Why the small wing?

Unread postPosted: 12 Jul 2007, 16:50
by sprstdlyscottsmn
The F-16 has encountered phenominal growth for a fighter from the YF-16 stage untill the F-16E/F. This increase in structural weight, and avionics weight, and fuel weight, is to allow for more ordnance to be carried greater distances. The Engines are also upgraded to allow these heavier falcons to get off the ground. So why does the Falcon still carry the small 300 sqft wing? Why not up grade to a 350 sqft wing or somthing like what the F-2 has? I spoke to an AIr FOrce pilot who said a larger wing was planned for the Blk50 but was not implemented due to cost. input anyone?

RE: Why the small wing?

Unread postPosted: 12 Jul 2007, 22:40
by Roscoe
1) A bigger wing would completely change the aero characteristics of the airplane requiring a huge flight test program
2) A 9g airplane puts tremendous loads on the wings. Bigger wings may spread out the load, but the bending moment at the root can actually increase.
3) F-2 had tremendous structural issues as a result of the bigger wing. USAF anticipated that and for that reason elected not to go with it. We recommended that Japan not...but they chose to listen to GD (LM?) instead.
4) More wing equals more drag.

RE: Why the small wing?

Unread postPosted: 13 Jul 2007, 02:04
by johnwill
I agree that an F-2 wing is not the way to go, but not because it is larger. As member of the F-2 program, I can tell you the issues with the wing have little or nothing to do with the larger wing area. The customer wanted a "new" technology wing (among other things) to help justify the cost of the program. They acquired or developed new capability in flight control, avionics, structural materials, etc. The wing design used a new composite technology called co-curing, where the understructure (spars, ribs) and lower skin were bonded together into one component. All of the wing issues are related to the new technology, not the increased area. If the wing area had not been increased, the same issues would still have occurred.

Surface area increases are well within the capability of competent engineers. As Roscoe correctly states, cost and drag are the only disadvantages. Even drag is questionable and could be lower due to reduced AoA required. I offer several examples - the F-16 Block 15 horizontal tail and the F-16XL. Both had serious increases in area and both were totally successful structures. There are other examples - F-111B, F-111C and FB-111A from the basic F-111A, plus the F-16A and B wing and horizontal tail derived from the YF-16.

As far as increased bending moment goes, the F-16C/D Block 60 bending moment is much more than the F-16A/B, but has the same area. So there is no consistent correlation between area and moment.

RE: Why the small wing?

Unread postPosted: 13 Jul 2007, 17:27
by sprstdlyscottsmn
more between pure weight and span for the moment?

RE: Why the small wing?

Unread postPosted: 13 Jul 2007, 21:30
by johnwill
Yes. For example, flexibility. One of the differences between the F-16 wing and the F-2 wing other than the material (aluminum vs. composite) is flexibility. When air pressure load is applied to the wing it will both bend and twist. The twist changes the local angle of attack. A more flexible wing will twist more, so that the outer wing will be at a lower angle of attack than the inner (for a swept wing). The tip area will generate less lift, thus moving the center of pressure inboard, which means less bending moment. Although carbon fiber composite is stiffer than aluminum, it does not mean a carbon fiber wing will flex less than an aluminum wing. Carbon fiber is also stronger, so the skins do not have to be as thick as for aluminum. Which means the carbon fiber wing may in fact be more flexible than an aluminum one.

Associated with flexibility is the effect of sweep angle. More sweep means more negative twist, thus less bending moment. Forward swept wings will tend to have positive twist, thus more bending moment. The big hazard with positive twist is divergence, where more load means more twist which means more load and so on until the wing fails.

Mach number has an important effect on bending moment. The center of pressure on a wing shifts aft as mach increases (causing more negative twist) and reduced bending moment. At higher angle of attack, the inboard part of the wing will generally stall first, causing the center of pressure to move outboard - more bending moment.

Another difference is planform taper. A rectangular wing will have a higher bending moment that a tapered wing. External stores have a big influence on wing bending moments, as does the presence of internal wing fuel.

As you can see, it is not a simple matter to determine the strength requirements for a wing or any other part of the airplane.

RE: Why the small wing?

Unread postPosted: 14 Jul 2007, 03:02
by sprstdlyscottsmn
Yeah I know what can go into it. I had to work on the lift and drag moments and sweep induced twist for a heavy lift cargo plane my senior year of school. We had an all composite wing but had a problem getting the skin onto the spars/ribs. I dont think that the sweep on the F-16 causes too much of a twist due to load when compared to a wing like on the F-86 of EE Lightning. Putting fuel in the outer sections of the expanded wing would help with the moments as would tip AMRAAMs.

I know increasing the wing area will increase the parasite drag, but the plan is to get it to reduce the induced drag enough to compensate. This of course would involve a new controll for the FBW.

Oh! If the F-2 wing was all composite did it offer any weight savings over the smaller aluminum wing? and with a ~20% increase in wing area, what kind of lift increase could be expected? Even if lift was created purely by the wing, which its not, the thickness would have to be the same for a 20% lift increase, so with a thinner wing (I assume F-2 had thinner wing ratio) and fuselage effects would it be closer to 10%? Sorry so many questions, I havent designed a fighter yet, only cargo.

Was the problem with the F-2 structure that it was composite, single piece, or trying to mate it to aluminum fuselage? Was the Fuselage even aluminum? Thank you johnwill for your input. I love the F-16 more than all other fighters and I always want to know conceptually how to make it even better.

RE: Why the small wing?

Unread postPosted: 14 Jul 2007, 04:49
by habu2
Why? Money.

RE: Why the small wing?

Unread postPosted: 14 Jul 2007, 05:54
by johnwill
Glad you are interested enough to ask good questions. I think you are right about twist effects compared to the F-86 and EE Lightning, but they still have to be considered. To show how powerful twist effects can be, the F-16 wing tip area is down loaded at low altitude supersonic level flight conditions. When you mentioned the problem of getting the skin load into the spars/ribs, you came close to one of the F-2 problems. As you know, the load between the skin and spars/ribs is shear. The lower skin is bonded to the spars/ribs, and that joint provided good shear strength. But at certain locations like the flaperon hinge ribs, the concentrated hinge load tries to pry the rib away from the skin, a tension load. Bonded joints are not very good in tension, so problems were found during ground test and redesign was required. Fuel in the wing doesn't help much, since it is the first internal fuel burned. That helps with roll maneuverability.

I am not familiar with weight comparisons, but wing weight is so small compared to maximum takeoff weight, it would not be very significant. The original F-16 wing structural weight was about 700 lb each, with a max weight of over 35000 lb.

Mating to the aluminum fuselage was no particular problem. Similar to the F-16, there are upper and lower aluminum brackets bolted to the wing and fuselage. There are also shear ties on the front and rear spars attached to fuselage bulkheads.

As to your original question, GD proposed an enlarged wing in the mid-80s to cope with current and expected weight increases. 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.

RE: Why the small wing?

Unread postPosted: 14 Jul 2007, 17:09
by sprstdlyscottsmn
during our lift load test of our composite wing, human error reared its ugly head. We bonded the bottom skin to the ribs/spars with composite fillets but had to use a bizzar epoxy with composite chunks to bond the top skin. At about 1700lbs load the top skin buckled off the structure causing rapid failure of the system. lesson learned? Dont be too fancy by putting camber where it wasnt needed (the reason we couldnt test it upside down) and PAY ATTENTION to which side gets the stronger bond. Had we filleted the upper skin the buckling resistance would have been phenominal. The poor bond on the lower skin would have bad little effect as that side was in tension.

Back to the subject. I can see how flaperons would complicate the issue. Our test was just of primary structure so ailerons were not modeled (mostly for time/money) but I could see shear posing an issue there. I did not know that the tips were down laoded during low level high speed flight. How much natural twist does the Vipers wing have?

Did putting holes for bolts in a compostie structure create any issues for delamination? I have only a rudimentary knowlege of composites.

Another topic, a pure delta at high alpha (say M-2000 at 30 deg) the upward deflection of the elevons creates the pitch up moment to gain alpha right? So if a roll input (left stick) is performed would the increase of lift on the wing with the lowered elevon(right wing) over come the reduced pitch moment. What effect does a differential pitch moment have on a plane in that situation?

RE: Why the small wing?

Unread postPosted: 14 Jul 2007, 20:32
by johnwill
The test you describe is very interesting. Upper skin buckling is always a problem. That is why the upper skin is normally thicker than the lower. The F-16C static test wing failed (like you described) at 137% of limit load due to buckling (150% is required). Since composites are stronger than aluminum, the composite skin can be thinner to resist bending moment, but then the buckling problem becomes much worse. Composite upper skins many times have more spars or other stiffening devices for stiffening the skin to resist buckling. When you talk about the lower skin bond having little effect since it was in tension, only the lower skin is in tension - the bond is primarily shear.

The static twist on the F-16 wing is -3 degrees. So if you are flying at 4 deg AoA with no elastic twist, the wing tip still has positive AoA. But as you approach and pass 0.9 mach, two things happen - AoA goes down due to higher dynamic pressure (q) and the center of pressure moves aft. If AoA is 3 deg and no elastic twist, tip load is zero, but the aft cp means there is elastic twist, making the tip AoA negative. Once that happens, the tip missile airload is also down. Since it sticks out far ahead of the elastic axis, that twists the wing even more nose down. It is very dramatic for the pilot to look at his tip missiles and see they are pointing several degrees nose down. The lift distribution on the wing is strange - down load on the tip, gradually going to zero then positive as you move inboard. The total lift (shear) is positive, but the root bending moment is negative.

One of the reasons for co-curing the F-2 wing was to avoid holes in the lower skin. I also am no expert on composite structure, but is seems reasonable that delaminations are more likely to start at a hole or any edge rather than away from an edge. Maybe not. Another reason is to reduce fuel leaks.

Rolling a delta wing airplane could cause some problems with pitch unbalance if the pitch elevon deflection is near maximum. With small pitch elevon deflection, roll commands could be sent equally to left and right elevons, thus maintaining pitch deflection. If the pitch deflection is large, a roll command could cause one elevon to reach its travel limit while the other elevon continued moving. Pitch unbalance would result, and the nose would drop.

Sample numbers - de max = +/-30 deg

Before roll de (L) = -10 de (R) = -10 Pitch de = -10
Roll command +/- 15 de (L) = 5 de (R) = -25 Pitch de = -10

But if
Before Roll de (L) = -20 de (R) = -20 Pitch de = -20
Roll command +/- 15 de (L) = -5 de (R) = -30 Pitch de = -17.5

Since de cannot exceed +/- 30 deg, the right elevon is travel limited, and de is reduced (the nose goes down)

This case is similar to the F-16 rolling with gear and flaps down, where only one flaperon moves in a roll command, reducing total lift. The test pilots said it felt like the airplane was rolling around an outboard x-axis. They soon became accustomed to it and it was never an issue.

Also in the F-16, horizontal tails are used to help roll the airplane. Below about 0.9 mach they don't help much, but as the flaperons become less effective at higher mach, the tail roll commands are increased and the tails provide most of the roll power. The same condition you describe for the Mirage 2000 could possibly occur in the F-16 tails. In this case the filght control people have given priority to pitch control over roll control. If the roll commands could cause limit tail movement, the roll commands are reduced so that pitch command is never limited.

RE: Why the small wing?

Unread postPosted: 15 Jul 2007, 00:11
by sprstdlyscottsmn
"The same condition you describe for the Mirage 2000 could possibly occur in the F-16 tails. In this case the filght control people have given priority to pitch control over roll control. If the roll commands could cause limit tail movement, the roll commands are reduced so that pitch command is never limited."

hence why rolling g is significantly lower than symetric G?

"The lift distribution on the wing is strange - down load on the tip, gradually going to zero then positive as you move inboard. The total lift (shear) is positive, but the root bending moment is negative. "

Man I want to see a chart of that, sounds crazy but I can see it. That moment arm out at the tip simply overpowers the net positive shear force. would that be one of those times that the body produces significant percentage of lift?

RE: Why the small wing?

Unread postPosted: 15 Jul 2007, 05:05
by johnwill
The conditions involving possible full deflection of control surface occur at very low airspeeds, where the airplane can't pull more than 2g anyway. F-16 roll g limit (6g clean) is lower than symmetric g limit (9 clean) primarily due to historical precedent. With strictly manual control systems, rolling at 1g was easily controlled, but as g increased control was more difficult and roll performance was degraded. At high AoA, roll commands turn into yaw commands and the airplane may exceed its yaw stability limits.

So the standard way to perform high rate rolls was to reduce g. With the arrival of jet engines and ever higher speeds, the problems became worse. Even as late as the F-4, roll performance was much better at lower g. So the AF normally specifies max roll g as 80% of max symmetric g. Obviously, the F-16 is different, because 6g is less than 80% of 9g. All US fighters up through F-16 were specified to be 7.33g symmetric and 5.86g roll. (I am not sure about F-22 and F-35 roll g, but I think it may be 7.2g). During development of the F-16, GD proposed adding 9g capability at a very low weight penalty (22 lb), so the AF bought it. However max roll g was left at 5.86g. High g rolls are difficult conditions for the structure to withstand and would have required extensive redesign. After flight test confirmation, the limit was raised to 6g primarily because it is easier to observe. Todays control systems make it easier to control yaw with automatic aileron/rudder interconnects. F-16 roll rates at 6g are not much different than at 1g.

Wing structure is primarily designed by bending moment and torsion (twist). Symmetric maneuvers are mostly bending moment and 1g rolls are mostly torsion due to flaperon load. 6g rolls are a combination of bending and torsion as you would expect. Of course in the roll one wing will have added bending and the other wing will have reduced bending. How much? Depends mostly on mach number and its effect on flaperon efffectiveness.

Well, I've gone on far too long on this topic and have only scratched the surface.

You've got it exactly right about wing load distribution with negative tip load. Fuselage lift is always significant, but it becomes more of a contributor at lower speeds. Look at it this way - the wing lift vs AoA is curved, so that it is reduced significantly above 12 - 14 degrees. Yet for some reason, the fuselage lift curve is almost linear up to about 20 deg AoA. So for high g conditions with AoA < 12 degrees, the fuselage lift is around 40% of the total. But at say 18 deg AoA, the wing has lost some of its capability and the fuselage is still doing its job at perhaps 50% of total lift.

Thanks for asking good questions.

RE: Why the small wing?

Unread postPosted: 15 Jul 2007, 05:42
by sprstdlyscottsmn
Thanks for giving good answers.

RE: Why the small wing?

Unread postPosted: 15 Jul 2007, 06:23
by sweetpete
Hey, your guys brains seem bigger than mine so ill ask a question not a single F-16 pilot i have asked has been able to tell me. What is the primary purpose of the Anhedral in the stabs? From my limited understanding of fixed wing aerodynamics dihedral in a wing is designed to help an aircraft return to a neutral "level" plane, as the wing lowered during the maneuver becomes closer to horizontal its lift vector becomes more vertical than the opposing wing thus righting the aircraft by design. Based on this theory Anhedral in the stabs would serve to make the aircraft more unstable in the longitudinal plane making the maneuver more pronounced (better roll rate)? Sorry for going off topic but I can see the one of you posting on this topic prolly has the answer.

RE: Why the small wing?

Unread postPosted: 15 Jul 2007, 15:11
by johnwill
The horizontal tails droop 10 degrees to provide more directional (yaw) stability. Look at the F-4 for an extreme case of the same idea. You can think of the tails as having a horizontal component (tail area x cos 10 deg) and a vertical component (tail area x sin 10 deg). The vertical component acts just like a vertical tail or a ventral fin - helps to keep the pointy end of the airplane in front. The vertical tail can be smaller (less drag, weight, cost) by doing this. Very little is lost in the horizontal component (cos 10 = .985), while a good gain is made in the vertical component (sin 10 = .174).

Agile Falcon

Unread postPosted: 16 Jul 2007, 23:20
by tmofarrvl
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.

RE: Why the small wing?

Unread postPosted: 17 Jul 2007, 02:43
by Roscoe
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.

RE: Why the small wing?

Unread postPosted: 17 Jul 2007, 05:07
by johnwill
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.

Re: Why the small wing?

Unread postPosted: 30 Mar 2019, 01:40
by n3sk
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?

Re: Why the small wing?

Unread postPosted: 30 Mar 2019, 06:16
by johnwill
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.

Re: Why the small wing?

Unread postPosted: 30 Mar 2019, 12:44
by madrat
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

Re: RE: Why the small wing?

Unread postPosted: 30 Mar 2019, 15:30
by Patriot
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?

Re: Why the small wing?

Unread postPosted: 30 Mar 2019, 15:40
by sprstdlyscottsmn
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!

Re: RE: Why the small wing?

Unread postPosted: 30 Mar 2019, 19:05
by johnwill
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.

Re: RE: Why the small wing?

Unread postPosted: 30 Mar 2019, 23:01
by Patriot
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 ?

Re: Why the small wing?

Unread postPosted: 31 Mar 2019, 04:59
by johnwill
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.

Re: Why the small wing?

Unread postPosted: 31 Mar 2019, 13:57
by Patriot
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)

Re: Why the small wing?

Unread postPosted: 01 Apr 2019, 05:13
by f119doctor
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.

Re: Why the small wing?

Unread postPosted: 01 Apr 2019, 06:26
by madrat
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?

Re: Why the small wing?

Unread postPosted: 01 Apr 2019, 15:03
by Patriot
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

Re: Why the small wing?

Unread postPosted: 01 Apr 2019, 17:50
by johnwill
madrat wrote: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?


LERX on a twin vertical tail airplane can be a can of worms due to interaction of the vortices with the tails. The Hornet found that out the hard way, so ever since then, designers have been very careful to avoid that problem while still meeting the RCS, lift, and drag needs. As always, the final design is a compromise between several competing requirements.

Several different shapes were studied during design of YF-16, but once the shape was selected it wasn't changed, even to the current airplanes.

Re: Why the small wing?

Unread postPosted: 02 Apr 2019, 23:29
by n3sk
So it seems the wing design of the F-16 is the best compromise. Any change causes issues that are not worth the effort or negatively affect another aspect of performance.

The CTF’s do cause minor yaw control issues and high speed. They do not and much meaningful lift.

Power is the answer to make up for the added weight.

One place I commonly see vapor vortices is the lerx and wings leading edge meet. If there is enough pressure to squeeze moisture out of the air, is this also a sign of turbulence and drag??

This question is a bit off topic but relates. These vortices mentioned which cause problems for twin tail jet, is that why the Su-57 has levcons. Is it an attempt to alleviate this issue and smooth out the airflow over the top of the fuselage. Will we see these in future design?

Re: Why the small wing?

Unread postPosted: 03 Apr 2019, 02:23
by madrat
Levcons seem to act like Handley-Page slats. keeping the front edge inline with airflow then smooths laminar airflow over the wing I reckon. Just from an eyeball guess, it probably helps smooth airflow at high AoA.

Instead of levcons, I think any future changes on a design should be to cut drag. I wonder how a Y-tail like on F-117A would function on an F-16 by eliminating both the vertical stabilizer and horizontal tail planes. Maybe the ventral side of the tail section could be slat free with some sort of aerodynamic package that more closely conforms to the body. Stuff like that. The downside is it's money thrown into the wind since we already have F-35A.

Re: Why the small wing?

Unread postPosted: 03 Apr 2019, 05:20
by saberrider
n3sk wrote:So it seems the wing design of the F-16 is the best compromise. Any change causes issues that are not worth the effort or negatively affect another aspect of performance.

The CTF’s do cause minor yaw control issues and high speed. They do not and much meaningful lift.

Power is the answer to make up for the added weight.

One place I commonly see vapor vortices is the lerx and wings leading edge meet. If there is enough pressure to squeeze moisture out of the air, is this also a sign of turbulence and drag??

This question is a bit off topic but relates. These vortices mentioned which cause problems for twin tail jet, is that why the Su-57 has levcons. Is it an attempt to alleviate this issue and smooth out the airflow over the top of the fuselage. Will we see these in future design?

The vortices is a s presense from the medium AOA in flight on F16 LEX's. Sweept angle (more 60 degree's) create them at lowAOA. They're being useful at high pitch angles when WINGS start to loose lift to drag ( increase)

Re: Why the small wing?

Unread postPosted: 21 Apr 2019, 09:53
by saberrider
The wings are small (wingspan wise) to accomplish minimum drag request for the payloads .

Re: Why the small wing?

Unread postPosted: 21 Apr 2019, 20:49
by vilters
The production F-16's wing is 10% bigger then the YF-16 wing, and many of us want to torture the clowns that shelved the XL-F-16 WIng.

Re: Why the small wing?

Unread postPosted: 19 Aug 2019, 13:18
by secretprojects
madrat wrote: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?


The answer to the last part is - yes, very much changed.

In the beginning, the LERX/strake was a wide forebody with blunt leading edges:

The aerodynamic design concept of obtaining high-lift coefficients at transonic speeds by the use of wide forebody shapes has been an integral feature of most General Dynamics fighter designs since before the FX (F-15) competition. The concept was initially wind-tunnel-tested in 1966. Sharp, narrow forebody strakes were also investigated briefly at that time; however, it was then considered an advantage to produce the lift with a blunt leading edge in order to maintain attached flow and greater leading-edge suction for lower drag.


When wind tunnel tested, this configuration (401F-0 with twin vertical tails) did give high lift, but had very poor directional stability at moderate to high angles of attack.

At this point, NASA/Langley Research Center aerodynamicists were consulted. They suggested that the lift of the wide forebody could be increased by sharpening the leading edge to strengthen the vortices rather than weaken them as
our earlier attempts had done. The point was that forebody separation is inevitable at very high angle of attack; therefore, the lift advantages offered by sharp leading edges should be exploited. This also would allow the forebody vortices to dominate and stabilize the high-angle-of-attack flow field over the entire aircraft, even improving the flow over the outboard wing panels.


Lots of different strake shapes were tested - I count more than 133 variations in the data tables.

Two series of parametric forebody strake tests were initiated. A series of delta planform strakes were designed for testing on the conventional forebody aircraft Configurations 785 (single vertical tail) and 786 (twin vertical tail). The second series had curved planforms (gothic and ogee) and were designed for testing on the blended configuration, 401F-5. These two parametric transonic tests provided the basic data for all the other evolutionary forebody strake tests, which throughout the YF-16 and F-16 development as various design changes required re-evaluation of the strake effectiveness.


However, once the final shape was settled, it hasn't been altered since.

Source:
NASA Contractor Report 3053
Aerodynamics of Forebody and Nose Strakes Based on F-16 Wind Tunnel Test Experience
Volume I: Summary and Analysis
C. W. Smith, J. N. Ralston, and H. W. Mann


https://ntrs.nasa.gov/archive/nasa/casi ... 019972.pdf

Re: Why the small wing?

Unread postPosted: 19 Aug 2019, 15:35
by sprstdlyscottsmn
Excellent find!

Re: Why the small wing?

Unread postPosted: 20 Aug 2019, 04:20
by zero-one
I always wondered why wasn't the F-35C design chosen as the base form F-35.

We've seen engine thrust added without an increase in fuel consumption. And the F-35 is expected to have this as well.

So wouldn't the C model eventually reach F-35A levels of acceleration and E bleed. The A on the other hand will never reach the C model's lift generation, fuel capacity, range etc.

Re: Why the small wing?

Unread postPosted: 20 Aug 2019, 13:29
by secretprojects
sprstdlyscottsmn wrote:Excellent find!

Thanks!

I've been researching the evolution of the YF-16 design for a while. Here's thumbnails of an 18 page PDF I've put together so far. Long term goal is a comprehensive history of the LWF program including design history of all proposals.

F-16DesignEvolution.jpg