Lift to Drag Ratio

Four Forces

There are four forces that act on an aircraft in flight: lift, weight, thrust, and drag. Forces are vector quantities having both a magnitude and a direction. The motion of the aircraft through the air depends on the relative magnitude and direction of the various forces. The weight of an airplane is determined by the size and materials used in the airplane’s construction and on the payload and fuel that the airplane carries. The weight is always directed towards the center of the earth. The thrust is determined by the size and type of propulsion system used on the airplane and on the throttle setting selected by the pilot. Thrust is normally directed forward along the center-line of the aircraft. Lift and drag are aerodynamic forces that depend on the shape and size of the aircraft, air conditions, and the flight velocity. Lift is directed perpendicular to the flight path and drag is directed along the flight path.

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L/D Ratio

Because lift and drag are both aerodynamic forces, the ratio of lift to drag is an indication of the aerodynamic efficiency of the airplane. Aerodynamicists call the lift to drag ratio the L/D ratio, pronounced “L over D ratio.” An airplane has a high L/D ratio if it produces a large amount of lift or a small amount of drag. Under cruise conditions lift is equal to weight. A high lift aircraft can carry a large payload. Under cruise conditions thrust is equal to drag. A low drag aircraft requires low thrust. Thrust is produced by burning a fuel and a low thrust aircraft requires small amounts of fuel be burned. As discussed on the maximum flight time page, low fuel usage allows an aircraft to stay aloft for a long time, and that means the aircraft can fly long range missions. So an aircraft with a high L/D ratio can carry a large payload, for a long time, over a long distance. For glider aircraft with no engines, a high L/D ratio again produces a long range aircraft by reducing the steady state glide angle at which the glider descends.

Lift Equation

As shown in the middle of the slide, the L/D ratio is also equal to the ratio of the lift and drag coefficients. The lift equation indicates that the lift L is equal to one half the air density rho (ρ) times the square of the velocity V times the wing area A times the lift coefficient Cl:

\(\LARGE L=C_l\cdot\frac{\rho\cdot V^2\cdot A}2\)

Drag Equation

Similarly, the drag equation relates the aircraft drag D to a drag coefficient Cd:

\(\LARGE D=C_d\cdot\frac{\rho\cdot V^2\cdot A}2\)

Dividing these two equations give:

\(\LARGE \frac LD=\frac{C_l}{C_d}\)

Lift and drag coefficients are normally determined experimentally using a wind tunnel. But for some simple geometries, they can be determined mathematically.

Application

This figure at the top of this page shows the balance of forces on a descending Wright  glider. The flight path of the glider is along the thin black line, which falls to the left. The flight path intersects the horizontal, thin, red line at an angle “a” called the glide angle.

The tangent of the glide angle, tan(a), is equal to the vertical height (h) which the aircraft descends divided by the horizontal distance (d) which the aircraft flies across the ground.

\(\LARGE \tan(a)=\frac{h}{d}\)

The tangent of the glide angle is also related to the ratio of the drag, D, of the the aircraft to the lift, L.

\(\LARGE \frac{D}{L}=\frac{c_d}{c_l}=\tan(a)\)

Where cd and cl are the drag and lift coefficients derived from the drag and lift equations and measured during wind tunnel testing.

What good is all this for aircraft design? If we combine the two equations into a single equation through the tan(a), and invert the equation, we get:

\(\LARGE \frac{L}{D}=\frac{c_l}{c_d}=\frac{d}{h}=\frac{1}{\tan(a)}\)

The lift divided by drag is called the L/D ratio, pronounced “L over D ratio.” From the last equation we see that the higher the L/D, the lower the glide angle, and the greater the distance that a glider can travel across the ground for a given change in height. Because lift and drag are both aerodynamic forces, we can think of the L/D ratio as an aerodynamic efficiency factor for the aircraft. Designers of gliders and designers of cruising aircraft want a high L/D ratio to maximize the distance which an aircraft can fly. It is not enough to just design an aircraft to produce enough lift to overcome weight. The designer must also keep the L/D ratio high to maximize the range of the aircraft.

Going up? What is the elevator in an aircraft, and how does it work?

An aircraft’s elevator is not the kind you find in a tall building. The elevator on a plane is a movable surface that directly influences pitch control and altitude management. In other words, it is critical in making the plane go up and down.

Quick Navigation to Elevators

The Basics

• 1. Definition and Explanation
• 2. Main Functions
• 3. Structure and Components

Operations

• 4. How It Works
• 5. Mechanisms and Their Functions
• 6. Aerodynamics and Dynamics

A Closer Inspection

• 7. Types of Elevators
• 8. Using Trim
• 9. Advancements and Innovations
• 10. Watch Our “Elevators Explained” Video!

Functionality

• 11. Controls and Forces
• 12. Designs, Composition, and Trends

Pilot Mastery

• 13. Flight Training and Simulators
• 14. Incidents and Accidents

Purpose and Definition of an Elevator

How does an elevator work? Like many other airplane parts, it functions interactively. Located on the horizontal stabilizer of the tail section, the elevator adjusts the nose’s angle relative to the longitudinal axis. The elevator ensures flight stability and maneuverability making it essential to airplanes.

When I explain it to our aircraft mechanic students, I tell them it acts like a seesaw at the back of the plane, tilting the nose up or down as needed.

Do you ever wonder how a pilot knows exactly when to tilt the nose for landing? The elevator makes it happen! An elevator is a flight control surface that manages the aircraft’s pitch angle, which affects the flight path and altitude control. Its primary purpose is to help the pilot to safely fly the aircraft through climbing, cruising, and descending.

Main Functions of an Elevator

• Pitch Control: First of all, the elevator adjusts the aircraft’s nose up or down for changes in altitude. Pitch control refers to the ability to control the up-and-down movement of the nose.
• Stability: The elevator also provides balance when a plane experiences turbulence or shifts in weight distribution. Stability refers to a plane’s natural tendency to return to its normal flight condition after encountering turbulence or other disturbances.
• Stall Prevention: Additionally, the elevator keeps the angle of attack within safe limits to avoid aerodynamic stalls. Stall prevention refers to a pilot’s actions to avoid an aerodynamic stall during flight.
• Flight Path Management: Lastly, the elevator allows pilots to achieve precise adjustments for takeoff, cruising, and landing phases. Flight Path Management (FPM) refers to planning, executing, and monitoring a flight.

Structure and Location of the Elevator

You’ll find the elevator at the rear of the aircraft. It is an integral part of the empennage, or tail. This includes the horizontal stabilizer and vertical tailplane. Working together, these components take care of directional and pitch stability. At Epic, you’ll notice the empennage on all the planes in our fleet is painted bright red for visibility/safety.

Main Components of an Elevator

• Horizontal Stabilizer: Also known as a tailplane, this is a fixed surface in the tail section of aircraft. It runs horizontal, hence its name, and provides balance and prevents the nose from bobbing up and down.
• Elevator Trim Tabs: These are small, adjustable surfaces attached to the elevator so pilots can precisely adjust pitch trim.
• Hinges: The hinges connect to the vertical stabilizer and allow the elevator to move.
• Mechanical Systems: These include cables, pulleys, or advanced fly-by-wire systems that connect the elevator to the flight controls.

How the Elevator Works

The elevator operates by changing the angle of attack of the horizontal stabilizer (tailplane). This directly influences the aircraft’s lift and balance. Through this action, the elevator controls the position of the nose of the aircraft and the angle of attack of the wing.

Mechanisms and Their Functions

Pilots control the elevator using a yoke or control column connected to mechanical cables or fly-by-wire systems. They make precise adjustments during flight to maintain the flight path.

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• Upward Deflection: This produces downward lift at the tail, which raises the nose for the plane to climb.
• Downward Deflection: This creates upward lift at the tail, which lowers the plane’s nose for descent.
• Trim Adjustments: Pilots use trim to fine-tune the elevator position for stable cruising. This nuanced action makes the ride smoother.
• Cable and Pulley Systems: These ensure mechanical reliability in conventional aircraft.
• Hydraulic Assist: These are found in big jets and provide smoother and more consistent control forces.

Aerodynamics and Dynamics

The elevator is designed to follow basic aerodynamic principles. This enables it to adjust lift forces and stabilize the aircraft. By adjusting the airflow over the horizontal stabilizer, the elevator impacts pitch and overall flight dynamics. The same forces of flight that impact the aircraft also act on the elevator:

• Lift: The wings generate lift, which is adjusted by elevator movements. Lift forces are directly altered by changes in the tailplane’s angle.
• Weight: Weight is counteracted by lift, which creates balanced flight. Weight distribution is critical to prevent overcompensation. Whether carrying cargo or people, every pound matters.
• Thrust: The power of thrust propels an aircraft forward, which allows pitch changes to change altitude.
• Drag: The design of the elevator helps to minimize drag. The forces of thrust and drag are balanced to maintain efficient flight paths.

Why Dynamics Matter

Dynamic forces, including upward and downward deflection, have to be balanced so the aircraft stays within its flight envelope (range of operations). A pilot seeks this balance to ensure stability and prevent overcorrection during turbulence or high-speed maneuvers.

Types of Aircraft Elevators

There are different types of elevators, each of which is designed to meet specific aircraft requirements and design plans. These include:

• Conventional Elevators: Like those found on Epic’s Cessna Skyhawks, these hinged movable surfaces are attached to the trailing edge of the horizontal stabilizer.
• All-Moving Tailplanes (Stabilators): On this elevator, the entire horizontal stabilizer moves. These are commonly found in high-speed jets and transonic and supersonic aircraft.
• Canard Elevators: These are placed at the aircraft’s front to enhance maneuverability in advanced configurations. Fun fact: The earliest canard was the Wright Flyer prototype.
• T-Tail Elevators: Mounted on top of the vertical tailplane, these provide stability in larger aircraft.
• Elevons: Used in delta-wing aircraft, this specialty elevator combines ailerons and elevator functionality. The Concorde is one example of a plane with an elevon.
• Three-Surface Designs: Some aircraft, such as the Grumman X-29, include both forward (canard) and rear elevators for exceptional stability and control.
• Slab: A “slab” refers to a type of horizontal stabilizer where the entire surface acts as an elevator.

Elevator Trim and Fine-Tuning: Finesse in Flight

The elevator trim system reduces a pilot’s workload by enabling small adjustments that maintain a steady pitch angle. This system uses trim tabs, small adjustable surfaces on the elevator, to ensure flight stability. Benefits of elevator trim include:

• Reduction of pilot fatigue, especially during long-haul flights
• Better fuel efficiency by maintaining optimal weight balance
• Providing smoother altitude changes and stability

“When we work on elevators, we often need to manually deflect them up and down, sometimes while someone is sitting in the cockpit moving the controls. This is to check for proper movement, balance, and hydraulic response. Also, some elevators have counterweights to prevent aerodynamic flutter at high speeds. If we remove these weights during maintenance, the elevator can feel surprisingly light and even difficult to control until reinstalled.” –Josh Rawlins, Chief Operating Officer and Aircraft Mechanic Program Director

Advancements in Aircraft Elevators

The trim systems in modern aircraft dynamically adjust the elevator’s position for maximum efficiency and safety. Also, advanced diagram-based control interfaces in cockpits provide pilots with more intuitive ways to manage trim settings.

Modern elevators integrate cutting-edge technologies to improve performance, reliability, and safety. More recently, fly-by-wire systems have replaced traditional mechanical cables. This has led to increased precision.

Innovations in Elevator Systems

Designers are using active, or smart, materials capable of adjusting stiffness dynamically. They also focus on integrated aerodynamic features to improve airflow management. This reduces drag and increases efficiency.

• Real-Time Diagnostics: This is a major win so pilots can monitor elevator performance to prevent potential failures.
• Active Control Surfaces: Pilots love this aspect because elevators automatically adapt to changing flight conditions. This improves stability.
• Enhanced Materials: Designers are using new smart composites to reduce weight and increase durability. Each of these innovations contribute to better maneuverability, fuel efficiency, and safety.
• Diagrams: These help pilots visualize elevator functionality. They are critical for understanding the elevator’s role and highlight:
  
  • the interaction between elevator, horizontal stabilizer, and other flight controls.
  • the effects of upward/downward deflection on the flight path.
  • how aerodynamic forces affect pitch adjustments.

Aircraft Elevators Explained: Watch Our 7-Minute Video

How Do Elevators Interact with Secondary Flight Controls?

An elevator operates in harmony with other secondary flight controls, which ensures robust maneuverability and stability. For example:

• Rudder: Controls yaw, which is the side-to-side motion about the vertical axis.
• Ailerons: Manage roll, or the tilting motion around the longitudinal axis.
• Flaps: Essential to enhance lift during takeoff and landing.

What is Coordinated Control?

Just as the name suggests, the pilot coordinates efficient interaction between the elevator, rudder, and ailerons to ensure seamless transitions between flight phases and precise control during maneuvers. These are critical during complex operations, such as a crosswind landings.

Challenges in Dynamic Forces

High-speed conditions and/or turbulence can increase dynamic forces on the elevator. Modern aircraft include feedback systems to help pilots effectively manage these conditions.

Elevator Designs: Considerations for Safety and Performance

Designers use careful planning to meet stringent safety and performance standards for elevators. Key considerations include:

• Material Selection: Manufacturers should use high-strength yet lightweight materials for durability.
• Failure Redundancy: Airplane safety often relies on backup systems to prevent catastrophic outcomes. When one feature fails, the backup system can make a world of difference.
• Aerodynamic Efficiency: An optimized camber and chord line enable precise lift adjustments. Smooth and steady!

Composition

Aircraft manufacturers construct elevators today from durable, lightweight materials like carbon fiber composites and aluminum alloys. These materials reduce weight yet ensure structural integrity. Larger carriers typically use advanced materials to guarantee durability and efficient performance under heavy loads.

What Does the Future Hold in Elevator Design?

Manufacturers tell us that designers are working to integrate advanced materials and automation for improved performance and increased safety from the flight deck. Some of the trends we’ve been hearing about include these exciting ideas:

• Flexible Wing Surfaces: They hope to replace separate control surfaces with adaptable materials. This could be a real game-changer.
• Integrated Systems: Designers are planning to combine elevator functions with ailerons for streamlined control. Pilots are going to love that!
• Automation: By leveraging AI-driven adjustments, designers believe they can reduce pilot workload and enhance safety. We are keeping an eye on all of this for our flight students as well as our aircraft mechanic students who will need to be trained on this new tech.

Pilot Training and Using the Elevator

Mastering elevator operation is a critical aspect of pilot training. Pilots must learn to coordinate pitch control with other systems for smooth flight and safe navigation. Since an aircraft moves in three dimensions, it operates along three distinct axes, each managed by specific flight controls. The aircraft pitches around its lateral axis using the elevator and rolls around its longitudinal axis using the ailerons.

Key training areas at Epic include:

• Trim Adjustment: Using elevator trim to reduce manual input during extended flights.
• Stall Prevention: Recognizing and recovering from dangerous flight conditions.
• Altitude Management: Maintaining steady levels during turbulence or emergencies.

Can Pilots Learn to Operate Aircraft Elevators in a Simulator?

Absolutely! We rely on flight simulators at Epic so our pilots can practice all types of maneuvers safely. Simulators provide hands-on experience for pilots to perfect the management of the elevator under all types of scenarios. Pilots can replicate extreme conditions in aviation training devices (ATD) making them an important tool in effective flight training.

Incidents and Accidents Involving Elevators

In class, we often share historical events to emphasize a concept. Both pilots and aircraft mechanics learn about the causes of aviation tragedies in the hope to avoid history repeating itself. Epic’s motto is “Safety first!”

Below I’ve shared three deadly incidents involving the aircraft elevator.

1. Alaska Airlines Flight 261 ()

• Incident: This tragic flight involved a McDonnell Douglas MD-83 that experienced a catastrophic failure of its horizontal stabilizer trim system, which directly affected the elevator’s functionality.
• Key Role of Elevator: The jackscrew, which controlled the position of the horizontal stabilizer, failed due to inadequate lubrication over time. Focus for A&P mechanics? The importance of routine lubrication. Without proper stabilizer control, the pilots struggled to maintain pitch control using only the elevators. They managed to regain some control temporarily by inverting the aircraft, but the situation worsened, and the plane ultimately crashed.
• Outcome: The investigation by the NTSB highlighted maintenance lapses and led to stricter regulations regarding inspections and lubrication of critical flight control components.

2. United Airlines Flight 232 ()

• Incident: A McDonnell Douglas DC-10 experienced engine failure that caused severed hydraulic lines. This disabled all conventional flight controls, including the elevators.
• Key Role of Elevator: With no hydraulic control over the elevator, rudder, or ailerons, the crew was forced to use differential engine thrust to control pitch, roll, and yaw. Despite having no functional elevators, the pilots managed to reach Sioux City, Iowa for an attempted emergency landing.
• Outcome: The crash resulted in fatalities, however, 185 out of 296 passengers did survive. This flight became a landmark case in human ingenuity and teamwork under extreme conditions. It also emphasized the importance of redundancy in flight control systems.

3. Japan Airlines Flight 123 ()

• Incident: A Boeing 747SR experienced a rapid decompression due to a faulty rear pressure bulkhead repair. The decompression damaged the rear vertical and horizontal stabilizers. This severely impaired the elevators.
• Key Role of Elevator: With damaged elevators and limited stabilizer effectiveness, the pilots could not control the aircraft’s pitch and descent rate. The flight ultimately crashed into a mountainside. Today, this incident remains a stark example of how critical the empennage (tail section) is to aircraft stability and control.
• Outcome: This was one of the deadliest single-aircraft accidents in history. It led to more robust scrutiny of maintenance practices and repair quality, a point we drive home in our classes.

Onward and Upward

I have worked on countless elevators myself as an A&P mechanic, and I can assure you the elevator is an indispensable part of an aircraft’s control system. The elevator ensures precise pitch control, safe altitude management, and overall flight stability.

Ongoing development in design, materials, and technology underscore its importance in the aviation world. As aircraft evolve, so will their control surfaces, making elevators more sophisticated, reliable, and efficient. Whether you’re piloting or maintaining an aircraft, understanding the elevator’s role is key to safety.

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About the Author

Josh Rawlins

Josh Rawlins, a native of New Smyrna Beach, grew up just a few miles from Epic Flight Academy. At age 19, he began working at Epic, sweeping hangar floors and assisting coworkers with various tasks. What started as a humble job quickly evolved into a lifelong career in aviation.

Over time, Josh demonstrated remarkable initiative. While working at Epic, he completed accounting courses, earned his A&P mechanic license, and obtained inspection authorization. These achievements propelled him to Lead Mechanic and, later, Director of Maintenance.

• Josh's dedication to safety, hard work, and commitment to Epic earned him the title of Vice President and, in , Chief Operating Officer and board member. In , he also took on the role Aircraft Mechanic Program Director.
• Josh has earned significant recognition for his contributions to aviation. In , the National Business Aviation Association honored him with the Business Aviation Top 40 Under 40 Award in Maintenance. He was also named a "Top 40 Under 40" honoree by the Daytona News-Journal in .

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