WEIGHT & BALANCING
Why take this course?
This course will provide you with the skills and experiences necessary to perform weight and balance calculations in your daily flight. You will be able to carry out aircraft weight analyzes, you will be familiar with the nomenclature and the necessary manuals, you will understand the effects of excessive weight and unbalance, you will know how the weighing process is, and most importantly, you will be able to safely and quickly carry out the checks. calculations before each flight. The ability to perform and understand these calculations and analyzes increases flight safety levels and makes you a better prepared professional for the market.
What will you learn?
- Understand how the weight and position of the CG change the characteristics of the flight.
- Learn how to safely calculate the CG weights and position for aircraft.
- Learn what documentation and information are needed to perform weight and balance calculations correctly.
It is common to find wheels of general aviation pilots immersed in loud conversations. The themes are as varied as they are animated. On these occasions, machine speed is a recurring issue. If airplanes were made to arrive fast, there is nothing more natural than discussing how fast each one is. It is not uncommon for some to confuse IAS with TAS and GS. Others speak of land miles instead of nautical miles. But in spite of everything, it seems to be a well accepted sport to compare models and their performances.
The speed of flow in the air depends not only on the machine's design, but also on some operational factors. Low flight, for example, tends to be slower, as the density of the air creates greater induced drag. The higher, the fewer molecules are in the atmosphere and the plane flows more freely. This phenomenon is eventually confused with another factor that also influences: the variation in engine power. With fewer air molecules, engines without compression at the intake tend to lose strength at altitudes and stop pulling the machine with energy. Performance graphs on a turbocharged aircraft indicate that higher is faster.
There is another factor, the importance of which is closely related to flight safety, but which greatly influences speed. It is the distribution of weights inside the aircraft. Loads out of position require the pilot to deflect the elevator to compensate for heavy nose or tail tendencies. Deflection increases the frontal drag area and slows down the speed.
Imagine a lamp, with several lamps distributed in a circle, hanging from the ceiling by a single wire. The part remains level if the total weight is well distributed. If a lamp is removed, the assembly will tilt to the opposite side. Now, imagine a large passenger plane, half occupied. If all the passengers decide to sit in the front rows, leaving the bottom half empty, the plane may not even be able to take off. In fact, it is important to remember that one of the commissioners' tasks is to keep them calm in their places, avoiding large internal displacements.
To better understand static balancing, let's understand the meaning of two important locations. The first location is in the center of the fuselage, equidistant from the two wing tips. It is called the pressure center (CP) and it is where the combined aerodynamic forces act to suspend or sink the plane. The CP is designed by aeronautical engineering to stay where it is. That is, it does not change the position in any flight configuration. It originates from the shape of the wings and the design of the fuselage and its warps.
Before defining a CP, engineering needs to find out where the aerodynamic center (CA) will be for each wing profile. Currently, most of the conventional profiles have already been studied by NASA, and engineering chooses one of them, whose characteristics meet the desired project. The AC, then, is a specific point for each wing design and is usually measured in terms of the percentage of the average chord, starting from the leading edge. Imagine yourself still a child traveling in the passenger seat of a car and with one hand feeling the wind outside. For each aerodynamic hand position, there would be pressure that would make the arm go up or down. Good times, right? A pity that many children today have little access to aerodynamics, as they travel in closed-glass cars, do not fly kites or play small planes.
The center of pressure (center of lift) is the point where all the aerodynamic forces of the wings, fuselage and warpage are concentrated, producing positive or negative lift. The CG can walk between front and rear limits. This limited ride allows the pilot to stabilize the aircraft as desired, using the elevator.
The center of gravity (center of gravity) is the point where the total weight of the aircraft is concentrated.
The support of the wings is concentrated in the CA. But the fuselage can also have an aerodynamic shape and its own AC, as well as external luggage racks or large antennas. The combination of the CA, of each aerodynamic surface, considering the wings and the rest of the fuselage, defines the CP. Therefore, it is in the CP that the “invisible finger” of the support acts on the aircraft, as a whole. It behaves like a seesaw's fulcrum. If the weights are not the same, one side goes up and the other goes down.
The seesaw effect helps to understand the second point of calculating the balance of an aircraft: the center of gravity (CG). This is where the weights of the aircraft parts and the cargo are concentrated. And this is where the lamp wire hanging from the ceiling should be attached.
CG and CP will remain active at some point along the aircraft's longitudinal axis. However, they are rarely located in the same position. In a traditional civil aircraft project, the CG must always be ahead of the CP. The distance between them generates a moment of pitching. The nose can rise or fall due to the difference in the forces acting, which in the CP is supportive (upwards) and in the CG it is weight (downwards). It is up to the pilot, or autopilot, to apply a correct deflection of the elevator to maintain stability. The distance between CP and CG does not remain the same. If the CP is fixed, the CG walks along the longitudinal axis. This tour is foreseen in the engineering calculations and happens due to the different arrangements that the pilot applies, in the distribution of weights, inside the aircraft. If a passenger comes next to you in the front seat, the CG will be ahead of schedule. If you decide to travel in the back seat, the CG runs towards the tail.
The positioning of the fuel tanks is a critical factor in the design of the aircraft. As a consumable input, emptying the tanks can generate an undesirable CG ride and put stability at risk. For this reason, the aircraft design must avoid additional tanks installed far from the CG, as in the luggage racks or tail cones.
Stall and loads
The CG must always remain ahead of the CP. In the case of a stall, and the consequent loss of support, the nose naturally points downwards, and returns to have aerodynamic flow in the wings, recovering the support.
CG and CP are opposed, as in a toy seesaw. The elevator has pitching control. If it is too deflected, it causes additional drag and reduces the aircraft's speed.
When the aircraft stalls, the aerodynamic forces are reduced or disappear from the CP. Gravity continues to act on the CG and the aircraft descends sharply. At that moment, the nose should point downwards, so that the vertical displacement causes the return of the air flow in the wings and the lift, rebalancing the aircraft. For this, the CG must remain ahead of the CP. Otherwise, the aircraft will never recover from the stall.
The displacement of the CG to a position after the CP is always feared by everyone. And it can be caused by unforeseen cargo shifts inside planes or wrong weight distribution. Several accidents have already occurred because of this. In 1987, a FAB C-130 Hércules crashed when taking off from the island of Fernando de Noronha at night, when the cargo mooring did not support the pitching angle applied on the climb. The cargo loosened, ran to the rear of the aircraft and took the CG behind the CP. The stall was unrecoverable.
In April 2013, an accident also occurred with a National Airliner Boeing 747 taking off from Bragam airport in Afghanistan to Al Maktoum airport in Dubai. The first reports raised the hypothesis that its cargo of military vehicles loosened during the climb and reached the interior rear of the aircraft, displacing the CG far beyond the planned ride. The effect caused a deep stall and was reported by the radio by a crew member, before the impact with the ground.
The displacement of the CP can also generate problems. Although its position is rigid, changing the shape of the fuselage or wings can cause the CP to also shift. Imagine a plane experiencing ice formation. In addition to getting heavier, the aerodynamic shape changes. From that moment on, the plane is different and nobody will be able to predict its reaction. Or if it collides with a bird, which deforms a wing. The aerodynamic effect and CP are also changed. In military aircraft, the alteration of the CP can be obtained by varying the angle of inflection of the wings. The F-14 Tomcat fighter is an example that totally changes the theory applied to airplanes of non-variable geometry.
In civil aircraft certification processes, the manufacturer must present the balance calculations to be verified. If approved, they become part of the aircraft's mandatory documentation. Each unit produced must be weighed and the information must appear on a weight and balance sheet. Throughout life, with each modification that changes the weight, such as a new painting, modification of equipment on board or alteration of the fuselage, a new plug must be produced.
For pilots, it is important to understand how to balance your aircraft. If done well, the aircraft flies faster, consumes less and, in the face of a stall situation, recovery will be easier. Here are four steps to calculate the balance:
The aircraft design includes a vertical plane, called “Datum”, which will be used as a reference. Leaving it, with a longitudinal line in the center of the aircraft, engineering defines distances to predefined points, called “Fuselagem Station” (FS). Each FS is distant from the Datum by a fixed-length arm (arm), seen in the picture below, in inches. There are the FS for the pilot's seat and its side passenger, for the rear passengers, for the luggage compartment and for the fuel. The weight applied to each of these FS, multiplied by their respective arm, generates a “moment”. This first step is performed by the aircraft manufacturer. The operator is given a weight and balance sheet, which contains the weight of the empty plane, and all arms, of all FS.
Here the pilot must fill in the weight and balance sheet. In each line he will insert the weight being applied to that FS. The moment will be defined by the multiplication of each weight by its respective arm, divided by 1,000.
Now divide the total moment by the total weight. The result should be multiplied by a thousand and it will be the distance from the CG, in inches, from the Datum.
(Total moment: 597.4 ÷ Total weight: 4149 lbs) x 1000 = 143.98 in. of Datum
Insert the data of total weight and CG position (in inches from the Datum) on the aircraft graph (called “envelope”). For each situation, the pilot will know how the balance is. In this example, the CG closest to the left will cause more weight on the nose, and, closer to the right, it will cause heavy tail. The maximum takeoff weight of the Piper Matrix is 4,340 lbs.
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