Ka50 fidelity?

[Bucic]

@A16

Could you post a link to the bug teport?

[sobek]

Could you post a link to the bug teport?

 

Sorry, those are private.

[Bucic]

Sorry, those are private.

Ah, OK. It would be really great if moderators could pass to Yo-Yo that there's need to clarify this and that issue. It would be great if this particular one gets clarified sometime.

 

Regarding the Ka-50 fidelity, what I said before, I'm not aware of higher fidelity simulator series than DCS. If anyone's interested in the fidelity of DCS I recommend exploring the http://www.digitalcombatsimulator.com/ as well as using the forum search function > advanced search, and searching for posts/threads by Yo-Yo with e.g. failure as the keyword.

 

Lurking through the forums is also a good idea.

 

How the simulator works..

[Yo-Yo]

Ah, OK. It would be really great if moderators could pass to Yo-Yo that there's need to clarify this and that issue. It would be great if this particular one gets clarified sometime.

 

Regarding the Ka-50 fidelity, what I said before, I'm not aware of higher fidelity simulator series than DCS. If anyone's interested in the fidelity of DCS I recommend exploring the http://www.digitalcombatsimulator.com/ as well as using the forum search function > advanced search, and searching for posts/threads by Yo-Yo with e.g. failure as the keyword.

 

Lurking through the forums is also a good idea.

 

How the simulator works..

 

I only can say that the airfoil lift and drag throughout the whole range of AoAs presents as a complicated-shaped curve representing the results of wind tunnel full range AoA test of a similar airfoil (I saw in this thread a note that most of airfoils have almost similar CL and CD in post-stall region. It's true.). The rotor itself is calculated as a sum of partial moments generatined at a number of elements having their own in-world velocities, so - their own AoAs.

 

If it can accelerate its rotation - I think, the real one can do the same regarding the actual blade elements velocities, AoAs and total aerodynamic force vector components causing rotating moment.

 

The model gives accurate results in the areas we can compare with real charachteristics, so we can expect that the results will be close to real in the areas we have no accurate real data.

Edited October 17, 2013 by Yo-Yo

[Bucic]

Thank you for the clarification, Yo-Yo. I'm curious though what evidence has A16 acquired that make him think the model demonstrates incorrect behavior in some regimes.

[Yo-Yo]

So, i have one ace in my sleeve... if you take a look at the light arcraft having a free rotating rotor for lift and a prop for thrust, you can see that inital rpm required to start autorotation is low. It is enough just to push it by the hand then incoming air does the rest.

[AlphaOneSix]

All of my evidence is essentially anecdotal, since I am not an aeronautical engineer, I'm just a mechanic. The people I talk to about this type of thing are pilots and mechanics, not engineers with degrees and laboratory experience. Obviously, this sort of thing is not tested on real aircraft with live crews, so my practical experience is basically zero. I submitted my "arguments" and some tracks, and I welcome my findings being confirmed or denied. Sadly, the tracks are gone.

 

My whole argument really hinges on my assumption that the amount of total driving force on the rotor in an autorotative state may not be able to overcome the friction of the drive system if the rotor RPM drops too low. In an autogyro, there is very little friction that the lifting rotor needs to overcome. Likewise, and autogyro has a very low inertia rotor that is easy to speed back up, while larger helicopters have much "heavier" rotor systems that should require more effort to recover lost RPM. My assumptions here may very well be wrong, and I'm okay with that, since my assumptions are al based on my training and experience, which is very different from the training and experience of someone like Yo-Yo, who I would absolutely defer to in this case.

[ShuRugal]

My whole argument really hinges on my assumption that the amount of total driving force on the rotor in an autorotative state may not be able to overcome the friction of the drive system if the rotor RPM drops too low.

 

what would lead you to this assumption? The rotor system of a helicopter in autorotative state is very nearly frictionless, provided the freewheeling clutch operates correctly. There's a reason helicopters have friction brakes to set when parked. without them, the wind could very easily spin the rotors up enough to tip the bird over.

 

In an autogyro, there is very little friction that the lifting rotor needs to overcome. Likewise, an autogyro has a very low inertia rotor that is easy to speed back up, while larger helicopters have much "heavier" rotor systems that should require more effort to recover lost RPM.

 

By that same token, a helicopter rotor system can store considerably more energy, which means more can be bled out before it loses enough RPM to be capable of lifting. The greater mass of a helicopter also makes it easier to recover lost RPM by flaring. Rotor RPM increases significantly during a flare, at the cost of forward speed. More mass in the helicopter means more momentum per speed, which means more energy transfer to rotors during flare.

 

Provided you have a bit of forward speed and drop the collective in a timely fashion, maintaining rotor RPM in autorotation is child's play. It actually requires more attention to avoid an over-speed condition, which could damage the rotors either through increased vibration, reaching an RPM that is resonant with the airframe (which would result in nearly immediate catostrophic failure) or even centrifugal action becoming great enough to rip the rotors loose (see this from time to time with model helis. Seen a 600mm CF rotor blade punch a hole in 11/32 plywood at 40 feet and keep going)

 

but yeah, baring a failure in the drive system, a rotor which is in autorotative state has almost no friction.

 

 

Another obvious point, but it's much easier to slow down the rotor system than it is to speed it back up, which is how this whole discussion regarding the Ka-50 rotor dynamics came up in the first place...that is, that it's too easy to regain lost rotor RPM after dropping it to a critically low speed.

 

 

Given sufficient altitude, no RPM is unrecoverable. A real crowd-pleasing stunt in competitive model helicopter flying is a

. In this situation, the pilot sets his throttle to Idle and leave the collectives maxed, which very quickly brings the rotor RPM down to just a few revolutions per second (or even a full stop). At this point, collective is lowered again, and airflow through the blades gets RPM back up to allow a successful landing.

 

Now, granted, a model has a few advantages in this area: Virtually negligible mass in the rotor system, so it accelerates very rapidly; very high ratio of rotor area to weight, requiring less RPM to produce working lift; wider collective pitch range (most competition-setup models are set for +15 to -15 degrees. Even my old Raptor .30, which is only a light sport flyer at best, can swing +-10 with no trouble). A full-stop autorotation in a full-scale helicopter would require more altitude to recover from than most helicopters are capable of flying.

 

Still, there is no reason a drop to 50%, or maybe even 30% RPM should be unrecoverable, given adequate altitude AGL to re-accelerate the rotors.

Edited October 18, 2013 by ShuRugal

[sobek]

what would lead you to this assumption? The rotor system of a helicopter in autorotative state is very nearly frictionless, provided the freewheeling clutch operates correctly.

 

Far from frictionless. The freewheeling clutches are situated just after the engines in the drivetrain, that means that the main gear box with all the accessory gearboxes attached to it has to be driven by the rotor, that includes the hydraulic pumps and the main generators.

[Bucic]

And yet 'irrecoverable RPM' is mentioned all over avia docs, probably including accident reports. Rather confusing... I'd imagine that for classic design helicopters there is RPMcrit for which Tfrict>Twindmill, so that RPM won't increase once below RPMcrit. At unfavorable flight trajectory that is - see Note1. This could be explained by some components of Tfrict~RPM^2 and airfoil sections not getting out of the miserable Cl/Cd region.

 

The question remains - what is so different in this regard in Ka-50 apart from lack of tail royor?

 

Another possibility is that an ambiguous 'irrecoverable RPM' term is used. After years of working with Western documentation I wouldn't trust the term at all to mean what it seems to mean.

 

Then there's also RPMcontrol below which you loose controllability, e.g. you won't be able to pitch down by pushing the stick forward.

 

Note1:

Blade section AoA (pitch, RPM, 'rotor disc AoA')

 

Note2:

Freewheeling clutches disengage only engines, AFAIK. The whole main gearbox as well as tail rotor are still driven.

 

RC models are completely out of place here, as are autogyros, Yo-Yo :)

Edited October 18, 2013 by Bucic

[sobek]

Maybe irrecoverable simply means that under all circumstances that can occur during a typical helicopter flight profile, RPM recovery would take so long that by the point where the rotor starts producing significant lift again, the flight trajectory will inevitably end in a ground collision.

[Bucic]

Gotta love Western technical writing...

 

What you wrote totally makes sense.

Here however it seems both terms have been distinguished and it seems it says something about regaining RPM specifically, not ambiguously.

The reaction to this low rotor speed warning must be timely and rapid - in the order of two seconds - since decayed rpm is not always retrievable, and rotor rpm below the minimum for controlled flight is irrecoverable.

http://c66.203.200.38.tidc.telus.com/eng/rapports-reports/aviation/2005/a05f0025/a05f0025_sec1.asp

Edited October 18, 2013 by Bucic

[sobek]

Hmm, well IMHO that just hints at loss of control (loss of cyclic effectiveness and therefore an uncontrollable airframe), not per se that the rotor can't be sped up under *any* circumstances (you won't have any say in it any more though because you have no control over attitude).

 

One issue i just discovered in a test flight with the BS is that, for me, it is next to impossible to significantly underspeed the rotor since everytime i tried, a blade intersection ensued.

[Yo-Yo]

Note2:

Freewheeling clutches disengage only engines, AFAIK. The whole main gearbox as well as tail rotor are still driven.

 

RC models are completely out of place here, as are autogyros, Yo-Yo :)

 

If friction losses are so high you never cool oil from the reductor gear... you must understand that in the model we use known efficiency figures for real helicopters.

 

RC models and autogyro ARE IN THEIR RIGHT PLACE HERE because somebody was in doubt if AoAs are correct to accelerate rotation. THis examples show that AoA in RL is sufficient to speed-up the rotor.

 

All recomendation to avoid rpm drop are due to side effects of it - increasing of cone angle, loss of contrallability, lost hydraulics and power supply, etc.

 

Moreover, it's a way better for pilot to control a helicopter at known conditions than to try to catch new control charachteristics due to low rpm.