hey guys!!
wow! thanks so much for your replies!! very much appreciated! Yes as you can see im no aerodynamics expert, far from it and this course is my first real introduction to this concept of flight. Here are the comments the lecturer made on last years group who did the assignment, ive been trying to use these when attempting to break down the sequence of events. For someone of my limited background in this area, it is quite hard as there is a lot of depth in regards to aerodynamics knowledge.
Here are the comments the lecturer made on last years group who did the assignment, ive been trying to use these when attempting to break down the sequence of events.
Fortunately, the majority of you seemed to understand most of the principles well enough. Areas where some of you still had difficulty were:
1. Making the distinction between angle of bank and load factor. Many of you insisted that “the stall speed increases with angle of bank” which is not always true (how does an aircraft do a slow roll?) and many of you used the formula “ sqrt(load factor)” to calculate a stall speed at a certain angle of bank ... which is NOT correct! It is possible to get to 90 degrees AOB without pulling ANY extra ‘g’ ... the formula is ONLY true in a STEADY, LEVEL TURN (where lift is being used to generate the CPF). If you look at the video again, you’ll see that not much of the manoeuvre fits that bill!
2. Many of you had a trouble distinguishing between not enough lift to maintain altitude, and stalling. They are completely different issues. Those of you who went for the simple “it stalled” solution really needed to state exactly ‘when’ (eg X degrees AOB at time Y), explain ‘why’, and then go on to explain why the bank angle continued to increase .. and the aircraft continued to ‘fly’ quite well, albeit in a descent. Those of you who said it didn’t stall needed to come up with a very convincing case indeed, and some of you did.
3. The stall characteristics caught a lot of you out ... many people just described ‘typical’ stalls from GA textbooks. Swept wing aircraft will likely stall wingtip first and pitch up, not ‘nose drop’ ... and recovery can usually be completed simply by ‘blasting out of it’ with lots of thrust ... well, that certainly worked in the 747 and 777 : )
4. A common misunderstanding was the spool up time of the engines. The eight seconds (which very few of you referenced ... where did it come from?) refers to the time from idle thrust to max thrust .... it is unlikely that either thrust setting was used. More likely, engine response was almost instantaneous from the thrust settings they were using. Similarly, just because the engines were at low thrust at impact does NOT prove that thrust was not added before then.
5. Many of you quoted the lift formula, then asserted that the ONLY way to increase lift in a turn is by increasing AOA. Well, what about increasing speed!? Also many stated that “at 90 degrees AOB the aircraft is not producing ANY lift.” Well, there is a LOT of lift coming from the wings ... but it just isn’t opposing weight, and there is also quite a lot of lift coming from fuselage, fin, thrust etc.
6. Many people did not understand how spoilers work. They do not work like elevators or rudders. They do NOT produce an angle of bank. They generate a roll RATE ... when the required AOB is reached, spoilers are retracted (and possibly deflected slightly on the other wing) to STOP the rate of roll, then retracted.
7. There were also a lot of people who stated that there is NO lift being produced after the wing stalls. Have another look at the CL v AOA curves ... there is a loss of lift, but the wing is still producing a LOT of lift ...
8. A small number seemed to want to relate the increase in induced drag to a corresponding increase in thrust ... without realising that parasite drag probably made up a larger proportion of total drag at the time.
9. Another small number did not understand the effect of flaps (no, they do NOT increase the stall speed, nor do they “reduce the available range of AOA” either)
10. Those of you who explored the spiral instability idea had their work cut out. Quite a few simply mentioned that swept wing and high wing are stable, and that anhedral was the opposite, then somehow ‘concluded’ that the overall effect was “unstable” ... that’s a bit of a leap of faith! There was much more to that argument, and a lot of referencing work would have to be done to support it.
11. The understanding of the effect of head/tailwind was poor. Many of you would have scored more marks by forgetting it completely.
Thanks again all for your input, again i cant emphasize how much it is appreciated!
Daniel