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RobertL18C wrote:

…it leads back to PoF because you are not weightless in the ISS. The ISS is in orbit, hence all objects are falling together
So far I agree, even though it’s a bit playing with words to say that you are not “weightless” in the ISS.

with centrifugal force balanced by centripetal (gravity), which leads back to forces in a (balanced) turn, and forces in accelerated flight.

This is not right. There is no “balance” of centripetal and centrifugal forces since these forces exist in different reference frames, and anyway the centrifugal force is not a real force. For the same reason there is no balance of forces in a turn. If there were, the aircraft wouldn’t change course!

ESKC (Uppsala/Sundbro), Sweden

Turning flight is by definition accelerated with the forces not in balance. However to centre the ball I would stand by that CPF is balanced by CFF.

The ISS is also in accelerated flight, albeit in space where fluid dynamics don’t apply.

On the ISS you are falling at orbital velocity, so by definition you are not weightless.

If centrifugal forces exceed centripetal in a turn, wouldn’t you no longer be in a balanced turn but in a skidding turn? Was not aware that there were different frames of reference, none of the ATP text books (and their aerodynamics and aero space engineering sources) mention this, but I do agree a lot of the concepts are simplified to their essentials.

Oxford (EGTK)

I see a slight danger with the gravity question, in that somebody giving a very correct answer might give the interviewer an inferiority complex and would not get the job. Hence this sort of thing might be selecting candidates whose IQ is within a certain range and who are “not too educated”. This already happens in certain professions (that have a public face) and not always with the best results

Administrator
Shoreham EGKA, United Kingdom

Peter, couldn’t agree more, most ATP interviewees will stumble on whether you are weightless on the ISS, but this will not disqualify them.

The most practical airlines have the SIM ride first (after the usual HR screening and background checks), if the candidate aces the SIM the interview consists of a version of what is the colour of a red rose? ie they will describe an emergency scenario which they regard as the highest risk to safety, and then ask what do we regard as the highest risk emergency scenario? Having a different opinion would not be career enhancing.

Oxford (EGTK)

RobertL18C wrote:

However to centre the ball I would stand by that CPF is balanced by CFF. [snip] If centrifugal forces exceed centripetal in a turn, wouldn’t you no longer be in a balanced turn but in a skidding turn? Was not aware that there were different frames of reference, none of the ATP text books (and their aerodynamics and aero space engineering sources) mention this, but I do agree a lot of the concepts are simplified to their essentials.

The CFF is opposite and equal in strength to the CPF at all times, but you really shouldn’t talk about the CFF because it is not a real force. It is appears to be a force when you are using a accelerating object as the frame of reference, e.g. if you are observing the world from the inside of an aircraft in a turn.

The CPF accelerates the aircraft which means that to the observer, there appears to be a CFF pushing her outwards, but this isn’t really a force — it’s the inertia of the observer which tries to keep her going in a straight line even though the aircraft it forcing her to follow it in the turn. Refer to this wikipedia article. It says explicitly that the CFF is always opposite in direction to the CPF. It doesn’t say explicitly that it always have the same strength, but that follows from its strength being calculated to be mrw^2 which is the same as the CPF (w being the angular velocity).

The balanced/slipping/skidding turn issue is completely distinct and applies to straight unaccelerated flight as well where the forces are certainly balanced.

Last Edited by Airborne_Again at 16 Mar 09:52
ESKC (Uppsala/Sundbro), Sweden

A_A must confess to only layman’s appreciation of physics and Newton’s third law. The relationship between CPF and CFF and whether a turn is slipping or skidding may be a small white lie told to innocent children, or semantics between CFF and inertia.

Once again the great American taxpayer kindly provides a free link to the FAA Pilots Handbook of Aeronautical Knowledge where the description of the relationship between CFF and CPF is covered in easy terms in chapter 4.

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/

I believe the Wright Brothers may claim credit to first really understanding horizontal component of lift and how the aircraft turns.

Oxford (EGTK)

RobertL18C wrote:

Once again the great American taxpayer kindly provides a free link to the FAA Pilots Handbook of Aeronautical Knowledge where the description of the relationship between CFF and CPF is covered in easy terms in chapter 4.

The description of forces acting on an aircraft in a turn is wrong and it makes me sad to see that this misconception is being propagated by the FAA. The text is even inconsistent. On page 4-19, Newton’s first law is correctly stated: "Newton’s First Law of Motion, the Law of Inertia, states that an object at rest or moving in a straight line remains at rest or continues to move in a straight line until acted on by some other force. An aircraft, like any moving object, requires a sideward force to make it turn. "

But only a few lines later, they write that the centrifugal force “acts equal and opposite to the horizontal component of lift” (i.e. the centripetal force). This is also clearly shown in figure 4-28. But if this was the case, there would be no net sideways force on the aircraft since the CPF and CFF would cancel out and by Newton’s first law, the aircraft would continue in a straight line!

This can be made to make sense if we consider that the CFF only “exists” in the turning (accelerated) reference frame of the aircraft. In that frame of reference, the aircraft does indeed continue in a straight line, while the rest of the world is turning outside it. But that hardly helps anyone understand how aircraft turn.

Compare with this figure taken from the Oxford Aviation Academy’s ATPL manual, chapter on “Flight Mechanics”:

Last Edited by Airborne_Again at 16 Mar 15:01
ESKC (Uppsala/Sundbro), Sweden
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