3-D instrument approaches
April 25, 2019
Overpressure valves: Stall or loose the torque?
November 10, 2019
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When is a PT-6 like a Barbeque?

N844X is just not as fast as it used to be.
Suddenly, my engine temperature is running 70 degrees C higher, my gas turbine is spinning 1% faster, and my fuel flow is a gallon per hour higher, just to maintain the same torque.
And since that temperature is so much higher, I have to fly at a low throttle to stay within temperature limits!
I didn’t buy this airplane to go Mach 0.40! I want Mach 0.50+!

So we washed the compressor, inspected the compressor, checked our temperature probes, EVERYTHING.
We can’t find any problems.

So what’s wrong with my engine?

Well, there is one thing we have not checked because it is hard to check: The over-pressure dump valves.

Here’s the deal: The PT-6-42m, which I use on my Evolution, was amped up to super-duper pressure when designed, to bat out crazy horsepower up to VERY HIGH ALTITUDES.
This lets you go super-fast up high.
BUT, at Pratt and Whitney they upped that compressor OOMPH so much to do this that it would MESS UP THE FLOW IN THE COMPRESSORS AT LOW THROTTLE!

SOOO Pratt hacked the system: They put in two OVERFLOW valves that open at LOW engine speed to dump out the extra pressure (in-efficient! Boo!), BUUUUT, when that engine turbine speed revs up to full turbine speed when I open that throttle, the valves close, the flow becomes perfect, and ZOOOOM that RACES to insane power, even at high altitudes!

So the trick is to have those valves OPEN at low speed to dump out the extra pressure, and closed at hi speed to funnel the flow perfectly and make maximum power.

And we THINK those valves are now STUCK OPEN, NEVER CLOSING!

And as a result, that poor engine is leaking pressure at high-power cruise, and not really delivering maximum torque.

And Pratt does NOT have any sensors on the engine to TELL you what the valves are doing! BOOO!

So how do we find out what the valves are doing?
We take some hoses and duct-tape them to the outflow valve, let them run along the wing to a bucket of water sitting on the ground beside the plane, and run the engine up! At low throttle, the hoses should be bubbling air into the buckets of water, and at high throttle, the bubbling should stop as the valves close!

So it’s just a ground run-up!

Oh wait.

I don’t even think I can do that.

Asking for that much turbine speed on the ground would be WAAYY too much torque!
We like to have full turbine speed up in the thin air of high altitude to keep our torque within limits!
Full turbine speed at sea level would be a disaster! Way too much torque!
And the damage to the prop and turbine from running at that high a power on the ground, ugh!
And the maintenance guy said the plane has to be full of fuel to even stand a CHANCE of not dragging the tires across the ground from the runup thrust!
Remember, we take off at HALF POWER in this airplane! Full power on the ground is unheard-of!

So how do we run the test? A really long hose to reach down to the ground when in flight, like string on a kite?

I have a better idea.

Let’s measure the TEMPERATURE just outside the overflow valve.
Izzit cold? Then the valve is closed.
Izzit hot? Then the valve is open.

But how do we measure this?
I don’t want to drill holes in the firewall to run temperature sensors around the plane.

Well, you know how some people get all obsessed with the temperature of their BARBEQUES?
Is it too hot? Too cold? Just right?
They need a TEMPERATURE sensor for their barbeques, and it has to be WIFI from the probe to a read-out, so they don’t have cables getting all melty going into their grills!

So I think you see where this is going:

We’ll attach these remote wifi temperature sensors ($39.99, amazon.com) just outside of each outflow valve (or two per valve for redundancy, why not?) and look at the temperature readings in the cockpit! YES! I will have outflow valve temperatures in the cockpit now!

So in flight I can just check the temperatures on those probes like any other engine instrument: If hot, the valve is open (needed at low throttle to avoid engine damage), and of cold, the valve is closed (desired at high throttle and altitude to send all that air to the prop where it can do some useful work!)

I can just leave this in the airplane, and then I will have the only PT-6-42 that I know of that has overflow valve temperatures displayed in flight to monitor the engine valve status in normal flight! Cool!!!!!!

I’ll report on how it goes when we run the test, which will be soon.