De-rated engine: how?

[TangoWhiskey]

The EADS Socata TBM850 has a P&W PT6A-66D, 1825 shp, derated to 850 shp.

How do they "do" that? Limit fuel flow? Is a mechanical modification required (swap out parts) or is it just an "adjustment" of some parameters?

Can piston engines likewise be "derated"?

I presume the reason for doing this is to under-work an engine and thus increase it's lifespan. Are there other reasons?

 

[Areeda]

Troy,

My only experience with derated PT6's is in the Bell 206 JetRanger. As far as i can tell all they do is put the red line in a different place and it's up to you not to exceed it.

For the B206, my understanding is that the transmission was designed for an older version of the PT6. Each subsequent version increased SHP and they derated them to the level the transmission could handle.

Joe

 

[mikea]

I thought for the Lycoming (I)O-540 235/260/300HP they varied the timing (?).

 

[KennyFlys]

I thought for the Lycoming (I)O-540 235/260/300HP they varied the timing (?).

It seems like that would reduce efficiency? I thought it was simply a govenor of sort in the fuel-air induction.

 

[ScottM]

Troy,

My only experience with derated PT6's is in the Bell 206 JetRanger. As far as i can tell all they do is put the red line in a different place and it's up to you not to exceed it.

For the B206, my understanding is that the transmission was designed for an older version of the PT6. Each subsequent version increased SHP and they derated them to the level the transmission could handle.

Joe

For the derated 172s (180hp) to 172r (160hp) my understanding is that they do the redline in a different place and use a different prop, but I do not think the prop has anything to do with hp.

 

[Skip Miller]

For the derated 172s (180hp) to 172r (160hp) my understanding is that they do the redline in a different place and use a different prop, but I do not think the prop has anything to do with hp.

Correct, but the higher pitch prop puts more of a load on the engine which restricts rpm, and that does have something to do with horsepower.

-Skip

 

[gismo]

The EADS Socata TBM850 has a P&W PT6A-66D, 1825 shp, derated to 850 shp.

How do they "do" that? Limit fuel flow? Is a mechanical modification required (swap out parts) or is it just an "adjustment" of some parameters?

AFaIK turbines can be "derated" (more often the word "flat" rated is used) at least a couple of ways. One is to establish a torque limit that's lower than what the full rated output would generate and as mentioned it's up to the pilot to observe that limit just like with some turbocharged piston engines the pilot must observe a manifold pressure limit manually.

On at least some turbines with FADECs the FADEC computer handles the limiting and the pilot simply pushes the thrust lever (I think they still call it a throttle in a turbine heli) all the way and the FADEC prevents the torque from exceeding the set limit.

Can piston engines likewise be "derated"?

Yes, and again there are multiple methods. If the application involves a CS prop the prop governor can be adjusted so the RPM cannot exceed a value that would allow the limited power with the throttle open. Sometimes the limit is accomplished by requiring the pilot to keep the manifold pressure at or below a value that could be exceeded if the throttle was opened all the way at low altitudes (or higher if there's a turbo).

I presume the reason for doing this is to under-work an engine and thus increase it's lifespan. Are there other reasons?

With a turbine the limit either to protect the drivetrain (e.g. the main transmission in a helicopter) or to prevent a handling problem that might preclude meeting a certification requirement. Sometimes when a higher power turboprop engine is STCd to an airframe, the power is limited to what the original lower power engine provided which eliminates much of the expensive flight testing that would otherwise be required to get FAA approval.

With a piston engine the reason could be the same as the last turbine related one (limit asymmetrical thrust etc) or to reduce the airplane's noise footprint to pass some kind of noise limit requirement. In that case this almost always involves lowering the maximum RPM.

When the derating means is the observance of a torque or manifold pressure limit, the result is normally such that the limited power is available at a higher DA.

 

[stingray]

A lot of engines are derated, The Moonie is derated to 285. The most popular engine right now is the 550 they derate this to what they want for HP. One engine for different applications less parts less approvals. higher TBO's.

Dan

 

[TangoWhiskey]

Thanks for all the replies. All of the certification problems mentioned above can be addressed by putting a lower-powered engine in. Why install an 1825 HP engine to get 850 HP? That's more than DOUBLE the intended usable HP. Are there no 850, 1000, 1200 HP turbines?

I just feel like some part of the story is still missing.

 

[gismo]

Thanks for all the replies. All of the certification problems mentioned above can be addressed by putting a lower-powered engine in. Why install an 1825 HP engine to get 850 HP? That's more than DOUBLE the intended usable HP. Are there no 850, 1000, 1200 HP turbines?

I just feel like some part of the story is still missing.

Like I said, the larger engine will make 850 HP at a much higher altitude than one that can only make 850 at sea level. Often there are other advantages such as greater efficiency and/or lower maintenance costs.

 

[sba55]

Lots of reasons I can think of. Lower RPMs, less wear and tear on the engine, and the engine can run cooler. For the TBM, I seem to remember reading that temperature was the main concern with the previous models, which is one reason why they are de-rated.

Isn't the TBM nice? I figure in 5-6 years an early TBM 700 is going to be quite attractive. It fits my mission perfectly, too.

-Felix

 

[Teller1900]

As it was explained to me in indoc:

We use the PT-6A-67D, flat rated to 1279 shp, or 1310 eshp. The -67D is actually capable of over 1500 shp (and uses all of it in other operations, such as running oil derricks as originally designed), but that requires approx 5000 lbs of torque. Our gauges, in fact, go up to 5000 lbs of torque, but there's a red line at 3950 lbs (MTOP) and a yellow arc starting at 3750 (MCP, restricted to NGT 5mins at a time). We hit those lines on the gauges at just a little over half of the power lever's movement range.

At higher altitudes, torque no longer is the limiting factor, and the little red line on the ITTs at 800ish C becomes our limit. We're not going to be producing max hp because we're at the rated limit for the engine. 3950 torques gives us (theoretically) 1279 shp at sea level, and we could easily exceed that (keep pushing the levers forward) but we'd have a lot of paper work to do.

Someone mentioned FADEC engines...the Q-400 is FADEC control. Push the power levers up to the MCP/NTOP/MTOP detents and the computer sets the right amount of power. If you need more than that (RA/terrain/windsheer escape procedures), you can push the levers farther...you'll exceed the "rated" power limit (and rumor has it, in the 1900 at least, you'll look like you have after-burners) but the engines will give you a bit more.

 

[gprellwitz]

If you need more than that (RA/terrain/windsheer escape procedures), you can push the levers farther...you'll exceed the "rated" power limit (and rumor has it, in the 1900 at least, you'll look like you have after-burners) but the engines will give you a bit more.

In an emergency, I'd like to be able to use my PIC authority to DEMAND that extra little bit, cost be d@mned! If it'll save lives, screw the plane! (May the Lords of Kobol never put me in that situation!)

 

[flyingcheesehead]

Lots of de-rated piston engines, Troy...

* O-320 in an older 172 is 160hp, but it's de-rated to 140hp in a Cherokee 140
* IO-360 in an Arrow (except the Arrow I) is 200hp, derated to 180hp in the DA40 and 172SP, and 160hp in the 172R. In fact, the 172's both use the same L2A variant, and the difference in HP is a difference in RPM - 2700 on the SP, 2400 on the R.

Lots of other examples too.

 

[Tom-D]

Most turbins are derated by limiting the TIT or in other words limiting the amount of fuel you can pour on the fire. HEAT is the turbins power because it converts heat into rotating motion. by being a constant speed engine the fuel control will add fuel when the engine RPM slows down, and it will take fuel when the engine goes over speed, because of the load applied to the output shaft by the prop.

So in a round about method, it is the propeller control unit that limits the load the engine will carry.

 

[kgruber]

The EADS Socata TBM850 has a P&W PT6A-66D, 1825 shp, derated to 850 shp.

How do they "do" that? Limit fuel flow? Is a mechanical modification required (swap out parts) or is it just an "adjustment" of some parameters?

Can piston engines likewise be "derated"?

I presume the reason for doing this is to under-work an engine and thus increase it's lifespan. Are there other reasons?

The PT-6 is not derated. It is flat rated, and there is a big difference.

A turbine engine is just like a normally aspirated piston engine, as it loses power as altitude is increased. Now, TBM wants this airplane to go fast at altitude so they take a great big 1825 HP PT-6, hang it on there, but only allows the pilot to extract 850 horsepower from it.

As the pilot climbs he can maintain that 850 HP from it just by moving the power levers forward. (just like a piston engine) There are torque limits and temperature limits. At some altitude, probably in the low 20,000s of feet, the pilot will reach the temp limit and from there on higher the horsepower will decrease from the 850.

But it maintained the full 850 all through the climb.

If TBM would have put only a 850 thermodynamic HP turbine on the TBM. it would have run out of power right from the get go and would never be able to achieve the climb rates and speeds it gets at altitude.

Best,
Karl
"Curator" N185KG

 

[TangoWhiskey]

In fact, the 172's both use the same L2A variant, and the difference in HP is a difference in RPM - 2700 on the SP, 2400 on the R.

Interesting... kind of makes you wonder how they get away charging $40K "more" for 200hp! Same engine = same cost. Put a different prop on it, adjust fuel flow, "done!"! But $40K "done"? $243K vs $283K

 

[Richard]

Teller, it is confusing to me why you provide SHP and ESHP. All I can think of is those values are serial number specific. Please elaborate.

PS: The values appear to be so close that you wouldn't see the spread on any gauge except perhaps a torque gauge in a test cell. So why would the pilot have this information?

 

[gprellwitz]

Interesting... kind of makes you wonder how they get away charging $40K "more" for 200hp! Same engine = same cost. Put a different prop on it, adjust fuel flow, "done!"! But $40K "done"? $243K vs $283K

200hp? The SP is 180hp. It also has leather standard, which is worth a few thousand. Also, did you notice that the 406MHz ELT is available on "international only" and at over $7K, at that!

 

[docmirror]

I believe in most cases of same displacement piston engines, the HP and torque values are defined by the cam profile, timing, and carb and jet size, as well as redline. This is true of the older Continental engines. An old C85 can be changed to a C-90, and to an O-200 (100HP) by some changes in cam and carb jetting. The different variants of the 470 cube are the same thing. It started life as a 165HP engine, and finished with something like 260HP. Is the early version a 'derated' engine? I know they have different cams, pistons, carbs, ignition timing, and counterweights on the crank.

This has nothing to do with turbines, but HP ratings on same displacement piston engines has a lot to do with different parts used inside the engine.

 

[TMetzinger]

Electronics in their various forms have made power management so simple a cave man could do it, all the while while extending engine life and reliability while lowering fuel consumption and mx costs.

Hi, I'm from Geico, and I'd like to talk to you about trademark infringement

 

[gprellwitz]

There are many ways both piston and turbine engines are "flat rated" or "de-rated". Families of a basic, modern modular engine design can vary widely in power output due to the addition or exchange of specific components.

Most turbine engines are thermodynamic (ITT or EGT Red line) or mechanically limited (horsepower/thrust). You either temp or torque out, or said another way, first red line is limiting.

Torque is an airframe limitation as initially designed and certified into the aircraft, or put another way, how much stress or twisting the motor mounts can take or because of aircraft aerodynamic controllability issues, like the size of the rudder in case of an engine out.

To find the horse power output of a turboprop, the simple formula is:

Torque (in foot pounds) X Propeller RPM
--------------------------------------
5757

Electronics in their various forms have made power management so simple a cave man could do it, all the while while extending engine life and reliability while lowering fuel consumption and mx costs.

Limiting, yes. The question I have is whether a pilot can exceed those limits in an emergency? If there's a governor of some sort, then the answer appears to be "no." OTOH, if it's just a painted red line...

 

[Teller1900]

Limiting, yes. The question I have is whether a pilot can exceed those limits in an emergency? If there's a governor of some sort, then the answer appears to be "no." OTOH, if it's just a painted red line...

As far as our engines go on the 1900, and even on the Q with the FADEC...yes, absolutely. But again, we're only flat rated, so it's an artificial limitation...the engine is designed to do a lot more, but only certified to a certain amount. In the sim, though, they purposefully give us either a terrain proximity or wind sheer escape event that we simply cannot get out of without exceeding engine limitations. They want us to see that the engines won't self destruct if we exceed the red lines, just that it's a bad idea to do routinely. Our torque limits (takeoff/climb power) are at less than 3/4 the full travel of the power levers, so we can definitely exceed the limitations if need be.

There's a big chart in our book that dictates, based on the phase of flight (starting/taxi/climb/cruise/cruise-climb/descent/approach/landing/shut down) and what limitation is exceeded (and for how long), just what action must be taken. We can exceed some limitations with no required action, so long as it's very brief (eg: we can cross the 800C red line on the ITTs during engine start, so long as it's less than 15 seconds) however some situations would kill the engine until it gets either a hot-section inspection, borescope, or full tear down (more than 7 seconds above 1000C, the engine is done; more than 20 seconds above 3950ft-lbs of torque, the engine and gearbox are done).

 

[Teller1900]

Teller, it is confusing to me why you provide SHP and ESHP. All I can think of is those values are serial number specific. Please elaborate.

PS: The values appear to be so close that you wouldn't see the spread on any gauge except perhaps a torque gauge in a test cell. So why would the pilot have this information?

No particular reason, just force of habit. Every time our book refers to the hp of the engine, it gives both. I just got used to reading it that way, I guess. The only difference is, ESHP = SHP + whatever jet thrust we get from the exhaust stacks. There's no way for the pilot to know how much of a boost we're getting, or what effect it's having (which, in the grand scheme, is not much at all). At less than maximum output, we have no idea what our ESHP is, nor do we particularly care...it's just something the P&W people put in our specs a lot.

 

[Teller1900]

To find the horse power output of a turboprop, the simple formula is:

Torque (in foot pounds) X Propeller RPM
--------------------------------------
5757

That's awesome! I'm going to annoy the crap out of a lot of captains (more than usual) with that bit of knowledge!!

 

[kgruber]

That's awesome! I'm going to annoy the crap out of a lot of captains (more than usual) with that bit of knowledge!!

Before you bug them too much, better get the constant right!

5252

 

[n20junkie]

In Chinooks, we use around 11,000 SHP. The engines will make much more than that, but like it was mentioned, "Flat" rating means that we will make AT LEAST 11,000SHP at sea level, and as we climb, and performance drops, we will continue to make that required 11,000SHP. This over powering (at sea level) gives us the ability to make rated HP at very high altitudes (for a helicopter) and so much so, that we blow the doors off of any other helicopter at altitude.
Also, the use of a PT6 is like using a small block chevy for cars, its so common and its parts are so common your almost stupid to use a lesser engine. Take the few pounds of weight added, and use the PT6, its maintenance and rebuild costs will be much lowered, and with its cooler tempratures during takeoff, she will run longer inbetween TBO's, and be more reliable due to the cooler TIT. Turbines use heat to make power, but less heat is better for the metal. Good thing in single engine applications.