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Servo Tabs

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					   This page is devoted to Trim Systems, or more precisely Servo, Anti-
                           Servo & Trim Tabs




Note : This is not the " Reflexing of trailing edges" discussion page. It is a close cousin, but for a
full discussion on why these silly trim tabs actually work, you need to click < here > That will
take you to the Reflexing discussion page. Please take the time to read this alternate page. it
contains the " why " that this page's " how" assumes you already know.




What is a Servo tab? The classic definition goes something like this: A servo tab is a small
portion of a flight control surface that deploys in such a way that it helps to move the entire flight
control surface in the direction that the pilot wishes it to go. it is power steering for control
surfaces. A servo tab is a dynamic device that deploys to decrease the pilots work load and de-
stabilize the aircraft.

What is a Anti-Servo tab? The classic definition goes something like this: An anti-servo tab is
small portion of a flight control surface that deploys in such a way that it works to oppose the
motion of the entire flight control surface from the direction that the pilot wishes it to go. That is to
say, an anti-servo tab is a dynamic device that deploys to increase the pilots work load and
stabilize the aircraft.

Note that the servo tab makes a pilots work load go down (and makes the aircraft less stable)
while the anti-servo tab makes the pilots work load go up and stabilizes the aircraft. A servo
assisted control surface system is at best a trade off. To get a high amount of lift from the
elevator, you must send it down into the air stream. To get it down into the air stream, you must
overcome the rotational moment that is created by the air hitting the elevator and trying to shove
it back up. This rotational moment gets higher the farther you try to put the elevator down into the
breeze. A servo tab (when deployed from neutral) creates a small zone of very negative
rotational moment that is added to the elevator's overall positive rotational moment. The sum of
the negative and positive moments on the elevator is brought to be near zero and the pilot can
overcome the forces of control that are acting on the elevator. All this helpful counter rotation
force does not come free. It creates drag. In effect, the pilot is getting lift and the ability to control
the aircraft at the cost of creating drag. And drag means more engine power needed to
overcome it. Essentially, you are trading engine power for the ability to control the process of
lift.

So why all this trading of this for that ??? Well...... the only thing the pilot has to overcome the
forces of flight is his upper body strength. In the tandem wing design, the canard's elevator is
rather large and is connected to the pilot through the control linkage. The problem breaks down
something like this : The pilot pulls back on the control stick, the elevator rotates down into the
air stream and the air stream does not want to be deflected. Air slams against the elevator and it
tries to rotate back up. That force is in turn delivered to the stick and the eventually to the pilots
arm. Without a trim system, the elevator will eventually win every time. The more lift you want
out of that elevator, the more resistive force you get. At high demands for lift, the rotational
moment forces created by the elevator will overpower the best efforts of any pilot.

So we need a trim system. We need something that will help pull the elevator down into the
breeze or let it cycle up (controlled) as we wish it. That is why "sparrow strainers" were invented.
Strainers are little up-side-down wings that " fly " the elevators down and reduce the pilot's work
load. They are true aerodynamic surfaces that are use the free stream to do work on the control
surface. They are slightly "anti-servo" in nature, in that they are always trying to return the
elevator to its neutral trim position. Being aero surfaces, they respond to speed changes as do
the control surfaces (forces proportional to the square of the speed). In this way they can
automatically adjust to changes in speed.

There is however another way to get there from here. By deflecting a portion of the trailing edge
of the elevator in the opposite direction from the rest of the surface, air pressure itself can be
used to balance the forces caused by lift. Airfoils with a portion of its trailing edge reflexed up will
still create lift. It will make less lift than it did with the trailing edge in neutral trail (or deflected
down), but it will still make lift. In the case of the Mark 2 Dragonfly, the plans built canard airfoil
is a GU25-5(11)8 modified If you take a bit of the trailing edge of this airfoil's elevator section
and move it upwards into the air stream, the whole elevator will want to move downwards in
response to the forces that have been applied by the trim tab's movement. When the pressures
over and under the elevator adjust and finally come to equilibrium, the new position of the
elevator is stable with respect to the aerodynamic forces acting on it.

There are four major types of this trailing edge tab control.

( 1 ) An " active anti-servo " trim tab system : Anti-Servo Trim Tab This system of trim
utilizes the "race ahead and help push it back" theory of trim. It does this by having the little trim
tabs race ahead of the elevator in both directions of travel and aerodynamically force the elevator
back to equilibrium. The linkage basically connects the trim tab to a fixed part of the airframe (or
servo motor attached to the airframe). The way you get reversed travel is to push on a lever that
is ahead of the trim tab's pivot point. From a trimmed position, if you bump the stick, this concept
aggressively returns the aircraft to its former state of flying. In general, the tab's position is
always up to offset the aerodynamic loads of the elevator. The tab travels farther up as the
elevator travels up, and the tab travels down as the elevator travels down. Note: this is the most
stable configuration you can come up with. The trim settings (once established) would neutralize
elevator forces to the control stick for a very small range of elevator travel. Once the pilot
exceeds this little range of neutral (low) forces, the stick loads would load up fast.

( 2 ) An " active " or " motor driven " trim tab system : Active Trim Tab This system of trim
utilizes the "motorized bent metal plate" theory of trim. The little tabs are inert as the elevator
goes up or down and do not actively assist in returning the elevator to equilibrium. The linkage
connects an electric motor (or mechanical trim wheel cable) to the movable trim tab at a point that
is aft of the trim tab's hinge point. In general, the tab's position is always up to offset the
aerodynamic loads of the elevator. From a trimmed position, if you bump the stick, this concept
neither hinders or helps return the aircraft to its former state of flying. Return to equilibrium is
achieved by a longer period porpoise effect as the aircraft "seeks its own" based upon power and
drag. This type of trim tab system is only slightly better than the " fixed " version in that the
servo motor can adjust the tab positions and affect a trim condition for the aircraft. The trim
settings (once established) would neutralize elevator forces to the control stick for a small range
of elevator travel. Once the pilot exceeds this little range of neutral ( low ) forces, the stick loads
would load up fast.

( 3 ) A " passive " or "un-powered" trim tab system : Same as above except that this is just a
bent metal plate sticking out in the breeze. It would be the same as above when the servo
motors fail. You bend the tab once and it is good for a small range of speeds and elevator
settings. There is no pilot control of the pitch trim. Once the pilot exceeds this little range of
neutral ( to low ) forces, the stick loads would load up fast.

( 4 ) An " active servo " trim tab system. Servo Trim Tab This system of trim utilizes the
"push in the opposite direction and help the elevator to go" theory of trim. It does this by having
the little trim tabs work opposite of the elevator and generate aerodynamic forces that aid in the
elevators travel. The elevator is helped away from equilibrium. From a trimmed position, if you
bump the stick, this system aggressively pulls the aircraft farther into the perturbed state. The
linkage essentially connects the trim tabs to some fixed part of the airframe (or servomotor
attached to the airframe) at a point that is aft of the trim tab's pivot point. In general, the tab's
position is up if the elevator is down, neutral if the elevator is neutral and down if the elevator is
up. The tabs travel up as the elevator travels down, and the tabs travel down as the elevator
travels up. Stick forces imparted by the elevator would be neutral the whole time. Note :
THIS IS AN AERODYNAMICALLY UNSTABLE CONDITION. This type of tab would be a great
as a booster for ailerons or really big elevators that overpower the pilot’s best efforts. In effect,
this type of trim system is "power steering".

In each of these cases, the lift being created by the elevator section will still be centered at the
elevator's 1/4 chord position. The benefit of servo and powered trim systems is that this lift will be
created with far less rotational moment. If enough of its trailing edge is reflexed correctly, the
elevator can be made to create all its lift with little or no rotational moment forces. In fact, you can
make lift and create positive rotational forces that actually pull the elevator down into the breeze.
This is what the servo tab does.

The price you pay for this aerodynamic magic is, of course, DRAG. Parasitic drag to be more
specific. In the trade to reduce the work load on the pilot, the designer is poring a huge amount
of power to the bottomless drag pit. That power is coming from your engine in one way or
another.




The image (seen
right) is a variation
of the R1145MS
airfoil with an
integrated powered
trim tab. This airfoil
uses a 38% chord
elevator to create a
tremendous amount of lift and rotational moment on the elevator torque tube. Being the airfoil of
choice for many canard experimental aircraft, it has proven itself to be an excellent replacement
for the problematic GU25-5(11)8.

This is the canard airfoil that was chosen for the Raptor. As shown here, a portion of the inboard
trailing edge of each elevator has a powered trim tab. These tabs act as powered trim tabs under
the pilot's control. They aid in drawing the elevator down into the air stream during high lift
operations. Integrated into the trailing edge of the elevator, the powered trim tabs " reflex " up to
about 60 degrees. Electrically driven, they do not automatically adjust to changes in
aerodynamic loading as true servo tabs would. Adjustment of the tabs if done in parallel at the
pilots command and can relive 100% of the aerodynamic forces on the pilots control stick. .




Perhaps an example will help clear things up :

Lets say you have a Mark II Dragonfly cruising along at 80 kts with its plans built GU25-5(11)8
canard. Lets also say you have no trim tabs, springs or sparrow strainers installed to help keep
the elevators deployed into the air stream. You are just really strong and that you are holding the
elevator is down 20 degrees. Lets also say that the elevator is making 100 units of lift, 100 units
of rotational moment force and 10 units of drag. The actual quantities of the units are not
important in this example. Now, we get smart and hit the magic button on the control stick and
deploy the servo trim tabs. Poof, the elevator now has 10% chord servo trim tabs along its entire
trailing edge and they are deflected upwards a few degrees. The elevators, still rotated down 22
degrees into the air stream are now making 90 units of lift, 10 units of rotational moment and 100
units of drag.

So What Happened?

Well for one thing, the pilot commanded a portion of the elevator's trailing edge (the trim tab) to
reflex upward. The aerodynamic forces on that little bit of tab are now added to the forces made
by the rest of the elevator. Remember, from the pilots point of view, the elevator is one thing,
with one force being delivered to that control stick. So now, there is a 10% decrease in lift and a
90% decrease in rotational moment. Of course that same elevator now generates many times
the drag it once did. Unless you are very strong and have great fortitude, you will think this a
good trade. In fact, a clever pilot would just deployed the elevator down into the free stream a bit
more ( say another 1 degree or so ) to make up for the loss in lift. It only cost drag. And drag
cost power. Eventually even flights of fantasy run out of thrust.

You do not get anything for free. In fact, you get far more drag from Reflexing the (trim tab)
trailing edges of a control surface than from attaching "sparrow strainers" to get the same job
done. So why would we choose to use motorized trim tabs in place of strainers?

Because : #1: The powered tab mechanism can can be used as elevator trim ( under pilot
control ) while flying. Aerodynamic assisted trim is the industry standard. It is speed proportional
and responds at the same natural frequency as the control surfaces. Springs resist in a linear
fashion while aerodynamic forces go up with the square of the speed. your trim system and your
flight system are responding to the same aerodynamically generated forces.

Because : #2 The powered trim linkage operating ratio is ground adjustable. That means that
the ratio of how much the trim tabs help at max elevator deployment to how much they work at
min elevator deployment can be adjusted to the pilots preference.

Because : #3 Sparrow strainers tend to get knocked off the aircraft at fly-inns or in very
turbulent flight conditions. This is very bad as it leaves the pilot with little or no help in fighting the
aerodynamic forces of the elevator.

Because : #4      It just looks way cooler to have trim tabs that move with the press of a button.

				
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posted:8/26/2012
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