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Brakes

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)

  1. 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:
  1. 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.
  2. 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 Vso
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