10
$\begingroup$

This question deals with flying by reference to old-fashioned "steam gauges", not a modern "glass cockpit".

I recall hearing some pilots very experienced (ATP) pilots advocate for the older-style "turn rate indicator" (senses rotation about the aircraft's up/down axis) over the newer-style "turn coordinator" (senses rotation around an axis canted about 35 degrees from vertical, to sense roll as well as yaw.) Specific reasons were given, but I don't recall them. In what ways is a turn rate indicator more useful than a turn coordinator, during "partial panel" flying (no working attitude indicator or heading indicator) in clouds?

The purpose of canting the gyro's axis of rotation to sense roll as well as yaw is to give an earlier warning to the first stages of a developing bank and turn, and in my experience this is indeed helpful, but apparently there are some drawbacks as well.

$\endgroup$
  • 3
    $\begingroup$ Yes ,comment above is correct. I'm sorry to change the question in a way that invalidates one or two sentences in a previous answer but it has only been up a few minutes and contained a glaring error as was pointed out in that answer.Perhaps a comment might have been a more appropriate way to point out a problem w/ the question that obviously needs fixing; hope it's not a problem that I changed the question in this manner. Thanks for the replies so far. $\endgroup$ – quiet flyer Oct 17 '18 at 15:23
  • 3
    $\begingroup$ Editing the question is definitely the right way to go -- that's how SE is designed. $\endgroup$ – Cullub Oct 17 '18 at 20:02
8
$\begingroup$

A turn coordinator's axis of rotation is tilted around 30 degrees and rotates in the opposite direction. Because of this, looping errror is reversed and it under-reads at more than 1g. This makes it essentially useless when recovering from an unusual attitude on instruments, which is one situation in which you can easily find yourself when flying partial-panel.

With a turn rate indicator the instrument will over-read when the pilot is pulling excess G's in a turn; all the pilot has to do is relax the backpressure on the stick or yoke to correct.

$\endgroup$
  • 1
    $\begingroup$ Well, I can say in the situation where I really really wished I had a working turn rate indicator or turn coordinator, there was definitely a strong G-load. $\endgroup$ – quiet flyer Oct 18 '18 at 3:11
  • 2
    $\begingroup$ Re the effects of "looping error" in the turn rate indicator versus the turn coordinator-- Is it really inherent in the concept of tilting the gyro axis to sense roll as well as yaw, that the gyro must spin in the opposite direction in the (canted) "turn coordinator" as in the (non-canted) "turn rate indicator"? If so, why? And if not, why did the designers of the turn coordinator choose to do this? Perhaps this would be appropriate as an actual question. $\endgroup$ – quiet flyer Oct 18 '18 at 3:55
  • 2
    $\begingroup$ Re the concept of "looping error", this post may possibly be of some interest-- apparently if the springs that try to hold the gyro axis fixed relative to the aircraft are stiff enough, "looping error" can be greatly reduced-- pprune.org/tech-log/… $\endgroup$ – quiet flyer Oct 18 '18 at 3:57
5
$\begingroup$

You have the yaw/roll switched. The TC's canted gyro adds roll to the yaw rate sensing, not the other way around.

The biggest drawback to the turn coordinator was the presentation of the aircraft, which to someone new to its use can tend to get mentally processed as a horizon bar, especially an old timer trained on aircraft with war surplus black face artificial horizons (with were widely installed on light aircraft into the 60s until surplus stocks ran out). The TC's little airplane tilting left could initially be mentally interpreted as a bank to the right in other words, until your conscious brain finished processing the information (Russian artificial horizons had this presentation where the gyro tilted an airplane symbol; transitioning to western style horizons can be a problem for Russian pilots).

A turn and bank with the vertical needle is easier to interpret at a glance. The problem with the turn and bank was since only yaw rate information was presented, and there is no internal damping, in bumpy air you ended up with too much "noise" in the information presented from the nose wagging back and forth. In an airplanes like V tail Beech Bonanza, which were quite bad for tail wagging in turbulence, a regular turn and bank could be nearly useless in the bumps with the needle slewing back and forth constantly in level flight. TCs avoid this by not just splitting the motion sensing between roll and yaw, but also by using internal dampers (little shock absorbers) to present only movement that is established.

One problem with TCs is that once you are stabilized in a turn, there is no more roll rate to sense, only turn rate and that has to be taken into account in the way the rate markings are placed.

My old '68 Cardinal had a TC that had gone bad because the internal damper had gone south. The thing jumped around like crazy because of the un-damped sensitivity to both roll and yaw. I replaced it with an older 12v electric turn and bank (way cheaper - Turn Coordinators are quite expensive because they are electric with internal inverters to convert 12vdc to 110v/400 Hz fixed frequency ac for the gyro) and found I had a strong preference for the vertical needle presentation.

Personally, I think the turn coordinator should have stuck with the vertical turn needle presentation, but with the damped canted gyro internals. Like I said, much easier to interpret at a glance.

$\endgroup$
  • $\begingroup$ Yes, thank you, re yaw/ roll switching $\endgroup$ – quiet flyer Oct 17 '18 at 15:21
  • $\begingroup$ I definitely agree re ease of interpretation at a glance. $\endgroup$ – quiet flyer Oct 17 '18 at 15:28
  • $\begingroup$ PS look at this piezoelectric turn rate indicator-- do you see the problem with the unmodified display on the left-- the pilot has to remember that the LED lights are supposed to represent the wingtips of the aircraft, NOT the horizon-- it is kind of backwards from a conventional "turn coordinator", which uses a somewhat similar display face. The other device shown at the end of the video may be mounted either flat to sense pure yaw, or tilted to mix roll sensing -- I prefer the latter. vimeo.com/64331923 $\endgroup$ – quiet flyer Oct 17 '18 at 15:31
  • $\begingroup$ Yeah that's even worse, although he's invented a great gadget. Imagine how instantly recognizable the indication would be with just a vertical pointer moving right for right turn and left for left turn. $\endgroup$ – John K Oct 17 '18 at 15:36
  • 1
    $\begingroup$ You'd be the first person I know who prefers the vertical needle indication. The tilting airplane is much more obvious for interpretation, even if you don't know exactly what it means. But both are 'good' in the sense that to 'fix the problem', you counteract (oppose) the indication with the stick/yoke. That's how all indication should be, because in a stress we instinctively behave that way. This is why Russian pilots have big problems with the 'western' (direct) AI while western pilots never have problems with the 'Russian' (reverse) AI. (And why the @quiet's indicator is just horrible). $\endgroup$ – Zeus Oct 18 '18 at 1:14
1
$\begingroup$

Some other reasons a turn coordinator is less useful than a turn rate indicator--

  • Due to the fact that the axis of the gyro is canted to sense roll as well as yaw, a turn coordinator cannot be calibrated to read accurately for all combinations of airspeed and bank angle.

  • In an inverted spin, the direction of roll is opposite the direction of yaw, so a "turn coordinator" with the canted axis of sensing may either give an indication of a turn into the direction of spin, or the opposite. For some discussion of this see https://www.pprune.org/archive/index.php/t-56241.html

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.