D17S Posted September 14, 2005 Posted September 14, 2005 General Electric TF-34-100/A engine start sequence needs a tweak. I was noticing the engine start sequence on the A-10 is very unusual for a normal turbofan startup sequence. First I’ll go through a normal turbofan start sequence, then what I saw wrong with the sim. In real life, a turbofan engine starts with starter engagement of the N2 spool. N2 then accelerates to 20%. Oil pressure is typically seen starting to rise during this initial acceleration. You want to see at least an indication of oil pressure before you give it the gas. During this initial acceleration, the fan (N1) may show some RPM indication, but generally N2 has not accelerated enough to begin to drive the N1 turbine/fan spool. At 20% N2, fuel and ignition is supplied to the engine. Fuel flow is indicated at this point and starts to rise. A second or 2 later, EGT begins to indicate. This shows that the fuel has ignited as the engine “lights-off.” N2 then continues to accelerate to 50% where the starter is disengaged. This also shuts off ignition. N1 RPM is now clearly increasing and EGT reaches a maximum as the both N2 and N1 continue to accelerate. EGT will then decrease to idle temp as both N2 and N1 stabilize at idle RPMs. A Fan engine will typically stabilize at an N2 RPM of 60% and an N1 of 30%. (more or less, depending on the engine). N2 is Always higher than N1 at idle. Such is the anatomy of a normal turbofan engine start. Boy, that’s not what happening with the LOMAC A-10 General Electric TF-34-100. The first time I really watched a start, I ejected. The engine was about to explode! I’ve got to admit that I’ve never run a TF-34, but I’ve been running CF-6-50s (-80s), JT-9s (all varieties) , PW4000s and CFM-56s for 20 years. These all exhibit the same start characteristics I described above. For instance, here are the things I see going wrong on the A-10 engine start-up. 1) I see fuel flow immediately on starter engagement. Wrong! If I saw that in real life, I would immediately abort the start. 2) Also, along with fuel flow, I see N1, N2 and oil pressure begin to increase immediately on starter engagement. Again, if I saw this I would abort the start and call maintenance. In the real world, the starter engages N2. As N2 drives the gearbox, oil pressure will begin to indicate at about 15% N2. The “Fan” (N1 RPM) will begin to indicate around 15-40% N2. If the Fan (N1) and N2 increased together with the initial starter engagement, they are mechanically jammed. This is an engine damaged beyond any hope of ANY further flight. 3) As the A-10 engine start sequence is currently programmed, light-off (EGT indication) correctly occurs at 20% N2. Good! But Fuel Flow had been increasing since starter engagement. If I threw a match into the burner cans (i.e., engaged ignition) with the cans that full of fuel, the engine would explode! I saw a 747 JT-9 actually catch fire for this kind of situation. That’s why I would have aborted the start if I saw fuel flow with starter engagement. Fuel (flow) and ignition come on together at 20% N2 (or max motor). 4) From 20 to 50% N2, the fan RPM (N1) stays pretty much even or even ahead of N2. If I saw this in real life, I would abort the start. Something has the N1 and N2 spools jammed together . . .but we saw this in the first 10 seconds of the start. Not good! This engine has a MAJOR mechanical problem. 5) You should not see any positive sign of N1 until at least 15-20% of N2. The fan (N1) and N2 are mechanically separate assemblies. The N2 turbine blows hot exhaust gasses across the N1 turbine on the way out the tailpipe. N2 has to get blowing pretty hard to get that N1 turbine to spin. The N1 turbine is mechanically connected to the (N1) Fan by a shaft . . . and around goes the Fan. There is no mechanical connection between N1 and N2. 6) If you want to make it really real, have the EGT continue to increase from (where ever it is at) 50% N2 (starter-cutout), Then decrease EGT to an idle EGT once N2 reaches idle RPM. All Fans (and jets in general) have a peak EGT during start and then a decrease to idle temps. 7) At idle, N2 RPM is at 35% and N1 RPM is at 70%. This is backwards! If the engine hadn’t already exploded, (or if I wasn’t already out of the burning plane, running for my life) this would end it for me. Time to get out of the plane! Maintenance!! You got a little problem here! Idle N2 should be around 60% and N1 around 30%. 8) Don’t forget Flight Idle once we get this beast in the air. N1 not below 35% and N2 not below 65% As it stands the A-10 is not airworthy . . . in a technical sense. The engines are severely malfunctioning. Actually, those engines will not run at all at the RPMs that are being indicated. They have crossed-up indicator wiring, are mechanically jammed and that fuel flow schedule is down right dangerous. To put in technical jargon, it’s really broke. No big deal really, It’s just a game, but I know you are spending a lot of time on flight modeling and realism. Thought your might want to know. If you really want to tweak this, you need to get the specs. As I said, I only know the fans I mentioned. The TF-34-100 RPM/temps and acceleration schedules will be similar. Just something to do in your spare time! Thanks for your efforts. Really looking forward to the 1.2 and then the F-16. :beer:
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