Why not another variable sweep fighter?
There seems to be a lot of F-14 nostalgia around. While it may have had a great deal of impact on how the US Navy conducted fleet defense, we have to consider the effectiveness of the concept of variable sweep aircraft. It is human nature to always want to look up to the past. The other reason may very well be that people find the F-14 to look visually attractive and want similar proposals.
The reason why we will not see future variable sweep fighters however is because there are very serious drawbacks compared to fixed wing aircraft.
Short Background
Variable sweep wings, known as “Swing wing” evolved as a solution for early jet engines. Experiments were being made as early as WW2 with wings that could change their sweep on the ground, such as the Messerschmitt P.1101.
Back then jet engines produced less thrust because they ran at lower inlet temperatures and were overall more primitive. Wings with a sharp sweep were desired for high top speed, but that left the aircraft vulnerable in dogfights, which as Vietnam revealed still happened, and also led to high take-off and landing speeds. High take off and landing speeds are less safe, which would result in increased number of crashes. They also led to long runways, limiting off road mobility and making it easier to disable for enemy forces, as there would be a far larger airport to protect.
In Europe, there were two key projects, the Panavia Tornado, which entered service as a mult-role interceptor/bomber, and the Dassault Mirage G, which never entered production. The US would build the F-111, which was a very heavy variable sweep multi-role aircraft. The famous F-14 was derived from the F-111. The USSR made several variable sweep designs, most notably the Mig-23 and the Su-24.
Bomber designs were also made by the US and USSR. The B1 Lancer from the US, along with the Tu-22 and Tu-160 from the USSR. All 3 bombers remain in service.
What do swing wing aircraft bring?
Their main advantage is that they can use that variable sweep wing to find the optimal wing swing angle (within their sweep limits) for a given airspeed. This can allow for fuel savings on the climb and landing during a fighter sortie.
On aircraft carriers, they have the advantage of having very low sweep on take-off and very high sweep when bursting with full afterburner. Variable sweep wings can also be folded for compact storage without compromising wing’s structural integrity (as is the case with folding wings like on F-18E).
On an aircraft carrier, deck-space is always going to be a bottleneck. While a carrier may look very large to an untrained eye, deck space is always at a very big premium.
So why not on fighter aircraft?
To achieve variable sweep aircraft, that requires a large gearbox in the fuselage of the aircraft. This gearbox adds a great deal of mass and makes the fuselage larger, causing drag. This means that fuel fraction on such aircraft is lowered a great deal.
In a dogfight, this heavy gearbox would mean that compared to a fixed wing, it would result in an unfavorable thrust to drag, even if the pilot could switch to what they felt was the optimal sweep right before combat. Switching the wing sweep during a dogfight would be risky, as it could cause a loss of energy.
This would mean:
- Higher wing loading due to mass of gearbox
- Faster fuel consumption due to gearbox
- Lower transient performance (very important in a dogfight)
This gearbox would also lead to lower G limits as well. On the F-14D, the symmetric limit at 50,000 lbs was 6.5G. The F-16 and F-15 were both capable of 9G. Navalized versions of the F-18 were capable of 7.5G, while certain land based variants of the F-18 could also perform 9G. For a comparison, Dassault Rafale can do 11G, with an ultimate limit of 16.5G.
The gearbox lowered the aircraft’s fuel fraction. An empty F-14D has a mass of 43,735 lb ( or about 19,838 kg) and can take on 16.200 lb of fuel. This results in a fuel fraction of 0,27, which is below 0,30 fuel fraction required for sufficient combat persistence.
Jet engines have become far more powerful than their 1960s and 1970s counterparts, allowing for much higher thrust to weight ratios. As such, they can achieve lower take distances, even more so on an aircraft carrier with a catapult. This fact somewhat negates swing-wing’s main advantage of high low-speed efficiency.
Modern computer control surfaces too have played a role in rendering variable wing sweep obsolete as they can adjust wing shape and size very rapidly, without the weight penalty.
Complexity and reliability problems
The more complex a system is, the more risk there is for failure.
When the US Navy opted to retire the F-14 in favor of the F-18, a big reason that was given was the appalling flight to maintenance ratio.
The decision to incorporate the Super Hornet and decommission the F-14 is mainly due to high amount of maintenance required to keep the Tomcats operational. On average, an F-14 requires nearly 50 maintenance hours for every flight hour, while the Super Hornet requires five to 10 maintenance hours for every flight hour.
I’ve been told that a newer F-14 would likely require 40 to 1 and on average, the F-18 requires 8 to 1, which is in line with the USN’s claims of 5-10 to 1. So in that regard, the F-18 would be able to generate much higher sortie rates. Keep in mind that the 50 to 1 is with after the General Electric F110 engines were put on the F-14. Early F-14s suffered from an unreliable TF-30 engine that was prone to flame-outs.
Compounding the problem, the high flight to maintenance ratios mean that there’s a good chance you will not have enough F-14s available when you need them the most (ex: if an enemy launches a surprise attack on your carrier battle group, you may need to scramble the aircraft very quickly).
There were other points of failure. Sometimes when one side of the gearbox worked properly and the other did not, it could lead to an “asymmetric wing sweep”.
While the aircraft could fly in such a situation and land with some difficulty, this leaves a point of failure. This could also be a weakness in combat, as the hydraulics could be damaged.
Much like this F-14, under Australian service, the F-111 did encounter a similar incident, and the B1 did once as well. I suspect that under Warsaw Pact service, Soviet variable sweep designs may have too.
Conclusions
The cons simply outweigh the pros when it comes to variable wing sweep. There are very significant penalties in terms of mass, cost, and complexity for variable sweep wings. While they may bring some advantages in the take-off and can have the “optimal” sweep for each scenario, the drawbacks outweigh these to the point where we are not seeing variable wing sweep aircraft on modern aircraft.
They are simply a dead end as far as aircraft design goes. While they may have seemed like a good idea on paper, when implemented in combat aircraft, they carried significant drawbacks that outweighed any advantages they brought.
History and War 
“There were other points of failure. Sometimes when one side of the gearbox worked properly and the other did not, it could lead to an “asymmetric wing sweep”.
It only happened twice in the life of the Tomcat and both cases they landed safely
http://www.airspacemag.com/flight-today/swing-wings-9189621/
The fuel fraction numbers, its quite rare to be above 0.30, the US navy when the Tomcat was around relied on big tankers and of course unswept wings were more fuel efficent at high altitude cruise.
LikeLike
Fuel fraction of 0,27 is minimum of minimum, it should be above 0,30 for adequate persistence, and ideally above 0,40 if you want a supercruising fighter.
Greater problem with gear box is the volume and weight taken up…
LikeLike
Actually, the f-14 tomcat saves weight due to it’s VG wings. Most people simply don’t realize the only reason the tomcat is so heavy is because it’s built like a tank, and it’s a navy plane, of course. The super hornets, while considerably smaller are not that far behind the tomcat in weight, with only a 10,000 lbs difference empty. The tomcat is a very big plane, and that really has nothing to do with it’s variable sweep wings. It’s to do with having an extra guy with 1,000s of pounds of avionics, messy and complicated electrical wiring and archaic hydromechanical flight control system. It is the last of the hydromechanical planes built in the USA.
Having fixed wings requires more connections, therefore more weight. The wings on the f-14 can pivot obviously, so there is only one connection it needs to make to the wing box. The flat and wide underbelly in the engine nacelle is not only where the tomcat would carry SIX very large phoenix missile, which no other plane in the world would be able to do and still remain aerodynamic and maneuverable, it is also the reason the tomcat has more wing area than either the f-22 raptor or f-15 eagle, thus a lower wing loading than any of our other air superiority designs..
So yes, you can blame the tomcat’s 1960’s technology for most of it’s weight problems. But you can’t blame it’s variable geometry wings.
Also the tomcat suffered from lack of funding to upgrade the aging electrical problems, which would have significantly lowered maintenance hours.
LikeLike
A few minor corrections as i did not proof read.
This should read like this:
“The tomcat is a very big plane, and ****it’s weight problems*** really have nothing to do with it’s variable sweep wings. It’s to do with having an extra guy with 1,000s of pounds of avionics, messy and complicated electrical wiring and archaic hydromechanical flight control system. It is the last of the hydromechanical planes built in the USA.”
another fix:
“The flat and wide underbelly in the engine nacelle is not only where the tomcat would carry SIX very large phoenix missile, *****with 2 on the glove pylons**** “
LikeLike
But having swing wing does reduce wing area compared to, say, delta wing. Also, issue with weight and maintenance is the mechanism needed to change wing position. And F-14 doesn’t remain maneuverable because it never was maneuverable in the first place.
LikeLike
The tomcat has superior turning sustainability to just about anything out there today. There are videos on youtube of old f-14s turning 9 – 10gs you can see on the max-g counter (generally the bottom number left of screen). The tomcat, despite having a very large wing area, due to it’s ability to sweep it’s wings back, actually burnt LESS energy in turns than the f-15 or f-22 raptor. And everyone knows the f-15 eagle romps all over the f-22 in a close-in dogfight. The f-22 is not maneuverable in the least bit, they try to only use it as a sniper. It’s not much of a dogfighter. The F-14D on the other hand will out climb, out accelerate, out turn and out gun the raptor any day of the week.
The tomcat notably made an 8.5g+ sustained turn during a flyoff for Iran’s leader so he could choose which fighter he wanted, the tomcat or eagle. He chose the tomcat. And this 8.5g sustained turn was in fact made by an f-14A (albeit with lowered fuel due to poor engine thrust). They were able to get it to be exactly the same thrust to weight ratio as the f-15 eagle it was flying against that day. After the show, the f-15 guys immediately started working on reinforcing the F-15 eagle’s frame. It was limited to 7.5gs at the time.
The only reason they say the tomcat’s maximum G-load is 7.5g (newer plane, or 6.5 for seriously older planes) is because that is peace time limitations. In war the tomcat can pull out sustain G both the eagle and raptor, and bleed less energy in those turns to boot. This is beacuse the frame is a seriously robust tank like structure with extremely low wing loading (44 psf would be the lowest you can get out of it. IN turns it’s around 72-73 psf at about 47-49 degree of wing sweep).
LikeLike
I generally believe that instantaneous turns are more important in air combat – maximum turn followed by maximum acceleration, nothing in-between. But yes, sustained turns are also important. And what you are telling me about F-14, it does seem to be better aircraft than I had thought, though it is not exactly surprising – after all, straight wings do have advantage in subsonic flight.
Now, I can’t speak for altandmain, but the main reason I don’t like the F-14 is its size and weight. To operate from road bases, fighter needs to be small (due to road width limitations) and with low logistics footprint. Neither of which describes the F-14.
LikeLike
The f-14 actually has generally better instantaneous turns as well. Remember it is a swing wing aircraft. It can change it’s flight envelope at any given time. with the wings some degree forward, turning the tomcat tightly will make it turn inside of itself in a fashion that can only be described as hitting a brick wall made of air. Of course, the tomcat’s wing loading also goes down the further forward you go, so in reality, the tomcat doesn’t flinch at the higher g-load. (I think it can do up to 13-14g, same as most actual living breathing birds).
LikeLike
A turn at higher G loads (the 13-14G you are talking about) would almost certainly damage the airframe.
The manual itself restricts to 7.5G to preserve airframe life. It has been taken to over 9G, but then again so have other fixed wing aircraft.
The F-14 could only sweep the wings aft of the position defined by the CADC (the main computer), but not forward as that would cause structural damage. Manually sweeping would be a tactic that you would have to use very carefully.
LikeLike
7.5gs is a peace time limitation. There is no navy aircraft that goes higher than this, because of hte abusive carrier landings and takeoffs. Navy planes take much more physical stress on their frames than airforce planes do, hence the lower G limit. However, the tomcat was already tested in the development stages at 13-14g, and it came back fine. The wing loading of the tomcat when the wings are 20deg is 44-46 p.s.f. That’s lower than the mig-21, which has about a 65. The f-15 was also tested to 12+gs, but again, it’s wing loading is much higher than the tomcat and will not put up with that amount of abuse for very long.
LikeLike
I’d like to bring up a few links to contribute to this discussion, but I don’t know whether or not comment moderation for links is still on here. Anyway, one thing I think this article’s author didn’t mention is that for all its complexity back in the day, the F-14 was actually finished on time and on budget. That’s rare in 21st century warplane procurement, and despite the initially underpowered engine, Grumman eventually did swap it out for a better one.
I heard that Russia is planning on reopening the Tu-160 “Blackjack” bomber production line, which they announced back in 2015. So it seems at least one world power still sees value in variable-sweep aircraft.
LikeLike
Correct that on the F-14 the TF30 was problematic and the GE engines did resolve the tendency to flameout. However, this does not address the other problems with the very heavy gearbox, nor the high maintenance to flight ratios associated with swing wing aircraft.
Agree that the Russians are re-opening their Tu-160. The big issue remains though with swing wing aircraft. The other big difference is that the Tu-160 is a strategic bomber, whereas the F-14 is a interceptor geared towards fleet defense that is more likely to find itself engaging enemy fighter aircraft at times.
LikeLike
Another consideration is that for fleet defense bomber interceptors, something closer to the Su-27 variants, like the Su-33 would work better. They would not have to deal with the cost and complexity of a variable sweep wing, while in many ways, their performance would be better because they would be able to fly more, not have to worry about a gearbox in a dogfight, and it is likely that a better airframe design combined with a high fuel fraction would more than offset the range drawbacks in such an aircraft.
LikeLike
Wow! Sounds very uninformed of NAVY requirements!
There is a reason they called it the “Grumman Iron Works”!
Grumman, with their long history of successful carrier fighters, attack & early warning aircraft knows how to build these! And they know they have to OVER-BUILD them!!! And they did!
They built one airframe that was never intended to fly and dropped that thing from heights of 15 to 30 feet to destruction, in order to find any weak points! Then fixed it!
As for wing sweep failure, no Tomcat aircraft was ever lost to it, period!
As for maint.; about 75% was avionics; lots & lots of avionics. Easily solved with todays technology! I was an Avionics Tech., I know what I am talking about.
I also know 3 things about F-14 agility: a) a well trained, 2 man Tomcat crew will defeat most anything in the sky in a dogfight! b)If it can (& it did) out manouver F-5’s, A-4’s, etc., an F/A-18 didn’t have a chance, & were routinely defeated in NFWS dogfights.
c)I was with my squadron in a mixed A.F./Navy Red-Flag event. Our squadron had the new GE engines and trounced everything thrown at us! Even A-10’s!!! That’s right, A-10’s which were doing their little trick of 400 mph, 2000 foot diameter turns,and our guys would match turns & gun them twice every 360! How? While the A-10’s kept banking hard over or even doing figure-8s, our F-14’s would come up behind, wings on auto-sweep, match the A-10 speed, “guns, guns, guns”, as they bled speed -then break & power up, roll back over in a turn and be right back behind them; “guns!” – twice each revolution! The A-10 pilots were apoplectic, trying to figure our how the hell huge Tomcat’s were pulling it off!
Secondly, the Tomcat never needed to get into a dogfight in real combat, because a) the enemy knew they’d get creamed by those long radar controlled missiles! And DID! So, the enemy ran away! Iraq learned to run after Iran used their F-14’s to shoot down many Iraqi aircraft (using Phoeonix & Sparrows)! The Soviets ran during the cold war, as Tomcat’s would surround a Bear bomber and “turn it”, they did so with a Tomcat locked on target & they knew what came next if they did not turn away! They always did! Now, the idiots in Libya didn’t run, they foolishly engaged Tomcat’s, shot at them, and got
their butts blown out of the sky!
Consider this FACT! NO U.S. Navy F-14 Tomcat has EVER LOST an air to air real combat situation!!! No other Fighter jet in the world an claim that, as far as I know!
The Tomcat needs to come back to the Navy! Redesigned, with new avionics, redesigned parts of the airframe to reduce weight (example, composite tail fins), and other mods to make it relevant again and for decades to come.
It dosen’t need stealth! In fact, we WANT our enemies to know it is there!
“Run damn you enemies, run! The U.S. Navy Tomcat’s are here, and you’re gonna die if you don’t!!!”
LikeLike
“John” is obviously a bot that is attempting to make anyone who supports the tomcat look stupid. He is saying some things that are true, but sprinkles BS on top of it in attempt to contaminate any positive sentiment about the tomcat.
What’s weird is the F-14 tomcat wikipedia page has changed it’s numbers, lowering the f-14’s engine thrust from 30,200 LBT per engine to about 28,000 LBT. The f-110-400 engine DID NOT have 28,000 LBT. It had 30,200 LBT. But that’s not whats’ weird. A few months ago I was able to access the F-110-400 page on GE’s website to see it’s specifications. They took the page down and now there is no specification information on the f-110-400 found anywhere else on their website.
It seems that nostalgia for the f-14 tomcat is in fact growing and that’s why there is a lot of fishy things going on with being able to access certain information about the tomcat on the internet.
LikeLike
I know this is an old topic, but iI just saw it and I wanted to add some details about the 40 to 50 maintenance hours to flight hours ratios.
First, keep in mind that these hours cited were being used in arguments justifying the F/A-18E/F over the F-14D. As such, the often rolled in the F-14A/B numbers, which were more maintenance intensive than the D, to push the ratio up.
Second, once the Department of Defense (not the Navy) ordered the F-14D program terminated, this lowered its priority as time went by for spare and replacement parts as well as facilities and availability of new trained personnel. This tended to mean attempts had to be made to repair things that normally would have been scrapped and replaced. it also meant that it took longer to do things than it would on an aircraft receipting normal priority and support. There were figures available that looked strictly at the F-14D’s numbers early on when it was still receiving normal support. Those numbers were around 17 mmh/fh and trending downward. Still more than the Hornet, although you got more capability, but nowhere near as bad as is often portrayed.
LikeLike
Regarding F110-400 thrust. It was generally rated @ an average 16,000lbs mil power and 27,000 lbs afterburner of static thrust. Installed thrust varied and was partly dependent on the speed at the time. Keep in mind that 27-28,000 lbs of static thrust was what the F-14 was originally designed for.
LikeLike
I meant to say, “… and 27-28,000 lbs. afterburner…”
LikeLike