REFLECTOR: Brake authority

[Pat Shea]

Hiroo,

There are two issues at play here: Braking Capacity
and Braking Power. Braking Capacity is the amount of
work the brakes can do before they become saturated
with heat and f-f-f-fade away. Braking Power is how
much friction can be generated between the pads and
discs. Limited Braking Power is an annoyance (i.e.
can't hold the plane still during run-up). Limited
Braking Capacity is a major safety issue (i.e. you
can't stop or steer). If you have air in the lines,
the brakes may feel spongy or soft an which can impact
Braking Power, but it will not affect Braking
Capacity. 

I'm using the Matco 600XT's on my XL. I maintain
85KIAS on final and based at a 3,200' airport. I've
done a extensive testing with my brakes and there are
some definite limitations which need to be contained
by energy management on the part of the pilot.

The bottom line is that the brakes are only good for
one full stop with substantial braking if they are
cool (ambient temp) to start with. If you want the
ability to abort a take-off from Vr, your bakes will
need to be ambient temp to start with. FYI - it takes
about 15-20 minutes for your brakes to fully cool down
from max temp (approx 700*).

So, if yu come in for landing and have to stomp on the
brakes right away and ride them down to a full stop,
you're not going to have much braking capacity left
for taxiing. Taxi with caution - take it slow for a
few minutes, giving the brakes a chance to cool a bit
so you have some capacity to work with.

I've found the most challenging scenario to be a
headwind with a x-wind component. If departing and you
need to taxi to get to the departure end of the
runway, it's difficult not to heat up the downwind
brake. The wind is pushing and turning the plane. Take
it slow. If landing, let the headwind slow you down on
the runway because you're going to need some braking
capacity to taxi back with that quartering tailwind...

Pat    

> ----- Original Message -----
> From: Hiroo Umeno<mailto:humeno at microsoft.com>
> To: Velocity Aircraft Owners and Builders
> list<mailto:reflector at tvbf.org>
> Sent: Monday, June 04, 2007 11:57 AM
> Subject: REFLECTOR: Brake authority
> 
> I had a situation this weekend that heightened the
> "pucker factor" a bit.
> 
> It was a rather warm day (80 or so at sea level) and
> I landed at a 3400ft long.  Upon touchdown, I braked
> more firmly than normal since it was a bit shorter a
> strip than normal.  About mid-way down the strip, I
> noticed that the rate of deceleration has decreased
> a bit.  I used the full length of the 3400ft to get
> it stopped and turned off the active.  The parking
> was at the other end of the field so I taxied back. 
> It was probably about 5 minutes in taxi and after
> getting to the other end of the 3400ft strip, I
> suddenly realized that I could only turn left.  To
> make matter worse, once I got the nose cocked left,
> I could not straighten it out, either.  I had to
> shut down, push the plane to the ramp, let the
> brakes cool off, then taxi.
> 
> Now, I've always had an impression that the braking
> on my plane was "softer" than other planes I have
> flown in the past.  For example, on the Pipers with
> castering nose gears, I could make a "pin-point
> turn" by locking one wheel or another by standing on
> the brake.  Not on my plane.  I have the let the
> plane start rolling and then nudge the nose over
> gradually.  Also at run-up, I can barely keep it
> from creeping forward at higher RPMs.
> 
> As I have the standard wing and fuselage (SUV), I
> have the Matco brakes.  My understanding is that the
> Cleveland upgrade is only necessary on the XL class
> aircraft.
> 
> How much brake effectiveness should I expect on
> these Matcos?  I am starting to think I might have
> an air bubble or two in my lines.
> 
> Hiroo



       
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