View Full Version : Turn co-ordinator vs Turn Indicator

11th Jun 2002, 19:13
Some advice please.

I have been told that a Turn Co-ordinator (the one with the little aeroplane symbol) is not the best instrument for advanced aerobatics - in particular inverted spinning - and should ideally be replaced by a Turn Indicator. I have tried to do a little research into this, but am still confused.

Does anyone have any knowledge/views on this, and can explain it in laymans language (without going into too much gyroscopic detail)

11th Jun 2002, 20:00
Instruments such as the turn co-ordinator are gyroscopes. Gyroscopes "topple" when thrown around by an aircarft doing aerobatics. They then take several minutes to resettle, before they give true readings again. Some gyroscopic instruments can be "caged", before performing aerobatics, to stop them toppling, but they do not show a correct reading when caged.

A turn indicator is not a gyroscope (but don't ask me the techie stuff on how it works :D ). Therefore a turn indicator cannot topple, so gives a correct indication during spinning & other aerobatics.

I hope that helps, but perhaps the techie experts can give you more info.

11th Jun 2002, 21:28
Thanks for your reply. However, I'm still confused. The following are selected quotes from an article on Gyroscopic instruments I found by searching the net:

Turn & Slip Indicator
"The turn needle is an electrically driven gyroscope which indicates the rate of turn"
"This type of instrument is usually found in older aircraft"

Turn Coordinator
"The Turn Coordinator is similar to the Turn & Slip Indicator. It is found in more modern aircraft. The main difference is in the presentation of the turn. A miniature airplane is used to show the bank instead of a needle"

Another article states that the T&S Indicator shows Rate of Turn only, whilst the Turn Coordinator also shows Rate of Roll.

However, what I am interested in is which of them is most reliable in an inverted spin situation

11th Jun 2002, 21:48
It is late and I haven't time to ensure that I have double-checked the science bit so in simple terms:

when I built my aerobatic biplane I installed a turn and slip (needle & slip ball) because:-


That is the important bit.

Having experienced most spins - I can recall occasionally checking the needle to ensure that it agrees with the known facts BUT I guess I would have to be totally disorientated to have to use that as my primary recovery tool. I spin regularly but the :-

Yawing to the left/rolling left (erect)

yawing left but rolling right (inverted)

discussions really confuse me - perhaps I am not spatially aware enough to comprehend them whilst in the classroom.

It just makes more sense when it is happening!

This year's BAeA sequences higher than beginners all incorporate spins of increasing complexity - I have tried them all and can not recollect ever having time to peruse the turn&slip!!



11th Jun 2002, 21:57
By turn indicator I presume you're referring to the standard 'Bat & ball' Turn & Slip' instrument (TI). This instrument most certainly contains a gyroscope!

The difference between the TI & the Turn Coordinator (TC) is that the TC has the axis of rotation of its gyro inclined by ~30deg. This makes the instrument sensitive to forces in two planes of motion: yaw & roll.

A spin involves moving all planes so the TC will respond to rolling forces, not just yawing/turning.

Send Clowns
11th Jun 2002, 22:21
Between Tinny and stiknruda you have the answer.

The turn co-ordinator is a horrible, misbegotten mix-up of an instrument, which has the axis of its gyro tilted to sense rate of rall mixed with rate of yaw. A yank attempt to make a more quickly-responding instrument I believe (impatient nation).

In an erect spin yaw and roll are in the same direction. In an inverted spin from the point of view of the upside-down pilot and instruments the roll direction is opposite to the yaw.

Here's the catch : the turn co-ordinator sees both, so may respond to one, or the other, or neither. The pilot has senses that are fallible, and tends to see the roll more than the yaw. Since the yaw determines direction of spin and direction of required rudder to oppose the spin, this can be disasterous.

The turn needle will always give correct direction of spin. Hence good spin recovery training (for inadvertant spinning, not aerobatic intentioal spins) teaches you always to check your needle before recovery action, not to trust your senses.

I did a lot of early training on aerobatic types, which of course had turn and slip indicators, hence my dislike of turn co-ordinators.

QNH 1013
11th Jun 2002, 23:00
A bit more info...

The Turn Indicator (and Turn Co-ordinator - Tinstaafl and Send Clowns have explained the difference) are normally electrically driven in the average light aircraft whereas the other gyros (DI and AI) are vacuum driven. The rotation speed of the TI or TC is, I understand, much higher than the DI and AI and because the former pair have only one degree of freedom, they won't topple. In a loss of control in IMC, the TI or TC is what you should rely on.

I'm assuming you are not inverted at this point. The check that you're not inverted should of course be your first action in the event of loss of control in IMC. If you get this bit wrong, you're in real trouble.

12th Jun 2002, 08:04
Thanks for the informative replies, Folks.

I have now learnt 2 things:

Firstly, all about the different instruments that indicate turns etc

Secondly, don't reply to threads, when I have no idea what I am talking about! ;)

12th Jun 2002, 08:53
My understanding of the reason for the invention fo the turn co-ordinator is that it helps in IMC during the entry to a turn. The turn indicator will not indicate a rate 1 turn until you are actually doing a rate 1 turn, whereas the turn co-ordinator, because it detects movement in the rolling axis, will indicate a rate 1 turn even before rate 1 is acheived, ensuring a smooth entry into the turn just by keeping the wings pointing at the rate 1 marker. Once the rolling action has stopped and the turn has stabilised, the turn co-ordinator will indicate the rate of the turn the same was the turn indicator does. I seem to remember something about it not showing any rate correctly except for rate 1, but can't remember why that is, so I might be wrong.

I'm certainly not an aerobatic expert, and I know there are people on here who are - but I'd be very surprised if you were looking at any of the instruments while spinning. It's true that instruments may help you if you become disoriented during a spin, but I'd suggest that if you're not comfortable enough with spins to not become disoriented during them, you shouldn't be doing solo aerobatics. The turn co-ordinator, I would have thought, would be far useful for a non-aerobatic pilot in recovering from an inadvertent spin. And since it's very hard to get into an inadvertent inverted spin (unless you're doing aerobatics), I think this thread is an academic one, although still interesting.

Finally, distaff, please don't stop posting on threads where you have no idea what you're talking about. I do, frequently (including this thread) - and it's often resulted in having my misconceptions put right, or occassionaly having something that I thought I knew, but wasn't sure about, confirmed!


12th Jun 2002, 09:37
Thanks for all the replies, things are a lot clearer now. The point of my original question was to clarify the situation about inadvertent spinning during aerobatics, especially inverted.

So it wasn't academic as far as I am concerned, as I intend to move more seriously into aerobatics, and the particular aircraft involved has a Turn Coordinator. It seems that, for peace of mind, I should get a Turn Indicator installed. Now the question is, should I replace the TC with the TI, or have it as an additional instrument?

12th Jun 2002, 10:11
If the aircraft is to be used mainly for aerobatics, why not make a straight swap? The co-ordinator might be handy if doing instrument flying, but most aerobatic aircraft don't do much of that. Have you got an aerobatic AI, with a good caging mechanism? We had to fit an expensive new one in our Cap 10 last year after someone renting the aircraft bust the old one, aerobatting without the gyro caged. There's a naughty video shot of the new AI rolling 360 degrees on the aircraft's website, but that's for marketing and we've told him not to do it again!

Lastly, if spinning inverted and confused, the rudder which offers most resistance is the one to stomp on for recovery.

Stan Evil
12th Jun 2002, 20:32
Just a couple of points.

FlyingforFun you said "I'm certainly not an aerobatic expert . . .". If you were you would know that the ONLY reliable way to tell the direction of a spin is to look at the turn needle and that the standard spin recovery is based on doing this. That is why, as Send Clowns says, the turn co-ordinator is such a dangerous instrument in a fully aerobatic aircraft. FNG, having flown an aircraft with an almost untoppleable AI (cost of AI/HSI around 100000), the old fashioned turn needle was still the recovery cue in a spin.

On the tech side, the turn needle and turn co-ordinator gyros actually spin a lot slower than the AI and DI. The reason is that the AI and DI rely on rigidity to give you the information you want and that needs a high rpm whereas the turn needle and co-ordinator rely on precession. This means that the turn co-ord or needle need only a low voltage DC supply to keep them working while the AI needs a lot of suck/blow or an AC supply (or DC plus an internal inverter).

Undertheweather. In answer to your question about keeping the TC as well as the TN I'd say no. The TC only has advantages over the TN if you plan on spending lots of time IMC with no AH. Under normal circumstances you're going to fly attitudes and ony use the TN to confirm that the AoB you're using is giving you a rate 1 turn. If you lose the AH then you'll be doing a little bit of flying on the TN to get VMC again so the slight loss of initial sensitivity to roll is hardly a big issue. If you keep both you could get very confused when they indicate opposite directions (as they will when you roll out of a turn).

12th Jun 2002, 20:35
Send Clowns - thanks for the clarification.

FNG - have been trying to contact you since I received your mail last week which I am unable to open. Drop me another note with yr work e-addy!



Send Clowns
12th Jun 2002, 23:06
It's incredible what you find out about instruments when someone expects you to teach them :D ! The spin recovery I knew from aerobatics training, but I never realised how many assumptions I had about various aspects of flying including basic instruments until I became a ground instructor.

13th Jun 2002, 09:18
distaff - see what I mean about having misconceptions put right when you post about something you don't know much about?

Thanks for putting me right about the turn needle being the only reliable way of telling the direction of the spin, Stan!


13th Jun 2002, 09:47
Stik, I think that we've re established contact through steam powered pprune mail.

Stan Evil: I wasn't for a moment suggesting that the AI was of the slightest relevance to spin recovery. My point was that, whilst considering instruments for an aerobatic aircraft, getting an AI which can be safely caged for aeros is a good idea as otherwise the thing gets damaged by all the leaping about.

13th Jun 2002, 10:51
FFF Rest assured, being totally wrong won't be sufficient to stop me posting :D

Anyhow, folks, all the very informative posts, above, leads me on to another question.

The last time I did any aerobatics, was about 6 years ago in a Citabria. It was a venerable, but docile taildragger, & the only electrical item in it, was its one radio. And it did not have any form of vacuum pump.

I therefore seem to recall that it did not have ANY gyros, but I stand to be corrected, AGAIN!

Its only instruments were: wet compass, airspeed indicator, VSI, RPM guage, oil temp & pressure guages (no DI, AH, or turn co-ordinator)
...... AND a very old fashioned Bank & Slip Indicator

Does a Bank & Slip Indicator have a gyro?
If so, or if not, how are they normally driven?

As I have booked up to go spinning again, in few weeks time, in a C152 aerobat, with an instructor ('cos I am not that brave/mad), I have been reading this thread with interest ;)

Stan Evil
13th Jun 2002, 19:22
Distaff. If the 'bank and slip' indicator you mention could give you a rate of turn then it was, assuredly, gyro driven. Even the Tiger Moth has one of these in the centre of the instrument panel. You don't need a vacuum pump, just a venturi somewhere on the fuselage. These normally look like a short cone and a long cone joined tip to tip. This system is nice and simple but relies on you having some airspeed and so won't work on the ground.

The use of venturis reminds me of a story, probably apocryphal: on the Harvard there were 2 venturi, one big one for the instruments and a small one to ensure that users of the p-tube didn't get their own back. Once day a big, instrument one was accidentally fitted to the p-tube and an unfortunate pilot experienced the 'Chartham Method' unexpectedly. He had to slow down to the stall to extract himself from the predicament. Makes my eyes water just to think of it!

13th Jun 2002, 21:27
Stan Evil Thanks for the reply. Yes, you must be right.

I dug out an old edition of Trevor Thom - Aeroplane Technical & from the illustrations, I think that what I was recalling in the Citabria, was probably a very old model of Turn Indicator. And, of course, if that is so, then it must have a rate gyro.

Right thats it, I am going to give up on trying to under the techie stuff, I just get back to flying :D

Mind you, I understand fully the workings of GPS
..... NO I am not going there! :D :p :rolleyes: :D

14th Jun 2002, 21:52
Just to clearify a few things!

Firstly both the turn & slip indicator and the Turn-coordinator is gyroscopic instruments.

Secondly Instruments of the such mentioned above is not designed for use in aerobatic aeroplanes, however they may well be used, but one should then be very aware of their limitations as with all mechanical gyros.

The turn and slip and turn-coordinator are mainly used in instrument flying as they indicate the rate of heading change. When flying IFR there are procedure turns, holdings, reversal procedures that are designed according to given rules. These being PANS-OPS (Old, New or even Newer), or TERPS; there are a bunch of these. Point being that all of these procedures are designed within a given airspace therefore also a given turn rate. these are referred to as rate one turns. 3 degrees / second. The istruments has dots on them to indicate when wether the aircraft wings of the instrument or the bat aligns with these dots they indicate you have a rate one turn, and therefore you are adhering to the desing of the procedure you are flying.

Regarding the question on how gyros are driven. There are four ways that is used today: They are Electrical, Vacuum, Laser or Piezo-elements. Electrical and Vacuum driven gyros are the old fashion gyros and are seldomly used in modern airliners, they are also called mecanical gyros as they have a mass spinning around an axis. The piezo-gyro is a piezo-electrical element welded in place with a silicon mass and sensors around it therby inducing electrical impulses that are sensed by the sensors around it and therby interpreted by a computer. This type of gyro is mostly used in model aeroplanes and helicopters and as far as I know not used in real-size aviation at all. The laser gyro is a ring or infact a triangle of two lasers. the lasers are directed opposite of one another and sends a beam of laser light hitting mirrors, reflecting the light in a triangle back to two sensors. The sensors registers the time from emmision to the time of reception for both lasers. Diffrence in the two emissions indicate that the laser platform has moved and therefore it is able to determine what way it has moved. These signals are then sent to the VDU by a computer (Visual display unit) telling the pilot the information he needs. This is generally used in modern glass cockpit designs.

Pretty technical, but explanatory I hope..

PS: And as M14P writes further down here, which is quite right, The time measured is not really what is measured. It is the Phase change of the laser beam. The phase change gives one the indication of TOT (Time of travel) for the beam which is necessary to calculate the amount of movment of the ring or (gyro platform).

Far Canard
15th Jun 2002, 04:46
The turn co-ordinator has its rate gyro precession axis inclined 30 degrees relative to the aircraft's longitudinal axis. This means it is precessed by roll and yaw. The benefit comes when flying IFR and having a major instrument failure. Flying on partial panel with a turn co-ordinator which responds to roll is a benefit.

Most purpose built aerobatic aircraft have no gyros fitted as the manoeuvring causes large rates of gyro precession and ultimate failure of the unit. If you fit one it would need to be a basic turn and slip indicator responding to yaw only.

15th Jun 2002, 06:04
I'm not a fan of the turn co-ordinator, although it is easier to use in IMC if the AH has failed.

It is certainly possible to use the Turn and Slip to fly IMC letdowns, but it takes a lot of practice and a fair bit of skill. The RAF IR for the Bulldog required you, without the availability of the AH or DI to achieve the overhead, and to depart on a particular track using the T&S and stopwatch and then to fly a radar letdown in simulated IMC. It wasn't too difficult given some practice and I certainly used it several times after aerobatting or tail-chasing over 8/8 cloud - although some wimps wouldn't ever consider that in case the AH didn't re-erect before entering cloud........

16th Jun 2002, 09:33
Sorry to be pedantic but the laser gyros don't measure the arrival time of the beam of light (a bit difficult to create a time difference at the speed of light) but rather the phase change as the gyro is moved in its plane of freedom. Llike the t/s they only have a single degree of freedom so you need three to get pitch/roll/rate of turn info.

I have seen a few instruments using Piezo elements for standby info in airliners. Absolutely fantastic as well as light and compact. Hopefully every light a/c will soon be fitted with similar systems thus providing accurate and error free attitude info!

PS - impure though it is I find the turn coordinator much more useful than a t/s when flying partial panel etc. This fact is borne out by the S-Tec rate based autopilots which only use the t/c as their reference.

16th Jun 2002, 20:55
I have seen a few instruments using Piezo elements for standby info in airliners. Absolutely fantastic as well as light and compact. Hopefully every light a/c will soon be fitted with similar systems thus providing accurate and error free attitude info!

Got $2k to spare for a sexy toy?

GA EFIS (http://www.pcflightsystems.com/pcefis.html)

Shaggy Sheep Driver
17th Jun 2002, 10:42
We were discussing Yak 52 spins at the weekend, and a very exerienced Yak aeros pilot said that the turn needle was not always to be trusted to tell the direction of the spin. In fact in certain '52 spins the turn needles in the front and rear cockpits can indicate diffrrent directions to each other.