Trim

[1776]

It seems to me that the trim tabs can fly the plane!!  Any pilots(RL pilots, that is) out there fly their planes with just the trim tabs?  I thought trim tabs were only for auto pilot trimming,is this correct??

[Toad]

Oz,

Yes, you can "fly" a plane with the trim. In fact, when I was somewhat, ahem, younger, we used to have squadron competitions to fly down the ILS in an RC-135 to a landing. You trimmed up the airplane straight and level on course inbound, prior to glideslope capture. When you declared yourself "trimmed" you got no power changes and only two "dits" or actuations of the the stab trim from there to landing. You just used gear and flap to slow down and maintain glideslope. You needed one "dit" when the gear came down and one "dit" in the flare.

After a while, nearly everyone could do it.

So, yeah, technically you can "fly" an airplane using only trim.

But no trim tab is a primary flight control and that's how you really "fly" and airplane, with the primary flight controls.

[1776]

Ok, then once you trim straight and level you shouldn't have to mess with the trim tabs in combat,right?  How come I have to mess with the elevator trim all the time in combat? Shouldn't have to, right?  I could understand how trim could be used at slow speed, but at high speed trim shouldn't be the governing control.  In AH in order to dive and climb at high speed adjusting the trim seems to be a "must"!!

Will give you undivided attention if this subject is discussed at the BBQ joint.  

Can smell it now!!    

[funked]

The aerodynamic balance of the aircraft changes as you go to different altitudes, speeds, configurations, and power settings.  So if you change any of these you need to retrim the plane.

[Toad]

Oz, what Funked said is true...sort of.

Trim changes with all of those things, that's totally true.

However, you generally use trim to "trim" the plane so that it will do what you want it to do "hands off". That is, trim it out for a hands off climb out, level flight, descent or maybe an extended turn. This is just so you can pay a bit less attention while you check out your chart or reach in the flight kit or write down a clearance or something. When you let of of the stick, nothing wild is going to happen...it'll just stay there.

So, you need to retrim the plane in these situations if you want it to fly "hands off".

I know of no trim tab, however, that is larger than or exerts more force than the control surface to which it is attached. Usually, they are a small fraction of the size of the trimmed control. So, for example, if you trim full nose down elevator and then are able to still pull the stick full aft, you are still going to get most of the "up force" that you would have gotten had the elevator been trimmed correctly for the maneuver. (Don't know why you'd do this tho, unless you're bodybuilding   )

I'm sure there are some aero engineers that could figure all this out for a particular plane lurking right here on this board.  

We can discuss this in great detail over some cereal malt and burnt ends.