Fighter jets maximum sea level speed specifications question
 

Hello guys,

I have a question about maximum speed specifications as quoted for jet aircraft such as the F-16, F-15, etc.

When, for example, a resource lists a sea level "maximum speed" is this speed maximum because that's as fast as the aircraft's thrust allows it to go (drag limited), or is it because there is a redline speed limit such as the VMO or the VNE and the pilot has to back off from the power?

I am having a discussion with a friend and I think we have very different opinions of concepts such as "ram effect" on jet engines. He's under the impression that ram effect on jet engines dramatically increases thrust at all altitudes and as such would allow an aircraft such as the F-15 to simply blow through its supposed maximum speed at sea level necessitating a retardation of the throttles, but I'm of the belief that net thrust actually falls off with higher speed at low altitude, but begins to increase at high altitude and higher mach numbers because of the ram recovery theory.

Anyway, I'm not trying to claim to an expert on jet engines, but I would like to know if I see some specs on a fighter jet that says its maximum speed is *whatever* at sea level, that this is because this is as fast as the jet can go because of thrust concerns, or if this is purely a structural limitation.

Thanks!

It can be either. The Lightning was IAS limited at low level due to the probe, yet at high level it was one of the best supersonic machines. The Tornado F2/3, when clean, was initially unlimited-other than by total thrust over total drag as you describe. I have certainly seen 840 KIAS at 250 feet over the sea, apparently the speedo stopped reading at 999! The F3 was subsequently limited to a lower KIAS for "other" reasons. During this sort of thing the fuel gauge was a blur.

In your opinion, on more modern jets would you say the quoted maximum speeds are drag-related?

I say this because you mentioned older jets such as the Lightning and the Tornado that might have had some structural issues, but I might assume that newer jets like the F-22 and the Eurofighter are more-or-less free of these issues.

Intake design reduces engine airflow velocity to within the design parameters of the engine in question.

DAL,

I think it's fair to say that in general, Computer Aided Design and computational fluid dynamics have aided in reducing the drag of modern aircraft to a far lower level than earlier jet aircraft. Therefore, it might seem reasonable to assume that less of the thrust produced is required to overcome drag, and more is available for other purposes. However, it's a complicated field which most aircrew touch on briefly and never fully explore. Suffice to say that the tefal-heads work out the handling limitations of the jet and you stick by them!

However, it is worth remembering that the stores hanging from the aircraft in question will also have a flight envelope of their own to be adhered to, and this might be the limiting factor in how fast you can go!

Blunty.

I have heard that the limitations imposed by underwing stores are quite large. Apparently an F-111 can supercruise clean, but stores dramatically reduce its top speed and manouverability. I have also been told it has a far more dramatic effect on the F/A-18 as this aircraft has far less kenetic energy to start with. Not being an FJ pilot, I don't actualy know myself.

Probally worth a post in Tech log or FTE threads.:ok:

I think the answer is - "it depends". Every aircraft is different. To give a few examples:

The Harrier is generally LL speed limited by drag - that huge fan which is designed to produce a lot of thrust at low (no) speed is not so great at 450 kts.

A Tornado with, say, external fuel tanks is LL speed limited by an aircraft limit - the tanks can't go as fast as the aircraft or they drop off. Drag is definitely not the limit!

Most FJs accelerate faster the faster you go (basic jet engine theory) but above about M0.9, transsonic (or whatever they call it these days)drag becomes a factor - perhaps to the extent that the aircraft can't get above M1.0.

Also, as mentioned above, engine design is a factor. The Tornado F3 has moving intake ramps, which operate at about M1.3. Acceleration is noticeably better at M1.4 than it is at M1.2

Hope that helps.

Certainly the P109s help, but the planets really have to align to be able to supercruise in the Piggie! She'll do it, but not with the wings fully swept because of the alpha, but if you brings the wings forward a notch you might get lucky. As long as a Pig stays high and clean, she'll go all day - slow her down and load her up, and just watch the fuel gauge plummet...:eek:

Well yes, a brick will only fly so fast! The 404s are good donks but they're little more than noise makers if you're loaded up!

And some aircraft are limited by airframe temperature, some by intake buzz, some by flutter, some by canopy limits, some by aerodynamic issues not associated with drag – the list goes on. It is not unusual for an aircraft to be max speed limited by a different reason in each part of its operating envelope/different config.

As far as the F3/GR1/4 goes, IIRC (been a while… Got a manual buried somewhere but can’t be r-s-d to dig it out), the ADC only calculated up to 850KTS (IAS) and the analogue ASI only read up to 800 (obviously you have TAS, but I don’t think that’s what people are getting at here), but you had the word ‘KNOTS’ to the right of it (top of instrument) and I suppose if the needle was between the N and the O then you could probably interpret that as 850.

In terms of super-cruise, many of the legacy jets would do it – but only juuuuuuuuust… And put any significant stores on it and you’re stuck under the Mach in dry (level). Useful super-cruise is the other side of transonic

IAS
 

I was led to believe the Harrier ( particularly Seajet or maybe 2+ which is better at altitude ) even carrying large weapons is good for a lot more than 450 kts, and due to the nature of the Pegasus is not so restrained by stores, it's basically a very big engine fighting drag all the time anyway and doesn't mind much what you hang on it !

More educated comment would be welcome.

I was told by an ex-Lightning Test Pilot that it suffered intake overheating at around 650 IAS, and he'd " love to fly an F-16 " as it can manage considerably faster on the clock.

Also a Rolls Royce engineer I knew reckoned the Tornado GR1 had the highest recorded low-level IAS ( distinctly different to highest speed ) at over 850 - I stand to be corrected by those who actually fly fast jets, my record was 400 or so as a passenger in a souped-up Hawk !

On the other hand, a/c such as the B-1, despite it's supposedly de-rated engines from the original spec', seem to achieve Mach .999 at low level with great ease; I've seen this done and always suspected there's a 'speed limiter' option to prevent sonic booms alerting the bad guys ?

I wonder what the actual IAS of Concorde was - surprisingly low I suspect. Damn, I used to know a Flight Engineer on them, but he only mentioned transferring fuel like mad, and before that tragically exploding Vulcans ( buried engines not being such a great thing if they throw a blade into a fuel tank ).

What'll she do mister?
 

According to my Concorde flightdeck DVD, IAS in the cruise was around 480kts, with a Vmo of 530.

Groundspeed was considerably faster . . .

Double zero

I am not a harrier pilot, so I stand by to be corrected by someone who is, but I have shot/chased a lot of them in my time (and ok I think once one may have shot me:uhoh:) and your comments on the speed/stores is, I believe true up to the 450ish mark. Somewhere around there they hit a "wall". I don't think I have ever seen a harrier do 550, for example (not saying it can't).

They tend not to run away because they can't, and if you integrate them into a COMAO you have to take into account that they are appreciably slower than most other FJs.

Also would be surprised (and hurt) if the GR1 was faster than the F3 - the F has tweaked engines, the extra length makes it more stable trans/supersonic.

Not sure about the SHar but Harrier with tanks etc would max out at 520ish and that was simply a drag isssue. Anything above 480 and you were just wasting gas cos lets face it, everybody else could go much faster. Speed was only really an issue for ASD considerations.

Hopefully the original poster is thoroughly confused by now. Now read this:

http://www.codeonemagazine.com/archi...tch/index.html

Now consider that all these limits (low alt, high alt, different configurations or "we just didn't test it") apply to one aircraft.

By the way, as for ram effects: the problem is that sooner or later the temperatures in the engine reach a point where it (in practice) blows up or wears out. Above a certain speed (usually supersonic on a fighter engine) the engine actually has to throttle back to stay within compressor exit temp limits.

Lots of very accurate and valid answers, but all could do with simplifying:

As far as I know: ALL 'modern' FJ aircraft with AB (Afterburner) will exceed their 'Q' limit (Airspeed / Dynamic pressure Threshold) at low level in level flight. The F3 / GR4 Does, the F16 / F15 / F18 Does........ they are all basically built the same.... so? Everyone is right, kind of..... but the bottom line is:

The days of 'parking the fun lever full forward, and waiting'...are long over! and have been since the 60-70s

By the way, Very Modern a/c like Typhoon' are likely to have lower limits, due to temperature effects on their materials. (a jet at Sea Level, at 800Kts has a skin temp in the order of 160C):cool:

Hope this helps,

Advo

PS

As one poster noted: The F3 ADC does indeed stop at 850Kts! (Nothing bigger than 850 in the HUD) althought the Navs TAS (on the TV Tab) continues to read..... I would guess anyhow.....:E

There might be a viewer out there, who, may or may not have seen 900+ at LL (Where TAS=IAS or is as close as who cares)....... You Know Who You Are........ (Good luck mate, and love to the family)

Keep safe folks,

Advo

Harrier with tanks is limited by the tanks limit - 495no. Tanked jet will max out about 520 level with 105 engine. Clean aircraft limited to 575ne. Won't do it level. Stick a 107 engine in and it'll break all of the ne limits with ease. Not much point though - you'll still be chased down, better to turn and fight. (or avoid in the first place of course).

So lets cut to the quick (lol) here....in their natural operating environment, low level, clean, what was/is fastest F111/F3 ??:}

Harrier performance
 

Thanks to you all for the info; I realise drop tanks, especially the large ones currently trendy, are a different issue and enough to stop anything...

Seem to remember an FRS1 involuntarily 'lofting' a pair of ( actually smaller ) tanks into an asparagus bed !

Are the speeds quoted here Harrier 1 or 2 ? I know the AV-8B was originally around 100 knots slower, but had the impression progress has caught up with the gap to an extent at least - and surely the Seajet was a lot faster than 520 kts, transonic in a slight dive ?

Or am I getting as I suspect into the original question of this thread, and confusing max' speed with max' IAS ?!

As for the old stories about stores drag, there was the classic ( and probably true ) tale that a fully loaded Buccaneer carrying it's weapons internally was in fact faster than a Phantom with a similar load - now I wish they'd have thought of that with the F-22 & JSF - Oh, hang on...

The speeds quoted are for the Harrier II (GR7/9) - about 575kts or 0.98 Mach. However I believe it can go a little bit faster accidentally :O

The SeaJet was faster I'm sure due to it's (slightly) more aerodynamic nose and thinner and smaller wing. The GR9 is a faster accelerating jet with the 107 engine, but once it reaches about 0.8M with bombs/tanks on, or 0.95 Mach it hits the wall and doesn't want to go much quicker.