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I fly a CRJ (one of those old 200s without FADEC) and was on the climbout thinking, "Why do I have to keep adding power?" I tried a constant indicated airspeed of 250 from around 5K feet to 25K. I kept pushing an extra percent every few minutes, but around 18K I didn't have to push anymore. The N1 carot finally decides to stay where it is. So my question is,
What computes where the N1 carot should be for climb power?
And for good measure: What item are we trying to maintain at a constant value (pounds of thrust, EPR, ITT, etc)?
The N1 carrot shouldn't stay put anywhere on the climb. It should start somewhere in the neighborhood of 89% N1 (at climb setting) at sea level to about 97% N1 at 37000 feet. The way I undestand is that the FMS computes target N1 based on temperature and altitude. I also don't think it is trying to maintain a constant value of anything on the climb out, but follows a schedule programmed into it that Bombardier determined to be optimal, although around 800 degrees ITT seems to follow the schedule pretty close, but not exactly. My guess on the ITT fluctuation in the climb is that the FMS does not take into account actual pressure at altitude. So, if the pressure is lower than standard I would expect to see a higher ITT for a given N1 following the climb schedule, and vice versa for higher pressure. Just my observations, please chime in with others.......
Yeah, it's designed to generate max power without hurting the engines. The titanium turbine blades start to abrade above 820-850 C, so the idea is to keep ITT no higher than 800 or so. This is not a catastrophic failure issue, but just a gradual wear issue...makes the engines last longer.
At TO/GA or APR power, you're not overstressing the engine, but you are wearing out the turbine blades.
Remember, the CF34 core is a de-rated military engine (A-10 and S-3).
For hi-bypass turbofans thrust available decreases with increase in both altitude and velocity. You are going to have to bump it up as you climb to get the same output. Not a lot- because thrust output has an exponential relationship to N1 speed as the latter increases. A complete analysis can be found in books on jet propulsion or "jetpro" as they say in school. And as Rick says there's a limit imposed by the materials in the engine on internal temps due to wear rate on blades and other parts. Bear in mind TAS increases with altitude, so 250 kts indicated at sea level isn't the same speed at altitude. If you try and hold the TAS the same I doubt you will find as much of a difference in power setting required.
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Last edited by Cubdriver : 12-27-2007 at 07:35 PM.
Thanks for the replies. I guess what I was getting as was more in the line of, "what moves the carot?"
Now that I've thought about it I think the DCUs must supply the data to the FCU, which then tells the EICAS what N1 setting it wants to maintain the proper efficiency as programmed into the FMS. So the EICAS in turn displays the little carot at the proper place for the pilot to then match up the actual N1 with the carot.
So with the increase in altitude, the increase in TAS, variables in the programmed climb profile, among other factors, the carot moves accordingly to maintain that efficiency.
Bear in mind TAS increases with altitude, so 250 kts indicated at sea level isn't the same speed at altitude. If you try and hold the TAS the same I doubt you will find as much of a difference in power setting required.
If he is a jet driver already, you are really insulting his intelligence with that statement..
Thanks for the replies. I guess what I was getting as was more in the line of, "what moves the carot?"
The carot is derived from engineering charts created by Bombardier for "optimal climb thrust". If you can ever get your hands on a factory performance manual (it's huge), it's in volume two of the aircraft operating manual. Essentially they are accounting for SAT changes and pressure altitude.
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