High TOT in flight EP’s
 

How would lowering the collective (that is, reducing power) help to cool the engine? I
understand that it is less fuel, but less fuel means a lower Ng, so less air coming in since the compressor is now giving a decrease in air pressure. So in a sense isn't fuel flow proportional to Ng which means a nearly equal TOT would occur?

As the collective is lowered, the power required out of the engine decreases, so fuel flow reduces and Ng falls, along with back end temperature. The fall in Ng reduces the internal power required to drive the compressor, so a further decrease in fuel flow and a further fall in back end temperature follow.

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How would lowering the collective (that is, reducing power) help to cool the engine? I understand that it is less fuel, but less fuel means a lower Ng, so less air coming in since the compressor is now giving a decrease in air pressure.

So in a sense isn't fuel flow proportional to Ng which means a nearly equal TOT would occur?

You kind of answered your own question in your first paragraph.

Lowering the collective reduces the load on the PT/N2/NR, which, without a corresponding reduction in fuel flow (Wf, governed by the FCU), would result in a PT/N2/NR increase. As such the FCU automatically reduces Wf, which results in a corresponding reduction of Ng/P3 & ITT.

So in a sense isn't fuel flow proportional to Ng. Yes and no. As a result of a number of factors, the correlation between Wf and Ng is not always a linear function.

I believe your last sentence is what stumps me. How could fuel flow and Ng not be linear assumed they are controlled by the FCU. If i could grasp how they are different, I believe that would help to answer the root of my question. I appreciate your reply!

I am assuming you are referring to a turbine engine with separate gas generator and power output turbines (Ng and Nf are not linked).

Remember that the majority of air is used to shape the flame and cool the combustion gases before they interact with the turbines (because the combustion product typically peaks around 2100°C). The efficiency of the compressor blades to generate a flow of air through the engine varies as Ng speed changes, so the correlation is not linear (in fact it is very complex, because air pressure, temperature, engine speed, engine acceleration, etc all factor into the temperature of the engine).

Hopefully, a simple way to understand it is that ITT / TOT / T4 increases as the collective is increased because an increase of fuel flow to the nozzles will increase the energy input into the engine. The main product of this energy is heat, while secondary products are kinetic and mechanical energy.

Thank you for weighing in Apollo. I believe we are kind of saying the same thing aren’t we? I am implying that that compressor efficiency is directly related to Ng speed. So as Ng decreases compressor efficiency decreases and they are proportional in that sense. So even though fuel is decreased….. so is Ng …. So is compressor efficiency. So I understand how it would be a little cooler with the reduction in fuel but not a significant amount. Am i seeing this correctly?

Fun fact true in most helicopter turbine engines. “About 70% of total engine power is dedicated to spinning the compressor section and engine driven accessories in an engine.
So if you are pulling 100% Q that equals 500 Hp on a turbine engine to drive your rotor system the total horsepower being generated inside that engine is +- 1667 Hp.

See I did remember a little from my 250 course all those years ago.

No….high TOT / EGT / T4 is not the result of “Flying too close to the sun”! (Actual answer on the course by a fellow having a major helmet fire during one early morning verbal quiz and upon having drawn a complete mental blank as to possible causes of abnormally high TOT and just went for giggles. It took 5 minutes before the instructor and class got themselves back to order. Especially humorous because he was a real smart fellow and a very switched on dude.)

I am implying that that compressor efficiency is directly related to Ng speed.

Again, yes and no.

As a general rule, compressors are designed to be at their most efficient at their optimum ‘Design Speed’, which is neither at its slowest nor fastest speeds.

Think of a commercial airliner engine. Cruise power is where the engine is designed to be at its most efficient, as this is where the engine will spend the majority of its operation, so ‘Off-Power’ settings below (Taxi) and above (Take-Off) the optimum ‘Design Speed’ would result in reduced efficiency, so faster does not necessarily mean more efficient.

As evidence, modern helicopter gas turbine engines can incorporate VIGV’s and Bleed Valves, so as to prevent stalls and surging during the compressor speed range and having a bleed valve open may well prevent a stall, but it does little for efficiency.

Also, during operation of the VIGV’s or Bleed Valves, the correlation between Wf and Ng is unlikely to be a linear function, due to airflow disturbances and bleeding.

Could you give a specific example of a POH inflight EP for high TOT where lowering the collective is the response.

I think lowering collective to control high TOT is SOP. under normal operating procedures.

Only time it happened to me the rapidly increasing TOT even as I lowered collective followed by a loud bang, rapidly decreasing Nr, sounding of horns, flashing lights and high rate of descent did lead me to execute the “Engine Failure in Flight” EP.

The fellow in the 500 behind me said it looked “really neat” what with the smoke and flame from the exhaust and everything else as I plunged toward the planet.

I think you may be overthinking it. Simply put more fuel = hotter.

there’s a lot going on in the engine, for one higher N1/Ng will mean higher intake temp.
Also primary combustion uses only approximately 20% of the compressed air then around 20% secondary air is used to complete combustion the rest is mostly used to cool the engine. I suspect part of your answer may come from the proportion of this ratio as it changes for different fuel flows and is not linear compared to compressor speed but that would be a guess.

A running engine with flat pitch will burn less than half the fuel but the compressor isn’t spinning at half the speed.

Definetly not linear. For example, I noted some NG/TGT valies in my last flight (GE CT7-8A):
Ng TGT
95.6 865 (cruise)
68.0 853 (idle, cooling down)
73.8 782 (cooling down, adjusted to keep NR outside the avoid range)

I agree 100% with the first part. I actually tried to find a chart showing the temperature gradient of compressed air. The only thing i could find were a few statements saying compressed air is approximately 500c.

Obviously as in your wonderful example Ng and TOT are not linear as those ratios would be approximately 1:9, 1:12.5, and 1:10.5 but fuel flow and Ng would be wouldn’t they? I believe SLFMS is correct when he stated, “ I suspect part of your answer may come from the proportion of this ratio as it changes for different fuel flows and is not linear compared to compressor speed but that would be a guess.”

That makes lot of sense as to why an engine would get hot without exceeding Ng limits. Because the compressor might be in the backside of its optimum efficiency at a high power setting.

Weads, I think you have been given the answers above.

Basic effects of controls sortie on a turbine helicopter shows raising the lever increases Ng and ToT and lowering the lever does the opposite.

You are unlikely to exceed an Ng limit because that will be set in the FCU with bleed valves to dump fuel pressure. You can easily overtemp an engine without exceeding Ng limits.

Most aircraft I have flown don't have a PTIT/ITT limiter so it is the one thing you have to keep an eye on. More modern aircraft may use a Power Index (PI) to show that you are near a limit without specifying which one.

Cooling the engine is critical so whilst the fuel flow/Ng/ITT relationship might be linear, or close to it, the cooling effect of even a reduced amount of air may well not be linear.

I don't think it's realistic at all to expect linear relationships between anything, given the complexities and interactions of the various processes - just look at a Brayton cycle graph for starters. Then, everything involved is mostly following a square or cube rule, some power consumption will be fixed, some variable. Forward speed versus stationary, IGVs, Bleed Air Valve thresholds etc etc