Question about airliners

nuuumannn · May 13, 2014

Isn't this reversed?

No. At subsonic speeds the intake opening is narrow and at supersonic speeds the intake lip opening is at its widest to enable shock waves to build up at its throat to slow the air down before it hits the compressor. Here's a definition from wiki:

"For higher flight speeds a moving cone becomes necessary to allow the supersonic compression to occur more efficiently over a wider range of speeds. With increasing flight speed the cone is moved to the rear, or into the intake. Due to the shape of the cone surface and the internal duct surface the internal flow area gets less as required to continue compressing the air supersonically. The compression occurring in this path is called "internal compression" (as opposed to the "external compression" on the cone). At the minimum flow area, or throat, a normal or plane shock occurs. The flow area then increases for subsonic compression, or diffusion, up to the engine face."

From here: https://en.wikipedia.org/wiki/Inlet_cone

I just realised I might have made it sound like the reverse in my previous description.

The Basket · May 13, 2014

727? Supersonic? On the level?
Need more input.
What about the never exceed speed?
That 727 I believe was a slat opened in flight.
And then hell started.

GregP · May 13, 2014

I have no more input Basket.

I have had conversations with over twenty 727 pilots and at least half said it was possible.

I make no claim either way. Just passing on what I have heard. It wouldn't surprise me either way ... but, if I had to pick one to try it in, it would be the 727, probably the longest one.

The Basket · May 14, 2014

In a dive yeah anything is possible.
On the level I cant see it.
But would these 727 guys actually do the deed?
When could they? They would get busted for sure.
Not calling you out GregP but gotta see it.

GregP · May 14, 2014

I'd like to see it, too. I doubt I'll ever see an attempt at it and would not have mentioned it if so many former 727 pilots hadn't SAID it in conversation. As stated, I make no such claim and merely passed on what I heard in conversations with people who flew them.

It takes on more believability when a guy who has flown 10+ different jet airliners (and several pistons and turboprops along the way) over 30+ years for a living says the 727 was only one that could do it. He flew B-47s in the USAF. Doesn't mean it CAN ... means he thinks it can.

I'll most likely never know for sure.

Jabberwocky · May 14, 2014

The Basket said:
In a dive yeah anything is possible.
On the level I cant see it.
But would these 727 guys actually do the deed?
When could they? They would get busted for sure.
Not calling you out GregP but gotta see it.

According to the US NTSB, a 727 may have indeed gone supersonic, but not in level flight.

In 1979, TWA Flight 841 (something of an infamous number) suffered a rejected slat retraction at 39,000 ft while operating from New York To JFK and cruising at Mach .816

The aircraft went into a right hand spiral dive during which it may have broken the sound barrier. Control was only re-established at about 8,000 ft, so I can imagine a few tight sphincters onboard.

A DC-8 definitely did break the sound barrier, again in a dive. This time it was planned, with the aircraft being water ballasted to aid supersonic recovery. The aircraft recorded Mach 1.03 at 43,500 ft in 1961, during testing of aerodynamic enhancements to the wing by Douglas.

It was taken up to about 52,000 ft and put in a dive of about 22 degrees. Supersonic flight was recorded for about 15-16 seconds.

The Basket · May 14, 2014

Tu-144 was supersonic. But only in reheat. So had fuel efficiency of a pig.
My view is that its all about the money. And no new supersonics until warp drive.

I mentioned the TWA flight. Big difference between out of control and deliberate

GregP · May 14, 2014

Actually I mentioned the TWA flight.

And they can do supersonic anytime they want to pay for it.

Aerion is slowly working on a supersonic bizjet as you might know, but the ongoing slow research on reducing sonic booms needs to be funded more heavily if supersonic overflight of a populated land mass is to be approved by anyone except maybe the Russians over Siberia, and the takeoff and landing speeds need to be more in line with existing subsonic airliners to avoid the Concorde's potential fate when a tire blows.

The technology exists, but the enticement to fund it must be able to show profit. Reducing the shock wave would help by extending the supersonic flight time. If that can be done, it just might pay for itself. The desire to have it happen must be present somewhere that is funded for it to come again.

FLYBOYJ · May 14, 2014

GregP said:
According to friends of mine who flew them, the Boeing 727 could go supersonic without too much fanfare.

Don't hold your breath on that one Greg - I worked 727s years ago and there were all kinds of inspections to be done if you exceeded speed limits on the aircraft. Additionally you would have the FE, who usually had a maintenance background, watch the "two dummies steering" like a hawk, making sure they did nothing funny to bend the aircraft. I'm not saying that these guys might be BSing you, but I would find it hard to believe that a flight crew of 3 would risk job and career by operating an airliner outside of FM parameters, especially if they were flying for a major airline.

The 727 slat failure/ circuit breaker incident is classic and I remember when that was going on.

The 727 was a solid, easy to maintain aircraft and in some areas superior to what's flying today. I do remember that the APU was a chore to remove and had to be thoroughly inspected on the bottom when first removed as it was easy to miss leaky and broken fluid lines and frayed cables.

fastmongrel · May 14, 2014

Dont some planes have mach indicators that show the highest mach number flown that can only be reset to zero by ground crew. I might have this mixed up with early jets though.

GregP · May 14, 2014

Hi Joe,

I was careful to say that I didn't make the claim myself.

And since you worked on them, do you recall the "puppy death" switch that depressurized the holds where the pets were carried? A friend said he inadvertently pushed it when he turned around to check something and froze all the pets on one flight. He felt very badly about that for years.

FLYBOYJ · May 14, 2014

fastmongrel said:
Dont some planes have mach indicators that show the highest mach number flown that can only be reset to zero by ground crew. I might have this mixed up with early jets though.

I don't think the 727 had this but they had something better - A Flight Engineer!

GregP said:
Hi Joe,

I was careful to say that I didn't make the claim myself.

And since you worked on them, do you recall the "puppy death" switch that depressurized the holds where the pets were carried? A friend said he inadvertently pushed it when he turned around to check something and froze all the pets on one flight. He felt very badly about that for years.

That I don't really remember - I do know that the entire fuselage except the wheel wells were pressurized including the cargo holds and I believe it was a customer option to have the holds made so animals could be transported in them. The only thing I could think of is if an outflow valve was dumped then that storage hold may lose pressure if it was seperated from the rest of the pressure vessel.

bobbysocks · May 14, 2014

hmmmm how about if you flew in the jet stream? we had a flight from alabama...always came in WAY early. which peeved me off as i had to schedule ground crews to bring it in. i accused the pilots of bugging out early but they ( and this came from several different crews ) claimed they caught one hell of a tail wind that shaved over an hour off of the flight time. so could you theoretically fly past mach ( ground speed ) but never break the sound barrier ( indicated air speed )??

gumbyk · May 14, 2014

bobbysocks said:
hmmmm how about if you flew in the jet stream? we had a flight from alabama...always came in WAY early. which peeved me off as i had to schedule ground crews to bring it in. i accused the pilots of bugging out early but they ( and this came from several different crews ) claimed they caught one hell of a tail wind that shaved over an hour off of the flight time. so could you theoretically fly past mach ( ground speed ) but never break the sound barrier ( indicated air speed )??

Yeah, with a big tail-wind, you could have a groundspeed that is technically over M1.0, but as sound travels through air, it is airspeed that counts. The only time groundspeed counts is when the wheels touch down...

Re: the 727 going supersonic, I'm sceptical. shockwave formation over the elevators will render them unusable at some point. Here's an interview with one of the pilots.
I Was There: When the DC-8 Went Supersonic | History | Air Space Magazine

It also talks about the ailerons both going about 5º up at 0.97 mach.

Strength is only one aspect of supersonic flight. Flight control design is a major part of it as well, and airliners just aren't aerodynamically designed for supersonic flight.

GrauGeist · May 14, 2014

Great story about the DC-8!

When the Me262 entered the "speed not to exceed" zone, it tended to nose down and the pilots were able to break it by pushing forward on the stick and recover. (Assuming they had the luxury of altitude)

GregP · May 14, 2014

When I visited the Stormbirds hangar in Everett, Washington as they were completing the first of the new-built Me 262s, there was a letter from Messerschmitt on the wall limiting the replicas to 540 mph Vmax. After that, the pilot became a test pilot and was on his own as far as handling went ... they had nothing whatsoever to say about handling past Vmax.

They DID issue the Stormbirds replicas consecutive werknummers from the cessation of Messerschmitt production. I understand one of them is flying in Germany owned, I believe, by either EADS or Messerschmitt. I've heard it both ways and have never checked since I rarely get to Germany.

gumbyk · May 15, 2014

GrauGeist said:
Great story about the DC-8!

When the Me262 entered the "speed not to exceed" zone, it tended to nose down and the pilots were able to break it by pushing forward on the stick and recover. (Assuming they had the luxury of altitude)

Its called 'Mach Tuck', caused when the centre of pressure moves rearward as the aircraft goes trans-sonic.
SKYbrary - Mach Tuck

m37b1 · May 15, 2014

"The 727 was a solid, easy to maintain aircraft and in some areas superior to what's flying today".

Agreed. . . I fly a lot. Almost exclusively Southwest, which means 737. I got thinking about the narrow-body 7x7's and wondered why Boeing dropped the 727. I know it had noise issues and center engine could not be easily upgraded to a modern, more quiet, high bypass fan.

Why not upgrade the two outer engines and eliminate the third? You would then have all the benefits of the clean wing that the 727 enjoyed, and avoid the FOD, vacuum cleaner issues presented by the 737.

GrauGeist · May 15, 2014

gumbyk said:
Its called 'Mach Tuck', caused when the centre of pressure moves rearward as the aircraft goes trans-sonic.
SKYbrary - Mach Tuck

I'm sure at the time these early jet pilots were discovering this event, they were referring to it by names that I can't print here!

GregP · May 15, 2014

I've heard it called "The Thing."

I also heard the term "The Thing" used to describe what can happen to an F-100 pilot or an F-104 pilot if he makes the "big mistake" and winds up in inertia coupling or the dreaded "Sabre Dance," which was used to describe the alternating stall wobble just above the runway of the early short-tail F-100's when they exceeded the angle of attack limit on either takeoff or landing and sometimes caused a compressor stall. It was invariably fatal.

Here is a "Sabre Dance:"

View: https://www.youtube.com/watch?v=mZL0x-gEDM8

And when the margin between stall and Mach Tuck gets sufficiently small, they call it the "Coffin Corner." In the early Lear jets, the coffin corner at high altitude was ±2 - 3 knots! Now that's cutting it close! Several did a lawn dart imitation before they figured it out and adjusted the margins. I've heard the margins on the new Gulfstream G650 are in the neighborhood of ±35 knots at 51,000 feet, making things much less hair raising. You can keep inside that margin even if you lose an engine.

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