Helicopter take-off

[Prudie]

When a helicopter is running and on the ground what happens to it physically to make it take off? Presumably the rotor speed is increased but is that all, do the blades alter angle (like flaps)? Just wondering.

[Philtaz]

No idea sorry methyl, would be guessing in that the front rotor is not as angled as the rear one or vice versa.

[Andyvon]

Sorry about the spelling above. I think the batteries in my keyboard need changing as the keys are sticking.

[Andyvon]

As I put in my post a Chinook has two jet engines mounted on either side of it's rear pylon and they power the rotors and produce forward thrust.

[Clanad]

So... Ok... where to start? First, the rotors on a helicopter are nothing more than moving wings. If you could see them up close they have the same upper contour as an aircraft wing has and produces lift in the same way. (Thank you Mr. Bernoulli). As the rotor rotates counter clockwise as seen from the cockpit (on American helicopters), the rotor produces lift on the right side of the circle, but as the rotor moves into the left side of the circle it must produce more lift than the right side, since it's moving away from the oncoming air (when the craft is flying forward). In fact there's a limitation on every helicopter on how fast in can fly (normal airplanes can fly too slow and stall). If it flys to fast the left side of the rotor arc cannot produce enough lift and that side will stall, creating a very dangerous condition called "retreating blade stall".
So (stay with me here) all of this is controlled by the pilot, but not directly. As AlBags introduced, the pilot is controlling the rotor head through the stick like control that moves up and down. This in turn controls the rotor head (the device at the base of the rotor system on the shaft from the engine). This device is ingenious in that it automatically alternates the angle of each blade as it rotates. As the blade on the right side of the arc moves forward, the cyclic decreases the blades angle, and as it moves into the left side or retreating side it automatically increases the angle (called the angle of attack and shortened by AlBags as AOL) thereby equalizing each blade with the other.
The other stick like control in the pilot's left hand can be moved freely in all directions. This one is called the collective and simply tilts the entire rotor head in the direction the pilot wants to move. (The previously described cyclic control also provides the pilot engine power control through a motorcycle like throttle).

On standard helicopters, there are also pedals on the floor that similarly control the tail rotor whose sole purpose is to provide ant-torque power to keep the body of the helicopter from spinning under the rotor on top (thank you Mr. Newton). In the case of the Chinook (and other similar designs), each entire overhead rotor system spins in opposite direction to the other, thereby cancelling the results of Mr. Newton's Third Law.
All very complicated, which is why "Rotorheads" (semi-affectionate name for helicopter pilots) all seem to very nervous... they'e just waiting for something to go wrong. It's often said that helicopters don't actually fly... they just beat the air into submission...
I'm sure my attempted word picture is clear as mud, but it's the best I could do without my proverbial greenboard and chalk...

[Andyvon]

Can you sleep with the distinctive thumping of those Chinooks Methyl?

[Andyvon]

My father says it was the same with B-36s Eddie. He remembers them being here in the 50s and you could hear the reverberation a couple of minutes before you saw them. Even when they were high they were so big he says he could still see them when all other planes were just a contrail.

How I would have loved to have seen those! Zeppelins too!

[ChuckFickens]

Fantastic explaination by Clanad.... apart from he got the names for collective and cyclic pitch the wrong way round.

[Prudie]

Thank you all for your answers, very special thanks to Clanad who has gone to a lot of effort to explain. It is all over my head to speak but obviously helicopter flight is incredibly scientific and a lot more complicated than one might think, more so than fixed wing craft it seems.

[ChuckFickens]

It not that much more complicated....

(says the person amusing himself at work this morning by flying an RC helicopter around the office :o) )

[AlBags]

The Rotor Head plate is called a swash plate .. Clever bit of engineering .. as are the blades. Chinook blades are absolutely massive ..! Incredible forces are transferred through the blade roots .. they are not made of tin plate!

[icemaiden]

I love Chinooks. Love the sound, Wocka Wocka!

I had a ride in a helicopter last year over London. Loved every minute of it. Will definately do that again before I die!

[DosAmigos]

Prudie. I just wonder how anything stays up.

[rowanwitch]

We get chinnooks quite often bringing in forces casualties to the centre for defence medicine.... terrifying.... somehow a primeaval response like an ancestral mammal seeing a pterodactyl..... and we get the odd Merlin and lots of air ambulances.... they seem so small and fragile by comparison.