Gunnars Driver

Just found a very nice writeup and description with pictures from A16: https://forums.eagle.ru/showthread.php?t=110313 Quite easy to the that the control rods are mechanichally connected to the swash. What we dont see is the 20% internal adjustment the autopilot can do. Either use the link below togheter with google.translate, or just believe it anyway :) http://cnit.ssau.ru/vertolet/mi8/

We had another thread going about cyclic position during a sustained turn. ( https://forums.eagle.ru/showthread.php?t=231698 It wasn't about the mi-8, but basic principles of flight and somehow the mi-8 entered the discussion. To sum it up, this discussion was basic helicopter without AP, and the conclusion is that all helos will have the cyclic close to center latterally in a turn and this have to be the same for the mi-8, at least with AP of. I found support for this in the flight manual, and also in some flight tests, where the lateral stick position was logged. Basically, we know that the mi-8 need stick input around center, often opposite lateral during turn(as with lot of helos and aircrafts) and the question from the OP i think was related to the fact that in DCS mi-8 with AP of, stick is around center in a turn but when AP on, you need to keep it deflected. The OP question was about hydraulics and mechanical link from stick to swash, trying to find a reason why the module changes behavior with AP on/off. The other thread had a long way to come to the conclusion that stick around center for a continuous banked turn is valid for virtually all helicopters( and aircrafts), I suggest reading this if the stick@center in turns isn't clear.

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.

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 - 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.

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).

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. "][/url]

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.

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.

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. My memory was slightly wrong, not that much though: Page 4-19: 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.

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.

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.

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.

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

That's right. 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. 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.

:thumbup:

Ehh…well. I wasn't sure how to handle this, to shoot down another statement… :cry: Saw it earlier but was due to a VR racing game. That statement isn't really true.(either). Its airspeed depending. To try to make an easy explanation, the need for higher lift causes the induced drag to increase. There's parasite drag(drag like a car at speed =no lift) this doesn't change when increasing lift. At high airspeeds the induced drag is low, making for a less part of total drag. --> If you are on the high side of the Lift/drag curve the torque increase is less then increased lift in percent. ---> If you are at bottom of the lift/drag curve the torque increase should be the samt as the increased lift. ---> If you are on the low side, the torque increase should be higher than the increased lift. Ill try to show this in a graph, I think. [EDIT] The Red boxed numbers is at higher speed, 83% torque /11000kg vs 59.5% torque /6900kg. From 6900kg to 11000 is 59% weight increase (= 59% lift increase). The "cost is" only 39% torque increase. The Blue boxed numbers is at bottom of L/d curve, 51.7% torque / 11000kg vs 32.5% torque /6900kg. 59% weight/lift increase, 59% torque increase. At the bottom of the L/d curve, Borchi_2b is correct. The Green bexed, hover 97.3%/10600kg vs 54.5%/6900kg. 54% weight/lift increase, 78% torque increase. The 2G turn needs a 100% lift increase, and at bottom of the L/d curve it will cost us double power, but at higher speeds it will not. It'll cost us more total power but as referred to the torque at 1G the increase will be lower.

:) I dont really have the time. I already got a couple pf modules that I didnt have the time to learn to use weapons etc. I’m connected to the guys developing the Bo105. I have helped, and will, to get the Bo as good as possible. Besides this, I try to keep up with the guys I do DCS with. I can only put a fraction of the time :cry:

That's a good approach! In my way of thinking, DCS isn't really a simulator but a game. Very many helos aren't that easy to fly or hover IRL(for a non trained person), at least the ones without a stab system. A flight model that allows for fun gaming without making it to hard is good. That said, I don't have the Mi-8 module and what I said about the mi-8 flight characteristics is not a complaint towards the DCS mi-8. In the case of me saying "thats not wright" it's about that there cant be to different ways to use the stick AP on or of( and IRL stick around center for roll rate zero in a banked turn). This is just for the sake of this thread and the OP question. We have people that never ever flew a real helo on the forum which have the DCS Mi-8 and/or the Huey and then they think they know how a helicopter fly IRL and from this they say that another DCS module isn't correct because of this or that and they really don't know what they talk about. There are also some, stating to have flown the type in question but from some statements its easy to see that they don't know what they talk about at all. For the developers its good to know how it really works, to be able to se the rubbish from 'hobby complainers' and not implement this to the module. I dont have the Gazelle either( the friends I game with have and tried to talk me into it). I knew I had read some complaints, that's the reason I got to this part of the forum. Yes, there are some problems with having a (joy-)stick that cant move the center position and simulate the real world stick-further-the-faster. In the Huey you loose throw, and might run out of cyclic if trimmed for Fast Forward Flight( I got that a few times befor I learnt to reset trim.). I think this way of trimming should change the expo curve instead so you can reach the cyclics 100% throw despite being trimmed for FFF. The other way around is to have it like I saw in the clip posted in this thread, and have stick att center for trimmed pitch up/down at all times. I would not have any problem with this. Its a good compromise I think. Not how you fly a real helo, but then again, its not a simulator, its a game. (I still think the DCS name is good, its a combat simulator, not a "flight simulator"). I have some hours playing around with RC helos also(long story, my brother in law….). Well, the RC helos (old fashioned) have a Bell hiller mixer with stub wings on the flybar that cancel the nose up with increasing speed. The same behavior is selected for the electronic flybarless gyros that superseeded the real flybar. For a RC helicopter with a transmitter where the stick also cannot be re-centered the only logical solution is to always have it trimmed with stick in center. So, for the modules no problems. But expressions like this: or They are wrong. And when someone like the OP makes a thread with a question he deserves the appropriate answer. That at least what I think.

Right, but in the same way wrong. Thats because the G-force and formula I referred to is actually the calculation to maintain the same lift vertical to the earth. Your description is another way to describe the same as G= 1/(cos bank angle). So it is not ’in addition’ :)

Yes, stick forward in forward flight is valid for all real helos. (Fact, not opinion) ”Adding power” is the same for all aircrafts. If you find a difference in litterature, it will be because how they express the issue. This is how it works. (Facts, not opinion): If you are to keep the same airspeed and altitude you need to add power in a turn. Easiest eay to understand this is that a turn will increase the G force. Small bank doesnt give that much G-force, but indeed there is. G = 1/(cos bank). For a 30 degree bank the G-force will be 1.15. Thats 15% more lift needed, and if you keep the power and altitude the speed will go down. But, because in many cases, the exact speed is not important you can just leave the set power and perform your turn. This is how I do the must of my turns. When learning to fly you need to learn this though, and for Fixed Wing in slower speed regime you might find yourself in a smoking hole in the ground if you dont apply power and try to keep the altitude without adding power :cry: W. J. Wagtendonk: heard of but I havent read his book. Cool name BTW :)

While this thread isn't about Mi-8 at all, not even the mi-8 forum I suggest someone knowing AlexanderT ( reference from earlier post) pass this information on, and that we leave it from now in this thread. (By the way, the description of pitching up vs down when entry a turn is also common knowledge in aerodynamics. It has to do with the blade flapping when commanding a turn. This is also the same on all helos but may be ATT hold function in an AP/SAS. Issue well covered by R.W.Prouty. Only thing bothers me, it should be the other way around on the mi-8, due to the clockwise rotor). [Edit] I actually accidently found a flight test with mi-171V made by the Polish armed forces.( google search from yesterday was still on my phone) http://yadda.icm.edu.pl/yadda/element/bwmeta1.element.baztech-article-BUJ5-0041-0038/c/httpwww_bg_utp_edu_plartjok42011jok42011317.pdf Of i terrest for this matter vi have fig.7 in page 323. This picture shows the swash plate position for the mi-171 in different kinds of turns at 3000m an different speeds. Negative values normally to the left so this tell us that the stick is virtually centered in right turns at speed 205km/h but at left turns the stick is to the right. Stick to the right is valid for left turns at all speeds, and for right turns at lower speeds stick will be to the right: This values are taken by using some of the tests that you need to do to certify a large rotorcraft according to FAR29. Only a fool will say these tests ”aren’t true”.

Source [/img] After this excellent contribution to the development of a better DCS I will call it day, and enjoy a good nights sleep. :)

Well, I might actually have :) From what i find on the net, Mi-8MTB-1 (the export version – Mi-171B). I could find proof of both using the same swash plate, thats a good indication of using virtually the same rotor system. Mi-171B Flight Manual page 4-53: https://storage35.uloz.to/Ps;Hs;fid=2408693;fide=EPJukX;hid=Rz36m6;cid=756726525;rid=111946803;up=0;uip=81.235.204.136;tm=1551743429;ut=f;aff=uloz.to;did=uloz-to;rs=0;He;ch=d9f92bea178d45d67ad7643a3a5883b9;Pe/!D22LfMug/kazan-mi-17-1b-flight-manual-full-version-pdf?bD&c=756726525&De&redirs=1 Well, thats seems to validate my thoughts/claims about the mi-8 as well. You can do whatever you wish with this information.

Well, finally post ≠64 settled the question. At least for those that can accept FAA and RW Prouty as valid sources.