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Is the question "An airplane has an engine that pushes its flight. What force pushes a glider to fly?" really a duplicate of "What produces thrust along the line of flight in a glider? "?

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  • $\begingroup$ The post in question was admittedly rather vague. It would have been better to leave a comment asking the questioner to clarify whether he was speaking about the flight path as viewed from the airmass reference frame or the earth reference frame or both, before leaving any answers. Also to clarify whether or not the question was intended to be confined to steady-situations (constant airspeed and constant trajectory.) $\endgroup$ May 1 '20 at 14:22
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I voted to close the question as a duplicate and I still think it is one.

I think you are fixating on a minor nuance in how the question is worded. Both questions ultimately ask about the forces involved and how these forces can have a forward component. A complete answer to either question would explain all of these forces and therefore be basically identical. xxavier even posted an answer to both questions and they are quite similar.

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I would suggest that, as a rule, different degrees of competence in formulating a question do not make for different questions.

For a litmus test, if the best possible canonical answer to two questions would be the same, they should be considered the same question.

Exceptions should be made when the new question asks for new information, which the old one did not, and which answers to the old question have not covered.

For example, "What is X made out of" and "Why is X made out of Y" should be merged, as any answer to the second question will completely cover the first. "What parts of X are made out of Y" will also cover the first, but may be separate from the second.

The two questions in question here stem from the lack of the same piece of knowledge, and there is no reason to address them separately.

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  • $\begingroup$ This argument would seem to strongly suggest that aviation.stackexchange.com/q/77429/34686 is a duplicate of line 2 of aviation.stackexchange.com/q/14263/34686 , and therefore the two questions are duplicates.The only difference I see is that the first question is a) broader and b) seems to suggest that the author has only recently become aware of the existence of this feature.I'm baffled that the comment suggesting that they were not duplicates received 3 upvotes within 45 minutes of being posted. Some things about the workings of this site will forever remain a mystery to me. $\endgroup$ Apr 22 '20 at 16:45
  • $\begingroup$ @quietflyer I somewhat agree, these questions would have been better off merged. The new one is included in the old one, just has not been answered. On the other hand, the old question was phrased incorrectly, with 4 questions in one, which should have really been 2 questions (point 1 is implied/basic search, points 3 and 4 the same, point 2 separate). $\endgroup$
    – Therac
    Apr 22 '20 at 18:38
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Is the question "An airplane has an engine that pushes its flight. What force pushes a glider to fly?" really a duplicate of "What produces thrust along the line of flight in a glider? "?

I would argue that the newer question is not a duplicate of the older question, and therefore it should have been re-opened.

A close reading of the actual body of the question, not just the title, gives some insight as to why.

(Note: this answer considers the situation as it stood before the March 22 edits to the question. The March 22 edits, which the original asker never elected to roll back, even though he was still engaged with the question (leaving a comment) a full week later, only strengthen some of the arguments given in this answer.)

A new question that is broad enough to encompass a situation addressed in an older question, as well as other situations, is not a duplicate of the older question.

In other words if the content in the most appropriate answer to an older question is a subset of the content in the most appropriate answer to a newer question, then the newer question is not a duplicate of the older question.

Similarly, if a question is broad enough that the answer to an older question is not a complete answer to the newer question, than the newer question is not a duplicate of the older question.

Clearly in such a case, well-crafted answers to the newer question might links to answers to the older question, but they would need to also include additional content to be complete answers to the newer question.

Note that the older question "What produces thrust along the line of flight in a glider?" ***specifies that the glider is in a steady-state condition (gliding along a linear flight path with zero acceleration), while the newer question "An airplane has an engine that pushes its flight. What force pushes a glider to fly?" does not specify that the glider is in a steady-state condition.

Note that ten hours after the newer question was closed, the asker of the newer question posted an answer to the older question that treated the more general case where the glider's tangential acceleration was not constrained to be zero, which really was outside the realm of the older question, but would have been a perfectly suitable self-answer to the newer question.

Therefore the questions cannot be duplicates.

Just because the other answers to the newer question only addressed the steady-state case where acceleration was constrained to be zero, does not mean that they were the best possible answers to the newer question. The answer noted above was actually a better answer to the newer question than any of the answers that were posted to it, but it could not be posted there because the newer question had already been closed at the time the answer was posted.

In addition to this fundamental matter of whether the glider is constrained to be in a steady-state condition or not, there is another fundamental reason why the two questions are different. By using the word "thrust" in the title and in the body of the question, the older question strongly implies that it is only asking about forces that act along the direction of the glider's flight path through the airmass. I.e., parallel to the direction that a thrust vector would act, to a first approximation (e.g. ignoring tilted thrust-line effects), in a powered aircraft.

In addition, the older question contained the sentence "The glider is on a straight line flight towards the ground." This seems to rule out any case where the glider is rising upwards, and seems to further imply that the asker is either presuming that airmass is not moving, or is only interested in considering the forces that act parallel to the glider's movement through the airmass, and not forces that act parallel to the glider's movement with respect to the ground.

The newer question has a more open wording that legitimately invites consideration of the force vector acting along the direction of the glider's flight path as seen from the ground. Naturally, this can be radically different from the direction of the glider's flight path through the airmass-- this difference is the whole reason why gliding flight is anything other than a quick trip to the ground. In strong lift or tailwinds, the flight path as seen from the ground can go straight up, or even backwards, relative to the glider's actual orientation in space.

This entire answer to the newer question took that point of view -- the view of asking what force acted parallel to the glider's flight path as seen from the ground -- An airplane has an engine that pushes its flight. What force pushes a glider to fly?. This answer was entirely appropriate for the question that it was posted to, but would not have been appropriate for the older question that is supposedly being duplicated, since the lift vector never contains any component acting parallel to the glider's fight path through the airmass.

If an answer would not have been appropriate for the older question, but was appropriate for the newer question, then the two questions cannot be the same.

The same can be said for much of the content in this answer-- An airplane has an engine that pushes its flight. What force pushes a glider to fly? . For example, see specifically the content that references the case where the glider is rising straight upwards relative to the ground.

Some further thoughts in response to some of the other answers to this ASE meta question --

If two people ask different questions without understanding that they are different, the lack of understanding doesn't invalidate the difference. If the most appropriate answer to each is not the same, then they are not the same question. And who are we to say that the asker of the newer question did not understand the differences with the older question?

Consider also-- would the proposed alternative questions (a) "what force moves a glider along its trajectory as seen from the ground?", or (b) "under what circumstances does a glider's lift vector exert force and perform work along the direction of the trajectory as viewed from the earth?", duplicates of (1) the earlier question or (2) the later question? I would say (a1 and b1) definitely not and (a2 and b2) borderline but arguably so. Therefore they are not the same question.

If the newer question were closed on the grounds that the question as it stood at the time (before the March 22 edits) was too vague (reference frame of ground or airmass not specified) rather than being a duplicate, I would have less objection. But the whole essence of gliding flight is that these reference frames are not the same, and that's why the newer question should not be presumed to be asking only about the airmass reference frame, which would make it a duplicate of the older question IF it had specified a steady-state case with no acceleration, which it did not.

In summary, the older question specified a steady-state case with zero acceleration, and strongly implied that it was only asking about force components that acted parallel to the drag vector, i.e. parallel to the flight path through the airmass. The newer question did not specify a steady-state case with zero acceleration, and also left open the possibility that the problem might be considered with respect to the ground reference frame, meaning that force components acting parallel to the trajectory as seen from the ground could be considered to pushing the glider along its trajectory. Either one of these differences would be sufficient to cause the newer question not to be a duplicate of the older one.

Update:

It may be a moot point now because a third question has now been asked (posted on April 6). Most readers would probably view the (closed) question under a discussion ( An airplane has an engine that pushes its flight. What force pushes a glider to fly? ) as being largely a duplicate of the combined effect of the older question What produces thrust along the line of flight in a glider? and the newest question When do the lift and drag vectors contribute a force component along a glider's path of travel as seen from the ground?. Taken together, these two question look at the forces acting parallel to the gliders trajectory through the airmass and the forces acting parallel to the glider's trajectory as seen from the ground.

The (closed) question under discussion still stands apart from the other questions in that does not specify that the glider is in a steady-state condition (i.e. with net acceleration equal to zero.) The answer What produces thrust along the line of flight in a glider? still stands as better answer to the closed question than to either the question it was actually posted to, or to the newest question. So I would still argue that the closed question either as it currently stands, or as it was originally asked, is still not a duplicate of either the older question or the newest one, and therefore should not be closed as a duplicate. It could be improved by editing to specifically emphasize that tangential acceleration is not constrained to be zero, i.e. the airspeed is not constrained to be constant.

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  • $\begingroup$ The post in question was admittedly rather vague. It would have been better to leave a comment asking the questioner to clarify whether he was speaking about the flight path as viewed from the airmass reference frame or the earth reference frame or both, before leaving any answers. Also to clarify whether or not the question was intended to be confined to steady-situations (constant airspeed and constant trajectory.) $\endgroup$ May 1 '20 at 14:21

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