Military aircraft compasses
Cunning Artificer
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The datum compass did indeed read in tenths - but it had uses beyond RAF Compass swings, in surveying.
The datum compass also required scheduled calibration and at Changi we used to swing the Watts Datum on an open area that doubled as part of the golf course. One day the inevitable happened and it was hit by a golf ball. I won the draw out of the hat for the trip to the Admiralty Compass Laboratory in UK to have it repaired - which gave me two weeks "leave" while it was returned to serviceability - one week at home and a second week in the "Route Hotel" at Lyneham, waiting for a flight. I promoted myself to commissioned rank during my stay, as the food was so much better in the officers' dining room and they had free newspapers in the lounge: nobody from the staff seemed to notice, and if they did they never said anything.
The datum compass also required scheduled calibration and at Changi we used to swing the Watts Datum on an open area that doubled as part of the golf course. One day the inevitable happened and it was hit by a golf ball. I won the draw out of the hat for the trip to the Admiralty Compass Laboratory in UK to have it repaired - which gave me two weeks "leave" while it was returned to serviceability - one week at home and a second week in the "Route Hotel" at Lyneham, waiting for a flight. I promoted myself to commissioned rank during my stay, as the food was so much better in the officers' dining room and they had free newspapers in the lounge: nobody from the staff seemed to notice, and if they did they never said anything.
They still are weighted pasta, and I don't know if you noticed but it leads to a problem which is known about in flying and more or less ignored in sailing. That added weight gets left behind if you move sideways. In boats it makes steering South harder, but in a side-slipping aircraft it's possible to fly in a complete circle with the compass apparently motionless.
My Pa's idea was to remove the weight but to stop the needle from dipping.
My Pa's idea was to remove the weight but to stop the needle from dipping.
How does your Pa's compass stop the needle dipping without inhibiting its rotation?
KenV - you're right of course - precision is the correct word, and I'm kicking myself for getting pulled up on that one. Particularly with regard to compasses with their many sources of inaccuracy...
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It was designed for use in gliders - early units were loosely mounted so that they could be tilted to match the bank angle in thermals. Later units got an external gimbal. The advantage over a floating ball type was the very low inertia, and they were pretty popular until the fluxgate arrived.
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Since the numbers are all "backwards" and E and W are opposite to where they "should" be, does that mean the needle is stationary and the dial rotates about the needle?
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KenV, you're going to kick yourself!
Nope, numbers are fixed to the body. Body fixed to glider panel and the needle stays pointing North.
Imagine flying North and turning right ninety degrees.
Nope, numbers are fixed to the body. Body fixed to glider panel and the needle stays pointing North.
Imagine flying North and turning right ninety degrees.
The original gyroscopic Direction Indicator had a couple of numbers visible through a window on a horizontal disc which you flew around. You had to cage it after landing, during aerobatics and then reset it to a massive compass between your legs.
All this with no autopilot!
All this with no autopilot!
Below the Glidepath - not correcting
Seeing that Chippie directional Gyro reminds me of when I was intellectually challenged in first trying to fly and think simultaneously. I used to have imagine a string tied from the top of the stick to the heading I wanted to turn on to so you would "pull" the compass heading over to the lubber line. In this case, to turn from a Northerly heading to 330 you would "pull" the string to the left until the number came around!
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IIRC the story about an A4G pilot hitting the gunsight during an arrest onboard MELBOURNE has been told elsewhere. Sadly 'Neptunus Lex' the USN NavAv story teller is no longer with us whilst some of his old stories have been archived here: https://thelexicans.wordpress.com/20...-neptunus-lex/ I could not find the '8-day clock [& wet compass]' story so here is the last bit about it - Skyhawkwise after a 'utility hydraulics failure'.
Face to Face with the 8-Day Clock [& Wet Compass] 23 Apr 2011 Neptunus Lex
“...I chose an off-duty runway for my straight in approach, and lowered the arresting hook to engage the approach end arresting gear. The wheel brakes would still have worked just fine, but without wing spoilers the Scooter could be a little tricky to handle on deck at high speed.
I had by that time a few hundred arrested landings, most of them in the FA-18. In the Hornet you were supposed to lock the restraint harness that kept you strapped securely in the ejection seat before landing - it was part of the landing checklist - but it was not big deal if you didn't: There was an inertial reel incorporated within the seats of all carrier aircraft that would sense sudden decelerations and apply the brakes. The Scooter had one too.
Only that inertial reel, like my poor, wounded utility hydraulic line, was also 35 years old. I landed safely at about 125kts, took the gear at maybe 115 or so and was thrown violently forward in the cockpit, getting a face-full of the 8-day clock [& wet compass] mounted on the canopy as the airplane came to a sudden stop, the arresting wire snaking and hissing behind it. It was a sunny day, fortunately, because my helmet visor was down and took the brunt of the collision, cracking in two places, or otherwise I would present an entirely different face to the world to this day. I was momentarily stunned and a little bruised, but none the worse for wear as the aircraft ticked down in the cable and the fire trucks raced up alongside me.
And I wasn't feeling quite so smug about checklists.”
http://www.neptunuslex.com/2011/04/2...e-8-day-clock/
OR: https://thelexicans.wordpress.com/20...-neptunus-lex/
“...I chose an off-duty runway for my straight in approach, and lowered the arresting hook to engage the approach end arresting gear. The wheel brakes would still have worked just fine, but without wing spoilers the Scooter could be a little tricky to handle on deck at high speed.
I had by that time a few hundred arrested landings, most of them in the FA-18. In the Hornet you were supposed to lock the restraint harness that kept you strapped securely in the ejection seat before landing - it was part of the landing checklist - but it was not big deal if you didn't: There was an inertial reel incorporated within the seats of all carrier aircraft that would sense sudden decelerations and apply the brakes. The Scooter had one too.
Only that inertial reel, like my poor, wounded utility hydraulic line, was also 35 years old. I landed safely at about 125kts, took the gear at maybe 115 or so and was thrown violently forward in the cockpit, getting a face-full of the 8-day clock [& wet compass] mounted on the canopy as the airplane came to a sudden stop, the arresting wire snaking and hissing behind it. It was a sunny day, fortunately, because my helmet visor was down and took the brunt of the collision, cracking in two places, or otherwise I would present an entirely different face to the world to this day. I was momentarily stunned and a little bruised, but none the worse for wear as the aircraft ticked down in the cable and the fire trucks raced up alongside me.
And I wasn't feeling quite so smug about checklists.”
http://www.neptunuslex.com/2011/04/2...e-8-day-clock/
OR: https://thelexicans.wordpress.com/20...-neptunus-lex/
Thanks for the replies so far, and in particular to SpazSinbad for the lengths he's gone to.
I should have made it clear that my interest is in compasses used for steering. And, more specifically, the fact that I may have cornered a 'Walt'.
He claims to have flown Sea Vixens and also mentions Meteors. I had my doubts, so mentioned that I had an E2.B (a standby compass that he would probably have sat behind). He blustered a bit over what an E2.B might have been and so I told him it was a compass. His immediate response was to ask whether it was graduated in Mils or degrees, and went on to explain that you had to know which it was when being given a course to fly. At which point the rat became a bit smellier.
I should have made it clear that my interest is in compasses used for steering. And, more specifically, the fact that I may have cornered a 'Walt'.
He claims to have flown Sea Vixens and also mentions Meteors. I had my doubts, so mentioned that I had an E2.B (a standby compass that he would probably have sat behind). He blustered a bit over what an E2.B might have been and so I told him it was a compass. His immediate response was to ask whether it was graduated in Mils or degrees, and went on to explain that you had to know which it was when being given a course to fly. At which point the rat became a bit smellier.
The aircraft compass system used in the Wessex HAS3 and the Sea King HAS1 was derived from the TSR2 and was a wonderful piece of kit in the Wessex, with a large cockpit display and easily flown to 1 degree increments. For some bizarre reason the same compass system in the Sea King saw the display replaced with a tiny instrument calibrated in 5 degree segments: made GCA and other IF work quite a challenge!
But always in degrees, as already opined by others.
Other than surveying and gun sighting, it's hard to imagine anything that could be done to an accuracy of a degree or better; certainly not steering a moving object of any kind.
Which was something I used to ponder on when doing a compass swing on a raw Norfolk February morning out on the airfield. I cant remember the accuracy of the headings I passed to the nav inside the aircraft after reading them off the Watts Datum Compass, but I am sure it read to less than 0.1 of a degree. Could never see the point of that, nor the laborious Fourier's Analysis which followed the swing. But then I was only a hack squadron nav - maybe somebody more versed in the mystical art of navigation might enlighten me!
Which was something I used to ponder on when doing a compass swing on a raw Norfolk February morning out on the airfield. I cant remember the accuracy of the headings I passed to the nav inside the aircraft after reading them off the Watts Datum Compass, but I am sure it read to less than 0.1 of a degree. Could never see the point of that, nor the laborious Fourier's Analysis which followed the swing. But then I was only a hack squadron nav - maybe somebody more versed in the mystical art of navigation might enlighten me!
On a cold winter's morning standing in the slipstream of an aircraft at full power, I think it unlikely if any heading we recorded was within 5 degrees. And as for a Fourier analysis, I scraped a C at O level maths, so that's all I'm going to say about that matter.
As approximately half of all RAF pilots passed through my hands between 1990 and 1993, I'm probably more responsible for the decline in RAF pilot navigation standards than anyone else!
Last edited by Dan Winterland; 26th Apr 2017 at 02:54.
It looks from the Sea Vixen photos that it had AHARS (or the AI and Compass fed from the MRG in non commercial speak). You could ask your Walt about that - he would know if he flew the thing.
I have never heard of an aircraft having a compass calibrated in mils. I used them in the Army, IIRC a mil was tens yards displacement at 1000 yards. Useful info if you are trying to drop a mortar bomb on someone's head.
I have never heard of an aircraft having a compass calibrated in mils. I used them in the Army, IIRC a mil was tens yards displacement at 1000 yards. Useful info if you are trying to drop a mortar bomb on someone's head.