A Very Basic Technical Question

[balaton]

Hi Dear Guys,


Please accept the appologies of a FW guy who just recently started to fall in love with helis and started to admire and envy helicopter pilots.
I went through many books on the basics of helicopter flying/physics/aerodynamics and have a couple of hours in a simulator, still one technical question remained which is still bothering me (probably stupid):
To my knowledge the rotor "disc" of a simple system (like the B206) can "pitch" and "roll" freely (as seen in some tutorial animations).
If one exceeds the mechanical limits of this free motion you will have "mast bumping".
Now, despite this free motion of the disc, the body (airframe) of a helicopter is obviously pitching and rolling, so, there must be some moment exerted by the disc to the rotor mast.
How is it possible if the rotor "disc" (rotor blades flapping) is moving freely? How is it possible without bumping the mast?
I just hope that my question is clear enough, appreciate your help,


Tamas

[puntosaurus]

First of all you're welcome to envy helicopter pilots, but I definitely wouldn't admire them :8.

Secondly, I was always told that the rotor bit is doing the flying, and the rest of it is just hanging on.

[Ascend Charlie]

The disc pulls on the top of the mast, which then exerts a force on the fuselage, which follows some time later, depending on the rotor head type. Fastest to follow is a rigid head, slowest is a teetering head.

[[email protected]]

Tamas - if you look for threads about mast bumping and negative G you should get a decent explanation.

As AC states, the rotor pulls the fuselage around but negative (or less than positive) G can allow a teetering head rotor to exceed its normal flapping limits with regard to the rotor mast because the fuselage isn't obediently following the rotor disc any more.

[dClbydalpha]

A demo is to tie two pieces of string to the neck of an empty drinks bottle. They are free to flop about. Now pull the ends in opposite directions you find you can lift the bottle. By keeping on the tension but having your hands at different heights you find you can alter the angle the bottle hangs at. Simplistic, I know, but a good first step to introducing rotor stiffness and hinge offset.
If the string was replaced with straws you would be able to push the bottle sideways. Because the neck of the bottle is above the centre of gravity the bottle will tip.
Again, simplistic, but it is the offset of the thrust from the cg that allows a rotor with no hinge offset to impart a moment. As the body rolls the force vectors come in to line and the roll or pitch stops.

[NickLappos]

balaton,
You hit on a major myth that training books leave entirely unexplored - how the rotor actually controls the helicopter. Most rotors are not hinged exactly at the mast like the simple illustrations in the training manuals, the rotor blades are usually attached to extended arms that are strongly attached to the mast. The arms allow strong bending forces to be transmitted from the blades to the mast, and then to the body of the helicopter, so the body must follow the rotor.
Here is a teetering rotor that is more rare, but often used. With this, only the lift of the rotor can control the helicopter, because the tilting of the lift imparts a control around the center of gravity of the machine.:
https://scontent-lga3-1.xx.fbcdn.net/v/t1.0-9/14433017_10207418009831213_7459747005934969429_n.jpg?oh=e695 e8c215564eee24ecf788b61c4c4b&oe=58782D11

Here is the more common hinged rotor (articulated), where the tilting of the disk makes the blades use their centrifugal force to twist the rotor and force the body to follow. This rotor can control the helicopter even if there is little or no lift being applied. That is why it is more modern, and now more often used.

https://scontent-lga3-1.xx.fbcdn.net/v/t1.0-9/14479675_10207418010431228_3078851641701825231_n.jpg?oh=d9fc 0a73b8a06a45369ecf21ca99fccf&oe=588412F0

I hate to be rough, but most of the explanations below aren't correct. For example, if you suspend a pop bottle from a string through a loop, at the center of the cap, that most nearly approximates a teetering hinge, and in that case no angle of the string can make the bottle tilt at all.

[chopjock]

Are there any 2 bladed articulated rotor systems out there, ie not teetering I wonder?

[RINKER]

Chop jock. I've often wondered that too. Large rc helicopters commonly use
Two bladed designs that are rigid with and without flybars. Just google something like rc tRex 3D flying to see what I mean.
I do know the flybarless ones use solid state 3 axis stability instead of the flybar to help control.

R

[dClbydalpha]

Nick, before being rough please read my answer again.
The drink bottle allows someone to physically grasp the concept of stiffness and hinge offset. Which as you point out is something that isn't often addressed in casual discussions.

Reducing the hinge offset to zero is the next step whereby you have to imagine the thrust vector in terms vertical and lateral. The second part of my answer, replacing the strings with a straw, therefore gives an appreciation of the interaction of the cg with an offset force.

I've always found the two taken together to be a good hands on introduction to help someone understand these fundamental points. Of course you are entitled to hold a different opinion.

On a personal note I'm glad that you are well and posting.

[Arm out the window]

For the original poster, the other part of the puzzle which it seems is being left out in some explanations above is drag.

A good and simple way of looking at it is this:

Imagine the spinning rotor as a disc, free to fly where it wants, and the body as a box hanging freely by the mast from the rotor head. As the disc tilts and flies off in any direction (let's say forward with respect to the body), it pulls the mast head along with it. As speed increases, drag builds up on the body trying to push it backwards and therefore aligning it into its proper orientation with the rotor disc - we have a force couple, the forward pull at the mast head vs the backward pull of drag working lower down.

Why doesn't the body just keep moving back? Because, as has been mentioned above, its center of gravity (which is lower than the mast head) is moving backwards from under its suspension point, and the further it goes, the greater the restoring force (an opposing force couple created by the upward pull on the mast vs the downward pull at the centre of gravity).

When the two force couples are equal and opposite, we get a steady state. The faster you go the greater the pull/drag couple becomes, so the body swings further back until the lift/weight couple increases enough to balance it.

That hopefully makes it clear, as long as you're OK with the concept of a force couple (two opposing forces separated by an arm, which creates a rotation - like two hands on a steering wheel, one pushing up and the other down - the bigger the wheel / longer the arm, the greater the twisting force). Apologies if you already knew all that!

[NickLappos]

dClbydalpha
You are correct, the cap's width serves as the offset hinge, so your illustration does illustrate the hinge effect. I mistook it for a demo of a teetering rotor, sorry.

There are no 2 bladed articulated rotors that I know of, I'd have to do some thinking about why, it could be the coalescence of the blades lead-lag frequency with the rotational frequency

For teetering rotors the body does follow at a long interval, mostly due to the effect of trim forces and drag (as arm discusses).

[WillyPete]

How is it possible if the rotor "disc" (rotor blades flapping) is moving freely? How is it possible without bumping the mast?

Are you asking how does mast bumping NOT happen if the blades flap in a teetering rotorhead?

The rotor head uses "feathering" to balance the amount of flap experienced due to dissymmetry of lift while the aircraft is moving. (This doesn't happen at a static hover with no wind. No flapping occurs)
Example: The advancing blade is at a decreasing pitch, while the retreating blade is at an increasing pitch when forward cyclic is applied.
This "feathering" of the pitch balances the lift across the rotor disc, reducing flapping.

As you apply a change in cyclic, the resultant thrust in the direction of the change tilts the shaft and aircraft with it, keeping the flapping within safe margins and the gap between shaft and rotor allows some teetering prior to the fuselage changing orientation.

This all fails during low-G, and causing the rotor disc to tilt to the left when the aircraft rolls to the right does NOT pull the fuselage and shaft into the same orientation, exceeding the shaft/rotor teeter gap, causing bumping.

[balaton]

Many thanks guys, indeed! I will print out your explanations and try to comprehend :)


Tamas