Wednesday 19 June 2013 10:04 UTC
The place for technical discussions about GA and flying.
Technical discussions about GA only, please.
Would trim have any effect if it's used in an opposite way on a stuck elevator emergency in a convential plane (eg Cessna 172)? It is not clear for me if the trim tab really acts as a mini elevator in this scenario. There is a guy (don't remember his name) who said he did tests holding the yoke in position and using trim the opposite way and unfortunately there was no effect. But a lot of books and safety articles state that it works.
What's the truth behind this? Is this more like a flying safety myth? Or a proved flying technique?
I hope none one ever is put in such a situation to test it on his own skin.
Yes, it does.
If the elevator is totally jammed, the trim tab will provide some ability to adjust angle of attack. It won't be a lot, and if the elevator is stuck away from "straight and level" it's not likely the trim tab will help a lot. if the elevator is (for example) frozen by ice at S&L, then the trim tab may be of some use - if it isn't frozen, too, which it probably would be.
I don't think holding the yoke in position would provide a valid test. Control forces would mask the elevator effect of the trim tab.
Moderatio in omnibus
Always assuming you've GOT a genuine physical trim tab and not some spring-and-sellotape arrangement a-la the dear old Tommyhawk and some others! (Just relieves the control forces - doesn't have any aerodymanic effect!)
"Let's go flying"
Scribblings of a novice PPL
Also, if the cable fails or anything in the flight control system, is there any logic to snap the elevator fully upwards or downwards? I've read some accident descriptions which states that elevator deflected upwards after cable failure. That doesn't seem plausible because as I know the airflow will streamline the elevator (assuming it will be freefloating, I don't figure out a problem where the elevator will stuck more than present position).
The answer is "it depends".
If a cable snaps or jams, it will certainly spoil your day. What the elevator will do will depend on the design, what other components are there, and the aerodynamics of the thing. You will almost certainly find that a (working) trim tab will allow you to exercise some control over the aircraft, but it will be type-dependent.
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?
It's far better to make a thorough preflight, and particularly to look at the control cables where you can see them. A snapped strand means "do not fly".
Moderatio in omnibus
I know they are different - but most of my "large" models have closed loop controls on the main surfaces.
You tend to find rudder works well in one direction when one of the cables snaps (happend once)
On the evlevator however i've never had one fail and the model survive to be able to tell... Of course this could be because we fly closer to the ground so have little time to realise what the issue is before it crashes or that one way elevator is much more of an issue...
Oh ya, I tried to figured out, but figure out nothing so, it would have any effect or no?
In a PA28 the trim tab is the bit wot moves the stabilator so if that's jammed you're stuffed as trying to trim won't move anything.
On a C152 it would work.
But you will have done full and free at least twice on the ground innit.
CPL IMC FI (A) and registering 9 on the bolometer.
No tab. Just springs IIRC. Leia mentions above.
I thought that trim tab forces elevator to stay in certain position. So if the cable itself is jammed, I find it hard to believe that you could force elevator to move that way. (if it's stuck).
If cable snaps, there were accidents where pilot tried to use trim wheel to control elevation (unsuccessfully tho).
<-- (yet another) ppl blog.
Prob75 this msg was sent from iPad, not any toy.
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