How does the rotordisc tilt the airframe?

[ColeTrickle]

Hello fellow aviators,

while toying around with my helo flightsim, a question rose in my mind:

I understand when I move the cyclic (e.g. in a stationary hover) I'm not altering airframe/fuselage attitude directly, but rather tilt the rotordisc.

Ok, but how come the fuselage follows after a (short) while?

I can easily (presumably wrongly) image the behind-the-scene mechanics of this effect on a rigid rotor system. But what about a teetering rotor (e.g. R22/44 style) with a trunion bearing, unable to transmit any moment to the (rather thin) mast?

Help from this group is greatly appreciated....

Best regards

Cole

[parabellum]

By whatever means, the rotors are attached to the mast, which is attached to the airframe, move the disc and the airframe will (usually, hopefully), follow.

[BusinessMan]

IIRC there are a couple of reasons:

1 If the disc itself tilts forward then it will start moving in that direction as the thrust vector tilts, hence it will induce a nose-down rotation in the fuselage about the aircraft's CofG because the directional thrust is applied offset from the CofG point (in effect dragging the a/c by it's hub).

2 In an articulated head (similarly to a rigid head, as you have said, but unlike a semi-articulated/teetering head) there is the additional effect of a rotating moment applied to the rotor hub itself as a result of the flapping hinges being offset from the centre of the (rigid) hub.

I stand ready to be corrected though!:O

BM

[Camp Freddie]

My recollection from instructor skool is that -

The rotor disk tilts forward and reaches an equilibrium dictated by forward thrust and lift, with the vector pointing up and forward

As the vectors have to eventually line up in a steady state of flight, the airframe vector is dictated by weight and drag, and will point down and aft, but aligned with the rotor disk vector.

The nose pitches down due to airframe drag, but this effect is modified due to the horizontal stabiliser which provides downforce on the tail, to maintain a relatively small variation in airframe attitude across the speed range.

Convinced ?

[311kph]

Agree with said.
Rotor force through CofG = steady hover.
Cyclic forward, rotor force inclines, creates arm from CofG, comes momentum that rotates frame, etc...
With speed increasing, various aerodynamic forces come into play, etc...

[Arm out the window]

It can be useful to think of force couples here.

For a teetering head where the fuselage pretty much just hangs off the mast, you might wonder why disc tilt would change body angle at all, but I think of it like this:

Start in steady hover. CG hangs under mast head.

Disc tilts and starts to move forwards, dragging mast head with it. Fuselage drag increases, giving a rearward force centred lower than the mast, pointing back.
The couple between the forward force at the head and the lower rearward drag causes a forward rotation around the CG.

This rotation will continue until an equal and opposite couple stops it.
With the fuselage tilt, the CG moves rearwards from out under the mast. Now weight pointing down from the CG and lift pointing up at the mast head have made an opposing force couple. The fuselage will keep on tilting forwards until the the two couples are equal and opposite.

[topendtorque]

pushimego,
pullimekum

[Arm out the window]

That's heaps easier mate, they should teach it like that all the time.:ok:

[311kph]

All this force and momentum talk reminded me of something from the past. We were told that when you catch the Gazelle by her tail, she cannot take off. First we thought that it cannot be truth, but then started to think, well, there is a lot of distance from head to tail, and a guy with 80+kg on the end… and there we were, colleague of mine hoverin’ at 3-4 feet, and me sneaking from behind, with headset on and cord in my suit so not to be tangled in fenestrone. I grabbed her on the tail, there is a handle in the back (instant thought: what a @#$% am I doing here!!!) and pulled her up and down to signal him. He started to push cyclic, and I felt tail is going up and slowly stretching me. Then, a thought crossed my mind: if he pulls me more, I’ll have to let her go, an who knows if he could compensate from sudden nose down surge while hovering so low. Luckyly, he thought the same, leveled her out and landed.
Young and stupid.., but if nothing, it was worth a laugh every time we remembered it.

[Arm out the window]

instant thought: what a @#$% am I doing here!!!

Great story! That would have looked good in the board of inquiry if you'd kept hanging on...

I remember as a young boggie 'helping out' to do a stick track on an Iroquois tail rotor, just unnatural going so close to that end of the machine while it was running, even if in your case it was a fenestron.