[Very technical question] Engine thrust, engine limits and max speeds.
-
Good day,
this is a very technical question that can probably only be answered by developers themselves, but I try neverthless.
Consider this chart:
As far as I understand it, the right side of the continuous black lines represent the thrust-limited max speeds, in other words where the max AB thrust equals aerodynamic drag.
Now, as we can see, at lower altitudes (below 22.000ft) the continous line is interrupted by a dashed line labelled “LIMIT AIRSPEED”. This, I reckon, represents the structural limit airspeed, and is the max speed at which the engine (and the aircraft as a whole, per manual) can safely operate, and is equal to 800 KIAS.
Now, from this chart we could deduce that the engine could in theory provide, say at sea level, a thrust which would allow the aircraft to exceed the limit speed of 800KIAS/M1.2. In other words, that the max operative speed at S.L. is not limited by thrust/drag (like it is above 22.000 ft), but by operative airspeed limitations for the engine/airframe.
My question is this: how does BMS model all of this?
In other words, does the engine in BMS produces higher thrust than that needed to reach M1.2 at S.L. (Basically, extending the continous black line in the chart down to sea level)?
Or, does the engine in BMS “loses” thrust below 22.000 ft, in order to match the max attainable speed to the LIMIT AIRSPEED dashed line in the chart?
And, supplemental question, what does the real aircraft/engine do? Would it be able to exceed M1.2 at S.L, with the pilot being responsible to not exceed speed limitations?
Or would the FADEC limit fuel flow (and hence thrust) once the airspeed approaches 800KIAS?
-
…you’re giving me flashbacks to my days at GE.
You’re nearly right…but there is one more issue to consider, and that is the difference between static thrust and dynamic thrust.
Static thrust is always going to be higher than dynamic operating thrust due to a deficit in momentum due to the aircraft/engine moving in the opposite direction from the engine’s ejected mass flow.
Your conjecture about the FADEC limiting thrust (and thus mass flow - the consideration is really about mass flow, and maintaining it) really has more to do with the design of the inlet - the worst thing you can do is create a choked flow ahead of the nozzle throat and stall the engine and/or the inlet - you generally want to keep the flow in the inlet sub-sonic to aide the thermodynamic cycle overall, and to keep shock systems out of the compressor.
Weather or not these design considerations amount to an inflection at 800 KCAS is anybody’s guess for a given airplane/engine…but if the book contains a limit, then it’s a “reasonable” limit. A real engine is designed to meet a requirement (primarily for MIL power), and they all differ.
-
@Murmur test pilot Joe Bill Dryden in the Semper Viper article “Dont Stretch the Limits” states that it was due original F100 engine -200 version would have to almost go into a dive to break the 800 knots but once there is over the limit of the engine control system since it ran in an open loop and could cause the engine to damage itself. Also talk about issues you could have with the later models of engines and then comes back around the air load on the canopy gets pretty large and it warms up over 800. But then talks about how in combat that would necessarily stop him from going over 800 KCAS. And no, the FADEC doesn’t automatically limit thrust to avoid the oversepeed. BMS will allow you to go over 800 KCAS with the right conditions.
-
Thank you gentlemen.
I learned the -200 version would be likely damaged when going over 800 KIAS, while the -220 and -229 are able to exceed that speed, having a closed loop control that limits the compressor discharge pressure.
-
I regularly exceed 800 kts in Falcon without issue. You will need to dive/hold zero g to get there quickly. In level flight it is quite a bit harder and consumes a LOT of fuel, so climbing, then diving while pushing zero g is the most efficient way.
I’ve destroyed a few engines that way, too… It’s also fun/interesting to load up with various AG stores and seeing what she can do. It can be useful to know how to maneuver while loaded.
-
To be honest I can’t remember since I have developped the HFFM in 2004
But considering I didn’t have access to code at that time and was unable to code failures in case of overspeed I would say that I think I limited the thrust to make sure the model didn’t go over in level flight
Some engine damage is modeled when overspeed though but that’s very basic
-
@Murmur said in [Very technical question] Engine thrust, engine limits and max speeds.:
Or, does the engine in BMS “loses” thrust below 22.000 ft, in order to match the max attainable speed to the LIMIT AIRSPEED dashed line in the chart?
And, supplemental question, what does the real aircraft/engine do? Would it be able to exceed M1.2 at S.L, with the pilot being responsible to not exceed speed limitations?
Or would the FADEC limit fuel flow (and hence thrust) once the airspeed approaches 800KIAS?
The F4.0 since the beginning uses thrust characteristics because in RL thrust is anything but a static thing…
The engine does not loose thrust at low altitude, the case is just the opposite. But the drag does it job…
https://www.quora.com/How-come-the-MiG-25-with-200kN-wet-thrust-achieve-Mach-2-8-3-while-the-F-22-with-300kN-wet-thrust-and-also-lighter-cant-Is-it-something-to-do-with-turbojet-vs-turbofan/answer/Balázs-Molnár-28?filter=5&nsrc=1&sncid=10693813142&snid3=15418125920
-
@Mav-jp Thank you for your incredible work on BMS. On the same subject, I have an additional final question:
what is the source of the engine thrust curves showed in the HFFM documentation? I’m talking about the one below (there’s 5 of them in total):
Are they estimated from known performance data? (Acceleration, max speed, etc.)
-
@Murmur said in [Very technical question] Engine thrust, engine limits and max speeds.:
@Mav-jp Thank you for your incredible work on BMS. On the same subject, I have an additional final question:
what is the source of the engine thrust curves showed in the HFFM documentation? I’m talking about the one below (there’s 5 of them in total):
Are they estimated from known performance data? (Acceleration, max speed, etc.)
Original model was rough I can’t remember where we found it , but we started with a Pw220
Then we spent months to loop between engine and aero from EM charts and accel / décelel charts and fuel flow / dilution charts
When when considered we had a coherent thrust vs aero model . We used this aero model and retro engineered the other engines from the EM charts and accel/decel charts