View Full Version : What is Flex?
3rd Aug 2004, 15:40
I keep hearing and reading it and I am yet to understand its basic relevance to anything.
All I know is that it is to do with taking off. A flex take off? For some reason I have "flex climb" and "flex cruise" in my mind too, but that might be my mind wandering. :ugh:
3rd Aug 2004, 19:59
In basic terms a 'Flex' Takeoff is a takeoff that uses less than full thrust (which is known as a rated takeoff) It is usually used whenever the aircraft is below it's max takeoff weight to reduce the thrust to a level where it will be less then full thrust but still meet all the performance requirments even in the event of an engine failure at or after V1. The benifits of this include increasing engine life and reliability largely as a result of having a lower EGT for takeoff.
4th Aug 2004, 00:02
Aircraft take-off performance is affected by many things. Aircraft weight, OAT, pressure height, runway available and obstacles to be cleared after becoming airborne.
Engine power basically reduces with increasing temperature.
One type of flex takeoff is the assumed temperature method where after consulting the RTOW (Regulated Take Off Weight charts - usually unique to each individual runway and take off flap setting ) one determines that for the atmospheric conditions on the day, and the weight of the aircraft, that the aircraft can take off.
Further it could take off at say 40 degrees C BUT this would require less than full power but the aircraft would still meet all regulatory performance requirements.
So, to conserve engine power and thus prolong engine life, the take off is performed as if the OAT is 40 degrees (in this example) despite the fact that it is lower.
On the 146s that I fly we can flex up to an assumed 50 degrees C. The limiting takeoff N1 for an ALF502 is up to 97.6% and we can flex back to 88%. The limiting take-off N1 for an LF507 is up to 97% and we can flex back to 89%. Both flex limits are company imposed and I believe more conservative that the flex limits approved by the manufacturer, although both allow signifficant power reductions without compromising safety.
The most signifficant difference is take-off run and subsequent climb to a safe, obstacle clear height. At Ref power (OAT & Pressure height dependant) the take-off run and subsequent climb will be shorter than at flex power.
p.s. We can and do use flex take-off at MTOW
4th Aug 2004, 07:34
So a Flex cruise is a myth I created :p I understand now. Just a bit curious as to what ALF502 and LF507 are? Engines I presume :confused:
You presumed correctly
Try this (http://www.globalturbine.com/components.html)
4th Aug 2004, 08:25
Try here (http://roger.ecn.purdue.edu/~propulsi/propulsion/jets/tfans/alf502.html) for data on the ALF502, and here (http://www.globalturbine.com/components.html) for a cut-away of an LF507.
I believe that the engine started life as a tank (military) and was then fitted to the Chinook helicopter, then fitted to the BAe146.
p.s. Sorry to double up on CR2's link. :zzz:
4th Aug 2004, 08:43
Oh, thanks. Interesting stuff. Amazing how someone drew these, made them and then screwed them onto wings.
Just one thing. "Flex". Is that a word on its own, or is it short for "flexible"?
4th Aug 2004, 11:24
Short for flexible.
Unless, I suppose, you're talking about the power cord of something electrical. Though even then, originally....
4th Aug 2004, 15:30
Flex Climb Thrust
I'm not aware of any users of Flex Climb Thrust, BUT, various levels of Derated Climb thrust are used in some aircraft.
For Example - The RR Trent uses CLB, CLB1, and CLB2, different ratings and pilot selectable. Above 10000 feet, all of these 3 become the same at full Climb Thrust (CLB).
Other operators may vary......
It's been a while since groundschool so forgive me for bugging you but could you clarify this:
"Further it could take off at say 40 degrees C BUT this would require less than full power but the aircraft would still meet all regulatory performance requirements."
I thought it wasn't so much that it requires less thrust, but that the higher ambient temperature means the engines run at a lower power setting because they are flat rated (I think that's the term used?). Hence the reason for forcing the engine management system whatever it maybe to think it's running at 40*C...the flex temperature?
That's how I've thought of it and think I've been taught up until now. I just want to clarify!
9th Aug 2004, 02:09
The aircraft requires less thrust (than is currently available) in the sense that at the higher temp. the engines produce less thrust BUT the a/c can still meet the performance requirements.
So, as temperature rises, there will come a point where the thrust available matches the thrust required to meet performance requirements. If you set exactly the same thrust in a lower temperature environment you will still meet the performance requirements. One way of doing this is by telling the system that the OAT matches this higher temperature limit. This is the assumed temperature.
The effect is that the engine produces an amount of thrust equivalent to what it would produce IF the temperature was at this higher amount however the thrust produced is still sufficient to meet performance requirements.
9th Aug 2004, 08:30
When I posted I knew what I meant. Now I see how I may have confused you. Tinstaffl has pretty well summed it up.
Thought that was how the system worked!
I understand! I think that way most of the time also.