Morgans wrote:
"flyncatfish" wrote

I have a russian Yak 52
and the starting, brake, and flap systems are all pneumatic. It's nice
not to have to worry about whether you battery will crank in the
morning.

How does the air make the brakes work? Please don't say very well! ba-doomp

More specifically, do they have a spring that puts the brake on, with the air
pressure pushing the brakes off, or more like a truck brake, with one air source
pushing the brakes off, and another pushing the brakes back on?

That's a good question. I think it's more like a truck brake. The
plane uses differential braking of the main wheels to turn/brake. The
plane has a castoring nose wheel. Braking is actuated by depressing a
brake handle on the stick. Air pressure is directed to the brakes via
the rudder pedals. You push the rudder pedal on the side you want to
turn toward and then start depressing the brake handle on the stick in
short 1 second intervals. You can hear the air going to the brakes
like an air brake on a truck. To release the brake you just release the
handle on the stick. I'm not sure but I think there are springs that
pull the brake shoes away from inside the brake drum when the air
pressure is released. You got me thinking and I'll see if I can confirm
that because that's something I should know.

I had always thought aircraft parking brakes were kinda' dumb. It would make
sense to have a system like cars, with a cable putting the parking brake on,
plus the advantage of having an emergency brake, but there goes the weight issue
again, I guess. No wonder that airplanes have to use wheel chocks.
--
Jim in NC

The Yak 52 has no parking brake, other than a latch on the stick handle
for keeping the brake handle compressed. My biggest concern with the
brakes is on startup. The starting, flaps, and brakes use the same
main pressure tank. There is a backup pressure tank for emergency.
Both tanks typically are set to hold about 50 - 60 bar or around 800
psi with an adjustable pop off valve. As the aircraft is started the
air pressure available for braking slowly decreases as the engine
continues cranking. Air pressure is replaced only after the engine has
started via a low volume high pressure gear driven compressor that runs
off the engine. Not a problem if you are proficient at getting the
engine started in a timely manner, but if you have some problem getting
the engine to turn over (forgetting to turn the mag switch on in the
second cabin is a typical gotcha) you could possibly start the engine
with insufficient air pressure remaining to hold the aircraft. This
could have serious consequences. This is why I always start the plane
pointed at the least expensive thing on the ramp. If the plane fails
to start on the first try I chock the wheels before trying again. Of
course that then requires somebody to remove the chocks for me once the
engine kicks over. Kind of a hassle at times, but the last thing I
want is a 360 hp airplane leaping forward on the ramp on startup. That
may sound like a design limitation but these aircraft were intended to
be used at primary flight training facilities where there was always a
ground crew around to help out if needed.

Greg in AL