Reasoning for the F-35's top speed?

Forgive me if this is a newbie-like question, but why exactly is the F-35's often quoted top speed Mach 1.6? We've heard of it's ability to supercruise at around M1.2 or so (probably a more practical metric), but what reason is the craft not (at least publicly) able to hit mach two? DSI intakes, overall transonic-optimized shaping, lack of thrust, or something else?

firstimpulse wrote:Forgive me if this is a newbie-like question, but why exactly is the F-35's often quoted top speed Mach 1.6? We've heard of it's ability to supercruise at around M1.2 or so (probably a more practical metric), but what reason is the craft not (at least publicly) able to hit mach two? DSI intakes, overall transonic-optimized shaping, lack of thrust, or something else?

Drag, lots and lots of drag.

The F-35 uses a relatively short bulky design which runs counter to most theories of super-sonic efficiency particularly the Whitcomb area rule.

As drag squares with speed a high drag object is going to suffer a hell of a lot more as speed increases then a naturally low drag object. The amount of thrust required to increase from 1.6 to 2.0 is massive.

Test pilots will have taken the F-35 beyond M 1.6 to paraphrase their words 'to ensure operational pilots do not have a problem at or about that speed'. Operational pilots may exceed that speed temporarily in a dive for example unintentionally. What / IF any problems beyond M 1.6 need to be known for NATOPS etc.

The top speed of the F-35 is 1.6 mach because that is what the customer specified. Capability beyond spec is costly, heavy, and reduces range and payload.

firstimpulse wrote:Forgive me if this is a newbie-like question, but why exactly is the F-35's often quoted top speed Mach 1.6? We've heard of it's ability to supercruise at around M1.2 or so (probably a more practical metric), but what reason is the craft not (at least publicly) able to hit mach two? DSI intakes, overall transonic-optimized shaping, lack of thrust, or something else?

There are a tone of factors. Wing sweep is a very big one. Basically but once the shock cone from the nose hits the wing tips, the amount of energy required move the aircraft faster increases dramatically. The wing design is intended to optimize a balance between maneuvering performance and top speed under mach 1.6.

The fuselage profile is important, but then again the F-35's profile isn't that much different from other faster fighters like the F/A-18E, Rafale, or Eurofighter. Whitewhale's claim that its short and bulky, then invoking the lack of area ruling is not very accurate. The F-35 has clear area ruling, its just not nearly as exaggerated as other aircraft due to the need for large internal carriage. Remember the F-35 has phenomenal range compared to other aircraft in its class because it has very good aerodynamics.



As for operational requirements,I believe 1.6 was selected based on fighter analysis done in the 1970s. I can't remember which study it was, but it critical for setting down the performance figures for the F-16 and YF-17 lightweight fighters. Going through that list, you'll see that many of the F-35's performance figures meet those specifications.

Looking at those profiles I notice two things:

1) The profile are very similar. The profile of the F-18E isn't that much bigger even though it has two engines.

2) If the F-18E carries the same weapons and fuel and equivalent and avionics pods (assuming they exist) as the F-35C can internally then what is its top speed? Probably less than Mach 1.8

hb_pencil wrote:Remember the F-35 has phenomenal range compared to other aircraft in its class because it has very good aerodynamics.
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A very very debatable statement for several reasons including its range is hardly 'phenomenal' compared to other fighters and is paid for with a much larger internal tank, not aerodynamics.

The Typhoon for instance has a internal capacity of around 5,700*kg compared to the F35's 8,300kg yet still manages to get over 90% of the range.




*Sources vary from 5000 to 5700, annoyingly classified.

hb_pencil wrote:Remember the F-35 has phenomenal range compared to other aircraft in its class because it has very good aerodynamics.

Oh, don't be ridiculous.
F35's aerodynamics is perhaps ok in the LO class, but is otherwise laughable.
Su27 carries similar fuel load, has ~20% more dry and wet static thrust, has Russian engines! and yet makes ~25% longer combat radius (at least according to some estimations).

whitewhale wrote:*Sources vary from 5000 to 5700, annoyingly classified.

Most I saw was 4996 (kg).

Isn't there also a factor of what altitude profiles those combat radii are determined for?

One other thing to consider, unrelated to frontal cross-sections and area rule usage, is that the Mach 2-capable jets in days of yore (think Zipper, Delta Dagger/Dart, Eagle, Tomcat, Phantom, etc.) were intended to be interceptors first and foremost. Get airborne, get uber-fast, go say "Howdy!" to Ivan before he even sees the Homeland on his radar. Most of 'em had mud-mover missions foisted upon them later (with the exeption of the Delta jets).

While the F-35 will probably play an air-defense/air superiority role, it was designed from scratch to be multi-role. Mach 1.6 made a good compromise top speed. Will it ever need to get to 1.6? Who's to say? But the fact it can supercruise slightly below 1.6 with a combat load is impressive in and of itself. IMHO.

LinkF16SimDude wrote:One other thing to consider, unrelated to frontal cross-sections and area rule usage, is that the Mach 2-capable jets in days of yore (think Zipper, Delta Dagger/Dart, Eagle, Tomcat, Phantom, etc.) were intended to be interceptors first and foremost. Get airborne, get uber-fast, go say "Howdy!" to Ivan before he even sees the Homeland on his radar. Most of 'em had mud-mover missions foisted upon them later (with the exeption of the Delta jets).

While the F-35 will probably play an air-defense/air superiority role, it was designed from scratch to be multi-role. Mach 1.6 made a good compromise top speed. Will it ever need to get to 1.6? Who's to say? But the fact it can supercruise slightly below 1.6 with a combat load is impressive in and of itself. IMHO.

I think the idea was that the jets capable of higher speeds historically didn't actually use them in combat, so there wasn't really any advantage to it.

If your going to optimize a fighter, what speed do you optimize it for; it's top speed or the speed where it spends the majority of its time, in the transonic cruise speed range?

hb_pencil wrote:The fuselage profile is important, but then again the F-35's profile isn't that much different from other faster fighters like the F/A-18E, Rafale, or Eurofighter. Whitewhale's claim that its short and bulky, then invoking the lack of area ruling is not very accurate. The F-35 has clear area ruling, its just not nearly as exaggerated as other aircraft due to the need for large internal carriage. Remember the F-35 has phenomenal range compared to other aircraft in its class because it has very good aerodynamics.

I wonder if it's easy to get a more accurate ratio number between the cross-section areas of these 2 aircraft. Like a count of pixels.
But super hornet is a poor transsonic accelerator anyway.

It's true that F-35 has its own priorities among performance parameters. Still, the transsonic acceleration is not as good as expected, or say, the actual wave drag is more than designed. Though I'm not very convinced if that's because of "short" or STOVL requirement as some suggested.

@megasun.... The transsonic acceleration statements released to press, might be just misinformation. I really can't see Lockheed Martin actually releasing that data publicly, why would they?

The DSIs in the F-35 and the lack of canted pylons provide a significant improvement in the F-35 aerodynamics, especially above M0.8

Flankers have respectable range, but I doubt they can perform airshow stunts with a full fuel load. The F-35 is an improvement in range over the F-16 and F/A-18E/F jets, but not designed to be an F-15 class long range strike interceptor