Coriolis effect

[Jack57]

Just thought I would throw this one out there for discussion.

 

I am wondering if coriolis effect is modelled in BS - can't imagine not really. If it is, it certainly isn't that noticeable. In small single rotor helicopters (eg: R22, R44, etc) it can be so great in high G manouvres that overspeeding of the rotor becomes a real issue. It is of course a lot less of an issue in helicopters with rigid and semi-rigid rotor systems. It may be that given the weight of the Ka50, the blades are flexed to near their limit in normal flight anyway and therefore further loading won't flex them a great deal more.

 

So, can anyone definitively answer the question of wether it is modelled in BS or not?

 

Jack :pilotfly:

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ps: for those who may not know, coriolis effect is caused by the conservation of angular momentum. It is the effect that causes the spinning ballerina or ice skater to speed up when they pull their arms closer to their bodies. In helicopters it results from the rotor blades flexing upwards which in turn result in their COG moving closer the the centre.

[eV1Te]

I don't think that what you are explaining is called Coriolis effect but instead conservation of angular momentum (that is what makes a spinning ice skater speed up when they move their center of mass closer to the spinning axis).

 

Coriolis effect is a very weak force (usaually) that has to do with motion of something on another moving body, for example moving clouds on the rotating earth and in which direction these clouds will rotate around themselves, and this force is proportional to what frame of reference you use, for example the force is different if the observer is rotating with the earth or if he is stationary and looks at the spinning earth from a distance. And in many cases the Coriolis effect has it's origin in physical laws like conservation of angular momentum but they are not the same.

 

Anyway, it would be interesting to get a complete list of what physical laws BS simulates!

Edited February 7, 2009 by eV1Te

[Jack57]

Well without getting bogged down with semantics, it is commonly known in the helicopter world as coriolis effect, even in the text books.

 

Just to put this back on track, can anyone definitively answer the question of wether it is modelled in BS or not?

[eV1Te]

Well without getting bogged down with semantics, it is commonly known in the helicopter world as coriolis effect, even in the text books.

 

Just to put this back on track, can anyone definitively answer the question of wether it is modelled in BS or not?

 

Sorry didn't know that, I'm more of a physics guy I guess :smartass:

 

But I still think it's a good question!

Edited February 7, 2009 by eV1Te

[DTWD]

I think it's the same thing:

 

http://www.geocities.com/flyingmouse1/Chapter_2.html

 

http://en.wikipedia.org/wiki/Coriolis_effect

 

And yes to the OP I can't imagine it woulnd't be moddled, it seems quite an inportant part of the flight model.

[Yo-Yo]

I don't think that what you are explaining is called Coriolis effect but instead conservation of angular momentum (that is what makes a spinning ice skater speed up when they move their center of mass closer to the spinning axis).

 

Coriolis effect is a very weak force (usaually) that has to do with motion of something on another moving body, for example moving clouds on the rotating earth and in which direction these clouds will rotate around themselves, and this force is proportional to what frame of reference you use, for example the force is different if the observer is rotating with the earth or if he is stationary and looks at the spinning earth from a distance. And in many cases the Coriolis effect has it's origin in physical laws like conservation of angular momentum but they are not the same.

 

Anyway, it would be interesting to get a complete list of what physical laws BS simulates!

THe blades movement as well as whole helicopter movement is modelled using the general laws of movement - so all effects are taken in account.

Coriolis effect as a part of movement is a cause of blade hunting for fully articulated rotor Ka-50 has. It can be seen in BS if you slow the time while helo flies at high speed and the flapping is maximal.

 

To be correct: Coriolis effect (Coriolis acceleration or force) is a term useful in non-inertial systems and generally it is a result of conservation and inertia laws.

Edited February 7, 2009 by Yo-Yo

[Jack57]

Well I'm pleased to report that there is indeed a noticeable spooling up of the rotors when performing rapid flared turns. Not that I ever really doubted ;) I believe I'm in the habit of pulling a little collective when performing such manouvres which would of course negate any spool up - prolly learnt that flying 'Robbies' to prevent overspeed.

 

Cheers, Jack

Edited February 7, 2009 by Jack57

[Yo-Yo]

Well I'm pleased to report that there is indeed a noticeable spooling up of the rotors when performing rapid flared turns. Not that I ever really doubted ;) I believe I'm in the habit of pulling a little collective when performing such manouvres which would of course negate any spool up - prolly learnt that flying 'Robbies' to prevent overspeed.

 

Cheers, Jack

 

THe effect you mentioned is not the Coriolis effect. It is the same that autorotation is.

When you perform any flaring maneuver not only turns you increase rotor AoA so aerodynamic (AD) forces begin to spool up the rotor. Increasing pitch you change the AD conditions preventing rotor from overrev.

[Jack57]

THe effect you mentioned is not the Coriolis effect. It is the same that autorotation is.

When you perform any flaring maneuver not only turns you increase rotor AoA so aerodynamic (AD) forces begin to spool up the rotor. Increasing pitch you change the AD conditions preventing rotor from overrev.

 

You may have a good point there.

 

If the flare is sufficiently abrupt to reverse the airflow through the rotor disc then there may be an increase in rotor rpm due to blades being driven by the airflow, a la autorotation.

 

This would combine with the coriolis effect which, without question, is also present. In the manouvres I am describing the G-force exceeds 1 which is why there is coning of the rotor disc. You can see both the rotor rpm and accelerometer values in the attached screenshot (both screenshots are of the same moment). In an autorotation you would not expect the g-force to exceed one except in the final phases.

 

Increasing collective pitch increases the drag which is why it decreases the rotor rpm, thereby preventing over speed.

 

 

 

Thanks for your interest,

Jack :)

Edited February 7, 2009 by Jack57
added screenshots

[Yo-Yo]

You may have a good point there.

 

If the flare is sufficiently abrupt to reverse the airflow through the rotor disc then there may be an increase in rotor rpm due to blades being driven by the airflow, a la autorotation.

 

This would combine with the coriolis effect which, without question, is also present. In the manouvres I am describing the G-force exceeds 1 which is why there is coning of the rotor disc. You can see both the rotor rpm and accelerometer values in the attached screenshot (both screenshots are of the same moment). In an autorotation you would not expect the g-force to exceed one except in the final phases.

 

Increasing collective pitch increases the drag which is why it decreases the rotor rpm, thereby preventing over speed.

 

 

 

Thanks for your interest,

Jack :)

 

You are not right. Of course rotor cone increasing during the flaring provides an amount of moment. But only during the increasing itself. No Coriolis effect presents while the G-factor and cone shape is constant. This transition is relatively small because Coriolis velocity (radial velocity) is a product of cosine function changing at low angles that known as small. So it causes only blade hunting in vertical hinges as it happens when the blade flaps during every revolution.

There is no need to REVERSE full air flow through the disk to change the momentum balance on the rotor. Blade AoA (not pitch!) is determined by air inflow velocity (depends on ROTOR AoA and helo TAS), inductive velocity and blade pitch.

In 1-g flight rotor AoA is NEGATIVE, air inflow is from upper side of rotor but if you flare - you decrease the negative value up to positive AoA values. It causes significant blade AoA decreasing so drag moment significantly decreases too. As the engines have lag in feedback loop they can not reduce power instanteniously. That's why you encountered rotor acceleration.

 

 

If you perform flare maneuver in Ka-50 you must notice that rotor rpm increases while you pulling the stick and decreases when you stabilises in upper point. As one can see the rotor AoA at this point becomes high negative.

[Jack57]

You are not right.

 

Delivered with so much tact ;)

 

Both my text books from flight school describe coriolis effect as I do. While the principles you describe are true, they doesn't discredit the effects of the conservation of angular momentum.

 

I think it best that we leave it there, don't you?

 

Fell free to have the last word.

 

Have a nice day,

Jack :)

[Yo-Yo]

Delivered with so much tact ;)

 

Both my text books from flight school describe coriolis effect as I do. While the principles you describe are true, they doesn't discredit the effects of the conservation of angular momentum.

 

I think it best that we leave it there, don't you?

 

Fell free to have the last word.

 

Have a nice day,

Jack :)

 

Sorry if I hurt you but please proove your statements with citations from your book - it wold be interesting.

[JasBird]

I have learned aswell that coriolis effect is part of helicopter aerodynamics. Upward coning of the blades like in a flare, speeds up rotor RPM. Rotor tips "get's closer to center" just like ice skaters curl togheter when they wanna spin faster in a circle. Don't wanna argue with a developer :) but I assume the Ka-50 has such a high inertia rotor system(heavy blades) so they are very slow to react to this, and therefore not a big issue/ for the pilot?

 

Take the R22 for instance with it's light blades, it's very easy to overspeed when flaring and I belive the coriolis effect is part of that.

 

Not saying the coriolis effect is main reason for increase in rotor RPM during flare :)

 

Cheers

[sobek]

I have learned aswell that coriolis effect is part of helicopter aerodynamics. Upward coning of the blades like in a flare, speeds up rotor RPM. Rotor tips "get's closer to center" just like ice skaters curl togheter when they wanna spin faster in a circle. Don't wanna argue with a developer :) but I assume the Ka-50 has such a high inertia rotor system(heavy blades) so they are very slow to react to this, and therefore not a big issue/ for the pilot?

 

Take the R22 for instance with it's light blades, it's very easy to overspeed when flaring and I belive the coriolis effect is part of that.

 

Not saying the coriolis effect is main reason for increase in rotor RPM during flare :)

 

Cheers

 

Any rotor reacts independently of it's mass to the coriolis effect. If there would be a delay as you stated, angular momentum would be lost and miraculously regained, which is not possible. If the rotor is coned, its angular speed must increase, there is no delay.

[EtherealN]

Agreed, I'd love to have some more bytesize reading on this. Combining physics and aviation is for the win. ;)

 

Sorry if I hurt you but please proove your statements with citations from your book - it wold be interesting.

[JasBird]

Any rotor reacts independently of it's mass to the coriolis effect. If there would be a delay as you stated, angular momentum would be lost and miraculously regained, which is not possible. If the rotor is coned, its angular speed must increase, there is no delay.

 

My point was that in a high inertia rotor system compared to a low inertia rotor system, the blades change rotor RPM much slower than in a low inertia rotor system. Going back to original question if coriolis effect is programmed or not (I assume it is) since there are several factors affecting rotor RPM, it would be more difficult to tell in a high inertia rotor system. Also I guess you couldn't tell anyways, since more factors come in to play :smilewink:.

 

Cheers

[Yo-Yo]

My point was that in a high inertia rotor system compared to a low inertia rotor system, the blades change rotor RPM much slower than in a low inertia rotor system. Going back to original question if coriolis effect is programmed or not (I assume it is) since there are several factors affecting rotor RPM, it would be more difficult to tell in a high inertia rotor system. Also I guess you couldn't tell anyways, since more factors come in to play :smilewink:.

 

Cheers

 

 

I'll try to show how much will be Coriolis angular acceleration for Ka-50.

 

For example, TAS = 0, w=32.5 rad/s, the initial blade elevation angle for 1 G load is about 3.5 deg (taken from the model). As we can consider this angle proportional to the G-load the blade elevation for 3 G will be 10.5 deg. Let's leave aerodynamic forces out of consideration then keeping in mind that MOI_new/MOI_old = (cos(10.5)/cos(3.5))^2 and w_new/w_old = sqrt(MOI_old/MOI_new) and

 

w_new/w_old = cos(10.5)/cos(3.5) = 1.015

i.e. 1.5%. It is not very much, isn't it?

 

And now some values that describe power balance due to flaring...

Level flight TAS - 250 kph, power required is 2700 hp. Flaring at rotor AoA 12 deg. with the same pitch gives about 2.6 g but the power required becomes negative, i.e. all engine power + external power start to accelerate rotor.

Edited February 16, 2009 by Yo-Yo