Ever find yourself wondering how a helicopter gets airborne and stays there? Sure, you've heard explanations like “it's so ugly the ground repels it,” or, “it beats the air into submission,” and other sordid lies along those lines. But there really are good explanations as to how a helicopter can fly. Really! Don't believe me? Check this out...

Most of you plank drivers (yeah, but then you call us “fling wing” drivers, so fair is fair) already know about that guy Bernoulli and his Principle, making you familiar with terms like airfoil, chord, relative wind, angle of incidence, angle of attack, and lift. If not, stop here, get a cuppa joe and review these terms. If not for me, at least for your next flight review or check-ride!

You back? Good! That means you're ready to learn the truth about how the chopper stays up there! On a helicopter, the “wing” is the main rotor-blade. Some have two “wings”, some have three and the really smooth riding have four; you could even take your cuppa joe along and not spill any on your lap in one of these! If you look at a rotor-blade, you will see it is an airfoil – some symmetrical and some asymmetrical. There are pros and cons for each, but I don't want to lose you too soon by getting into that. The important bit? Just as your wing travels through the air producing lift, the rotor-blades perform in like manner. As the blade passes through the air, some molecules go over the top and some go underneath, producing lift (and a little drag, too).

You know that rotational motion is a vehicle's ability to rotate about one of its axes. Let me introduce a word now that I will use a lot: translational motion. It is simply linear motion without rotation. Like a vehicle moving fore or aft along the longitudinal axis without any roll about the lateral axis. Fixed wing aircraft translate along the longitudinal axis when moving forward, but rotate about their longitudinal and vertical axes when banking, and rotate about their lateral axis to go up and down. Although helicopters also move in rotational motion about their axes, they also can translate about their lateral axis to move sideways, about their longitudinal axis to move forward/backward, and about their vertical axis while moving up and down. You can see this interplay while the helicopter is hovering. If it's sitting on the ground with blades turning everything is pretty much even. The relative wind is the same all the way around the blade travel, hence the lift/drag produced is relatively even at every point around the disc. But what happens once that aircraft starts to move? Now the lift requirements of the blades are constantly changing throughout their travel to be able to produce the desired results. How does it do this? Each blade is constantly changing angle of attach through out its path around to produce the resultant lift vector in the direction the pilot wants to go.

You read that right: that blade is in constant motion all the time. There is a name for all this. It is called Dissymetry of Lift. Here are the basics. Picture the helicopter transitioning from stationary to forward flight. The advancing blade begins to move faster (add the speed of the helicopter to the blade speed) while the retreating blade goes slower (now you have to subtract the speed of the helicopter from the blade speed). Can you see how there is more lift produced by the advancing blade vs the retreating blade? The main rotor blades flap and feather automatically to compensate for this. Flap is that up and down motion like a bird's wings, and feather is a pitch change. Rotor systems are designed to compensate for this by introducing hinges in the rotor-head or use of flexible materials such as composites to allow the blade to twist. The advancing blade produces more lift thru the increase in relative wind. Hence it flaps up, decreasing the angle of attack as it flaps up. The retreating blade flaps down due to the reduced relative wind, and therefore increases its angle of attack. Easy right?

By the way, did you ever wonder why helicopters are slower than airplanes? It's the slower relative wind on the retreating blade that limits the maximum forward speed of the helicopter. Staying in the air depends upon keeping all of those blades from stalling! Wanna see what kind of motion a blade has to go through in rotation? It's impressive. Check this out!

These are the basic principles that keeps a helicopter flying. To tweak this to make the ride nice and make the helicopter a viable utility vehicle, there are a few other aerodynamic principles you have to be aware of and compensate for. The rest of these little ditties kind of clean up the whole story. Torque is the tendency for a body to rotate in the opposite direction of the rotating blades. The tail rotor compensates for this, but has a translating tendency where there is a lift component off the tail rotor blades laterally. Some manufacturers tilt the main mast a degree or so to compensate. You hold the cyclic a little offset too. Ground effect takes place from the ground to up to about one half the rotor's diameter and is an improvement in performance due to the ground cushioning all that air being pushed down thru the blades as well as interrupting that drag producing tip vortex. Translation lift is present anytime there is a horizontal flow over the blades and effective translation lift occurs when the rotor accelerates out of its own vortices into undisturbed air, greatly improving performance. A more horizontal airflow reduces induced flow and drag. Induced flow is that large volume of air that is pushed downward through the rotors.

So, now that you've got the formal aerodynamic explanation you'll need to repeat for phase checks, written tests, oral exams and check rides, you're eady to hear the truth. Or are you? What really keeps the helicopter in the air? MAGIC. Yes, magic. You just have to believe...