Help with Instruments
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Join Date: Sep 2000
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Help with Instruments
I wonder if anyone who has sat Instruments in June, or recently could help me.
I sat my nav subjects for ATPL at Gatwick in June, and failed Instruments, Nav by 4%
and Flight Plan by 1% 
. In the instruments paper, near the end, there were about 5-6 questions about Tachometers (I seem to remember). I couldn't answer any of them. Everyone I spoke to didn't have a clue either. Can anyone remember these questions? The people at my school can't remember as they were all so bizarre. Tachometers aren't covered in my schools notes under Instruments, and they don't appear in my notes from Oxford either.
I really hope someone can help me, I don't want to fail Inst again Also any advice on passing this bugger of an exam, or the other two, would be most welcome 
. Please could you post here, or mail me at my new address below (as I haven't used it yet 
). Thanks for reading.
I sat my nav subjects for ATPL at Gatwick in June, and failed Instruments, Nav by 4%
and Flight Plan by 1% . In the instruments paper, near the end, there were about 5-6 questions about Tachometers (I seem to remember). I couldn't answer any of them. Everyone I spoke to didn't have a clue either. Can anyone remember these questions? The people at my school can't remember as they were all so bizarre. Tachometers aren't covered in my schools notes under Instruments, and they don't appear in my notes from Oxford either.I really hope someone can help me, I don't want to fail Inst again
Also any advice on passing this bugger of an exam, or the other two, would be most welcome . Please could you post here, or mail me at my new address below (as I haven't used it yet ). Thanks for reading.
Join Date: May 2001
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I just scraped that one with 75%.
The tacho questions were about induction tacometers which i only knew about from work and hadn't been taught them by oxford. Which are magnets get swept past a coil inducing a voltage. The faster the drive the bigger the voltage.
The others were to do with various set ups you can have.
If you have long wire you don't want to be running high currents because you will get large errors due to voltage drop because of wire resistance. So high voltage low current systems will be better. And it is easier to feed a voltage into A/D converter for logging than a current is.
I had only been told about the drag cup design before by oxford.
MJ
And the instruments for engines arn't covered in the instruments folder they are at the back of the engine notes i think.
[ 09 July 2001: Message edited by: mad_jock ]
The tacho questions were about induction tacometers which i only knew about from work and hadn't been taught them by oxford. Which are magnets get swept past a coil inducing a voltage. The faster the drive the bigger the voltage.
The others were to do with various set ups you can have.
If you have long wire you don't want to be running high currents because you will get large errors due to voltage drop because of wire resistance. So high voltage low current systems will be better. And it is easier to feed a voltage into A/D converter for logging than a current is.
I had only been told about the drag cup design before by oxford.
MJ
And the instruments for engines arn't covered in the instruments folder they are at the back of the engine notes i think.
[ 09 July 2001: Message edited by: mad_jock ]
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The in-depth tacho questions are relatively new which is probably why they are not covered in your notes. This is a bit from our notes, sorry no diagrams, I can't work out how to include them.
Tachometers
The most primitive form of RPM measurement is the mechanical tachometer now found only on light piston singles and twins.
A direct drive is taken from the engine to the indicator. In the instrument itself the drive turns a magnet in an aluminium cup. The rotating magnet sets up eddy currents in the cup which have their own magnetic fields. The force of the rotating magnet on the fields set up in the aluminium cup tends to turn the cup. This turning force is opposed by a hairspring so that the movement of the cup is an indication of the magnetic force and therefore rpm.
Mechanical tachometer dials indicate engine rpm, mostly up to about 3000rpm as light aircraft engines are fairly low revving.
The green sector on the dial indicates the normal operating range and the yellow sector between 2000 and 2200rpm shows a caution area where prop vibration is likely. Operation in this range should be avoided. Some early aircraft like the C180 had a red section here rather than yellow.
The maximum length of the drive is about 2m. It is this which limits the use of mechanical tachometers to small aircraft where the engines are close to the cockpit.
On more modern aircraft where a direct drive can be taken from the engine RPM is usually measured by a small generator, called a tachogenerator.
The generator supplies a three phase alternating current which drives a synchronous or squirrel cage motor at the instrument.
The motor at the instrument drives a magnet rotating in a cup. Eddy currents pull the cup round against the restraining force of a hairspring to move the indicator needle and display RPM.
The tachogenerator system requires no external power supply
The display is usually calibrated as a percentage of the maximum RPM. The indicator may have coloured arcs on it that show normal readings in green, caution areas in amber and overspeed limits in red.
More modern instruments also have an overspeed pointer fitted. This stationary pointer is initially positioned at the maximum RPM limit. If this limit is exceeded the RPM needle carries the overspeed pointer with it and leaves it at the maximum rpm achieved.
The overspeed pointer can not be reset in flight
Where a gas turbine engine has several spools RPM is indicated for each one separately. The dials are labelled N1, N2 and N3 for the speed (N) of each spool.
[img]null[/img]
Tachometers
The most primitive form of RPM measurement is the mechanical tachometer now found only on light piston singles and twins.
A direct drive is taken from the engine to the indicator. In the instrument itself the drive turns a magnet in an aluminium cup. The rotating magnet sets up eddy currents in the cup which have their own magnetic fields. The force of the rotating magnet on the fields set up in the aluminium cup tends to turn the cup. This turning force is opposed by a hairspring so that the movement of the cup is an indication of the magnetic force and therefore rpm.
Mechanical tachometer dials indicate engine rpm, mostly up to about 3000rpm as light aircraft engines are fairly low revving.
The green sector on the dial indicates the normal operating range and the yellow sector between 2000 and 2200rpm shows a caution area where prop vibration is likely. Operation in this range should be avoided. Some early aircraft like the C180 had a red section here rather than yellow.
The maximum length of the drive is about 2m. It is this which limits the use of mechanical tachometers to small aircraft where the engines are close to the cockpit.
On more modern aircraft where a direct drive can be taken from the engine RPM is usually measured by a small generator, called a tachogenerator.
The generator supplies a three phase alternating current which drives a synchronous or squirrel cage motor at the instrument.
The motor at the instrument drives a magnet rotating in a cup. Eddy currents pull the cup round against the restraining force of a hairspring to move the indicator needle and display RPM.
The tachogenerator system requires no external power supply
The display is usually calibrated as a percentage of the maximum RPM. The indicator may have coloured arcs on it that show normal readings in green, caution areas in amber and overspeed limits in red.
More modern instruments also have an overspeed pointer fitted. This stationary pointer is initially positioned at the maximum RPM limit. If this limit is exceeded the RPM needle carries the overspeed pointer with it and leaves it at the maximum rpm achieved.
The overspeed pointer can not be reset in flight
Where a gas turbine engine has several spools RPM is indicated for each one separately. The dials are labelled N1, N2 and N3 for the speed (N) of each spool.
[img]null[/img]
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All these question were about induction tachos which are a bit different the squirrel cages relys on a resonable current flowing. The induction tacho has a small current so hence it is not as suceptable to line drop.
There must have been 5 questions on it relating the voltage drops etc. Send me a email if you want me to phone and explain my self.
MJ.
There must have been 5 questions on it relating the voltage drops etc. Send me a email if you want me to phone and explain my self.
MJ.
