F-15 Performance

Hello all,

Been playing since Flanker 1....just upgraded to Flaming Cliffs 3 and the F-15C.

Question on performance.....The F15C has almost 50,000lbs of total thrust in AB. The AC while clean with no external stores weighs significantly less. I believe the real AC is capable of a true vertical climb while accelerating at the same time. The Flaming Cliffs AC is not even close to this capability.

Am I missing something? Never flow a fighter but have lot's of real experience in Boeing, Airbus and various business jets....

That may be static thrust, when you're actually in the air at speed it will decrease, so it may not be as significant as you would assume.

From a quick google, it is only 1.04 when "nominally" loaded, which I would guess means at half fuel, and half its weapons used.

Thanks!

Makes sense....the thrust of the engines would decrease a lot as you climb.....

I also found on Google, the supersonic climb while vertical was a specially stripped down model called the Streak Eagle...

That may be static thrust, when you're actually in the air at speed it will decrease, so it may not be as significant as you would assume.

I might be wrong, but I'm pretty sure the thrust should be greater at speed than static if at the same height. If you don't beleive me, just compare the fuel flow at different speeds, it increases.

I might be wrong, but I'm pretty sure the thrust should be greater at speed than static if at the same height. If you don't beleive me, just compare the fuel flow at different speeds, it increases.

Thrust does go up with speed, to a point. However the engine installed in the aircraft may not see the same thrust as the engine stand alone.

As far as vertical climbs go, the TWR doesn't necessarily tell you if you can climb, even if you know what the engines are making exactly. You need Thrust-drag to weight ratio.

Just carry 3 external fuel tanks. He perform great with heavy payload. Including acrobatics maneuvers. Just test it

The thing with thrust is that the engine isn't the only thing to consider, as you increase speed the difference between the speed of the air leaving the engine and the free stream velocity is reduced, leading to a decrease in thrust, imagine the hypothetical case where the air leaving the engine was going exactly the same speed as the free stream airflow, you'd have no thrust at all, in reality you never get to that point because drag will stop you ever getting there, ensuring that the exhaust speed is always greater than the freestream, except when you suddenly close the throttle.

Fuel flow will increase with speed, but that does not necessarily mean thrust will in proportion, a modern engine will vary the fuel flow to maintain the optimum conditions in the engine, if you consider drag aswell you will come to the realisation that that is where the extra fuel consumption comes from, to go even a little bit faster means a large increase in thrust would be needed, and that would need to have more fuel burned to do it.

Also as Exorcet says, an engine on a test rig and an engine in a plane are two different things.

Consider the case where you put the throttle at a particular position, your plane will accelerate until thrust and drag balance out, you won't just keep on accelerating, that would be magic, you may get some initial increase in thrust with the increase in air flow through the engine, but you also get a much greater increase in inlet drag, which has to be deducted from the total thrust.

Regarding the vertical climb you also need to push against drag besides gravity so having just thrust to weight greater than 1 doesn't mean you can climb vertically. I have heard it has to be something like 1.5 to truly climb vertically.

You want to go fast? Take it up to 35,000ft and level out, you'll get there...at mach 2.35.

Hehe… Another one of these :D

Just carry 3 external fuel tanks. He perform great with heavy payload. Including acrobatics maneuvers. Just test it

The Streak Eagle profiles didn't included a pure vertical climb. The nearest to vertical was 80 degrees and only in the 3km (10kft) and 12km (40kft) trajectories. Yes, the F-15 could achieve slight acceleration in a vertical climb at high T/W ratio.

You can test it out in game with a clean jet and a few thousand pounds of fuel. Start out in level flight, around 2-3kft at 220 knots. Smoothly pull the stick aft. Around 180 knots increase thrust smoothly to MIL. Once you have the nose pointed straight up to 90 degrees, plug in full AB. Your airspeed will start increasing! So, briefly, you can accelerate in a vertical climb in DCS. Should you? How much energy will you come out with on top? You're in a state of decreasing energy and exiting the maneuver means you need to spend more energy pulling back down towards the horizon. Not a very efficient flight path.

The Streak Eagle's mission was minimum time to target altitude. The climb angle is an output, not an input. A time optimized climb is achieved at the velocity where maximum excess thrust occurs (not necessarily L/D max).

The Streak Eagle profiles didn't included a pure vertical climb. The nearest to vertical was 80 degrees and only in the 3km (10kft) and 12km (40kft) trajectories. Yes, the F-15 could achieve slight acceleration in a vertical climb at high T/W ratio.

 

You can test it out in game with a clean jet and a few thousand pounds of fuel. Start out in level flight, around 2-3kft at 220 knots. Smoothly pull the stick aft. Around 180 knots increase thrust smoothly to MIL. Once you have the nose pointed straight up to 90 degrees, plug in full AB. Your airspeed will start increasing! So, briefly, you can accelerate in a vertical climb in DCS. Should you? How much energy will you come out with on top? You're in a state of decreasing energy and exiting the maneuver means you need to spend more energy pulling back down towards the horizon. Not a very efficient flight path.

 

The Streak Eagle's mission was minimum time to target altitude. The climb angle is an output, not an input. A time optimized climb is achieved at the velocity where maximum excess thrust occurs (not necessarily L/D max).

On a side note, the strike eagle was considerably lighter then a combat ready aircraft. No redundancies minimal fuel all non essential equipment removed. On top of that, the jet was tethered to allow maximum thrust before explosive bolts severed the tether allowing the jet to take off as fast as possible.

Yes, the streak eagle was about 1,800 lbs lighter IIRC. Additionally it carried the non-DEEC F100's where's the DCS F-15C has the newer -220 performance modeled.

The streak eagle actually only reached 75% of its maximum energy height in the zoom climb, 104,000ft vs 141,000 ft. In DCS we aren't constrained by the airframe's dynamic pressure limit. We also aren't required to fly a trajectory that allows return to the same airfield (or any airfield for that matter). I've had the eagle quite a bit higher than the real life record. ;)

Additionally it carried the non-DEEC F100's where's the DCS F-15C has the newer -220 performance modeled.

The -100's have EEC's and a mechanical unified fuel control (UFC) which were a serious PITA to operators and maintenance.

http://foxtrotalpha.jalopnik.com/how-to-win-in-a-dogfight-stories-from-a-pilot-who-flew-1682723379

"Accelerating straight up? That's a myth. First off, the old coal-burning Pratt F100-100 proved troublesome. When the throttles were pushed into afterburner you weren't 100% sure if the flame would come out of the back end or the front end. Sometimes, it came out of both ends. I've flown a twin-engine glider, meaning that both engines, while still operating but producing no thrust, as they had stalled. I had maybe 30 hours in the jet at that point. While the Pratt F100-220 in the F-15C is more trouble free, it only produces about 23,500 pounds of thrust. Installed thrust-to-weight is slightly less than 1:1 with eight missiles and no external fuel. If you point the jet straight up and start climbing thrust starts to fall off as the air's density starts to decrease. Weight is not decreasing, as fuel is burned off faster than thrust is decreasing. So stop it everyone – there's no accelerating going straight up in the F-15."

The F-15 can't accelerate straight up empty, for example? I read that very useful and interesting document about this guy who flew the F-15, F-16 and MIG-29, yet he says that the T/W ratio was less than 1/1 due to loadout and perhaps he didn't benefit from the 25000lbf engines that the DCS F-15 should have. The real F-15C should be able to prove a T/(W+D), where D is drag force (it matters even at null lift AoA) above 1 even with full fuel empty weight while starting a vertical climb (90 degrees pitch) from sea level if he manages to remain with at least 300-400km/h after reaching 90 pitch. In this condition, the F-15C with 25000lgf engines should normally find an initial TAS (not IAS) increase up to 1.5..2km height before it starts to decelerate (TAS decrease). Under the mentioned conditions, our DCS F-15 has it's TAS drop quite quick instead of slightly increasing. I have the feeling (at least by the numbers) that our DCS F-15C has it's engine thrust lower than it should at low altitude. At high altitudes, it can reach Mach 2.5 so the thrust to drag ratio is relatively correct, even if the drag and thrust may not be correct/realistic, their ratio is as long as it reaches Mach 2.5.

For some reason, in DCS, the F-15 has a lower T/W and also T/(W+D) than the Mirage 2000C from RAZBAM and also lower than the Harrier O.o from RAZBAM. Who hasn't inputted the right engine performance data, ED or RAZBAM? If you try a Mirage vs Eagle player vs player battle in DCS, the Mirage outaccelerates the F-15 in the vertical. I've done a test in which I had both planes with 50% fuel, started from 1000MSL and at 700km/h IAS I started a 3G climb until I reached 90 and held 90 pitch until the tailslide took place. The Eagle only reached 8400 meters, while the Mirage passed through 9000m. How come?

Regarding the vertical climb you also need to push against drag besides gravity so having just thrust to weight greater than 1 doesn't mean you can climb vertically. I have heard it has to be something like 1.5 to truly climb vertically.

At what speed and altitude? If you fly vertically at a low enough TAS (to reduce the drag),let's say 300km/h starting from SL, even 1.1 T/W should get you at least a higher than 1 T/ W+D) and slightly accelerate (TAS increase, not IAS) in the vertical.

Does any of this matter once the F-18 comes out? Are any of us going to be flying the F-15 anymore? haha

Actually yes.

Does any of this matter once the F-18 comes out? Are any of us going to be flying the F-15 anymore? haha

The F-15 can't accelerate straight up empty, for example?

... why this necropost?

I read that very useful and interesting document about this guy who flew the F-15, F-16 and MIG-29, yet he says that the T/W ratio was less than 1/1 due to loadout and perhaps he didn't benefit from the 25000lbf engines that the DCS F-15 should have.

Missiles, fuel + 1 bag puts you at 50000lbs, thrust is 47000. That's before we account for the store drag.

The real F-15C should be able to prove a T/(W+D), where D is drag force (it matters even at null lift AoA) above 1 even with full fuel empty weight while starting a vertical climb (90 degrees pitch) from sea level if he manages to remain with at least 300-400km/h after reaching 90 pitch. In this condition, the F-15C with 25000lgf engines should normally find an initial TAS (not IAS) increase up to 1.5..2km height before it starts to decelerate (TAS decrease). Under the mentioned conditions, our DCS F-15 has it's TAS drop quite quick instead of slightly increasing. I have the feeling (at least by the numbers) that our DCS F-15C has it's engine thrust lower than it should at low altitude. At high altitudes, it can reach Mach 2.5 so the thrust to drag ratio is relatively correct, even if the drag and thrust may not be correct/realistic, their ratio is as long as it reaches Mach 2.5.

Wrong technique. To get maximum thrust out of the engines (installed thrust is 80% of rated, typically) you need to get moving. If you want to accelerate straight up you need 500kts before you pull up. You'll accelerate for a short time, as you'll soon reach a density altitude where again you're not ramming enough air into the engines to maintain all this thrust.

For some reason, in DCS, the F-15 has a lower T/W and also T/(W+D) than the Mirage 2000C from RAZBAM and also lower than the Harrier O.o from RAZBAM. Who hasn't inputted the right engine performance data, ED or RAZBAM? If you try a Mirage vs Eagle player vs player battle in DCS, the Mirage outaccelerates the F-15 in the vertical. I've done a test in which I had both planes with 50% fuel, started from 1000MSL and at 700km/h IAS I started a 3G climb until I reached 90 and held 90 pitch until the tailslide took place. The Eagle only reached 8400 meters, while the Mirage passed through 9000m. How come?

The F-15 is fine. It might be the other aircraft that need some adjustment ;) OTOH the Harrier shouldn't have anywhere near the performance of an eagle at medium to high altitude, and neither should the mirage. Down low it's another story.

Also, the mirage is light. Very different airframe.

If I remember correctly, the Harrier does hold the time-to-altitude record for 15,000ft.

The Mirage with old flight model had indeed superior supersonic acceleration compared to the F-15, but that was fixed with the new flight model.

The F-15 can't accelerate straight up empty, for example? I read that very useful and interesting document about this guy who flew the F-15, F-16 and MIG-29, yet he says that the T/W ratio was less than 1/1 due to loadout and perhaps he didn't benefit from the 25000lbf engines that the DCS F-15 should have. The real F-15C should be able to prove a T/(W+D), where D is drag force (it matters even at null lift AoA) above 1 even with full fuel empty weight while starting a vertical climb (90 degrees pitch) from sea level if he manages to remain with at least 300-400km/h after reaching 90 pitch. In this condition, the F-15C with 25000lgf engines should normally find an initial TAS (not IAS) increase up to 1.5..2km height before it starts to decelerate (TAS decrease). Under the mentioned conditions, our DCS F-15 has it's TAS drop quite quick instead of slightly increasing. I have the feeling (at least by the numbers) that our DCS F-15C has it's engine thrust lower than it should at low altitude. At high altitudes, it can reach Mach 2.5 so the thrust to drag ratio is relatively correct, even if the drag and thrust may not be correct/realistic, their ratio is as long as it reaches Mach 2.5.

 

For some reason, in DCS, the F-15 has a lower T/W and also T/(W+D) than the Mirage 2000C from RAZBAM and also lower than the Harrier O.o from RAZBAM. Who hasn't inputted the right engine performance data, ED or RAZBAM? If you try a Mirage vs Eagle player vs player battle in DCS, the Mirage outaccelerates the F-15 in the vertical. I've done a test in which I had both planes with 50% fuel, started from 1000MSL and at 700km/h IAS I started a 3G climb until I reached 90 and held 90 pitch until the tailslide took place. The Eagle only reached 8400 meters, while the Mirage passed through 9000m. How come?

I started typing a response and realized I typed nearly the same thing in this very thread....3 years ago :megalol:

 

You can test it out in game with a clean jet and a few thousand pounds of fuel. Start out in level flight, around 2-3kft at 220 knots. Smoothly pull the stick aft. Around 180 knots increase thrust smoothly to MIL. Once you have the nose pointed straight up to 90 degrees, plug in full AB. Your airspeed will start increasing! So, briefly, you can accelerate in a vertical climb in DCS. Should you? How much energy will you come out with on top? You're in a state of decreasing energy and exiting the maneuver means you need to spend more energy pulling back down towards the horizon. Not a very efficient flight path.

When did you do this test vs the mirage? Before or after the Mirage FM update?

I’ve performed climb tests/levelacceleration before and after the flight model update using a constant .9~.95 Mach climb profile. As you know, maintaining a constant mach number through varying the pitch angle in a climb means the aircraft it constantly accelerating while climbing. in a In my testing the mirage no longer out climbs the Eagle to 40,000ft (12km). Also, the mirage no longer out accelerates the eagle in level flight at 40,000 ft.

When did you do this test vs the mirage? Before or after the Mirage FM update?

It seem's he used the AI FM of the Mirage as a data source if his test description in the Mirage forum is correct...

I also can't confirm that the Mirage out accelerates the Eagle at high alt.