can you answer any of the following questions?
Thread Starter
Joined: Jan 2002
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From: UK and proud of it
can you answer any of the following questions?
Hi there,
Could any of you be kind enough to answer any of the following questions.
1) what is a JET UPSET?
2) what happens to a stall at very high altitudes?
3) what are active controls?
4) what is the purpose of a mach trimmer?
5) what is a direct lift control?
6) why is EPR set by 60/80 kts?
7)where is the best place for C of G in flight?
8)what is dithering?
9) difference between diff/non diff spoilers?
10) what are the requirements for ETOPS?
Thanks for any replies to the above.
PL
Could any of you be kind enough to answer any of the following questions.
1) what is a JET UPSET?
2) what happens to a stall at very high altitudes?
3) what are active controls?
4) what is the purpose of a mach trimmer?
5) what is a direct lift control?
6) why is EPR set by 60/80 kts?
7)where is the best place for C of G in flight?
8)what is dithering?
9) difference between diff/non diff spoilers?
10) what are the requirements for ETOPS?
Thanks for any replies to the above.
PL
Joined: Apr 2002
Posts: 23
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From: Netherlands
Jinkster your right about dithering, i believe it was a method used by the US military to alternate the times in the atomic clocks in the gps satellites so that your position would be less precise when you are using a "civil" gps-system. The military did have the special code to reveal what the real timing was so they could be more precize, and maintain the advantage on users of the civil-system.
And ETOPS requirements have to do with reliability and maintenance of the engines of the aircraft, they have to be proven to be within certain limits (i'm sure you can find them at google)
And ETOPS requirements have to do with reliability and maintenance of the engines of the aircraft, they have to be proven to be within certain limits (i'm sure you can find them at google)
Usual disclaimers apply!
Joined: Nov 1999
Posts: 843
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From: EGGW
No 3 Active controls.
Ailerons respond to gust load imposed on the wing. The L1011-500 had them. The outboard ailerons (normally locked out with the flaps up) respond simultaneously and symetrically to absorb the increased wing root bending moment, limit the load on the wing and negate the need to beef up the wing structure.
No 5 Direct lift control.
Another L1011 thing. Used on the approach the spoilers are deployed to a null position and modulate around that point, improves pitch axis control and handling.
No10 ETOPS
Not only engine reliability but systems reliability and redundancy also, including, for instance, the ability of the APU to be started in flight after a cold soak.
Engine health monitoring is also a requirement.
Ailerons respond to gust load imposed on the wing. The L1011-500 had them. The outboard ailerons (normally locked out with the flaps up) respond simultaneously and symetrically to absorb the increased wing root bending moment, limit the load on the wing and negate the need to beef up the wing structure.
No 5 Direct lift control.
Another L1011 thing. Used on the approach the spoilers are deployed to a null position and modulate around that point, improves pitch axis control and handling.
No10 ETOPS
Not only engine reliability but systems reliability and redundancy also, including, for instance, the ability of the APU to be started in flight after a cold soak.
Engine health monitoring is also a requirement.
Joined: Oct 1999
Posts: 398
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From: United Kingdom
Q 1 From Boeing FCT Manual.
An upset can generally be defined as unintentionally exceeding the following
conditions:
• pitch attitude greater than 25 degrees nose up, or
• pitch attitude greater than 10 degrees nose down, or
• bank angle greater than 45 degrees, or
• within above parameters but flying at airspeeds inappropriate for the
conditions.
An upset can generally be defined as unintentionally exceeding the following
conditions:
• pitch attitude greater than 25 degrees nose up, or
• pitch attitude greater than 10 degrees nose down, or
• bank angle greater than 45 degrees, or
• within above parameters but flying at airspeeds inappropriate for the
conditions.
Joined: Jun 2001
Posts: 889
Likes: 0
From: Australia
Since no one else has replied to this one...
What happens to a stall at very high altitudes?
As an aircraft climbs, TAS becomes much greater than CAS. This occurs because CAS is an airspeed based on dynamic pressure, but assuming sea level air density. It is not an accurate measure of airspeed at high altitude (low air density).
As an aircraft climbs the stall speed (CAS value does not change much with altitude) gets closer and closer to the maximum allowable speed - based on TAS or Mach Number.
At an aircraft's absolute ceiling the airspeed indicator will show a value just above the aerodynamic stall buffet, but TAS will be high enough to be just below max TAS or Mach buffet.
If the aircraft were to be allowed to stall, how will the pilot recover? You must lower the nose to reduce angle of attack, but this will inevitably result in an increase in airspeed so you exceed max TAS or even get Mach buffet. Both of these (of course) are very bad for the aircraft structure, and may result in structural limits being exceeded and/or bits falling off.
To fix this, aircraft capable of flying at very high altitudes have a certificated maximum altitude to avoid getting near this flight regime (sometimes called "coffin corner".)
Note 1: there are many other reasons why an aircraft may have a specified max altitude - this is just one aerodynamic one.
Note 2: as far as I know supersonic aircraft do not have the same problem - it's a civil air transport issue.
O8
What happens to a stall at very high altitudes?
As an aircraft climbs, TAS becomes much greater than CAS. This occurs because CAS is an airspeed based on dynamic pressure, but assuming sea level air density. It is not an accurate measure of airspeed at high altitude (low air density).
As an aircraft climbs the stall speed (CAS value does not change much with altitude) gets closer and closer to the maximum allowable speed - based on TAS or Mach Number.
At an aircraft's absolute ceiling the airspeed indicator will show a value just above the aerodynamic stall buffet, but TAS will be high enough to be just below max TAS or Mach buffet.
If the aircraft were to be allowed to stall, how will the pilot recover? You must lower the nose to reduce angle of attack, but this will inevitably result in an increase in airspeed so you exceed max TAS or even get Mach buffet. Both of these (of course) are very bad for the aircraft structure, and may result in structural limits being exceeded and/or bits falling off.
To fix this, aircraft capable of flying at very high altitudes have a certificated maximum altitude to avoid getting near this flight regime (sometimes called "coffin corner".)
Note 1: there are many other reasons why an aircraft may have a specified max altitude - this is just one aerodynamic one.
Note 2: as far as I know supersonic aircraft do not have the same problem - it's a civil air transport issue.
O8
Last edited by Oktas8; 20th July 2002 at 20:37.
Thread Starter
Joined: Jan 2002
Posts: 35
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From: UK and proud of it
BIK, as requested the edit has been done. I just feel that sometimes on pprune you ask a simple question and you get some very stupid replies!
And to all the guys that have hepled in answering my questions, I thank you.
Take care
PL
And to all the guys that have hepled in answering my questions, I thank you.
Take care
PL
Joined: Feb 2001
Posts: 60
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From: London, Ontario, Canada
One more use of "dithering", although this may show my age.
Back in the 60s and 70s, dithering was used in the earliest electrohydraulic servo controls to minimize the breakout friction present when the control was first commanded to move.
A high frequency, very low amplitude electrical signal was continually sent to the EHSV (electrohydraulic servo-valve). This was called a dithering signal. This kept the moving parts (EHSV coils, control spools and hydraulic actuator pistons) in constant motion, but of such small amplitude that there was no effect on the airplane (or whatever) the system was installed in.
When a real control input came along, there was no static friction to overcome before things started to move.
Back in the 60s and 70s, dithering was used in the earliest electrohydraulic servo controls to minimize the breakout friction present when the control was first commanded to move.
A high frequency, very low amplitude electrical signal was continually sent to the EHSV (electrohydraulic servo-valve). This was called a dithering signal. This kept the moving parts (EHSV coils, control spools and hydraulic actuator pistons) in constant motion, but of such small amplitude that there was no effect on the airplane (or whatever) the system was installed in.
When a real control input came along, there was no static friction to overcome before things started to move.
Joined: Feb 2001
Posts: 1,064
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From: Hants, UK
Mutt: Does it matter? If you have the issues explained as thoroughly as they have been here then you can answer any question on them in any paper, can't you? I've always believed understanding the subject is better than trying to question-spot!
