Why pull cyclic for hovering?

[Jinker]

Hi, can someone explain me clearly why does the pilot need to pull the cyclic in hovering? The explanation in the manual (highlighted in the attachment) is really confusing to me.

[lemoen]

The rotor tilts forward a bit when the cyclic is centred. You need to pull back on it to make it level. Also a bit left, to make up for tailrotor's effect on main rotor wash.

[Holton181]

The main rotor is tilted slightly forward when the cyclic is dead center, to help out a little when flying forward. Due to this you need to hold the cyclic slightly behind dead center when hovering to not move forward. Hope it makes it a little bit clearer.

 

Edit

Sniped by lemoen

[Panthir]

Holton is right. The main rotor mast is tilted slightly forward. Is this context, a cyclic hold back compensation is needed to deny forward movement. It's simple. Moreover, depending on the main rotor spinning direction clockwise or counterclockwise the mast is tilted left or rightside as well.

Edited May 17, 2018 by Panthir

[Jinker]

Thanks for the answers.

This also means that normally the CG of the helicopter is in front of the main rotor axis, right?

[Holton181]

Thanks for the answers.

This also means that normally the CG of the helicopter is in front of the main rotor axis, right?

Not necessarily. The CG should be close to it, but not necessarily in front of it. I don't know the details for the Huey though.

[BlacleyCole]

For every input of one control (cyclic,collective, trim pedals) the other two will need to adjust to counteract the forces. The more collective the more left pedal. And left and rear cyclic.its best to read the manual throughly to understand the hueys control process. Because when you lift off slowly like you should you pull back and to the left for. A reason the first thing to happen is the skids toes clear the ground, then the rt skid and finally the left skid. Finally you have slipped the surly bonds of the earth.the you have to deal with thins likenige, one etl before you get into forward flight. Then remember what a friend told me “It’s easy to fly the huey, but it’s diffi to control her”

[Gunnars Driver]

Thanks for the answers.

This also means that normally the CG of the helicopter is in front of the main rotor axis, right?

 

 

Yes.

 

 

The CG limits usually is in front of the rotor mast. Stability in forward flight comes from balancing forces against each other. CG in front of rotor mast and stabilisator on tail boom gives a downward force in forward flight.

This is mainly the same principle as an normal stable fixed wing aircraft also.

 

 

For the UH1, the longitudinal cg limits is 130-144 inch rear of reference datum. Rotor mast is at 133.5 inches at the top, but is 5 degree tilted forward, making the longitudinal limits to be situated forward of the rotor mast shaft on the floor.

 

 

[Edit]Of course a typo did sneak in, the CG limits is 130-144(corrected above from typo, 140-144)

 

 

[Edit]Before getting shot down by anyone, when I say CG limits usually is in front of the rotor mast, this is a simplification of reality. But just to make it easy to understand. If I remember it correct the UH60 has the mast in front of the CG limits, and the last two helos i did fly and still fly has the rotor mast around the middle of the CG limited area. ( I sell CG calculation apps at app store, so I have at least some knowledge about it, http://sevenbytes.se/ )

Edited May 17, 2018 by Gunnars Driver

[Holton181]

...but is 5 degree tilted forward, making the longitudinal limits to be situated forward of the rotor mast shaft on the floor.

Thanks, this is something I never thought of!

[Gunnars Driver]

From FM:

 

https://s7.postimg.cc/820csp9mj/UH1_CGlimits.png

 

https://s7.postimg.cc/r5to92p1n/UH1_CG2.png

Edited May 17, 2018 by Gunnars Driver

[Chic]

You'll find that armaments in the DCS Huey will shift CoG forward.

[WildBillKelsoe]

without back cyclic and centered cyclic the rotor disc is slanted forward, so when you increase collective the disc is already forward, so the aoa of disc is increased causing lift with forward thrust by rotor disc being angled.

[Sharkku]

Those of you explaining this with the rotor disc being slanted forward, you have not thought it through completely. It is true that the slanted disc requires you to apply aft stick when taking off, but that's just for the initial small pitch up required until the rotor disc is horizontal. After that, when you are in the air in a stable hover, what's important is the balance of forces, not the angle between the disc and some arbitrary "level" plane of the helicopter body. What matters then is the horizontal distance between the force vector of the rotor and the center of gravity of the heli, as well as trim and mechanical properties of the cyclic system. That's what's really causing the need for aft cyclic in a stable hover.

[PeaceSells]

I suspect the reason to be designed like this is that the increase in speed results in a increase in pitch up tendency. I guess the rotor being centered in a slight pitch down position is to partially compensate that. If you take off without trimming (or hit trim reset), you'll see that after you gain a certain amount of speed you don't need to hold cyclic forward anymore and can leave it at rest in the center, and the helicopter will fly level. If you continue increasing speed, you'll have to push the stick forward instead of pull back, to keep flying level. Maybe if the rotor wasn't a bit forward when centered, you'd run out of cyclic travel forward in order to keep level flight at high speeds...

[Chic]

Sharkku makes a good point. Once in "level" flight, the built in forward tilt of the rotor mast serves mainly to allow the pilot to comfortably look forward without fatiguing his neck. Lateral tilt biasing on the other hand has more aerodynamic purpose.

[Sharkku]

@PeaceSells: Nope, the forward slant would not be able to compensate for the pitch-up tendency at higher forward speeds by itself. It's there for the reason Chic mentions, to keep the ride more comfortable for the pilot and passengers. In normal cruise you need a forwards propelling force, that in a helicopter comes from angling the thrust vector of the main rotor forwards. Compensating for the pitch-up tendency is accomplished by cyclic trim and tail elevator angle.

[PeaceSells]

I wasn't talking about rotor slant forward, I was talking about pitch foward moment, not the same thing, despite one might contribute to the other.

 

Plus, whatever is causing a pitch foward moment without the need to pull the stick (or trim) is already naturally compensating for pitch up moment.

[Sharkku]

Ah, gotcha.

[CHPL]

Yes.

 

 

The CG limits usually is in front of the rotor mast. Stability in forward flight comes from balancing forces against each other. CG in front of rotor mast and stabilisator on tail boom gives a downward force in forward flight.

This is mainly the same principle as an normal stable fixed wing aircraft also.

 

 

For the UH1, the longitudinal cg limits is 130-144 inch rear of reference datum. Rotor mast is at 133.5 inches at the top, but is 5 degree tilted forward, making the longitudinal limits to be situated forward of the rotor mast shaft on the floor.

 

 

[Edit]Of course a typo did sneak in, the CG limits is 130-144(corrected above from typo, 140-144)

 

 

[Edit]Before getting shot down by anyone, when I say CG limits usually is in front of the rotor mast, this is a simplification of reality. But just to make it easy to understand. If I remember it correct the UH60 has the mast in front of the CG limits, and the last two helos i did fly and still fly has the rotor mast around the middle of the CG limited area. ( I sell CG calculation apps at app store, so I have at least some knowledge about it, http://sevenbytes.se/ )

 

 

In fact its pretty simple.

Because control inputs are limited in any direction. It make sens to give the Helicopter a tendency in a direction you need it most. In Fact forward. Because nobody fly's with 80 Knots backwards.

Main rotor masts usually rigged to the left or to the right for compensating tail rotor trust, which not only controls the direction in which the Nose points. It also pushes the helicopter sideways.

 

CG may be any place in front or behind the main rotor mast as long it is in limits.

Limits are Coarsely defined by the ability of the controls to counteract the unbalance in load.In fact by burning fuel CG is moving during flight.

 

 

 

For example . If your GC is out of limits to the front. Pull back the cyclic all the way, will not prevent the helicopter from moving forward in a hover.