Speed Is Life

November 21, 2010 8:39 PM

The SR-71 Blackbird reconnaissance aircraft was designed to cruise at speeds in excess of Mach 3. But what's the slowest speed attainable by an airborne Blackbird?
posted by CrunchyFrog (49 comments total)

Stall speed?
posted by squorch at 8:45 PM on November 21, 2010

Let’s just say that the plane truly loved speed and effortlessly took us to Mach numbers we hadn’t previously seen.

That breaks my heart all over again.
posted by clarknova at 8:47 PM on November 21, 2010

Spare Parts...Get your Red hot
posted by clavdivs at 8:52 PM on November 21, 2010

The flight manual at SR-71.org suggests that when nearly empty, final approach speed was 175 kts and landing speed was 155 kts. So, despite the bluster, this is not too outrageous that they got down to 155 or so. I mean, I wouldn't want to do it, but it's still within operating limits (which is evident because they did not stall and crash).

I'm a little surprised at the approach speed; it's not drastically higher than most wide-body airliners.
posted by kiltedtaco at 8:56 PM on November 21, 2010

From reading that story, I think I would have been glad to have some Spare Pants, had I been there.
posted by idiopath at 8:57 PM on November 21, 2010

can someone explain the incident to me? What was so glorious about going to almost stall speed? Does it look cool or something? Were they really low? What does "falling in a slight bank" mean?
posted by spacediver at 9:00 PM on November 21, 2010

I'm a little surprised at the approach speed; it's not drastically higher than most wide-body airliners.

It's made of a lot lighter material, and the fuselage is also mostly lift surface. You'd think it would be even less.
posted by clarknova at 9:02 PM on November 21, 2010

Of course the SR-71 went slow; it's the slow version.
posted by NortonDC at 9:03 PM on November 21, 2010

Stalling close to the ground is scary since you don't have any altitude to recover, but these guys didn't stall. So, uh... Don't let your scan break down?
posted by squorch at 9:03 PM on November 21, 2010

True Story -- we had an Industrial Tech teacher in 7th grade who had a bit of a ... problem. Had a steel girder had smashed in his head, post-nasal drip... eccentric, to say the least. The poor man had kids pouring oil and steel shavings into his coffee (not by me, btw... not even my class, but I had heard from friends that another class did).

Anyways.

He had all sorts of classic sayings... Things like "A cube is a 6 dimensional object. It has 6 dimensions. A top, bottom, left, right, front and back"

I recall in my 7th grade industrial tech class he made the claim that the "Blackbird was the fastest plane on earth. It goes Mach 3. That's 3 times the speed of light." I corrected him "That's the speed of sound." He looked pissed off "No... It's the speed of light" (I'm thinking "No... that's Warp 3! Don't you watch Star Trek?")

He back in the late 90s due to some form of blood poisoning. My guess is that it had to do with the years of oil and metal shaving ingestions. Those kids, IMO, were accessories to murder.
posted by symbioid at 9:16 PM on November 21, 2010 [1 favorite]

Brian Shul is one of my favorite copyright examples. He has a lot of stories to tell, and based on the few I've read, he tells them well. I would love to read (and own) his book. But as the author, he decided to limit the print run and charge $495 per copy, so I will probably die never having read it. That's copyright for you. I respect his decision, but it sucks.

This is my favorite of his stories. (If the link dies, then Google the phrase, "we were the fastest guys on the block and loved reminding our fellow aviators of this fact.")
posted by cribcage at 9:20 PM on November 21, 2010 [17 favorites]

I suspect that the fact they were banking also contributed to how scary it was, since (IIRC) the further off level you are the higher your stall speed.
posted by Grimgrin at 9:41 PM on November 21, 2010

cribcage: That story was great. Thanks for sharing it.
posted by Lafe at 9:43 PM on November 21, 2010

Everyone is focusing on the wrong sentence of the FPP.

The SR-71 Blackbird reconnaissance aircraft was designed to cruise at speeds in excess of Mach 3.

My grandfather was a draftsman by trade, assisting with the buildout of an agricultural school into a major state university. As a member of that "greatest" generation, he had the privilege of working that same job from shortly after The War until his retirement in the late 70s. As a draftsman, you might imagine that he would be something of an admirer of engineering feats, and he was. But despite his longtime professional focus on architecture, he only ever conveyed one engineering artifact to us, his grandchildren: a scale model of the Lockheed-Martin SR-71 Blackbird.

The SR-71 was an engineering marvel, and in many ways a perfect machine in its time. Square in the middle of the cold war, the US needed (or perceived that it needed) a reconnaissance vehicle that could be deployed and withdrawn more quickly, and provide better photographic resolution, than an orbiting satellite. The SR-71 cruised at 80,000 feet, defined the term "jet black" against the near-total lack of atmosphere at that altitude, and peaked at mach 3 - more than 2,200 miles per hour (something like 6 times your transcontinental flight speed - LAX to NYC in just over an hour). If it was targeted by an anti-aircraft missile, its evasion strategy was simply to accelerate and outrun the rocket - it was nearly impossible to shoot down, and in fact was never shot down. I see from Wikipedia that they lost a helluva lot of them to accidents, however.

Along with some dinosaur books, the SR-71 was one of the main spurs to my interest in science when I was a kid. It flew faster and higher than any other jet, and it didn't shoot anyone down. It was retired from service before I ever took a calculus class, but its story was so great. It was one of those mid-to-late 20th century machines that just excelled so much it made you think human ingenuity could tackle any problem. I'm not surprised the RAF guys wanted to see it up close.
posted by rkent at 9:46 PM on November 21, 2010 [13 favorites]

I wish that happened in the age of cheap cameras and YouTube. It must have looked sensational.
posted by ambient2 at 9:47 PM on November 21, 2010 [1 favorite]

rkent, the SR-71 really is the slower and lower version of the A-12. Both have the same engines, but the A-12 is significantly lighter and smaller, letting it fly faster and higher than the SR-71.

Compared to the A-12, the SR-71 was about six feet longer, weighed 15,000 pounds more fully loaded, had more prominent nose and body chines and a two-seat cockpit, and carried additional optical and radar imagery systems and ELINT sensors in interchangeable noses.

With the added weight, the aircraft flew slower and lower than the A-12 or the YF-12A, but it carried more fuel and had a longer range. (source)

The SR-71 did serve much longer, and accomplished much more for the nation, but its A-12 predecessor was faster.
posted by NortonDC at 10:16 PM on November 21, 2010

What was so glorious about going to almost stall speed? Does it look cool or something?

Do you ever see big hawks or eagles just drifting slowly on thermal updrafts? Impressive in their grandeur, right? OK, take that, magnify it by a million and add flames and lots and lots of noise.

Planes fly because of lift, and lift is generated by the movement of air over the wings. To get movement of air, you have to, you know, move. To stall means to go so slowly that you're not generating enough lift to keep you in the air. When that happens, gravity does its thing and pulls you down. No lift and you become a very large brick.

They were essentially operating right on the edge of just falling straight out of the sky. The SR-71 was designed to go fast at high altitudes, therefore it didn't have a whole lot of surface area from which to draw lift. It drew lift from going fast with smallish wings. Other planes draw lift from going slower with really big wings.

To a viewer, this must have looked amazing -- the aircraft is not doing what it's supposed to do, not doing anything you've ever seen other aircraft do. And plus, this plane was sooper secret and looked like the goddamn Batmobile, so there's that.

What does "falling in a slight bank" mean?

Turning and losing altitude at the same time. One of the many ways to stall is to turn too sharply -- lose enough forward velocity and you'll get closer and closer to a stall speed.
posted by Cool Papa Bell at 11:02 PM on November 21, 2010 [5 favorites]

Fun fact about the SR-71: It leaked fuel. On purpose.

The plane was meant to handle enormous levels of heat in the atmosphere. When things get hot, they expand. Solution? Design the plane so it's "loose" at sea level and "tight" at high speed.

You've probably heard the SR-71 is a severe leaker, and I'll try to put this into perspective. Once LN2 is serviced a few hours prior to launch, the fuel system becomes pressurized, and that's when the real leaks start. Normally, about five or six steady fuel leaks (about the width of a drinking straw) show up coming from both inboard wings, falling about six feet to the ground. ... Why all the leaks? High temperature fuel sealant was especially designed for the SR-71, and there's no other substance known in existence to replace it. Once the aircraft is as cruise speeds, it tends to seal itself.
posted by Cool Papa Bell at 11:08 PM on November 21, 2010 [1 favorite]

I love the engineering that went into the SR-71. I love that the plane was built to leak fuel through parts of its skin because the skin would tighten up at speed. I love that they had to build a specialized cooling system that used the fuel as a heat sink because of how hot the inside of the windscreen could get (250°F!). And the engine air inlet designed with a retractable spike, to manage the shock wave at speed.

I mean:

The backside of this "normal" shock wave was subsonic air for ingestion into the engine compressor. This capture of the Mach 1 shock wave within the inlet was called "Starting the Inlet". Tremendous pressures would be built up inside the inlet and in front of the compressor face. Bleed tubes and bypass doors were designed into the inlet and engine nacelles to handle some of this pressure and to position the final shock to allow the inlet to remain "started". Air that is compressed by the inlet/shockwave interaction is diverted around the turbo machinery of the engine and directly into the afterburner where it is mixed and burned. This configuration is essentially a ramjet and provides up to 70% of the aircraft's thrust at higher mach numbers.

And this thing was designed in the 60s? This + the space program = awesomeness that always excites me to think about.

On preview, Cool Papa Bell beat me to the leaky bit. Christ, awesome.
posted by disillusioned at 11:19 PM on November 21, 2010 [3 favorites]

My wife's grandfather, a WWII flying ace and retired Air Force colonel, is a pretty amazing man. 90 years old, perfect vision, and sharp as a tack. Still plays jazz trumpet like a muhfuggah. Dude oozes honor, integrity, and class. Seriously, when I spend time with him, that "Greatest Generation" phrase summons itself, unbidden, and rattles around in my head with zero irony.

We toured the Hill AFB museum with him once, where an SR-71 is (of course) one of the centers of attention. Only after being specifically asked--because he had once been the second-in-command of Hill AFB, and had flown in many of the aircraft now on display there--did he casually mention that he had actually piloted it.
posted by rodeoclown at 11:48 PM on November 21, 2010 [9 favorites]

35 miles a minute...

Yeah, the SR-71 has always been badass.
posted by azpenguin at 11:53 PM on November 21, 2010

My favourite Blackbird story is the one about the navigational error and the light bulb.
posted by dansdata at 12:30 AM on November 22, 2010 [5 favorites]

spacediver: it looks cool* and requires testicles made of steel; its the sort of trick you see at airshows with sub $1M planes at high altitude, as opposed to a $1B black technological marvel near ground. Also, the article says they reached 152 knots, which is lower than the 175 minimum operating speed reported above.

(* it looks & sounds cool w/ an average aircraft;, with an sr-71 i guess "awe inspiring" is a more fitting description)
posted by 3mendo at 1:07 AM on November 22, 2010

thank 3mendo, but I'm still having problems visualizing what exactly happens. What's so exciting about a slow moving aircraft? Is the fact that it's hanging on the edge of catastrophe (i.e. about to stall) visually apparent in its movement?

And based on the article, I couldn't make heads or tails about its altitude. Something about an invisible field of grass that the pilots couldn't make out due to haze.
posted by spacediver at 1:34 AM on November 22, 2010

The SR-71 cruised at 80,000 feet

I knew they operated above FL 60, but did not realize it was as high as 80,000 feet. IIRC USAF Aviators who fly at 70,000 qualify for Astronaut wings?
posted by mlis at 2:14 AM on November 22, 2010

Brian Shul is one of my favorite copyright examples. He has a lot of stories to tell, and based on the few I've read, he tells them well. I would love to read (and own) his book. But as the author, he decided to limit the print run and charge $495 per copy, so I will probably die never having read it.

He's also a right-wing semi-literate moron. Save yourself $495 and just read his site.
posted by Wolof at 3:05 AM on November 22, 2010

Meh. It's not as cool as the MiG-31.
posted by obiwanwasabi at 3:44 AM on November 22, 2010

rodeoclown, your wife's grandfather is awesome! Thanks for the link. What a privilege to have someone like that in your family.
posted by madred at 3:58 AM on November 22, 2010

landing speed was 155 kts. So, despite the bluster, this is not too outrageous that they got down to 155 or so
uhm... he had just refueled and was most likely over the max landing weight. that is a slightly different situation.

spacediver: he was crashing. not enough air was passing over the wings to produce lift. he recovered by ading power just before he would have hit the ground.
posted by krautland at 4:03 AM on November 22, 2010

But what's the slowest speed attainable by an airborne Blackbird?

Is that an African Blackbird, or a European Blackbird?
posted by Naberius at 4:47 AM on November 22, 2010 [10 favorites]

My father was an aerospace engineer who designed planes for the Navy. There was a time, after spending a good chunk of my childhood at his office and many,many airshows, that I could ID just about any military plane that flew by.

In my dad's book, the SR-71 was the pinnacle of plane design. He finally got the opportunity to take a ride in one, and we heard the story about his experience a million times after that.

And, as an aside, "slow" is an important concept to fighter jet designers. Adjustable wings ( I think the real modern push began with the F-14) are designed to allow a jet to operate at slower speeds.
posted by Benny Andajetz at 5:29 AM on November 22, 2010

Variable geometry wings aren't used anymore due to the immense cost of keeping such a dynamic and critical part of your aircraft in working order. The F-15 still has variable geometry intakes, but that's about it. Fly-by-wire for inherently unstable aircraft is the current tech, with thrust vectoring being next-gen. (see f-22, f-35)
posted by squorch at 5:55 AM on November 22, 2010

Spacediver, the whole scale, very slow moving thing is a very awesome effect. Even watching MD-10's and A300's landing with some strong winds is pretty cool. Just seeing this plane that is huge moving around sharply to try and stay balanced looks not very natural and therefore is amazing.

I think the details that really make this sort of flyby stand out so much was that it was overcast so they couldn't see it coming. They couldn't find the airfield so they were flying slower and slower and had the engines down real low so the people at the field couldn't hear it coming. The critical moment being when it appeared out of the clouds right in front of them (it is pretty vague about how close they actually were, but it seems like it was way too close). At the very moment they could see it, the pilot sees the ground and tower and the speed and rams the throttle up, which if this silent alien bird just dropped out of the sky and all of a sudden roared away with all it's might I imagine I would probably get a huge rush without even having any knowledge of how dangerous it just was.

I remember one year I was at the Indianapolis 500 when the B-2 Spirit was the flyover. I was on the front straightaway so it would fly right over me and I couldn't wait. Well the day was very overcast with the clouds pretty low. Well the national anthem had ended and a few seconds went by and all of a sudden this monstrously huge (at least it seemed) delta wing appeared out of the clouds silently and once it was over my head this massive roar filled the air and everyone stopped talking and doing whatever and just watched it go down the straightaway and slowly bank away into the clouds. On that occasion I believe the delayed sound was merely from the design of the bomber so that even at slower speeds it doesn't project much sound forward as part of its stealth design.
posted by Phantomx at 6:13 AM on November 22, 2010

So many ambiguous 'they's but I'm sure you can figure out who was flying and who was observing =)
posted by Phantomx at 6:14 AM on November 22, 2010

Since we're lovin' on Blackbirds here, I'd recommend folks in the Seattle/Portland area check out the Evergreen Aviation Museum in McMinnville, Oregon. They've got a SR-71 with all the recently-unclassified recon gear and other guts exposed.
posted by RobotVoodooPower at 6:24 AM on November 22, 2010 [1 favorite]

I had the privilege of meeting one of the more senior engineers involved in the project a few years ago when he spoke at my Rotary club. The complexity of the task really humbled me. They set up fake bicycle companies to import Titanium from Russia (the major source of the metal), had to use the fuel as a coolant and all manner of things never done before.

The engineer stated that he offered up a $500 reward (out of his own pocket, no small amount in the 60s) for anyone who could something that wasn't extraordinarily complex about the plane. He never paid it out and said they had to rethink the build of the aircraft right down to the dials and buttons used in the cockpit.

I wish I could remember more. One thing for sure is that I left feeling astonished at how we used to build amazing stuff in relatively short spans of time.
posted by dgran at 6:38 AM on November 22, 2010 [1 favorite]

To stall means to go so slowly that you're not generating enough lift to keep you in the air ... lose enough forward velocity and you'll get closer and closer to a stall speed.

A stall is not due to low air speed. A stall is due to a high angle of attack (nose too high) which disrupts the smooth flow of air across the wing. A stall is only indirectly related to airspeed. You can have almost no forward velocity and still not be stalled because a stall depends on angle of attack, so an airspeed below 155 knots does not necessarily create a stall.

The lift produced by a wing is related to airspeed and angle of attack. Increasing airspeed or increasing angle of attack increases lift. If you decrease airspeed, you must increase the angle of attack to compensate and produce the same amount of lift. If you decrease the airspeed too much and attempt to produce the same amount of lift, you may increase the angle of attack to the critical stall point.

Lift is created perpendicular to the wing surface, that is, straight up when in level flight. When in straight and level flight, the wings must provide lift equivalent to 1 g to support the airplane. When banked in a turn, the lift is still perpendicular to the wing surface, but because of the bank, the lift is not straight up but instead at an angle. If you resolve the tilted lift vector into its vertical and horizontal components you find that there is less vertical lift. Therefore you need more total lift to support the airplane. To increase the lift while in a turn, you must either increase airspeed or increase the angle of attack.

For example, in a 60 degree bank, if you draw it out, you can see that the vertical component of lift is cosine(60) = 0.5. That is, in a 60 degree bank, the vertical lift is only one-half the total lift. In order to keep the airplane from descending in a 60 degree bank, the wing must produce lift equivalent to 2 g. This is true for any airplane -- SR-71 or Cessna -- 60 degrees = 2 g.

If you do not provide sufficient lift, you don't stall. You simply descend. That is the state described by the pilot. He was at low speed, which produces low lift. He also was in a banked turn which requires more lift. The lift produced at that airspeed and angle of attack was insufficient to maintain altitude so the plane was in a banked descent, not a stall.

The pilot makes no mention of the stick shaker or stall horn which means he was not in imminent danger of a stall, despite the low airspeed. He would have been in danger of a stall if in reaction to discovering the rapid descent he had tried to abruptly increase lift by pulling up the nose (increase the angle of attack) to increase lift. Instead, he did the proper thing which was to increase lift by increasing airspeed -- he fired up the afterburners.

The point is that low airspeed does not necessarily create a stall. Low airspeed creates less lift and that is how an airplane descends and lands. It does not create a stall. The so-called stall speed is just a number that indicates the minimum speed at which you can maintain altitude in level flight. Below that airspeed, in order to maintain altitude, the angle of attack (nose up) will be too high and a stall will occur. An airplane can still fly at well below "stall speed" if it keeps the angle of attack below the critical stall angle. The airplane will simply descend because it is producing less lift than the equivalent of 1 g.
posted by JackFlash at 8:25 AM on November 22, 2010 [6 favorites]

Nice explanantion, Jack. You a CFI, perchance?
posted by Thistledown at 9:12 AM on November 22, 2010

I'd never get a pilot's license (some unfortunate family history behind that), but nevertheless, a phrase like "As we scooted across Denmark in three minutes" still makes my heart beat a little faster.
posted by Halloween Jack at 9:33 AM on November 22, 2010

I love that we have the POH for the SR-71. Page 6-4 says

There is no stall in the classic sense where an abrupt loss in lift would occur at a critical angle of attack. Instead, a nose-up pitching moment develops as angle of attack increases, which becomes uncontrollable (even with full nose-down elevon) as the critical angle of attack boundary is reached. ... The SAS will tend to maintain apparent stability about all three axes until pitch-up occurs, then aircraft control is lost with little or no warning

There's more there about "engine stalls" at subsonic speeds, but I think that's a different (more frightening!) problem.

Stalls in a little airplane are generally benign; you lose lift, the nose drops, you push it down until you get enough angle of attack / airspeed that the wing is flying again. Student pilots practice this all the time completely safely and you can recover something like a Skyhawk after dropping about 500'. Sounds like the SR-71 had a whole different failure mode at high angle of attack. OTOH you're also sitting on a rocket and can just thrust your way out of trouble, as the story above.

You know how they send fighter jets out to intercept little airplanes that blundered into restricted airspace? Those poor jet jockeys have to throttle back and escort planes that may be flying < 90kts. They must hate that.
posted by Nelson at 9:35 AM on November 22, 2010

I currently work in Kalamazoo, michigan, and its Air Zoo also houses an SR-71B, among many other awesome aircraft.
posted by Gelatin at 10:54 AM on November 22, 2010

I can't get to page 2. =(

(Tried Chrome AND FF)
posted by Eideteker at 10:56 AM on November 22, 2010

I can't get to page 2.

Same here. Something must have busted.

Luckily, Google finds many copies of the story.
posted by CaseyB at 11:05 AM on November 22, 2010

The fuel-leak is apparently so severe that the Blackbird leaves a "shadow" on the ground when it leaves the hangar. Somehow that just adds to the mystique.

My favourite Blackbird story is the one about the navigational error and the light bulb.

My god, that link says the SR-71 had a refrigerator-sized computer, programmed with punch-cards. Such an amazing machine.
posted by lekvar at 1:44 PM on November 22, 2010

Phantomx, thank you :)
posted by spacediver at 2:07 AM on November 23, 2010

There is a SR-71 on the parade ground at Lackland AFB. I was shocked at how small it was.
posted by Daddy-O at 6:16 AM on November 23, 2010

Daddy-O, they've got one at the Air and Space Museum Annex at Dulles in DC. It doesn't look so small sitting next to a MiG-21F. Each engine is about the same size as the MiG. Of course, to be fair, the MiG engine is also about the size of the MiG.
posted by atbash at 11:15 AM on November 23, 2010

I just always imagined it to be a lot bigger.
posted by Daddy-O at 5:40 PM on November 23, 2010

We have a Blackbird at the Flight Museum in Seattle as well -- actually, the M-21 variant of the A-12, the one with the drone, built in 1963. My boys always make for the Blackbird first...even though they're 16 and 20 now.

The thing is frickin' huge...it dominates the floor level of the Great Gallery, and that's no small space.
posted by lhauser at 6:14 PM on November 24, 2010