Discussion: F-14, F-15 Hybrid Idea

[KJ_Lesnick]

Mr. Mayerle,

That large glove on the Tomcat 21 basically kept the plan view of the extended glove vanes and extended the glove around it; near as I can tell, the new glove wasn't any thicker so there was no real increase in frontal area; you had a bit more wetted surface area, but not overly much more.

So the glove extension was as fat as the regular glove right?  Well, the regular glove was swept back a lot... this glove wasn't swept really at all at the front although further outboard it swept back more than the regular glove.  The point I was getting at is that the areas that had little sweep were not razor sharp (and generally you leading-edges with little sweep you want razor sharp -- like the F-104 kind of,  with highly swept shapes you can see sharp set ups too but generally you see thicker set-ups, somewhat blunter leading-edges and even conical camber)


KJ

[elmayerle]

I believe the new extension functions as much as a large LERX as anything else.  It's got the same sweep as the deployed maneuvering vanes on the basic F-14 have as far as I can tell.

[KJ_Lesnick]

Are you talking about the fixed glove-vane?  (It looks like a canard)

KJ

[elmayerle]

Yeah, we're talking about the vane that deploys from the fixed glove on some versions of the F-14 (they were later locked in the retracted position when it was found they weren't needed).  If you use the plan view of the aircraft with the vane extended, the leading edge of the vane lines up nicely with the leading edge of the extended glove on the Tomcat 21.  I've always thought that to be a right clever use of aero data they already had to make the aircraft even better.

[KJ_Lesnick]

Mr. Mayerle,

Which airplane's leading-edge extensions work the best, the F-15's or the F-14's in terms of their effects on maneuverability (high alpha), lift at supersonic speeds, potential trim-drag reductions, and effectiveness versus size? 

I could use guesses from a more knowledgeable person than just myself and my own guesses (which are as follows = the F-15's LEX seems to have a higher sweep, and seems to use twist and inverse camber inboard on the surface which is conducive for reduced trim-drag, and compression lift; the F-14's do seem to allow higher alphas, has plenty of room for fuel, and due to it being swept behind the shockwave even with it's thick size seems to be able to generate more lift up top on the wing's surface, although little to no compression lift benefits) -- note I'm not counting the pancake (F-14), the wing-body fairing (F-15) aft of the LEX and the glove vanes (F-14). 


KJ Lesnick
BTW:  How easy was the F-4 to maintain?  I was told it was pretty good.. (which strikes me as highly confusing considering the F-14, also a Navy Plane was very maintenance intensive)

I also found a drawing of the Grumman's F-14 concept without swing-wings.  I'm wondering which characteristics from it and the F-15B/D should be used for this concept?

[elmayerle]

I'd say the F-14's configuration has the edge on high-AOA aerodynamics and I suspect that a tailoring of the glove using CFD backed by tunnel testing could make good use of that thick glove with a drastic increase in drag; too, the vortex flow around it could also help in maintain control under more extreme conditions.  The F-15 was designed for both speed and dogfighting capability (if the speed had been relaxed - MiG-25 was a threat at that time - it'e likely have been a bit smaller aircraft) and has a lot of aerodynamic tailoring for high maneuverability.

From what I've heard, the F-4 wasn't the worst nor the greatest aircraft in the world to maintain and had a few features that drove maintainers to drink (I've heard that there's a heat exchanger right behind the cockpit that can be installed when you're building the plane but is a decided pain to replace later and even then the surrounding structure makes fastening it fully firmly down impossible, but this same structure also prevents it from shifting much).  I do believe that the F-14 has better maintainabilty/serviceability provisions that the F-4 though the F/A-18 outshines them both as it was designed from the beginning with the "-illities" in mind (mind you, there's a lot that way on the B-2, also; when opening up to get something and then sealing the bay afterwards takes so long, you get very inspired to make the rest of the maintainer's/sevicer's life as easy as possible). 

I think the thing I'd add to the non-swing-wing F-14 version would be a good set of LERX, either from the F-15 or the F-18 - though the design of the LERX on the F-20 was pretty good, too.  Since the root of the wing is on the thick side anyway to accomodate the main landing gear, it'd be easy for the LERX to blend in and give you something similar to the -15's wing-body fairing at least back to the trailing edge of the wing.

[KJ_Lesnick]

Does anybody know how much extra lifting-area the MiG-29's tunnel (the gap between the engines) adds to the design (For example the F-14's pancake/tunnel adds 443 extra square feet, with the wing-area being 565 square feet, which produces a total lifting area of 1,008 square feet)?


Kendra Lesnick

[KJ_Lesnick]

Mr. Mayerle,

I'd say the F-14's configuration has the edge on high-AOA aerodynamics

Makes sense... it's low-speed handling exceeded that of the F-15

I suspect that a tailoring of the glove using CFD backed by tunnel testing could make good use of that thick glove with a drastic increase in drag

I take it you did mean a drastic decrease in drag?  Because I don't know why anyone would want an increase in drag... 

too, the vortex flow around it could also help in maintain control under more extreme conditions.

Makes sense. 

The F-15 was designed for both speed and dogfighting capability (if the speed had been relaxed - MiG-25 was a threat at that time - it'e likely have been a bit smaller aircraft) and has a lot of aerodynamic tailoring for high maneuverability.

Well, the MiG-25 was a perceived threat:  It would seem some people thought the thing was some kind of superfighter!  Which was one thing it was not.

From what I've heard, the F-4 wasn't the worst nor the greatest aircraft in the world to maintain and had a few features that drove maintainers to drink (I've heard that there's a heat exchanger right behind the cockpit that can be installed when you're building the plane but is a decided pain to replace later and even then the surrounding structure makes fastening it fully firmly down impossible, but this same structure also prevents it from shifting much).  I do believe that the F-14 has better maintainabilty/serviceability provisions that the F-4 though the F/A-18 outshines them both as it was designed from the beginning with the "-illities" in mind (mind you, there's a lot that way on the B-2, also; when opening up to get something and then sealing the bay afterwards takes so long, you get very inspired to make the rest of the maintainer's/sevicer's life as easy as possible).

That's kind of funny, I remember reading that the F-14 was really hard to maintain and the F-4 was fairly easy.  I won't say that you're wrong though.  Regardless, the F-15 was designed to be more maintainable than either. 

I assume the limitations of the radar and electronics to be used on the F-14 pretty much made it impossible to make it any easier to maintain?

I think the thing I'd add to the non-swing-wing F-14 version would be a good set of LERX, either from the F-15 or the F-18 - though the design of the LERX on the F-20 was pretty good, too.  Since the root of the wing is on the thick side anyway to accomodate the main landing gear, it'd be easy for the LERX to blend in and give you something similar to the -15's wing-body fairing at least back to the trailing edge of the wing.

You know, I don't know why the F-14 had to wedge it's landing gear in the thick wing-side...  The F-15 managed to stuff it right under the engine bays.

In either case, you make a good point about the LERX.  I was thinking (It's really hard to explain) of something that's kind of got elements of the F-15, and the F/A-18, and X-32

- X-32 JSF Concept:  Basic top-view sweep basic angle of LERX (it's a double-delta, the forward delta part), nice and highly swept with good vortex producing capability; also has some thickness to it

- F-15 Eagle:  Highly swept in most areas, thick and highly swept, low-sweep in the front which might be partially outside the conical flow-field (the design does use inverse camber and a slight twist -- the inverse camber might produce compression-lift effects, and the twist and basic contouring might reduce trim-drag a tad).  LERX is not particularly long... fairly compact.

- F/A-18 Hornet:  Highly swept outboard, exploits conical cambering; features very sharp low-sweep area on the chine inboard which might be able to exploit inverse-camber and compression-lift effects (possibly better than the F-15 as it's sharper up front).  Conical cambered LERX could produce extra lift up front supersonic and deal with trim-drag at at least upper speed-range. 

Not entirely sure if my logic is correct here but that's kind of the idea I have, feel free to add your insights.


Kendra Lesnick

[elmayerle]

I suspect that a tailoring of the glove using CFD backed by tunnel testing could make good use of that thick glove with a drastic increase in drag

I take it you did mean a drastic decrease in drag?  Because I don't know why anyone would want an increase in drag... 

That should have been "...without a drastic increase..."  I think it'll make more sense now.

The difference in landing gear locations are due to a difference in maintenance needs for engine removal.  With the tight environment of a hanger deck, the Navy needs to be able to open the engine bay door(s) on the underside of the fuselage and drop the engine down onto a cart and then raise another engien into place from another cart while the USAF (F-15 for example) has the engine on rails so that a cart can be positioned behind the aircraft and the engine slid out.  This approach was carried over the the F/A-18 and then, because of the complicated structure for low obesrvables, the engine for the B-2.

Considering the state of the electronic art at the time, I suspect the radar was a major driver in maintenance requirements.  Ease of maintenance as a contractual requirement stems from later than when the F-14 was designed (I suspect that experiences with it and the F-4 may have helped drive those requirements being imposed on the VFAX and subsequent programs.  Besides the obviously means of providing good access, there are a number of simple things that can be done in the design phase to make an aircraft easier to service and maintain, translating into lower MMH/FH and reduced life-cycle costs.

[KJ_Lesnick]

Mr. Mayerle,

That should have been "...without a drastic increase..."  I think it'll make more sense now.

It does.

The difference in landing gear locations are due to a difference in maintenance needs for engine removal.  With the tight environment of a hanger deck, the Navy needs to be able to open the engine bay door(s) on the underside of the fuselage and drop the engine down onto a cart and then raise another engien into place from another cart while the USAF (F-15 for example) has the engine on rails so that a cart can be positioned behind the aircraft and the engine slid out.  This approach was carried over the the F/A-18 and then, because of the complicated structure for low obesrvables, the engine for the B-2.

I did not know that the USN changed their engines that way -- I thought they just slided them out like the USAF, and the commercial aircraft industry did it -- Regardless though, if I recall, wasn't the F-15's gears located ahead of the engine position?  If so, whether they folded rearwards or sideways inwards just under the engine-bay it wouldn't have made a difference no?

It is interesting to know that the F/A-18 uses the USAF method for engine installation and removal...

Considering the state of the electronic art at the time, I suspect the radar was a major driver in maintenance requirements.

Makes sense...

Ease of maintenance as a contractual requirement stems from later than when the F-14 was designed (I suspect that experiences with it and the F-4 may have helped drive those requirements being imposed on the VFAX and subsequent programs.  Besides the obviously means of providing good access, there are a number of simple things that can be done in the design phase to make an aircraft easier to service and maintain, translating into lower MMH/FH and reduced life-cycle costs.

Generally ease of maintenance is always a good quality -- less time fixing them, more time can be spent flying them.


KJ Lesnick

[elmayerle]

I'm afraid there's a mis-understanding, the F/A-18 removes it's engine out the bottom of the fuselage, as does the B-2 (30 minute swap time, there, not counting dealing with all the activities that low observables features require (you wouldn't believe the number of fasteners on those access doors, nor the other activities required).

The F-15's gear retracts forward under the inlet trunks (a lot of them retract this way for some very valid reasons).  GIven teh overall geometry of the F-14, it's present gear location makes sense.  I'm not saying it couldn't be elsewhere, just that for the other design decisions, it makes sense.

Designing for maintainability takes some forethought in locating components relative to access panels/doors and to each other (the low-MTBF units go in front of high-MTBF units so you don't damage the more reliable parts while removing/replacing the others).

[KJ_Lesnick]

Mr. Mayerle,

I'm afraid there's a mis-understanding, the F/A-18 removes it's engine out the bottom of the fuselage, as does the B-2 (30 minute swap time, there, not counting dealing with all the activities that low observables features require (you wouldn't believe the number of fasteners on those access doors, nor the other activities required).

Okay, I misunderstood what you said -- I got it now.

The F-15's gear retracts forward under the inlet trunks (a lot of them retract this way for some very valid reasons).

What reasons? 

Out of curiousity, the inlet trunks are ahead of the engine itself right?

GIven teh overall geometry of the F-14, it's present gear location makes sense.  I'm not saying it couldn't be elsewhere, just that for the other design decisions, it makes sense.

Out of curiousity, can I ask why?

Designing for maintainability takes some forethought in locating components relative to access panels/doors and to each other (the low-MTBF units go in front of high-MTBF units so you don't damage the more reliable parts while removing/replacing the others).

What's MTBF mean?


Kendra Lesnick

[elmayerle]

Why do so many aircraft go with a gear that retracts forward?  Well, in the event of hydraulic failure, they can unlatch the gear and doors as they're preparing to land and once the gear falls far enough for the airloads to hit it, it'll extend fully.  Yes, the inlet trunks are ahead of the engine (the term signifies that portion of the air path between the main inlet and the engine face(s).

Given that the basic F-14 layout is two inlet-engine pods plus the center fuselage, putting the main gear into those pods would likely result in a somewhat heavier structure in that area as well as some challenges to the area-ruling.  The existing location allows, too, for nice long struts that have the travel room to take carrier landings.  If I was adapting a F-15 for carrier use (and I am), i'm use main gear similar to the F-18's to help absorb the load of carrier landings; the long and sturdy main gear legs fo the F-14 can be a bit simpler.  Since, even on the fixed-wing concept, the main gear ties directly into heavy structure that'd be there anyway, it doesn't have the penalties associated with putting the main gmeear under the engine pods and the thick wing roots allow the wheel to rotate and lie flat while leaving room fore and aft of the gear for equipment or tankage.

MTBF = Mean Time Between Failure; you want the low-MTBF components in front of the high-MTBF ones, realative to the bay access, so that you're less likely to damage healthy components when you remove and replace one that needs fixing.  It gets a lot of emphasis in aircraft design these days (say over the last 30 years or so - to the best of my knowledge, the F/A-18 was the first one this was a consideration in).

[KJ_Lesnick]

Mr. Mayerle,

Well, in the event of hydraulic failure, they can unlatch the gear and doors as they're preparing to land and once the gear falls far enough for the airloads to hit it, it'll extend fully.

That makes complete sense.

Yes, the inlet trunks are ahead of the engine (the term signifies that portion of the air path between the main inlet and the engine face(s).

Understood.

Given that the basic F-14 layout is two inlet-engine pods plus the center fuselage, putting the main gear into those pods would likely result in a somewhat heavier structure in that area as well as some challenges to the area-ruling.

How would putting the gear into the pods result in a heavier structure?  The F-15 manages it just fine...

Also, what challenges would there be to area-ruling? 

The existing location allows, too, for nice long struts that have the travel room to take carrier landings.

Shock absorbtion ability and springy-ness? 

Interestingly I remember a concept for a navalized F-15 and while it had a nose-gear that could be jacked up to increase alphas for takeoffs, it's main legs looked the same -- I could be wrong, but I have a pretty good visual memory...

If I was adapting a F-15 for carrier use (and I am), i'm use main gear similar to the F-18's to help absorb the load of carrier landings; the long and sturdy main gear legs fo the F-14 can be a bit simpler.  Since, even on the fixed-wing concept, the main gear ties directly into heavy structure that'd be there anyway, it doesn't have the penalties associated with putting the main gmeear under the engine pods and the thick wing roots allow the wheel to rotate and lie flat while leaving room fore and aft of the gear for equipment or tankage.

From what I remember, there was such a concept for an F-15N.  However it's main gear legs looked the same... the nose gear however could be jacked up.

Regarding the F-14's landing gear set-up, was the design reasonably light?

MTBF = Mean Time Between Failure; you want the low-MTBF components in front of the high-MTBF ones, realative to the bay access, so that you're less likely to damage healthy components when you remove and replace one that needs fixing.  It gets a lot of emphasis in aircraft design these days (say over the last 30 years or so - to the best of my knowledge, the F/A-18 was the first one this was a consideration in).

This was physically possible to do even back in the 1960's with design like the F-14?


KJ Lesnick

[elmayerle]

Actually, the F-15 has it's gear attaching to full width fuselage frames rather than to just frames around the inlet; putting the gear into the inlet/engine pods would've required strengthened frames there as well as some enlarging of the contours to stow the gear.  I can't prove it, but I rather would expect the difference between the main gear on the F-15 and the proposed navalized F-15 to be similar to that between the proposed F-18L and the production F'A-18; i.e. the carrier version in each case would be a trailing beam gear while the strictly land-based version suffices with a much simpler design.  I suspect the navalized F-15 may well have an extendable nose gear for launch, much as the F-4K did.  I'd say that the F-14's gear was as reasonably light as it could be and still meet the strength and life requirements.

The design philosophy I mentioned MTBF components was somewhat possible to do back when the F-14 was designed, though it would've required several detailed mockups before it could get to the level that CAD allows you today.  I'd hazard a guess that Grumman did the best they could with the tools available at the time.  I'm in a situation now where I'm working with everything from ink-on-mylar drawings to the latest CAD models and it's quite an "interesting" experience.