[CORRECT AS IS] Maximum RPM at high altitude?

[cuervo111]

While trying the Viper I realised that around 30k ft the max RPM is around 95%, both with and without afterburner. This happens both in today's beta as well as the previous version.

 

Is this the expected behaviour?

[Strikeeagle345]

the RPM gauge is off in AB as well as the Nozzle gauge. probably something to do with it.

[mvsgas]

While trying the Viper I realised that around 30k ft the max RPM is around 95%, both with and without afterburner. This happens both in today's beta as well as the previous version.

 

Is this the expected behaviour?

 

Yes, Also After Burner (AB) should not affect RPM. RPM is not set and depends on many factor. The only thing to worry about is make sure RPM does not exceed 108% (plus or minus 1%) But AFAIK, nothing says RPM should be a specific number.

 

Obviously if you have throttle all the way forward to max AB and you only have 25% RPM, that is different. But 90% to 106% RPM at mil power and above I would consider normal.

Edited October 11, 2019 by mvsgas
spelling

[cuervo111]

I noticed it's dependent on airspeed, which kind of makes sense.

 

Are other jets in DCS having this behaviour?

[mvsgas]

AFAIK, all of them.

[mattag08]

100% RPM is based on the design engine limitations. Because turbine engines spin at ~25,000 RPM it's pointless to show the actual engine speed, so the gauge depicts a percentage. The engine will still spin more slowly than this 100% rated RPM at high altitude for the same reasons that all engines produce less power at altitude.

[bbrz]

Interestingly the RPM changes significantly even at low altitude in the F16.

100% <300ktias, 102% from 400 to 500kias and 97% at 900kias.

 

On the F-5 the RPM doesn't change with speed at low alt.

[bkthunder]

On 10/11/2019 at 7:07 PM, mvsgas said:

AFAIK, all of them.

That's not correct, I just tested F-14B, F-15C, F-18 and M-2000. 

The F-14B which notably uses the same engine as the F-16 (adapted to the F-14 of course), maintains 100% RPM at MIL power both at 6k feet and at 35k feet, given the same CAS (250kts).

Same for all the others, except for the F-18 which has a 5% rpm reduction between 6k feet and 35k feet.

 

So, all high fidelity modules. Which one is wrong?

Edited September 22, 2023 by bkthunder

[throAU]

20 hours ago, bkthunder said:

That's not correct, I just tested F-14B, F-15C, F-18 and M-2000. 

The F-14B which notably uses the same engine as the F-16 (adapted to the F-14 of course), maintains 100% RPM at MIL power both at 6k feet and at 35k feet, given the same CAS.

Same for all the others, except for the F-18 which has a 5% rpm reduction between 6k feet and 35k feet.

 

So, all high fidelity modules. Which one is wrong?

 

the f14 has variable geometry intakes, the f16 does not.

 

so while it’s the same engine potentially, the end result is going to differ. 

[bkthunder]

On 9/18/2023 at 12:15 PM, throAU said:

 

the f14 has variable geometry intakes, the f16 does not.

 

so while it’s the same engine potentially, the end result is going to differ. 

And that's precisely why I tested at 250kts IAS, at that speed and Mach the variable intakes have no effect.

[Dragon1-1]

It's not going to be the same mach at 35kft, though. Definitely enough for the ramps to have an effect. 

[bkthunder]

250 Kts at 35k feet at 15 degrees C is 0.74 Mach, not a speed at which the ramps even start to move on the F-15, and I doubt they have any effect on the F-14 at that speed. 

However, since the ramps on the F-14 should start to move >M 0.5 (according to NATOPS), I tested again at M 0.4 @35k feet, and the results are exactly the same as reported. 

[throAU]

On 9/22/2023 at 4:14 PM, bkthunder said:

And that's precisely why I tested at 250kts IAS, at that speed and Mach the variable intakes have no effect.

Sure. 
 

but the f16 intake is compromised to work at all speeds. So the variable intake is likely more efficient before it starts to move and indeed at all speeds. 
 

maybe it’s not? It’s a 1960s design whereas the viper is a 70s design. 
 

main point being; the combination of engine and intake is the power plant. 
 

same engine mounted in different airframe with different intakes amongst other things = probably not comparable. Because the performance will be different. 

Edited September 26, 2023 by throAU

[bkthunder]

On 9/26/2023 at 3:26 PM, throAU said:

So the variable intake is likely more efficient before it starts to move and indeed at all speeds. 
 

 

Source?

[throAU]

See the rest of my post.  Maybe it is, maybe it isn't.

Point being, the engine is not the only thing that influences performance - and as a result, comparing engine performance across airframes (especially fixed vs. variable geometry inlet) is apples to oranges.  

Look up the SR71 for example, a huge amount of thrust was generated by the intakes at speed.

Again: the rest of the aircraft - especially inlet design - influences engine performance.

 

The sr71 is an extreme example, but sometimes extreme examples are useful to illustrate that other things matter

 

http://www.enginehistory.org/Convention/2014/SR-71Inlts/SR-71Inlts.shtml

Edited September 30, 2023 by throAU

[bkthunder]

You know, it's funny. Most of the time, using general (let alone extreme) examples to explain something on these fora and ED in particular, is brushed off as "it's not the exact airplane / year / serial number / conditions". 

And here you are bringing an SR-71 in a F-16-related conversation... 

 

I appreciate the thread has now been labeled "correct as is". Can we at least get an informative explanation as to why it is correct as is (other that the SR-71)?
Can we get the same accuracy in being told why something is correct as is, as we are required to provide when reporting a bug?

I, for one, would like to know and learn from the devs, why the F-16 engine is running 90% RPM at MIL power at higher altitudes, and that explanation should be referred to the F-16 itself.

Seems only fair...

 

EDIT: and just to be extra clear, I am not saying it isn't correct as is. I am pointing out it does behave differently from any other jet engine in game, and I would like an explanation as to why the F-16 in particular sees this effect.

Edited October 6, 2023 by bkthunder

[Pavlin_33]

I am also very interested in the mechanics of it. MiG-29, for example also has it's engines RPM limited at altitude and it has intake ramps.