Military Training Wrong ... ?
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Joe,
I have been in for 19 years of which 11 have been as a QHI and I have never been taught or taught others anything other than the fact that recirculation can occur when hovering next to any obstruction. The AP 3456 (Mil-Bible) does not claim an attitude change occurs in the direction of the obstruction - where were you taught this idea?
I have been in for 19 years of which 11 have been as a QHI and I have never been taught or taught others anything other than the fact that recirculation can occur when hovering next to any obstruction. The AP 3456 (Mil-Bible) does not claim an attitude change occurs in the direction of the obstruction - where were you taught this idea?
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Joepilot - are you just an anti-christ, or do you just like antaganising real pilots?!!!
Think basic PofF =- when hovering next to any building, cliff, wall etc, where will all the downdraught go from the side of the disk nearest to the obstruction? - It must go up then fed back into the disc along with normal downdraughting air thereby increasing the induced flow on the side of the disc nearest the obstruction - basic P of F again increase in induced flow = decrease in lift = aircraft drifts into obstacle because the other side of the disc remains unafected thereby producing more lift than the other side hey presto A/C drifts into cliffs
Think basic PofF =- when hovering next to any building, cliff, wall etc, where will all the downdraught go from the side of the disk nearest to the obstruction? - It must go up then fed back into the disc along with normal downdraughting air thereby increasing the induced flow on the side of the disc nearest the obstruction - basic P of F again increase in induced flow = decrease in lift = aircraft drifts into obstacle because the other side of the disc remains unafected thereby producing more lift than the other side hey presto A/C drifts into cliffs
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To: Joe Pilot
This is taken verbatim from the FAA Rotorcraft Flying Handbook. “ When flying an approach to a pinnacle or ridgeline, avoid the areas where downdrafts are present, especially when excess power is limited. If you encounter downdrafts, it may become necessary to make an immediate turn away from the pinnacle to avoid being forced into rising terrain”.
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The Cat
This is taken verbatim from the FAA Rotorcraft Flying Handbook. “ When flying an approach to a pinnacle or ridgeline, avoid the areas where downdrafts are present, especially when excess power is limited. If you encounter downdrafts, it may become necessary to make an immediate turn away from the pinnacle to avoid being forced into rising terrain”.
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The Cat
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OK, just for the hell of it...
If close to an obstruction air will be recirculated to the disc as induced flow moves up the side of the obstruction. This will happen in proportion to the height and density of the obstruction (trees with space in between would cause less recirculation than solid walls). Gyroscopic precession (I know at least one person who will like this !) will carry the effect of loss of thrust (lift if you like)90 degrees on.
So - Obstruction on Helis. right with anti clock rotor = Front of disc or skids lower, etc. Most recirculation is generally quoted as happening around one third of the rotor diameter away from the tips and that's close enough for most.
BUT this change in attitude is only good for the recirculation at the tip area of the disc. If the disc gets closer to the obstruction more of it is affected. If half of the disc is affected the loss of thrust is still precessed, but now the blade over the tail(still obstruction on the right) will become low TOWARD the obstruction as well. End effect = disc low at front and to the right.
This is why some exam Q's I have seen include this as a correct answer. It is one of those 'paper correct' theory things that means little in the real world.
We'll all be close enough to an obstrction to have a slight change in hover attitude at some time or other, depending where we operate from, but as for the 'sucking in' of the Heli to the obstruction I have heard quoted, I beleive it to be impossible as the disc would have to be so close for 1/2 of it to be affected in such a way (or already be buried IN the obstruction!) if 1/3 of the diameter is accurate.
Yes, agreed fully. Common piloting sense is one thing and answering an exam question that has little relevance to flying is another. No pilot is going to get so close to an obstruction that it becomes 'sucked in', if it is possible at all. I'm sure all will give the maths a think and maybe arrive at the conclusion I did - For the aft blade to be affected by 1/3 of rotor diameter max. recirculation whilst it is 2/3rds of the radius away from the obstruction, it will have already struck the obstruction?
SPS, sometimes disenhchanted with having to teach one thing for the student to pass exams and another to put it right afterward.
If close to an obstruction air will be recirculated to the disc as induced flow moves up the side of the obstruction. This will happen in proportion to the height and density of the obstruction (trees with space in between would cause less recirculation than solid walls). Gyroscopic precession (I know at least one person who will like this !) will carry the effect of loss of thrust (lift if you like)90 degrees on.
So - Obstruction on Helis. right with anti clock rotor = Front of disc or skids lower, etc. Most recirculation is generally quoted as happening around one third of the rotor diameter away from the tips and that's close enough for most.
BUT this change in attitude is only good for the recirculation at the tip area of the disc. If the disc gets closer to the obstruction more of it is affected. If half of the disc is affected the loss of thrust is still precessed, but now the blade over the tail(still obstruction on the right) will become low TOWARD the obstruction as well. End effect = disc low at front and to the right.
This is why some exam Q's I have seen include this as a correct answer. It is one of those 'paper correct' theory things that means little in the real world.
We'll all be close enough to an obstrction to have a slight change in hover attitude at some time or other, depending where we operate from, but as for the 'sucking in' of the Heli to the obstruction I have heard quoted, I beleive it to be impossible as the disc would have to be so close for 1/2 of it to be affected in such a way (or already be buried IN the obstruction!) if 1/3 of the diameter is accurate.
Yes, agreed fully. Common piloting sense is one thing and answering an exam question that has little relevance to flying is another. No pilot is going to get so close to an obstruction that it becomes 'sucked in', if it is possible at all. I'm sure all will give the maths a think and maybe arrive at the conclusion I did - For the aft blade to be affected by 1/3 of rotor diameter max. recirculation whilst it is 2/3rds of the radius away from the obstruction, it will have already struck the obstruction?
SPS, sometimes disenhchanted with having to teach one thing for the student to pass exams and another to put it right afterward.
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Sure it's a bit theoretical ... but it does help pilots understand what is happening with their rotor disc...
Blades on the 'cliff' side experience more ind. flow ... and therefore flap down in that half ... therefore they are lowest by the end of that half ... therefore the attitude WOULD NOT change towards the 'cliff', but Parallel to it (some people call this gyroscopic precession).
The attitude does not change however because the pilot uses the cyclic to maintain the desired attitude.
CLEVER BIT:
- in doing so he is running more Pitch on the 'cliff' side (to combat the higher ind. flow) ... and a slightly increased collective ... and so just MORE POWER!
Very elegant really...
Swerve: is that a revelation to you?
Eden: Spot-on , agreed
Helisphere: Yes! (but no need to hide behind the term Gyroscopic Precession)
SARcastic: Just curious... do you mean it's a tedious subject?(agree) Or did/do you dissagree with the assertion?
Purple: Expand please.. (see above)
Lu: Missed the point...
SPS: Approximately correct. the hole in your arguement is (for your orientation) to treat the 'extra' ind. flow over the tail, on getting closer, as different from that at the front. - or in your language; 'the SUM of the precessions of blades experiencing increased ind. flow (the right hand half still results in a low point over the front...' -
Blades on the 'cliff' side experience more ind. flow ... and therefore flap down in that half ... therefore they are lowest by the end of that half ... therefore the attitude WOULD NOT change towards the 'cliff', but Parallel to it (some people call this gyroscopic precession).
The attitude does not change however because the pilot uses the cyclic to maintain the desired attitude.
CLEVER BIT:
- in doing so he is running more Pitch on the 'cliff' side (to combat the higher ind. flow) ... and a slightly increased collective ... and so just MORE POWER!
Very elegant really...
Swerve: is that a revelation to you?
Eden: Spot-on , agreed
Helisphere: Yes! (but no need to hide behind the term Gyroscopic Precession)
SARcastic: Just curious... do you mean it's a tedious subject?(agree) Or did/do you dissagree with the assertion?
Purple: Expand please.. (see above)
Lu: Missed the point...
SPS: Approximately correct. the hole in your arguement is (for your orientation) to treat the 'extra' ind. flow over the tail, on getting closer, as different from that at the front. - or in your language; 'the SUM of the precessions of blades experiencing increased ind. flow (the right hand half still results in a low point over the front...' -
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To: All
Here is a theory that will really get you to thinking. Assuming the pressure differential between the cliff side of the rotor and the opposite side of the rotor. If there is a differential of pressure between the sides of the disc there is a differential of pressure on the fuselage and the greater pressure is on the side of the fuselage away from the cliff. Nature being what it is, the pressure differential acting on the fuselage will push the helicopter towards the cliff in order to compensate for the differential. The closer to the cliff the greater the differential and the greater the sidward force.
Don’t laugh or, I’ll have to use rocket science. If you have ever seen a large liquid propellant rocket engine you will have noticed circumferential rings around the nozzle. These rings give stiffness to the bell and help maintain the shape of the engine. Sometimes, during the start of the combustion at sea level the gasses can’t fully expand which creates a pressure differential between the inside and the outside of the bell. This is called Jet Separation, This pressure gradient can be so strong as to shift the engine sideways and force the control servos up into the structure or, it could cause the nozzle to collapse. The rings increase the resistance to collapse by altering the hoop stresses on the nozzle. I am not sure but the Space Shuttle Lox / Hydrogen engines may have a two stage hydraulic system to allow the engine movement but the force is attenuated by a spongy servo. Next time they show a closeup of the engines you can see them move at ignition start.
The same is true for aircraft wings on aircraft flying in close formation and the wings overlap. The air between the wing tips increases in speed due to the venturi effect and causes a low pressure. The result is that the wings “lock”. These illustrations prove that pressure acting over a large surface can generate a great deal of force and in this case move the helicopter, which is a free body in space into the cliff.
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The Cat
[This message has been edited by Lu Zuckerman (edited 23 January 2001).]
[This message has been edited by Lu Zuckerman (edited 23 January 2001).]
Here is a theory that will really get you to thinking. Assuming the pressure differential between the cliff side of the rotor and the opposite side of the rotor. If there is a differential of pressure between the sides of the disc there is a differential of pressure on the fuselage and the greater pressure is on the side of the fuselage away from the cliff. Nature being what it is, the pressure differential acting on the fuselage will push the helicopter towards the cliff in order to compensate for the differential. The closer to the cliff the greater the differential and the greater the sidward force.
Don’t laugh or, I’ll have to use rocket science. If you have ever seen a large liquid propellant rocket engine you will have noticed circumferential rings around the nozzle. These rings give stiffness to the bell and help maintain the shape of the engine. Sometimes, during the start of the combustion at sea level the gasses can’t fully expand which creates a pressure differential between the inside and the outside of the bell. This is called Jet Separation, This pressure gradient can be so strong as to shift the engine sideways and force the control servos up into the structure or, it could cause the nozzle to collapse. The rings increase the resistance to collapse by altering the hoop stresses on the nozzle. I am not sure but the Space Shuttle Lox / Hydrogen engines may have a two stage hydraulic system to allow the engine movement but the force is attenuated by a spongy servo. Next time they show a closeup of the engines you can see them move at ignition start.
The same is true for aircraft wings on aircraft flying in close formation and the wings overlap. The air between the wing tips increases in speed due to the venturi effect and causes a low pressure. The result is that the wings “lock”. These illustrations prove that pressure acting over a large surface can generate a great deal of force and in this case move the helicopter, which is a free body in space into the cliff.
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The Cat
[This message has been edited by Lu Zuckerman (edited 23 January 2001).]
[This message has been edited by Lu Zuckerman (edited 23 January 2001).]
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Lu, the clue in your quote from the FAA is "downdraught" - this is not recirculation but turbulence on the lee side of a mountain feature where the air is going downwards and takes the aircraft with it - standard reaction is to turn away from the obstacle(high ground) and establish a maximum rate climb configuration. Then you wait (and pray)that the severity of the downdraught really does reduce as it nears the ground!
Your rocket science sounds like COANDA effect which the NOTAR aircraft use to exploit pressure differentials on opposite sides of the tail boom to produce anti-torque thrust in the yawing plane. They enhance it by blowing air down the inside of the tail boom and out through a slot along it's length. Fine yaw control is achieved by installing a movable bucket on the end of the tail to direct the remainder of the air in an appropriate direction.
Back to the cliffs and the Vortex ring thread highlights how recirculation around the tip vortex causes a loss of lift which in this case would be on the side nearest the cliff. Nice theory, never seen it practice and I have hovered close to a lot of cliffs.
Your rocket science sounds like COANDA effect which the NOTAR aircraft use to exploit pressure differentials on opposite sides of the tail boom to produce anti-torque thrust in the yawing plane. They enhance it by blowing air down the inside of the tail boom and out through a slot along it's length. Fine yaw control is achieved by installing a movable bucket on the end of the tail to direct the remainder of the air in an appropriate direction.
Back to the cliffs and the Vortex ring thread highlights how recirculation around the tip vortex causes a loss of lift which in this case would be on the side nearest the cliff. Nice theory, never seen it practice and I have hovered close to a lot of cliffs.
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Lu:
Your analogy of fixed wing flying in close formation is not quite right. The resultant pressure differential actually causes the trailing A/C to fly away requiring a trim change into the lead aircraft (ie. echelon right requires trim left).
Quite apparent when your doing a tight (wing overlap formation) at 120/3 and 240+ knots. (BTW 120/3=120 degrees of bank and 3G).
Cheers, OffshoreIgor
[This message has been edited by offshoreigor (edited 23 January 2001).]
Your analogy of fixed wing flying in close formation is not quite right. The resultant pressure differential actually causes the trailing A/C to fly away requiring a trim change into the lead aircraft (ie. echelon right requires trim left).
Quite apparent when your doing a tight (wing overlap formation) at 120/3 and 240+ knots. (BTW 120/3=120 degrees of bank and 3G).
Cheers, OffshoreIgor
[This message has been edited by offshoreigor (edited 23 January 2001).]
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Don't know about COANDA.... but if you're blowing too much air anywhere it was probably the CORIANDER effect of too many PHALL blowouts and the resultant application of STOKES LAW to the viscous fluid deposited thereof.
Ooooooeeer - nothing like a flying suit full...suits you sir!
Ooooooeeer - nothing like a flying suit full...suits you sir!
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To: Offshoreigor
What do I know, I saw it in the movies and on TV. In one case the wings locked and on another the pressure on top of the wing was used to support the wing of another aircraft. Do you think they used artistic license to make the movies more interesting?
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The Cat
What do I know, I saw it in the movies and on TV. In one case the wings locked and on another the pressure on top of the wing was used to support the wing of another aircraft. Do you think they used artistic license to make the movies more interesting?
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The Cat
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Lu:
The term, wing overlap does not mean that the wings are on top of each other. It refers to the fact that the trailing aircrafts wingtip is inboard of the lead aircrafts wingtip.
If you were to look from directly above or below a formation such as the Snowbirds or Red Arrows, you would see this. Due to the fact that most spectators only ever see such a formation from the ground at an offset angle, the illusion of the wings being "locked" as you put it is observed.
Standard Mil training in Canada uses about three feet of overlap, ie a wingman would have his wingtip inboard of the lead by three feet and slightly below. The resultant vortice created by the lead requires that the wingman trim into the lead aircraft to maintain station.
Cheers, OffshoreIgor
The term, wing overlap does not mean that the wings are on top of each other. It refers to the fact that the trailing aircrafts wingtip is inboard of the lead aircrafts wingtip.
If you were to look from directly above or below a formation such as the Snowbirds or Red Arrows, you would see this. Due to the fact that most spectators only ever see such a formation from the ground at an offset angle, the illusion of the wings being "locked" as you put it is observed.
Standard Mil training in Canada uses about three feet of overlap, ie a wingman would have his wingtip inboard of the lead by three feet and slightly below. The resultant vortice created by the lead requires that the wingman trim into the lead aircraft to maintain station.
Cheers, OffshoreIgor
