Why do we Lose Airspeed in a Turn and What Causes This?
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Correct, so if you hold a steady bank angle in a continuous turn you don't ever need to look at the compass/DI/E2 etc! I am not referring to any effects when rolling in or out of the turn.
The point about conventional gyroscopic AI turning errors is that the bank and pitch indications on the AI vary throughout the 360deg turn during a constant rate turn. They are not "rolling in or out" errors!
So in a high-rate, high-bank-angle turn, flying to maintain constant pitch attitude and constant bank angle on the AI means the aeroplane is continually varying in pitch and bank throughout the 360 degrees (with maximum error every 180deg). Sorry, but it is those turning errors, combined with wind velocity fluctuations, that cause your IRT candidates problems with airspeed variation, AND CERTAINLY ISN'T ANYTHING TO DO WITH ALLEGED DIFFERENCES BETWEEN TURNING UPWIND AND DOWNWIND!!
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Going back to the Ark Royal example, if the ship could decelerate instantly to a standstill, the plane wouldn't loose any airspeed would it?
So why then when you experience real windshear the plane does loose airspeed?
So why then when you experience real windshear the plane does loose airspeed?
Gyro Errors
Islander 2, H Peacock,
Just to expand on my last, as i said, the effect that Beagle was alluding to is Gimble Error. This manifests itself as a cyclic speeding up or slowing down of the rotation of the compass card in an aircraft flying a steep turn. It is caused by the difference in the aircraft's anlge of bank to the orientation of the Gyro platform and if memory serves me, the stock A2 / IRE explanation involves a dinner plate, a whiteboard maker and a swivel chair. You are quite correct to say that it is only really a snag when attempting to roll out on a specifc heading and has no effect on the other parameters of Altitude, Bank Angle and Speed.
Islander 2's comment that pitch and bank indications also cycle during the turn is way off the mark. The closest I've seen to that is the Stby AI on the hawk which suffers from a pitch / roll couple, but thats due to its instalation. With a normal AI, pitch and bank indications are acurate throughout the turn.
Cheers.
Just to expand on my last, as i said, the effect that Beagle was alluding to is Gimble Error. This manifests itself as a cyclic speeding up or slowing down of the rotation of the compass card in an aircraft flying a steep turn. It is caused by the difference in the aircraft's anlge of bank to the orientation of the Gyro platform and if memory serves me, the stock A2 / IRE explanation involves a dinner plate, a whiteboard maker and a swivel chair. You are quite correct to say that it is only really a snag when attempting to roll out on a specifc heading and has no effect on the other parameters of Altitude, Bank Angle and Speed.
Islander 2's comment that pitch and bank indications also cycle during the turn is way off the mark. The closest I've seen to that is the Stby AI on the hawk which suffers from a pitch / roll couple, but thats due to its instalation. With a normal AI, pitch and bank indications are acurate throughout the turn.
Cheers.
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Going back to the Ark Royal example, if the ship could decelerate instantly to a standstill, the plane wouldn't loose any airspeed would it?
Islander 2's comment that pitch and bank indications also cycle during the turn is way off the mark.
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Your completely right Islander, I don't know why I thought otherwise.
Thanks for answering all my questions, from the perspective of quite a young aviator, you've made a lot more sense than most on this thread.
Thanks for answering all my questions, from the perspective of quite a young aviator, you've made a lot more sense than most on this thread.
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"Try walking along a train carriage, against it's direction of travel (i.e. 'upwind'), at say 5mph ('airspeed'). Now ask the train driver to slam its brakes on. What will your trajectory be? Still think the plane won't lose any airspeed?"
You don't have 'airspeed' - your feet are on the floor!?! If you suddenly jump when the brakes are slammed on - what happens then ;-)
The hangar on a ship analogy breaks down when you do the 'sudden stop' thing because the air is 'trapped' (taken along) inside it - you are 'tying' the reference frames together (the air and the container) and this makes the particular analogy invalid.
Can someone please take up an instrumented Tincan in a bit of wind and fly some slow speed turns - please.....???!!
You don't have 'airspeed' - your feet are on the floor!?! If you suddenly jump when the brakes are slammed on - what happens then ;-)
The hangar on a ship analogy breaks down when you do the 'sudden stop' thing because the air is 'trapped' (taken along) inside it - you are 'tying' the reference frames together (the air and the container) and this makes the particular analogy invalid.
Can someone please take up an instrumented Tincan in a bit of wind and fly some slow speed turns - please.....???!!
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And there was me thinking I'd understood things
As far as the ship idea is concerned this is how I understood it:
When the ship decelerates the air inside it decelerates, therefore the plane looses airspeed, just like it would in windshear (which after all in a odd sort of way is the aircraft's frame of reference decelerating, just like the air in the ship)
I don't really see how tying frames of reference together comes into it.
As far as the ship idea is concerned this is how I understood it:
When the ship decelerates the air inside it decelerates, therefore the plane looses airspeed, just like it would in windshear (which after all in a odd sort of way is the aircraft's frame of reference decelerating, just like the air in the ship)
I don't really see how tying frames of reference together comes into it.
I suppose the standard instrument texts (try Pallett: Aircraft Instruments) devote several pages to the turning errors in gyroscopic attitude indicators for the hell of it, then!
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You don't have 'airspeed' - your feet are on the floor!?!
If you suddenly jump when the brakes are slammed on - what happens then ;-)
The hangar on a ship analogy breaks down when you do the 'sudden stop' thing because the air is 'trapped' (taken along) inside it - you are 'tying' the reference frames together (the air and the container) and this makes the particular analogy invalid.
Whilst Mike O's grasp of fundamental physics is not what one would expect from someone with his academic engineering attainment, he did wisely and correctly predict that this thread will run, and run, and run, and run ........!!!
I've given my twopence worth, I'll just be repeating myself and getting ratty with people from here onwards. So I'll bail out now. You all take care with those downwind turns!
Islander 2. Please tell me more about this interesting phenomenon.
Last edited by Islander2; 19th May 2007 at 22:04.
A steady wind of 120kts at typical holding altitudes is as big a myth as the downwind turn!
"Typical" holding altitude, possibly- But FL 290 over Bindook during a two hour delay due fog and traffick into Sydney in winter? The core of the jet was at 370 and was 190kts. If you've ever flown in a jetstream (which you wouldn't in an islander!) you'll know that at a constant altitude it is one of the smoothest, most constant winds you will ever encounter.
I'm sure the vaguries of the internet makes things sound a little blunt-but I don't appreciate having it implied that I'm telling porkies!!
See http://www.allstar.fiu.edu/aero/GSI.htm for a simple explanation of turning errors.
Basically, 'centrifugal forces' cause the vertical gyro to precess towards the inside of the turn; the precession increases as the bank steepens.
Basically, 'centrifugal forces' cause the vertical gyro to precess towards the inside of the turn; the precession increases as the bank steepens.
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Thank you all for an hour of amusement while wauting for the dust storm to die down. I have advised the KISS principle many times in the 23 years since I got my A2. Beagle is right, but the Lift Pixie theory gets most marks for humour.
Thanks for the link BEagle.
However, the errors above are all to do with the gyro erection mechanism, not the actual spinning rotor. Remember that an AI has vertical gyro axis, and therefore does not not know it is turning. However, the AI needs to know where the true vertical is, and usually uses gravity/pendulum. A 'good quality' AI will ignore any perceived changes to this vertical if beyond a certain angle of bank (7deg-ish) called pitch erection cut-out. If it doesn't ignore it then this false datum will cause the gyro to precess, but only very slowly. Futhermore, if the gyro spin axis has moved to a false daum this error will not 'disappear' (as quoted above) as you roll out - it will either need fast erecting or leaving a while to return to the true vertical
I do have a big humble pie ready to eat, but it will have to wait for another day!!!
Principal Attitude Indicator Errors
TURN ERROR
During a normal coordinated turn, centrifugal force causes the gyro to precess toward the inside of the turn. This precession increases as the bank steepens; therefore, it is greatest during the actual turn. The error disappears as the aircraft rolls out at the end of a 180 degrees turn at a normal rollout rate.
Therefore, when performing a steep turn, the pilot may use the attitude indicator for rolling in and out of the turn, but should use other instruments (VSI and altimeter) during the turn for specific pitch information.
ACCELERATION ERROR
As the aircraft accelerates (e.g., during takeoff), there is another type of gyro precession which causes the horizon bar to move down, indicating a slight pitch up attitude. Therefore, takeoffs in low visibility require the use of other instruments such as the altimeter to confirm that a positive rate of climb is established immediately after takeoff.
DECELERATION ERROR
Deceleration causes the horizon bar to move up, indicating a false pitch down attitude.
TURN ERROR
During a normal coordinated turn, centrifugal force causes the gyro to precess toward the inside of the turn. This precession increases as the bank steepens; therefore, it is greatest during the actual turn. The error disappears as the aircraft rolls out at the end of a 180 degrees turn at a normal rollout rate.
Therefore, when performing a steep turn, the pilot may use the attitude indicator for rolling in and out of the turn, but should use other instruments (VSI and altimeter) during the turn for specific pitch information.
ACCELERATION ERROR
As the aircraft accelerates (e.g., during takeoff), there is another type of gyro precession which causes the horizon bar to move down, indicating a slight pitch up attitude. Therefore, takeoffs in low visibility require the use of other instruments such as the altimeter to confirm that a positive rate of climb is established immediately after takeoff.
DECELERATION ERROR
Deceleration causes the horizon bar to move up, indicating a false pitch down attitude.
I do have a big humble pie ready to eat, but it will have to wait for another day!!!
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Islander,
No need to get ratty my dear chap, it's a discussion not an argument (isn't it?!) :-)
(1) Of course I agree that you have speed through the air, but as your feet are on the ground I would call it groundspeed (or 'footspeed' in this case).
(2) Ok, there are two of you walking abeam each other when the train decelereates, one carries on 'feet on floor' walking and the other jumps having been walking at exactly the same pace as the other. Are you telling me that they will remain abeam each other? The bloke with his feet on the floor will experience a force (through his feet) and the bloke (who we'll imagine stays airborne for a bit) who jumps feels no such force and will, relative to the train (reference frame) move. And you're telling me 'I' don't get it! ;-)
(3) I think we might be having a screaming agreement on the ship/hangar thing! I agree with your steady state (35Kt) argument, what I was saying was (bady expressed I admit) that stopping the ship and consequently the air inside isn't representative of environmental reality (because you are 'tying' the air to the ship) but it is physically correct - it's the reality of the analogy that I don't agree with. The model will suddenly change airspeed, as you say. Stopping the ship and the air is more akin to windshear (from the viewpoint of the model). The air versus the container tends to increase the price of bread by the way (historically).
My head hurts now and I think I will join the 'lift pixies' brigade.
No need to get ratty my dear chap, it's a discussion not an argument (isn't it?!) :-)
(1) Of course I agree that you have speed through the air, but as your feet are on the ground I would call it groundspeed (or 'footspeed' in this case).
(2) Ok, there are two of you walking abeam each other when the train decelereates, one carries on 'feet on floor' walking and the other jumps having been walking at exactly the same pace as the other. Are you telling me that they will remain abeam each other? The bloke with his feet on the floor will experience a force (through his feet) and the bloke (who we'll imagine stays airborne for a bit) who jumps feels no such force and will, relative to the train (reference frame) move. And you're telling me 'I' don't get it! ;-)
(3) I think we might be having a screaming agreement on the ship/hangar thing! I agree with your steady state (35Kt) argument, what I was saying was (bady expressed I admit) that stopping the ship and consequently the air inside isn't representative of environmental reality (because you are 'tying' the air to the ship) but it is physically correct - it's the reality of the analogy that I don't agree with. The model will suddenly change airspeed, as you say. Stopping the ship and the air is more akin to windshear (from the viewpoint of the model). The air versus the container tends to increase the price of bread by the way (historically).
My head hurts now and I think I will join the 'lift pixies' brigade.
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What does whether you have your feet on the floor matter?
If two people are moving forward against the direction of travel (one on the ground and one in the air) inside the train and the train suddenly stops dead (instant deceleration) both will move backwards at the same rate. They both had the speed of the train, for example 40 knots (regardless of their airspeed) and when the train stops they will for a moment retain that 40 knots and hence fly backwards towards the front of the train. The reason why they might not end up abeam each other is because the on the floor (having fallen over) will stop faster.
If two people are moving forward against the direction of travel (one on the ground and one in the air) inside the train and the train suddenly stops dead (instant deceleration) both will move backwards at the same rate. They both had the speed of the train, for example 40 knots (regardless of their airspeed) and when the train stops they will for a moment retain that 40 knots and hence fly backwards towards the front of the train. The reason why they might not end up abeam each other is because the on the floor (having fallen over) will stop faster.
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Yes, given the conditions you quote, they will. I wasn't talking about an instant deceleration.
If you have a deceleration that is 'train typical' then the chap with his feet on the floor will feel the deceleration through his feet (and decelerate with the train), the chap airborne will not feel the decel (force) through his feet.
If you have two baloons, one is blu taked to the floor, the other is floating above it - the train decelerates, they do not remain abeam each other.
If you have a deceleration that is 'train typical' then the chap with his feet on the floor will feel the deceleration through his feet (and decelerate with the train), the chap airborne will not feel the decel (force) through his feet.
If you have two baloons, one is blu taked to the floor, the other is floating above it - the train decelerates, they do not remain abeam each other.
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Fair enough, but the example of Ark Royal stopping instantly was being given to explain windshear which is often an almost instant deceleration of the wind, i.e. the air which the plane is flying through stops very quickly.
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To quote Jeremey Pratt (author of The Private Pilot's Licence Course PPL3 Naviagtion & Meteorology):
The real danger of a strong windshear is the dramatic and rapid effect it can have on a plane's airspeed. Imagine the wind velocity changing so that an aircraft transitions from experiencing a 20 knot headwind component to a 10kts tailwind component in a 100 foot height band. As it descends with an airspeed of 90kts, experiencing a 20kts headwind, its groundspeed is 70kts. As the aircraft passes throught the windshear zone, the headwind of 20kts becomes a tailwind of 10kts. Due to intertia, the aircraft retains its original groundspeed for a short time, so the airspeed drops suddenly to 60kts.
Bearing in mind what has been said on this thread the above doesn't sound completely correct. Surely the momentum of the plane has nothing to do with the groundspeed. (that last sentence has been repeated over and over again by almost everyone)
Pratt also claims that larger planes are more vunerable to windshear than small ones, because they have more momentum. Why on earth is that true?
The real danger of a strong windshear is the dramatic and rapid effect it can have on a plane's airspeed. Imagine the wind velocity changing so that an aircraft transitions from experiencing a 20 knot headwind component to a 10kts tailwind component in a 100 foot height band. As it descends with an airspeed of 90kts, experiencing a 20kts headwind, its groundspeed is 70kts. As the aircraft passes throught the windshear zone, the headwind of 20kts becomes a tailwind of 10kts. Due to intertia, the aircraft retains its original groundspeed for a short time, so the airspeed drops suddenly to 60kts.
Bearing in mind what has been said on this thread the above doesn't sound completely correct. Surely the momentum of the plane has nothing to do with the groundspeed. (that last sentence has been repeated over and over again by almost everyone)
Pratt also claims that larger planes are more vunerable to windshear than small ones, because they have more momentum. Why on earth is that true?
No, in this case he is quite correct.
As we have been saying, the aircraft flies relative to the air around it. If the air suddenly changes velocity relative to the aircraft (note the ground doesn't come into it!!) you have winshear.
The aircraft does have momentum which is mass times velocity. Velocity is relative to the observer. The velocity that counts in this instance is that relative to the airflow. A larger aircraft has more mass, therefore more momentum.
Ground speed is a useful reference in this case, as the magnitude of the change in the airmasses velocity relative to the aircraft will be the same as it's change realative to the ground.
You could, however, reference the aircrafts speed relative to the moon, sun,or alpha centaurus with the same result, it's just that the ground is nearer and therefore more convinient!!
As we have been saying, the aircraft flies relative to the air around it. If the air suddenly changes velocity relative to the aircraft (note the ground doesn't come into it!!) you have winshear.
The aircraft does have momentum which is mass times velocity. Velocity is relative to the observer. The velocity that counts in this instance is that relative to the airflow. A larger aircraft has more mass, therefore more momentum.
Ground speed is a useful reference in this case, as the magnitude of the change in the airmasses velocity relative to the aircraft will be the same as it's change realative to the ground.
You could, however, reference the aircrafts speed relative to the moon, sun,or alpha centaurus with the same result, it's just that the ground is nearer and therefore more convinient!!