Info required RE Mach Tuck in an interview situation
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Info required RE Mach Tuck in an interview situation
Hello All,
Having reviewed all the information with the use of the search function already. I thought I might trouble you all to elaborate on Mach Tuck a little further.
I'm preparing for an interview and have dug out and blown the dust of the old CAA ATPL POF stuff and I have also been reading Aerodynamics for Naval Aviators, which can be hard going but very enlightening in areas.
I'm currently flying a straight wing jet so I just know that they are going to ask a few pointed questions about swept wing theory.
So please, for those of you in the know, what would you ask in an interview situation and what would your answers to those questions be.
I have printed off some of the previous threads and am using the same as revision material, I would be very grateful if we could briefly revisit things here.
Thanks in advance for any helpful info
Having reviewed all the information with the use of the search function already. I thought I might trouble you all to elaborate on Mach Tuck a little further.
I'm preparing for an interview and have dug out and blown the dust of the old CAA ATPL POF stuff and I have also been reading Aerodynamics for Naval Aviators, which can be hard going but very enlightening in areas.
I'm currently flying a straight wing jet so I just know that they are going to ask a few pointed questions about swept wing theory.
So please, for those of you in the know, what would you ask in an interview situation and what would your answers to those questions be.
I have printed off some of the previous threads and am using the same as revision material, I would be very grateful if we could briefly revisit things here.
Thanks in advance for any helpful info
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I think there were 2 main "tuck" eras, though the first was the only true problem:
1. P38 era, where the formation of shock waves on the main wing, at Mcrit of about .65 or so. When the shock wave is strong enough (depends on the airfoil shape) you get separation of the airflow over the wing, causing a sharp reduction of wing lift. That's a first contribution to the tuck problem. Secondly, the reduction of lift means less downwash produced, less downwash on the elevator, meaning the elevator is no longer producing as much down force (remember, the tail always pushes down, except for FBW fighters). These two were enough in some cases to totally prevent recovery from dives once a high enough speed was attained, even with full aft elevator. I seem to call some test pilots killed troubleshooting the behaviour. The only option was to reduce thrust, and perhaps add drag, to reduce airspeed.
2. DC8/707 and onward era. the following I'm not 100 % sure of, but I understand the tuck problem was solved, however, if you were to take one of these to mach .90 plus, the elevator was found to be "blanked". Ie again, full aft elevator would not recover the aircraft. But no problem, since they had a trimable stab, and trim would allow recovery. What I'm not so sure of is why the loss of downwash is no longer an issue with a trimable stab. I'm thinking the flow over the horizontal tail itself of the DC8 at mach .90 plus becomes supersonic, developing shock waves ahead of the elevator, and thus rendering the elevator ineffective. Thus trim can recover the aircraft.
These days we have mach trimmers, which function well before tuck onset. This is because the mach trim actuates to ensure the stick gradient of so many lbs of forward force required per knot of airspeed increase. You'd have to look up FAR 25 to get the number. AFAIK, airbus and 777 (due to FBW) are exempt.
Perhaps some gurus can comment on the trimmable stab implications to mach tuck.
1. P38 era, where the formation of shock waves on the main wing, at Mcrit of about .65 or so. When the shock wave is strong enough (depends on the airfoil shape) you get separation of the airflow over the wing, causing a sharp reduction of wing lift. That's a first contribution to the tuck problem. Secondly, the reduction of lift means less downwash produced, less downwash on the elevator, meaning the elevator is no longer producing as much down force (remember, the tail always pushes down, except for FBW fighters). These two were enough in some cases to totally prevent recovery from dives once a high enough speed was attained, even with full aft elevator. I seem to call some test pilots killed troubleshooting the behaviour. The only option was to reduce thrust, and perhaps add drag, to reduce airspeed.
2. DC8/707 and onward era. the following I'm not 100 % sure of, but I understand the tuck problem was solved, however, if you were to take one of these to mach .90 plus, the elevator was found to be "blanked". Ie again, full aft elevator would not recover the aircraft. But no problem, since they had a trimable stab, and trim would allow recovery. What I'm not so sure of is why the loss of downwash is no longer an issue with a trimable stab. I'm thinking the flow over the horizontal tail itself of the DC8 at mach .90 plus becomes supersonic, developing shock waves ahead of the elevator, and thus rendering the elevator ineffective. Thus trim can recover the aircraft.
These days we have mach trimmers, which function well before tuck onset. This is because the mach trim actuates to ensure the stick gradient of so many lbs of forward force required per knot of airspeed increase. You'd have to look up FAR 25 to get the number. AFAIK, airbus and 777 (due to FBW) are exempt.
Perhaps some gurus can comment on the trimmable stab implications to mach tuck.
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Grab a copy of D.P. Davies Handling The Big Jets 
They'll never stump you on an airline interview then
Interviewer: What is Mach Tuck/ and How is it dealt with? and how is it dealt with after failure of the Mach trim system?
You: blah blah, but in this case ...blah blah blah blah and if that happens blah blah blah blah blah blah blah blah, reduced aerodynamic damping and loss of tail effectiveness also can blah blah blah blah blah blah... which leads me to this digression blah blah blah blah Now on to Stalling
Interviewer:
They'll never stump you on an airline interview then
Interviewer: What is Mach Tuck/ and How is it dealt with? and how is it dealt with after failure of the Mach trim system?
You: blah blah, but in this case ...blah blah blah blah and if that happens blah blah blah blah blah blah blah blah, reduced aerodynamic damping and loss of tail effectiveness also can blah blah blah blah blah blah... which leads me to this digression blah blah blah blah Now on to Stalling
Interviewer:
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Mach tuck was very pronounced on the 'ole 707, above about .84, and would often result in the stab trim running near to the full stop, if not caught in time...that is, if it didn't bind up the stab jackscrew beforehand.
Definitely NOT a good scenario.
Correction technique, power off then split the spoilers to get the nose up, and it was right in the QRH...clearly it had offered up exciting occasions before.
Definitely NOT a good scenario.
Correction technique, power off then split the spoilers to get the nose up, and it was right in the QRH...clearly it had offered up exciting occasions before.
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I own one, rhovsquared, and a fine aeroplane it is, wet bar and all.
Your reference to David Davies book is a good one, and in my opinion, every new jet transport pilot should read it, if for no other reason than to appreciate the very good handling qualities of new(er) jet transport types.
Your reference to David Davies book is a good one, and in my opinion, every new jet transport pilot should read it, if for no other reason than to appreciate the very good handling qualities of new(er) jet transport types.
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What's the interview for, aircraft designer or pilot?
If it's for a pilot position, the interviewer will most likely be evaluating whether you are someone they could sit next to for 10 hours at a time.
If it's for a pilot position, the interviewer will most likely be evaluating whether you are someone they could sit next to for 10 hours at a time.
Per Ardua ad Astraeus
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You will also know, having reviewed all the information, that the primary cause of the early Mach Tuck, as it was named, was the rearward movement of the CofP on the wing as it became transonic - from about 25% chord to about 50% chord. Even without 'elevator blanking' this trim change was too strong to be controlled and killed a few.
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Can it be that it was all so simple then?
back then Flying was,
Through the fire, to the limit, to the wall
For a chance to be with you
I'd gladly risk it all
Through the fire
Through whatever, come what may
For a chance at loving you
I'd take it all the way
Right down to the wire
Even through the fire, To the wire, to the limit
Through the fire, through whatever
Through the fire, to the limit
Through the fire, through whatever
Through the fire, to the limit
Through the fire, through whatever
Chaka Khan's word's 707/DC-8 pilots' Job
back then Flying was,
Through the fire, to the limit, to the wall
For a chance to be with you
I'd gladly risk it all
Through the fire
Through whatever, come what may
For a chance at loving you
I'd take it all the way
Right down to the wire
Even through the fire, To the wire, to the limit
Through the fire, through whatever
Through the fire, to the limit
Through the fire, through whatever
Through the fire, to the limit
Through the fire, through whatever
Chaka Khan's word's 707/DC-8 pilots' Job
It was severe uncontrollable mach tuck that caused three Australian built Vampire Mk 30's to go in like darts near the RAAF fighter base at Williamtown near Sydney in the very early Fifties. With no ejection seats yet fitted, the three pilots were unable to abandon their aircraft. The problem was shock waves forming over newly installed air intakes called "Elephant Ears situated just to the rear of the canopy on top of the fuselage. Around Mach 0.75, the shockwaves drastically affected elevator effectiveness. I understand the coming September issue of the UK magazine "Todays Pilot" has an article about these aircraft.
Originally Posted by rhovsquared
Grab a copy of D.P. Davies Handling The Big Jets
My hat goes off to him
The Chuck Yeager of big jets!
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TruBlu351 I'd like to meet the guy who took the DC-8 to M 1.0, the first sonic flight on an airliner (i think) and Douglass Co's strongest lady. and RT in flight for speed control
I think Boeing's was the 727-she's seen some high mach no flight
Re-entry. I'll try to keep out of jets forn the future, but then I'll have no place to talk about ladies and if I stay on the ground then I talk to ladies about JETS
hence no chicks
RADAR REQUIRED ,
rhov
I think Boeing's was the 727-she's seen some high mach no flight
Re-entry. I'll try to keep out of jets forn the future, but then I'll have no place to talk about ladies and if I stay on the ground then I talk to ladies about JETS
hence no chicks RADAR REQUIRED
,rhov
