ASI wrote:It's worth noting that if you have a jammed elevator (with a trim tab arrangement), the trim wheel will work in the opposite sense to the placard, versus a snapped elevator cable, where it will work in the normal sense.
Then we're back the original question, how effective would it be as an elevator control on its own?
I think it depends on the aeroplane and the trim tab in question.
That would be fairly easy to ascertain. Hold the stick in a fixed position and wind the trim wheel to see what happens (NB...I wouldn't advocate what is essentially a flight test exercise in quite such a flippant way!).
My suspicion is that the throttle would like have a far greater pitch effect, both in terms of propwash and speed stability effects.
I'm not flying again until into the New Year - May give it a try then.
ASI wrote:My suspicion is that the throttle would like have a far greater pitch effect, both in terms of propwash and speed stability effects.
And that too would work in a counterintuitive sense in my current mount. Odd little microlight thing that she is - with the engine perched up overhead between the wings the high thrust line means a pitch DOWN when opening the throttle...
I think the main thing to commit to memory from this thread is "Know how what you fly works!" and especially where it differs from the bog standard what's in the PPL textbooks
Keef wrote:Have a read of the (counter-intuitive) bits of aerodynamic theory: for example, if you hold two pieces of paper a couple of cm apart and then blow down the gap between them, will they move apart or move together? What does this mean your elevator might do with more windspeed on one side than the other?
Oh ya, I tried to figured out, but figure out nothing so, it would have any effect or no?
It's Mr Venturi's Astounding Effect. It will come up in one of your exams...
Oh ya, passed some exams without knowing a lot about Venturi (my fault not knowing it in details). I would appreciate if you'll tell me if it works or no, sorry, no sense to make theoretical assumptions, as I told you, I can't figure out from your paper sheet example. Sorry again.
Thank you, useful stuff to know, but I think the practical trim issue regarding this thread is beyond any basic theoretical aerodynamic assumption given the fact that it seems to be more or less a proven emergency flying technique.
What I've read is the fact that the cable stretch or the sponginess in the flight control system can cancel the opposite trim effect in a jammed elevator scenario (also, this might make impossible any attempts to test it by holding the yoke in position), but I don't figure out how can this happen because in that case the trim would work the normal way if the elevator moves a bit due to cable stretch, the only logical asumption would be that the range from strecthed cable in a jammed elevator would be too little to have any effect, but this doesn't seem true at all because when te cable stretched at max the trim would begin to works in the debated opposite way, what do you think guys, am I missing something in this assumption?
If your elevator jams in any position, you are in the deep doo-doo. You absolutely don't want to go there, and if your aircraft is maintained properly and you do proper preflight checks, you shouldn't need to.
What happens next will depend on the aircraft type and how the trim tab (if any) works. With the spring-loading of the elevator types there's not a lot you can do with the trim control. With a moveable trim tab, and with the elevator firmly stuck in S&L, you may or may not have a little control ability. If one elevator control cable is broken and the elevator free to move, you may find that the trim tab works in the "correct" sense.
Power settings and flaps may do more for you. Flaps may make it better, or worse, depending on type and where the engine thrust line goes.
It's a very theoretical case: file it away for the extremely unlikely event you need it, and if you end up with a stuck elevator, give it a try.
I know one person who's had jammed elevators (someone in the aeroplane dropped a mobile phone, which ended up in the elevator bellcrank). He survived. In his case (Yak 52) the trim did nothing. He got the aeroplane down, slightly damaged by an extremely fast landing, using power to adjust attitude.
Yea, you are right, you can take it more like my personal technical curiosity, not really an emergency preparation, I don't worry for this kind of emergency being really rare and avoidable through careful preflight, but I simply don't understand some debated aspects, detailed above in my last post, that's why I insist on this issue and that's why I wrote it here. For me, the only logical asumption for not working is that the tab is too small to have any effect, maybe the case for yak 52 which has a convential trim system, but despite this haven't work. Really didn't understand why cable stretch and system sponginess would be a problem. I'm sorry and I apologize if I upset you with my tiring ideas.
David36 wrote:For me, the only logical asumption for not working is that the tab is too small to have any effect, maybe the case for yak 52 which has a convential trim system, but despite this haven't work. Really didn't understand why cable stretch and system sponginess would be a problem. I'm sorry and I apologize if I upset you with my tiring ideas.
I think you can assume that
1. The trim tab will have some effect, but this will vary from aeroplane to aeroplane. It won't be as effective as the elevator but can make some difference.
2. Slack in the cable and system play, if the jam is at the yoke/stick end, may mean that some of the affect may be cancelled out as the trim tab moves the elevator a bit in the "correct" (though wrong for the emergency situation) direction. Obviously this will vary from one to the next.
3. All you will do with any of the above is to vary the speed of the aeroplane. You may be able to flare a bit but you'll be turning quite fast and will only have a limited authority compared to the elevator. It might be best to trim for the slowest speed you can before starting the approach. Vary descent rate with power. Depending on how slow you can get it, it might be best to aim for a long runway.
Ah yes, got your PM, thanks, but will reply here in case anyone else is interested.
The normal way the trim tab works is that it works by changing the position of the elevator. If you pitch for a certain speed by holding the yoke/stick, you then move the tab to hold the elevator in that position. Put another way, if you move the trim tab down, the elevator goes up and the aeroplane pitches up and vice versa. When the elevator is jammed, you only have the trim tab moving up or down, and this is all the pitch control you have, and will work in the opposite direction, as stated in the initial post.
So what happens if there's a bit of play? When you move the trim tab, for a small part, the elevator will move in the direction it's supposed to move...so moving the tab down will move the elevator up until the slack has been taken up and the aeroplane will pitch up. Then further movement of the tab will make the aeroplane pitch down as the elevator can't move any more. And vice versa.
But there really shouldn't be all that much slack in the system, only a tiny bit. Try moving the elevator with the control lock in and see how much it moves. Too much slack would be picked up at the annual and adjusted accordingly.
Paul_Sengupta wrote:Ah yes, got your PM, thanks, but will reply here in case anyone else is interested.
...so moving the tab down will move the elevator up until the slack has been taken up and the aeroplane will pitch up. Then further movement of the tab will make the aeroplane pitch down as the elevator can't move any more. And vice versa.
Here comes my curiosity.
I'm thinking that further movement will work the trim tab the opposite assummed way and that will give you further the desired pitch effect. Am I right? If not, what is the explanation of not working? That further movement which remains is too small and too close to maximum trim stop range? Why it won't work further this opposite way to have any effect? That further movement of the trim would be close to max trim tab deflection and it won't be enough to have any effect except canceling the initial undesired pitch caused by slack? Basically, you'll reach maximum trim before having any effect? In fact what remains is too small to have any further pitch effect?
This is the only logical assumption which I figured out about not working. (Of course except the case when the system has a trim tab when the tab itself is too small to have any effect, but that was discussed above.)
Last edited by David36 on Sat Dec 22, 2012 11:40 am, edited 1 time in total.
Depends. As I say, there shouldn't be much slack. Then I would have thought it would depend on the size of the trim tab, the moment of the elevator around the C of G, C of L, etc.
It doesn't take much of an elevator deflection to move the aeroplane pitch a little, and some trim tabs are quite large.
Have a look at these pictures. The camera angle in the first one makes things a little odd, but look at the size of the trim tab compared with the size of the elevator. However the trim tab is only on one side on this aircraft.