PPL Video Lectures covering Aerodynamics, General Knowledge, Performance, Meteorology And Navigation are now available through our website see front page for details.
Welcome to the CPL Aerodynamics question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
shreyasrawat created the topic: Exam Style Questions
I have a question with regards to Vortex Generators. Which of the following will VG be beneficial to?
a) Improve Cruise flight
b) Improve gliding performace
c) Improve climb performace
d) Improve short field landing
I suspect C. Not 100% sure on the explanation behind it if that is the correct answer.
The question I have is why do high wing aircraft have less dihedral than low wing? Is it more to do with the down wash of airflow over the lower tail wing or more to do with the CoG and CoP relationship?
Lastly, when looking at the total drag vs TAS graph. Lets assume at point X (min total drag) an aircraft is flying in a clean config. During an approach, the retractable landing gear gets stuck in the down position, will the appropriate speed required;
a) remain at point X
b) be greater than point X
c) be less than point X
Several people have had an attempt at this answer but no one has provided a confident answer or explanation besides parasite drag will increase (interference drag).
John.Heddles replied the topic: Exam Style Questions
VGs are a bit difficult when it comes to broad brush questions such as the exams favour.
VGs are a fix-it thing to correct a flow problem or improve flow control and so forth. For example -
(a) one light twin had a problem stall during certification and it took a lot of CFD study to figure out that the problem was associated with some unexpected nacelle separation .. a few suitably placed VGs and the problem went away.
(b) you will see a a large single VG (usually called a chine) on wing-mounted jets with large diameter engine nacelles necessarily tucked up close under the wing due to ground clearance problems. This is there to generate an intense vortex in the stall region to control flow separation on the upper rear wing surface behind the engine installation.
(c) the not uncommon light aircraft VG mod kit installations are intended to improve separation characteristics in the stall and near stall environment. End result is a lower POH stall speed and a bunch of performance flow on advantages from that. Also they usually are able to improve the OEI climb performance (by reducing a bit of drag) for light twins to advantage.
(d) you will often see VG arrays down the back end of an aircraft. These could be for low speed empennage flow control or just to tuck in the wake for cruise drag benefits.
(e) on most modern fighters, you will have seen high alpha manoeuvring flight in high humidity air and the resulting condensation in the vortex flow shed from the LEX (leading edge extension). Generally, these two vortices are lined up nicely with twin vertical tails so that the aircraft is able to manoeuvre at very high alpha. (As an aside, the associated vortex turbulence generally causes no end of trouble for tail end structural fatigue).
.. and the list goes on.
It follows that the question is a bit rubbery without some additional details as to the specific VG installation of concern. It is important to note that all the options can be correct for appropriate VG array designs.
However, for the question you have posed, it is probably likely that the examiner is looking at the light aircraft VG mod situation. In this case there should be benefits reference (c) (reduce the drag a bit with the VGs) and (d) (reduced the POH stall speed a bit with the VGs). As to which is the more important for a given case, well .. that would depend on the design imperatives for the specific mod installation.
To do with the influence of the fuselage in sideslip flow and resulting rolling moments - it's a bit more complicated than this but this is the sort of answer the examiner will be looking for. The flow moving around the fuselage experiences a change in angle so far as the wing is concerned and this is quite different for a low and high wing. Plenty of graphics on the net to look at to see what the likely flow is going to be.
3 Stuck gear
For the simplistic drag curve encountered in pilot training, the stuck gear will have a much more significant effect on the lift-independent drag. If you consider the two main components of the drag curve, you have the lift-dependent drag curve part in the low speed range and the lift-independent drag curve part in the high speed range. If you increase the lift-independent curve a bit (ie move the curve up the graph a bit), then the intersection of the two will occur at a slightly lower speed. Presuming the point of the question is to stay at the min drag point, (c) is the answer you would be looking for.
Engineering specialist in aircraft performance and weight control.
This is a little video of the effect of VGs on the behaviour of the boundary layer of a typical GA wing. The VGs are installed towards the outboard leading edge of the wing. Separation is delayed and the stalling speed is reduced.
shreyasrawat replied the topic: Exam Style Questions
Thank you John for your detailed answer. Much appreciated.
Also thank you Bob for your response. I passed the exam which is good.
There were 2 very similar questions which popped up which I got the answer right on one of them but not sure which one i got correct. The question was a light aircraft is lining up on 36 and a 747 has just conducted a missed approach on 27. The 747 initiated the missed approach 200ft AGL 1000ft before the intersection. Which of the following is correct with regards to wake turbulence?
a) 3 min delay
b) 10 min delay
c) 6 min delay
d) no extra delay necessary
I'd be interested to know...Departure full length behind a heavy such as a 747 is 2min separation, but obviously this is a perpendicular runway situation... I cannot seem to find anything in the AIP ENR 1.4 that mentions what to do here... as it kind of saying in there separation standards are applied by atc when using the same or parallel runways....?