Operational Performace Comparison: Viper, Beagle, and Stubby

The F-35 compared with other modern jets.
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marsavian

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Unread post10 Feb 2019, 10:01

Residual pylon drag plus CFT drag is DI 75.


Is that the CFT-E rather than the CFT-C ? Also wouldn't the F-15X be carrying one Legion pod instead of two Lantirns these days ?
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Unread post10 Feb 2019, 10:32

marsavian wrote:
Residual pylon drag plus CFT drag is DI 75.


Is that the CFT-E rather than the CFT-C ? Also wouldn't the F-15X be carrying one Legion pod instead of two Lantirns these days ?

That a good point, without IRST pod, F-15X won't know F-35 location to follow
Do you know Legion pod is 1/2 size of TALON HATE pod
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zero-one

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Unread post10 Feb 2019, 14:23

Question about EFTs.

What has a bigger penalty on performance, 1 center line tank or 2 wing tanks.
Common sense will tell you 2 tanks have more penalty but is this the case?
Or does it depend on the aircraft. (i.e. F-16 is less affected with a lone center line while F-15 is less affected by 2 wing tanks)
Thanks in advance :mrgreen:
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kimjongnumbaun

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Unread post10 Feb 2019, 15:09

zero-one wrote:Question about EFTs.

What has a bigger penalty on performance, 1 center line tank or 2 wing tanks.
Common sense will tell you 2 tanks have more penalty but is this the case?
Or does it depend on the aircraft. (i.e. F-16 is less affected with a lone center line while F-15 is less affected by 2 wing tanks)
Thanks in advance :mrgreen:


2 tanks assuming they are the same size as the centerline tank. It doesn't matter what airframe it's on. Parasitic drag will kill you.
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sprstdlyscottsmn

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Unread post10 Feb 2019, 16:12

kimjongnumbaun wrote:
zero-one wrote:Question about EFTs.

What has a bigger penalty on performance, 1 center line tank or 2 wing tanks.
Common sense will tell you 2 tanks have more penalty but is this the case?
Or does it depend on the aircraft. (i.e. F-16 is less affected with a lone center line while F-15 is less affected by 2 wing tanks)
Thanks in advance :mrgreen:


2 tanks assuming they are the same size as the centerline tank. It doesn't matter what airframe it's on. Parasitic drag will kill you.

This is not the case with the F-15. The two wing tanks have less drag than a single centerline.
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zero-one

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Unread post10 Feb 2019, 17:09

sprstdlyscottsmn wrote:This is not the case with the F-15. The two wing tanks have less drag than a single centerline.


So it does depend on the airframe? I always wondered why F-15s taking off to do AA training seem to always carry 2 wing tanks, even aggressors.
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Unread post10 Feb 2019, 17:44

Interference drag?
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Unread post10 Feb 2019, 19:17

zero-one wrote:
sprstdlyscottsmn wrote:This is not the case with the F-15. The two wing tanks have less drag than a single centerline.


So it does depend on the airframe? I always wondered why F-15s taking off to do AA training seem to always carry 2 wing tanks, even aggressors.


What do you mean by airframe? F-15 vs F-16? F-15C vs F-15E?

sferrin wrote:Interference drag?


the F-15 Centerline tank has a DI of 12.2 on it's own and an additional 3.3 for the pylon for a 15.5 no matter anything else. C, E, CFT, no CFT, stores on CFT, no matter.
The F-15 Wing tank is 5.5 (C/E) without CFT, 6.0 (C/E) with CFT (this is interference drag), 8.2 (E) with bombs on inboard CFT stations, 12.3 (E) with bombs on outboard CFT stations.

So really, unless a Strike Eagle has bombs on the outboard position even with interference drag having two wing tanks has equal or less drag than a single centerline while carrying twice the relative fuel. This is why you always see Eagles of all types with two wing tanks. Once they are dropped it is less drag than a centerline.

marsavian wrote:Is that the CFT-E rather than the CFT-C ? Also wouldn't the F-15X be carrying one Legion pod instead of two Lantirns these days ?

The CFT-E as that is what I have seen in every shot of the F-15X (not artists rendering, actual shots). Also, I only counted the weight and drag of a single targeting pod when I did that calculation.
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Unread post11 Feb 2019, 07:42

sprstdlyscottsmn wrote:What do you mean by airframe? F-15 vs F-16? F-15C vs F-15E?


Yes, do 2 wing tanks always have less drag than 1 center line tank or is this unique to the F-15?
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Unread post11 Feb 2019, 14:31

I think it is unique to the F-15. Let's have a look.

The F/A-18A-D show 10.5 DI for a 330gal centerline and 14.5 DI each for the same tank on the wing. Pylons add 7.5 DI for each wing and 3.0 DI for centerline.
The FA-18E-G show a 480gal tank as 21 DI each midboard, 30 DI each inboard, 22 DI centerline if wing tanks are present and 19 DI centerline with no wing tanks. The E-G -1 also seems to indicate Pylons are additional, not included. This adds 6.5 DI to each midboard, 8.5 DI to each inboard, and 1 DI to the centerline.
F-16s are different in that they do not have a common tank. The 300gal centerline is 15 DI with nothing on inboard pylons or 18 with inboard pylon stores. The Centerline also has a 7 DI pylon. The 370gal wing tanks are 27 DI each with no stores or A2A stores at the midboard, 35 DI each with single A2G munitions midboard, and 39 DI with multiple munitions midboard.

So yes, this is unique to the F-15. It may have to do with the distance between the wing and the fuselage, the size of the pylon, or the sweep of the wing. I don't know.
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Unread post11 Feb 2019, 15:22

sprstdlyscottsmn wrote:So yes, this is unique to the F-15. It may have to do with the distance between the wing and the fuselage, the size of the pylon, or the sweep of the wing. I don't know.


Couldn't this be somehow related to the fuselage/body lift design/capability of the F-15?

I'm asking this because according to what I've read, even the F-15 designers seem to have been "caught by surprise" regarding the amount of lift that the F-15 fuselage generates, this specially after the well know Israeli F-15 incident/mid-air crash with (an also Israeli) A-4 where one of the F-15's wings was completely ripped off but it still managed to return to the base (due to the large amount of lift that the F-15 fuselage generated which compensated the loss of a wing).
Anyway my point is that since the fuselage itself seems to generate large amounts of lift - as opposed to other aircraft like the F-16 or F/A-18 - there would be far more interference drag with a Center fuselage external fuel tank as opposed to wing mounted external fuel tanks.
However I'm not sure if this is the case? Nevertheless I believe it could help "justify" this "unique property" regarding the F-15.
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, 15:47

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 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. Not sure. Again, what it would take is a dedicated CFD or wind tunnel study to determine the answer.
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Unread post11 Feb 2019, 15:57

sprstdlyscottsmn wrote:
sferrin wrote:Interference drag?


the F-15 Centerline tank has a DI of 12.2 on it's own and an additional 3.3 for the pylon for a 15.5 no matter anything else. C, E, CFT, no CFT, stores on CFT, no matter.
The F-15 Wing tank is 5.5 (C/E) without CFT, 6.0 (C/E) with CFT (this is interference drag), 8.2 (E) with bombs on inboard CFT stations, 12.3 (E) with bombs on outboard CFT stations.

So really, unless a Strike Eagle has bombs on the outboard position even with interference drag having two wing tanks has equal or less drag than a single centerline while carrying twice the relative fuel. This is why you always see Eagles of all types with two wing tanks. Once they are dropped it is less drag than a centerline.


What I meant was is interference drag the reason for the centerline tank having more drag on the aircraft than a pair of wing tanks?
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Unread post11 Feb 2019, 16:05

sferrin wrote:
What I meant was is interference drag the reason for the centerline tank having more drag on the aircraft than a pair of wing tanks?


I see. FMs often list multiple drag index values depending on neighboring stations, and the Hornet manuals go so far as to have dedicated interference drag tables, so when I read interference drag I think of other stores as a result.

Sorry about the misunderstanding, by post just above covers that though toward the end.
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Unread post11 Feb 2019, 16:26

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?
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