Discussion: F-14, F-15 Hybrid Idea

[elmayerle]

I rather suspect that the Typhoon's inlet location and canards play a large part in its being able to do well with only a single vertical tail.  Oh, for an idea of how much of the A-5's vertical tail folds, check out the lates issue if Flypast with the Flying Tiger on the cover and a major article on the Vigi inside.

[KJ_Lesnick]

El Mayerle,

I thought a canard and a strake were used for the same exact purpose?  I'm guessing the inlet position has to do with the airflow over the top of it?


To Everybody,
Back to swing-wings for a sec,

R-16 awhile back mentioned the Mirage-G which weighed 40,000 pounds and had swing wings.  The B-1A also was fairly light for the fact that it had swing-wings as well.  What I'm wondering is, could either design swing it's wings forward and back enough quick enough for air-to-air combat (like the F-14), and could sustain full G's in any wing-position (even in transit like the F-14) 

Do those features (able to swing wing forward and back fast enough for air to air combat, and able to withstand full G's in any wing position even in transit) add a tremendous amount of weight?


Kendra Lesnick,
BTW:  Does anybody have a picture of the A-5's tail-folded?

[elmayerle]

Canards are essentially control surfaces up front, in many cases they could be considered equivalent to horizontal tails moved forward.   Strakes, on the other hand, are used simply to guide airflow and tune aerodynamics and even then you have to be careful with your airflow (witness the fences required on all first-generation F-18s).  I don't have a scanner with me, but I can provide an issue and  page reference for the recent issue of Flypast with a picture of the RA-5 with folded tail tip as well as other folding.

[KJ_Lesnick]

The F-16 had one vertical tail and a strake/chine and no canard and manages just fine.  Would things have been different if it had intakes that were more F-15 like (huge boxy intakes with the chines on the side of the box)?  Or would it depend on specifics?

KJ Lesnick

[elmayerle]

The F-16 had one vertical tail and a strake/chine and no canard and manages just fine.  Would things have been different if it had intakes that were more F-15 like (huge boxy intakes with the chines on the side of the box)?  Or would it depend on specifics?

KJ Lesnick

It also had a single engine and a comarpativelylarge vertical tail (GD did study, as part of the run-up on the F-16XL, an all-moving vertical tail that would be smaller).  That it didn't have the inlet arrangement of the F14 or F-15 likely helped some; it also doesn't hurt that it doesn't have the rear fuselage width that two engines in  that power range would necessitate.  The F-18 has chines/strakes and would've had balancing vorteces on the vertical tails of the slots had been left in the LERX.  The wind tunnel study I've mentioned before was done strictly on twin-engined aircraft so any read-across to single-engined aircraft should be done carefully.  For that matter, a F-16-sized aircraft using two smaller engines gets away with with a single vertical for much the same reason (the Chung-Kuo fighter from ROC).

The "everything folded" Vigilante is pictured in the August, 2008 issue of Flypast on page 55, in the upper right hand corner of the page.

[KJ_Lesnick]

ElMayerle

It also had a single engine and a comarpativelylarge vertical tail (GD did study, as part of the run-up on the F-16XL, an all-moving vertical tail that would be smaller).  That it didn't have the inlet arrangement of the F14 or F-15 likely helped some; it also doesn't hurt that it doesn't have the rear fuselage width that two engines in  that power range would necessitate.  The F-18 has chines/strakes and would've had balancing vorteces on the vertical tails of the slots had been left in the LERX.  The wind tunnel study I've mentioned before was done strictly on twin-engined aircraft so any read-across to single-engined aircraft should be done carefully.  For that matter, a F-16-sized aircraft using two smaller engines gets away with with a single vertical for much the same reason (the Chung-Kuo fighter from ROC).

What slots in the LERX?  The F-18 had no aerodynamic slots I'm aware of...

Also, what caused General Dynamics to reduce their data wrong for the TFX program that caused such high drag on the aft-fuselage?  What would have looked different had they reduced their data properly?

The "everything folded" Vigilante is pictured in the August, 2008 issue of Flypast on page 55, in the upper right hand corner of the page.

Okay...  I guess the next time I got to Barnes and Noble, I'll check the magazine out.  It's hard to believe on the web I can't find a single picture of the folding fin.

Since I'm on the topic of the A-3J/A-5 Vigilante.  How much drag did the aft fuselage produce on the A3J/A-5?  And do you have any guesses on what the drag figures would have been had the two engines been right next to each other and that "bump" (The weapons-train, weapons-package) not been there?


KJ Lesnick
BTW:  Out of curiousity, would the Navy have preferred a swing-wing aircraft who's tails did not need a folding mechanism, or a straight wing-aircraft that offered the same performance as a swing-wing (hypothetically) in terms of range, maneuverability, top-speed, and was lighter, although it had folding wing-tips and a folding tail?

[elmayerle]

Look at the prototype F-18's in their initial build configuration, there are slots between the LERX and the fuselage; these were removed as part of the design development and the space used for equipment.

What happened on GD's part to cause the inaccurate winde tunnel data reduction?  I have no idea and if any one does, they're not talking other than to say there was an inaccuracy.  I rather suspect that the bump on the A3J for the weapons package likely didn't hur the afterbody drag of the Vigi all that much, it's a quite clean and well-tested effort. 

As to your hypothetical question, it depends on whether airborne loiter time plays a major role in the reqirements, if it does, then a v-g wing has definite advantages, despite the higher wieght needed for v-g.  Folding wingtips are a fairly known quantity and I don't see that bothering anyone (even the F-35C has folding tips, but they're outboard of the wing primary structure).  A folding vertical tail, on the other hand, is used soley, as far as I can remember, on the A/RA-5.  I rather suspect that if an EA-5 had been made, that folding would've complicated the installation of a fin-mounted antennae set, as on the EA-6B and EF-111A, a good bit (I've got doodles for both an EA-5D and a KA-5E tanker).

[KJ_Lesnick]

Elmayerle,

Look at the prototype F-18's in their initial build configuration, there are slots between the LERX and the fuselage; these were removed as part of the design development and the space used for equipment.

I can't seem to find any picture of a prototype F-18...

I rather suspect that the bump on the A3J for the weapons package likely didn't hur the afterbody drag of the Vigi all that much, it's a quite clean and well-tested effort.

Would it have been better hypothetically (since I'm thinking up a concept here) if the bump was removed and the two engines spaced together with no seperation?

As to your hypothetical question, it depends on whether airborne loiter time plays a major role in the reqirements, if it does, then a v-g wing has definite advantages, despite the higher wieght needed for v-g.

But hypothetically -- if the fixed wing could do everything the swing wing could in terms of performance and loitering, why would a swing-wing have advantages?

A folding vertical tail, on the other hand, is used soley, as far as I can remember, on the A/RA-5.

Understood


KJ Lesnick

[elmayerle]

The original F-18 prototypes had slots between the LERX and the fuselage much like its YF-17 predecessor did.

Looking at the lines of the A3J, the weapons package makes very little bulge and I don't believe that spacing the engines closer would reduce drag as you'd still need a fairing between the two engine exhausts to prevent interference effects from causing drag.  NAA-Columbus clearly did their homework very well, here.

In theory, a fixed wing would have the advantage of the swing-wing if it could do everything in terms of both loiter and performance that a swing-wing can do; but loiter requires a large wingspan while performance requires a small one.  You can trade off, of course, but if you have a need for a lot of both, a swing wing starts looking attractive, despite the structural requirements and extra systems requirements.

[KJ_Lesnick]

The original F-18 prototypes had slots between the LERX and the fuselage much like its YF-17 predecessor did.

Those long skinny slits at about half way down the length of the plane on the top of/inboard of the strakes?

Looking at the lines of the A3J, the weapons package makes very little bulge and I don't believe that spacing the engines closer would reduce drag as you'd still need a fairing between the two engine exhausts to prevent interference effects from causing drag.  NAA-Columbus clearly did their homework very well, here.

Why do you need the fairing in between the engine exhausts to avoid drag?  I didn't think the exhaust itself would cause interference effects. 

In theory, a fixed wing would have the advantage of the swing-wing if it could do everything in terms of both loiter and performance that a swing-wing can do; but loiter requires a large wingspan while performance requires a small one.  You can trade off, of course, but if you have a need for a lot of both, a swing wing starts looking attractive, despite the structural requirements and extra systems requirements.

Of course I meant if the plane could achieve the loiter time and high-speed performance as well.  The idea I was thinking of was kind of was a wing that essentially through droops and flaps/flaperons could vary it's camber to achieve optimum L/D ratios to boost it's ability to loiter, and fly at low-speeds, with the basic wing and thrust-figures allowing high-speed performance. 


KJ Lesnick

[elmayerle]

Yep. those slits/slots are what I was referring to.  With regard to the A3J, the high-energy exhausts tend to entrain the air around them to move with them (the ejector pump effect) and without a carefully done fairing between them or other care taken in managing the airflow (the YF-17 and F-18 use some careful aerodynamics, the results of lots of development work and test time), the two entrained streams with interfere with each other.  Now, separated exhausts, as on the F-14 don't have near the problem with this.

As to your hypothetical fixed wing with lots of slat/flap variability, it might work, but the mechanisms and sevicing would be a whole 'nother problem.  What you *might* consider is a wing with a suitable composite skin, like the X-29 used, where the mechanism is all internal to the wing and you don't get as adverse an environment for the mechanisms.  Granted, you can't necessarily droop the "flaps" as much, but it does have definite points in its favor.

[KJ_Lesnick]

ElMayerle,

Yep. those slits/slots are what I was referring to.

Okay, now I know what you're talking about. 

Just out of curiousity, what were the slots supposed to do?  The high-sweep angle looks like it would produce enough vortex lift to avoid any need for slots...

With regard to the A3J, the high-energy exhausts tend to entrain the air around them to move with them (the ejector pump effect) and without a carefully done fairing between them or other care taken in managing the airflow (the YF-17 and F-18 use some careful aerodynamics, the results of lots of development work and test time), the two entrained streams with interfere with each other.  Now, separated exhausts, as on the F-14 don't have near the problem with this.

Okay, I sort of understand the ejector pump effect.   How exactly did the YF-17 and F-18 manage to circumvent the problem basically, out of curiousity?

As to your hypothetical fixed wing with lots of slat/flap variability, it might work, but the mechanisms and sevicing would be a whole 'nother problem.

How would it be that big a deal on servicing?  Slats/droops and flaps are used on fighters before the F-14 and F-15 entered service, and I don't recall them being serious maintenance problems. 

The system as I envision it would use pretty ordinary manual-settings for low-deflections of either the LED's or flaps, to the takeoff and landing settings, a variable-camber setting which is selected after the flaps and droops are brought "up", which maintains the optimum camber in flight (ideally can be over-ridden by the pilot) based on IAS and mach-number readings, obviously the droops and flaps are all the way up when flying supersonic at full speed!  And of course a dedicated "UP" setting for use on the deck (so the flaps are up and all that before folding the wings)   

What you *might* consider is a wing with a suitable composite skin, like the X-29 used, where the mechanism is all internal to the wing and you don't get as adverse an environment for the mechanisms.  Granted, you can't necessarily droop the "flaps" as much, but it does have definite points in its favor.

Good point, the idea I was going for was kind of a alternate-history VFX that would have been fixed-wing, lighter and a better dog-fighter.  I don't know how far composites were along back then...


Kendra Lesnick

[elmayerle]

As I understand it, the slots were to create a second set of vortices that would allow plenty of vortex lift under all attitudes as well as provide a balancing set of vortices in the inboard sides of the vertical tails to balance the forces the LERX-produced vortices produced on the outboard sides; when these were done away with, it resulted in the condition that necessitated the fences on the LERX - or so I'm told.

Th YF-17 and F-18 built on that extensive round of afterbody drag tunnel testing and adopted a configuration that minimized afterbody drag (worked as well in the real world as it did in the tunnel).

I'm thinking that extended use of a slats/flaps configuration, or a conbination of leading edge and trailing edge flaps, is likely to see more usage than flaps currently do and will also be exposed more to the exterior environment, and contamination; that's why I suggestsd a flexible wing such as the X-29 uses.  I won't say a fixed-wing design couldn't have matched the specs (as I recall, the NAA entry was fixed wing) but it'd take a lot more detailed tailoring of the wing.

[KJ_Lesnick]

As I understand it, the slots were to create a second set of vortices that would allow plenty of vortex lift under all attitudes as well as provide a balancing set of vortices in the inboard sides of the vertical tails to balance the forces the LERX-produced vortices produced on the outboard sides; when these were done away with, it resulted in the condition that necessitated the fences on the LERX - or so I'm told.

Understood.  Out of curiousity, did the fences add significant amounts of drag to the design?

Th YF-17 and F-18 built on that extensive round of afterbody drag tunnel testing and adopted a configuration that minimized afterbody drag (worked as well in the real world as it did in the tunnel).

This afterbody drag issue, pertaining mostly to entrained air due to the exhaust... is this applicable at supersonic as well as subsonic speeds?  Honest, I'm not trying to sound dense, or stupid or anything

I'm thinking that extended use of a slats/flaps configuration, or a conbination of leading edge and trailing edge flaps, is likely to see more usage than flaps currently do and will also be exposed more to the exterior environment, and contamination; that's why I suggestsd a flexible wing such as the X-29 uses.  I won't say a fixed-wing design couldn't have matched the specs (as I recall, the NAA entry was fixed wing) but it'd take a lot more detailed tailoring of the wing.

The flexible-wing does sound like the best idea for a variable-cambering wing.  My question is, was there such technology available during the time of the VFX that could allow such a design to work?

Regarding the use of the flaps (and leading-edge devices) in small deflections to produce optimimum L/D in cruise (in addition to the normal use of flaps for takeoff and landing) -- how much more stress (and wear) would such a flap/slat-set up actually see over a conventional design?  Could any methods be made to strengthen them without excessive gains in weight?

I'm not sure I understand your statements regarding contamination?  Do you mean like dust and crud getting into the gaps or something (because I don't think the slot in the flap or slat would be exposed at deflections this small), or sea-corrosion (the only other thing I could think of)

What kind of detailed tailoring of the wing would be required to meet the performance requirements in addition to variable-cambering of the wings (through either a flexible-wing or small flap/slat use during cruise)?

 
KJ Lesnick
BTW:  Since I'm curious as to afterbody-drag design, I'd like to just quickly touch back on the A-3J/A-5/RA-5:  Would the aerodynamic-fairing on the tail have been significantly different in size or shape if the plane had two tailfins instead of one?

[KJ_Lesnick]

Anybody have any answers?

KJ