Why specifically Mach 1.4?

[Sarix]

Hello all!

The F1's engine is scheduled to go to overspeed mode (~8,900 RPM) once it hits Mach 1.4 and afterburner should not be disengaged above this mach number.

What is the technical reason behind both the overspeed mode and the A/B limit?

[gulredrel]

I'm nowhere near into the flow dynamics, but I assume, with the incoming air flow, the engine performance has advantages going into higher RPM.

You can disable A/B above Mach 1.4 and being in the overspeed regime. But you need to stay at MIL power with the throttle until RPM goes out of overspeed when speed decreases to below Mach 1.4.

[BitMaster]

Likely a reverse of airflow could happen, stalling the engine. That's usually the problem with Turbines, airspeed and throttle changes in the mix.

[Bremspropeller]

As others have stated, the A/B can be disengaged at any speed/ Mach. Just don't retard the throttle out of MIL.

The overspeed regulation exists in most jets. Difference here is that the overspeed will kick in automaticly, but there's no protetction against retarding the throttle in place, like on most other jets. In other jets, if you retard the go-stick, the engine will stay at high power and schedule back with lower Mach automaticly. In the F1, the engine will just go back and *poof* stall.

The overspeed is a mechanism to account for the shock-losses that become a thing at Mach 1.4 - why at 1.4? Well, I have forgotten too much supersonic flow theory to tell you all about it, but it's most probably related to some specific shock-table value. In essence, it's a matter of ram-rise (temperature goes up) and hence a loss in static pressure/ density, so you'll have to run the compressor at a higher speed to make the suck-squeeze-bang-blow magic happen.

Keep in mind that the ATAR 9K50 is basicly a glorified BMW 003 from WW2 - it's only got 9 compressor stages, so it's a very simple engine with a low compression-ratio compared to other contemporary engines. For comparison: The J79 has 17 compressor stages and a complex VSV/IGV system to have it run at higher compression ratios.