How do elevator trim tabs work?

Recently I noticed (when using the outside view during flight) that when the elevator trim tab is adjusted they appear to work in the opposite direction to the elevator ie when the elevators are in the up position the tail drops and the aircraft climbs (and vice versus) BUT when the trim tab is in the up position the tail goes upwards and the aircraft dives.

I find this quite confusing.

I tried to find how they actually work but could not.

Could someone please advise or give a link to an appropriate web site.

Thanks

Trim tabs are used in real life to balance control forces that the pilot feels, and also to balance the aeroplane into a set angle of attack. Properly set, a pilot could if he wished let go of the controls and the aeroplane will fly onward as he has balanced it.

 

How do they work? By deflecting airflow. This adds an extra force acting directly on the elevator so the pilot can choose how much elevator is being used by default. Think of using the trim tab not as a separate control, but as a means of balancing the forces you have to deal with.

 

So, if you want level flight, instead of constantly and inconveniently having to hold the joystick just-so, use the tab to get rid of the need to hold it by setting the nose attitude. The same thing applies on climb and descent, so you can use the trim tab to control your airspeed by altering the nose up or down. Practice this until it seems more natural.

 

Hope that helped.

Here's an illustration of Caldrail's description above:

 

Thanks

 

Yes I use the trim tab to level the aircraft with hands off the controls.

 

The graphics makes it easier (for me) to understand ALTHOUGH

When I noticed that the trim tabs move in the opposite direction to what I imagined the elevator remained 'neutral' (neither up or down) and didn't seem to move when I moved the trim tabs to full up or down. Imagine the graphic in the previous post with the elevator in line with the tailplane and only the trim tab up or down. Doesn't that happen? When the trim tab is moved does it also move the elevator without input from the pilot eg hands off.

Be cautious about thinking that what you see on any outside view of a PC based flight simulation is a completely accurate representation of what is going on in a real airplane.

 

The control surface movements at any speeds much beyond climb speed (say around 70 knots in a Cessna 172) are quite small and may well not be noticeable to anyone more than a dozen or so feet away from the surface. And the animations that exist in flight simulation (and for the most part control movements are animations programmed in as a sort of eye candy, since in the real world you can't see much of anything in the way of control surfaces from inside anything other than a Cessna) are merely reflective of what the surface might be doing with such and so displacement of the wheel or rudder pedals. The effect of trim tab movement is probably even more of an approximation.

 

That being said, on a real airplane the actual control surface movements and trim tab movements are small - on the order of a few inches at most and usually less than an inch in cruise flight unless we are talking about an Extra 300 being flown by someone like Sean D. Tucker! When the elevator is moved by the yoke, it leaves its streamlined position and pushes against the airflow, which in turn pushes back and results in an opposing force felt on the yoke. A pilot who has taken quite a few of the old Charles Atlas courses (good old dynamic tension!) may well feel comfortable simply holding the yoke against the opposing force of the air, but most pilots are seekers after comfort and sloth - they would rather just sit back and keep at most a finger or two on the yoke! Thus the trim tab.

 

It would really be great if the Cessna and Piper dealers still offered those very inexpensive first flight experiences - a few minutes in a real Cessna would give you all the insight you would need!

Ah yes FSX aircraft representation.

Still it baffles me when say I am 'flying' (say) a single engined prop aircraft and I let go the 'yoke' and the aircraft begins to dive (and increased engine power/prop pitch doesn't seem to correct it) I can level the aircraft by adjusting the elevator trim tab 'down' - if I adjusted the elevator down it would increase the dive. I guess it doesn't really matter what I see but I only did that as I had noticed that negative elevator tab (resulting in the trim tab moving downwards) caused the aircraft to pitch up.

Does moving the trim tab down cause the elevator to move upwards (with no hands on the yolk)?

Does moving the trim tab down cause the elevator to move upwards (with no hands on the yolk)?

 

Yes.

 

With the elevator at an aerodynamically neutral state, moving the trailing edge of the trim tab down exposes the lower side of the trim tab to the air stream moving under the elevator.

 

The air stream pushing on the lower side of the trim tab will force the elevator up as it seeks a new neutral state.

 

peace,

the Bean

Aha

That is what I thought - the elevator trim tab affects the position of the elevator.

Thanks bean

...a few of the old Charles Atlas courses (good old dynamic tension!)

"He thinks Dynamic Tension must be hard work! Such strenuous living I just don't understand..."

 

Bonus points for whoever can ident the movie that's from! :p ;)

Has nothing to do with aircraft. Or trim tabs, for that matter.

As a side note, the F-14 and F/A-18 don't even have trim tabs. The entire tail surface moves in response to trim commands...

Pat☺

Are you talking about Stabilators where the entire tailplane surface can be adjusted?

Yup, lots of Pipers flying with no elevator trim tab. The trim moves the horizontal stabs up and down to hold attitude.

 

Some planes, like the DC-9 family, have free floating elevators. The control yoke is hooked up to the trim tabs which fly the elevators. Looks kinda weird to see one taxiing with one elevator up and the other down.

 

peace,

the Bean

Are you talking about Stabilators where the entire tailplane surface can be adjusted?

 

Yup. The F-14 doesn't even have ailerons, although for very strong roll commands the spoilers on one will will lift slightly to assist the stab. They are called spoilerons. The F-14 and F/A-18 have stabilators, as do the F-15 and F-16. All the "teen" series fighters. Huge tail surfaces that do most of the work for pitch and roll. The '18s do have ailerons, but the stabs have most of the roll authority. The NATOPS even specify not to pull the nose up before the nose lifts on it's on, because the stabs are so big they can create enough drag to prevent acceleration on the take-off roll. They work very well for slowing the plane down during landing though :)

The concept, I think, of a tail surface moving in toto, was really perfected on the F-86 for production aircraft, although I believe Col. Yeager got them started on the X-1. Just didn't have enough pitch authority near Mach speeds, unless he moved the entire tail. Same problem on the P-38 and P-51. In his case by using the trim to move it. He got the idea to make the entire horizontal stab move to get the control authority off that. I think (although rarely :D).

Pat☺

Rocky Horror Picture Show, of course!

 

By the way, flying an airplane without any pitch inputs (elevator or trim) using only the power (throttles) is not well modeled in MSFS, but it certainly can be done in real airplanes - just ask Captain Al Haines.

 

This is one of the areas where the higher priced (like A2A, pmdg and the like) add-ons and/or XPlane have a bit of an edge. The default Cessna in XPlane 11, for example, appears to have a much better pitch response to power changes, and can be flown with throttle alone just like the real airplane can, albeit still with not quite the perfect response compared to the real thing. Such flying is a dire emergency procedure, not generally taught or practiced outside of major airline simulators, which themselves are probably not 100% perfect in duplicating this.

 

As to the stabilator, one of the first applications of it in general aviation came on the type airplane I own - the Thorp T-211 SkySkooter. John Thorp was an early advocate of the stabilator, mostly because the power of the all flying tail could lead to a smaller and lighter structure (simpler too, since it has no elevator and the associated hinges). This found its way onto the Piper Cherokee line, which was descended directly from the T-211. Thorp and Piper stabilators do indeed have trim tabs which function the same way that they do on airplanes with elevators.

What about adjustable stabilisers on many passenger jets? Do they work similarly and the pilot adjusts for the COG of the loaded aircraft.

 

John Thorp was an early advocate of the stabilator, mostly because the power of the all flying tail could lead to a smaller and lighter structure

What about adjustable stabilisers on many passenger jets?

Biggest difference between those and the Stabilators the military aircraft have, is that the military aircraft predominantly use them for pitch AND roll authority. At least the F-14 and F/A-18. I am pretty sure the F-15 is the same, but I am not sure about the F-16.

Also, generally, the commercial aircraft have trim tabs, where-as the military planes, the entire Stab (or slab, as they're called), is adjusted for trim, for both pitch and roll.

Easy to see it in the Navy birds. Just watch a launch video. When they "wipe" the controls, you can easily see the slabs performing in both pitch and roll.

Have fun!

Pat☺

Yes I have seen war planes do that.

Yes it does. I do think though that you need to concentrate on the effect on the aircraft's balance rather than learning which way the tab operates.

As a side note, the F-14 and F/A-18 don't even have trim tabs. The entire tail surface moves in response to trim commands...

 

The Super Cub does that too, Pat (and the J-3, for that matter), though not with a stabilator (see below), along with a number of other aircraft.

 

Are you talking about Stabilators where the entire tailplane surface can be adjusted?

Pat described things pretty well, but there is also a jackscrew that moves the leading edge of the horizontal stabilizer up and down, in aircraft such as the Super Cub. A stabilator is a different animal yet, and it uses an anti-servo tab for trim and to put some resistance into the feel on the stick/yoke, though it's a different mechanism on light aircraft from what Pat describes above. There are even a few aircraft that just use an adjustable bungee or spring to put up or down tension on the elevator.

I do think though that you need to concentrate on the effect on the aircraft's balance rather than learning which way the tab operates.

 

Not sure who this is directed to but as I started the thread maybe it was me.

My initial question was directed to how elevator trim tabs work as I initially thought that they would work in the same direction as the elevators do (to change the pitch of the aircraft) but now have learned that they actually force the elevators in the opposite direction. My initial post has been answered, thanks, although there have been some digressions along the way.