How Do Helicopters Fly?

Margie O’Connor

During my fixed-wing flight training, I witnessed what I thought to be a miracle – a Blackhawk helicopter landing at the same airport I was learning about the theory of airplane flight. Captivated by the powerful sound of the rotor blade, I instantly began to wonder. How did the rotor blades work to produce the lift necessary to keep the aircraft afloat? And were helicopter rotor systems susceptible to any of the same by-products of flight as the airplane?

I would soon discover the significance of words like flapping and feathering; that hunting was more than traipsing through the woods towards the nearest tree stand, and that coning and twist weren’t always referring to ice cream.
Helicopter flying has often been equated to rubbing your belly while patting your head and walking, all at the same time. There’s no doubt rotary wing flying involves a bit of manipulation unfamiliar to the fixed-wing crowd but proper manipulation of the wild yet fascinating components of the helicopter lead to the successful creation of lift, just like flying an airplane.

So how do helicopters fly? First, let’s decipher some helicopter vocabulary. Maybe in doing so, you will gain an appreciation (or at least a sense of awe, like I did) for helicopter flight.

Helicopter Rotor System Characteristics

Helicopters really come with two rotor blade systems – the main rotor system mounted above the cockpit and connected to the engine and the tail rotor, affixed to, well, the tail section (more on that in a future article). These two rotate simultaneously to produce and counteract lift, among other talents.

Like airplanes, helicopters must create enough lift to overcome weight to fly…it’s really all about balancing the forces. This vertical vector combined with centrifugal force produces a resultant force that’s not completely opposite the downward component of weight. So while your helicopter’s main rotor system is still creating lift, centrifugal force is stealing the thunder. If the goal is to take off vertically, the resultant vector needs some adjustment.

How Do Helicopters Fly, Figure 1 - The resultant force of centrifugal force and lift.

How Do Helicopters Fly, Figure 1 – The resultant force of centrifugal force and lift.

Coning

To make the resultant force more effective, the blades cone! Coning occurs to counteract our sneaky friend, centrifugal force. Ask and you shall receive…more lift that is. The blades flex upwards to more effectively concentrate the lift vertically. But beware – coning only augments lift to a certain point, after which it can actually degrade the amount of lift. Excessive coning can creep in at low RPMs, high gross weights, or high G maneuvers.

How Do Helicopters Fly, Figure 2 - Blades coning.

How Do Helicopters Fly, Figure 2 – Blades coning.

Blade Twist

Helicopter rotor blades move fast! And they create a great deal of lift but the lift is not consistent along the blade so engineers design a twist into the blade. Twisting the blade distributes this lift more evenly along the length of the rotor blade.

Dissymmetry of Lift

A look at dissymmetry of lift is necessary to lay the groundwork before moving forward. Dissymmetry of lift is essentially the difference in the lift between the advancing half of the rotor disk and the retreating half. When the speed of the blade combines with the airspeed of the helicopter (wind affects both here), The advancing blade pulls ahead in the race as it moves much faster and acquires greater lift. Conversely, the retreating blade slows down and loses lift. And the closer you get to the tip of the blade, the faster the blade moves!

How Do Helicopters Fly, Figure 3 - Dissymmetry of lift as viewed from above.

How Do Helicopters Fly, Figure 3 – Dissymmetry of lift as viewed from above.

Although lift is a good thing, if half the helicopter has more than the other half, the aircraft may end up in a rolling situation (literally rolling over). To prevent the advancing blade from overpowering the retreating blade, we have to equalize lift. Several mechanisms exist to counteract this undesirable condition.

Flapping

As the main rotor blades travel, they want to fight off the dissymmetry of lift while having some fun. So they climb (flap up) as they advance around the right half of the rotor’s path and dive (flap down) as they round out the left side. This is flapping. They can do this because they teeter on a hinge. You can see this when the helicopter is sitting on the ground, not running. The blades actually droop (and no, not because they’re sad).

How Do Helicopters Fly, Figure 4 - As viewed from the back of the helicopter – advancing blade flaps up while retreating blade flaps down.

How Do Helicopters Fly, Figure 4 – As viewed from the back of the helicopter – advancing blade flaps up while retreating blade flaps down.

The advancing blade flaps up, eventually decreasing its angle of attack. Conversely, the retreating blade flaps down, eventually creating an increase in the blade’s angle of attack and winning the battle against dissymmetry of lift.

Feathering

Feathering, like blade flapping, has a role in countering dissymmetry of lift. Feathering is the rotation of the blade about its span wise axis, by collective or cyclic inputs, which causes a change in blade pitch angle.

How Do Helicopters Fly, Figure 5 - Feathering rotates the blade around the span wise axis.

How Do Helicopters Fly, Figure 5 – Feathering rotates the blade around the span wise axis.

Primary feathering occurs when you manipulate the cyclic, which in turn moves the thrust vector in the direction of movement (left, right, forward).

Leading and Lagging (also known as Hunting)

While the blade flaps up, the CG moves closer to the rotor mast. Why does this happen, you ask? Well, it’s all about Coriolis force. If you’ve ever watched ice skaters, you are familiar with Coriolis force (which simply states that as a mass moves closer to the center of rotation, it gains speed). So when the ice skater moves her arms closer to her body as she spins, her speed increases. The same thing occurs on a spinning rotor blade.

The faster blade also experiences a change in pitch and an increase in drag. If these stresses continue too long, the rotor blades risk excessive bending. Leading and lagging can give the blades some room to relax and unwind from their overstressed condition.

During leading and lagging, the rotor blade moves fore and aft (or hunts) in the plane of rotation. But this feature only frequents fully articulated rotor systems, so you may not encounter this when first learning to fly a helicopter.

In Conclusion

If the thought of learning to tackle a new, yet challenging mode of flight involving rotor blades seems intriguing, then maybe the time is ripe for you to leap into the world of helicopter flying.

Get Started With Your Flight Training Today

You can get started today by filling out our online application. If you would like more information, you can call us at (844) 435-9338, or click here to start a live chat with us.

References:

Dole, C. E. (1994). Flight Theory for Pilots. Redlands: Jeppesen Sanderson.

Headquarters, Department of the Army (2007). Fundamentals of Flight. Washington, D.C: U.S. Government Printing Office.

Knowing Simple Aerodynamics Helps Your Aviation Career

When it comes to teaching someone about aerodynamics, it is possible to teach simple aerodynamics in a way that doesn’t require a physics course as a prerequisite. I am a firm believer that as your flight career progresses, so should your knowledge. However, we need to take a simple to complex, known to unknown approach. If you are a Flight Instructor, you know this is a key fundamental of instruction. Take ‘Lift’ for example; at what point should someone be expected to know the Coefficient of Lift (CL)? Here’s what NASA says regarding the lift coefficient:

“The lift coefficient is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and some flow conditions on lift. This equation is simply a rearrangement of the lift equation where we solve for the lift coefficient in terms of the other variables.

The lift coefficient Cl is equal to the lift L divided by the quantity: density r times half the velocity V squared times the wing area A. Cl = L / (A * .5 * r * V^2)

The quantity one half the density times the velocity squared is called the dynamic pressure q. So Cl = L / (q * A)

The lift coefficient then expresses the ratio of the lift force to the force produced by the dynamic pressure times the area.”

Above, NASA states this equation is “simply a rearrangement of the lift equation”.  Perhaps if you have a PHD and work for NASA this equation is simple. Why is this complex lift equation shown to brand new student pilots across the Country? If we know that a key fundamental principle of instruction is to go from simple to complex, known to unknown; why would we ever introduce a complex physics equation to a new student? Instead, when introducing lift, ask your student a few simple questions:

“Have you ever been driving down the highway with your hand out the window? Have you noticed that if your raise your hand up slightly, your whole arm wants to shoot up like you’re waving to oncoming cars? That is lift…simple”

When learning simple aerodynamics, where should one start?

The first thing I have my students learn, are the basic definitions of helicopter aerodynamic terms. I focus on their rote memorization of bullet point definitions. Once they have these memorized, I then focus on their understanding and application. I present aerodynamic terms to my students in a question / answer format. Before a Student Pilot is ready to take their Private Pilot exam, they will need to be able to describe aerodynamics much more in depth. However, if you’re new to aerodynamics, I recommend you start by memorizing the key definitions below. Writing these questions / answers down on index cards is a good idea to aid in your memorization of these key aerodynamic terms.

What are the forces of flight?

A. Lift, Weight, Thrust and Drag

How is lift developed?

A. LIft is developed by creating an area of positive pressure beneath the airfoil and negative pressure above the airfoil.

What is Bernoulli’s Principle?

A. Bernoulli’s Principle states that as velocity increases, pressure decreases. This is also known as the Venturi Effect.

What is Newton’s Third Law?

A. For every action, there is an equal and opposite reaction.

What is Angle of Attack?

A. The angle between the chord line and relative wind.

What are the three types of drag?

A. Profile, Parasite and Induced drag.

What is Profile Drag?

A. Drag caused by the frictional resistance of the blades moving through the air. Composed of Form Drag and Skin Friction.

What is Parasite Drag?

A. Drag caused from all Non-Lifting surfaces of the aircraft.

What is Induced Drag?

A. Drag that is a result of developing lift. Also known as Vortex Drag.

What is Coriolis Effect?

A. As the center of mass moves closer to the axis of rotation, the blades have a tendency to accelerate.

What are the two external factors that cause Coriolis Effect?

A. Coning and Blade Flapping.

What is Coning?

A. Coning is the result of two forces acting at the same time; Centrifugal Force and Lift.

Why do helicopter blades Flap?

A. Helicopter Blades are allowed to Flap to compensate for Dissymmetry of Lift.

What is Dissymmetry of Lift?

A. Unequal lift between the advancing and retreating halves of the rotor disc.

What is Retreating Blade Stale?

A. Due to Dissymmetry of Lift, at high forward airspeeds the retreating blade exceeds its critical angle of attack causing the blade to stall.

What is Translating Tendency?

A. The tendency of the helicopter to drift in the direction of tail rotor thrust.

What is Translational Lift?

A. Improved rotor efficiency resulting from directional flight or surface winds.

What is Effective Translational Lift (ETL)?

A. ETL occurs at approximately 16-24 knots when the rotor system completely outruns the recirculation of old vortices.

What is Transverse Flow Effect?

A. Occurs at speeds just below ETL. Induced flow drops to near zero at the forward disc area and increases at the aft disc area.

What is Gyroscopic Precession?

A. Gyroscopic Precession states that when an outside force is applied to a rotating body, the result of the outside force will occur 90 degrees later in the plane of rotation.

Begin introducing the “Why?”

450px-Clear_light_bulbOnce you have memorized some of these key definitions, it will be time to start asking “Why?” The short list above is exactly that…short. Notice I did not include aerodynamics of autorotations, conservation of angular momentum or other complex aerodynamic principles. There is a lot more to learn, but building a foundation of key definitions is where we should start. Once you have the definitions memorized, the next step is to gain an understanding of what is actually happening. Helicopter Aerodynamics can be made simple and enjoyable to learn. Start with the basics and develop a good foundation to build on. This is my approach to teaching aerodynamics to a brand new student. There are other good approaches that Instructors use and they are successful in their teaching. With a goodFlight School you will be surrounded by a large amount of Flight Instructors ensuring that your individual learning needs will be met.

Get Started With Your Flight Training Today

You can get started today by filling out our online application. If you would like more information, you can call us at (844) 435-9338, or click here to start a live chat with us.

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