Turbofan operations/limits/aerodynamics

jrh

Well-Known Member
I'm currently in training to fly as SIC on Citation II/V series aircraft. I'm trying to wrap my head around a lot of the technical aspects of turbofan engine operations. I have experience with all kinds of piston engines, but very little turbine experience.

I think I'm understanding the mechanical side of things no problem, such as how the engines are physically put together and overall principles for turbine powerplants. It's more the background of how various limitations are developed.

For instance, power settings for takeoff, climb, and cruise. Under some conditions power will be set at 96% N1, 98% N1, 101% N1, etc. Sure, I can read tables and push the power levers to the appropriate setting, but I have no idea of what's causing the number to be higher or lower under a given situation. If the engine is rated to 104% N1, why can't we push the power to 104% all the time and not worry about it?

Any recommended reading for this sort of information? When learning about aerodynamics, seeing graphs of lift over drag, lift versus power available, etc. really helped me understand the deeper relationships with Vx, Vy, best glide, stall speed, and so on. I'm looking for those kinds of graphs to explain what happens with turbofan engines throughout flight for power output, fuel efficiency, limitations, etc.

Thanks!
 
We use flex temperatures to reduce takeoff N1 to achieve adequate performance at lower ITTs. Keeping engines cool(er) helps them last longer.

As far as thrust deviations from a set percentage I would point you to temperature as being the biggest contributing factor. However, even though there is no "M" in FADEC, I assure you there is magic. Whatever the FADEC calculates is prohpehcy and will be interpreted as nothing but divine.

Good luck.
 
This is WAY oversimplifying it, but engines can take higher power settings if temps are lower. This would be caused by being at climb or cruise rather than beginning a takeoff roll with no airflow. Another is that most engines are only rated for certain power settings for a certain period of time. So it may be time limits, and temp caused by airspeed and altitude or a combination of those things that change the limits.
 
I'm currently in training to fly as SIC on Citation II/V series aircraft. I'm trying to wrap my head around a lot of the technical aspects of turbofan engine operations. I have experience with all kinds of piston engines, but very little turbine experience.

I think I'm understanding the mechanical side of things no problem, such as how the engines are physically put together and overall principles for turbine powerplants. It's more the background of how various limitations are developed.

For instance, power settings for takeoff, climb, and cruise. Under some conditions power will be set at 96% N1, 98% N1, 101% N1, etc. Sure, I can read tables and push the power levers to the appropriate setting, but I have no idea of what's causing the number to be higher or lower under a given situation. If the engine is rated to 104% N1, why can't we push the power to 104% all the time and not worry about it?

Any recommended reading for this sort of information? When learning about aerodynamics, seeing graphs of lift over drag, lift versus power available, etc. really helped me understand the deeper relationships with Vx, Vy, best glide, stall speed, and so on. I'm looking for those kinds of graphs to explain what happens with turbofan engines throughout flight for power output, fuel efficiency, limitations, etc.

Thanks!
The power setting you are aiming for with the sub 104% is a MINIMUM power for the day (conditions). Your take off numbers are predicated on that power setting being achieved by 80 knots (as well of a whole host of other things).

If you want max power out of that engine, then you push the levers up until you hit the first engine limitation, N1, N2, ITT, etc. Essentially you are setting a poor man's FLEX power in these engines and the whole rationale is over haul/hot section inspection times. To sell the engine as a, say, 3000 hour engine, every take off can't be at MAX power.

Now, this isn't true of all bizjet engines. The Challenger 300 gives you 6800 pounds (round numbers) of thrust on every take off that is within the SL and ISA +15 window (you can figure the density altitude). The newer engines can get away with this because they are essentially derated. The HTF7000 (CL30 engine) is capable of something north of 9000 pounds of thrust. Again, MX times are a big factor. The Lear 45 engines are the same at 3500 pounds of thrust. On these two engines, you don't see max N1 or ITT/EGT because the engines are derated or more simply, just capable of much more thrust than required by design and performance. The Lear 45 has two engine types an AR and BR model. Both engines are identical, the AR can be "upgraded" to the BR via a software updated to the DEEC, performance package upgrade in the FMS and some rerouting of fire loops. All this upgrade does, is allow the engine to run hotter and slightly faster N1, giving you 3500lbs of thrust up to ISA+20 (from sea level). MX intervals stay the same.

Don't ask me why the max limit is 104% and not 100% somebody somewhere thinks they're funny.
 
I was told, and I don't know if it's true or not, but engine failures in turbine engines typically happen at max takeoff thrust. A FLEX takeoff reduces your chances of an engine failure essentially.
 
I'm currently in training to fly as SIC on Citation II/V series aircraft. I'm trying to wrap my head around a lot of the technical aspects of turbofan engine operations. I have experience with all kinds of piston engines, but very little turbine experience.

I think I'm understanding the mechanical side of things no problem, such as how the engines are physically put together and overall principles for turbine powerplants. It's more the background of how various limitations are developed.

For instance, power settings for takeoff, climb, and cruise. Under some conditions power will be set at 96% N1, 98% N1, 101% N1, etc. Sure, I can read tables and push the power levers to the appropriate setting, but I have no idea of what's causing the number to be higher or lower under a given situation. If the engine is rated to 104% N1, why can't we push the power to 104% all the time and not worry about it?

Any recommended reading for this sort of information? When learning about aerodynamics, seeing graphs of lift over drag, lift versus power available, etc. really helped me understand the deeper relationships with Vx, Vy, best glide, stall speed, and so on. I'm looking for those kinds of graphs to explain what happens with turbofan engines throughout flight for power output, fuel efficiency, limitations, etc.

Thanks!
im not a jet engine guy, but I would guess it's analogous to a density controller controlled turbo system-at a colder temp you're making the same thrust at a lower N1 because of less dense air, hence for rated power your N1 limit is lower. I don't THINK that with something like a CII or CV all this other complex stuff guys are talking about regarding flex thrust etc plays into it.
 
I was told, and I don't know if it's true or not, but engine failures in turbine engines typically happen at max takeoff thrust. A FLEX takeoff reduces your chances of an engine failure essentially.

I believe it was a study by Boeing. The probability of an engine failure was higher when demanding max rated power.


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I was told, and I don't know if it's true or not, but engine failures in turbine engines typically happen at max takeoff thrust. A FLEX takeoff reduces your chances of an engine failure essentially.
TBH that sounds like the old wives tale about piston engines failing during the power reduction after takeoff. Now, I THINK flex takeoffs still reduce your aggregate chance of engine failure due to putting less stress on the engine.
 
@jrh,
For simplicity's sake, if you have a good grasp on piston engine performance then you'll understand Turbine performance. Just relate the differences in takeoff power settings (N1) in a turbine to the calculations you had to make when Density Altitude changed while in your piston prop. As DA changes, so does the performance in both engines.

You'll notice from your checklist charts that as you climb, you can place the power settings higher (just like in your prop plane).

As far as the FLEX discussion, I think it's a good idea only from the aspect of keeping the throttles matched. For me, it's easier to manage with one hand and to understand what is going on in all modes of flight.....this is technique only so find one that works for you.
 
I'm currently in training to fly as SIC on Citation II/V series aircraft. I'm trying to wrap my head around a lot of the technical aspects of turbofan engine operations. I have experience with all kinds of piston engines, but very little turbine experience.

I think I'm understanding the mechanical side of things no problem, such as how the engines are physically put together and overall principles for turbine powerplants. It's more the background of how various limitations are developed.

For instance, power settings for takeoff, climb, and cruise. Under some conditions power will be set at 96% N1, 98% N1, 101% N1, etc. Sure, I can read tables and push the power levers to the appropriate setting, but I have no idea of what's causing the number to be higher or lower under a given situation. If the engine is rated to 104% N1, why can't we push the power to 104% all the time and not worry about it?

Any recommended reading for this sort of information? When learning about aerodynamics, seeing graphs of lift over drag, lift versus power available, etc. really helped me understand the deeper relationships with Vx, Vy, best glide, stall speed, and so on. I'm looking for those kinds of graphs to explain what happens with turbofan engines throughout flight for power output, fuel efficiency, limitations, etc.

Thanks!
Don't flame out at the top of the loop... other than that it's just all logical extensions of suck, squeeze, bang, blow ;)
 
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JRH welcome back man. Haven't seen you around in a while.

I used to fly the 560 and 550 series citations so if you have any questions shoot me a PM.
 
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