Why the small wing?

Always wondered why the F-16 has a tailhook, or how big a bigmouth F-16's mouth really is ? Find it out here !
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tmofarrvl

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Unread post16 Jul 2007, 23:20

On the topic of adding more wing area to the F-16 to accommodate growth, some of us will remember that the USAF actually considered just such a development effort during the late 1980s, labelled "Agile Falcon". The concept died following the end of the Cold War and the collapse of the Soviet Union. This would have offered an airplane somewhat like the Japanese F-2, but without resorting to co-cured composites.

I should add that in retrospect, the USAF made the right decision when they elected not to pursue this option. The cost of developing an all-new wing structure such as this (new tooling, flight test, software modifications . . .) was better spent on buying MORE F-16s. At some point (and the Navy crossed this line on the Super Hornet), you are no longer developing a derivative: you are developing a new airframe with its own inherent problems and learning curve.
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Roscoe

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Unread post17 Jul 2007, 02:43

10 degress doesn't buy you much side area. I don't know this but I suspect that the anhedral has to do with the mainwing downwash...as in clearing it. The ventral strakes were added for directional stability.
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johnwill

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Unread post17 Jul 2007, 05:07

I don't have access to the horizontal and vertical tail areas, but would guess that the area of two horizontal tails is about 125% of the vertical tail area. Multiplying by the sine of 10 degrees (HT droop) you get an area 22% of vertical tail. That seems significant to me. And it was significant to the YF-16 designers, because that is why the tails are drooped. They would have dropped them further, but were limited by runway clearance of full trailing edge down deflection, maximum pitch angle (engine nozzle touching the runway), one main gear shock strut fully compressed, with a flat tire on that gear. Remember the damage to the right tail on YF-16 flight "zero". When the block 15 tail was enlarged, the aft outboard corner was clipped for runway clearance.

The ventral fins were indeed added for directional stability.

If the reason for drooping was to move the tail out of the wing/flaperon downwash, they were spectacularly unsuccessful. Angle of attack and flaperon angle are very powerful influences on tail load - verified by flight test.

The F-16 has some maneuver limitations at high supersonic conditions, primarily due to reduced directional stability. Due to vertical tail and rudder flexibility, tail and rudder effectiveness is reduced. Without the contribution of horizontal tail droop, the limitations would be more severe.
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n3sk

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Unread post30 Mar 2019, 01:40

If I may conduct a Voodoo ritual...

I am curious how the conformal tanks on the F-16 affect its lift? Does it mirror the original characteristic discussed earlier, or is it vastly different as far as ratio of wing lift/body lift? I believe I remember reading about the F-15E’s CTF’s. At high speeds high AOA it had more of a tendency to depart flight. After a couple of mis haps and subsequent testing change in the FCS alleviated the issue. Curious if the F-16 with CTF’s has any similar issues?

Where engineers able to extract more body lift out of the viper and avoid the issue of larger wings while increasing lift to deal with the extra weight?
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johnwill

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Unread post30 Mar 2019, 06:16

Twelve years later, and I'm still around to rack my brain for an answer. The CFT came along long after I left the F-16 program, so I cannot help very much. However I did have some conversations with folks who did the analysis and flight test to verify CFT integration on the airplane. The only comment I can remember that applied to lift effects of the CFT was that it significantly increased the horizontal tail loads under some conditions, forcing tail and fuselage attachment area redesign. I don't recall specific conditions, but I suspect it would be supersonic high g rolls, since that is where tail loads are maximum. Wing redesign was not required, but the added weight of CFT plus fuel could have resulted in some reduction in g limit until the fuel was burned off, depending on external store load.

Your second question about improving fuselage lift to help counter increased gross weight is interesting. As far as i know, no effort was made to do that, but I suppose it could have been tried if necessary. One change to the fuselage did make a small difference in fuselage lift. When the inlet was redesigned to accommodate the GE engine, forward fuselage lift was increased under some conditions. Some local redesign was required, but the added lift was not of much help to counter the weight growth.
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madrat

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Unread post30 Mar 2019, 12:44

I always think the dsi intake experiment on an F-16 looks like it would have induced some form of positive pitch movement. Inversely the intakes on XF-103 and XF8U looked like they would induce a negative pitch.

Image
Last edited by madrat on 30 Mar 2019, 15:53, edited 1 time in total.
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Unread post30 Mar 2019, 15:30

johnwill wrote:The AF had no money for that, so the wing has been redesigned for strength increase as required for block weight increases. Instantaneous g capability can be maintained that way, but not sustained g.

Does it mean that a heavy Block 50/52+ or Block 60 even more so lost its inherent ability to sustain 9g due to substancial increase of wing-loading when compared to light Block 15?
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sprstdlyscottsmn

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Unread post30 Mar 2019, 15:40

Wow, this was a fun trip down memory lane. Back when I don't even know if I understood who Roscoe and johnwill were. That was even almost a year before Gums named me "spurts" and johnwill complained about how long my name was.

I know a lot of work was done to try and minimize the impact of the CFTs.

Lastly, Holy thread resurrection Batman!
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johnwill

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Unread post30 Mar 2019, 19:05

Patriot wrote:
johnwill wrote:The AF had no money for that, so the wing has been redesigned for strength increase as required for block weight increases. Instantaneous g capability can be maintained that way, but not sustained g.

Does it mean that a heavy Block 50/52+ or Block 60 even more so lost its inherent ability to sustain 9g due to substancial increase of wing-loading when compared to light Block 15?


Not necessarily. Increased thrust can make up for increased weight and drag to help maintain sustained g capability. Whether it did or not, I don't know. Caution - with no intention of insulting your knowledge, be sure you understand what "sustain" means, as opposed to instantaneous g.
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Patriot

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Unread post30 Mar 2019, 23:01

johnwill wrote:Caution - with no intention of insulting your knowledge, be sure you understand what "sustain" means, as opposed to instantaneous g.

Easy peasy John. Im not that offensive at all. I gues it's pretty straight forward... sustained g is a g-load that an aircraft is able to produce and maintain for a little while like when makes a turn either 180 or 360 degree. Also if I have this right, the sustained g is strictly corelated to sustained turn rate, the greater the STR is the greater the g-load at any given speed. The greater speed the less AoA/STR is needed to achieve substancial g as opposed to the less speed the more AoA/STR is needed to get greater g. Did I confused AOA with STR? It's not one and the same thing right? The F-16 theoretically can turn 25°/sec through entire 360 and the AOA would be much less than 25° right?

The Instantaneous g is when the aircraft rapidly hits its max AOA and immediately drops the angles back considerably so does the g drops, right?

As far as the F-16 goes. Tell me if I get the physics right. The greater wing-loading in the later Blocks makes that "system" to lose energy more easily than early Blocks did upon turning BUT the stronger engines of the later ones add that lost energy back to the system.. maybe even with some surplus? Maybe that means later Blocks with better engines can actually obtain more sustained g that early ones?

Final question. How much more the legacy F-16 design we all know can gain on weight before its well established performance get ruined? Theoretically, what would have happened if say the proposed F-21 or any other future F-16 have the weight of the classic Hornet and the thrust of F-35 ?
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johnwill

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Unread post31 Mar 2019, 04:59

You are close on the understanding of sustained g, but it is a little more rigorous than you think. In addition to holding a constant g, the airplane must not lose airspeed or altitude during the turn. And instantaneous g capability can limited by a g limiter, an AoA limiter, or a structural limit.
Your comments on the relation between turn rate and g are correct. Turn capability is highly variable with weight, speed, and altitude due to the wide range of AoA required.
A very heavy F-16 with lots of thrust would have a much reduced range, worse runway performance, and sluggish maneuverability. Just guesses of course.
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Unread post31 Mar 2019, 13:57

Thanks man.

In regard to the topic, Ive noticed something intriguing a while ago. The early Blocks 15 and late Blocks 52 fly different, I mean airshow wise, clean config and low. The light early blocks seem to have a better (faster) both pitch and roll onset and also are able to decelerate and stop roll rate faster. It is not a huge difference but it's noticable, at least it's my feeling.

Im not quite sure how much it's a matter of digital FLCS (vs. analog one) and how much it is a rule saying: the heavier the object the more flight limits it has imposed on it by physics/flight mechanics.

A Block 15 with the PW-229 engine would be a sight to see tho! 8)
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Unread post01 Apr 2019, 05:13

Patriot wrote:A Block 15 with the PW-229 engine would be a sight to see tho! 8)


In 1990-91, an early F-16B Block 15 at Edwards had a PW229 installed for flight test. The forward engine mount had not been beefed up for the increased weight of the PW229, restricting the aircraft to a 7.5 G load factor.

This aircraft was a beast. During low altitude high Q performance testing at 500 ft above sea level on the range west of Pt. Mugu, the test pilot could not hold the 800 KCAS Max AB Test point, with the aircraft continuing to accelerate even while pulling to the 7.5 G limit.
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madrat

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Unread post01 Apr 2019, 06:26

Johnwill-

The LERX on F-16 has always intrigued me. I notice the F-16 has a very narrow LERX similar to the Hornet. Was a wider LERX, like on a Super Hornet, counter productive? The F-35 seems to use a decoupled LERX with the chine from the upper intake forming sort of a bi-plane effect not really dissimilar than how Rafale shapes the canard shoulder over the leading edge of the main wing. I have to imagine again, the net effect just wasn't a benefit or we would have seen something similar backfir to the F-16. Was the LERX in the F-16 changed much from the concept to the final product, e.g. to get to its current shape?
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Unread post01 Apr 2019, 15:03

The LERX in the Hornet is much much wider than the Viper's, Id say. I think it's because Viper is more of a vanguard blended body-wing design whereas Hornet seem to be more old fashioned. To have a satisfactory lift Hornet designers had to make LERX considerably wide where in Viper the underside of the fuselage contributes much to the overal lift.

Im not sure but I think Super Hornet's shortened and widened LERX is made like that for two reasons:
1) its more stealthy
2) vortices create wider apart and thus they're not hitting the tail and that was an issue before, thats why SH dont need vortex fences and thats also benefitial from a stealth point of view
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