Exam question
Thread Starter
Join Date: Apr 2006
Location: scotland
Age: 64
Posts: 10
Likes: 0
Received 0 Likes
on
0 Posts
Exam question
Hi I`m an engineer currently studying for my module 12 [helicopters]. I have come across a question which i don`t understand and the pilots in work suggested this site for an answer. question is when a helicopter has a main rotor which rotates anti-clockwise when viewed from above , what happens if a loss of anti torque drive happens in cruise speed in flight?
a. Nose pitches slightly up and a/c yaws left
b. Nose pitches slightly up and a/c yaws right
c. No appreciable change.
I know it will yaw right but i am assuming it pitches slightly because of the loss of drive [power] to the tail being transferred to be main rotor and thus a slight change in attitude. Is this correct or does any of you pilot types know more than the ones i asked at work?
a. Nose pitches slightly up and a/c yaws left
b. Nose pitches slightly up and a/c yaws right
c. No appreciable change.
I know it will yaw right but i am assuming it pitches slightly because of the loss of drive [power] to the tail being transferred to be main rotor and thus a slight change in attitude. Is this correct or does any of you pilot types know more than the ones i asked at work?
As the rotor RPM will be maintained by the governor, the total rotor thrust will stay the same. However, transiently while the aircraft is yawing to the right, the advancing blade will experience a slight increase in lift (rotational speed plus speed of rotation of the fuselage) and the retreating blade a slight decrease. This would cause the disc to flap back a little more than in the steady state which would cause a slight (but, I suspect, very slight) pitch up. Vaguely similar to the spot turn situation where a spot turn to the right causes a slight climb and to the left a descent (all valid of course only for proper helicopters where the rotor rotates clockwise!).
I guess the exam setter hasn't had practical experience of the event (neither have I, fortunately!)
The last person I spoke to about his experience of a tail rotor drive failure in the cruise described the subsequent experience as being akin to a Lomcevak, with gyrations in all three axis that only eased when the SSLs were retarded. Another ex-colleague described a tumbling so violent that both crew's headsets flew off.
I accept that not all failures are that dramatic but I think to start quibling about hypothetical aerodynamic theories is misleading. There will be many simultaneous inputs and the precise interaction will vary from type to type and on the precise speed/power regime at the time.
The last person I spoke to about his experience of a tail rotor drive failure in the cruise described the subsequent experience as being akin to a Lomcevak, with gyrations in all three axis that only eased when the SSLs were retarded. Another ex-colleague described a tumbling so violent that both crew's headsets flew off.
I accept that not all failures are that dramatic but I think to start quibling about hypothetical aerodynamic theories is misleading. There will be many simultaneous inputs and the precise interaction will vary from type to type and on the precise speed/power regime at the time.
Join Date: Mar 2006
Location: On the move...
Age: 58
Posts: 358
Likes: 0
Received 0 Likes
on
0 Posts
I thought it would relate to why the anti-torque drive failed. If the TR drive shaft broke and was flapping around in the tail shaft, then it would chew the machine to bits. If it is due to a less dramatic reason then the pitch/yaw would be as has been described.
At cruise speed I thought that the yaw should be minimal due to the effectiveness of the tail/vertical stabiliser doing what it's name implies.
I think for exam purposes though, apply the KISS principle. Keep It Simple Stupid! You've already described the answer, it will yaw right.
The trick as a pilot in the real world is to realise what has gone wrong and know how to fix it.
Good luck on the exams.
At cruise speed I thought that the yaw should be minimal due to the effectiveness of the tail/vertical stabiliser doing what it's name implies.
I think for exam purposes though, apply the KISS principle. Keep It Simple Stupid! You've already described the answer, it will yaw right.
The trick as a pilot in the real world is to realise what has gone wrong and know how to fix it.
Good luck on the exams.
I agree with 212. The effective change in rrpm will have a miniscule effect on pitch attitude, totally swamped for instance by the rolling resulting from disc flap-back as the aircraft yaws violently and presents the side of the disc to the forward airflow. Thinking along the same lines I woud say it would pitch nose down at that point because the cylic position was set to overcome the flapback whilst pointing ahead, but with the yawing its now pointing into a reduced (or zero) airflow. Less flapback for a given cyclic position will cause a marked nose down pitch. As 212 says, after the initial yaw all hell is likely to break loose.
Regarding CYHeli's comment, of course the effectiveness of the vertical stabiliser depends on the model, but I think that most of us would not expect to be able to maintain control short term following loss of tr drive (as opposed to following loss of tr control, where a number of models have some sort of device to set the tr to a sensible position when cables break etc). Vertical stabilisers are generally designed to add yaw stability and perhaps to unload the tail rotor in fast forward flight, but not to replace it.
Sounds to me like the question was set by someone who had not only not experienced the failure, but in fact by someone who has never flown at all!
HC
Regarding CYHeli's comment, of course the effectiveness of the vertical stabiliser depends on the model, but I think that most of us would not expect to be able to maintain control short term following loss of tr drive (as opposed to following loss of tr control, where a number of models have some sort of device to set the tr to a sensible position when cables break etc). Vertical stabilisers are generally designed to add yaw stability and perhaps to unload the tail rotor in fast forward flight, but not to replace it.
Sounds to me like the question was set by someone who had not only not experienced the failure, but in fact by someone who has never flown at all!
HC
CYHeli - what you say is true for lots of helis, but some have little tail area and small fins. The old Puma with huge intakes in front of the mast is a classic example - they are likely to self destruct in seconds following a drive failure unless the lever is lowered and the speed selects pulled back. Something like a Wessex would be more manageable (probably!).
Exactly, Old beefer. The examples I gave were for a 332 and 330 respectively. I imagine the effect would be similar for a 76 B/C model (smaller fin than A model) or 212/412 too, as neither types have particularly large vertical fins.
I don't think loss of thrust per se is the issue in the high speed cruise; it's loss of disc effect as the rotor rpm decays.
I don't think loss of thrust per se is the issue in the high speed cruise; it's loss of disc effect as the rotor rpm decays.
Sounds to me like the question was set by someone who had not only not experienced the failure, but in fact by someone who has never flown at all!
What earthly reason would an engineering quiz have such a question?
It is a well know fact that many exam questions have naught to do with the real world.
My favorite was the one on the Air Law exam I first took...."If a lighter-than-air ship is underway but without power at night, what signals and/or lights shall be displayed?" Granted, there were no lighter than airships in existence at the time....but every pilot sure needed to know that I guess.
Good point SAS but I suppose the thinking might be to give the Engineer some insight into what might happen if he made a mistake or the component failed. So the thinking would be along the lines of:
"I suppose I had better tighten up these tail rotor drive bolts properly otherwise the pilot might experience a yaw right and slight nose-up pitch, and that would never do".....
Anyway, as we all know exams simply measure one's ability to pass exams, no-one said they had any bearing on the real world!
HC
"I suppose I had better tighten up these tail rotor drive bolts properly otherwise the pilot might experience a yaw right and slight nose-up pitch, and that would never do".....
Anyway, as we all know exams simply measure one's ability to pass exams, no-one said they had any bearing on the real world!
HC
