# The speed ability of an airliner to approach or surpass Mach 1

Is it true that many commercial airliners are* technically capable *of surpassing Mach 1 in straight and level flight if they put their engines to max thrust (of course, that would run immense danger of structural damage?)

There has seldom been a better pairing of username and topic.

No. In a dive, yes, but regardless of altitude no commercial airliner currently in service is capable of 1+ speeds in level flight. They can all get pretty close though.

Not sure where you’d get an idea like this, but no. Not even close.

Down low where the engines are powerful max approved speed is around .5 Mach. Drag goes up at roughly the square of the speed. Yes, the engines can physically push the airplane faster than the approved limit (assuming nothing else breaks) but doubling the speed and quadrupling or more the drag takes lots more excess thrust than we have.

Up high where the engines are much less powerful but the air is also much less dense we can cruise at M0.82 to M0.88 depending on the aircraft. Once again the incremental power required to accelerate goes up at the square of the incremental speed, but meantime we’re already using damn near max thrust just to cruise.

Breaking Mach 1 going downhill is easy. Too easy.

And the power required to overcome the drag goes up as cube of speed. (Drag force goes up as square of speed, and power = force x speed.) And of course this goes up even faster as you approach the speed of sound (hence the term “sound barrier”).

There was the Concorde but has since been retired in 2003.

This is true until you get somewhere in the vicinity of Mach 1; as you get close, the drag increases much more dramatically. here’s a plot of drag coefficient vs. Mach number for a particularly shaped bullet; the specifics of the transonic/supersonic part of that curve will vary depending on the shape involved, but it’s pretty much always going to be a flat line in the subsonic regime. The fastest commercial airliners fly right at the point where that curve starts to deviate from a flat line; as you’ve noted upthread, this is around Mach 0.85.

Sea-level thrust-to-weight ratio for a commercial airliner seems to be around 0.2 or 0.25, depending on how much weight is aboard. What is the full-power thrust-to-weight ratio at cruising altitude? Assuming it’s 0.1 (SWAG), then a dive increases thrust by a factor of ten, which, yes, should be more than enough to break on through to the other side.

Can the engines on an airliner even operate above mach 1? Assume you go into a dive to pass mach 1 and the airframe doesn’t rip apart. Can the inlets/compressors deal with supersonic airflow?

No they can’t really. Exactly how fast the engine inlets could run isn’t something I have any good info on. My bet is the upper limit would be below 1.0 just inside the inlet lip.

Which might be a little above or below M1.0 for the whole airframe. So at an airplane speed of maybe M0.95 or maybe M1.05 the inlets would “unstart” and we’d have supersonic flow at the fan face. At which point the engines pretty much quit even if they don’t come unglued.

The F-16 had a fixed inlet with no moving parts. Which was pure aerodynamic voodoo inside. That limited the airplane to M1.8 because that’s all the much supersonic flow the inlet could reduce to subsonic ahead of the engine.

The F-15 had the identical engines. They were interchangeable with the F-16 except for some accessories unrelated to the air path. That airplane could go to M2.0, i.e. 10% faster. The difference was entirely due to the variable geometry inlets and internal dump doors.

For round numbers the F-16 inlet roughly doubled in cross section from about 5 SF to about 12 SF in a 20 foot tunnel. By contrast a 767 inlet is about 50SF cross section by about 3 feet deep and has no taper at all. A very, very different shape.

Not really. Remember airliners use turbofan engines, military fighters use turbojets. From Wiki:

Since the efficiency of propulsion is a function of the relative airspeed of the exhaust to the surrounding air, propellers are most efficient for low speed, pure jets for high speeds, and ducted fans in the middle. Turbofans are thus the most efficient engines in the range of speeds from about 500 to 1,000 km/h (310 to 620 mph), the speed at which most commercial aircraft operate.

For reasons of fuel economy, and also of reduced noise, almost all of today’s jet airliners are powered by high-bypass turbofans. Although modern combat aircraft tend to use low bypass ratio turbofans, military transport aircraft (e.g., C-17 ) mainly use high bypass ratio turbofans (or turboprops) for fuel efficiency. The higher the bypass ratio of a turbofan, the lower the mean jet outlet velocity, which in turn translates into high thrust lapse rates (decreasing thrust with increasing speed)

It goes on to say that turbofans in general can actually be efficient up to Mach 1.3 but not the designs that are used on airliners.

I’m assuming that “aerodynamic voodoo” means it blows your mind what’s going on.
What’s going on?

Well, for the record,wiki reports (with cites) that at least one non-SST airliner have broken the sound barrier, in a dive, and a 747 may have reached Mach 1 (again, in a dive), accidentally, during an in-flight emergency. With passengers aboard. :eek: ETA: It survived, but took damage.

Interesting - now I wonder, if a 747, 777, A330, etc. surpasses the sound barrier (for, let’s say, one minute), what’s the likelihood of total airframe disintegration?
33%?

50%?

It’s mostly about speed. M1.01 might be tolerable in smooth air for awhile, ignoring the issue with engines. Whereas you might not get to M1.1 before you lost control or big parts broke off.

In post #4 of this thread I referenced another thread with this post in it: http://boards.straightdope.com/sdmb/showpost.php?p=8444574&postcount=9

The point being that once you lose tail effectiveness you’re going to be going straight down in just a few seconds. At which point you’ll break up at most a few seconds later.

So the question becomes: How closely can you tickle that dragon’s tail? It might be M0.98. It might be M1.04. You can probably stay just below the critical point for a long time. But pop over the limit for just a moment and you’re off the cliff doing the high dive.

Boeing & Airbus engineering staffs certainly have a pretty good idea. I don’t, but my general aerodynamic knowledge tells me it isn’t very far over M1.0. And it might even be a little less than M1.0.