Any Ideas on this one?

[swamp monkey]

The other day whilst we're bored flying along we discuss what we'd do in IMC if AH & TC fail. Amongst other things, someone comes up with the idea to put a bottle of water on the dash. Surely it'll show some sort of movement banking or pitching up/down.

So we try it, using anywhere from 5deg through to 60deg, the water in the bottle stays level with dash through a range of movements.

Being the rocket scientists that we are, any ideas as to why.

[itchybum]

You should refresh yourself with a look through your PPL Flight Instruments notes for this one. I could do with the same.

To my recollection, which is not the best, the AI (Attitude Indicator) and the T&B (Turn and Balance), otherwise known as AH and TC, are instruments driven by gyros, and before you say "Do you mean chicken gyros or beef gyros?", no I mean gyro-scopes.

The bottle of water, to my knowledge, is NOT gyro-stabilised and cannot reference anything other than gravity.

This is how they are able to reference a geometric plane. The gyro in a simple AI gets it's cues from gravity and holds its position by making use of the gyroscopes property of Rigidity in space. Because it is influenced by gravity, however, if you fly around in a nicely balanced turn for long enough the simple gyro-driven AI will begin to (very slowly) re-align to indicate you are wings level.

As for the T&B Indicator, the Balance Ball or Skid Indicator part of it is a very basic instrument whose information is to be used in conjunction with the Turn info (which is why the two together are known in civvy terms as a Turn Co-ordinator) and also the Attitude information.

The Skid Ball is unpowered and uses a spring. The Turn Indicator uses some sort of fancy lever action on a gyro and the gyros property of Precession (I think) to indicate a turn as it is happening. It can, however, be influenced by gravity: Pull enough 'G' during a turn, then reverse the turn and the Turn Indicator will continue to indicate the previous direction of turn.

If, in IMC with your multiple instrument failures, you descend in a nicely balanced spiral dive (a likely outcome for a novice) your bottle of water will sit nicely level and tell you all is well. Until the end, that is, where it will "indicate" a tremendous rate of deceleration to anyone still capable of observing it. It's only use then might be in the fire-extinguishing role.

If, on the other hand, you are like me and become immediately panic-stricken and begin to throw the controls around like Maverick trying to recover from his flat spin, your bottle will soon contain white water and become un-readable.

The only way the water is going to tell you anything is if you know how to read tea leaves and happen to have some to throw in.

You'd be better off carrying around one of those stupid "AI"s they put in Land Cruisers to tell morons when they're about to roll the thing. Or better yet, pull out your Emergency Duck from your nav-bag and have him give you his opinion on matters. It is well known ducks can fly in cloud although they prefer not to.

[apache]

Actually, I had heard that you need a CAT!

Cats always land on their feet, and have an aversion to being anything less than perpindicular to gravity. Of course, the old GPS is a fantastic tool, as it gives tracking information.... Keep the track steady, and you won't be turning (more or less).

Then there was the story relayed to me about how good "NATIVE australians" are in the bush. Story goes, that he was on his first charter, starting to sweat about exactly where this remote strip was. He was looking around outside, as one does, and happened to notice that all the "natives" seemed to be looking in one direction. So he points the plane in that direction, and all the heads look forward.As he got closer to the strip, the heads semed to be moving far more frequently.... so he followed their stares, and lo and behold... there was the strip! Dunno about yer AH... but apparently these fellas make a GREAT NDB!!!!!

[itchybum]

Yeah that's true, the abbos are bluudy awesome in the bush. Could spot air-traffic miles before I ever could, too.

As for the cat, I forgot about them. Apparently when you don't need the cat, you can keep his batteries from going flat by putting a rubber band around his gut. This inhibits his stabilisation functions.

[Milt]

If you are a very smooth pilot you should be able to fly a loop followed by a barrel roll and not see the water in the bottle move except for some sloping of the surface fore and aft as the aircraft accelerates and decelerates.

[Tinstaafl]

The water won't work because, unlike a gyroscope, it's not constrained to remain rigid in space while the airframe moves around it. Just like hanging a pendulum from the roof, it will align itself to the sum of ALL forces acting on it - gravity, lateral & longitudinal. In other words, it will move with the airframe, instead of staying aligned with the horizon.

WRT solving the problem there some alternatives. Recall limited panel IF. Power, ASI, altimeter & VSI are used together to derive pitch information. The pilot has to interpret the data to figure out the pitch attitude eg for any given power setting then if the a/c speed is reducing, altitude is decreasing & there's a RoD then the a/c must have a pitch that is too much ND for level flight. It's not as easy as having the pitch information directly presented, but it's certainly do-able.

For AoB another derivation needs to be done by the pilot. Again, direct AoB is no longer available, however if the balance ball is kept in the centre then the turn information from the rate gyro in the T&B or TC (& the compass or DI) can be used to figure out if the a/c is banked or not ie if the ball is in the centre AND the a/c is turning then the a/c must be banked.

If I was in the unfortunate situation of losing attitude & rate gyros then I suspect my only resort would be to resort to 'normal' limited panel IF techniques for pitch with the extra workload of deriving turn info from the DI or compass alone while keeping the ball centred.





whot speling erors?

[Arm out the window]

Just to clarify how the simple turn needle or coordinator works, it's a gyro with spin axis pointing side to side, spinning 'up and away from the pilot' as the old saying goes.

It can tilt side to side only, and that is constrained by a spring, so the more tilt, the more the spring tries to stop the tilt.

Now imagine you're yawing left. This turns the gyro mount anticlockwise when viewed from above, which has the same effect as pushing the rim of the gyro left at the front.
Precession makes the gyro lean to the right. The gyro is geared to an indicator needle that will indicate a left turn when this happens.

Why doesn't the gyro just fall over then? This is where the spring comes in. As the gyro leans right, the spring tries to stop it, with an effect that is like pushing the top of the gyro back to the left.
Remembering that it's spinning up and away from you, that would tend to make the gyro want to turn left when viewed from the top, the same as the original aircraft yaw. This is secondary precession, and when the spring force gets to a certain point the system reaches an equilibrium so that the secondary precession rate is the same as the yaw rate, and a stable rate of turn is indicated.

Getting your head round the precession / secondary precession thing takes a bit of thinking, but it's pretty straightforward once you try.

The reason why the gearing is there to 'reverse' the indication of turn direction (ie why don't they just have the gyro spin the other way and indicate directly) is that if you're also rotating the gyro mounts nose up, as in a spiral dive, it would tend to reduce the turn indication. Having it set up with the gearing means that nose-up pitching will increase the turn indication, making it obvious which direction you're turning.

This exaggeration of turn indication is called looping error.

As an aside, I far prefer turn needles to the little aeroplanes on turn coordinators - yaw isn't the same thing as roll, of course, so it seems stupid to have an instrument indicating a roll when it's not necessarily happening. Call me old fashioned if you will!

[Woomera]

Reminds me of two film footages I saw.

The first was Bob Hoover in his Shrike - had a glass of water sitting (not stuck or held) ontop of the panel through a loop and roll. The glass or water didn't move! The second was a Pitts pilot who poured water up (not down) at the top of a loop and didn't spill a drop.

Now, that's coordination! :ok:

Woomera

[OZBUSDRIVER]

Woomera mate, your showing your age now. A young Bob Hoover on "Thrillseekers". You forgot to mention the ball on the end of a string immediately below the cup. The ball does not waver thru the entire sequence.Was so impressed to finally see the man do a similar demonstration at the '99 Avalon show.

Understand a lot of radiostacks were fried trying to imitate the great man:D

Regards

Mark



Yes, one wonders where all the years of my miss spent youth went!! :{ :{

Woomera

[I Fly]

I think some people here are a bit loose with their definitions / interpretations.
A T&B is not the same as a TC, hence the different presentation.
A T&B (turn and balance indicator) has only 2 axis around which it can rotate. 1 is the axis around which the rotating mass rotates. The other is the 'fore and aft' axis (parallel to the longitudinal axis) around which it has limited movement constrained by springs (as some one explained). The T&B ONLY indicates yaw.
The TC (turn co-ordinator) again has 2 axis around which it can rotate. However the 'fore and aft' axis is inclined 36º to the longitudinal axis (forward and up). Now when you yaw OR ROLL the aircraft, that produces a compound angle and the TC indicates yaw AND ROLL (but due to the 36º not in equal proportions).
I do not believe that Gravity has anything to do with the operation of a T&C or TC.
swamp monkey ask a question in relation to a AH.
A AH is not the same as a FAI and hence again the different presentation. (what is an AI, a AH or a FAI?)
In a AH the 'bank angle card' stays fixed with the 'wings'. It has a pointer that either points down and in the correct direction of the turn or up and in the OPPOSITE direction of the turn (I.e. in a 30º Bank turn to the left, the 'sky pointer' points to 30º to the right).
In a FAI the Pointer stays fixed with the 'wings' and the 'bank angle card' moves and hence indicates the Bank angle in the correct direction.
Now back to swamp monkey's question.
You need a cat AND a duck. The cat as apache explained and if the duck walks with his/her peak to the ground - then you have a nose high attitude. If he/she walks with the bum to the ground - you have nose low attitude. The other way you can tell. When the **** runs out of you collar - your upside down.
But seriously, if you are flying anything with a fixed pitch prop -
You get pitch information from the AH / FAI, ASI, VSI, ALT and Tacho and to some degree engine noise.
You get roll information from the AH/FAI, TC and the further effect of roll on the T&B, DI/HSI, Compass and if the ADF is tuned to a station more than a mile or two away, the ADF. As already stated the GPS is also very useful. However keep in mind, that without an air data computer, the GPS is indicating a track rather than a heading. It uses rate of track change and groundspeed to compute a "Bank angle".
You get yaw information from the T&B, TC and the further effect of yaw, on the DI/HSI and compass and perhaps on the ADF and GPS.
With most light aircraft the T&C/TC is powered electrically, so the question you need to ask is "has only the T&C/TC failed or is it a complete electrical failure"? Your life depends on it.
Again, with most light aircraft the AH/FAI and DI has an air driven gyro (either by blowing or sucking). So if the AH/FAI fails, you need to ask yourself "has only the AH/FAI failed or have I lost the DI as well. There are some HSIs that are air driven but the 'card' is electrically connected to the gyro. So it's susceptible both ways.
To disprove the 'water bottle theory' you just need to put the water bottle in your car on a flat straight road and see what it does when you accelerate/decelerate and/or change lanes.

[air-hag]

some wild theories there.......

[Arm out the window]

Thanks Ifly, I shouldn't have mentioned turn coordinators in my post as being the same as turn needles.

[Runaway Gun]

SwampMonkey,

No insult intended, but did you ever do Physics or Science at School?

I really thought this would have been answered at the pre-solo groundschool level.


I have had this happen before (cause - a hard-arsed instructor simulating such an emergency, in real IMC).

What he taught was to stare at the mag compass, for turning errors, and the VSI/Altimeter - for pitch errors.

Correct slowly but surely. It was a brain fart - but it did work. I hope that I never have to do it again though.

[Cougar]

Now the question is: HOW do birds fly in cloud? Some type of migrating goose flies in large formations in cloud asleep apparently (Richard Attenborough Doco). What the?

Anyone care to explain?

[Arm out the window]

Probably because a lot of them seem to be naturally stable, lots of dihedral, low c of g giving pendulous stability, and I'm guessing that they would be very sensitive to which direction the airflow's coming from and have natural reflexes to adjust their flight feathers to suit, enabling them to 'straighten up and fly right'...ahh OK then, I dunno!