The height of the rotor on a helicopter

General Questions
#1

A helicopter went over my car today - I think it was a traffic 'copter considering the pattern was making above the highway.

One thing I noticed was the long shaft between the helicopter and its rotor. I saw a similar helicopter on TV the other day being used to chase animals on the African plains (for relocation).

My best guess is that a long shaft increases the ability of the 'copter to hover since both applications might do a lot if that - probably something to do with the down-wash. On the other hand, maybe it’s just easier to walk under the blades.

Is there any reason for a long rotor shaft? I would think that it’d be better to keep it short and lessen the wobble that a long spinning shaft might generate.

#2

For a chopper that’s mostly going to be used in urban areas, like a traffic copter, I’d guess the customer can specify a raised rotor so it can land in more places.

#3

My guess is that it needs enough clearance so that the blades wont smack anyone that walks under them.
Also, the higher above the center of gravity the blade is, the more stable it is.
http://www.bellhelicopter.com/en/aircraft/commercial/bell206B-3.cfm

#4

Just curious: do they ever make the rotor blades so low that you can get a crew cut if you’re above a certain (reasonable) size and you exit the copter standing straight? I ask because every everytime I see someone getting on or off one, he’s always all hunched over. Sorta “I know the blade is two feet above my head, but why take chances?” posture. Is this really necessary?

#5

I think it’s more of a reaction to the noise and wind and dust.

#6

The Robinson’s seem to have what looks to be an unusually till rotor mast http://www.robinsonheli.com/r44clipperii.htm and I have noticed more of them recently.

#7

Yeah - like that - maybe a bit slimmer & taller even. Hard to say from a distance.

#8

Have you ever walked under spinning helo rotor blades? I’m a reasonably short guy and it freaked me out so much even I was hunched over.

#9

Yes, I have. Of course, I was about 3 foot seven at the time, so no worries…

That’s sort of my point–I was wondering if it’s purely psychological or if there’s any actual danger if you stood up straight and tall. Seemingly, it’s all psychological.

#10

Last time I flew in a helicopter I was told to take that pose as the blades droop- they are closer to the ground at low revs. Hence the height when you get out is not the same as that at the end of the blade.

#11

Most, if not all, blades are high enough that you won’t hit your head, but there can be some danger. Some helicopters have rotors that are tilted forward slightly, making them lower in the front. The blades may also flex downward when powering down, and if the helicopter is sitting on a hill, one portion of the blades may be lower than the rest. I believe the Huey had blades that were 8 feet high. If parked sideways on a hill, a 6ft tall person walking uphill would only need to gain 2 feet of altitude to be in danger.

#12

Thats exactly right. We were taught to walk away from the helicopter at a 45 degree angle. That is both to avoid the slightly lower blade in the front and so the pilot can see when you clear the blades. In one of my safety briefs at NTC (Ft Irwin) they had multiple examples of different mishaps which occurred there, most with pictures. A pilot was once killed there because his OH-58 was on a slope and he walked uphill into the rotor. Also high winds may cause the rotors to bounce a bit. Also as mentioned with the engine off in helicopters without a rotor brake the blades tend to dip before coming to a stop.

To answer the OP on most commercial and military helicopters there is plenty of clearance. The mast (that is what the shaft is called) is a couple of feet above the transmission. The height of the transmission and the engine are enough to give the clearence. No need to add in extra height. The mast is high enough so that you should be able to walk under the blades without ducking and high enough that the blades won’t cut off the tail during all but the hardest of landings. They have another term for a landing that hard.

#13

Helicopters with coaxial rotors also have a rather tall shaft, although I doubt that’s what you saw.

#14

Rotor blades flex a fair bit. One reason for a high mast is to avoid having the blades hit the tail boom.

#15

A helicopter thread, two days old, and no word from Johnny L.A.; do you think we should contact the authorities?

#16

Slight hijack…

The front mast of a Chinook is tilted forward. This does bring the blade quite close to the ground. The blades are big and heavy, and quite flexible.

The one time I rode in one I was told that we must stay away from the front of the heliopter. The rear too because of the jet exhaust. Go up the rear ramp from the sides if the thing is ‘running’.

This was jungle penetrator exercise, so I exited it on a cable from about 50 feet. It was very windy. :smiley:

#17

There is a helicopter manufacturer that really wants to cut people’s heads off, and that is Kaman. If you want to exit to the side from a Kaman HH-43 Huskie or K-MAX, you might want to crawl instead of duck. These have two intermeshing rotors that are tilted to the side.

(ok, the rotors are actually pretty high but these are still helicopters that give me the heebee-jeebees)

#18

Many (ALL smaller ones, AFAIK) helicopter blades are hinged. The blades teeter as they rotate in order to balance the lift due to differential speed when translating. It is very important that they NOT contact the tail rotor or tail boom, and a tall mast may be required to insure adequate safety margins.

#19

Ha! Actually I just saw the thread earlier today, and didn’t think I had anything to add.