"Burble" and "mush" in flying/aerodynamics: little more info

Factual Questions
1

Just saw a very informative and clear, even to me (!:)) pilot-training intro film from WWII era on the A-26 Invader.

The pilot talks about “burble” and “mush” in situations of handling during a stall or near-stall situations.

I understand them individually as the way the controls will handle (and with burble, the entire airplane responding to the flutter of the wings?), and it’s nice the way it’s described from the human sensory standpoint.

  1. Do pilots still say that? I don’t recall it ever being tossed around in the many SD threads with the resident flyboys. [1a: Do they still say “fly boys?”]

  2. Everything that flys can stall, but in some modern jets or configurations (I don’t know, a B2 maybe) will these two effects be buffered out by the control software as direct actual physical “signals” to the pilot?

  3. Just to be complete for, e.g, the level of short dictionary entry, what word or words would best summarize the two situations as aerodynamic descriptions?

ETA: The word “burble” is a new one to me, and I like it a lot. Can’t think of what else it would just fit. Stomach rumbles from indigestion? Any others?

2
  1. Both terms are still used.
  2. As a general matter, fast swept wing airplanes do not have benign stalls like straight wing low speed WWII airplanes did. It’s far more important to stay well away from the stall or even near-stall environment in swept wing airplanes (setting aside fighters & such). Actually getting a bizjet, airliner, military transport, or bomber to the aerodynamic stall is a very bad idea.

So fly-by-computer airplanes will have envelope protection systems that will prevent the airplane from getting close enough to the edge to feel the actual aerodynamic burble.

Lesser bizjets and heavies will have artificial warning systems intended to fire off before the burble starts. If those fail or are ignored you’ll get to feel it.
3) “Burble” is a gentle rhythmic turbulence-like buffeting or vibration of the whole airplane caused by turbulent flow along part of the upper wing surface that’s physically shaking that part of the plane. And hence shaking the whole plane at least some. In a low-speed straight-winged airplane it feels kinda like driving over the rumble strips they have along the edge of the freeway to warn you that you’re drifting into the shoulder. But more like how those feel at 30mph than at 75. A shake, not a buzz.

FYI - “flutter” has a specific aerodynamic / structural dynamic meaning. Your use of it as a layman’s descriptive term was apt at that level, but dead wrong at the technical level. In a burble, the wings will be vibrating, but not “fluttering”.

“Mush” is used in a couple of different but allied senses.

  1. In any non-computerized airplane as you get slower the controls become less responsive. It takes more and larger pushing and pulling to make the nose move. At the same time, for non-powered controls as found in current lightplanes and WWII planes, the feedback is softer. At high speeds it feels like stirring a pot of fudge. At low speeds it’s like stirring a pot of beer foam. The control impression changes from a direct connection where you push against typical resistance and the airplane moves as expected to instead you push against little feedback and the airplane slowly wallows sorta where you’re trying to herd it towards. The polar opposite of “crisp” is “mushy”.

The other sense relates to power available versus power required. Which requires some background explanation.

Maintaining altitude requires different amounts of power at different speeds. The faster you go towards max speed, the more power it takes to do that. Eventually at max power you’ll discover the max level flight speed for the conditions you’re in. The only way to go faster from that condition is to start downhill.

So far, so like a car. Now it gets more complicated.

Because of the nature of aerodynamic lift, the *slower *you go below a middling speed, the more power it takes to hold altitude. In essence as you slow down the wing becomes less efficient and you need to make up for that decreasing efficiency with more power.

Depending on how powerful your airplane is, you may have enough power and then some to hold altitude while slowing all the way down to just above a stall. Or you may find that you run out of power before you get slowed all the way to stall. But unlike the high speed case that’s not stable. You’re slow and have insufficient power to maintain speed. What happens next is you continue to slow uncontrollably until you stall. Or you augment your power with a descent. Sliding downhill is another form of power to let you maintain speed above stall.

An unstoppable descent caused by lack of enough power at low speed is also called a “mush”. Near the ground a “mush” sometimes turns into a “crash.”

Note that both forms of mush occur together when a simple airplane gets slow. First the controls get increasingly uncrisp = mushy. Then if carried far enough beyond your power available, you completely lose the ability to maintain altitude and *will *descend gently or less so until something changes. Inherently low powered airplanes or even high powered airplanes when operated at extreme high weights, temperatures, and/or high altitudes are prone to mushing.

3

From Jabberwocky, 1871:

4

Although it’s one of the few words in that stanza that is not Carroll’s invention.

5

LSLGuy: Beautiful, as usual.

markn+: :smack: Of course! And that’s why I fully understood the feeling he was conveying, but with a transposition to …

Not sure right there, in the poem, if the sound or physical activity the Jabberwock is being pointed out. Not unlike whifflling. Interesting places, bulgy woods.

6

More SD for me.

OED:

ˈburble, v.1 Obs.

Also 4 burbull, (5 brobill), 6 burbyll, -bul.

[Found c1300. There are several similar forms in Romanic: It. borbogliare to make a rumbling or grumbling noise, Pg. borbulhar, Sp. borbollar to bubble forth, also mod. Picard borbouller to murmur (Diez); all apparently imitative words, though Diez thinks the Sp. and Pg. possibly formed on L. bulla bubble. The Eng. word can hardly have any actual connexion with these, exc. as a parallel onomatopœia, expressing the sound made by the agitation, issuing forth, or flowing of a liquid mixed with vesicles of air or gas. Of this the later bubble appears to have been either a simple variant or a conscious modification. In the later use of burble there is more of the notion of flowing than in bubble, as though burble combined the notions of bubble and purl; but the n. burble was in 14–16th c. exactly = L. bulla ‘bubble’.]

  1. a.1.a intr. To form vesicles or bubbles like boiling water; to rise in bubbles; to flow in or with bubbles, or with bubbling sound.

1303 R. Brunne Handl. Synne 10207 As þoȝ here yȝen shulde burble out. c 1440 Promp. Parv. 56 Burblon [1499 burbelyn], as ale or oþer lykore, bullo. 1470–85 Malory Arthur x. ii, A fayre welle, with clere water burbelynge. 1530 Palsgr. 459/2 To boyle up or burbyll up as a water dothe in a spring, bouilloner. 1577 W. Vallans Two Swannes in Leland’s Itin. (1759) V. 10 To Whitwell short, whereof doth burbling rise The spring, that makes this little river runne.

b.1.b To form bubbles in water, etc., to gurgle; cf. burl v.2

c 1400 Destr. Troy 5760 Hom was leuer‥be brittnet in batell, þen burbull in the flod. c 1440 MS. Lincoln A. i. 17 f. 115 (Halliw.) Many a balde manne laye there swykede, Brobillande in his blode.

7

I suspect that "babbling brook"s are actually some 18th Century poet’s corruption of "burbling brook"s. Or else a 20th century elementary school teacher’s corruption of same.

It may have also been a clever metaphor, where he likened the soothing stream noise everyone called burbling to the incomprehensible but somehow soothing babble of a baby. They’re both sort of the audio equivalent of fire: always changing and yet always the same.

When this hypothetical piece was written, everybody got the metaphor. In the intervening centuries we remembered the babble description and forgot the burble source of the clever word play.
The burbling flow over a nearly-stalled wing is very similar to the turbulent flow downstream of a rock in a brook or stream. In a stream you have the air/water interface which induces a bunch of noise you can hear. in a wing there’s just air, but the fluctuating local pressures induce similar noises into the structure.

Which is almost certainly where the term came from.

8

You have to love the English language. I know nothing about aeronautical engineering but just from the title I imagined what those two words might mean in relation to how a plane might “feel” and LSLguy describes exactly what I imagined.
Is there a term for when a word “feels” like the phenomenon it describes? Onomatopoeia is for sounds but this is more tactile than that (as when believe Hugh Laurie coined the term “spoffle” for the spongey thing on a microphone)

9

I finally got around to watching Leo’s video. Great stuff. Both cool nostalgia and clearly explained aviation tips.

10

Funny the way memory works and creative assonation works (not sure if that is a word before this). It was on the tip of my tongue, and last night I remembered where I had seen it: in Nabokov, unsurprisingly:

ˈborborygm Med.

? Also in Lat. form. borborygmus.

[ultimately ad. Gr. βορβορυγµός, f. βορβορύζ-ειν to have a rumbling in the bowels. In 16th c. F. borborygme (Cotgr. -igme).]

See quot. 1719. Also fig. Hence borboˈrygmic a., characterized by borborygms or rumblings.

1719 Glossogr. Nova, Borborygm, a rumbling Noise in the Guts. 1794–6 E. Darwin Zoon. (1801) II. 530 Hence the borborigmi, or rumbling of the bowels. 1880 Beale Slight Ailm. 97 Borborygmi‥are a serious annoyance. 1927 H. G. Wells in Sunday Express 1 May 12/4 Elephant hunters say that they can tell the proximity of a herd by the borborygmic (see dictionary) noises the poor brutes emit. 1928 A. Huxley Point Counter Point xi. 170 The stertorous borborygms of the dyspeptic Carlyle! 1938 Times 24 Nov. 12/3 The borborygmic note of the Arabian camel. 1959 E. Fenwick Long Way Down vii. 54 The room was very quiet, except for its borborygmic old radiator.

Great rhythm in “The stertorous borborygms of the dyspeptic Carlyle!”

11

I’ve actually experienced the burble in the KC-10 once (the flight manual calls it the “stall buffet”). Bad, bad feeling, especially when you know what the implications of that buffeting are.

IIRC our gross weight at the time was around 540k. We were the lead tanker in a multi-tanker cell, dragging a flight of F-16s to the desert; while we were getting topped-off on fuel from a “force extension” KC-135, their autopilot kicked off, and as the -135 is notorious for, it dove into us. Our copilot executed the breakaway procedures a little *too *well, and kept the throttles at flight idle a few seconds too long for our weight. We all heard and felt the airframe rumble for a couple/few seconds, and I think all four of us on the flight deck yelled “POWER!” at the same time. Interestingly, while debriefing the incident after we finished getting our gas, none of us could recall the stick shakers going off.

12

I’ve not had the “privilege”, but I’ve talked to other of our pilots who have. While riding in the back as passengers. As you say, not fun.

The MD-80 was/is famous for this. Under increasing AOA the system should trigger an impending stall warning light, then a horn, then a stick shaker, followed by aerodynamic buffet and finally aerodynamic stall. The problem is the warning system is calibrated and certificated for low altitude low Mach operations. There’s no FAA requirement that it work seamlessly at all points in the flight envelope.

Turns out with that airplane (like the KC-10, another upsized & up-weighted Douglas fuselage attached to a non-upsized wing and barely upsized engines) at high altitude with high Mach at low IAS the warnings occur in another order: aerodynamic buffet, aerodynamic stall, warning light, horn, stick shaker. Oops. :smiley:

Pressing up near the max altitude for current weight & ambient temp didn’t leave much room for airspeed excursions.

13

Gliders likewise will buffet and controls get mushy when a stall is imminent. The terms we use are “buffet” (shaking), “mushy” (how the controls get when plane is about to stall), and “incipient stall” (a stall just beginning to happen).

I’ve heard “burble” used to refer specifically to the turbulent airflow along the upper surface of the wings as the airflow separates (delaminates) from the wing; “buffet” refers to the resulting shaking of the whole aircraft.

We get the six signs of impending or incipient stalls drilled into our skulls:

From here among many other sources you could find on-line.

I posted a 28-minute video of a glider flight last December. Here it is again. (Yes, that’s me in the multi-colored floppy hat.) In the middle of this flight, we play with some stalls. (Sorry, I can’t display videos on this machine to find the exact minutes and seconds into the video.)

First I do a slow stall. Then Dan (the back-seat pilot whom you hear but don’t see in this video) takes the controls and demonstrates a more aggressive stall. In both cases, watch carefully – you can see the whole plane start to shake just in the moment before the nose starts falling. (Watch the canopy strut at the right – it’s most noticeable there I think.)

ETA: Note the “Quietness” too. As the glider slows down and is about to stall, you can hear how much quieter the wind noise gets.

14

(Missed second-edit window) –

As student glider pilots (student power pilots too), one of the things we practice is airspeed control with the air speed indicator covered up. The clues are pitch (vertical position of the nose with respect to the horizon) and, for gliders, how loud the wind noise is. The instructor will tell the student to fly at 45 knots, then 50 knots, then 60 knots, etc.

I got to practice that for a substantial portion of a 2-hour flight over Lake Tahoe just two days ago – thermal flying with the air speed indicator covered! (I’m still a bit mediocre at it, but I’m steadily getting better.) Instructor (almost constantly) telling me: “Too slow, speed up! Too fast, slow down! Don’t stall!” More details of this flight in this post, et seq. And yes, if one gets a bit too slow, one can feel the whole plane start shaking as a stall becomes imminent.

15

And this is why you can land the older C-150, C-170, C-172, C-180 at night with no electrical power in a totally blacked out cockpit with just enough star shine or moon light to make out the runway. You just have to listen and feel. Those and other aircraft, but I have done it many times in those particular aircraft. Those airplanes actually talk to you and to mess it up means you are unable to listen. :cool:

It is not a good idea to try this in a Swift. Just sayin… :eek:

16

Good post and good vids.

Tying this thread in with the GA omnibus thread …

You’ll find as you get into aerobatics that stalls actually are signaled only by item 6. Plus item 1 for positive G stalls but excessive *forward *pressure for negative G stalls.

Items 2-5 on the list, despite being bog-standard piloting pedagogy, are entirely properties of stalls *only *when in roughly 1G straight-and-level upright totally vanilla flight.

Which is admittedly the vast majority of the flying even aerobatic pilots and aircraft do. But that pedagogy does a disservice to the aerobatic pilot and to the ordinary pilot who suddenly finds him/herself in something other than vanilla flight.

I think you’ll really enjoy your first steps into a larger world.

17

Hi Senegoid!

18

Hi, Leo!

Okay, for anyone who wants to see those stalls without watching the whole video – I’m at the Library now where I can view it and get the exact times.

Stall #1 (done by me): Starts at 15:35 where I pull the nose up just above the horizon and let the airspeed bleed off. Notice the buffet (shaking) at 15:43, then the nose falls back down.

Stall #2 (more aggressive, done by Dan): At 15:58 he puts the nose down to gain some speed, then pulls the nose up higher above the horizon than I did. The buffet happens at 16:10 then the nose falls.

@LSLGuy: Yes, I’ve seen various flying-safety articles on-line about this, questioning the standard pedagogy – even questioning the eternal verity that an aircraft can stall at any airspeed and any attitude. (Heard that one?) Most especially, the worst case stalls (and spins) – namely turning base or final, or when climbing out – can happen with no warning at all :eek: