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Introduction
The suitability of the brakes for the Cozy MKIV has been
raised on a number of occasions. The designer, Nat
Puffer of Co-Z Development
Corporation, insists that the brakes are adequate for the MKIV model and
opposes any recommendation to change to a brake with more braking capacity
even though such a brake exists and at similar cost. Nat has repeatedly
maligned me for raising a question mark against the suitability of the
brakes, yet he has never disclosed any design data to support the adequacy
of the design, moreover he has listed brakes from Piper's, Cessna, Beechcraft,
etc. as examples of adequacy. These aircraft use a larger model of brake
albeit from the same manufacturer (Cleveland), and are thus not representative
of the Cozy MKIV configuration. In contrast to Nat Puffer's design the
Aero Canard, which has an almost identical take off and landing performance
envelope, is configured with the larger 6 inch Cleveland wheels & brakes
which I believe is an acceptable option available for this aircraft albeit
at slightly increased cost and drag.
The FAR's
There is a book in the EAA library "Landing Gear Design for
Light Aircraft" authored by Ladislao Pazmany ISBN 0-9616777-0-8 and Published
by:
Pazmany Aircraft Corporation
Post Office Box 80051
San Diego, Calif. 92138
On Page 77 of this book the brake sizing is given and
this is a direct extract from this page.
"According to FAR Part23, the brakes of a light aircraft
should meet the following requirements: (23.735)
-
Brakes must be provided so that the brake kinetic energy
of each main wheel brake assembly is not less than the kinetic energy absorption
requirements determined under either of the following methods:
-
The brake kinetic energy absorption requirements must
be based on a conservative rational analysis of the sequence of events
expected during landing at the design landing weight.
-
Instead of a rational analysis, the kinetic energy absorption
requirements for each main wheel brake assembly may be derived from the
following formula:
K.E. = 0.0443 W Vso2
N Where K.E. = Kinetic Energy per wheel (ft lbs)
W = Design Landing Weight (lbs)
Vso = Power off Stalling speed in knots, of the aeroplane at sea level, at
the design landing weight, and in the landing configuration: and
N = Number of main wheels
The text goes on to state that, under the FAR, the
designer of landing gear does not have to consider an aborted take off
at maximum load and under the worst case density altitude."
Designers Obligations
The designer of home built aircraft does not have a legal
obligation to abide by the FAR's however the designer is ultimately responsible
for any design he/she may market. In this context the designer has an obligation
to its clients to provide them with a design that is of merchantable quality
which includes the ability to stop, and manoeuvre whilst stopping, using
the means provided that are both within the piloting capabilities of the
expected user and within the expected range of take-off & landing scenarios.
Limitations of the Cleveland 5" Wheel/Brake System
When an aeroplane lands it has kinetic energy that the brakes
convert to heat. This energy relates to the speed that the aircraft is
traveling when the brakes are applied, and the mass of the landing aircraft.
There is some aerodynamic braking that is not peculiar to the Cozy as Nat
would have everyone believe, there is also some rolling friction. To offset
these drag components there is some addition thrust from the still spinning
propeller. On average the turning propeller almost offsets the other two
frictional forces so designers of braking systems usually design as if
neither existed. The ferocity with which the pilot applies the brakes does
not impact the heat generated, it simply changes the period over which
the heat is generated. The temperature rise is solely a function of the
heat, the ambient temperature and the thermal capacity of the braking system.
If no heat were to be lost in the landing process, the worst case scenario
would be the protracted landing where the pilot applies light braking.
(In this condition the propeller thrust dominates the landing energy) not
the hard braking condition, contrary to popular belief. In the real world,
the brakes have some air cooling, although the tightly cowled wheel pants
reduce this cooling efficiency. It is therefore necessary that adequate
ventilation is applied to the wheel pant design even though it is not the
most efficient aerodynamically.
The Pilots Operating Handbook (POH) gives a touchdown
speed of 65 kts when at a gross weight of 1400lbs. Increasing the gross
landing weight to 1900 lbs translates to a touchdown speed of 75.7kts.
In this configuration, and with zero head wind, the landing energy is 481,000
ft lbs. If the landing is free of cross wind, the energy is distributed
evenly to both wheels resulting in each wheel having to absorb 240,500
ft lbs of energy. The MKIV is specified with the Cleveland Heavy Duty 5"
wheels & Brakes each of which are rated at 192,000 ft lbs capacity.
The brake, in this landing scenario, is thus being operated well beyond
it's rated capacity. The following set of curves illustrates the required
wheel/brake capacity Vs landing ground speed for various landing weights.
The last value of landing weight (light brown) is the maximum gross weight
value for the Cozy MKIV and is presented to account for an aborted take
off condition at maximum gross weight.
The red and blue broken lines represent the braking capacity
of the Cleveland and Matco 5" wheel/brakes respectively. The magenta line
is the locus of the appropriate landing energy and correct touchdown speed
when no headwind is present. For example if the landing speed was 67 knots,
then this would correspond to a landing weight of 1500lbs which in turn
corresponds to a landing energy of 150,000 ft lbs. Another way of looking
at the locus, is where it crosses the solid lines. This is the appropriate
landing speed for that landing weight. The curves indicate that the capacity
of the rated Cleveland brakes are exceeded for landing weights in excess
of 1700 lbs. The Matco brakes, or indeed the 6" Cleveland wheel & brake
set, are still operating within their rated capacities.
The black broken line represents the condition when there
is a 10 knot headwind. Under these conditions the Cleveland brakes are
adequate for all but the aborted take off condition. When Cleveland manufacture
their brake set they specify a maximum rated stopping energy. This figure
is the value in which Cleveland are prepared to assure their customers
of the performance of the brakes. Typically the capacity of any given brake
set will be greater than this figure however an event illustrating that
one aircraft can exceed this figure by a given percentage does not guarantee
that the same will be true for another aircraft of the same design. The
only thing that may be assured is that the rated figure is a safe number
and should not be exceeded.
Impact of Cross Wind
The Cozy MKIV steers through the use of differential braking
applied to the main wheels. Under cross wind conditions the aircraft weathervane's
into the wind. To compensate the pilot must apply additional braking to
the leeward brake. This increased load to the brake results in a higher
capacity being required to this leeward brake. In a condition of zero headwind,
and 1700 lb gross landing weight the leeward brake is now overloaded and
may result in a loss in performance manifesting itself as brake fade. This
fade will cause the pilot to loose directional control and the aircraft
may steer off the runway in a direction corresponding to the windward side
of the aircraft. This effect happens very quickly giving little time for
the pilot to take emergency action by raising the nose gear. It is likely
that a ground loop will occur and because the turnover angle is so poor
for this design, significant damage may result.
Impact of Density Altitude
All pilots should be aware of the increase in take off and
landing speeds associated with high density altitudes. The following figure
indicates the percentage increase in landing, or take off speed, necessary
as a result of non-MSL density altitudes. For example if the conditions
yield a density altitude of 5,000 ft then an 7.7% increase in TAS is required.
This increases the landing energy by 16.1%, i.e. not trivial.
The figure below is similar to the one shown above except
that the density altitude is 5,000ft. Notice how the black dotted line
and the magenta dotted line intercepts the four gross weight energy curves
now further to the right. Even at 1500 lb gross landing weight the Cleveland
5" wheels and brakes are operating beyond their rated capacity.
The addition of a ten knot headwind still helps the situation
but now 1700lbs is the maximum safe landing weight as limited by the braking
system.
Conclusion
Whilst Cleveland produce excellent aviation braking systems,
the selected 5" model would appear to be inadequate for the purpose of
stopping and steering, whilst stopping a Cozy MKIV in most landing scenario's.
A direct replacement manufactured by Matco
provides a stopping capacity of 337,932 ft lbs and also provides a lower
profile than the standard Cleveland products thus reducing drag. Builders
who prefer the Cleveland products could move over to the 6" wheel &
brake sets but compensation for the length of the main gear hoop is necessary
and an additional drag component should be expected because of the physically
larger assembly.
Last Updated:
Thursday August 31, 2006 |
