Cyclic curiosity

[Block137]

What happens during forward flight when swashplate tilts forward?
Is it
A) Gyroscopic precession thing that tilt the rotor disc forward so lift force also pulls heli forward.
B) Blades pitch change in some way that the rotor create forward thrust.
C) Both
D) Neither
E) "It depends"

Been wondering if a heli can drift horizontally without banking.

[Fauropitotto]

Doesn't the tail rotor provide lateral thrust as well?

[Mateyhv]

If the heli is in forward flight the rotor disc is creating an equal uplift force. When you apply forward cyclic the rear half of the disc increases pitch creating more lift and the front portion of the disc decreases pitch and lif consequently.

The increase and decrease of pitch is equal so on its own it won't translate into translation, but only rotation around its axis.

[ARM_Coder]

Originally Posted by Mateyhv

If the heli is in forward flight the rotor disc is creating an equal uplift force. When you apply forward cyclic the rear half of the disc increases pitch creating more lift and the front portion of the disc decreases pitch and lif consequently.

The increase and decrease of pitch is equal so on its own it won't translate into translation, but only rotation around its axis.

In reality the side of the disc where the pitch is effected is 90 degrees ahead of the desired direction of tilt. In a CW rotating rotor, forward cyclic increases the pitch at the right side, and decreases at the left side. This is needed because of the gyroscopic precession effect.

[Jmann841]

Pitch changes every time the blades spin around. Tilt your swash plate with the heli off and spin it around and watch the change in pitch of the blade as it spins.

The 90 degree offset is done by the blade grip. Notice it is 90 degree's offset from where the blade actually is? This allows the swashplate to move in the direction we want to go while compensating for phase lag.

Contrary to popular belief, Gyroscopic precession is not the dominate force at play, it is phase lag which is an aerodynamic effect. If it was Gyroscopic Precession, multiple bladed heli's would still have a 90 degree' offset, however, many do not as the phase lag is different.

Here is one of the better explanations I have found to date:

"Some time in the past, it was decided to draw an analogy between a rotor and a gyroscope. Unfortunately, this analogy is incorrect and has caused confusion ever since. A rotor is not a gyroscope, due to the articulation, real or virtual, of its blades. A rotor is a system of whirling pendulums, and its reaction to displacement inputs is governed by the laws of vibration. In accordance with those laws, the only time a rotor blade responds with a 90 degree phase lag to a cyclic control input is when that blade is hinged at the axis of rotation and does not include any pitch-flap coupling. As the flapping hinge is displaced away from the axis of rotation, or as pitch-flap coupling is applied, the phase lag steadily reduces below 90 degrees due to the increase in natural frequency of the blade. In a given helicopter rotor, the phase lag also changes with the Lock number, i.e. increased aerodynamic damping reduces the blade natural frequency and thus increases the phase lag, such as would occur if flying one day in freezing conditions in Death Valley, and another day in hot weather at the top of Pike's Peak.

In summary, a rotor is a dynamic system in resonance, i.e. an oscillating system responding to periodic disturbances. All else follows.

The phase lag may be referred to as "precession", but this is a less than rigorous - and confusing - use of this term. "

[Block137]

I feel like it is getting a little off topic

The reason I'm asking this:
I read Wikipedia about Chinook's yaw mechanism.
It said CH-47 achieve yaw by applying opposite roll/aileron to each rotor.
And I believe the answer is 'A' (according to my first post)

A) Gyroscopic precession thing that tilt the rotor disc sideway so lift force also pulls heli sideway.

Which means....every time a Chinook turns it has to TWIST..
Probably just 2-3 degrees

[Jmann841]

Gyroscopic precession is not a major force on rotors nor does it control the Yaw on a CH-47

[myxiplx]

Originally Posted by Block137

What happens during forward flight when swashplate tilts forward?
Is it
A) Gyroscopic precession thing that tilt the rotor disc forward so lift force also pulls heli forward.
B) Blades pitch change in some way that the rotor create forward thrust.
C) Both
D) Neither
E) "It depends"

Been wondering if a heli can drift horizontally without banking.

The whole point of the swashplate is that it's directly linked to the pitch of the blades. When you tilt the swash, it causes the blades to tilt as they rotate around.

While the overall effect may feel like it's gyroscopic precession, it really isn't. There's a tiny bit of gyroscopic force, but the dominant factor is the aerodynamic forces in play. The overall effect is similar, but the details are very different (such as it will rarely act at exactly 90 degrees).

So basically B. is the answer.

[Block137]

Originally Posted by myxiplx

.........
So basically B. is the answer.

So a heli can fly forward while staying completely level to the ground?
^^^Simply all I wanted to know^^^

[ARM_Coder]

Originally Posted by Block137

So a heli can fly forward while staying completely level to the ground?
^^^Simply all I wanted to know^^^

Helis with rigid rotors (like Trex et al) can't sustain forward speed without tilting the body forward because the rotor disc is always orthogonal to the mast.

Full-scale helis don't force the disc to stay like this. So it's possible to keep forward motion with the body level, and even with the nose slightly up.

I once saw a R44 with 4 POB taxiing nose up. Funny.

[Smoggie]

Originally Posted by Block137

Which means....every time a Chinook turns it has to TWIST..
Probably just 2-3 degrees

No, it doesn't twist and it's nothing to do with gyroscopes either. The rotor blades can 'flap' so when opposite aileron input is applied to the front and rear rotors this blade 'flap' allows the rotor disks to tilt in opposite directions, and so the heli yaws. The heli fuselage doesn't twist.

[Block137]

Originally Posted by ARM_Coder

Helis with rigid rotors (like Trex et al) can't sustain forward speed without tilting the body forward because the rotor disc is always orthogonal to the mast.

Full-scale helis don't force the disc to stay like this. So it's possible to keep forward motion with the body level, and even with the nose slightly up.

I once saw a R44 with 4 POB taxiing nose up. Funny.

So...
No for rigid rotors,
Yes for others with dampers / flaps /etc?

[nightstalker]

Yaw in the Chinook is accomplished by raising one side of the swash - the other side is stationary. - Different from aileron (roll) where one side lowers as well. That might explain why they don't twist apart.

[hANDYman]

Originally Posted by nightstalker

Yaw in the Chinook is accomplished by raising one side of the swash - the other side is stationary. - Different from aileron (roll) where one side lowers as well. That might explain why they don't twist apart.

Interesting thread, but if the Chinook controlled yaw by raising one side of the swash whilst the other remains stationary (as opposed to one side raising and the other side lowering) then you are basically adding collective to both swashes as well as opposed cyclic commands. So every time the pilot turns the heli it goes up? That doesn't make any sense to me.

I understand that as some of the thrust is converted from vertical to horizontal, the vertical thrust will be reduced, but does the pilot not account for that manually by pulling more collective, or is it built into the pedals?

And as others have already suggested, the frame of the heli would not need to twist, the blades would simply flap such that the rotor disks tilt in opposing directions.

[extrapilot]

Handy

Nightstalker is wrong.

The 47 uses differential cyclic for yaw, via complex mixing. In fact, in you want to yaw the 47 around the tail or nose in still air, you have to manually add opposing cyclic, to change the bias front to rear of yaw inputs.

The airframe does experience a torque, because the rotors are not free-teetering, and the flap hinges are offset. Any cyclic input that causes blade flap imposes a moment on the head, and that is transferred via the normal mechanicals to the fuselage. Since here the torques oppose, the fuselage reacts those twisting forces.

[hANDYman]

Originally Posted by extrapilot

Handy

Nightstalker is wrong.

The 47 uses differential cyclic for yaw, via complex mixing. In fact, in you want to yaw the 47 around the tail or nose in still air, you have to manually add opposing cyclic, to change the bias front to rear of yaw inputs.

I understand the differential cyclic, and I believe Nightstalker does also, but it sounds like he is suggesting the complex mixing is also raising the collective both fore and aft. I would think that adjusting the collective to maintain the proper amount of lift would be left to the pilot, and not mixed into the yaw as NS was implying. That's the impression I'm getting anyway.

Originally Posted by extrapilot

The airframe does experience a torque, because the rotors are not free-teetering, and the flap hinges are offset. Any cyclic input that causes blade flap imposes a moment on the head, and that is transferred via the normal mechanicals to the fuselage. Since here the torques oppose, the fuselage reacts those twisting forces.

I guess that makes sense, since the Chinooks are 3-bladed it could not be a centrally located flapping hinge but would need offset hinges.

Thanks for the insight and correction.

[nightstalker]

Extrapilot is totally wrong, Again! Torquing the airframe??? Stupid ignorant comment - bias cyclic? Wth are you talking about? Just because you read something somewhere, it doesn't make you an expert. You haven't even got the first clues about how a Chinook is designed, much less how it actually does its work. You should stick to your rambling physics thing.

[Xrayted]

A tandem heli yaws by one rotor tilting one direction while the mixing tilts the other rotor the opposite way.

This opposing aileron force of the rotors keeps the heli from banking either direction, but instead causes the heli to spin around on its axis, but it can lose a little lift at the same time when this is occurring.

My buddy had a very nice Hirobo tandem RC model that works the exact same way, but he would adjust the collective during yaw himself. I'm sure he could have created some mixing in the TX to do this for him.

I'm not sure if there is any collective mixing involved on the full scale, or if the pilot takes care of this.

[extrapilot]

There is a difference between the lateral component of thrust (from the rolled rotor), and the torque that rotor roll imparts on the fuselage.

It is an important difference; people get lost with the whole translating tendency thing (where with a conventional machine you get tend to get fuselage roll from a canted rotor offsetting TR thrust).

For tandems, with a pure yaw in hover (pedal only), both rotors see a lateral component of motion, which increases their efficiency. Smaller scale, it doesn’t work well, just because the effect (translational lift) is trivial at very low lateral speed. Large scale, there is a fair offset- and the loss of lift is not huge to begin with (i.e. 2 deg differential rotor roll for yaw drops the vertical component of thrust in hover about 0.15% if you exclude any gains from translational lift).

[Xrayted]

Interesting.......so on the full scale the translational lift in the yaw is enough that there is very little loss of altitude.

I know I could see my buddy's model drop a little when doing a hover yaw, but he was just used to dealing with that when flying it. They are very cool models to see and hear fly on the RC side of things.

His required additional mixing electronics for the dual swash setup back when it was built, but things are a little easier for us these days, as my Skookum FBL units that I use to fly have a tandem mixing function already built into the unit and ready to go for a mode such as this.