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Aerodynamics

  • jrcawaki
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jrcawaki created the topic: Aerodynamics

Hi Sir,
Just preparing for my aerodymics exam and have come across these two challenging questions:
1. What does vortex generators affect?
A. Glide range
B. Glide endurance
C. Climb performance
D. Cruise speed
I would go for D. as after some research, Vg's can slow cruise speeds up to 2kts, not forgetting the numerous benefits it has. However not sure if it affects your climb performance.

2. Ground effect is the result of?
A. Reduction in upwash, downwash and vortices
B. Air traped beneath the aircraft when close to the ground
I go with B. as to my understanding A. is the RESULT of ground effect and B is what GROUND EFFECT RESULTS FROM.

Your assistance would be much appreciated
Regards
JOELI
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  • John.Heddles
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John.Heddles replied the topic: Aerodynamics

Probably not terribly good questions ..There exist plenty of papers on the net and in the literature covering both areas and I recommend those more intended for the pilot audience for your reading.

1. What does vortex generators affect?
A. Glide range
B. Glide endurance
C. Climb performance
D. Cruise speed

This is a very broad brush question.

VGs are used to give a slowing boundary layer (which infers a destabilising pressure gradient and an increasing likelihood of major separation) a bit of a kick along by introducing higher energy flow from outside the boundary layer. Depending on where they are located their intent may vary somewhat.

Most which the pilot will have read about relate to lifting surface VGs and micro VGs which are concerned with low speed flow control. Others may exist to introduce a strong shed vortex to delay separation downstream (eg nacelle chines on larger underslung wing mounted engines where the wing-nacelle gap necessarily is reduced due to undercarriage geometric constraints) or provide useful airflow over tail surfaces at high alpha (eg the use of shed vortices from the LEX on aircraft such as the F18). Others may be looking at delaying or reducing separation in the aft most regions of surfaces to reduce drag associated with pressure losses (eg VGs around the aft sections of a fuselage).

If we consider the more general application, the general result is a reduction in drag at high alpha (although an increase, say, in cruising flight) and an increase in L/D. Indeed, NASA/Douglas showed results up to 100% increase in L/D for micro VGs as I recall.

As far as I can see, all the answers may be considered correct. I suggest that the "correct" answer sought will be that which relates to the text(s)/course(s) to which the questions relate .. ie give the examiner the answer he seeks. If I were to hazard a guess in this regard, I probably would incline to (c) climb performance.

2. Ground effect is the result of?
A. Reduction in upwash, downwash and vortices
B. Air traped beneath the aircraft when close to the ground

Again, this is somewhat too general a question. In general, GE becomes noticeable at reducing dimensions approaching semispan. However, there exist several regions where further decreases .. down to quite miniscule dimensions demonstrate increasing GE phenomena.

Answer (a) is more appropriate to conventional aircraft, which don't get much below semispan above the ground surface, while answer (b) is more appropriate to those GE machine which exploit the phenomena, eg ground effect aircraft such as pioneered by Lippisch.

Which is "correct" ? Again, which text(s)/course(s) are being examined ? Given that you are studying for fixed wing licences, I would incline to (a).

My response probably is not overly useful. However, hopefully, it provides some avenues for your further research reading.

Engineering specialist in aircraft performance and weight control.
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