F-5E Tiger II – Unrealistic Acceleration Spike Around 430–440 KIAS

[VPS_Choki]

While flying the F-5E in level flight at partial throttle (approx. 92–93% RPM) — below both military and afterburner power — the aircraft experiences an unrealistic and sudden increase in acceleration when passing through approximately 430–440 knots indicated airspeed (KIAS). No throttle change occurs, yet the aircraft begins accelerating far more rapidly than expected at this specific speed range.

[Bucic]

This is interesting! Could you please test the RPM only, eliminating the airspeed variable? I wonder if it isn't tied to the nozzle operation logic instead...

[VPS_Choki]

6 hours ago, Bucic said:

This is interesting! Could you please test the RPM only, eliminating the airspeed variable? I wonder if it isn't tied to the nozzle operation logic instead...

Thx for the reply

 

 

 

so the lead aircraft doesnt change the rpm it stays steady at this point and we also tried different rpm but the lead aircraft always has a a sudden acceleration boost at the mentioned speed and level flight thats why the POV plane needed to put alot of throttle. the airspeed is just something i found out after the flight when the others told me theres always this weird acceleration

[PawlaczGMD]

It might be auto flaps going up at that speed. I don't remember where the flaps schedule is in docs, but you can try doing your test with flaps UP  and see if this still happens.

 

Alternatively, the engine indicators spike around that acceleration spike. More fuel flow, RPM, nozzle open. Unless you opened up the throttle to catch up? But then why does the lead outpace you in the same jet?

Edited 23 hours ago by PawlaczGMD

[nairb121]

On 5/12/2025 at 4:46 PM, VPS_Choki said:

While flying the F-5E in level flight at partial throttle (approx. 92–93% RPM) — below both military and afterburner power — the aircraft experiences an unrealistic and sudden increase in acceleration when passing through approximately 430–440 knots indicated airspeed (KIAS). No throttle change occurs, yet the aircraft begins accelerating far more rapidly than expected at this specific speed range.

Can you replicate this and post the video so we can see it from the affected plane's POV? There could be clues in the engine indicators.

9 hours ago, Bucic said:

This is interesting! Could you please test the RPM only, eliminating the airspeed variable? I wonder if it isn't tied to the nozzle operation logic instead...

Nozzle should be controlled only by throttle position and exhaust gas temperature - unless the EGT is unstable at that speed for some reason (which I wouldn't rule out), the nozzle in theory shouldn't be moving. Observing the nozzle position indicator would confirm though.

1 hour ago, PawlaczGMD said:

It might be auto flaps going up at that speed. I don't remember where the flaps schedule is in docs, but you can try doing your test with flaps UP  and see if this still happens.

At the AOA shown in the video (about 5 units), auto flaps should always be up. There's also no indication of a flap shift (barber pole) by the flaps indicator, and visually they also appear to be up on the other planes in the formation.

[Dragon1-1]

2 hours ago, PawlaczGMD said:

It might be auto flaps going up at that speed.

At this speed? Not likely. The flaps don't start dropping until way below that speed. Not seeing anything odd with the nozzles, either.

[Bucic]

This or that, a modless track file is required either way. Also, I'd activate TacView recording. It might not have a high polling rate but having the ability to look at graphs is really helpful.

Edit:

The only other issue this weird I can think off is the RPM force impulse at around 88%

 

Edited 13 hours ago by Bucic

[VPS_Choki]

i send the video of the lead aircraft ill also try to get a version this week/weekend without mods so i can send the track

[Bucic]

As for videos it's helpful to enable extended info bar. It can be brought up even in-cockpit.

One important thing. The F-5E has been optimized to a specific dogfighting airspeed range. That includes the air intakes and ducts. You can read about it in a published mildoc

http://ciar.org/ttk/mbt/air/air.usa.f5.f5e.f-5e_case_study_in_aircraft_design.pdf

What I see from the lead is exactly what I'd expect. Not so much in case of the first video.

 

 

[VPS_Choki]

I've done some testing and attached the track files below—one for the lead aircraft and one for the wingman.

What I found is that the behavior isn't dependent on speed. As soon as the lead aircraft reaches 95% RPM, regardless of how gradually I apply the throttle, there's a sudden and significant boost in acceleration.

I'm not sure if this is intended behavior. Could someone clarify whether it's normal for such a sharp increase in acceleration to occur exactly at 95% RPM?

Toward the end of the track, I tested this by repeatedly moving the throttle back and forth between 92% and 95% RPM, and each time I crossed 95%, I experienced a noticeable surge in speed.

F-5 ACCELERATION TEST-20250514-205417.trk F-5 ACCELERATION TEST-20250514-205634.trk

Edited 55 minutes ago by VPS_Choki