Control linkages and hydraulics question

Bonjour gents and gentettes. The Mi-17 Dash-10 manual states that:

Control inputs are transferred from the cockpit to the rotor blades by mechanical linkages and hydraulic servos.

But what does this mean exactly? Are there two separate systems for redundancy, one mechanical and the other hydraulic? Or does it mean that the control rods are connected to the servos/actuators and never interact with the swashplate directly?

Hmmm... has to be the latter, or perhaps a DCS bug if not. Because when there's no hydraulic pressure in the system you can stir the cyclic to your heart's content, yet nothing moves up there. Am I right?

not 100% but I believe if you look at it like a digger for example, you use hydraulics to go to a system then it turns it mechanical, i.e hydro lines to the control surface of the linkage which in turn pushes the necessary items

That's what I thought too. But a certain individual here on this board almost chewed me a new rear orifice recently for thinking so, because according to him, all helo control schemes use mechanical link rods and the hydraulics are only used for power boosting and assistance.

But is that really so? Because at least our Mi-8 here seems to behave just as if the cyclic was hydraulically linked to actuators connected to the swashplate control rods. So what gives here?

The controls in the cockpit are linked via control tubes to the hydraulic servos (or actuators or boosters whatever you want to call them). The servos then connect to their respective bellcranks or whatnot that connect them to the main rotor swashplate (lateral and longitudinal bellcranks for the cyclic, collective lever for the collective, and a pulley and cable system for the tail rotor).

When the hydraulics are working, your inputs are boosted and the flight controls move. When the hydraulics are not working, the servos act like a static link in the chain, and your inputs go directly to the "control surfaces" (that seems like a good way to describe it). If you're strong enough, the controls will move with no hydraulics. I can do it with the tail rotor although I almost have to stand on the pedals with all my weight to do so. I have never been able to move the cyclic or collective without hydraulic power. Maybe it's easier to do with the rotor actually spinning but I'm not going to try it so who knows. Well, someone knows, just not me.

Well well, you live and learn. Thanks A16. So there is a direct mech control link there as well. Thanks again.

The controls in the cockpit are linked via control tubes to the hydraulic servos (or actuators or boosters whatever you want to call them). The servos then connect to their respective bellcranks or whatnot that connect them to the main rotor swashplate (lateral and longitudinal bellcranks for the cyclic, collective lever for the collective, and a pulley and cable system for the tail rotor).

 

When the hydraulics are working, your inputs are boosted and the flight controls move. When the hydraulics are not working, the servos act like a static link in the chain, and your inputs go directly to the "control surfaces" (that seems like a good way to describe it). If you're strong enough, the controls will move with no hydraulics. I can do it with the tail rotor although I almost have to stand on the pedals with all my weight to do so. I have never been able to move the cyclic or collective without hydraulic power. Maybe it's easier to do with the rotor actually spinning but I'm not going to try it so who knows. Well, someone knows, just not me.

That is not exactly correct for all helicopters. I can't speak for the Mi-8, but I can definitely attest to the fact that there are several models of helicopters flying around out there that depend entirely on hydraulics in order for the pilot to manipulate the flight controls. Sykorsky is big on this design concept. Their S-58T, S-61 and S-64s all require hydraulic pressure in at least one of the flight control systems for the pilot to be able to control the rotor systems. From the cyclic stick to the servos are a series of control tubes, bellcranks, mixing units and even some cables that basically move a pilot valve on the servo, which actually makes the swashplate move. There is no mechanical linkage between the cyclic and the swashplate and there isn't a pilot out there strong enough to move the flight controls without sufficient hydraulic pressure.

Lighter helicopters such as Jet Rangers and Astars, even the Huey can be flown with the complete loss of hydraulic pressure, albeit with some difficulty. Like I said, I can't say with any authority that the Mi-8 was flyable without hydraulics but given it's size, I would bet you that it is not and without hydraulics you will not see the swashplate move when moving the cyclic and/or collective around. :thumbup:

There is no mechanical linkage between the cyclic and the swashplate

Interesting. So how does the hydraulic link behave when you're turning / rolling such a chopper? Do you have to center the cyclic once you've reached your bank angle, or does it behave more like a fixed-wing aircraft where you more or less hold the stick in place during the turn?

I'm asking, because someone on this board claims there's a bug in the Hip's roll behaviour with the stab system / autopilot engaged. According to him, even with the AP on you should still have to center the cyclic after having reached your bank angle and even give some opposite stick in order to stop her rolling; but as she's modelled now, you can indeed just fly her as you would a fixed-wing AC because she returns "wings level" all by herself. With the AP / stab off however, she does behave like a conventional helicopter would, i.e. deflect the cyclic, return to center / slightly opposite to stop rolling, and level the machine yourself with the cyclic when approaching your new heading.

Now the pertinent question is whether he is right or wrong, because he seems to think all choppers, always, have a mechanical connection between the stick and the rotor blades, and this is why the Hip just couldn't behave IRL as she does here. Is he right?

That is not exactly correct for all helicopters. I can't speak for the Mi-8, but I can definitely attest to the fact that there are several models of helicopters flying around out there that depend entirely on hydraulics in order for the pilot to manipulate the flight controls. Sykorsky is big on this design concept. Their S-58T, S-61 and S-64s all require hydraulic pressure in at least one of the flight control systems for the pilot to be able to control the rotor systems. From the cyclic stick to the servos are a series of control tubes, bellcranks, mixing units and even some cables that basically move a pilot valve on the servo, which actually makes the swashplate move. There is no mechanical linkage between the cyclic and the swashplate and there isn't a pilot out there strong enough to move the flight controls without sufficient hydraulic pressure.

 

Lighter helicopters such as Jet Rangers and Astars, even the Huey can be flown with the complete loss of hydraulic pressure, albeit with some difficulty. Like I said, I can't say with any authority that the Mi-8 was flyable without hydraulics but given it's size, I would bet you that it is not and without hydraulics you will not see the swashplate move when moving the cyclic and/or collective around.

I don't question your statement per se, but I think you misunderstood the question discussed a bit (or I did). My interpretation of the question is if there is at least a theoretical possibly to move the controls without hydraulics, even if it might be basically impossible in practice. My understanding is that it indeed is even for big Sykorsky ones, in theory, but not in practice. As you can see in msalama's last post above the underlying reason for his questions is to understand some aspects of flights with AP/stabilization ON in contrast to OFF.

How it works in the Mi-8 by AlphaOneSix:

https://forums.eagle.ru/showthread.php?t=152786 Mi-8 Flight control systems

https://forums.eagle.ru/showthread.php?t=152670 Hydraulic systems

I know those documents by A16, they're all a great read. Alas, none of them provides an answer to my question here.

NB: the chopper doesn't quite autolevel even with the AP / stab system on, you still have to give her some slight opposite cyclic in order to level her after a turn. But going into the turn you can indeed fly her roughly the same way you'd do with a fixed-wing AC, i.e. deflect the stick laterally and just more or less leave it there during the turn. Just wonder if this is correct :unsure:

But going into the turn you can indeed fly her roughly the same way you'd do with a fixed-wing AC, i.e. deflect the stick laterally and just more or less leave it there during the turn. Just wonder if this is correct

Isn't that what you'd expect with mechanical linkages ?

If there's a mechanical linkage, then if the cyclic is off to one side, the blade angles are uneven and if it's in the middle, the blade angles are (as) symmetrical (as they're designed to be for normal flight).

So, if in flight you deflect the cyclic off to the side (ignoring the AP for a minute), it generates a torque that tips the aircraft over to that side until the torque in the opposite direction caused by the body of the aircraft being displaced out from under its suspension point counters the torque caused by the asymmetrical blade angles. & There you stay until you centre the cyclic...

If while banked you were to then put the stick back to centre, blade angles become symmetrical(ish), the torque that caused the roll would disappear and then the weight of the body of the aircraft displaced from its natural position hanging under the rotor, would provide a torque that rolled the aircraft back to level with the COG under the point of suspension.

The only exception I could see to that would be if you were turning coordinated at high speed, and the centrifugal force on the body might be enough to hold the COG off at an angle to its point of suspension....

Yes, thought along similar lines too, but someone supposedly in the know disagrees. According to him, a laterally deflected cyclic left in position should make the helo roll continously.

If while banked you were to then put the stick back to centre, blade angles become symmetrical(ish), the torque that caused the roll would disappear and then the weight of the body of the aircraft displaced from its natural position hanging under the rotor, would provide a torque that rolled the aircraft back to level with the COG under the point of suspension.

This completely makes sense to me. A heavy helicopter hanging under a rotor with symmetrically angled blades should roll back more or less level.

Yes, thought along similar lines too, but someone supposedly in the know disagrees.

Wish I could up-rep you for your patience.

That is not exactly correct for all helicopters. I can't speak for the Mi-8, but I can definitely attest to the fact that there are several models of helicopters flying around out there that depend entirely on hydraulics in order for the pilot to manipulate the flight controls.

That's right.

There is no mechanical linkage between the cyclic and the swashplate and there isn't a pilot out there strong enough to move the flight controls without sufficient hydraulic pressure.

Thats wrong, and right.

There is mechanical linkage to the swashplate. But it is servo assisted with hydraulics. You have to think like a car with servo assisted steering.

I have flown two types that could not be flown without at least one working HYD system.(Bo105 and Superpuma). Both have 100% mechanical connection to the swash and you can move the swash without HYD with the rotors not spinning. But the forces are to big with the dynamic system spinning so there is no chance of controlling them without HYD.

This basically have to be true for all hydraulic assisted flight controls( unless Fly-By-Wire). If you think of construction machines with HYD only they have either a spring loaded valve, making the hyd fluid flow when operating(Excavators) and the stick position will have no relation to the position of the hydraulic cylinder.

For wheel loaders you have a steering wheel that makes hyd fluid flow only when you move it. Still, if you trap oil to be sure of newer changing the steering position of the wheel compared to the cylinder, temperature change will change the volume of the fluid and the stick position would be completely different between +40 and -30 Celsius. I hope this is clear to everyone.

= There is a solid mechanical link between the stick and the control surface/ swash that is supposed to move.

For detail to others( I think you know): there's a small flex built into the steering that makes the hydraulic actuator to sense the pilots movements and reinforce it.

Lighter helicopters such as Jet Rangers and Astars, even the Huey can be flown with the complete loss of hydraulic pressure, albeit with some difficulty. Like I said, I can't say with any authority that the Mi-8 was flyable without hydraulics but given it's size, I would bet you that it is not and without hydraulics you will not see the swashplate move when moving the cyclic and/or collective around. :thumbup:

Yes, I did absolutely also think the same about the mi-8 size, but I found information in the manufacturers data telling the pilot that "you might in some cases be able to fly without hyd". I don't know if this is some eastern way of keeping the crew happy not telling the true or if its really so.

This completely makes sense to me. A heavy helicopter hanging under a rotor with symmetrically angled blades should roll back more or less level.

Nope. Not a chance. I hope we dont need to go there again, I think msalama with me on this.

You should read this thread. At least the first post and #64, #68, #69

Its already sorted out. We might wanna leave it out of this thread ?

https://forums.eagle.ru/showthread.php?t=231698

Well be that as it may, she actually doesn't autolevel fully even with the stab / AP system on (see #11).

you can indeed fly her roughly the same way you'd do with a fixed-wing AC, i.e. deflect the stick laterally and just more or less leave it there during the turn.

In what fixed wing aircraft can you leave the stick deflected during a turn ?

(Cause in the very most cases, you cant)

Final stick position will depend on speed, power and the prop turning direction(causes dissymmetri of drag) etc, but it still will be around the center position. Just the small offset we already discussed in helos, nothing more.

No, that was an inaccurate expression, not fully deflected by any means, but there's a difference in how you'll need to execute a turn in a helo compared to a fixed-wing AC.

PS. Done nitpicking yet, or did you just come here to stand on my dick again? Since most folks here seem to have understood what I meant.

Nope. Not a chance. I hope we dont need to go there again, I think msalama with me on this.

 

You should read this thread. At least the first post and #64, #68, #69

Its already sorted out. We might wanna leave it out of this thread ?

 

https://forums.eagle.ru/showthread.php?t=231698

Then the Gazelle is also wrong since it seems to have a preference to hover with the skids towards the ground when keeping the cyclic centered (stabilization deactivated). It might drift around a bit, but not start rolling over.

No, that was an inaccurate expression, not fully deflected by any means, but there's a difference in how you'll need to execute a turn in a helo compared to a fixed-wing AC.

 

PS. Done nitpicking yet, or did you just come here to stand on my dick again? Since most folks here seem to have understood what I meant.

I hope you don't take my comments as personal. It is absolutely not meant that way.

I think most here would agree that misunderstandings about the principle of flight in posts about principle of flight should be straightened out ?

I fly both aircrafts and helos since early 90's and I would say a normal turn with forward speed is performed the same in an aircraft and helo.:

-Stick deflection to achieve bank, then stick to the position where the bank angle doesn't change( = close to middle, and depending on speed and bank etc.)

-Backwards stick pressure to keep height

-Small adjustments with rudder pedals to keep ball centered.

-To roll out opposite deflection

-Ease on the backpressure

-Center stick to keep wings level when wings level.

Yes, that's about it. Didn't mean that either, just that the movement series are subtly different and corrective inputs needed as well. You of course turn them basically the same way.

Anyway, what do folks say about the Hip's turning / levelling behaviour with the AP / stab on? Anyone around who has flown the real thing and cares to compare and comment?

Then the Gazelle is also wrong since it seems to have a preference to hover with the skids towards the ground when keeping the cyclic centered (stabilization deactivated). It might drift around a bit, but not start rolling over.

You can read of the helicopter stability in the other thread(already linked), post #42.

Well be that as it may, she actually doesn't autolevel fully even with the stab / AP system on (see #11).

Anyway, what do folks say about the Hip's turning / levelling behaviour with the AP / stab on? Anyone around who has flown the real thing and cares to compare and comment?

The manual (link in the other thread) say a bit of the stabilization.

It doesn't seem to have a real Attitude mode, mor of a stab system( SAS or someting like it). The manual states that it might role out to wings level if the air is smooth and conditions is favourable.

Its in the autopilot part of the manual.

You can read of the helicopter stability in the other thread(already linked), post #42.

The post there seems to be about turning when going forwards, but what about hovering?