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The loss of speed at higher altitudes in f-15c


Punisher74
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Based on true fight characteristics of a f-15c at max altitude the max speed of a F-15 is High altitude: Mach 2.5+ (1,650+ mph, 2,665+ km/h) and at Low altitude: Mach 1.2 (900 mph, 1,450 km/h). In game at 33k feet I'm barely reaching 300mph at 96% (no Burn) and with full burner only 350 to 400mph. Not to mention the rapid loss of speeds in turns or the fact the the plane losses all control in any basic combat maneuver at high altitude. I do understand that the less air at higher altitudes would mess up fuel mixtures. But the with less drag the plane shouldn't rapidly drop airspeeds in turns. I feel that the avionics Lua might need some minor adjustments. One other main issue the thrust to weight ratio on a f-15 is 2x as much, and the f-15 is capable of vertical takeoff. Not in dcs the speed rapidly drops below 200 in seconds. But there is no section within the f-15.lua file which defines the maximum thrust of the engines. Just my 2 cents.

 

Raptor

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Lt. Commander Jason "Punisher" M

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Fl330 might be a bit low.

Have you tried over Fl400?

 

Have you tried vertical takeoff with less than 1\3 fuel?

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To clear this up, here are the following things you might want to consider:

 

1. Jet engine power drops drastically as altitude increases, to the point where the engines are producing 1/4 the thrust that they do at lower altitudes. That's why you lose speed.

 

2. More speed means more thrust, up to a point. But less speed definitely means less thrust.

 

3. Getting yourself to top speed requires a clean aircraft and a carefully practiced flight profile. If you're expecting to go from 250IAS to M2 at 40000', don't hold your breath. If you're carrying ANYTHING AT ALL on the pylons, again, acceleration will be slower due to drag.

 

4. HuD displays IAS, not TAS. Your best indicator for performance up there is your mach needle, with IAS as a secondary for maneuvering.

 

5. F-15's cannot perform a vertical take-off, period, end of story. They can accelerate straight up for a short time IF you're already moving at a speed that generates max thrust at sea level (around M0.8 ). Attempting to go pure vertical at 200kts is a recipe for suicide. The reason for all this is that jet engine INSTALLED thrust is some 20% less than bench-rated thrust (the thrust you usually see listed in brochures and wikipedia). This thrust recovers with speed.

 

6. This really needs to be repeated: The thrust generated by the jet engines is not a static number. The faster you go, the more thrust you get - up to a point (there is a peak mach number for thrust, for the F-15 it starts around M0.7-0.8 at low altitude and remains at around M0.9-0.95 at high altitudes). The higher you go, the less thrust you get.

 

7. The F-15's TWR was never in danger of being 2:1. That's just not possible. With typical combat payload (8 missiles, 1 bag) you won't even be taking off with 1:1.

 

8. Wikipedia is useless as a source for aircraft performance, or how you put it ... "true flight characteristics".

 

9. If you feel like learning a thing or two about how the F-15 flies, google the Streak Eagle flight profile and try to fly that, exactly as instructed.

 

Based on true fight characteristics of a f-15c at max altitude the max speed of a F-15 is High altitude: Mach 2.5+ (1,650+ mph, 2,665+ km/h) and at Low altitude: Mach 1.2 (900 mph, 1,450 km/h). In game at 33k feet I'm barely reaching 300mph at 96% (no Burn) and with full burner only 350 to 400mph. Not to mention the rapid loss of speeds in turns or the fact the the plane losses all control in any basic combat maneuver at high altitude. I do understand that the less air at higher altitudes would mess up fuel mixtures. But the with less drag the plane shouldn't rapidly drop airspeeds in turns. I feel that the avionics Lua might need some minor adjustments. One other main issue the thrust to weight ratio on a f-15 is 2x as much, and the f-15 is capable of vertical takeoff. Not in dcs the speed rapidly drops below 200 in seconds. But there is no section within the f-15.lua file which defines the maximum thrust of the engines. Just my 2 cents.

 

Raptor


Edited by GGTharos

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As this seems a recurring post, people now and then claiming that DCS F-15C cannot do Mach 2.5, there you go, just did a quick flight:

 

37,680 ft

Mach 2.524

767 KIAS

 

It was starting to shake, so it seem that it will not go beyond that at this altitude.

Screen_150408_221523.thumb.jpg.0ec4ef2e9e3c5266adb5a9dd89b54332.jpg

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5. F-15's cannot perform a vertical take-off, period, end of story. They can accelerate straight up for a short time IF you're already moving at a speed that generates max thrust at sea level (around M0.8 ). Attempting to go pure vertical at 200kts is a recipe for suicide. The reason for all this is that jet engine INSTALLED thrust is some 20% less than bench-rated thrust (the thrust you usually see listed in brochures and wikipedia). This thrust recovers with speed.

 

Just like this one

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Performance varies depending on conditions. Performance benchmarks without specific test conditions and procedures are pretty much meaningless.

 

Combat ready planes are not as fast as planes modified for setting performance records, and planes modified for setting records are usually not suitable for taking into combat. The problem with Wikipedia is that it typically does not distinguish between operational and record setting configurations.

 

A manual for the aircraft is a much better guide, and will have charts for different payloads, temperatures, and altitudes. In many cases the pilot is expected to calculate the additional effects of barometric pressure and humidity on their own.

 

In general if your thrust to weight ratio is more than 1:1 in a 4th generation fighter it's probably because you're out of munitions and extending for all you're worth. A nice time to have that sort of power, but hopefully not something that's categorized as a normal flight condition.

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Most certainly not. As an example a 43000lbs eagle is > 1:1, full of weapons and enough fuel for a fight.

 

In general if your thrust to weight ratio is more than 1:1 in a 4th generation fighter it's probably because you're out of munitions and extending for all you're worth.

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37,680 ft

Mach 2.524

767 KIAS

 

It was starting to shake, so it seem that it will not go beyond that at this altitude.

 

 

That's my sweet spot for getting to Mach 2.5 as well, the 37,000 Ft range.

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So streak eagle says at 500mph pull up to 90degrees and climb vertically. Now in DCS once you climb vertical your speed quickly drops to below 200 and down you go. That's nothing on your plane. In the Lua files for all aircraft there are no values for max thrust of the aircrafts engine. Only Max speed, fuel, takeoff weight, etc. i real don't care about vertical takeoff, it is basic combat maneuvers. For example a Split S is to provide speed and the expense of altitude correct, what I get once maneuver starts a substantial drop in speed to about 190mph from 300. Just to start the maneuver. At this point the plane attempts to rights itself and I just lost 15,000ft and have only 250mph to start the burn and attempt to recover a level flight. And once again that's on a empty plane. God forbid I'm in a real combat situation. As for the top speed and 37K. I've flown in an F-15 trainer. These are not issues in the real thing. In game At 1500ft I've reached Mach 2+ but never at altitude. I'll try again tomorrow but I'm afraid it's a mute point.

Thanks,
Lt. Commander Jason "Punisher" M

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Check out the -1 and try to duplicate the time to climb table. I thought the same as you at first but realized I was losing airspeed due to lack of finesse and realized the game matches the -1 pretty damn well. The -1 leaves a lot out but if that's all the devs had to work with they did an awesome job matching it. And P.S. the F-15 trainers are made by people just like ED so don't mistake your experience for the real thing.

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So streak eagle says at 500mph pull up to 90degrees and climb vertically. Now in DCS once you climb vertical your speed quickly drops to below 200 and down you go.

 

If you can't fly the Streak Eagle profile in DCS you don't know what you're doing, nor what you're talking about. In other words, you lack basic flying skills and knowledge. I don't care how many trainers you've flown.

 

For example a Split S is to provide speed and the expense of altitude correct, what I get once maneuver starts a substantial drop in speed to about 190mph from 300.

 

Lack of basic flying skills. At high altitude you're going to lose speed quickly with any amount of g, and if you're flying as slow as 300KCAS, then you have to finesse a lot. At low altitude you can pull more g per calibrated knot. If you're flying at 300KTAS, it's even worse. And BTW, it's kts, not mph

 

I'll try again tomorrow but I'm afraid it's a mute point.
I'm afraid you don't understand the subject. Get yourself a -1 and figure it out, but until you do so, quit arguing.

 

If you can show that the in-game performance deviates significantly from the -1 then we'll have something to talk about.


Edited by GGTharos

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The F-15 is quite capable of flying at mach 2.5+ at altitude, just like the real one.

 

 

 

 

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So streak eagle says at 500mph pull up to 90degrees and climb vertically.

 

You should stop using mph, it's confusing. Neither the F-15 or DCS will report speed in mph, so I'll assume you mean knots. However you have multiple airspeeds to read out. TAS gives you the actual speed. IAS is maneuvering speed.

 

Now in DCS once you climb vertical your speed quickly drops to below 200 and down you go.

Sounds normal with a combat load. With full fuel and 3 tanks (there is essentially never a reason to use 3 tanks in DCS, but many people do) you're not going to do vertical maneuvers.

 

If you want to climb with a heavy load out, you need to build up speed and climb at a reasonable angle. You'll never go straight up, that requires a very light plane.

 

That's nothing on your plane. In the Lua files for all aircraft there are no values for max thrust of the aircrafts engine.

There won't be, the flight model calculates thrust dynamically because thrust is dynamic.

 

 

For example a Split S is to provide speed and the expense of altitude correct, what I get once maneuver starts a substantial drop in speed to about 190mph from 300. Just to start the maneuver. At this point the plane attempts to rights itself and I just lost 15,000ft and have only 250mph to start the burn and attempt to recover a level flight. And once again that's on a empty plane. God forbid I'm in a real combat situation.

This sounds like poor technique. I've killed myself trying to split S in the F-15 when it first got AFM because the energy retention was so high. If you're losing nearly half of you speed entering the Split S, you're pulling far too hard. You don't need to yank the stick back all the way and it's not really advisable in most situations.

 

As for the top speed and 37K. I've flown in an F-15 trainer. These are not issues in the real thing. In game At 1500ft I've reached Mach 2+ but never at altitude. I'll try again tomorrow but I'm afraid it's a mute point.

I can also vouch for Mach 2 at altitude, it's not an issue in DCS. It sounds like you're mistaking IAS for TAS. Press the 2 key to bring up the Mach reading to actually find your Mach number.

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So streak eagle says at 500mph pull up to 90degrees and climb vertically.

 

1) That is incorrect, see the climb profile below.

 

2) The performance figures of the streak eagle can not be used as reference for the F-15C. IIRC the streak eagle had 3,000lbs of equipment removed(not fuel). Thats basically the weight of a car it didn't have to haul up to 40,000 feet.

 

8lPFCtY.jpg

 

 

In game At 1500ft I've reached Mach 2+ but never at altitude. I'll try again tomorrow but I'm afraid it's a mute point.

 

:huh: What is a moot point? Unless you typed something incorrectly mach 2+ is not possible at 1500 feet. It is only possible at altitude. The doghouse plot below is taken directly from the DCS F-15C manual.

0PNVoaZ.png

 

For example a Split S is to provide speed and the expense of altitude correct, what I get once maneuver starts a substantial drop in speed to about 190mph from 300. Just to start the maneuver. At this point the plane attempts to rights itself and I just lost 15,000ft and have only 250mph to start the burn and attempt to recover a level flight.

 

Press 2 on your keyboard. Look in the lower left of your HUD. That is your mach number. You must know your mach number at all times to stay within the flight envelope.

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Yes, they can be. The -220's are more powerful than the -100's, and you just have to lock down the fuel at the right amount. You can fly this profile with the in-game F-15C.

 

2) The performance figures of the streak eagle can not be used as reference for the F-15C. IIRC the streak eagle had 3,000lbs of equipment removed(not fuel). Thats basically the weight of a car it didn't have to haul up to 40,000 feet.

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  • 1 month later...
Yes, they can be. The -220's are more powerful than the -100's, and you just have to lock down the fuel at the right amount. You can fly this profile with the in-game F-15C.

 

Correct me if I'm wrong but the -220's offered better reliability at the expense of having slightly less maximum thrust than the -100's.

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You're wrong, but not your fault. That's the way it was written up, but it's not how things are.

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  • 2 weeks later...

No, is more like advertising material doesn't meet real world. The 220 can be tuned up to 25000lbs last I checked.

 

At supersonic speeds it runs better than a 100 with the vmax switch on, all thanks to meet materials. Generally this engine is better able to handle the temps that result in higher thrust.

 

That's just my amateur explanation based on what I know about the engine.

 

In any case, the 220s are on record as performing better: the -1 performance graphs show the 220s are superior, especially supersonic.


Edited by GGTharos

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