F-15 Stores Drag

[Exorcet]

I've noticed something that seems strange with the F-15. The semi-conformal missiles on the fuselage seem to generate more drag than weapons mounted under the wings, and missiles in general seem to be more draggy than external fuel tanks.

To test this, I measured acceleration between a specific range and took the result every 0.1 Mach, I've attached tracks for the Wings only missiles and Fuselage only missiles. Wings only (with 2 tanks) was the fastest by a few seconds, then Fuselage with 2 tanks, then all missiles with 0 tanks, then all missiles with 2 tanks. I would have expected the no tanks run to be the fastest and for the fuselage missile loadout to beat the wings loadout. Supposedly the fuselage missiles are low drag, especially the rear two, or is this not correct?

EagleM14F.trk EagleM14W.trk

[Spurts]

AFAIK in the current implementation missiles have a set drag no matter where they are stored.  what missiles are you using for your test?  AIM-120C in all cases?

 

[Exorcet]

I had figured the FC3 drag model was simplified, and a few other posts have brought it up. Since the F-15's underperforming radar was given attention I decided to bring this up as well.

For my tests I used the 6 AMRAAM 2 Sidewinder loadout as it's the most common one carried.

[Spurts]

for consistency you should use 8 AMRAAMs so that your wing missiles are exactly the same as the fuselage missiles, otherwise it introduces the error of Sidewinder drag vs AMRAAM drag.  While that difference is IRL quite small for the purposes of a test we should eliminate it if we can.  Another interesting data point, IRL the centerline tank has the same drag index as two wing tanks.

[Exorcet]

The Sidewinders do actually produce less drag than the AMRAAM. 4 wing AIM-120 is exactly the same as 4 fuselage 120's.

44 minutes ago, Spurts said:

Another interesting data point, IRL the centerline tank has the same drag index as two wing tanks.

That one is correctly modeled in DCS, in fact twin fuel tanks are actually slightly less drag than the center tank. 3 tanks are a nightmare to fly with, yet I see so many F-15's loaded that way in the sim.

[=4c=Nikola]

On 10/19/2021 at 8:14 PM, Exorcet said:

The Sidewinders do actually produce less drag than the AMRAAM. 4 wing AIM-120 is exactly the same as 4 fuselage 120's.

That one is correctly modeled in DCS, in fact twin fuel tanks are actually slightly less drag than the center tank. 3 tanks are a nightmare to fly with, yet I see so many F-15's loaded that way in the sim.

It flies just fine with 3 tanks, just remember to drop them before engaging an opponent.

[Exorcet]

13 hours ago, =4c=Nikola said:

It flies just fine with 3 tanks, just remember to drop them before engaging an opponent.

It's all relative. Sure the F-15 can take off and fly around with 3 tanks, but it's much slower than taking 2 which already give you a 600 mile combat radius.

Anyway, is there any chance the weapons drag will be looked at? Or is this something that will have to wait for a FF Eagle? @Flappie @NineLine @BIGNEWY

[Csgo GE oh yeah]

I am surprised this isn't stored under "reported" yet ? I thought this was already widely known . 
F15's are currently flying around at 55k , mach 2.5, with 3 tanks and 8 amraams. 
While funny, at some point it should be fixed. Can't imagine it's a difficult repair so ....

Edited November 7, 2021 by Csgo GE oh yeah

[Exorcet]

Technically this thread was posted before the patch that removed stores drag, ironically pointing out that the F-15 has too much stores drag potentially.

But I guess with the bug firmly in place I guess that can take precedence for now.

[henshao]

F-15's stores drags disappeared simultaneously with the introduction of over-g damage, I imagine they're somehow related

[Hardcard]

Now, after the last patch, it's the opposite situation again: F-15C (without bags) struggles to reach the mach 2+ speeds at altitude that it's known for.

You can consider yourself lucky if you reach mach 1.7, almost feels like accelerating a hornet, tbh. 

It's either too much drag or too little... we don't seem to get it right 

 

Edited November 19, 2021 by Hardcard

[henshao]

I tested the Eagle against the -1, a certain loadout to a certain speed in a certain time. It was within the margins of error

 

44k lb bird, 4 sidewinders 4 sparrows, 300 seconds to mach 2 from mach .8 at 40000 feet on a standard day. Unfortunately there are no amraam accelerations to test against. might be able to "simulate" such a thing with drag indexes but per-station drag indexes are not modeled on the Eagle (the fuselage stations are a much lower penalty than wing stations)

Edited November 19, 2021 by henshao

[Exorcet]

15 hours ago, henshao said:

I tested the Eagle against the -1, a certain loadout to a certain speed in a certain time. It was within the margins of error

 

44k lb bird, 4 sidewinders 4 sparrows, 300 seconds to mach 2 from mach .8 at 40000 feet on a standard day. Unfortunately there are no amraam accelerations to test against. might be able to "simulate" such a thing with drag indexes but per-station drag indexes are not modeled on the Eagle (the fuselage stations are a much lower penalty than wing stations)

 

Interesting then, I guess it was tuned for 8 AAM loadout, at least with Sparrow. Perhaps then the discrepancy would show more with other loadouts? The other alternative is that the drag savings from the semi-conformal missiles is so small that it doesn't matter, but that goes against what I've heard isn't what I'd expect.

[henshao]

3 hours ago, Exorcet said:

Interesting then, I guess it was tuned for 8 AAM loadout, at least with Sparrow. Perhaps then the discrepancy would show more with other loadouts? The other alternative is that the drag savings from the semi-conformal missiles is so small that it doesn't matter, but that goes against what I've heard isn't what I'd expect.

Drag index of a 120 on fuselage is 1.3, and 2.4 on a pylon, so almost double. (for reference a sidewinder is 2.0 under a wing) This is also not considering the pylon with launchers itself, which has an index of 3.3. So 4 amraams on the wings is (2.4*4)+(3.3*2)=16.2. edit: i forgot the lau-128s, 1.1 each. so the wing amraam penalty is a total of 20.6. for contrast 4 AMRAAMs in a hotrod configuration, fuselage only, is a drag index of only 5.2. (!)

 

Old man McDonnell knew what he was doing, see also Phantom conformal sparrow carriage

Edited November 20, 2021 by henshao

[henshao]

after testing the acceleration and watching more closely, the F-15 is definitely lacking some acceleration on the "bottom end", just eyeballing it...after 100 seconds the Eagle should be mach 1.37, DCS eagle is ~mach 1.29. after 200 seconds the Eagle should be mach 1.75, DCS eagle is mach 1.76, after 300 seconds should be ~mach 2, DCS eagle is mach 2.05. so it has probably slightly too much acceleration outside the transonic region and quite a bit too little in it.

 

Also the all AMRAAM loadout drag index is 25.4, 4 sparrows 4 sidewinders is slightly more at 26.2, yet it accelerates slower with 8 amraams. only reaches ~mach 1.96 after 300 seconds with 8xamraam, reaches ~mach 2.05 with 4sparrow/4sidewinder

[Exorcet]

Yeah the Eagle has always seemed to be plowing in a brickwall around M1.1 in DCS, it's something I've wondered about. From what I remember they gave up the optimal nose cone shape for the sake of radar performance, but transonic should still be where the Eagle shines. Also despite the better TWR and dynamic inlets the Eagle can't climb as steeply as the F-16 at supersonic speed from my casual testing, although the higher absolute speed and better radar (if it was modeled properly) at least let it take advantage of higher altitudes when engaging.

[henshao]

Well as loathe as I am to acknowledge it, the -129 Viper is a bit quicker than the Eagle under similar circumstances

 

@GGTharos have you noticed this transonic acceleration/amraam drag issue

[GGTharos]

If you mean stores drag, I would be going off of memory.   There are no charts for AMRAAM payload but it should be very similar to the 4+4 config, certainly within a few seconds I imagine.

The 4+4 config has been verified against acceleration charts.  I won't have time to re-check so if someone would like to try doing so it would be great.   The F-16 has a very powerful engine and if it's slick I would expect it to have pretty spiffy acceleration.  The thing about the F-16 is that it's going to be optimized for a narrower speed band whereas the eagle's intakes optimize engine airflow for a much larger range.

Maybe this is all correct or maybe it's the F-16 that's being too good, who knows?

Edited November 21, 2021 by GGTharos

[henshao]

Yes, I ran several acceleration tests. There are 3 problems identified in this thread: 1) the OP believes wing 120s drag more than fuselage 120s, when it should be the other way and by a large margin. I haven't checked this. 2) the F-15 with 8 missiles of any kind reaches mach 2 roughly on schedule but is too slow in the early part of the run and too fast in the late part to make up for it. I have measured this several times. 3) 8 amraams drag more than the cold war 4 sparrow/4 sidewinder loadout, when the -1 asserts 8 amraams should have a lower drag index. I have measured this twice

[Exorcet]

4 hours ago, henshao said:

Well as loathe as I am to acknowledge it, the -129 Viper is a bit quicker than the Eagle under similar circumstances

The F-16 being faster straight and level is fine with me, I just found it curious it wins the in climb angle as well. The Eagle still has the better TWR as far as I know, but this is probably the most minor thing.

 

3 hours ago, henshao said:

 the OP believes wing 120s drag more than fuselage 120s, when it should be the other way and by a large margin.

Just a small clarification, it seems that missile type is the only thing to impact drag. Wing mounted 120's are the same drag as fuselage ones. I initially said less because I was using a 2+6 loadout with Sidewinders, which through off the results. At the suggestion of another poster I tested 4 wing AMRAAM and 4 fuselage AMRAAM. They result in the same performance. I don't know how the wing mounted launchers factor into this though. You can't independently load them on FC3 modules. I think they might be visual only?

[GGTharos]

7 hours ago, henshao said:

Yes, I ran several acceleration tests. There are 3 problems identified in this thread: 1) the OP believes wing 120s drag more than fuselage 120s, when it should be the other way and by a large margin. I haven't checked this.

AFAIK each missile has a single drag value for FC3.  Where you put it does not matter.

7 hours ago, henshao said:

2) the F-15 with 8 missiles of any kind reaches mach 2 roughly on schedule but is too slow in the early part of the run and too fast in the late part to make up for it. I have measured this several times.

7 hours ago, henshao said:

3) 8 amraams drag more than the cold war 4 sparrow/4 sidewinder loadout, when the -1 asserts 8 amraams should have a lower drag index. I have measured this twice

I would say that 2 and 3 are the real issues then, good work on doing those tests.   Do you have some data to post?

[henshao]

You did the test with a centerline bag? I tested without one

On 11/20/2021 at 11:42 AM, henshao said:

44k lb bird, 4 sidewinders 4 sparrows, 300 seconds to mach 2 from mach .8 at 40000 feet on a standard day.

 

...

after testing the acceleration and watching more closely, the F-15 is definitely lacking some acceleration on the "bottom end", just eyeballing it...after 100 seconds the Eagle should be mach 1.37, DCS eagle is ~mach 1.29. after 200 seconds the Eagle should be mach 1.75, DCS eagle is mach 1.76, after 300 seconds should be ~mach 2, DCS eagle is mach 2.05. so it has probably slightly too much acceleration outside the transonic region and quite a bit too little in it.

 

Also the all AMRAAM loadout drag index is 25.4, 4 sparrows 4 sidewinders is slightly more at 26.2, yet it accelerates slower with 8 amraams. only reaches ~mach 1.96 after 300 seconds with 8xamraam, reaches ~mach 2.05 with 4sparrow/4sidewinder

 

[draconus]

5 hours ago, henshao said:

You did the test with a centerline bag?

That's how it is on the chart.

[GGTharos]

@BIGNEWYPlease look at the chart above.  The acceleration discrepancy is quite significant for this aircraft.   It's too slow in acceleration up to about M1.65 and too fast after.

For example, to reach M1.5 you should take about 190 sec but it takes 220.   30 seconds is a very large deviation; this trend is similar from M1.1 (from 0.8 to 1.1 there is a deviation also, but much smaller) up to M1.5-1.6 or so.

 

@Exorcet does the clean acceleration match?

Edited November 22, 2021 by GGTharos

[Snappy]

3 hours ago, GGTharos said:

@BIGNEWY Please look at the chart above.  The acceleration discrepancy is quite significant for this aircraft.   It's too slow in acceleration up to about M1.65 and too fast after.

For example, to reach M1.5 you should take about 190 sec but it takes 220.   30 seconds is a very large deviation; this trend is similar from M1.1 (from 0.8 to 1.1 there is a deviation also, but much smaller) up to M1.5-1.6 or so.

 

@Exorcet does the clean acceleration match?

 

OT but BTW, For whatever reason both your pings didn’t work, otherwise the members names would‘ve gotten a light blue background.Maybe re-edit and try again. Had the same thing happen to me before , seems to happen when you type quickly.