The type of explanation which is most appropriate depends upon the audience and the purpose of the discussion. For PPL and the initial stages of ATPL, Principles Of Flight training, the explanation which I produced in my initial post (post 7) to this thread is perfectly adequate. Note that I did not actually include the requirement for equal transit time for the two air streams. This omission was quite deliberate, because I am aware of the fact that this line often results in heated arguments, particularly when the audience includes Aerospace Engineering Graduates.
In deciding what is or is not an acceptable explanation when teaching a subject I think that we need to consider the following basic requirements:
1. The explanation must be sufficiently simple to enable the students to understand it. If they cannot understand it then it worthless and possibly harmful.
2. The explanation should aid the understanding of related subjects which will be covered in subsequent lessons. As an example the “lift Fairies” explanation may amuse the students, but it would not help them to understand more complex aspects of lift theory such as stalling or induced drag.
3. The simplification process must not result in dangerous misunderstandings.
Derfred
I do understand why you are so vehemently opposed to the “long path/short path - equal transit time” argument. For most students it is easy to understand, is quite convincing, and fits in well when moving on to the effects of increasing angle of attack, stagnation point movement and stalling.
NASA did not actually argue that this explanation should be eliminated from use. They simple said that it is one of the most commonly used. It is worth considering why it is so popular. Could it be that it satisfies all of the above requirements?
A far worse, but very common incorrect explanation is:
“Propellers produce thrust by accelerating air rearwards.”
Not only is this untrue, but it suggests that the rearward acceleration of the air is a good thing, when in reality it is the means by which vast amounts of energy are wasted.
A far better explanation would be something along the lines of:
“Propellers produce thrust by exerting a rearward force on the air. This causes the air to exert a forward force (the thrust) on the propeller as predicted by Newton’s Third Law.
Unfortunately the air is not rigid enough to resist this rearward propeller force, so it is accelerated rearwards, thereby giving kinetic energy to the air. This transfer of energy from the aircraft to the air represents a waste of fuel. We would get far more miles for each gallon of fuel if we were able to exert the rearward force on something which was rigid enough to remain stationary.”
A campaign to eliminate the lie that thrust is produced by accelerating air rearwards is far more justifiable than your campaign to eliminate the “long path /short path” explanation of lift.