Define and discuss the four forces acting on an aircraft in straight-and-level and unaccelerated flight. Give examples of how the combinations of these forces act on the airframe.
- Thrust—the forward force produced by a powerplant/propeller or rotor. It opposes or overcomes the force of drag.
- Drag—a rearward, retarding force caused by disruption of airflow by the wing, rotor, fuselage, and other protruding objects. Drag opposes thrust and acts rearward parallel to the relative wind.
- Weight—the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage. The earth’s gravitational force, which creates the weight, pulls the aircraft downward.
- Lift—overcomes the downward force of weight to allow flight to occur and is produced by the dynamic effect of the air acting on the airfoil and acts vertically through the center of gravity.
Lift
A very easy way to confuse new flight students is to throw a lot of obscure information at them with no concrete references or examples. Aerodynamics can be very difficult for the new student to understand because it is difficult to visualize what is happening to the rotor blades or tail rotor in flight. When teaching the student about lift and how the helicopter is able to obtain lift, the instructor must be creative and find ways to explain the theories, such as Bernoulli’s Principle and Newton’s Laws of Motion, in direct relation to the helicopter and how every flight control movement affects lift.
Bernoulli’s Principle
Instructors should introduce Bernoulli’s Principle to the student in simple terms and attempt to relate the theory directly to the production of lift that is created from the main and tail rotor blades. The discussion should begin with Bernoulli’s initial discovery that air moving over a surface decreases air pressure on the surface, and show the student an example of the differences in air pressure when an object moves through the air. Further discussion should include the following points and examples:
- Show the student a picture of an airfoil and how the air pressure changes when the air is disrupted. A picture of an airfoil is usually a small cutout or slice of the entire wing or rotor blade. The instructor should explain that the entire rotor blade(s) are essentially one large airfoil.
- As airspeed increases, surface air pressure decreases accordingly and this difference in pressure around the airfoil is directly related to the flight of an aircraft.
- As an airfoil starts moving through the air, it divides the mass of air molecules at its leading edge. The distance over the top of the blade with the angle of attack is greater than the distance along the bottom surface of the rotor blade. Air molecules that pass over the top must move faster than those passing under the bottom to meet at the same time along the trailing edge. The faster airflow across the top surface creates a lowpressure area above the airfoil.
- Air pressure below the airfoil is greater than the pressure above it and tends to push the airfoil up into the area of lower pressure. As long as air passes over the airfoil, this condition exists. It is the difference in pressure that causes lift. When air movement is fast enough over a wing or rotor blade, the lift produced matches the weight of the airfoil and its attached parts. This lift is able to support the entire aircraft. As airspeed across the wing or rotor increases further, the lift exceeds the weight of the aircraft and the aircraft rises.
- Not all of the air met by an airfoil is used in lift. Some of it creates resistance, or drag, which hinders forward motion. Lift and drag increase and decrease together. They are affected by the airfoil’s angle of attack in the air, the speed of airflow, the air density, and the shape of the airfoil or wing.
Newton’s Laws of Motion
Newton’s laws of motion provide the foundation for the student’s understanding of basic aerodynamic principles. The instructor should develop multiple ways of explaining these laws to ensure that if the student does not comprehend one explanation, the instructor has an alternate explanation that relates to something that the student will understand. Begin with relating the laws to helicopter flight, such as the requirements for lift, thrust, and power to overcome the effects of the three laws and the energy state of the helicopter. If the student has a difficult time understanding flight examples, try using an example that is more familiar, such as a car or motorcycle. This helps the student better understand the laws when the instructor applies it to flight.
First Law—the Law of Inertia
A body at rest remains at rest, and a body in motion remains in motion at the same speed and in the same direction unless acted upon by some external force. The key point to explain is that if there is no net force resulting from unbalanced forces acting on an object (if all the external forces cancel each other out), then the object maintains a constant velocity. If that velocity is zero, then the object remains at rest. And, if an additional external force is applied, the velocity changes because of the force.
A helicopter in flight is a particularly good example of the first law of motion. There are four major forces acting on an aircraft: lift, weight, thrust, and drag. If we consider the motion of an aircraft at a constant altitude, we can neglect the lift and weight. A cruising aircraft flies at a constant airspeed and the thrust exactly balances the drag of the aircraft. This is the first part sited in Newton’s first law; there is no net force on the helicopter and it travels at a constant velocity in a straight line.
Now, if the pilot changes the thrust of the engine, the thrust and drag are no longer in balance. If the thrust is increased, the helicopter accelerates and the velocity increases. This is the second part sited in Newton’s first law; a net external force changes the velocity of the object. The drag of the helicopter depends on the square of the velocity. So, the drag increases with increased velocity. Eventually, the new drag equals the new thrust level and at that point, the forces again balance out, and the acceleration stops. The helicopter continues to fly at a new constant velocity that is higher than the initial velocity. We are again back to the first part of the law with the helicopter traveling at a constant velocity.
In this example, only the motion of the helicopter in a horizontal direction is explained and as the student becomes comfortable with aerodynamics, further discussions should include the effects of the thrust on weight and on lift. For example, increasing the throttle setting increases the fuel usage and decreases the weight, and the increase in velocity increases the lift as well as the drag. Each of these changes effect the vertical motion of the helicopter.
It is important to point out the role of engine power when explaining the law of inertia. Power is used to accelerate the helicopter, to change its velocity, and thrust is used to balance the drag when the helicopter is cruising at a constant velocity. When a helicopter is on a normal approach, the power demand is generally in the middle range and the total drag is at the lowest. As the aircraft decelerates to effective translational lift airspeed and terminates to a hover, the power demand is quite significant, generally the highest of all maneuvers. An airplane makes minimal power demands at the termination of its approach through the flare and landing.
Second Law—The Law of Acceleration
A change in velocity with respect to time. The force required to produce a change in motion of a body is directly proportional to its mass and rate of change in its velocity.
For example, for a given helicopter, acceleration would be slower when loaded to maximum gross weight than when loaded to a lesser gross weight. During a normal takeoff, the power margin available between maximum torque available and hover power can be quite small based on helicopter weight and environmental factors. During the transition to forward flight and through effective translational lift airspeed, acceleration is limited until the aircraft is in smooth undisturbed air and the influence of induced drag begins to subside. Once the aircraft reaches its maximum endurance/rate of climb airspeed, acceleration potential is increased as total drag is at its lowest point. [Figure 1, Point E]
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| Figure 1. Drag graph |
Total drag is the sum of parasite drag and induced drag as shown in Figure 1, Point A and C. The total drag curve can also be referred to as the thrust required curve because thrust is the force acting opposite drag. At the point where total drag [Figure 1, Point D] and thrust required are at a minimum, the lift-to-drag ratio is maximum and is referred to as L/DMAX. At L/DMAX, the entire airframe is at its most efficient, producing the most lift for the least drag. Maximum endurance is found at L/DMAX, because thrust required and thus fuel flow (fuel required) are at a minimum, giving maximum time airborne.
Third Law—Action and Reaction
For every action, there is an equal and opposite reaction. The instructor should relate the third law to the amount of power applied to the rotor system and the need for the antitorque or tail rotor to supply the equal and opposite reaction to the torque of the engine(s) applied to the main rotor. The rotor system of a helicopter accelerates air downward, resulting in an upward thrust. A single-rotor helicopter demonstrates this law perfectly. Consider a helicopter on floats that is not moored to a dock. As the main rotor begins to turn counterclockwise during aircraft start, the fuselage reacts by turning in a clockwise direction until the point at which the tail rotor has reached sufficient rpm to provide the thrust necessary to counteract that force.
Torque effect is a result of Newton’s laws and an aspect of helicopter flight that a student must thoroughly understand. The turning of the helicopter’s main rotor blades in one direction causes the helicopter to turn in the opposite direction. In most helicopters, this is counteracted by the use of a second rotor (tail rotor) to provide the thrust to limit the rotation. Some helicopters use vectored air, while others use a counterrotating main rotor system. All have one thing in common—a method of counteracting the torque of the main rotor system. [Figure 2]
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| Figure 2. Rotation direction |
At some point in training, the instructor should have the student bring the helicopter to a high hover and explain that work load is greater and an increased left pedal requirement exists to hold a constant heading. The opposite can be shown at a lower hover with a decrease in left pedal requirement to hold the same heading.
Weight
As weight increases, the power required to produce lift needed to compensate for the added weight must also increase. This is accomplished through the use of the collective. Most performance charts include weight as one of the variables and students must be aware of the importance of managing aircraft weight to obtain optimum performance. By reducing weight, the helicopter is able to safely take off or land at locations that would otherwise be impossible.
Explain to students how maneuvers that increase the G loading such as steep turns, rapid flares, or pulling out of a dive create greater load factors and act as a multiplier of weight. The load factor is the actual load on the rotor blades at any time, divided by the normal load or gross weight. [Figure 3] At 30° of bank, the load factor is 1G, but at 60°, it is 1.8G, an increase of 80 percent. If the weight of the helicopter is 1,600 pounds, the weight supported by the rotor in a 30° bank at a constant altitude would be approximately 1,600 pounds. In a 60° bank, it would be 2,880 pounds and in an 80° bank, it would be 8,000 pounds. Emphasize to students that an additional cause of large load factors is rough or turbulent air. The severe vertical gusts produced by turbulence can cause a sudden increase in angle of attack (AOA), resulting in increased rotor blade loads that are resisted by the inertia of the helicopter.
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| Figure 3. Load factor |
Thrust
Thrust, like lift, is generated by the rotation of the main rotor system. Point out to the student that in a helicopter thrust can be forward, rearward, sideward, or vertical. The direction of the thrust is controlled with the cyclic. If cyclic control to produce thrust is too great, lift is lost and the aircraft descends. Conversely, if too little cyclic control is made, the aircraft begins a climb. Using visual aids, demonstrate how the resultant lift and thrust determines the direction of movement of the helicopter. [Figure 4]
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| Figure 4. Thrust |
Explain to the student that the tail rotor also produces thrust. The amount of thrust is variable through the application of the antitorque pedals and is used to control the helicopter’s heading during hovering flight and trim during cruise flight.
Drag
No discussion of aerodynamics is complete without its defining the three types of drag, how drag is created, and its effect on the aircraft. A certificated flight instructor (CFI) must become intimately familiar with the drag chart and how it relates to airspeed and power demands. Demonstrate this during the performance planning phase as the student has actual torque values to compare. Then, when the student is flying the helicopter, apply the values that were computed and show the effect on the helicopter. A technique is to show how each flight control is affected by simple hover flight maneuvers. Demonstrate the change in torque that occurs between left and right pedal turns and explain why. Discuss how the cyclic is utilized to hold position over the ground, while the pedals rotate the fuselage and control heading. When excess power is available, demonstrate how the collective pitch can be applied to vary the hover height, or to accelerate the helicopter. It would be prudent to discuss here that if no excess power is available, application of the collective then may be used to control the rotor rpm. This is done by changing the pitch in the blades. Over application of the collective in a low power margin setting results in rotor rpm decay and a loss of lift. Rotor rpm is the key to sustaining the aircraft in a steady state profile and should never be allowed to decay below minimum operating levels. It is the key to life for a helicopter pilot!
The types of drag are:
- Parasite drag—drag created by the fuselage or any nonlifting components (e.g., strut, skin friction, interference).
- Profile drag—caused by the frictional resistance of the rotor blades passing through the air.
- Induced drag—results from producing lift.
a. Blade tip vortices—pressure differential at tips of blades trying to equalize and produce a stream of vortices (turbulence).
b. Induced flow—causes lift and total aerodynamic force to tilt further rearward on the airfoil.
c. Total aerodynamic force tilted further backward at higher angles of attack. - Total drag—sum of induced, profile, and parasite.
Use a graph that depicts drag/power relationship, and have the student identify the power requirements to overcome drag at various airspeeds. [Figure 1]
The following describes the relationship of each of the different types of drag to the airspeed of the aircraft.
- Parasite drag—lowest point at a hover, but increases with airspeed. The major source of drag at higher airspeeds.
- Profile drag—remains relatively constant at low airspeed, but increases slightly at higher airspeed ranges.
- Induced drag—major source of drag at a hover, but decreases with forward airspeed.
- Total drag—the sum total of induced, profile, and parasite drag.
a. Total drag decreases with forward airspeed until best rate of climb speed is reached. [Figure 1, Point E]
b. Speeds greater than best rate of climb causes a decrease in overall efficiency due to increasing parasite drag.
Once the student understands the forces acting on the helicopter, provide examples of balanced and unbalanced flight forces. For example, when hovering stationary in calm wind at a constant altitude, thrust is equal to drag and lift is equal to weight. The aircraft is not moving vertically or horizontally. The aerodynamic forces are balanced. [Figure 5]
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| Figure 5. Balanced forces in hover |
The student will also notice during hovering flight in a calm wind condition that with smaller American made helicopters like the Robinson R-22, Bell 206, and Schweizer 300, the left side of the aircraft will probably hang lower than the right. This is due to the direction of the tail rotor thrust and the engineered mast tilt to compensate for translating tendency.
On much larger helicopters such as the BH-205, S-76, and BK-117, in which an additional gearbox is used to raise the tail rotor up to the main rotor plane, the tilting of the fuselage is not as prevalent.
The pitch attitude will vary depending on the loading of the helicopter. Many helicopters when flown single pilot will be nose high at a hover. Conversely, they may be nose load when fully loaded. The center of gravity (CG) of the helicopter determines which portion of the landing gear will come off the ground first. The CFI must pay particular attention to the attitude of the helicopter as the student lifts it off the ground. If excess power is applied in other than a level attitude, the helicopter may proceed to roll beyond its dynamic rollover limits. When lifted off the surface correctly and safely, the pilot has the opportunity to lower the collective if a portion of the landing gear is attached or hung on the surface, thus preventing a rollover incident from occurring. It is imperative that the CFI closely monitor the attitude of the helicopter and not the actions of the student. This simple action may determine whether or not the helicopter is allowed to stray beyond the comfort level of the instructor to recover from a particular action by the student. Never allow a student to go beyond your comfort level.
Several inputs are required simultaneously as the aircraft is brought to a hover. Stress to the student that these actions must occur without delay or coordinated flight will not occur. For example, as the collective is increased to lift the helicopter off the surface, the throttle must also be increased. Even if a governor accomplishes that action, the pilot still must monitor the power instruments to ensure that no limits are exceeded. With the increase in power, there is also an increase in torque and the tendency for the nose to turn to the right. The pilot must apply sufficient left pedal to maintain the helicopter heading. While this is occurring and the lift in the rotor system is changing, the pilot must apply cyclic to maintain position over the ground and not allow the helicopter to drift in any one direction. The helicopter bank attitude might not be level due to crosswinds and translating tendency. The pitch attitude might not be level due to tailwinds or CG. The pilot must ensure that the tail rotor is clear of all obstacles and is not allowed to hang so low that it impacts the ground or other objects.
For example, in steady state flight, the aircraft is maintaining a constant airspeed and constant altitude. The aerodynamic forces are balanced. Although the helicopter is moving, it is not accelerating or climbing. [Figure 6]
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| Figure 6. Steady state—balanced forces |
Any time opposing forces become unequal (unbalanced), acceleration results in direction of the greater force. If lift is greater than weight the helicopter climbs. If thrust is greater than drag, the helicopter moves horizontally. Point out that thrust can occur in any or all directions. For example, if the helicopter is moving sideways or backwards, thrust is in the direction that it is moving. [Figure 7]
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| Figure 7. Acceleration or deceleration—unbalanced forces |