Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

Turn Gyro Axis

Tech Log The very best in practical technical discussion on the web

Turn Gyro Axis

Old 26th Feb 2003, 09:57
  #1 (permalink)  
Thread Starter
 
Join Date: Feb 2002
Location: uk
Posts: 37
Likes: 0
Received 0 Likes on 0 Posts
Turn Gyro Axis

This might seem a pretty pointless and academic question with not much relationship to being a practical pilot, but the JAA ask it as an ATPL exam question anyway and I’d like to know if anyone can find an answer (or even a reference book).

Using a Turn and Slip indicator in a balanced turn, are you measuring angular rate with reference to the vertical or with reference to the aircraft’s yawing axis?

You see, in a balanced turn, the precession will cause the rotor of the rate gyro to go to the vertical. I think you’re measuring with respect to the vertical, ie, the rotor’s orientation at the time. If this is correct, then your Rate One turn will be measured with respect to the vertical axis and should then correspond to your stopwatch and the amount of heading change. If measured about the yawing axis, there will be a small difference (admittedy, not very much in a gentle turn, but it would build up over several continuous turns).

I can’t find any British pilot textbook that gives the answer. A university textbook on gyrodynamics will probably have the answer and maybe John or Ghengis know it anyway. Anyone know?

The thought does occur to me that if British pilots have not needed to answer this question for the many years we have had national exams, why does this suddenly become essential information when the French set the questions? I think that they must have different pilot textbooks. I wouldn’t mind if they’d only give us a clue as to what they are.

Last edited by greengage22; 26th Feb 2003 at 12:40.
greengage22 is offline  
Old 26th Feb 2003, 11:48
  #2 (permalink)  
 
Join Date: Sep 2001
Location: Toronto
Posts: 2,403
Likes: 0
Received 0 Likes on 0 Posts
Thumbs down

Definitely one of the all time totally irrelevant questions.

Especially considering that the last airliner that had a turn and slip was likely the DC-3 or similar vintage.

Maybe the Aviation History and Nostalgia folks might be more knowledgable.
RatherBeFlying is offline  
Old 26th Feb 2003, 15:23
  #3 (permalink)  
 
Join Date: Jun 2001
Location: UK
Posts: 3,972
Likes: 0
Received 0 Likes on 0 Posts
Yaw must surely be about the normal axis (ie not the vertical!),
fireflybob is offline  
Old 26th Feb 2003, 16:04
  #4 (permalink)  
 
Join Date: Aug 2000
Location: UK
Posts: 3,648
Likes: 0
Received 0 Likes on 0 Posts
Question

You see, in a balanced turn, the precession will cause the rotor of the rate gyro to go to the vertical.
Hold on a minute. It may cause it to go slightly out of plane, but not vertical. The angle by which the rotor precesses will depend on the elasticity of the strings used to tie it. Use very stiff springs (with a highly geared indicator) and it will remain very close to its original axis.

The question I've always wondered about is which axis is the turn gyro spinning about (in equilibrium). Clearly it has to be perpendicular to the yaw axis of the aircraft, but it could be fore/aft or port/starboard. My (very dusty) groundschool notes indicate port/starboard, but both the construction of the cases I've seen and credible source suggest fore/aft. Anybody taken a modern turn indicator apart?
bookworm is offline  
Old 26th Feb 2003, 20:20
  #5 (permalink)  
Eckhard
Guest
 
Posts: n/a
Not a modern one, but the 'cut-away' we used in groundschool had the axis across the case, with the rotor spinning forwards at the top. This ensures that the precession caused by the turn will tend to keep the rotor vertical (or at least closer to vertical) so that the yaw rate will be measured about the vertical to the horizon, not the normal axis of the aircraft.

Now if you want a really arcane discussion, let's talk about the turning errors of the artificial horizon!?!
 
Old 26th Feb 2003, 23:24
  #6 (permalink)  
 
Join Date: Sep 2001
Location: UK
Posts: 763
Likes: 0
Received 0 Likes on 0 Posts
Once a turn rate has been established and the rotational plane of the gyro has been displaced away from the vertical, there is a component of pitch rate that can be resolved perpendicular to the plane of rotation of the gyro. This increases the precessional moment generated by the yaw rate, and therefore the angular rate measured is greater than the pure yaw rate of the aircraft. I think, therefore, that it must measure turn rate with respect to the vertical earth axis but I have yet to work through the maths to prove it. It certainly indicates greater than the yaw rate.

The axis of a turn rate gyro is athwartships i.e. parallel to the lateral axis of the aircraft, with the gimbal giving freedom in roll (yz plane). If the axis were fore and aft, the gimbal would have to have freedom in pitch (xz plane). Therefore, it would only indicate yaw rate.

As for relevance, whatever happened to old skills such as limited panel IF? It may not be taught anymore but, if you do have a turn needle, one day this technique might just save your neck! And if you can fly limited panel, your full panel IF will doubtless benefit.

On a lighter note, I was told when in training that the turn gyro rotated up and away from you so that if it fell of its bearings it would roll away from you and not hit you in the nether regions!

Last edited by LOMCEVAK; 27th Feb 2003 at 07:22.
LOMCEVAK is offline  
Old 27th Feb 2003, 13:25
  #7 (permalink)  
 
Join Date: Dec 1998
Location: Escapee from Ultima Thule
Posts: 4,278
Likes: 0
Received 0 Likes on 0 Posts
...if it fell of its bearings it would roll away from you and not hit you in the nether regions!

Probably not true but a laudable sentiment nonetheless...



Last edited by Tinstaafl; 27th Feb 2003 at 13:36.
Tinstaafl is offline  
Old 27th Feb 2003, 17:31
  #8 (permalink)  

Do a Hover - it avoids G
 
Join Date: Oct 1999
Location: Chichester West Sussex UK
Age: 89
Posts: 2,206
Likes: 0
Received 0 Likes on 0 Posts
When you fly a turn and gradually increase the bank angle, it is not long before the turn needle gets to the stop.

If you continue to increase the bank angle and back pressure up to 90 deg of bank then the needle stays pinned in the max stop. But now the aircraft is only pitching.

I think this answers the question – it must measure turn rate about the earth’s vertical.

But I am sorry I do not have a reference I can quote.
John Farley is offline  
Old 27th Feb 2003, 19:52
  #9 (permalink)  
 
Join Date: Aug 2000
Location: UK
Posts: 3,648
Likes: 0
Received 0 Likes on 0 Posts
Question

I think there's a big leap between identifying an error in turn indicator output at high bank angles (which I don't dispute for a moment, John) and the idea that it is measuring a rate around the vertical. How does it even know where the vertical is? It doesn't have an erection mechanism, does it?
bookworm is offline  
Old 27th Feb 2003, 20:15
  #10 (permalink)  

Do a Hover - it avoids G
 
Join Date: Oct 1999
Location: Chichester West Sussex UK
Age: 89
Posts: 2,206
Likes: 0
Received 0 Likes on 0 Posts
Good point bookworm.
John Farley is offline  
Old 28th Feb 2003, 11:55
  #11 (permalink)  
 
Join Date: Feb 2002
Location: oxford
Posts: 247
Likes: 0
Received 0 Likes on 0 Posts
I've had to look in several references to piece an answer together, but I THINK I've got it right. I'm willing to be corrected, though.

Firstly, this website definitely says it measures about the vertical, and I don't see how the turn rate would check against compass and stopwatch if it didn't.

Pallett ('Aircraft Instruments' by E J Pallett) agrees with Bookworm. To quote, he says 'In practice the gimbal ring deflection is generally not more than 6 degrees, the reason for this being to reduce the error due to the rate-of-turn component not being at right angles to the spin axis during gimbal ring deflection'. So he is saying that the gyroscopic effect is measured about the yaw axis, or at least, not more than 6 degrees from it, which is acceptable.

But, hang on, depending on your TAS, a Rate One turn could be 30 degrees of bank or more. This is where the pitch precession comes in.

If you banked to, say, 30 degrees and yawed, the axis of yaw is no longer perpendicular to the horizon. So you would be yawing towards the ground, if you took no corrective action. This is why the pilot applies back pressure during the turn and, if necessary, increases power as well. In doing so, he is altering the pitch attitude of the aircraft relative to its yawing axis.

This pitch produces a second precession, as LOMCEVAK says. In a rather elderly version of RAF publication AP1234 which I have in front of me, it calles this 'Looping Error'. It then goes on to say 'The indicators are calibrated for rate 1, 2 and 3 turns at specified angles of bank and TAS, and looping error is taken into account in this calibration'.

So I think that the answer is that the gyro actually measures with respect to the yaw axis (or within 6 degrees of it) but the combination of looping error and linkage to the needle gives a read-out of rate of turn with respect to the vertical.

But what a hell of a question! I'm still not sure what answer the JAA want.
oxford blue is offline  
Old 1st Mar 2003, 13:11
  #12 (permalink)  
 
Join Date: Aug 2000
Location: UK
Posts: 3,648
Likes: 0
Received 0 Likes on 0 Posts
Thumbs up

I think you've got it, oxford blue.

Feels like a thread that benefited from a little operational experience, a little bookwork, a little theory and a little logic. Thanks to all for the contributions -- I learned a great deal from this one.
bookworm is offline  
Old 1st Mar 2003, 18:18
  #13 (permalink)  
 
Join Date: Jun 2001
Location: Fragrant Harbour
Posts: 4,780
Likes: 0
Received 0 Likes on 0 Posts
Oxford blue has the answer.

'Looping Error' leads to the instrument over reading at anything more than 1g. Significant if recovering from an unusual position on instruments using the turn indicator - you must unload to about 1g to be able to find the wings level position.

A turn co-ordinator (as fitted as a factory item on most spamcans) has the axis of rotation tilted about 30 degrees and rotates the other way. As a result, looping errror is reversed and the instrument undereads at more than 1g which makes it near to useless when recovering from an unusaul position on instruments. For this reason, the club where I instruct has replaced turn co-ordinators with turn and slips on our fleet of PA28s.

Also, if you consider that in a balanced turn you have yaw roll and pitch, the pitch element makes the instrument over-read as the aircraft will be at more than 1g. For this reason, the turn indicator is calibrated to read correctly with this increased g by adjusting the tension on the springs, but at one speed only . The instrument fittted to the Chipmunk in RAF service was actually calibrated to 180 knots, so I don't think this correction is too significant.

But, if an aircraft is flying a turn at 90 degrees angle of bank (theoretical, I know), the turn will be just pitch with no roll and no yaw, and the instrument will read no turn. Therefore the yaw component reduces with increase in angle of bank, so this explains the angle of bank calibration Oxford Blue mentions.

Not an easy subject to be examined on - and of very little significance these days, as if a turn needle is incorporated into a modern instrument display, it's almost certainly derived from INS / IRS information.
Dan Winterland is offline  
Old 1st Mar 2003, 20:51
  #14 (permalink)  
 
Join Date: Jun 2001
Location: UK
Posts: 3,972
Likes: 0
Received 0 Likes on 0 Posts
A very interesting thread to say the least!

When I was doing my commercial flying training in 1969 we were taught to "calibrate" the turn needle whilst on full panel before going to limited panel,ie do a rate one turn as indicated by the turn needle and time same and see whether this was correct. If not make an adjustment and check again etc. Quite often there was a small error - although I would not claim that my if/timing were that accurate!

Dan, very interesting to hear that you have had your a/c converted back to the "old" turn indicator. The turn coordinator also has a rate of roll element (as well as rate of yaw) due to the 30 degree tilt, etc. In theory should a wing drop the turn coordinator will show this before the turn needle will since the latter has to detect yaw before indicating. This is supposed to make it easier to fly limited panel (other than recovery from unusual positions) but I have yet to be convinced!
fireflybob is offline  
Old 2nd Mar 2003, 03:59
  #15 (permalink)  
Moderator
 
Join Date: Apr 2001
Location: various places .....
Posts: 6,886
Likes: 0
Received 0 Likes on 0 Posts
.... quite amazing the detail of things one can learn by reading PPRuNe threads ..... is it not ?
john_tullamarine is online now  
Old 2nd Mar 2003, 09:54
  #16 (permalink)  
 
Join Date: Jun 2001
Location: Fragrant Harbour
Posts: 4,780
Likes: 0
Received 0 Likes on 0 Posts
....indeed, but perhaps we should all get out a bit more!
Dan Winterland is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.