Control linkages and hydraulics question

role out to wings level if the air is smooth and conditions is favourable

 

That's a good catch, haven't noticed this myself in any of the Mi-8 manuals I've perused. Also, makes me think that perhaps there's nothing wrong with BST's modelling of this.

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

 

Yes, forward speed is the point in thread. Read the link in the thread that covers stability.

 

In short: Instable in hover. It will start to deviate by roll and or pitch. This gives a speed in that direction. This wil, cause the part of rotor aganst moving direction to lift when gaining airspeed.

(The same as in forward flight) The increased lift/disymmetri will raise that part of helo, speed will deceease and it will start to move the other way passigb the original point( if no wind) and speed increase-raise that part speed increas etc. This is how it should work but not easy to test because its scary, and not that safe.

 

That's a good catch, haven't noticed this myself in any of the Mi-8 manuals I've perused. Also, makes me think that perhaps there's nothing wrong with BST's modelling of this.

 

My memory was slightly wrong, not that much though:

Page 4-19:

"K" on the autopilot controller.

(6) In steady conditions of horizontal flight, climb or descent the helicopter holds its attitude with the free controls, but slowly deviates from a se- lected airspeed since the autopilot holds the pitch angle butnot the air- speed. Therefore in extended flying with the controls free, restore a se- lected flight condition periodically by application of the control stick and pedals. It is not recommended to free the controls when flying at a height below 50 m.

(7)

(8)

 

 

If you set the attitude correct on a true ATT stabilisation system, the airspeed will maintain around that number. Normas exact as when using IAS as attitude reference though.

So, if a true att system it will keep speed around rhe same, this is a bit different.

 

So its about the same as an ATT system, and keeps the attitude( bank and nose level).

Not self uprighting.

 

We also talked about the stock position earlier. Stick will be at around center for a turn, and need opposite stick to roll put of turn. And this can not differ if AP is on or off.

I think we are talking past each other. In your Wikipedia quote even you say that a helicopter is statically stable (it will roll back level) but dynamically unstable (increasing oscillations around the statically stable point). I’m not an expert but in a turn you of course have other forces acting such as centripetal force (changing what the helicopter perceives as being the down direction) and maybe gyroscopic precession because of the turn rate that might counteract this statical stability.

I think we are talking past each other. In your Wikipedia quote even you say that a helicopter is statically stable (it will roll back level) but dynamically unstable (increasing oscillations around the statically stable point).

 

Static stabillity as in the picture. Think it as a position. Not attitude. It will leave position but come back, and fly over. Lot of times.

It will not roll back wings level until airspeed from drift causes dissymetri of lift, and thats exactly like I wrote earlier.

 

Need to go to work, a little emergency. I can develope this later.

Stick will be at around center for a turn, and need opposite stick to roll out of turn

 

What I'm seeing in the Hip with the stab on is approximately as follows: 1) deflect the cyclic 2) having reached the bank angle, back off slightly towards the center 3) if you center the stick completely, AC will not go 100% wings level 4) to return to wings level, apply slight opposite stick. With it off, it behaves more or less as explained by you in that other thread.

What I'm seeing in the Hip with the stab on is approximately as follows: 1) deflect the cyclic 2) having reached the bank angle, back off slightly towards the center 3) if you center the stick completely, AC will not go 100% wings level 4) to return to wings level, apply slight opposite stick. With it off, it behaves more or less as explained by you in that other thread.

 

Just a thought her. If considering it a stabilization system and if centering the stick will make it not stay in bank but retur towards center (but not fully as you say), my thought of it is that part of the banking is considered by the stabilization system as an unwanted attitude change and is therefore (wrongly) corrected by the system and the bank is reduced (but not to "wings level"). It might be a reasonable, minor flaw to accept to get a working stabilization system (rather old design too) to help you in most other aspects of the flight.

Changes in attitude caused by the pilot's deflection of the controls are recognized as "wanted" attitude changes and are not counteracted by the autopilot system in the Mi-8.

Changes in attitude caused by the pilot's deflection of the controls are recognized as "wanted" attitude changes and are not counteracted by the autopilot system in the Mi-8.

Yes, it seems to do that for when initiating the bank. But when returning the stick to nearly centered to keep the bank, the bank is redused if the system is active but not if it is turned off. If it is turned off the bank stay as set when returning the stick to near center. This difference in behavior is, if I have understood correctly, the core of the discussion. Your input regarding this would be highly appreciated.

Yup Holton, that indeed is the salient point.

@AlphaOneSix

 

The question here is more, is there a direct mechanical linkage from collective and cyclic to the swash plate albeit with servo assist in normal operation but if you were to get into the pilots position with no engine/systems running and moved the cyclic and or collective would that move the swash plate? :)

but if you were to get into the pilots position with no engine/systems running and moved the cyclic and or collective would that move the swash plate? :)

 

Realistically, the answer is no. The documentation says yes, but it assumes sufficient strength. However, I have yet to meet a human that was capable of doing so. I can only move the pedals without hydraulics. I have never tried doing it with two people.

 

According to the TSMO -10, if you lose both hydraulic systems in flight, "sporadic control movement, response and extreme control stiffness will be felt." And just in case that wasn't scary enough, "loss of both hydraulic systems may result in a loss of aircraft control."

 

The Russian flight manual is a little less optimistic and says that in the event of a dual hydraulics failure, "prepare the crew for bailout."

Edited March 5, 2019 by AlphaOneSix

Yes, it seems to do that for when initiating the bank. But when returning the stick to nearly centered to keep the bank, the bank is redused if the system is active but not if it is turned off. If it is turned off the bank stay as set when returning the stick to near center. This difference in behavior is, if I have understood correctly, the core of the discussion. Your input regarding this would be highly appreciated.

 

I don't have enough hours behind the controls to answer that with any confidence, I'll ask one of our stick-wigglers and get back to you.

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.

 

In the case of the Sikorsky S-64, the mechanical linkage from the flight controls in the cockpit are connected to the pilot valve of the main rotor servo. This is, I suppose, what you would call a mechanical linkage but I can say with absolute certainty that you can not move the swash plate with brute force alone. If you tried, all you would accomplish would be to bend or deform the control tubes leading up to the servo. The linkage just moves the little pilot valve on the servo, which in turn actuates the piston in the servo.

 

 

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.

 

I have a colleague that was a Mi-17 instructor over in Afghanistan. He says that the only difference between the Mi-8TV and his Mi-17 was basically cosmetic. Anyway, he said that without hydraulics you could not move the flight controls. I found reference to this in the flight manual emergency procedures that indicated that with a primary hydraulics failure the aux system would automatically take over but the pilot was to land as soon as possible. I'll see if I can remember to bring it with me to work and scan that section for you.

I don't have enough hours behind the controls to answer that with any confidence, I'll ask one of our stick-wigglers and get back to you.

This is a cut from the training manual from a 11ton helicopter.

It has a conventional flight control system with the flight controls connected all the way to the swash plats and tail rotor control. Its FAR29 certified with dual reduntant hydraulic systems and I would say the hydraulic servos is built like *any other helicopter* where the hyd system is vital for the ability to control the helicopter.

 

Left side of picture, the control rod is attached to the lever "7". Via the hinge in the middle the lever is connected on the other side to the output of the servo. You can see the direct link on the right actual picture. Control rod in bare steel color is connected to the lever and on the other side you can see a blue anodized par the connects to the output of the servo, and is connected to the swash plate.

= There is a direct link from the controls to the rotor system.

 

If hydraulic system is unpressurized if you move the controls, the small flex movement in the lever allows for the valves to open, thereby making it possible to move the controls, and thereby also the swaschplate and the blades pitch.

 

 

The two hydraulic system in connected to the pink parts, one to the upper, "4" and one to the lower "10".

 

If you don't know another system in exact detail I suggest you consider any hydraulic assisted system on a helo to work this way, including the Mi-8. ( except for the part of being able to move the swash without hydraulic pressure, that might differ, as we already read in this thread). If its hard to understand, think that it works like a servo assisted steering on a car

 

 

 

There's one thing I add later, the mixing unit. I guess most don't even know about it.

Edited March 5, 2019 by Gunnars Driver

The same for the tail servo:

 

There's wires from "behind the pedals" to the control unit, which can be sen on this picture.

There is a direct link from the pedals to the tail rotor pitch control. Servo assisted.

 

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Realistically, the answer is no. The documentation says yes, but it assumes sufficient strength. However, I have yet to meet a human that was capable of doing so. I can only move the pedals without hydraulics. I have never tried doing it with two people.

 

According to the TSMO -10, if you lose both hydraulic systems in flight, "sporadic control movement, response and extreme control stiffness will be felt." And just in case that wasn't scary enough, "loss of both hydraulic systems may result in a loss of aircraft control."

 

The Russian flight manual is a little less optimistic and says that in the event of a dual hydraulics failure, "prepare the crew for bailout."

 

Yes I would have expected a lot of resistance even with the rotor static given the size of the rotor head and blades etc, can I assume from that there is a fixed correlation to cyclic position and swash plate relative angle? And that it is always good to have the hydraulics working.

 

That at least answers my question, thanks. :thumbup:

can I assume from that there is a fixed correlation to cyclic position and swash plate relative angle?

 

Yes.

This is one part from the mi-17 FM: I do not wish to say againsk anyone about the possibility to control the A/C, just what the manual say. As I wrote earlier, somewhere it was written a text in the manual stating "it might in some cases be possible to control the mi-8 without hydraulics. (it feels more like a nice writeup to keep the crews happy).

 

In case of failure of the main and auxiliary hydraulic systems in

flight (indicated by severe creeping of the control stick and depressurization

of both hydraulic systems) the crew should abandon the

helicopter upon the Captain's order. If the altitude does not allow to

abandon the helicopter proceed as follows:

1) Do not release the collective pitch control lever lock.

2) Reduce airspeed.

3) Change the engine power setting by means of the throttle

control twist grip.

4) Overcome the control pressure together with the PilotNavigator.

5) Execute landing.

6

Yes.

 

But shouldn't the control responses and the relative controller positions be identical regardless of whether the AP / stab system is on or off then? Because if so, there indeed is something wrong with it now since they differ a lot.

But shouldn't the control responses and the relative controller positions be identical regardless of whether the AP / stab system is on or off then? Because if so, there indeed is something wrong with it now since they differ a lot.

 

 

I found some info in the DCS manual stating that the AP can move around 20% of total control throw without the stick moving.

 

s214 and on:

https://srv0files.eagle.ru/dcs/manuals/DCS-Mi-8MTV2_FlightManual_EN.pdf

 

The autopilot control panel is located on the center console and provide the following functions:

 zeroing of autopilot control input prior to engaging the autopilot;

 individual engagement/disengagement of autopilot channels;

 entering of small corrections (±10%) using the centering knobs for pitch, roll, and yaw channels;

 

In this mode, autopilot affect small cylinder of the steering unit, which is, in its way, an adjustable rod, included into the control system. Due to that, resulting movement of the controls is a sum of pilot’s and autopilot’s control inputs. Movements of the executive steering units, due to signals from the autopilot, are not transferred back to cyclic and collective.

 

In case of automatic helicopter stabilization, the actuating rods of the KAU-30B combined hydraulic boosters can move themselves within 20% of their full travel range, wherein control sticks (cyclics) do not move and are hold in the same fixed positions by the spring load mechanism. Collectives are hold by the friction mechanism.

 

- So what we see is that the AP has a limited authority.

- It can not move the stick, so it has a trim funktion that alters the "length of the control rod"/actually done inside the servo actuator).

- If you make the Mi-8 turn with the AP ( "Roll channel centering knob") the helo should turn gently but the stick should be centered.

 

Its a very old style AP and it does not have any of the modes a less old autopilot/coupler has.

But shouldn't the control responses and the relative controller positions be identical regardless of whether the AP / stab system is on or off then? Because if so, there indeed is something wrong with it now since they differ a lot.

 

 

I read a bit to try to sort this out. For a while I was worried that I had been wrong in the Mi-8 case. My fear was that the autopilot was made to "not listen to the pilot commands" and thereby counteracting the pilots wich to make a turn with the 20% authority.

If that had been the case:

1) I had been wrong, and I would have needed to use a smiley like this :cry:

2) You would have to continuously hold the stick displaced latterally to turn., and the helo wouldn't really start turning until the cyclic had overcome the 20% AP authority.

 

 

But:

 

When pilot intervene roll or pitch, it is necessary to remove impact of signals from angular and angular velocity sensors on hydraulic boosters of roll and pitch channels. For this purpose the compensation sensors, kinematically connected with lateral and longitudinal controls, which provide signals, equal to ones received from attitude indicator, but with opposite sign, are installed. In other words, when pilot moves cyclic, the reference value of roll and pitch (which autopilot will maintain) is constantly being updated.

 

The text I made bold tell us that this shouldn't be the case, and thereby the AP should follow the pilots wish and not counteract. That should make the stick work like without AP for the position part.

(Just reading the DCS manual, and writing what this should mean to us.

I read a bit to try to sort this out. For a while I was worried that I had been wrong in the Mi-8 case. My fear was that the autopilot was made to "not listen to the pilot commands" and thereby counteracting the pilots wich to make a turn with the 20% authority.

 

If that had been the case:

 

1) I had been wrong, and I would have needed to use a smiley like this :cry:

 

2) You would have to continuously hold the stick displaced latterally to turn., and the helo wouldn't really start turning until the cyclic had overcome the 20% AP authority.

 

 

 

 

 

But:

 

 

 

 

 

 

 

The text I made bold tell us that this shouldn't be the case, and thereby the AP should follow the pilots wish and not counteract. That should make the stick work like without AP for the position part.

 

(Just reading the DCS manual, and writing what this should mean to us.

With that said, turning the hip with AP on you need to keep the stick laterally deflected, i.e. the stick position actually seems to give bank angle and not roll rate. At least in DCS, just verified it a moment ago.

With AP OFF you fly it like a normal helicopter, i.e. stick gives roll rate.

Could it be that as long as you don't exceed the 20% authority, the AP allow te pilot to move the swashplate as long as the cyclic is in motion, but as soon as the pilot holds the cyclic in position (deflected or centered) the AP work under the hood and returns the swashplate to "center" and cancel any roll rate?

The autopilot will attempt to hold the pitch and bank angles if the cyclic is not moving. That is, if the cyclic is stationary, the pitch and roll channels of the autopilot will attempt to hold the pitch and bank attitudes within its 20% authority.

 

If the cyclic is moving the autopilot still tries to hold the attitude, but essentially it's holding the attitude around a moving target, since the movement of the cyclic is being removed from the equation.

 

This is why it's more similar to a SAS-type system than what most people would consider a traditional "autopilot".

 

And so it follows that if you, for example, move the cyclic to the left to enter a 10 degree angle of bank, and you stop moving the cyclic when you achieve that angle of bank, the autopilot will attempt to, within its 20% authority, hold that 10 degree angle of bank, with the cyclic still being deflected to the left. With the autopilot off, you're now dealing with just the standard aerodynamics. I believe on the Mi-8 it will tend to roll out of shallow banks by itself. That is, for shallow banks, you may need to keep the cyclic deflected or else it will just naturally roll back to level. Then at some intermediate angles of bank, it will naturally hold that bank angle and the cyclic would be returned to center (or very nearly center) and the bank angle will just sort of hold itself. And then beyond some intermediate bank angle you will get the overbanking tendency, where the aircraft will tend to bank even more and you will have to apply slight opposite cyclic (opposite side of being centered) to keep the bank from continuing to exaggerate. I do not know what the angles involved are, but if memory serves (and it probably doesn't), an shallow bank angle is 15 degrees or less, intermediate is from maybe 15-40 degrees, and beyond 40 degrees you'll get overbanking tendency. I could be very wrong on those so as soon as someone argues with me I'll just edit this post and delete it. :D

 

I could be thinking of the over and underbanking tendencies in my Cessna.

 

EDIT: I asked one of our pilots about it and he said, basically, "Heck I don't know, I just do whatever makes it point where I want it to point." SO in other words he doesn't think about it enough to answer offhand.

Edited March 6, 2019 by AlphaOneSix

The autopilot will attempt to hold the pitch and bank angles if the cyclic is not moving. That is, if the cyclic is stationary, the pitch and roll channels of the autopilot will attempt to hold the pitch and bank attitudes within its 20% authority.

 

If the cyclic is moving the autopilot still tries to hold the attitude, but essentially it's holding the attitude around a moving target, since the movement of the cyclic is being removed from the equation.

 

This is why it's more similar to a SAS-type system than what most people would consider a traditional "autopilot".

 

And so it follows that if you, for example, move the cyclic to the left to enter a 10 degree angle of bank, and you stop moving the cyclic when you achieve that angle of bank, the autopilot will attempt to, within its 20% authority, hold that 10 degree angle of bank, with the cyclic still being deflected to the left. With the autopilot off, you're now dealing with just the standard aerodynamics. I believe on the Mi-8 it will tend to roll out of shallow banks by itself. That is, for shallow banks, you may need to keep the cyclic deflected or else it will just naturally roll back to level. Then at some intermediate angles of bank, it will naturally hold that bank angle and the cyclic would be returned to center (or very nearly center) and the bank angle will just sort of hold itself. And then beyond some intermediate bank angle you will get the overbanking tendency, where the aircraft will tend to bank even more and you will have to apply slight opposite cyclic (opposite side of being centered) to keep the bank from continuing to exaggerate. I do not know what the angles involved are, but if memory serves (and it probably doesn't), an shallow bank angle is 15 degrees or less, intermediate is from maybe 15-40 degrees, and beyond 40 degrees you'll get overbanking tendency. I could be very wrong on those so as soon as someone argues with me I'll just edit this post and delete it. :D

 

I could be thinking of the over and underbanking tendencies in my Cessna.

 

This basically answers that question and also verifying the behavior of the DCS Mi-8 to be correct.

Thanks!