yak 55 air start valve

It would have been interesting to have been a fly on the wall in the 1980’s when design requirements for the Yak 55 were being drawn up at “the Bureau”

Its lines are so different from the sleek, curvy single-seat fighter appearance of its predecessor, the Yak 50.

But as the seventies turned into the eighties, the Yak 50’s were being found too sleek, too slippery for the slower style of world championship aerobatics that was emerging. It would slice its way through the aerobatic box too fast, denying advantage to Russian team pilots.

No, it was clear. A lighter airframe with more drag was needed. Nothing short of National prestige was at stake.

And so it was that one cold Friday night in January the three members of the Yakovlev 55 core design team made their way to Gorky Park, with pencil and paper, a bottle of Green Label Vodka and a Tab of acid -  and the rest...is history.

The ugliest plane imaginable.

At least that’s what I thought too, until one spring morning several years ago when Vladimir let me degrease the belly of a customer’s 55/M.

You couldn’t have made this up.
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Almost as though the 55 was infecting me with its subtle pheromones, by the time it was beginning to sparkle, I was already besotted with its ridiculously fat Vertical stabilizer, the bulbous shoulder wing, spindly model airplane gear and cool raked-back bubble canopy.  

Just can’t explain it.

There are certain viewing angles from which the 55 can appear less than beautiful. Like saggy gear. You don’t want saggy gear - and your propeller tips certainly don’t want to be that close to the ground either. (Thank heaven for TYC’s gear extensions! )

But soon afterward I had the opportunity to join those few, those happy band of brothers who have taken a 55 aloft. 

And that changes everything.

What an underrated, undiscovered plane this is!  

Back at Gorky Park, they had been able to scrawl a few things down on the vodka-soaked notepad, before starting to notice their breath was now making technicolor ripples in the night air. One item on the ‘clean sheet of paper’, was to ditch the weight associated with retractable gear.

With fixed metal gear, you could toss the emergency system, get by with just one tiny air tank for engine start, even ground-fill it!. (“We won’t even hook that compressor up”).

It was to be the end of the line for air brakes too.

Hydraulic puck brakes!...Once more three re-filled shot glasses crashed together.

Head held high, tovarich Kondratiev announced with certainty that the electro-pneumatic start valve had outlived its usefulness. Always requiring perfect tank pressure and a fully charged battery - Bah! - so ‘Western’. “A push-button air start” he declared to the frosty stars.

About the same time, tovarich Drach had become aware of spiraling iridescent carousel horses, corkscrewing up into the shimmering night sky above their beloved Capitol, above the amusement park, above the three huddled designers, none of whom were now feeling any pain.

“Even your thumb cannot push against 50 bar of air pressure, Comrade” noted Sergei Yakovlev dryly.

There was a long pause.  The beautiful Moskva river before them pulsated black and silver.

“We will have a lever” said Kondratiev with finality.

 ​Today, you can’t actually see this lever, or the valve, since they got hidden behind the panel. The push button is by the Magneto switch where you’d expect it though, under its black metal guard.

50 and 52 owners anticipate a characteristically prompt motor start when the button is pressed in their planes.

In the 55, it’s different. 

Here the mechanical push button slops around like some handmade key in a medieval lock, requiring half an inch of travel before you get any reaction.

That, unfortunately, is how it is with levers.  

But let’s give the guys their due, they certainly condensed the air start operation down to one valve.

Full Air tank pressure bears down on the backside of a piston whose other side houses a rubber seal hard against a raised lip in the valve body, preventing air leakage.

The aforementioned starting lever ( via the push button) rocks against a convex nut on the end of a shaft, moving a shuttle forward against the metal center of the sealing piston, pushing it off that lip. 

https://www.screencast.com/t/e0aqeO4Ouwe

Full pressure air from the tank rushes forward and up the sidearm of the start valve, out to the air start spider, onward to join a well-timed shower of sparks above primed pistons.

Kaboom!

But 50 bar of air pressure wants out of our little start valve in every direction.  

They had to design in a rubber shaft seal on the downstream side of this air start valve to reduce lost air during the start. ( item 10 in the diagram)

Then, once the engine was running and the Pilot had released the push button, re-seating the piston, the downstream side of this air start valve was still pressurized.  The push shaft ( item 9)  had to be drilled to vent the post-start pressure to atmosphere.

High level simplicity had bred lower level complexity.

Maybe because this start valve is hidden behind the panel and not in your face, it’s easy to forget that the rubber face of this Main sealing Piston ( item 8)  sees tank pressure 24/7 ( or for as long as there IS tank pressure) - because the guys decided to go without a main air valve too.  

Yep, there isn’t one on a 55.

And just like PRV pistons, eventually the sealing rubber takes a set. 
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And starts to leak. 

With only 3 liters of air on board, some 55 pilots get range anxiety for good reason. 

Heard any air hissing from behind the left side of the panel on your 55 recently? 

TYC is now overhauling these valves, pressure testing them for the absence of leaks before yours is returned to you.

Items 3,8 ( including its rubber insert, a rather tired one is shown above), 10,12,13 and 14 in the factory diagram above are replaced with new parts, sourced or made in the US.

(Item 13, the set screw is replaced with a Socket Head style and a suitable Allen key is included.)

This won’t fix the entire push button/lever linkage sloppiness, but it will return sobriety to your Air gauge while dialing down the pilot’s range anxiety.

The Yak 55 - possibly the last in a grand tradition of Yakovlev competition monoplanes - and almost certainly designed using a Slide Rule.
 
Before the “big nineties”, before the sun rose on the Sukhoi Designers with their computer graphics, exotic blends of titanium for this, carbon fiber for that, their constantly irritated red eyes and permanently itchy noses.

Love your 55 - it's an exotic masterpiece which asks only that it be viewed correctly-through the hangar doors of perception.