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.
Text and Photographs © 2009 Gremline & Hill House
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