F-111 mach 3?

[Scrape]

Recently I made a post about the top speed of the F-111 being a mach three jet. Some of you may remember that...

 

I was wrong...:doh:

 

I talked with older pilots who have had flight experiance with the airframe and they broke it down for me. They said it was pretty tough for the 111 to get past mach 2 compared to most jets. It was explained that the engines (I'm not sure if this differs from other countries or if they use the same engines) but as far as the US version was concerned the engines were designed for low level flight. Down low the F-111 ws said to be "wicked fast" and there wasn't a fighter that could keep up with it.

 

I heard the following numbers from one colonel and a major with whom the 111 was thier first jet earlier today. It could reach 1.7 mach at 100 feet or 30 meters Above Sea Level. Most fighters like the F-15 are designed to perform well at high altitudes and so they were outpaced down low in thicker air. This limitation is largely based on engine design and not the airframe.

 

Playing a bit of devils advocate on my behalf I asked if it were possible to max out at high altitude then dive and reach mach 3. They hesitated and said maybe, and they were sure the airframe could take it, but they didn't know of anyone who tried. 111s went up high to get fuel or for ferry flights, but for mission training it was mostly low level NOE type flying staying within the engines designed performance envelope.

 

If you were wondering where I got mach 3 number from my first supervisor recieved a ride in the 111. He described a scenario where the pilot maxed out at 30,000 feet about 9100 meters rolled inverted and pulled into a dive. He explained watching the mach needle hit three and how the pilot laughed at him when he freaked out. He was a man who never indulged in embellishment so I believe him, but I was still wrong about the 111 being a mach three jet.

 

:notworthy:

[GGTharos]

If you dive the aircraft to mach 3 you WILL fry the engines well before you hit that number.

 

Even in the F-15, going up above mach 2.3-2.4 is a 'time limited' operation where you're allowed to exceed this speed for a minute or so before requiring engine replacement. Keep it there a bit longer and they'll fry on you in flight - we're talking engine fire.

[arneh]

Recently I made a post about the top speed of the F-111 being a mach three jet. Some of you may remember that...

 

I was wrong..

 

:thumbup:

It's not often you will hear someone on the internet admit they were wrong :)

 

 

Down low the F-111 ws said to be "wicked fast" and there wasn't a fighter that could keep up with it.

 

I heard the following numbers from one colonel and a major with whom the 111 was thier first jet earlier today. It could reach 1.7 mach at 100 feet or 30 meters Above Sea Level.

 

The F-111 is certainly among the contenders for the fastest aircraft ever a low level, but I do have to be very sceptical about the 1.7 mach number. What I've seen other places is from about 1.2 to 1.3. And other aircraft which are designed to be really fast at low level (like the Tornado, Su-24 and F-104) max out at around 1.2. That the F-111 should be able to do 0.5 faster than anything else at that low level where the drag is immense sounds unreasonable.

Also the official world record at low-level was set by a modified F-104 at 1,590.45km/h, which is around mach 1.3. Of course millitary aircraft don't always compete in such contests, but if they had something which shattered the record then I'm sure they would...

 

Playing a bit of devils advocate on my behalf I asked if it were possible to max out at high altitude then dive and reach mach 3. They hesitated and said maybe, and they were sure the airframe could take it, but they didn't know of anyone who tried.

 

I have to be sceptical about mach 3 in a dive too. The temperatures at such speeds is immense, and the few aircraft that can do it have been specially designed to withstand it.

After having read quite a bit about the designs of the SR-71 and XB-70 and how they were designed around flying at mach 3, and all of the problems they had to overcome and build into the design to achieve it, it seems really implausible that other aircraft that weren't designed for it can just accelerate or dive to mach 3, just because they have the engine power for it.

Admittedly the SR-71 and XB-70 were designed to cruise at mach 3, not just a quick dash before decelerating, so that is more demanding.

Edited May 21, 2009 by arneh

[Mugenjin]

I'm by no means an expert at this, but iirc even mig-25, which could go beyond mach 3, pilots weren't advised to go over 2.6-2.7 mach, since anything above would seriously damage the aircraft.

So it may be possible for the 111 to reach mach 3, but then it'd over.

[GGTharos]

The MiG-25 could suffer 'run away acceleration' of the engines above certain speeds, IIRC - that means you couldn't rev them down.

[Panzertard]

Scrape, never saw your initial discussion, still - kodus for coming back and clearing up your own interpretation of the issue. ;)

Disclaimer: This is all my own conclusion - with no reference to any pilots or designers notes anywhere

 

Airframe temperatures and stress.

Just to throw in a few more opinions, I assume the F-111 being built for low level high speed is built to withstand the highpressure/turbulence of NOE.

It means it's rugged - and can "take a beating".

 

However, I would speculate that it can't cope with higher temperatures > 2-3 Mach for sustained periods.

If that was supposed to happen, it probably would need another type of design to withstand the temperature contraction and expansions, like the SR-71. Such a design of airframe may be more flexible, and logically less robust.

 

Summary of speculation; That would more or less cancel out the ability to go low and fast.

Edited May 21, 2009 by Panzertard

[spyda]

interesting read i have no idea myself,

 

used to see them fly over once and a while when i was living in brisbane, Australia. they sound great they have this real loud screeching noise when they fly by

[Scrape]

I had no reason to believe that the pilots were hamming it up with story telling about the 1.7 low level mach because it wasn't the atmosphere and they are more professional than that at work.

 

A C5 corvette wasn't designed to drive at speeds above 200mph but it can, with modifications, and it won't explode if it does so its still a high performance machine.

 

Yes the stress on an airframe at mach three is significant, but not impossible. The jet wouldn't explode and neither would the engine. Its not like going past the redline with a car engine. Doing 1.7 mach at 100ASL is mighty tough on an airframe, much more so than mach 2 at high altitude.

The wing spars and frame of the F-111 is beastly compared to other fighters, it was very robust, but I'm not carrying the torch of my old argument. The pilots said maybe, and I'll leave it at that.

 

To my understanding one of the Mig-25's problems was couldn't control its shockwave properly which would cause the damage from high speed flight. Around mach three the shockwave from the sound barrier would start to angle down the intake and contact the engine blades creating severe turbulence in the compressor stage and damaging the engines. Not all aircraft had this problem. The cone protruding from the SR-71 engine intake is on a track that allows it to manipulate the shockwave. The F-15 intake changes shape for this same reason.

[Scrape]

The MiG-25 could suffer 'run away acceleration' of the engines above certain speeds, IIRC - that means you couldn't rev them down.

 

 

Do you mean a compressor stall?

 

Cutting fuel always does the trick, and that's connected to the throttle, could you explain?

[Scrape]

Scrape, never saw your initial discussion, still - kodus for coming back and clearing up your own interpretation of the issue. ;)

Disclaimer: This is all my own conclusion - with no reference to any pilots or designers notes anywhere

 

Airframe temperatures and stress.

Just to throw in a few more opinions, I assume the F-111 being built for low level high speed is built to withstand the highpressure/turbulence of NOE.

It means it's rugged - and can "take a beating".

 

However, I would speculate that it can't cope with higher temperatures > 2-3 Mach for sustained periods.

If that was supposed to happen, it probably would need another type of design to withstand the temperature contraction and expansions, like the SR-71. Such a design of airframe may be more flexible, and logically less robust.

 

Summary of speculation; That would more or less cancel out the ability to go low and fast.

 

For that speculation we'd have to know the temperature difference of the speeds mentioned. An object traveling at mach at sea level is going to be hotter than an object at 50K ASL traveling at the same speed. More air molecules at lower alt. means more friction, and that means more heat...doesn't it?

 

The SR-71 uses an unusual design but was also designed a long time ago, and much has been learned from its weaknesses. Looking at the technology available and the design of the airframe from an aeronautic point of view it probably maxed out closer to mach 6. (There's my speculation) The X-15 A2 achieved 6.2 mach before the space race began and it didn't use the corrogated design style that the SR-71 did to the same extent. Alloys have become more advanced since then, so maybe a maybe is appropriate.

 

NASCARs are more robust and much heavier than F1 cars, but they easily cruise at high speeds with comparable horsepower, so there is more to it wouldn't you agree?

[Vault]

The F-111 has an aluminium airframe, aluminium starts to deform from the physics of creep at mach 2.61, this is the reason why the SR-71 has a titanuim alloy airframe. The F-111's performance will be limited to the very maximum of mach 2.61.

[GGTharos]

Nope, not a compressor stall. Uncontrollable engine over-rev/over-temp.

Apparently it had something to do with the MiG-25's engines originally having been made for cruise missiles, but don't quote me on that - I barely remember.

 

Do you mean a compressor stall?

 

Cutting fuel always does the trick, and that's connected to the throttle, could you explain?

[arneh]

I had no reason to believe that the pilots were hamming it up with story telling about the 1.7 low level mach because it wasn't the atmosphere and they are more professional than that at work.

 

I won't speculate about how the 1.7 number came about, but it's just so far beyond any other number I've heard that I can't take it seriously.

 

Other F-111 pilots talk about speeds around 1.1 to 1.2 at low level. E.g. there are several stories by F-111 pilots at http://www.fb-111a.net/Storybook.html which talk about those sorts of speeds.

 

And in one exceptional circumstance one F-111F pilot told about doing 890 kts at 3500 ft (i.e. not quite low level, but still low), which was mach 1.37. That was so exceptional that he took a photo of the instruments to have proof that he went that fast. That the ones you talked to went 25% faster at lower altitude in the same aircraft doesn't seem very likely to me...

 

That's not to take anything from the F-111, it's certainly one of the fastest aircraft ever at low level. But exageregating its performance doesn't help, and isn't necessary, it's impressive enough as it is.

[arneh]

The SR-71 uses an unusual design but was also designed a long time ago, and much has been learned from its weaknesses. Looking at the technology available and the design of the airframe from an aeronautic point of view it probably maxed out closer to mach 6. (There's my speculation) The X-15 A2 achieved 6.2 mach before the space race began and it didn't use the corrogated design style that the SR-71 did to the same extent.

 

 

The X-15 was basically a manned missile. It was built from a special heat-resistant steel alloy called Inconel-X which kept its strengt up to 650°C. It only flew for a few minutes at a time, and it used liquid nitrogene for cooling that time. Still the surface temperature reached over 700°C in places. But since it was just momentary it could stand it.

 

For the SR-71 skin surface temperature of around 200°C was a huge problem. I seriously doubt it could take the heat at mach 6.

 

BTW, the maximum speed the X-15 reached was mach 6.70, but some reports of damage from that flight:

Ball nose temporarily froze at high temperature

Extensive heat damage (burnthrough) to the pylon and ramjet

Heating failed on control-gas line so residual propellant could not be jettisoned

 

Later calculations showed that temperatures where the extensive heat damage was reach 1500°C!

Edited May 22, 2009 by arneh

[AussieFX]

I won't speculate about how the 1.7 number came about, but it's just so far beyond any other number I've heard that I can't take it seriously.

You've never seen a dump'n burn then?

It's quite impressive if you've never seen one. It is accomplished with a massive fueldump into the tailpipe.

[nscode]

but what does it have to do with anything :D

 

it's good if you want to bbq some meat, but it's not gonna make you go any faster :)

[Vekkinho]

I wouldn't bet on that, mach 3 is a bit too much for a F-111, I'm not saying impossible but it would cause some serious engine and airframe damage so basically it would turn it into a write off!

[AussieFX]

but what does it have to do with anything :D

 

it's good if you want to bbq some meat, but it's not gonna make you go any faster :)

Time to read up on your physics. :book:

[GGTharos]

Physics dictates that the dumped fuel will lighten the aircraft and thus it will go just a smidgen faster.

 

Sadly, your implication that this provides additional thrust is sorely mistaken ;)

[RvETito]

The MiG-25 is rated at Mach 2.86 sustained (~3000km/h at alt). It has flown faster but I assume only in certain conditions like test flights and stuff. The engine intake panels which form the shock waves work rather good actualy, you can see the same system on the F-15 and F-14 later on. The R-15 engines are designed with very weak compressors- only 5 stages I think, and the engine is supposed to give maximum thrust at high supersonic speed where 80% of the air compression occurs in the air intake. Almost like a ramjet. That's why the MiG-25 has been so unsuccessful when tried to fight at subsonic speeds.

 

Regarding the service of the MiG-25RBT in Bulgaria I've never heard of any time limits regarding Mach number, except for fuel cunsumption- at that speed the engines eat the fuel like crazy. Oh yeah, one more thing- at Mach 3 the turn radius is such that my country has barely fitted that beast:D

 

Materials are also not an issue since the MiG-25 aiframe is made of welded stainless steel exclusively.

Edited May 23, 2009 by =RvE=Tito

[Scrape]

BTW, the maximum speed the X-15 reached was mach 6.70, but some reports of damage from that flight:

 

 

Later calculations showed that temperatures where the extensive heat damage was reach 1500°C!

 

NASA's goal for the X-15A2 was mach 8 in atmosphere and there was damage done at mach 6.7 cutting the test short however it was a relatively minor design flaw and the engineers could solve the problem. They were excited and confident that they'd reach their goal, but then the Mercury project began and NASA wasn't allowed any buget for any project out side of landing on the moon. Using the lessons learned as a stepping stone and lessons learned from the A-12 could it be that this problem had been solved to some extent?

 

SR-71 wings do have to be replaced after three top speed runs due to heat stress so the aircraft isn't immune to heat. uuhh BTW

Edited May 28, 2009 by Scrape

[borchi_2b]

nicer reading here, and alot of speculations.

 

when i read the first post i really thought, what an idiot, but well, there are circumstnaces that might give the f111 the possibility to travel m 1.7 at low level.

 

first to the design of this bird.

it is, from deigners point of views and physical calculations able to got about mach 1.7

 

what would concearn me most are the shockwaves and airpressure produced at such alt.

one thing that can be said about the f111. at low level it was at the alt it was designed for and it had the most stable flightpath of all planes designed to fight low level. 2nd in line is the tornado, which has an awesome design for low level abut not quiet that good like the f111.

 

one thing is sure, the engines of the f111 were very very strong, by far stronger then the once of the tornado, which can fly mach 1.2 at low alt, without any trouble.

 

an tornado pilot of the german air force once said, when the eurofighter was close to compleation, that they could, after repositioning a small box in the rear of the tornado, use the new eurofighter engines, cause they were designed to fit into the tornado.

the conclusion of the german air force testcenter was, that the tornado could exceed mach 1.4 at low level with these engines.

 

now back to the f111. i do not believe that it could go mach3 at low level but a speed of 1.7 is a number that is not impossible.

 

guys, just think about the fact, that most of us only know facts from book, that are controlled form the militaries in this world.

nobody ever thought that an f15c could withstand a max g of about 40 either, but it was taped and recorded by an f15 driver when he got into a no SA situation and pulled down instead of pulling up. his flight was recorded by the hudcam and the cam failed to record at about 32 or 38 g´s. cannot remember, have to see if i find the video again.

 

no matter what, i believe that untill it is not porven to be impossible, it stays possible for me that the f111 could go m1.7 at low level.

it has the deisgn and also the enignes to got faster then any other bird at that alt ever before

[GGTharos]

nobody ever thought that an f15c could withstand a max g of about 40 either, but it was taped and recorded by an f15 driver when he got into a no SA situation and pulled down instead of pulling up. his flight was recorded by the hudcam and the cam failed to record at about 32 or 38 g´s. cannot remember, have to see if i find the video again.

 

I remember. It was 15g ... not 30, not 32, not 40, 15.

[joey45]

the airframe can withstand more Gs then the human holding the stick...

[borchi_2b]

hey ggtharo, please do not correct people when they know more about a plane then you abviously do.

sometimes it is even for you better to not write anything.

 

this what i talk about was recorded on the hud cam, no harder evidence can prove me wrong, not even the biggest dick in the world.

 

the pilot himself did not even realised that he pulled so many g´s. he actually passed nearly out, but the fear of death kept him awake.

 

i am still surching for the tape though