of overshoots during pitch oscillation by acting as a pitch damper, thus improving dynamic pitch stability.
       The more longitudinally unstable gyroplanes are the more difficult they are to control and the more likely the pilot is to enter a pilot-induced-oscillation (PIO) in pitch. In a PIO the pilot’s inputs are out of phase with the response of the aircraft. A PIO in a gyroplane, if not recognised and immediately stopped by the pilot, can have fatal consequences. The study by the University of Glasgow demonstrated that when a gyroplane is pitching up and down the rotor speed is also oscillating up and down. If the rotor slows too much, retreating blade stall (also known as in-flight blade flap) can occur. The rotor blade then becomes unstable and usually strikes some part of the airframe, tail or pusher propeller with catastrophic results.
      Blade flap can result if the pilot pushes the control stick forward too rapidly causing the disk angle of attack and lift to reduce, thus unloading the rotor. The rotor will slow down, eventually causing blade stall and blade flap. This situation is aggravated if the propeller thrust line is above the vertical CG because as the Rotor Thrust Vector (RTV) reduces the propeller thrust causes a further pitch down, unloading the rotor even more. This phenomenon is referred to as ’power pushover.’
      The type of control stick employed can affect the aircraft’s susceptibility to PIO. The ‘pump action’ type of control stick fitted to the accident aircraft translates up and down during forward and aft stick movements. A PIO could be aggravated if the vertical motion of the aircraft’s pitching is coupled with the vertical stick motion as the pilot tried to control the pitch. A keel-mounted stick as opposed to a pump-action stick may help to alleviate PIO susceptibility.
     
In summary, gyroplanes can be designed with inherent longitudinal stability. Aligning the propeller thrust line at or slightly below the vertical CG improves longitudinal stability, as does a properly sized and located horizontal tail. The use of a keel-mounted stick as opposed to a pump-action stick may also help alleviate PIO susceptibility.

 


Stability Characteristics of the Accident Gyroplane
The gyroplane involved in this accident had a number of characteristics that probably would not have met the longitudinal dynamic stability characteristics required by Section T of BCAR’s. The thrust line was 4.8 ± 1.2 inches above the vertical CG. This is in the unstable direction and outside the 2-inch limit recommended by the University of Glasgow. The aircraft was not fitted with a horizontal tail surface and was modified with the fitting of a nose cone. The drag vector of this nose cone could have been destabilising. The aircraft also had a ‘pump-action’ control stick that could have increased the aircraft’s susceptibility to PIO. An additional feature of the accident aircraft that could have induced or aggravated PIO was the short throttle lever controlling the Rotax 532 engine. The Rotax 532 has a non-linear relationship between power output and throttle position at high rpm so that small movements of the throttle cause large power changes. Power changes affected the pitch response of this aircraft because the thrust line was so far above the CG. The standard Montgomery B8MR kit-built gyroplane has a longer throttle lever that partially alleviates this problem. The combination of all these features indicate that the aircraft would probably have been difficult to fly, particularly for an inexperienced gyroplane pilot.
      The University of Glasgow modelled the stability of the accident aircraft. The computer model showed that the aircraft responded to a fore and aft stick input at 45 mph airspeed with a stable and lightly damped pitch oscillation. However, at 65 mph the model showed an unstable rapidly divergent departure in pitch. These results, taken together with the design characteristics of the accident aircraft, indicate that it would have had an unstable mode in pitch and probably did not meet the longitudinal stability criteria of BCAR Section T.

The Safety Record of Gyroplanes
The safety record of gyroplanes is very poor compared to other types of aircraft. There have been 15 fatal gyroplane accidents in the United Kingdom since 1989. There were only between 200 and 265 gyroplanes on the UK register in that period. This gives a fatal gyroplane accident rate of 27.1 per 100,000 hours flown. This compares unfavourably with just 2 fatal accidents per 100,000 hours flown by microlights and 1.1 fatal accidents per 100,000 hours for light fixed wing general aviation aircraft. The fact that the fatal gyroplane accident rate is more than 13 times greater than that for similar weight microlight aircraft raises serious concern about the design of gyroplanes and the training of gyroplane pilots.
     A review of the 15 fatal accidents showed that 13 of the pilots held a licence for fixed wing aircraft or helicopters. Eight of the pilots killed had less than 50 hours gyroplane experience and six had less than 10 hours.
      There were 17 fatal gyroplane accidents in the USA between 1999 and 2002. Eight of these listed pitch instability as the primary cause. All of these eight accidents occurred in aircraft without a horizontal tail. “Deficient Pilot Proficiency” was considered a shared cause when pitch instability was involved.

Notes
Jeffrey Quill, the renowned Supermarine Spitfire test pilot, defined stability as ‘the tendency of an aircraft when disturbed from a condition of steady flight to return to that condition when left to itself; conversely instability is the tendency of the aircraft to diverge further away from the condition of steady flight if once disturbed.’ Fundamentally there are two kinds of stability:
Static stability, which determines whether or not the aircraft will initially tend to return to its trimmed condition.
Dynamic stability, which refers to the subsequent behaviour after the initial response of the aircraft to the static restoring moment.
      Both are affected by the position of the control surfaces.
Stick-fixed means the controls were held rigidly throughout the disturbance. Stick-free means the controls were allowed to take their own position. This may have either a negative or positive affect on the restoring moment, depending upon the position where the control settles after the disturbance.

 

 

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