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Posted
So you're saying that its so aerodynamically different because of the larger canopy that looses almost from 25-50% of its STR performance at low subsonic speeds?

 

The E is a lot heavier at empty for one and almost always wearing CFTs - has more thrust as well.

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Posted

No, I'm saying the CFTs and everything that's hanging off of them add a lot of drag. Drag has to be overcome by thrust. Same thing for empty weight, just as mentioned above, and who knows whether the stability changes as well (ie. less AoA available)

 

So you're saying that its so aerodynamically different because of the larger canopy that looses almost from 25-50% of its STR performance at low subsonic speeds?

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Posted

F-15E's always carry CFTs. Not carrying them is the exception, not the other way around.

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

Posted (edited)
F-15E's always carry CFTs. Not carrying them is the exception, not the other way around.

 

In practice they rarely fly without, sure, but the chart I'm talking about mentions no CFTs, whilst further down charts with them on are presented (-4 & -5 CFT), where weight is noticably higher as well.

 

Let's compare the F-15E (4x AIM-7s) vs F-15C (4x AIM-7s & 9s) at 10 kft:

 

Mach

0.2 = 4 / 7.5 dps (??)

0.4 = 9.8 / 11.2 dps

0.5 = 10.6 / 12 dps

0.6 = 11.2 / 12.5 dps

0.7 = 12 / 13.3 dps

0.8 = 12.5 / 13.7 dps

0.9 = 13 / 13.9 dps

 

Now considering that the E is 1,000 lbs heavier an overall 1-1.5 dps difference is to be expected across the board, and indeed a difference of 0.9 to 1.4 dps is observed at all but one place which sticks out with much higher difference (infact it is almost double) and that is the 7.5 dps at mach 0.2. This leads me to think that this is a mistake on the -15C chart, and that the F-15C in actuality couldn't do more than 5 to 5.5 dps at Mach 0.2 (exactly 1 dps more than the -15E), and that later testing which revealed this led to the charts being corrected for the later aircraft such as the F-15E.

 

To back up this assumption I will refer to the stall speed, ITR & TW ratio of the aircraft. It would make very little sense if the F-15 was able to sustain a noticably higher turn rate at speeds where it could barely fly in comparison to the F-14 which features a much lower stalling speed.

Edited by Hummingbird
Posted

If it's a mistake, then it is present in two separate F-15 manuals.

Modules: FC3, Mirage 2000C, Harrier AV-8B NA, F-5, AJS-37 Viggen, F-14B, F-14A, Combined Arms, F/A-18C, F-16C, MiG-19P, F-86, MiG-15, FW-190A, Spitfire Mk IX, UH-1 Huey, Su-25, P-51PD, Caucasus map, Nevada map, Persian Gulf map, Marianas map, Syria Map, Super Carrier, Sinai map, Mosquito, P-51, AH-64 Apache, F4U Corsair, WWII Assets Pack

Posted
Now considering that the E is 1,000 lbs

 

More like close to 3000. Empty. More with CFTs.

 

This leads me to think that this is a mistake on the -15C chart, and that the F-15C in actuality couldn't do more than 5 to 5.5 dps at Mach 0.2 (exactly 1 dps more than the -15E), and that later testing which revealed this led to the charts being corrected for the later aircraft such as the F-15E.
Critical mistakes in these manuals are corrected immediately if they even happen - errata are issued at least yearly for everything else AFAIK.

 

It would be pretty interesting though to see a chart like that with that type of mistake in it ... after all, they only flew the aircraft a bunch of times and averaged it all to get that curve.

 

To back up this assumption I will refer to the stall speed, ITR & TW ratio of the aircraft. It would make very little sense if the F-15 was able to sustain a noticably higher turn rate at speeds where it could barely fly in comparison to the F-14 which features a much lower stalling speed.
The only thing you should refer to is your burning desire to have the F-14 dominate the entire envelope.

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Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

Posted

I think he is wrong about the CFT's and the DPS. Here is a copy of the F-15E, 41500pds, with 4 AIM-7's. Note that even though the load manager says only 4 Sparrows, in the description it clearly states CFT present.

 

xgukyyN.jpg

 

Also note, the 10000ft STR starts at 7.5 DPS and mach 0.2

Modules: FC3, Mirage 2000C, Harrier AV-8B NA, F-5, AJS-37 Viggen, F-14B, F-14A, Combined Arms, F/A-18C, F-16C, MiG-19P, F-86, MiG-15, FW-190A, Spitfire Mk IX, UH-1 Huey, Su-25, P-51PD, Caucasus map, Nevada map, Persian Gulf map, Marianas map, Syria Map, Super Carrier, Sinai map, Mosquito, P-51, AH-64 Apache, F4U Corsair, WWII Assets Pack

Posted

From the data on that specific page, there is something wrong. I'm just going to run this off the SL line, 2.0G of G, and and a 60 degree bank angle:

 

2.0G/60 degrees of bank becomes available at essentially 0.25 Mach on the SL line.

 

With Mach at SL being 661.5 knots:

 

661.5kts * 0.25 = 165.375 kts

 

We can proof this against reality through figuring for stall. When we want to find how much speed a given G requires, it's

 

sqrt(G) * stall

 

In this case, we know the speed at which we can first generate 2.0G- 165.375 kts. So we perform the equation in reverse:

 

165.375 = stall*sqrt(2)

 

sqrt(2) = 1.414

165.375/1.414 = 116.937

 

116.937 = F-15 Stall @ SL.

 

Considering that normal takeoff procedure has the pilot rolling at 120 knots, we're not exactly out of the ballpark on our calculations.

 

Now, here comes the problem:

 

At 0.25 Mach and SL, the F-15's chart as above claims a *sustained* turn rate of approximately 13 degrees. And that is sustained = no altitude loss to make up for a deficiency in Ps. This constitutes trouble thusly:

 

Rate of Turn = [1091 x TAN(BA)]/KTAS,

 

where TAN = tangent, BA is bank angle, and we're talking knots true.

 

Now, why is this a problem?

 

1. The F-15 can't use a higher bank angle than 60, lest the nose fall and negate it being a "sustained" rate turn as presented. In fact, it's stating this clearly on the first half of the chart- 2.0G = 60 degrees of available BA. And that is a problem, because...

 

2. The math:

 

Rate of Turn = [1091*TAN(60, which is 1.732)]/165.375

 

For those following along at home, that's 11.426 degrees per second.

 

Now, do I think that anybody is going to get a hard on over rate numbers at the absolute bleeding edge of sustained rate and Mach? Not really, because at 0.25 Mach, you're in pebbles, boulders, and all sorts of garbage on that stick if you try standing around for a maximum sustained rate turn. That is to say, you're not going to go there, and you're not going to fight there.

 

But is the chart perfect along both axis at all points? Doubtful. These things are estimated, and not every single point is hit. They don't need to be to be operationally useful. Give a pilot everything above 0.35 solid, and with time in the cockpit, he'll be fine with everything underneath by feel, rather than a chart.

Posted

One explanation for the 11.4 to 13 deg/s difference could perhaps be that in the unbanked stall the wing and power component of the engine thrust is doing all the lifting but in a banked turn the wing lift could be supplemented by a vertical lifting component due to a yaw angle beta coupled with the resulting engine thrust component which theoretically means that you could have a slightly higher bank angle than 60 deg and still maintain altitude.

Old Crow ECM motto: Those who talk don't know and those who know don't talk........

Pilum aka Holtzauge

My homepage:  https://militaryaircraftperformance.com/

 

Posted (edited)
More like close to 3000. Empty. More with CFTs.

 

Critical mistakes in these manuals are corrected immediately if they even happen - errata are issued at least yearly for everything else AFAIK.

 

It would be pretty interesting though to see a chart like that with that type of mistake in it ... after all, they only flew the aircraft a bunch of times and averaged it all to get that curve.

 

The only thing you should refer to is your burning desire to have the F-14 dominate the entire envelope.

 

Huh? Or you've got a burning desire NOT to have that? Seems more realistic considering how combative you are, and frankly it's pretty disturbing to see a moderator being so accusing of others without reason.

 

I'm just trying to make the charts make sense, that's all I'm doing.

 

Read the chart for yourself (1,000 lbs difference, 4x AIM-7's, no CFTs mentioned at all):

vXlcyNE.png

 

Going further charts with the CFT -4 & -5 are presented with a noticable weight increase.

Edited by Hummingbird
Posted
I think he is wrong about the CFT's and the DPS. Here is a copy of the F-15E, 41500pds, with 4 AIM-7's. Note that even though the load manager says only 4 Sparrows, in the description it clearly states CFT present.

 

xgukyyN.jpg

 

Also note, the 10000ft STR starts at 7.5 DPS and mach 0.2

 

Nope, I'm using a different chart.

 

The chart u just presented actually starts even higher at almost 8 dps, which again screams at a mistake.

 

Looking at the lift curves of the aircraft there's absolutely no way it could do 7.5-8 dps at Mach 0.2, the thing is barely flying at that speed, and it's got no devices to keep the wings from experiencing a boundary layer seperation at the AoA it would need to pull off to achieve it.

 

If an aircraft like the F-14 with a lower LW ratio can sustain a higher STR from 0.33 to 0.74 (and higher ITR across the board until G limitation), then it would make little sense if it lost that edge below 0.33 considering its higher lift curve and lower stall speed.

 

You can check this yourself by doing the exact same calculations as Lunaticfringe has made.

Posted (edited)
Huh? Or you've got a burning desire NOT to have that? Seems more realistic considering how combative you are, and frankly it's pretty disturbing to see a moderator being so accusing of others without reason.

 

I'm just trying to make the charts make sense, that's all I'm doing.

 

Read the chart for yourself (1,000 lbs difference, 4x AIM-7's, no CFTs mentioned at all):

 

 

Going further charts with the CFT -4 & -5 are presented with a noticable weight increase.

 

Could the F-15E actually carry 4 x AIM-7 on its body without CFTs? was it actually wired up for that?

 

Also note that weight will also vary depending on current fuel load - E carries 18000+ lbs with CFTs

Edited by Basher54321
Posted
One explanation for the 11.4 to 13 deg/s difference could perhaps be that in the unbanked stall the wing and power component of the engine thrust is doing all the lifting but in a banked turn the wing lift could be supplemented by a vertical lifting component due to a yaw angle beta coupled with the resulting engine thrust component which theoretically means that you could have a slightly higher bank angle than 60 deg and still maintain altitude.

 

Completely logical.

Completely reasonable.

Completely negates the raison d'être of the chart.

 

What the chart says: "put X G and Y bank angle into aircraft at A Mach and get Z turn rate."

What the chart doesn't say: "put X G (with a bit of special sauce) and Y bank angle (with the appreciable mustard) into aircraft at A Mach and get Z turn rate."

 

Given its gradation, it actually *has* the capability to model exactly what you said. It doesn't. It simply says at 165 kts at SL, put a 60 degree bank angle on the airplane and generate 13DPS.

 

That's mathematically wrong. Sorry. It's especially wrong (and therefore incorrect to claim so) when the chart could be modified to present exactly what you say.

Posted

It might not necessarily be strange for the sensor to say something other than what's happening, but there's no reason to believe such a thing.

 

The discrepancy might might come from data-collection techniques, ie. AoA making it appear that you're a few degrees 'early', etc.

[sIGPIC][/sIGPIC]

Reminder: SAM = Speed Bump :D

I used to play flight sims like you, but then I took a slammer to the knee - Yoda

Posted

I'm not disputing that; however, that's why I'm stating- what matters is the meat of the envelope, not the bleeding edge of flight capacity.

 

If those charts were 100% correct low, that end of the chart would be almost illegible. Instead, it's fudged, with the understanding nobody is paying attention. No great shakes.

Posted
Could the F-15E actually carry 4 x AIM-7 on its body without CFTs? was it actually wired up for that?

 

Also note that weight will also vary depending on current fuel load - E carries 18000+ lbs with CFTs

 

I don't see why it couldn't, and it sure looks like it was tested in this configuration as no CFTs are mentioned.

 

A chart with 4x AIM-7's & -9's + a -4 CFT is also presented where performance is noticably worse, much much worse than what the addition of a mere 4x AIM-9s would constitute.

 

Also a chart with the F-15E in clean condition, no missiles or CFTs, shows 7.5 dps at Mach 0.2 at 10 kft, and that's at 39,500 lbs + with more thrust than the F-15C. Again pointing heavily towards the F-15C chart being at fault, and that the F-15C most likely did not exceed 5 dps with 4x AIM-7's & -9's. Anything else would make no sense.

Posted
Nope, I'm using a different chart.

 

AS we are looking at the same manual, there are only 2 points to be considered.

 

1. I think the charts always include CFTs (although not entirely sure), what the number next to the missiles states is if they actually contain any fuel.

 

2. And more important, there is "disclaimer" next to the PW229 introduction in appendix B. It states performance with that engine is not yet known, and will be added later on. In the meant time, what follows are estimates.

 

Still largely pointless as lunaticfringe points out, fighting at 140 knots is not something you want to do.

Modules: FC3, Mirage 2000C, Harrier AV-8B NA, F-5, AJS-37 Viggen, F-14B, F-14A, Combined Arms, F/A-18C, F-16C, MiG-19P, F-86, MiG-15, FW-190A, Spitfire Mk IX, UH-1 Huey, Su-25, P-51PD, Caucasus map, Nevada map, Persian Gulf map, Marianas map, Syria Map, Super Carrier, Sinai map, Mosquito, P-51, AH-64 Apache, F4U Corsair, WWII Assets Pack

Posted (edited)
AS we are looking at the same manual, there are only 2 points to be considered.

 

1. I think the charts always include CFTs (although not entirely sure), what the number next to the missiles states is if they actually contain any fuel.

 

2. And more important, there is "disclaimer" next to the PW229 introduction in appendix B. It states performance with that engine is not yet known, and will be added later on. In the meant time, what follows are estimates.

 

Still largely pointless as lunaticfringe points out, fighting at 140 knots is not something you want to do.

 

I see your points however I doubt that the aircraft wasn't tested without the CTFs.

 

Either way wouldn't you agree that it would make little to no sense if the F-15 somehow magically overtook the F-14 in STR below Mach 0.33 when it lags so much behind the rest of the way until ~0.74? It's not like it suddenly is able to increase wing sweep and span (and thus aspect ratio) to gain extra lift and a better L/D like the F-14 actually can for better low speed agility.

 

The F-14 features more lift pr. weight (confirmed via lift curve, ITR & stall speed), thus it should/will need less thrust to maintain the same STR as the F-15C at any subsonic speed if we look past G limitations. = and that's the point I'm trying to get at here, that this sudden transition at mach 0.3 makes no sense and I believe it to be a mistake, one perhaps accepted due to pretty much all combat taking place above this speed, as Lunaticfringe pointed out.

 

Bottom line is that I'm trying to make sense of the charts so that they add up with side by side calculations based on real world figures, and fact is that below Mach 0.3 they currently don't.

Edited by Hummingbird
Posted
Completely logical.

Completely reasonable.

Completely negates the raison d'être of the chart.

 

What the chart says: "put X G and Y bank angle into aircraft at A Mach and get Z turn rate."

What the chart doesn't say: "put X G (with a bit of special sauce) and Y bank angle (with the appreciable mustard) into aircraft at A Mach and get Z turn rate."

 

Given its gradation, it actually *has* the capability to model exactly what you said. It doesn't. It simply says at 165 kts at SL, put a 60 degree bank angle on the airplane and generate 13DPS.

 

That's mathematically wrong. Sorry. It's especially wrong (and therefore incorrect to claim so) when the chart could be modified to present exactly what you say.

 

Gee, why come out with guns blazing? Why the bold text and extra sauce added? I'm not saying the chart is 100% correct. Just floating some ideas here so no need to get defensive. ;)

 

Taking things to the extreme:Say I have a bank angle of 90 degrees in a turn and pulling max aoa in the turn on the edge of stall. In this case, if I have a sufficiently powerful engine I can still maintain altitude by body lift. In this case I will have a certain finite turn rate and load factor that could be measured by flight instrumentation. However, mathematically I am dead since I am flying with 90 degree bank and therefore at an infinite load factor.

 

So maybe both the load factor and turn rate were measured in flight tests? If so then that could explain the difference we see in the charts: While the load factor (acceleration perpendicular to wings) measured by flight instrumentation was 2, the bank angle was actually slightly more giving a higher turn rate and the altitude in the turn was maintained by a slight beta. Just a theory of course, and it could also be that the turn rate in the figure is higher than it should but I can't really see that how you can be so dead sure either way just because the figures don't tally.

Old Crow ECM motto: Those who talk don't know and those who know don't talk........

Pilum aka Holtzauge

My homepage:  https://militaryaircraftperformance.com/

 

Posted

When in doubt it's a matter of using the hard figures available, such as lift curves and stalling speeds, and here it becomes quite clear that some of the charts are inaccurate at low speeds.

 

Furthermore looking at the lift curves & stalling speeds of the F-14 & -15, the fact that the F-14 features an ITR & STR superior to that of the F-15 from lift off to Mach ~0.74 becomes rather clear. Physics dictate this to be true, UNLESS the F-14 somehow experienced a dramatic decrease in thrust below Mach 0.3, which doesn't really seem plausible.

Posted (edited)
Gee, why come out with guns blazing? Why the bold text and extra sauce added? I'm not saying the chart is 100% correct. Just floating some ideas here so no need to get defensive. ;)

 

Defended? There's nothing here to be defended, including an incorrect chart.

 

So maybe both the load factor and turn rate were measured in flight tests? If so then that could explain the difference we see in the charts: While the load factor (acceleration perpendicular to wings) measured by flight instrumentation was 2, the bank angle was actually slightly more giving a higher turn rate and the altitude in the turn was maintained by a slight beta. Just a theory of course, and it could also be that the turn rate in the figure is higher than it should but I can't really see that how you can be so dead sure either way just because the figures don't tally.[/Quote]

 

I can be dead sure because the statement made (60BA at 0.25M and 2G generates 13dps) is a physical impossibility. The chart doesn't read 62 degrees, it doesn't read 64 degrees, and it also doesn't read 63 degrees (63.11, to be exact), which is the required bank angle to generate 13DPS at 165.375 knots at SL.

 

The chart describes an impossibility. It could have projected the input curve higher, and the output curve substantially lower, and been made correct. It didn't. And if you're paying attention to what I've been stating, rather than wasting time trying to make excuses for it being wrong, you'd have noted that I have made it abundantly clear that its being wrong in that regime doesn't matter, because that region of the chart is immaterial to the conversation the chart is for.

 

*This* conversation has devolved into multiple booths at the Topeka County Fair arguing over which one can inscribe the smallest, and most, Chinese characters on a grain of rice, when it doesn't matter because no one at the county fair in Topeka, Kansas can read Chinese.

 

Let me ask a question: which is a better use of people's time-

 

a. understanding that some numbers on a chart are wrong, and directing their focus to the part of the chart that is both correct, and significantly more important to the task at hand, or

 

b. playing hypothetical games over why the chart is wrong in a region, when it has absolutely zero bearing on the employment of the aircraft?

 

The idea that this data was somehow collected, then intentionally traded in, by the flight test pilots, engineers, and directive authors, is not simply insulting to their methodology, it is insulting to their competency. Data outside the laws of physics would have been immediately eliminated from contention. AoA onset rate showing an output value for turn rate that is beyond possibility? Tossed.

 

They made a chart with the knowledge of what matters to the pilot, what doesn't matter, and with the aim of making it as quickly as possible with as few data points as required, and present it as legibly as they can. Within that framework, charts as such that we see, with errors of 15-20% are negligible because the region where the mistakes are exist as a known quantity.

 

Seriously- you folks would be better served defending map projection methods over this, because it doesn't factor in to the person strapping the jet on.

Edited by lunaticfringe
Posted
I'm just happy we have four or so incredibly smart guys here to shut down the flight model whiners when the Tomcat is finally released!

 

It's going to be an awesome bird to fly ingame I'm sure, and probably only second to the Su27 in ACM :)

 

The devs have already been provided with excellent primary source material by gents such as Lunaticfringe, and they no doubt know their stuff when it comes to aerodynamics & physics, so we're in for a treat :)

Posted

I can understand what the fuss is all about, but it wouldn't be the first time such a thing happened. If you look at the F-18/C Clmax-Ps=0 relations, you can see another such case. The 4AAM F-18's Ps=0 seams to ride the Clmax from 0 to 180KIAS or mach 0.27 ASL. And accordingly it states over 3g sustained at that point. What method was used to generate it? Is the data 100% accurate? I have no idea, to be honest. But does it matter?

 

It's going to be an awesome bird to fly ingame I'm sure, and probably only second to the Su27 in ACM :)

Oh no, you are not getting me into an argument about what is the best ACM platform out there ;)

Modules: FC3, Mirage 2000C, Harrier AV-8B NA, F-5, AJS-37 Viggen, F-14B, F-14A, Combined Arms, F/A-18C, F-16C, MiG-19P, F-86, MiG-15, FW-190A, Spitfire Mk IX, UH-1 Huey, Su-25, P-51PD, Caucasus map, Nevada map, Persian Gulf map, Marianas map, Syria Map, Super Carrier, Sinai map, Mosquito, P-51, AH-64 Apache, F4U Corsair, WWII Assets Pack

Posted
Defended? There's nothing here to be defended, including an incorrect chart.

 

I can be dead sure because the statement made (60BA at 0.25M and 2G generates 13dps) is a physical impossibility. The chart doesn't read 62 degrees, it doesn't read 64 degrees, and it also doesn't read 63 degrees (63.11, to be exact), which is the required bank angle to generate 13DPS at 165.375 knots at SL.

 

The chart describes an impossibility. It could have projected the input curve higher, and the output curve substantially lower, and been made correct. It didn't. And if you're paying attention to what I've been stating, rather than wasting time trying to make excuses for it being wrong, you'd have noted that I have made it abundantly clear that its being wrong in that regime doesn't matter, because that region of the chart is immaterial to the conversation the chart is for.

 

*This* conversation has devolved into multiple booths at the Topeka County Fair arguing over which one can inscribe the smallest, and most, Chinese characters on a grain of rice, when it doesn't matter because no one at the county fair in Topeka, Kansas can read Chinese.

 

Let me ask a question: which is a better use of people's time-

 

a. understanding that some numbers on a chart are wrong, and directing their focus to the part of the chart that is both correct, and significantly more important to the task at hand, or

 

b. playing hypothetical games over why the chart is wrong in a region, when it has absolutely zero bearing on the employment of the aircraft?

 

The idea that this data was somehow collected, then intentionally traded in, by the flight test pilots, engineers, and directive authors, is not simply insulting to their methodology, it is insulting to their competency. Data outside the laws of physics would have been immediately eliminated from contention. AoA onset rate showing an output value for turn rate that is beyond possibility? Tossed.

 

They made a chart with the knowledge of what matters to the pilot, what doesn't matter, and with the aim of making it as quickly as possible with as few data points as required, and present it as legibly as they can. Within that framework, charts as such that we see, with errors of 15-20% are negligible because the region where the mistakes are exist as a known quantity.

 

Seriously- you folks would be better served defending map projection methods over this, because it doesn't factor in to the person strapping the jet on.

 

I think I’ll limit myself to answer this objectively and skip the diatribe:

 

Returning the compliment about paying attention: If you pay attention to what I have said it is this: Just because the top g-load/bank angle chart does not tally with the turn rate chart this does not automatically mean the lower turn rate chart is wrong: Yes, it could mean that the curve in the lower chart showing a turn rate of 13 deg/s is too high. However, could also be that the upper curve showing 2 g 60 deg bank should be a tad higher so you would read off 2.2 g 63 deg bank instead. So to me it means there is an inconsistency and that one of the charts is incorrect but you can’t really tell which.

 

Finally, as to insulting, the only insults I can see here have been about wasting other people’s time, references to Topeka Country Fair and that people who don’t agree should study map projections.

Old Crow ECM motto: Those who talk don't know and those who know don't talk........

Pilum aka Holtzauge

My homepage:  https://militaryaircraftperformance.com/

 

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