Re: can a helicopter fly freely in a tunnel?

Hello Ariel,

That's a good question!

The answer depends on the size of the tunnel and the size of the helicopter. Let's start from the basics. The main rotor of a helicopter is essentially an air pump: it takes air molecules from above and forces them downward at high speed. Every action has an equal and opposite reaction, in this case, the rotor blades are pushed up, along with the rest of helicopter. Of course, the air has to come from somewhere and then go somewhere. If you look at wind tunnel tests, what actually happens is that air circulates around the edge of the rotor disk. Much of the air is caught in an endless vortex going through the rotor over and over.

If the helicopter is small compared to the tunnel (or other enclosure) there is very little interaction with the surroundings and air flow is no different from that of an open field. A radio controlled helicopter will probably not be affected by a large enclosed space, like a hangar. The critical factor is the ratio of the enclosed space to the rotor diameter. Significant air pressure changes can be observed several rotor diameters above and below a hovering helicopter. As long as there is nothing within several rotor diameters, there is no problem.

If the helicopter is about the same size as the enclosed space, then there are potentially lots of problems. It is likely that enough pressure difference will be developed to lift the helicopter, but it may be very difficult to control. Any number of vortecies could develop in unpredictable ways, causing the helicopter to jerk from one side to the other. Humans are very good at adapting to changing conditions, but we have limits on how quickly we can react. It is likely that even a seasoned pilot will strike the sides leading to catostrophic results. Helicopter blades are made to do only one thing: lift and control the helicopter. For a number of reasons, they are not nearly as strong as the wings of a plane. Any contact with a solid object like a concrete wall means permanent damage to the rotors. Even if the rotors are not seriously damaged, they may become imbalanced slightly, in which case, they will tear themselves apart in short order. There is a condition known as ground resonance which can occur with multibladed helicopters. If a pilot slams a helicopter down on one skid and then the other, the shock can be transferred to the rotor head and cause an imbalance. If you are lucky, the blades will rebalance themselves, if not the entire helicopter will be destroyed and there is nothing you can do about it. One of my flight instructors was an army pilot and knew of a case where a pilot of tandem rotor Chinook got into serious ground resonance with both rotors. He shut down the engines hoping it would quiet down, but the violent stresses tore the ship apart. It threw body panel, fuel tanks, engines, everything. All that was left was a frame, a seat, and a pilot.

In general, good helicopter pilots, or any good pilot for that matter, is always paranoid. You never take chances. Helicopters are very unforgiving of any mistakes, so it is best not to make any. Flying into a tight tunnel qualifies as a serious mistake.

I hope that answers your question.