Sunday, March 27, 2011

The Helicopter: A Fragile Fish

Helicopters do not have a lot going for them.  They are expensive to buy as well as operate, somewhat odd-looking, and, if movies like Die Hard 3 and Cliffhanger are reliable sources of information, they can be brought down with a hand gun or a rock or some rope... pretty much anything nearby.

Helicopters are fairly vulnerable pieces of hardware, but it takes more than a knock in the rotor to drop them out of the sky.

I have traveled by planes, trains and automobiles, but never by helicopter.  From an engineering standpoint, they scare me.  Where possible, it is good to avoid single-point-failure scenarios.  If one airplane engine were to fail, most airplanes can still be flown with the remaining engines.  If the main rotor blade of a helicopter were to fail mid-flight, down it goes.  The main rotor is responsible for the lift force, and without it, it is time to look for a good parachute (and then a good lawyer).

How does a helicopter navigate around?  The principal force is generated by the main rotor, which spins very quickly, displacing vast amounts of air in exchange for lift upwards.  If it wants to maintain altitude, the lift force upwards must be equal to the weight downwards. 

If a helicopter wishes to move forward, it must angle its pitch downwards slightly and spin the rotor a tiny bit faster.  The slightly rotated orientation causes a small component of the lift force to push horizontally.  As some of the lift force no longer pushes directly upwards, the rotor speed must be increased to maintain equilibrium in the vertical direction.

Helicopters are quite inefficient because the majority of the fuel is used not to thrust forward, but to fight gravity.  An airplane’s turboprops are aimed horizontally, and the lift force comes from an offshoot of the plane’s velocity.  What makes an airplane inefficient is the drag force, which is very high because it is proportional to the square of the speed.  Even though planes travel much faster, they are more energy efficient than helicopters.

Helicopter bodies are not all that aerodynamic, which is alright since they tend to move fairly slowly.  However, wind thrusts affect the aerodynamics of the main rotor (one side of the rotor has a different relative speed to the air than the other), which causes a headache for the pilot.

The small rotor on the tail of the helicopter allows it to rotate about its vertical axis (alter its yaw) to change its direction of travel.  It does this by applying a moment (torque) about the center of mass of the helicopter.

I used to see more helicopters in the air.  They used to be helpful to get a picture of the traffic, although you could never decipher a word of information from the reporter due to the extremely loud background noise.  Today, cameras placed in convenient locations accomplish the same goal while saving a great deal of energy and money.

There are some helicopters in use for emergency health situations, and others used to find a missing person.  The vast majority of helicopters are used by military forces at three million dollars a pop. 

The main advantage of a helicopter is that it requires no streets, like an airplane, but also does not require a runway for take-off or landing. 

Helicopters are also unique in that they can perform “station-keeping”.  A subsonic airplane cannot hover; the moment it stops moving forward, its lift force disappears and it plummets downwards.  An airplane is like a shark, which must remain in perpetual motion, even while it sleeps, as no oxygen can be absorbed by its gills if it remains static.  Helicopters are more like fish: agile, but vulnerable.

For all of the agility of helicopters, they tend to be portrayed as targets in the movies, and Rambo has taken down more than a few.  In fact, I challenge you to name a movie that involves helicopters, but does not show one exploding.

Yes, helicopters are fragile creatures, but I roll my eyes whenever an action hero attempts to take one down with dental floss.

1 comment:

Rob said...

Yeah pretty interesting, we've found plenty forms of lift, both dynamic and static, yet we are still to find anti-gravity, or something that allows very cheap lift. It's no easy task to keep tons of metal aloft, and the more power we put on an aircraft, the more weight we also add.

I hope we eventually truly defeat gravity and enable capabilities to send mountains worth of hardware, materials and personnel up to space so we can finally and cheaply explore at least this solar system.