SurClaro Flight Simulator Resources

Posted: Mon Apr 03, 2006 5:13 pm Post subject: Thrust settings vs time...

A small question. In a real aircraft like a 747 or A300/310 etc. or any jet for that matter, I know that there are different recommended thrust settings like take off thrust, climb out thrust, cruise settings, etc., all maybe dependant on factors like payload, weather etc.

But over and above all these, I have heard that there are other limiting factors. What I mean is, for example, there is a time limit for how long takeoff thrust can continously be applied, safely. Is it true? Is there a limiting value for take off thrust, climb out thrust etc in terms of time? I believe that cruise thrust can be applied for as long as you want (of course!), but what about take off thrust? Or is it just dependant on Engine temperature values?

Posted: Mon Apr 03, 2006 7:25 pm Post subject:

In modern airliners you have what's called "EEC" and that's Electronic Engine Control. This system allows a lot easier operation with a lot less strain on the pilots and eliminating the possibility of a flameout due to sharp throttle lever movements causing rapid increase in engine fuel demands. Another thing it does is that it gives out the maximum "safe" engine rpm settings. It will limit the N1 (or EPR depending on engine type) to certain values and they are almost a direct function of outside temperature. There are other factors, such as the runway slope, barometric pressure and such. For takeoff on modern jetliners, it's rare that the absolute full power is used. Most of the time it's calculated so that, with the given weight, and given runway length, you use as little thrust as possible to keep it a safe takeoff but still as little stressing the engines as possible. The top most limit for an aircraft that is already airborne is called the "CON" and it's the maximal continous power that can be applied without the risk of an engine damage or fire. On older jets, such as the 727, you have charts and tables and according to temperature and pressure altitude of the aircraft you have limits that ensure you won't over stress them.

This is engine limitations in a nutshell, don't hesitate to ask if you'd wish to know more. Wink

Posted: Mon Apr 03, 2006 7:27 pm Post subject:

I believe that that is true. I know engine temperature in jet engines limit the amout of thrust applied, but I don't know about time. I have HEARD that engines on a 747-400 can only be on a certain thrust setting for a certain amout of time, although I'm not sure if that is true.

I heard that in a Google Video that was linked through Surclaro right here in the forums. It was about passing cars behind a 747-400's engines to prove how powerful they were.

EDIT: I started typing this post before Max posted his.

Posted: Mon Apr 03, 2006 7:32 pm Post subject:

whoa! I wanna see that movie clip!!! Where did you get that?? I am curious as I am a big fan of the 747, now I want to see its true power lol.

Capt. El Very Happy

Posted: Mon Apr 03, 2006 7:44 pm Post subject:

www.flightlevel350.net is the website, but I forgot which one.....do a search.

Posted: Mon Apr 03, 2006 8:49 pm Post subject:

Thanx for the site, but there are thousands of them and can't find the specific one. Can you at least give me a key word(s)? Thanx in advance.

Capt. El

Posted: Mon Apr 03, 2006 9:05 pm Post subject:

One is at: http://www.alexisparkinn.com/photogallery/Videos/Jetblast.mpeg

But this was not the one I was referring to earlier. The one I was referring to had a British narrator and it was a "documentary like" with a Virgin Atlantic 747-400.

I can't find it!!

Posted: Tue Apr 04, 2006 12:05 am Post subject:

You can find it here:

http://www.youtube.com/results?search=747&search_type=search_videos&search=Search

Scroll down untill you see:
Boeing 747 Crosswind

have you ever imagine how does it feel like to be behind a 747 when take off, watch the video and you will know
Tags // Boeing : 747 : Crosswind : take : off : thrust
Added: 3 months ago by harryleung
Runtime: 03:23 | Views: 3510 | Comments: 2

You will need MAcromedia Flash Player to watch it though.

Posted: Tue Apr 04, 2006 2:41 am Post subject:

Thanks Max. Now on these older jets, after you calculate takeoff power setting, do you also have a maximum time duration for applying that power? And does that time vary with the actual power setting?

And in a modern jet with EEC, does the system also tell you what is the maximum safe duration for applying this power, say like take off power? If you apply takeoff power for a longer than safe duration, how does EEC react? And is there an override for this system? Will you have to override this in case of an engine failure during a critical phase?

By the way, awesome Link!

Posted: Tue Apr 04, 2006 5:54 am Post subject:

The EEC simply "locks" the throttle....actually you can imagine it as a joystick and the EEC re-calibrates it so that in full forward position it doesn't make the engines go 110% N1 but only 103%....and that power is safe to be applied for un-limited amount of time. What ever the EEC limits, it's safe to use it. If the worst happens, like the engines heating up more then they should....the EEC Will lower the limits to maintain safe operation.

The EEC can always be disengaged using the guarded switches usualy located on the most top part of the overhead (Boeing aircraft)

As for the older jets, here's the data directly from the 727-200 manual. Takeoff maximum EPR at Sea Level with outside air temperature at 10 degrees centigrade is 2.00, while the maximal continous EPR at sea level with temperature 10 is 2.12 (or 2.14 if the Bleed Air is off). From theese simple figures, you can notice that takeoff power can be kept for unlimited amount of time, but it is always our best interest to keep the engines at high RPM for as little as we need to safe their life....that's why for climb, a normal EPR of 1.88 until 25000 feet.

Data quoted here applies only to te described situation, with airport elevarion being around sea level and outside temperature being +10 *C
Any other situation would mean a different calculation and different results.

Posted: Tue Apr 04, 2006 1:10 pm Post subject:

Thanks. That is clear.

Posted: Tue Apr 04, 2006 2:00 pm Post subject:

WHOAW!!!! Now that's power!!! The true power of the 747 Cool

. Here is another one. This is an approach on St. Maartens, the airports that is a FlightSim favorite:
http://www.youtube.com/watch?v=J-ozpizVhSQ&search=747

Capt. El

Posted: Tue Apr 04, 2006 2:17 pm Post subject:

BTW Max, why is it that on older aircraft people mostly talk about engine power in terms of pressure ratio (EPR) while on newer a/c everyone talks in terms of fan speed % N1? You have done that even in your post. The older a/c never displayed N1 or what? I would have expected N1 to be more reliable since rpm measurement is less prone to errors than pressure measurement. (You dont normally hear of a car tachometer clogged by ice do you?!)

Posted: Tue Apr 04, 2006 2:51 pm Post subject:

EPR or Engine Pressure Ratio is not only displayed on the old airplanes. The most modern 747-400 with GE CF6 or Rolls Royce RB211's have the EPR. What it actually repsresents is the difference in pressure in front and the back of the engine. So how much does the actual turbine (turbines actually) increase the air velocity. I dont think there is any unique reason why some do and some don't have them, I am not entirely sure what would be the most precise way of indications, and which would be more reliable....

Posted: Tue Apr 04, 2006 7:35 pm Post subject: