Off Centre trim for straight and level cruising

[Essah]

by Using the little controller position indicater (right control + Enter) I've noticed that when trimming for straight and level cruise at high speed I end up with a stick trimmed forward and left and the rudder/rotation trimmed to a position right.

How come this is neccecary to maintain straight and level flight?

I was wondering if it was due to the asymmetrical layout of the helicopter with the cannon being located on one side or is it something else?

 

It makes rapid banking to the left quite dangerous due to the blades passing each other very close on the right side.

[PeterP]

Dissymmetry of lift

 

Same effect can be observed in a co-axial helicopter, not as strong and dangerous as in a conventional single rotor design, but at higher speeds strong enough for a highly off-centerd cyclic and rudder.

 

 

The upper rotor Disk in a co-axial helicopter is the "dominant" one, and produces more lift.

So, dissysmmetry of lift can't be nullified by a co-axial rotor design.

 

Edit:

Entering states when this effect gets more prominent needs high caution of the pilot because both rotor disks are getting very close together at the right side because of this effect:

Upper right side moves down, because of loose of lift / lower right side moves up, because of high lift.

Photo of a Ka-50 at high speed. Notice the different spacing of both rotor-disks at opposite sides.

 

 

 

This mod helps a little to prevent unintended piloting errors that result in a rotor-clash at higher speeds:

Unchain Rudder from Trim

 

 

 

Edited December 30, 2012 by PeterP

[Griffin]

This is normal due to the dissymmetry of lift. A little right rudder is also needed.

 

It's even more pronounced on helicopters with conventional tail rotor as on the coaxial the rotors cancel each other for most of the part.

 

EDIT: Damnit.

[yodosha]

I've been thinking about this recently too, but thus far couldn't find a plausible reason why this is happening. Even weirder: the manual doesn't mention blade clashing almost at all. The only time it does, it's to say that tail strikes are more probable than disc intersection and that "those who are against the coaxial configuration" say this happens (disc intersection). Other than this, there's no other reference to blade clashing in the manual at all, even though it's quite easy to do in the sim.

It can't be the cannon, because it would make you use left cyclic during hover too. (the position of the cg is speed independent)

I have to say though, that these control inputs, that are necessary at speed, are very similar to those of western single rotor helicopters (CCW main rotor). With single rotors you need cyclic forward to balance out the rotor's drag and tendency to tilt back (both caused by dissymmetry of lift, but one is the spawn of drag while the other is from lift), while left cyclic is necessary to counter the effects of transverse flow, dissymmetry of lift and the tail rotor(though this is compensated for to some degree). Right pedal is usually necessary to counter the tail rotor's higher lift at speed, but then again, these are highly dependent on how the helicopter is constructed.

[Bushmanni]

Dissymmetry of lift causes pitch up moment which needs to be countered by pushing the stick forward. This is because of precession. In a rotating system a force causes movement 90 degrees after the point where the force acts. Both rotors have more force acting on them at the advancing side and this force causes motion for both rotors when they are at forward position. Dissymmetry of lift works in exactly similar manner for coaxial and single rotor helicopters. Sideways tilt is caused be different phenomenon.

 

In single rotor helicopters while at forward flight the blade tips in forward phase move in undisturbed air and generate more lift and because of precession they lift up when at the retreating side. This needs to countered by moving the stick to the retreating blade side. In coaxial choppers something weird happens and the rotors tilts to just the opposite direction but I haven't been able to find or figure out why this happens but that it definitely happens. NASA had some paper published where this phenomenon was discussed but it's cause wasn't revealed. For some reason blades generate extra lift while at aft position in forward flight which causes the advancing side to lift up but where that extra lift comes from is mystery to me.

 

To recap you need to have stick offset on aft-forward direction because of asymmetrical lift in left-right direction and left-right offset because of asymmetrical lift in aft-forward direction.

 

Here's a link tot the NASA paper:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970015550_1997024330.pdf

 

US Navy Helicopter aerodynamics workbook:

http://www.cnatra.navy.mil/pubs/folder5/th57/p-401.pdf

[PeterP]

Dissymmetry of lift causes pitch up moment which needs to be countered by pushing the stick forward. This is because of precession. In a rotating system a force causes movement 90 degrees after the point where the force acts. Both rotors have more force acting on them at the advancing side and this force causes motion for both rotors when they are at forward position. Dissymmetry of lift works in exactly similar manner for coaxial and single rotor helicopters. Sideways tilt is caused be different phenomenon.

 

 

It is not a pitch up in the Ka50 - you have a more prominent side-tilt in the KA-50/coaxial rotor design because the both disks aren't producing the same lift.

The upper disk is moving also clockwise. (instead of counter-clockwise as in a conventional singel-rotor)

 

 

So what results in a pitching up and than left roll in a normal helicopter is different what happens in a coaxial design.

 

 

Because both disks aren't producing the same lift the Ka-50 is pitching right.

Edited December 30, 2012 by PeterP

[yodosha]

Bushmanni: Although I agree with 99% what you said I'd like to add that precession has a lesser effect (the main shaft and hub) on articulated rotors and as such, most rotors don't have a 90 degree phase lag. (eg. 78 degrees for a R22 which, even though got a teetering rotor, is designed with a small amount of delta-3 which changes the undamped flapping frequency of it, thus changing the phase angle) This is because a rotor is not a single solid body, but rather multiple bodies connected together through connections with angular degrees of freedom.

The "increased lift behind rotor hubs" happens to single rotor helicopters too. I can't explain it but here's a (very theoretical) paper about something similar: http://naca.central.cranfield.ac.uk/reports/arc/rm/2642.pdf

[Bushmanni]

PeterP: Sorry, I can't understand what you are trying to say.

 

Yodosha: Thanks for the link. I still need to study more to understand the math but I'm hopeful that my program will help me get there some day. But the plots were something I could possibly understand. If I got it right, there's air flowing upwards ahead of the rotor and also through the central part where there are no blades or the blade speed is so slow that they basically don't generate lift. In forward flight this upwards flow gets dragged aft and increases lift at the aft sector of rotor disk. It could be assumed that in coaxial rotors this phenomenon is more pronounced and actually capable of overpowering the transverse flow effect.

[PeterP]

PeterP: Sorry, I can't understand what you are trying to say.

 

 

Sorry.

Means:

You can't simply adapt the effect and the resulting behaviour from a conventional single-rotor design (pitching up followed by left roll) to a coaxial-design.

[EvilBivol-1]

I think the issue to understand first is more basic and one that may not be appreciated enough by many new sim pilots. The positions of the controls of any aircraft at any point in flight depend on the conditions of that moment - aircraft weight, airspeed, winds, payload, fuel load, etc. This is why there is trim control and why it is adjusted all the time during flight. Centered control position does not necessarily mean straight and level flight. In fact, it rarely does for most aircraft in general. It is the other way around - you set the controls to whatever position they need to be to maintain straight and level flight at that moment and then trim them to stay there.

Edited December 30, 2012 by EvilBivol-1

[PeterP]

I think the issue to understand first is more basic and one that may not be appreciated enough by many new sim pilots.

 

Well , if this is the case for the OP and some that participate in this thread ,

I invite everyone (once again) to read this:

 

So here comes my "standard Ka-50 newbie reply" :

 

 

First download Bigfoots interactive training missions

>>> KA-50 Black Shark 2 Training missions

 

 

 

Read also besides the manual this:

DCS: Black Shark...

Technical, Simplified

-Coaxial Rotor Aerodynamics

-Trimming the Ka-50 Black Shark <<<!!!! very important to understand this fully!

-Part 1 - Autopilot <<<this too!

-Part 2 - Autopilot

 

and when you are confident about your cyclic-trim,

you wan to read about your rotor-pitch trim:

Is it even possible to maintain speed and/or altitude?

And when you still have time , read this too:

A Stick and Rudder Man's Guide

to DCS: Black Shark

 

Here is a useful tweak if you using real rudders:

How to unchain the rudder from trim - solution

 

Things you want to have some day too:

 

KA-50 collective: another picture-tale

 

Two MS FFB2 with Cougar Grip

 

Welcome aboard!

[Bushmanni]

Sorry.

Means:

You can't simply adapt the effect and the resulting behaviour from a conventional single-rotor design (pitching up followed by left roll) to a coaxial-design.

 

Laws of physics are the same for both. The same laws that explain single rotor behavior must explain coaxial rotor behavior when you add another rotor to the analysis. I'm not just applying single rotor behavior to the coaxial but using it to illustrate the laws of physics that govern the behavior because single rotor systems and the physics of them are much better described in western literature.

[Humvee28]

So here comes my "standard Ka-50 newbie reply"

 

Very good Info for Newcomers like me. Thanks Peter! :)