Welcome to the world of turbines when an oil sample costs an AMU

A SOAP kit (oil sample) costs $1025?

I think it has a filter in the kit, but still, that seems excessive. I recall being annoyed when they were $400/each.

Mike C.

Hilgard - glad you went with Brian at Arkansas, I really like him. He's fair, he knows his stuff and he can source parts.

Did I tell you abut my buddy who "tuned" his, with a full Lebow? They found another 100hp aside after that and the old 681 gained like 10-15kts in speed. Worth doing.

Pray tell, Adam!

To my knowledge, the only way to increase power on a properly functioning TPE331 is to reduce the clearance between the compressor impellers and the housing. This is done by flame spraying additional material onto the housing. With the clearance reduced (to basically zero) the compressor moves more air, lowering EGT which allows more fuel, hence more power. The problem is that there are situations where the different parts of the engine are at different temps (mostly on the ground) and the compressor impeller contacts the housing, reestablishing the original clearance and you are back to where you started. 100hp seems aggressive but it depends on the condition the engine started in

5he process is referred to as “reshimming the compressor”

As I understand it it's a torque test on stands and the equipment referred to measure that is called a Lebow Test Cell. Many engines are temp limited, but not necessarily also torque limited and it's quite common that they're under-torquing. So you can bring the torque up to match. In conjunction with other smaller improvements like FCU tuning and perhaps compressor tweaks, this can unlock old performance that had degraded.

For instance, turning the FCU up max on the unit gauge is not the only way to increase FF, there is also an internal one where it can be adjusted. This is useful for older FCU's that might loose "oomph" at altitude. Normally they always push you towards a $30K+ overhaul once you've reached the max external setting, but there are ways to tune these things that avoids that (or at least keeps you flying a bit longer until you have to).

With smaller shops like Brians etc, they seem to enjoy helping owners extend the usefulness of their engines like that.

Your understanding is not quite correct. The engines all have both torque (mechanical strength) and temp (material temperature(deformation or melting))limitations .

The torque is limited by the strength of the gearbox and additionally derated by the airframe manufacturer for VMC or other airframe limitation.

The EGT temperature is limited by the materials in the engine. In particular, the turbine blades will tend to grow length wise (“creep”) as they are exposed to tension from rotation at elevated temperatures.

The “thermodynamic power “ is what the engine could produce if there were not gearbox or airframe limitations. It is what we are trying to increase. It is what determines the *available* torque. We want more power without exceeding the EGT limit

In order to produce more power the engine has to be able to burn more fuel which requires more air for cooling. You can think of diluting the combustion products with cooler air to reduce the temperature to the limit of what the hot section can take. It doesn’t matter what adjustment is used, for any amount of fuel going into the engine, there will be an EGT which will vary with the amount of air moving through the engine. Of course environmental conditions affect this (colder OAT will result in lower EGT both because it’s colder and because it’s denser) but this is independent of the engine

Of course none of this applies if the engine isnt running or adjusted properly but in this case , you aren’t increasing power by tuning, you would just be repairing the engine to get it to function properly. In short, if the engine doesn’t have a defect, additional power comes from getting more air in, allowing more fuel to be utilized.

Hmm, what he is really doing is setting things so you can run the engines harder.

The common trick to doing this is to play with the ET compensation so the readings are artificially lower than before. This allows you to push more fuel into the engine but see a lower EGT.

If there isn't any changes to the compressor and turbine section, then more power comes only from more fuel which means more temperature.

This will not extend the engine life, but it may make it more "useful" (more powerful).

There is no magic "tuning" one can do on a turbine engine to restore lost power from wear. The only thing that truly restores power is improving the rotating parts which costs much $$$.

Mike C.

Hmm, what he is really doing is setting things so you can run the engines harder.

The common trick to doing this is to play with the ET compensation so the readings are artificially lower than before. This allows you to push more fuel into the engine but see a lower EGT.

If there isn't any changes to the compressor and turbine section, then more power comes only from more fuel which means more temperature.

This will not extend the engine life, but it may make it more "useful" (more powerful).

There is no magic "tuning" one can do on a turbine engine to restore lost power from wear. The only thing that truly restores power is improving the rotating parts which costs much $$$.

Mike C.

Tanks for the clarifications.

The EGT temps on the early Commanders is a calculated value, taking lots of things in, put through a compensator to get the number. It's not an ITT value. Therefore it's entirely possible that you can be limiting your power, or running way short of max temp.

I think the gains my friend saw was all of these factors adjusted, corrected or "tuned" up. He was ecstatic after picking it up at how much more power it seemed to have.

Depends on what you mean by "harmful".

If an engine is run at max temp, it doesn't last as long as one run cooler than max temp. Honeywell presented this data at a number of MU2 PROP sessions showing that being 50 C off limit made a significant difference in wear on the engine hot section parts. Basically, it was always the case that hotter was higher wear.

If the engine is "tuned" to use up all the compensation margin, so that you are operating at the actual red line when the gauge is there, you will be getting less life from the parts. They should still make TBO, but maybe they won't pass HSI and have to be replaced when they might have passed otherwise.

There is no free lunch, more power is more wear, one should be aware of this.

The other risk is that whatever the compensation range is on the DSC (data sheet customer, the card that gives the compensation numbers from the test cell run) may not apply to an aged engine any more, and/or the mechanic makes the adjustment incorrectly. So you could end up over the limit some amount. Additionally, you have used up the safety margin for times when you might not be super attentive to temps and go slightly over by mistake.

My -10 engines had 38 and 49 C compensation margins which meant I could "overtemp" them by those amounts on the gauge before I really reached the actual temp limit in the engine. That gave a little peace of mind so that I had some margin to play with.

Mike C.

Correct that the ITT is not measured in front of the first stage turbine wheel. That is where you really want to know the temperature but a thermocouple in that location doesn’t last long. So EGT is measured and then “corrected” so that redline on the gauge is the actual ITT limit. The system is somewhat complicated because the relationship between ITT and EGT is not linear. The P2T2 sensor (inlet conditions) is connected to an air data computer and all the results are then run through the SRL computer. The only input that is adjusted is the temperature compensating resistor. As mentioned previously noted, it can be adjusted in a range listed on the DSC sheet for that engine.

A 100hp improvement would likely be the result of replacing a failed compensating resistor. It is unlikely that this magnitude of improvement could be realized any other way on an airworthy engine

A turbine engine at its most basic is a very simple device. Air blowing over the little blades on the turbine wheels produces power. The amount of power is simply a function of the mass and pressure of that air. The mass is determined by how much the compressor can deliver and the pressure is a function of how much that air has been heated. The improvements in turbine engines have almost all been higher compression ratios and getting the turbine blades to withstand hotter air flowing over them. The latest engines utilize tiny holes in the turbine blades with bleed air blowing out. This creates a film of air and the combusted air flows over that so the the blades never actually contact the combusted air. The result is efficiency close to a diesel engine

A comment on Lebows:

They are normally used for ground runs, but we flew with one once at Cessna (required modifications at front of engine) and it did not like the decreasing temperatures as the airplane climbed, resulting in decreased accuracy.

To my knowledge, the only way to increase power on a properly functioning TPE331 is to reduce the clearance between the compressor impellers and the housing. This is done by flame spraying additional material onto the housing. With the clearance reduced (to basically zero) the compressor moves more air, lowering EGT which allows more fuel, hence more power. The problem is that there are situations where the different parts of the engine are at different temps (mostly on the ground) and the compressor impeller contacts the housing, reestablishing the original clearance and you are back to where you started. 100hp seems aggressive but it depends on the condition the engine started in

5he process is referred to as “reshimming the compressor”

Had a gearbox done at Arkansas Turbine in my MU2 days & they done a great job.