Twin engined F-8-like variable incidence wing fighter?

[zen]

Curious thought and maybe worth a little AH....
Why wasn't there a twin F8-like Variable Incidence wing design?

Arguably such a design could have competed with the F8U-III and F4.

But trawling around it seems such a design never emerged.....?

Yet later on there was a twin engined Corsair design....

So what if there was?

 

[SSgtC]

There's no real reason you couldn't design a twin J79 powered ATL F8U-3. The biggest issue is the generally limited payload ability of the variable incidence wing. The F-8 only had one underwing pylon on either side, limiting it to 4,000 pounds of bombs combined. The F-4 had two underwing pylons per side. And while it couldn't carry the 2,000 pound bomb that the F-8 could, it could carry more bomb weight in total.

 

[isayyo2]

You could have the best of both worlds with an up-sized Phantom mounting a high shoulder variable wing and twin J75s. Designed as a A-5 replacement and proto-Tomcat with Eagle missiles.

 

[zen]

I'd start earlier, a twin J57 version with two seats, and Sparrow III.

 

[Pioneer]

There's no real reason you couldn't design a twin J79 powered ATL F8U-3. The biggest issue is the generally limited payload ability of the variable incidence wing. The F-8 only had one underwing pylon on either side, limiting it to 4,000 pounds of bombs combined. The F-4 had two underwing pylons per side. And while it couldn't carry the 2,000 pound bomb that the F-8 could, it could carry more bomb weight in total.

If I may SSgtC, the original F8U design was proposed with four underwing hardpoints (armed with early beam-riding Sparrow I AAM's), so I'm wondering if it wouldn't be too hard to incorporate these four underwing hardpoints again in a new build F8U-3, to customer requirements.

As for the issue of the wing design itself, I can't help wonder if there is scope for Vought mating their studied/proposed 'Attack Crusader' wing design to the legacy F8U-3 fuselage? [Greg/GTX from memory posted a drawing showing this wing design vs legacy F8U-3 wing somewhere....].
For as much as this said revised 'attack wing' design would cost the F8U-3 some speed, from my interpretation, it still gave supersonic performance and greater wing area, which perhaps will also elevate the said variable incidence wing requirement for carrier ops.....

P.S. I'll try to search out that 'Attack Wing F-8' picture/drawing

Regards
Pioneer

 

[Yellow Palace]

The biggest issue is the generally limited payload ability of the variable incidence wing.

Is that a general limitation of variable incidence wings, though, or specifically one of the Crusader's wing? AFAIK, since in flight the aircraft is supported by the wing, rather than vice versa, it's often structurally preferable to add loads to the wing rather than to the fuselage.

 

[MadRat]

The only twins that seem to fit the scenario are either J52 (F-101?) or the J79 (F-4). Those niches were taken.

 

[Scott Kenny]

Is that a general limitation of variable incidence wings, though, or specifically one of the Crusader's wing? AFAIK, since in flight the aircraft is supported by the wing, rather than vice versa, it's often structurally preferable to add loads to the wing rather than to the fuselage.

In general, you have a hydraulic cylinder and a hinge pin bearing the entire aircraft's weight with an in-flight-variable incidence wing.

 

[Yellow Palace]

In general, you have a hydraulic cylinder and a hinge pin bearing the entire aircraft's weight with an in-flight-variable incidence wing.

Anything that's hung off the wing, rather than the fuselage, presumably won't be applying load to that interface in flight, but probably is on the ground. I guess the question is, do the flight loads or the ground loads size the hydraulic system and hinge?

 

[red admiral]

do the flight loads or the ground loads size the hydraulic system and hinge?

Almost definitely flight loads (and landing) rather than static ground loads by a large margin

Given the likes of dH's GOR.339 submission had big engine pods and tanks in the variable incidence wing, I don't think it's a general conclusion that you can't carry heavy liads

 

[Scott Kenny]

Anything that's hung off the wing, rather than the fuselage, presumably won't be applying load to that interface in flight, but probably is on the ground. I guess the question is, do the flight loads or the ground loads size the hydraulic system and hinge?

Flight loads, and by a large margin. This is a fighter, so the wing and hinge/hydraulics gets to carry +7x the weight of the aircraft.

 

[Yellow Palace]

Almost definitely flight loads (and landing) rather than static ground loads by a large margin

Flight loads, and by a large margin. This is a fighter, so the wing and hinge/hydraulics gets to carry +7x the weight of the aircraft.

Which, in my mind, puts to bed the idea that variable incidence wings can't carry stores. If the Crusader couldn't do it, that's because of the design of its wing specifically, rather than a general limitation of the concept.

 

[SSgtC]

Which, in my mind, puts to bed the idea that variable incidence wings can't carry stores. If the Crusader couldn't do it, that's because of the design of its wing specifically, rather than a general limitation of the concept.

The Crusader did carry bombs on its wings. About 4,000 pounds plus the weight of the hardpoints and ejector racks. It was the only fighter in the Navy that could lug the Mk 85 2,000 bomb around. Something even the Phantom couldn't do. I just don't know if it could do more than that.

 

[zen]

Almost definitely flight loads (and landing) rather than static ground loads by a large margin

Given the likes of dH's GOR.339 submission had big engine pods and tanks in the variable incidence wing, I don't think it's a general conclusion that you can't carry heavy liads

View attachment 719852

Call me nuts but I have a soft spot for this design.

 

[Scott Kenny]

Which, in my mind, puts to bed the idea that variable incidence wings can't carry stores. If the Crusader couldn't do it, that's because of the design of its wing specifically, rather than a general limitation of the concept.

Maybe I need to be a bit clearer. The hinge and hydraulic cylinder carry up to 7x the weight of the fuselage.

A potential disadvantage of the Vought VG wing is that all those parts are in tension, while a VG wing that isn't carried as a "surfboard" on top of the aircraft won't have the entire VG assembly in tension.

 

[MadRat]

Metal under tension is normally advantageous. Fatigue is more likely from unloaded metal being repeated loaded. Cracks will form across tempoarary tension lines.

 

[MadRat]

 

[Scott Kenny]

Metal under tension is normally advantageous. Fatigue is more likely from unloaded metal being repeated loaded. Cracks will form across tempoarary tension lines.

Yes, but if you put the wing at the middle of the fuselage you have less load in tension total, because some of the load is in compression instead.

Put the wing on the bottom of the fuselage and you have almost no tension loads on the VI bits, all compression in normal flight.

 

[Scott Kenny]

That's a big load, I'm assuming 2x 2000lb bombs, 4x Zuni rockets and 2x Sidewinders...

 

[red admiral]

Does anyone have any pictures of how the variable incidence wing on F8U was mechanised? I've had a look around but can't find any. I assume that's its a slow moving hydraulic screw jack, but was also wondering whether it had a locking mechanism.

 

[Scott Kenny]

Does anyone have any pictures of how the variable incidence wing on F8U was mechanised? I've had a look around but can't find any. I assume that's its a slow moving hydraulic screw jack, but was also wondering whether it had a locking mechanism.

I was actually assuming a pair of hydraulic cylinders to push the whole wing section up to the "increased" height and pull it back down to the standard height.

 

[HoHun]

Hi,

Given the likes of dH's GOR.339 submission had big engine pods and tanks in the variable incidence wing, I don't think it's a general conclusion that you can't carry heavy liads

Perhaps the idea that the variable incidence wing cannot carry heavy loads stems from a consideration of wing loading. Any increase of incidence of the wing increases the currently available lift, but only until the maximum lift coefficient of the wing is reached - which doesn't increase when incidence is increased.

The main benefit of the variable incidence wing thus is to control the fuselage angle relative to the wing - to good use in carrier aviation, obviously.

To get an increase of the maximum lift coefficient, a variable incidence wing would additionally have to be equipped with high-lift devices like any other wing, but high-lift devices often are an alternative way of adjusting the fuselage angle relative to the wing in a landing configuration, so a variable incidence wing would be a competing mechanism for the same purpose.

In conclusion, it seems likely to me that the variable-incidence wing is an attractive solution only for an aircraft with a fairly light wing loading which is not trying to extract the greatest lift from the available wing area. While it would work in combination with high-lift devices just fine, it would be sort of redundant in such a configuration, adding complexity and weight with no real benefit, which probably why it's not considered worth it with high wing loadings.

Just a quick opinion, I might easily be wrong

Regards,

Henning (HoHun)

 

[Manuducati]

I was actually assuming a pair of hydraulic cylinders to push the whole wing section up to the "increased" height and pull it back down to the standard height.

 

[Scott Kenny]

Looks like I guessed right.

You'd want a hydraulic screw for something like fowler flaps, because they have multiple positions. The F-8 wing is a two position thing, full up or full down. Anything like that is simplest done as a hydraulic cylinder with mechanical stops. And maybe a down position lock.

 

[_Del_]

Just have the outboard portions of the wing pivot. Then you don't have to worry about hanging any ordinance there.

 

[Scott Kenny]

Just have the outboard portions of the wing pivot. Then you don't have to worry about hanging any ordinance there.

I think that's actually more complicated than an F-8 like system.

 

[red admiral]

@HoHun

In my mind, the variable incidence wing is independent of the maximum lift coefficient of the wing. The main effect is in changing the relationship between the body axes and the stability (or wind if you ignore sideslip) axes.

For take-off and landing then you're generally not able to use the maximum lift coefficient of the aircraft; e.g. tailscrape generally limits you to about 12 deg AoA, whereas for swept wings then max lift coefficient can easily be above 20 deg AoA. Another limit may be visibility over the nose. It's more correct to state AoA as AoA+wing rigging angle i.e. there may be a difference between the body axes and the "wing" axes over which the majority of the lift is being generated.

So one way to improve Take-off and Landing performance is to generate a higher lift coefficient for a given body angle. A variable incidence wing does this by varying the wing rigging angle in such a way that minimises the performance (drag) impacts in other parts of the flight envelope. From the Cl Vs AoA chart attached then what we're doing is horizontally translating the line. It doesn't change the maximum lift coefficient but it changes the lift coefficient for a given AoA.

I don't see why a variable incidence wing couldn't be combined with high lift devices (slats, flaps) in order to further improve the lift coefficient for a given AoA. It definitely looks like F8U has flaps and slats.

An other thought in my mind is that the likes of Fowler flaps also induce large pitching moments when deployed, which have to be transmitted through the variable wing mechanism. These torsional loads act in a different direction to those from lift. But some wing body joints can be very simple so I don't see this as a major issue - you're going to have two beefy attachment points for the variable wing mechanism anyway (hinge + moving arm)

 

[HoHun]

Hi,

I don't see why a variable incidence wing couldn't be combined with high lift devices (slats, flaps) in order to further improve the lift coefficient for a given AoA. It definitely looks like F8U has flaps and slats.

I'd say we're largely in agreement

Thanks for posting the flap lift coefficient diagram, it is great for illustrating the concept ... assume we desire to land at Cl = 1.25 at a 0° angle to our line of reference (fuselage centreline, for example).

We can achieve this either by using 15° of flaps, which gives Cl = 1.25 at 10° angle of attack on the wing, so we need 10° of variable incidence to get our fuselage to the 0° alignment.

Or we can use 50° of flaps, which gives Cl = 1.25 at 0° angle of attack on the wing, which aligns with our centreline right away, and omit the variable incidence mechanism.

There's a sweet spot where simple flaps alone will not give the desired combination of Cl and landing attitude, and that's where a variable incidence wing might be the best solution, since the beefy structure you pointed out can be combined with simple and thus probably robust and low-maintenance flaps.

Once you desire a higher Cl, you might be forced to use complex flaps, and that's where my speculation comes into play: I suspect that once you go for complex flaps, the addition of a variable incidence mechanism is not the best solution as it's easier to tailor the complex flaps themselves to get to the desired Cl.

That's no hard law of course, just an arm-waving estimation of the trade-offs potentially considered by the aircraft designers facing this challenge during conceptual design of a new type for specific requirements.

Regards,

Henning (HoHun)