Elevator trim and turning, myths and facts

[Ack-Ack]

Great Post Bighorn . I think I am finally beggining to understand trim with this help

There is another excellent write up over at Netaces.org[/i] that was written a couple of years ago by a former lead trainer (lephturn) that might also help you understand the trim system a little more.


ack-ack

[Murdr]

First, in a P-38, the control surfaces still move while in a compression dive...  They are just not effective due to the turbulent air both over and underneath the wing surfaces...  So, technically, not amount of trim should work.. 

P-38 compressibility was mainly an issue at altitude.  It is documented that trim would assist in dive recovery down in the denser air, which is where typical MA fights take place.

 

Also, even if the speed at which the plane is moving negates movement of the elevators because the pilot can't pull the yoke/stick hard enough, how would he be able to do so with a little trim wheel?  Unless the pulley system used for the trim wheel uses a compound pulley system which is "stronger" than the one connected to the stick/yoke/rudder pedals (which wouldn't make sense)..  I dunno...Just brainstorming why stuff works the way it does...

I can't seem to come up with a diargram, though I know one is out there....See if I can take a crack at the verbally.  Ok, picture your elevator with the trim tab a zero deflection.  With an airflow around the stab at speed, and no stick input, the control surface is going to be held in a position, by the airflow, at a point where the air pressure is equal on the top and bottom.  Basically with no deflection of the elevator.  Now if the trim tab is cranked down, that will create more pressure underneath the control surface, which means the elevator will be deflected up to rest in a position where the air pressure is equalized top and bottom. So it takes a lot less force to move that smaller trim tab, than it does to move the whole elevator surface.  The tab however does change the balance of airpressure over the control surface enough to deflect it in the opposite direction.

[goober69]

P-38 compressibility was mainly an issue at altitude.  It is documented that trim would assist in dive recovery down in the denser air, which is where typical MA fights take place.

 I can't seem to come up with a diargram, though I know one is out there....See if I can take a crack at the verbally.  Ok, picture your elevator with the trim tab a zero deflection.  With an airflow around the stab at speed, and no stick input, the control surface is going to be held in a position, by the airflow, at a point where the air pressure is equal on the top and bottom.  Basically with no deflection of the elevator.  Now if the trim tab is cranked down, that will create more pressure underneath the control surface, which means the elevator will be deflected up to rest in a position where the air pressure is equalized top and bottom. So it takes a lot less force to move that smaller trim tab, than it does to move the whole elevator surface.  The tab however does change the balance of airpressure over the control surface enough to deflect it in the opposite direction.


sweet explanation i always wonderd about that

[Hazard69]

An important point here: so as to explain how a smaller tab can move and maintain deflected a larger control surface.

Lets assume:
The force acting on the elevator is F.
The force generated by the tab is f.
F obviously much larger than f.

The distance between F and a the pivot of the elevator is L.
The distance between f and the pivot of the elevator is l.
But L is smaller than l.

So when we look at moments: FxL = fxl

So although the force generated by the tab is much smaller, the moment it creates is larger, and in a trimmed condition (be it level, nose up, nose down.....whichever) the two moments balance and the control surface remains deflected.

If the pilot wanted to move the elevator directly (i.e. via control column), he needs to apply force F, which maybe beyond his capability. But if he operates the tab, by applying a small force f, he can get the same surface deflection as if he had applied the larger force F on the controls. This is how trim may be able to hold a control surface deflected, even when the pilot cant.

I'll try and post a pic later to elaborate further.

[EDIT]
Found a simple one online. Ignore the balance tab bit.....although that also works on the same principle.

[muzik]

I can go into a turn, hit trim, and watch my nose swing around faster than without. Obviously the speed at which you use it has an effect on how responsive it is. And as you say, once the elevator reaches max deflection it can have no further effect.

I dont know why this works but I speculate that pilot strength may be a factor as mentioned.  I cannot say for sure but when I get into a flat turn with some opponents in same plane duels, I watch my nose come around on them slowly but surely even if they seem to be flying with good control. Of course I am talking about turns at just above stall where there is no energy differential. There are a couple of other factors that could account for this though, bad turn coordination possibly.

But the slow flat turns are not where I primarily see the difference. It is at a higher speed, Im guessing around 250 or so, just after entering the turn.

I have asked probably hundreds of opponents if they use manual trim. The ones that have asked me how I manage to out turn them are usually the ones that say no. The pros that I encounter more often say yes. And this is not only the 38 in my experience. I cant think of an aircraft that I haven't noticed the effect.

This being said, the reason I found this post is because after a short time away from the game I have noticed that my elevator trim is not having the same result as before and I came to find out why. The trim gauge wont even go to full deflection anymore. And the effect I have known for years is hardly even noticeable now. My edge may have just ran out.