This
page is devoted to Trim
Systems,
or more precisely
Servo, Anti-Servo & Trim Tabs
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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.
page information was last updated on 11/01/02.
