Fuel Flow Question

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delta2014

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Unread post12 Jan 2021, 19:36

If a pilot is flying a fighter at sea level at full 100 MIL power, in straight and level flight, and glances at the airspeed gauge and it reads 300 KIAS but the fighter is accelerating, will he see his fuel flow gauge keep increasing as his airspeed keeps increasing? Does an increase in airspeed increase fuel flow?

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Unread post12 Jan 2021, 20:10

delta2014 wrote:If a pilot is flying a fighter at sea level at full 100 MIL power, in straight and level flight, and glances at the airspeed gauge and it reads 300 KIAS but the fighter is accelerating, will he see his fuel flow gauge keep increasing as his airspeed keeps increasing? Does an increase in airspeed increase fuel flow?

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Delta2014

From 300KIAS up, I would expect to see it increase as MIL thrust will increase up to a certain point as well.
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Unread post12 Jan 2021, 22:51

Spurts,

The F-104's GE J79-19 produces 11,870 lbs of thrust at full MIL power.

So, when the F-104 is flying slow at, say, 300 KIAS, at full MIL power it would be producing less than 11,870 lbs of thrust, and then as speed increases and increases an airspeed is reached when full 11,870 lbs of thrust is attained?

So, maybe at 300 KIAS the engine might only be able to produce, say, 8,000 lbs of thrust. Then at 400 KIAS maybe 9,500 lbs of thrust. Then maybe at 550 KIAS the engine might be producing it's full 11,870 thrust output?

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f119doctor

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Unread post12 Jan 2021, 23:36

The J79 Mil power thrust rating is for zero airspeed, sea level, 59F standard day conditions, uninstalled (test cell zero restriction bell mouth, no horsepower extraction). With the engine installed, thrust will be less due to accessory loads and inlet flow restrictions at zero airspeed.

Once the aircraft begins forward movement, the intake flow restrictions go away as the airflow gets aligned with the intake. Different for every design, but you are probably near zero restriction conditions by 150 knots.

As you fly faster at the same altitude, ram airflow increases the inlet pressure to the engine, fuel flow goes up, thrust goes up. But, as you go faster, the inlet ram also increases the air inlet temperature until the turbine temps and rotor speeds reach a limit, after which thrust goes down. This second effect will reduce the amount of increased fuel flow and thrust you get with the higher ram pressure.

Of course, as you increase altitude, inlet pressure goes down, and fuel flow / thrust go down. The good news is the air is colder, delaying the inlet temperature cut back to higher Mach number. And the thinner air means less drag, so you don’t need as much thrust to get the same speed at altitude.
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delta2014

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Unread post13 Jan 2021, 20:33

F119 Doctor,

Thank you very much for explaining this to me.

Here are some questions:

(1) If the J79-19 was tested on top of a 40,000 foot mountain (just pretending), would it still be able to run at zero airspeed at 40,000 feet?

(2) If the J79-19 is able to run at zero airspeed at 40,000 feet, would thrust be reduced down about 3,798 lbs? That would be 32% of its 11,870 thrust at sea level. Does 3,798 sound about right?

(3) Now, in flight, flying at sea level, at about what airspeed would the J79-19 be able to produce its maximum MIL power thrust? It will definitely be less than 11,780, because you only get 11,780 when testing on the ground at sea level with zero airspeed. So would you think in flight the engine might be able to produce, maybe, 10,000 lbs of thrust? If 10,000 was the maximum amount of thrust the engine was capable of producing in flight at sea level, at what airspeed would the F-104 need to be flying at to get the J79-19 to produced its maximum MIL thrust?

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f119doctor

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Unread post13 Jan 2021, 22:06

delta2014 wrote:F119 Doctor,

Thank you very much for explaining this to me.

Here are some questions:

(1) If the J79-19 was tested on top of a 40,000 foot mountain (just pretending), would it still be able to run at zero airspeed at 40,000 feet?

(2) If the J79-19 is able to run at zero airspeed at 40,000 feet, would thrust be reduced down about 3,798 lbs? That would be 32% of its 11,870 thrust at sea level. Does 3,798 sound about right?

(3) Now, in flight, flying at sea level, at about what airspeed would the J79-19 be able to produce its maximum MIL power thrust? It will definitely be less than 11,780, because you only get 11,780 when testing on the ground at sea level with zero airspeed. So would you think in flight the engine might be able to produce, maybe, 10,000 lbs of thrust? If 10,000 was the maximum amount of thrust the engine was capable of producing in flight at sea level, at what airspeed would the F-104 need to be flying at to get the J79-19 to produced its maximum MIL thrust?

Thanks,

Delta2014


1) I don't have any personal experience with the J79 engine, but it should be able to run at Mil power at 40K, zero airspeed, at least with a test cell bell mouth on your hypothetical 40K mountain. With it installed, the altitude independent horsepower extraction will push the engine operating line closer to the compressor stall line, and the aircraft inlet will restrict the flow, also pushing it toward stall. Any angle of attack will further distort the inlet flow, resulting in compressor stall and flameout.

2) According to my handy P&W Vest pocket handbook, inlet pressure at 40K, 0 Mn would be 2.72 psia. Ratio the sea level thrust of 11,780 lbf by that pressure over standard sea level pressure of 14.7 psia would give you a thrust of approximately 2180 lbf. This assumes that the J79 doesn't increase its airflow significantly below standard inlet temperature. Since the inlet temperature has dropped from 59F to -70F at that altitude, the thrust would be a little less since the EGT would also be lower, and exhaust velocity drops with the square root of the temperature ratio (in absolute degrees). I would give a guess that Mil thrust would be somewhere between 2000-2200 lbs under these conditions.

3) If you are flying at sea level at Mn of 1.0, you have an inlet pressure of 27.82 psia. If there were no engine limits, 27.82 / 14.7 x 11780 = 22293 lbf thrust. But your inlet temperature has gone up to 163F, more than 100F above standard day conditions. Engine limits on rotor speed, turbine temperature, and main burner pressure all will reduce the "corrected" rotor speed and airflow through the engine. Assuming the engine maintains 100% rpm on the gauge, the corrected rotor speed would only be 91%. I would provide a wild guess that the engine would be putting out something in the order of 16K-18K thrust under these conditions, with fuel flow around 15,000 lbs / hour (reflecting the 0.85 Mil SFC). Of course, airframe drag is also huge under these conditions, so you might never get to 1.0 Mn at sea level with Mil power. The other effect that occurs with increasing speed is the decreasing delta between exhaust velocity and airspeed - not a huge effect subsonic for fighter engines, but significant in transonic and more so supersonic

As I noted in the beginning, I don't have any direct J79 experience, so this is sort of a generic answer for jet engines in general, using the J79 thrust numbers as a basis.
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delta2014

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Unread post13 Jan 2021, 23:38

Assuming the engine maintains 100% rpm on the gauge, the corrected rotor speed would only be 91%.


When a fighter pilot pushes the throttle to full mill, and the tachometer says 100% RPM, or a little over 100% RPM, the actual RPM of the engine may be less? The true RPM could be, say, 91% with the tachometer saying 100%?

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Unread post14 Jan 2021, 00:37

Temperature Corrected RPM is an indication of the airflow thru the engine -you won’t see it on any gauge.

Corrected RPM = Indicated RPM / Square Root of Theta. Theta is the ratio of the inlet temperature (in absolute degrees R) to Standard Day absolute temp in degrees R.

At 163F (622R), Theta = 622/518 = 1.21. 100% / Square Root of 1.21 = 91%

An engine turning 100% rpm at 163F has the same airflow as that same engine turning 91% rpm on a standard 59F day.

FYI - Square Root of Theta is also how Mach Number is affected by temperature. For the same true airspeed, Mn is lower as the air temp goes up.
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Unread post14 Jan 2021, 00:58

Here is a chart from Robert L. Shaw's book, "Fighter Combat:"

Image

On page 396 Shaw states:

"The effect of speed on engine thrust is illustrated in Figure A-7. Propeller thrust is usually greatest in the static condition (i.e., zero airspeed) and falls rather rapidly with increasing airspeed. Jet thrust also may be expected to diminish slightly as speed increases above the static condition. As airspeed rises farther, however, ram compression in the engine inlet generally results in significant increases in thrust until engine and inlet design limits are approached. It is quite obvious from this plot why jet fighters exhibit superior high-speed performance."

Let's say the F-104 with the -19 would be producing 18,000 lbs of thrust at sea level at full MIL at Mach 1.0. What do you think the thrust would be at these other airspeeds?

Mach 0.4
Mach 0.6
Mach 0.8
Mach 1.0: 18,000 lbs

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Chris
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Unread post14 Jan 2021, 03:40

It is difficult to say, but I would guess that you could put the top of Shaw’s jet curve around Mach 1 and you wouldn’t be too far from the truth for J79 at sea level. Whether that Mil thrust is enough to accelerate the aircraft to those conditions is another question.
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Unread post14 Jan 2021, 19:25

I added in two blue arrows to Mr. Shaw's chart:

Image

What do you think the Mach number would be for the first blue arrow, where thrust begins moving upward?
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Unread post14 Jan 2021, 21:25

I suggest you pull up the 1F-104A-1 flight manual that was linked to by Spurts in the “F16 vs F104” active thread and study it. Not sure if it will give you the answers directly, but it will provide authoritative info on the J79 performance in the F104.
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Unread post14 Jan 2021, 23:40

Let's say a fighter, at full AB power at sea level, can pull 7Gs in a level turn without losing any airspeed. But, this is only possible if the fighter is flying at, say, 500 KIAS. If the fighter starts the 7G turn at, say, 400 KIAS, it will bleed airspeed.

Is this because of these two reasons?

(1) AOA is lower at 500 KIAS (less lift-induced drag), and AOA is higher at 400 KIAS (more lift-induced drag)
(2) The turbojet engine is producing more thrust at 500 KIAS, and less thrust at 400 KIAS
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Unread post15 Jan 2021, 15:17

Generally yes
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Unread post15 Jan 2021, 18:26

delta2014 wrote:Let's say a fighter, at full AB power at sea level, can pull 7Gs in a level turn without losing any airspeed. But, this is only possible if the fighter is flying at, say, 500 KIAS. If the fighter starts the 7G turn at, say, 400 KIAS, it will bleed airspeed.

Is this because of these two reasons?

(1) AOA is lower at 500 KIAS (less lift-induced drag), and AOA is higher at 400 KIAS (more lift-induced drag)
(2) The turbojet engine is producing more thrust at 500 KIAS, and less thrust at 400 KIAS


Although induced drag may be lower at 500 kt, total drag may be higher. So the ability to maintain speed at 7g/500 kt may be entirely due to increased thrust. Spurts, I know you already know this, but I think delta2014 needs to know it also.
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