Operational Performace Comparison: Viper, Beagle, and Stubby

The F-35 compared with other modern jets.
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ricnunes

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Unread post11 Feb 2019, 16:34

sprstdlyscottsmn wrote:I don't think it has anything to do with lift, per say. Also, in the Israeli incident roll commands were also being used. That horizontal tail was helping to hold up the right side.


I see.
But I would say that if the F-15 wasn't an aircraft whose fuselage didn't generate big amounts of lift then it would almost certainly have crashed since there was nothing that the horizontal tail could help and/or the pilot could do in terms of roll commands in order to prevent the aircraft from crashing. I would say that if this happened to a F-16 for example than the aircraft would almost certainly have crashed (however I admit that there's a good deal of speculation on my part here), just like it happened with the A-4.

By the way I wasn't implying that the (fuselage) lift would be a direct reason for the increase in drag regarding the center external fuel tank. But couldn't it be that the drag/airflow from the center fuselage external fuel may be "messing up" with the lift generated by the fuselage? Or resuming what (I believe) sferrin mentioned as being interference drag - and perhaps this could help justify what you mentioned below:

sprstdlyscottsmn wrote:I honestly thing the center stations has so much drag on the F-15 because it is packed under such a broad fuselage while the wing stations are so exposed.


I don't know if what I'm posting makes sense, or not?
Last edited by ricnunes on 11 Feb 2019, 20:58, edited 1 time in total.
A 4th/4.5th gen fighter aircraft stands about as much chance against a F-35 as a guns-only Sabre has against a Viper.
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Unread post11 Feb 2019, 16:42

zero-one wrote:Okay so its official. For the F-15, 2 wing tanks have less drag than 1 center tank. But what about weight? surely the 2 wing tanks will be heavier. I guess what I'm asking is, what has more negative effects on performance? Will the lighter weight of the single center-line tank offset the lower drag advantage of the 2 wing tank configuration?


I believe that you bring up a good point.
However and while I could be wrong, but I believe that would be one of those parasitic drag Vs lift-induced drag cases. So I would say that at best the 2 wing tanks would add more lift-induced drag (compared to only 1 center tank) exactly due to the reason you mention - heavier weight. However lift-induced drag is considerably reduced as the aircraft speed increases.
So if there was to be a bigger impact on performance in having two wing tanks compared to having only one center fuselage then this would only happen at low or very low speeds and definitely not at medium to high speeds.
Again, my 2 cents...
A 4th/4.5th gen fighter aircraft stands about as much chance against a F-35 as a guns-only Sabre has against a Viper.
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Unread post11 Feb 2019, 19:24

zero-one wrote:Okay so its official. For the F-15, 2 wing tanks have less drag than 1 center tank. But what about weight? surely the 2 wing tanks will be heavier. I guess what I'm asking is, what has more negative effects on performance? Will the lighter weight of the single center-line tank offset the lower drag advantage of the 2 wing tank configuration?

I've looked at it both ways. I generally find that the two wing tanks is always preferable. Operationally this bears out as well as 95% of all operational photos of Eagles I have seen show two wing tanks no matter the Eagle variant or mission.
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Unread post12 Feb 2019, 13:42

sprstdlyscottsmn wrote:
zero-one wrote:Okay so its official. For the F-15, 2 wing tanks have less drag than 1 center tank. But what about weight? surely the 2 wing tanks will be heavier. I guess what I'm asking is, what has more negative effects on performance? Will the lighter weight of the single center-line tank offset the lower drag advantage of the 2 wing tank configuration?

I've looked at it both ways. I generally find that the two wing tanks is always preferable. Operationally this bears out as well as 95% of all operational photos of Eagles I have seen show two wing tanks no matter the Eagle variant or mission.


2 bags makes sense...

In the 1980's, you almost always saw Eagles flying with the centerline tank. DS pics showed at least 2 and many times 3, at least for CAP missions. What would be interesting is to find out if the F-15 vs. Mig-29/25 dogfights of DS were flying with any. The depictions in the youtube Dogfights of DS series show missiles only, although I doubt they did any homework on whether they in fact were flying with tanks, jettisoned those tanks etc.. These days, I ALWAYS see F-15's flying with wing tanks (both locally out of Westfield, MA and in pictures)

I have to believe any Eagle flying with 2 bags (in war time) is jettisoning them, especially if merging with a Flanker...
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Unread post12 Feb 2019, 16:29

sprstdlyscottsmn wrote:If you think so. I had to make sure the envelope, acceleration, and STR specs lines up for both a clean aircraft and one with CFTs, LANTIRN, 4 AAMs, 12 MK82s, and a centerline pylon. I quite literally spent over 100 hours calibrating this model against the -1.


"The book" t'sonic accel on a clean 'E' w/ PW 229s at 40K' and not much gas is about 45 secs. That's why a similar number for a jet w/14 weps, CFTs etc -- even at 30K' -- is a bit of a head scratcher.

"Legend" always had it that the CFTs reduced the drag profile of the Eagle; we used to joke that only the USAF could find a way to add 'tanks' and make a jet faster. :thumb:

Later we were told that the legend only held true for subsonic performance.
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Unread post12 Feb 2019, 17:00

quicksilver wrote:
sprstdlyscottsmn wrote:If you think so. I had to make sure the envelope, acceleration, and STR specs lines up for both a clean aircraft and one with CFTs, LANTIRN, 4 AAMs, 12 MK82s, and a centerline pylon. I quite literally spent over 100 hours calibrating this model against the -1.


"The book" t'sonic accel on a clean 'E' w/ PW 229s at 40K' and not much gas is about 45 secs. That's why a similar number for a jet w/14 weps, CFTs etc -- even at 30K' -- is a bit of a head scratcher.

"Legend" always had it that the CFTs reduced the drag profile of the Eagle; we used to joke that only the USAF could find a way to add 'tanks' and make a jet faster. :thumb:

Later we were told that the legend only held true for subsonic performance.


I appreciate your skepticism. I am adding Thrust/Drag curves for 20kft, 30kft, and 40kft. 20kft was calibrated against Sustained Turn performance. 40kft was calibrated against the acceleration spec. 30kft is estimated to fit between them in the speed range between .4 and 1.4M (those thrust values are determined by the flight envelope calibration). Drag is based on F-15X with full 16 AAM load.

Capture.PNG


As you can see, the 30kft thrust values are fairly evenly split between the 20kft and 40kft values. But when you look at the difference between the 40kft thrust and drag lines compared to the 30kft thrust and drag lines you can see how being at lower altitude increases acceleration drastically.
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Unread post12 Feb 2019, 18:53

Thx.

I would have a more conspicuous drag rise in the transonic region.
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Unread post12 Feb 2019, 19:04

quicksilver wrote:Thx.

I would have a more conspicuous drag rise in the transonic region.

That would require a more conspicuous thrust rise in the transsonic to match. What I match isn't so much the raw Thrust but the delta between my calculated drag and the calculated thrust. Raw Thrust is a byproduct. By using charted transsonic performance I can make sure my drag and thrust models result in a good curve fit to the original material, meaning my (T-D) is more or less correct. Change one because "I don't think it should look that way" and now the curve doesn't fit anymore so I have to change the other.
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Unread post12 Feb 2019, 19:16

Why so much drag around Mach 0.4-0.5 ?
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Unread post12 Feb 2019, 19:36

For a jet with CFTs, I expected a steeper slope in each between .8 and ~1.1 IMN, and after 1.1, a more gradual rise in thrust relative to drag to the intersection out there just shy of 1.5.

As a sidenote, at 40K' the thrust increase seems to 'anticipate' the drag rise.
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Unread post12 Feb 2019, 19:42

marsavian wrote:Why so much drag around Mach 0.4-0.5 ?

Induced Drag, drag due to lift. At those speeds (most notably at 40kft) the F-15 is using so much AoA to generate lift that the drag rise exceeds the engines ability to overcome it. Drag is pretty much always bowl shaped. On the left, it is dominated by drag due to lift/weight, on the right it is dominated by drag due to form and wave drag. The lowest point is L/Dmax and, nominally, your speed for best endurance. Changes in TSFC with speed and setting come into play. If you draw a straight line from the "0,0" point of the graph and found where it touched a single point (usually just in front of L/D max, but can coincide with it if Induced Drag and Wave drag but up against each other) that is nominally your best cruise speed.
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Unread post12 Feb 2019, 19:46

quicksilver wrote:For a jet with CFTs, I expected a steeper slope in each between .8 and ~1.1 IMN, and after 1.1, a more gradual rise in thrust relative to drag to the intersection out there just shy of 1.5.

As a sidenote, at 40K' the thrust increase seems to 'anticipate' the drag rise.

You can expect whatever you want, but the -1 says otherwise. I expected that as well, but it didn't play out that way. I don't build a model to what I expect unless I don't have raw data. With the F-15 I have raw data. I had to build unique wave drag properties for each munition type so that the various envelopes listed in the -1 were all reasonably represented.
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Unread post12 Feb 2019, 20:25

Thrust is only a proxy for drag at Ps=0, no?
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Unread post12 Feb 2019, 20:28

correct, other wise there is a measured acceleration or deceleration rate. If you can find the rate of speed change and you know the weight then you can find the difference between the thrust and drag in pounds.
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Unread post15 Feb 2019, 08:25

sprstdlyscottsmn wrote:enjoy
The attachment Strike Fighters 2025 F-15SA CAP.pdf is no longer available

I found this video of Rafale demonstration, i hope it can be useful to estimate the upper bound of its agility
https://www.aeronewstv.com/fr/evenement ... isses.html
Almost haft a circle in 7 seconds => 26 °/sec ? do you think it is a combination of pitch rate and ITR?
Rafale.PNG

F-35.PNG

https://www.youtube.com/watch?v=7XeViHEWzLk


Haft a circle in 10 seconds => 18 °/sec
rafale 180.PNG


Full circle in 17 seconds => 21 °/sec
Rafale 360.PNG
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