Why does increasing bleed air load increase ITT?

[ravisurdhar]

Yes, more like 75 percent is cooling and 25 percent is for combustion. The 60 percent Beef referred to is the amount of the energy, post combustion stage, that is lost to drive the compressor blades.

Perfect, that is exactly what I was looking for! Makes perfect sense now!

 

[BEEF SUPREME]

Yes, more like 75 percent is cooling and 25 percent is for combustion. The 60 percent Beef referred to is the amount of the energy, post combustion stage, that is lost to drive the compressor blades.

When I hang with the ladies I say "yeah I'm a pilot, and other pilots refer to me as Beef."

 

[z987k]

To answer your question it is because the 700 model uses p3 to pressurize. The 850 p2.5. They figured out the cabin size was small enough to use the lower pressure air and thus allowed the engine to be cooled better in a hot takeoff with the bleeds on. A bleeds off takeoff is not a tbm published procedure but is a commonly practiced one in the real world. Once you get good airspeed reduce the power a tad and flip the bleeds on. I think it's partly due to the smaller air inlet opening compared to other pt6 installed planes.

Wtf? It uses 2.5 for pressurization? So what does it do when it's using all the air from the axial compressor to feed the centrifugal compressor? P2.5 needs to be closed.

 

[auw2fly]

The 850's normal bleed air is P2.5. If system demand is high enough, it will automatically switch to "Bleed High", which is P3 air.

 

[z987k]

The 850's normal bleed air is P2.5. If system demand is high enough, it will automatically switch to "Bleed High", which is P3 air.

Ah, that works. It even.... makes sense. Which is way more credit than I would normally ever give the Canucks.

 

[DPApilot]

When you are talking about bleed air, the only reference is from the "core" or gas generator part of the engine. Commonly this seems to be denoted as "the core" "N2" or "Ng".

The bleed air extraction can come from 1, 2, or 3 places along the compressor section, often referred to as "LP" - Low pressure, "IP" - Intermediate Pressure or "HP" - high pressure. Engines usually have a source of bleed air for their own internal use ( pushed through the turbine stages to keep them cooler). Also there is a means for systems required extraction, (anti-ice, packs, hydraulic reservoir head pressure).

As pointed out by the other posters, most air going through the core is used for cooling and flame control. As you pointed out, there is advanced research into flame patterns and burning all the fuel versus pushing it unburned out the back (think of old school jets with thick black smoke trails). However, the more air we take from the engine, the less we have to control flame propagation and cooling the turbine stages. If we take too much air, the flame will actually touch the burner can and the resulting effect will be much the same as taking a cutting torch to that metal.

When you accelerate a turbine engine, you first see an EGT increase as there is more fuel added, then as the rpm increases, you get more airflow and the egt temp decreases as there is more cooling air once the engine stabilizes at a set rpm. You can also see the effect of HP and LP valves opening and closing. The best way is on an engine that automatically switches, in the ground. It's a hot day outside, so you want as much air as possible through the packs. You can push up the power on one engine, watch the EGT rise, then settle back. However, if you keep pushing the power up, you'll see the EGT suddenly cool as the HP valves close and the LP valves open. Same reason on a cross-bleed start, most engines require a specific N2 to start the opposite engine. It is close to the limit of the HP valve. If you push the power too far, you'll have the HP valve closed then the LP valve opens and you may have more power on the engine (80% N2 vs 70% N2) but less air to actually start the engine.

Perfect description!

 

[ryan1234]

Of course if you had a engine not designed by BACKWARDS Canadian's up front, N1 = N2 and we don't have Np gauges because that'd be pointless. N1 gauges tell you N1,2,p.

Well sorta... we have this whole PMU deal that overly simplifies the entire system so IPs and students can't mess it up. It's a tremendously simple system once it gets to the operator, even has a mostly linear power demand. All we see is Torque/Np/N1

There's just one big Power Control Level..... just like on a lawn mower.... Rabbit/Turtle sorta thing. No condition or prop levers.... as a matter of fact if you even mentioned a condition or prop lever to most of the IPs, they would look at you like you had three balls. It's a really, really simple airplane from the flying perspective... pretty docile.. the TAD tries to keep up with trim adjustments.. I don't really care for the trim system itself though, not during aerobatics or full power. The Radio Management Unit and GPS (if that's what you call it) are examples of lowest bidder options. For a fraction of the price of those fossils, they could have 530s. Overall the T-6 is a POS... but it does have a nice engine and ejection seat!

 

[Autothrust Blue]

Ah, that works. It even.... makes sense. Which is way more credit than I would normally ever give the Canucks.

The Brasilia is the same way, although the bleed air stage being used is more a function of where you have the bleed switches and how much power is being demanded from the engines. Unless you put the bleeds in AUTO and the packs in HI, but that's only done when you want "all the heat, right now" and then only briefly ("Smell that?").

(I think you can't give the Canadians credit for this one, is what I'm saying. Seems like it's more of an airframe-power plant interface deal.)

 

[CirrusMonkey]

You guys with all your smart talk.

 

[SpiraMirabilis]

Not only a majority, but the vast majority of air does not combust. I'm not sure about the numbers but I wouldn't be surprised if it was over 95% of the air goes in and out.