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Why does the F-18 suffer so much from stores-drag?


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Posted
On 5/29/2023 at 4:23 AM, Temetre said:

Okay, Ive actually done a test of straight line flying speeds. Now, I can definitely say the differences are less pronounced than I expected. Maybe its because a lot of my experience with the Hornet is from before the last aero update, which IIRC helped it a bunch. But I did find a big difference in missiles and fuel bags:

See bottom of my post for scenario and more numbers, but heres what stood out to me, at 10k altitude, 30% of empty weight in fuel (+full bags if listed) and full mil thrust in straight flight:

F16, clean: 627 TAS

-> 2xMK-84: 615 TAS
-> 6xAim-120: 626 TAS
-> 3x Fuel Pod (2x330,1x300 gallon), 6xAim-120: 607 TAS

F18, clean: 625 TAS

-> 2xMK-84: 617 TAS
-> 2xAim120+2xAim9 (clean): 620 TAS
-> 2xFuel Pod (330 gallon): 609 TAS
-> 2x2 Aim120 (inner wing station): 605 TAS

So the clean air speed of F-16 and F-18 is almost the same, im sure thats an improvement after the last update. But heres what stands out to me:

1. MK-84s cause more slowdown on F-16 than F-18, making the Hornet just a bit faster. This is what I would expect, a smaller low drag plane suffers more from the same amount of drag in bombs.

2. Putting 6x Aim-120s on the F16s pylons, slowed it down much less than the clean 2xAim120+2xSidewinder of the F-18. Here the opposite happens, a less draggy loadout had more impact on the F-18.

3. The F-16, with 3x pods, giving it more fuel than the F-18, and 6x missiles, was faster than the F-18 with 2x 330 pods. Again, the F-18 loses from a smaller fuel+bag load, where the F-16 has an easier time handling a bigger increase in weight+drag.

 

Like, does that make clear what confuses me so much? Similar clean speed, and with MK-84s, the Hornet even is faster. Makes sense, same drag, bigger plane is less affected. 

Yet then any missile or fuel bag issuch a big deal. A clean Hornet with 4x missiles in low drag slots, loses 5 nots, where an F-16 with 6x missile in more draggy slots loses 1 knot. And the fully loaded F-16 with three bags and six missiles is just 20 knots slower, just like a Hornet with its 2x smaller underwing bags. Surely this heavy F-16 loadout here is more drag than two bags on an F-18? 

Suddenly it is reverted; where the MK84 benefits the Hornet, missiles and bags penalize the F-18 to a much greater degree. That is even ignoring the dragg Hornets underwing stations.

Can someone explain me, or at least make a guess why that happens? To me that seems so counter-intuitive. There either must be something big happen (or maybe something is inaccurate, I have no clue).

 

 

--------------------------------------

Here the more specific numbers I checked. Mind the internal fuel is 30% of empty weight, not of capacity:

  Reveal hidden contents

F16 -> 30% empty weight internal fuel fraction, military thrust, 10K
-> No stores:  627 TAS
-> 2xMK-84: 615 TAS
-> 2xFuel Pod (370 gallons): 614 TAS
-> 6xAim-120: 626 TAS
-> 3x Fuel Pod, 6xAim-120: 607 TAS


F18 -> 30% empty weight internal fuel fraction, military thrust, 10K
-> No stores:  625 TAS
-> 2xMK-84: 617 TAS
-> 2xFuel Pod (330 gallons): 609 TAS
-> 2xAim120+2xAim9 (clean): 620 TAS
-> 2x2 Aim120 (inner wing station): 605 TAS
-> 2/2 (clean), 2x2 inner Aim-120,  central pod: 595 TAS


F16 -> 30% empty weight internal fuel fraction, military thrust, 20K
-> 3x Fuel Pod, 6xAim-120: 592 TAS
-> Above in full AB: 822 TAS


F18 -> 30% empty weight internal fuel fraction, military thrust, 20K
-> 2/2 (clean), 2x2 inner Aim-120,  central pod: 585 TAS
-> Above in full AB:  732 TAS

 

 

did you know a fighter with a 650 gallon drop tank is like carrying a 1955 chevy in terms of weight... except the fuel tank is more aerodynamic. and just streamlining an object doesn't mean it is invisible.

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Posted
On 6/4/2023 at 7:22 PM, AngelAtTheTomb said:

4000 lbs is a lot, and will requires 4000 more lbs of lift. This means you need more AoA for that lift, and you get more induced drag. Top speed - at least in the real word - is not just a factor of drag, but of weight.

No this isn't true of any high performance fighter since WWII. At top speeds the airframes produce so much lift that parasite drag is much more of a factor at military power than induced drag is from gross weight.

F16 at 15,000 ft stores gross weight knots at MIL
500 lbs of fuel clean 20,693 533
7163 clean 27,355 531
500 7,507 27,906 481
500 8,512 28,911 478
       
F18 at 15,000 ft stores gross weight knots at MIL
432 lbs of fuel clean 25,986 539
10,479 clean 36,032 536
432 6526 32,051 479
432 10,512 36,037 474

(For the Hornet the stores are 8*mk82 and 8*mk83, the difference in speed is but 5 knots yet the difference in weight is 4000lbs. So at MIL 4000lbs is nothing. But those pylons a human could play football with are what slows fighter jets down... at MIL.)

Comparing the Viper and Hornet at military power isn't fair because the Viper's airframe produces much less lift and therefore has much less parasite drag than the Hornet. But as you can see at military power neither aircraft is much affected by gross weight. So comparing the airframes at corner speed rather than MIL would be much more apt. The parasite drag would have less of an effect on speed and the induced drag from the added weight would get pretty nasty I bet. Then you could really compare the airframes 😀

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