An examination question has been giving myself and my students trouble for some time now, and I'd like to see if anyone has an opinion to share on it.
The question appears in both P of F and tech. papers (CPL) here in NZ. It goes like so;
What is the main function of the Centrifugal Clutch (CC) in a Helicopter?
Aside from the two silly choices (it disconnects the tail rotor or ejects the passenger(s) and spins the Xmas tree round) your choice is down to two others,
(A) To off-load the engine at low RPM.
(B) To disengage the engine from the main rotor should the engine fail.
I have problems with it.
Firstly it seems hardly even relevant in this day and age (I only know of the Bell 47 having such a system, although my knowledge of older types is limited) but I accept that it is within mechanical understanding of the Heli so I leave that aside.
Secondly, I'm not keen on the use of the word 'MAIN'. On the ground (A) is clearly the function of any type of clutch (it does not have to be a piston engined Heli. either, eg. Gazelle)
On the ground we have no primary interest in disengagement, hopefully our engine is to get further than that! Of course, the CC has the virtue of disengaging the engine from the rotor system if engine RPM become less than RRPM and in that way it can also perform the task more usually trusted (of late) to a freewheel device (be that a sprag clutch or another type) so it seems that the CC has TWO definate functions. (Especially if no other type of freewheel device has been fitted)
Going back to that word 'MAIN'.....
Which is it now?
For me (on the ground, the first operation) it has to be (A), as (B) is of little impact until the aircraft is flying.
I think that the designer chose to fit this type of clutch to the B47 (and others before it) when at the early stage, eg. 'How to start the engine without X inertia of rotor system with X mass (no pun!) stopping it doing so".
That choice must have been an easy one to make back then. It was easier to manufacture, (small , high powered electric motors were not as easy or impossible to produce) no control levers or similar to be fitted in the pilot's reach, etc. etc. The CC is automatic, reliable and has a wonderful extra benefit - It can disconnect the rotor from the engine if the engine stops. That bonus allowed the designer to omit any freewheel device and save the weight of it. But was that his main and original aim? I have to say I doubt it.
However, in the air none of us would want to fly a Helicopter with no freewheel device included in the drivetrain so (B) purpose of the CC must be awarded 'MAIN' by default.
So when is the main main? On the ground (where the Heli must spend most of its existence and will not leave unless purpose (A) is fulfilled, allowing flight to begin, it won't if it cannot be started) or in the air (when the greatest need for engine failure management is plain)?
When I was going through my written exams, and now I watch students do the same, (sometimes struggling like I did/do now and then) I have to say that I become pretty disenchanted with some exam questions being more about comprehending or understanding one person's use of English than knowing your subject back to front.
I beleive that questions such as this are ambiguous and unhelpful to either side. What we should be interested in is whether the candidate knows what must be known. Trickery proves little more than they may become better at solving crosswords after being trained to think in this manner. It has nothing to do with what Bloggs (the candidate) will do if trouble looms one day (and I hope it does not).
If a Pilot has demonstrated an ability to guess correctly (passing the exam) has his problem solving ability been improved by this process? It certainly has not performed the original purpose (if I have this right) which is to insist that he should know 70% or 80% of a finite amount of material and prove this by passing an exam. If this is to be the crucial question (and there always is one if you are just one question away from the passmark) then Bloggs has only proven that he is a good guesser. The not - so - good - guesser will be excluded until he can guess better.
I can't help thinking that this question would be fairer and more effective if it were worded slightly different, eg. "What is the main function of the CC IN THE AIR or ON THE GROUND"
Anybody have any comments to make?
I'd be very pleased to read them.
SPS (Pedantic but means well!)