The elevator is arguably the most important of the three primary flight controls. It is, after all, the one that gets you up off the ground. And once you’re off the ground, it’s the one that makes the aircraft stable and keeps you flying at your altitude. It’s one of the basic components of flight.
Ok, all three flight controls are important. But fly along with us while we dive–hopefully not nose dive–into why the elevator is critical to flying and aircraft design.
What is the Elevator on an Airplane?
The elevator is the flight control that makes the aircraft pitch around the lateral axis. That means it makes the nose go up and down. On takeoff, the elevator pitches the nose up during rotation. In essence, just like an elevator in a building, the elevator makes the plane go up and down.
The elevator is located on the empennage–the plane’s rear section. There are several designs, but it is usually mounted to the trailing edge of the horizontal stabilizer.
It is operated in the cockpit by moving the stick or control yoke forward or backward. For example, pulling back brings the nose up, and pushing forward puts the nose lower.
How Does the Elevator Work?
The operation of the elevator is pretty simple, at least in small aircraft. It is usually connected to the cockpit control column through a series of cables, pulleys, and bell cranks.
The aerodynamics that makes it work is much more interesting. The ailerons and rudder work on their own, creating an aerodynamic force that moves the plane around each axis.
The elevator is different, however, because it is one part of a complex system that keeps the airplane stable and upright in the sky.
Stability is the tendency of an aircraft to stay put–to keep flying in the attitude you want it to fly in. If you’re the typical pilot, that attitude is straight and level most of the time.
Airplanes have design features to keep them stable around each flight axis. The horizontal and vertical stabilizers are key–they keep the aircraft pointed forward, working like the feathers on an arrow or the fins on a rocket.
Longitudinal stability is the stability of the aircraft around the lateral axis–therefore, it is controlled, at least in part, by the elevator.
Factors Affecting Longitudinal Stability
Longitudinal stability is one of the more complex things going on with an airplane during flight, so it’s important to understand each component.
Three elements make it happen–the lift from the wings, the center of gravity, and the tail-down force.
Lift from the Wings
As you may have guessed, the airplane’s wings make lift. This is the force that pulls the plane up into the sky. You can imagine a giant hand reaching down to pick up the airplane.
When you average all the lift the wings make, the point is called the “center of lift” or sometimes the “center of pressure.” This is the point at which the giant hand grabs the plane and pulls up.
CG (Center of Gravity)
Since we aren’t talking about spaceships yet, gravity also plays a part. If the lift is a giant hand pulling the plane into the sky, gravity is a giant hand pulling it to the ground in a constant game of tug-of-war.
The spot at which the hand grabs is called the “center of gravity” or simply “the CG.”
The tricky thing about the center of gravity is that it moves around. One day you might load the plane with two people, but on another, you might load it with four. As you fly, fuel is used up, and the weight and CG change.
So aircraft designers need to build a system that allows enough flexibility to load the aircraft in different ways.
Tail Down Force
In the example of giants playing tug-of-war with the airplane, there is no flexibility. The goal is to have the plane remain straight and level as it flies. Unless the two giants are lined up (the center of lift is directly over the center of gravity), the plane cannot remain level.
So a third force is added. The horizontal stabilizer is designed to make a tail-down force. So, as long as gravity pulls forward of the wing, the tail-down force made by the horizontal stabilizer will counter the nose-down pull of gravity.
Of course, there are limits. The amount of tail-down force is a factor of the stabilizer’s design and the plane’s speed.
If the plane is going too slowly or the force of gravity is too great (as with an overloaded airplane), then the plane may not be able to make enough tail-down force. The same can happen if the CG is located too far forward.
Putting It All Together
How does all of this relate to the elevator and how it works? The horizontal stabilizer creates tail-down force–but is controlled by the elevator. So the elevator actually increases or decreases the amount of tail-down force being applied to the plane.
If you reduce the amount of tail-down force, the force of gravity pitches the airplane down. If you increase it, the elevator fights gravity, and the nose goes up. This is easily accomplished thanks to the elevator.
Moreover, since the elevator can produce different amounts of tail-down force, it can correct for different loading conditions. The elevator allows you to fly a lightly loaded or at maximum takeoff weight.
It can also correct for where you put the weight, within the limits you see on the loading charts when you do your weight and balance.
Alternative Designs of Elevator
Everyone is probably familiar with the conventional design of an airplane, like the Cessna 172. The empennage has a vertical stabilizer with a rudder, and the horizontal stabilizer and elevator are mounted below.
But the empennage is one of those things that aircraft designers love to tinker with. There are tons of variations, each with intriguing benefits and disadvantages. Each design also has interesting implications for the elevator and how it works.
A stabilator is a combination horizontal stabilizer and elevator. So the entire surface is hinged and moves as one piece. You’ll see stabilators on many Piper aircraft, fighters (like the F-16), and even some airliners (like the Lockheed L-1011).
Because the moveable control is such a huge surface, stabilators are extremely effective. However, that means that control inputs may need to be very small. Therefore, the design often incorporates an antiservo tab to make it easier to fly—more about tabs in a minute.
On the t-tail, the elevator or stabilator is moved to the top of the rudder. Sure, it looks ridiculously cool–but is it any better? T-tails are often used on jets with rear engines like Learjets and Gulfstreams, or on seaplanes where keeping the elevator out of the salt spray is important.
One important thing to realize about a t-tail is that the stabilizer and elevator will be above the propeller’s wash.
Without this extra airflow, the elevator will be less effective than others. It will need larger control deflections. It will also mean that the plane is less likely to experience pitch changes when power is added or reduced.
T-tail designs may also experience control surface fluttering during high angles of attack or stalls. In extreme cases, a t-tail may be more likely to become deeply stalled with a high angle of attack and aft CG since the disrupted air over the wing will also disrupt the airflow over the elevator.
What’s even cooler than a t-tail? Obviously, the answer is a v-tail! Once famous for the v-tailed Beechcraft Bonanzas, the V is in vogue again thanks to the Cirrus Vision Jet.
The v-tail is efficient because it reduces drag–you only have two surfaces instead of three. Each of the surfaces on the V combines a horizontal and vertical stabilizer, and each has a control surface called a ruddervator.
But the controls in the cockpit work just like regular flight controls, thanks to some complex linkages in the system.
A canard design is a completely different way to design an aircraft that shifts every aspect of the longitudinal stability equation around.
Instead of a horizontal stabilizer making tail-down force, a lifting wing is added to the front of the aircraft. This secondary wing is called a canard. The elevator is mounted on the canard to move the nose up or down.
With two lifting surfaces, one on each side of the CG, each can fight gravity to keep the plane stable. It’s an efficient design since more lift is good, while tail-down force is a wasted effort when you think about it.
Canard designs might seem revolutionary, but it is actually the OG.
The Wright Flyer was a canard design, with the elevator/horizontal stabilizer mounted forward of the wings. Burt Rutan is famous for his modern canard designs, including the Long-EZ and the now-retired Beechcraft Starship.
The Piaggio P180 features a modified canard design that combines a conventional t-tail and elevator on the plane’s rear and a lift-producing canard on the forward end.
As we’ve discussed, the elevator is integral to the aircraft’s stability. Therefore, it must be used in different amounts at different phases of flight, depending on speed, power, angle of attack, weight, and the location of the center of gravity.
Since the precise control input required will change with all of these variables, there can’t be one designed-in setting to make it easy on the pilot. So to keep the pilot from having to apply constant nose up or down pressure on the stick, a tab is provided.
Tabs are secondary control surfaces that help you fly the plane.
The most basic type of elevator tab is the trim tab. A common installation in small planes is that found on the Cessna 172–a single tab on one side of the elevator. It is controlled in the cockpit by a trim wheel that includes an indicator to show how the trim tab is set.
Trim tabs work by aerodynamically controlling the surface upon which it is mounted.
For example, when you set nose-up trim, the tab deflects down and moves the entire elevator up, giving the airplane a nose-up tendency. You might think of the trim tab as a miniature elevator for your elevator.
Pilots will adjust the trim constantly throughout the flight to neutralize the force they need to put on the control wheel. In addition, autopilots are normally connected to the trim system, allowing them to control pitch.
A balance tab might be used in planes with an overly effective elevator. A balance tab looks like a trim tab but works in the opposite direction. Its job is to reduce the effectiveness of a control surface to make it easier for the pilot to hold a control input and to move the surface more easily.
If a balance tab is used, it is usually linked so that the balance functions are automatic and the trim tab functions work as a regular trim tab.
A servo tab looks and acts like a trim tab, but it is actually the control that the pilot moves when they move the control column.
They’re used on large aircraft where the surface itself would be too heavy to use. By moving the servo tab, the larger control will move and control the plane.
An antiservo tab is installed on a stabilator to make it easier for the pilot to control. It works like a balance tab–it moves in the same direction that the control surface does to dampen the control’s effects. When the pilot moves the stick, the stabilator and the antiservo tab both move.
Antiservo tabs are also linked to a trim wheel for trim adjustments.
Some aircraft forego the tab altogether. Instead, to accomplish basic trim functions, the entire horizontal stabilizer moves up or down. This is usually accomplished with a jackscrew that moves the whole stabilizer assembly.
This is a common setup on transport-category aircraft.
You can learn about elevators, their various designs, and airplane stability in the FAA Pilot’s Handbook of Aeronautical Knowledge, Chapter 6: Flight Controls.
Leave a Reply