Fuel flow process

[reilar]

Hello! Can anyone explain to me what the pilot is regulating with the throttle in the Sabre. From diagrams in the F-86F manual (the same picture that you can find here: http://www.digitalcombatsimulator.com/en/products/sabre/?PAGEN_2=3#739739) I can see that the throttle quadrant regulates a fuel control valve (or actually the fuel regulator) in the by-pass fuel flow. I assume that the pilot goal is to set a specific engine rpm. He then set this rpm value by changing the throttle quadrant until he gets the rpm he wants.

 

 

Do I understand the process correct by saying that the pilot sets the fuel flow to get a wanted rpm? The fuel regulator then regulates the fuel flow to keep a constant rpm regardless of changing altitude and airspeed. Is this correct?

[drPhibes]

You're almost right. The throttle sets the desired engine RPM, the regulator determines the ammount of fuel necessary to maintain the RPM for different operating conditions (altitude, airspeed etc.), and sends some fuel back to the low pressure part of the fuel system through the by-pass system.

[reilar]

Ok, thanks for the answer.

 

So I ran some tests to see if I had understood the process correct.

I started out at 30k feet with a throttle setting that gave me 90% rpm.

I then descended to 20k feet without touching the throttle. I waited for the airspeed to settle. Now the rpm had decreased to 88%.

I did some more measurements on 10k feet and 1k feet (see table bellow).

I finally descended to 500 feet still not touching the throttle and waited for the airspeed to settle. The engine rpm had now decreased to 76%.

 

To summarize:

30k feet -> 90% rpm (2200 pph fuel flow)

20k feet -> 88% rpm (2900 pph fuel flow)

10k feet -> 84% rpm (3000 pph fuel flow)

5k feet -> 80% rpm (2400 pph fuel flow)

500 feet -> 76% rpm (1600 pph fuel flow)

 

So it seems that the engine rpm is changing even if I don’t move the throttle. Is this behavior expected and within operating tolerances or is this perhaps a faulty behavior in the simulation?

 

Another strange behavior is the fuel flow (the values inside parentheses). I guess it is expected that the fuel flow shall increase with decreasing altitudes and so it seems to do. But under 10k feet the fuel starts to decrease again with decreasing altitudes. I’m not sure about this because I don’t fully understand how the fuel flow is regulated by the fuel flow regulator. But it sure looks strange.

 

Can someone confirm this behavior? Is it expected or is it a faulty behavior of the sim?

 

And by the way, I love these kind of simulations and I’m not trying to pick on the fidelity of the sim. I’m fully aware that I’m not fully understand the ins and outs of the process here and I’m asking this first and foremost to be able to understand how things are supposed to work. And if this is a bug I’m only glad that I can help the beta process to rule it out.

[mvsgas]

I have very little knowledge on old jet engines. With that said; jet engine RPM will vary depending on many, many factors. Air temp, air density, throttle , etc. So it is not uncommon for RPM to vary. So, in my experience, there is rarely, if ever, a set RPM for a specific speed, altitude etc. Not even modern computer managed jet engines have a specific RPM nor fuel flow. Basically, AFAIK, you adjust the throttle to proved what ever speed you need at what ever flight conditions. The only thing you need to worry as a pilot is not to exceed the limits. referring to max or minimum RPM, fuel flow, etc.

[bart]

Ok, thanks for the answer.

 

So I ran some tests to see if I had understood the process correct.

I started out at 30k feet with a throttle setting that gave me 90% rpm.

I then descended to 20k feet without touching the throttle. I waited for the airspeed to settle. Now the rpm had decreased to 88%.

I did some more measurements on 10k feet and 1k feet (see table bellow).

I finally descended to 500 feet still not touching the throttle and waited for the airspeed to settle. The engine rpm had now decreased to 76%.

 

To summarize:

30k feet -> 90% rpm (2200 pph fuel flow)

20k feet -> 88% rpm (2900 pph fuel flow)

10k feet -> 84% rpm (3000 pph fuel flow)

5k feet -> 80% rpm (2400 pph fuel flow)

500 feet -> 76% rpm (1600 pph fuel flow)

 

So it seems that the engine rpm is changing even if I don’t move the throttle. Is this behavior expected and within operating tolerances or is this perhaps a faulty behavior in the simulation?

 

Another strange behavior is the fuel flow (the values inside parentheses). I guess it is expected that the fuel flow shall increase with decreasing altitudes and so it seems to do. But under 10k feet the fuel starts to decrease again with decreasing altitudes. I’m not sure about this because I don’t fully understand how the fuel flow is regulated by the fuel flow regulator. But it sure looks strange.

 

Can someone confirm this behavior? Is it expected or is it a faulty behavior of the sim?

 

And by the way, I love these kind of simulations and I’m not trying to pick on the fidelity of the sim. I’m fully aware that I’m not fully understand the ins and outs of the process here and I’m asking this first and foremost to be able to understand how things are supposed to work. And if this is a bug I’m only glad that I can help the beta process to rule it out.

 

The increase in rpm with increase in altitude is called "Flight Idle" and is normal on all gas turbines. To maintain the correct air fuel ratio in the combustion chamber (or chambers as the Sabre engine is a multi can system) and take into account the drop in air pressure at altitude the engine has to idle at a much higher rpm than at say 1000ft. If this didn't happen the engine would just flame out.

 

I'm not sure on the fuel flow figures, what you have quoted proves that the fuel flow decreases above 10k, but shows an increase between 500 and 10k which is strange.

 

To quote the Sabre Pilot's manual :-

 

Altitude (ft)........ speed (kts)......... rpm %.......... fuel flow(lb/hr)

sea level.............. 195................... 67................. 1750

5000................... 195................... 69................. 1600

10,000................ 195................... 71................. 1450

20,000................ 195................... 76................. 1350

40,000................ 195................... 88................. 1400

45,000................ 190................... 91................. 1350

 

Gas turbines is a very complicated subject and they are governed by many gas laws.

 

My thinking is the pilot sets his throttle to a given position to give him the airspeed he requires. The fuel control unit on the engine does the rest and maintains the correct fuel flow to the burners in the combustion chambers for any given altitude, speed etc. he happens to be at. The fuel flow is just an indication to the pilot of how much fuel is being used at his current altitude to give him the airspeed he is demanding. From this he can work out what range he's got from the remaining fuel he has and whether he can reach his destination or achieve his required flight endurance at the airspeed/altitude he is at.

Edited August 23, 2014 by bart