Question about autorotation for helicopter professionals

[Pirke77]

 

THE TRUTH IS SOMEWHERE OUT THERE . . . :alien:

[Vecko]

This thread seriously questioning DCS epithet "simulation" and superior physics and flight dynamics.The main impression is that developers just won't answer ...:huh:

Edited October 26, 2010 by Vecko

[Mikoyan89]

then no answer? the matter is closed ?:dunno:

[WynnTTr]

This thread seriously questioning DCS epithet "simulation" and superior physics and flight dynamics.The main impression is that developers just won't answer ...:huh:

Superior to waht's out in the market? Most definitely. Name the next helicopter simulator out there that comes close.

 

Would you rather fly FSX helicopters?

[Panzertard]

This thread seriously questioning DCS epithet "simulation" and superior physics and flight dynamics.The main impression is that developers just won't answer ...:huh:

Perhaps they are busy doing something more important than surfing on the internet all day, or reading all the threads on this forum. ;)

Seriously, I hope they are busy with DCS :)

[Bucic]

Extremely interesting. Any news on the subject?

[Zolitnitsky]

Any change in BS2? Just asking, as it seemed in need of revival! (Or in case I've missed some more rescent thread on the subject somewhere ells).

[ShuRugal]

Any change in BS2? Just asking, as it seemed in need of revival! (Or in case I've missed some more rescent thread on the subject somewhere ells).

 

 

there's an easy way to find out: grab some altitude, max the collective, and shut your fuel valves off.

[AlphaOneSix]

there's an easy way to find out: grab some altitude, max the collective, and shut your fuel valves off.

 

Just tested this. The helicopter explodes on impact.

[Dejjvid]

Didn't Yo-Yo write in another thread that rotor had to much weight and that it is adjusted in 1.2.0?

[Vecko]

Just tested this. The helicopter explodes on impact.

 

Sure thing...But the choper is still very governable even on extremely low rotors RPM. It is impossible to lose lift completely and to drop from the sky like rock. According to Pirke, this should not suppose to happen, but he isn't crazy enough to test this in the RL... ;)

[ShuRugal]

Sure thing...But the choper is still very governable even on extremely low rotors RPM. It is impossible to lose lift completely and to drop from the sky like rock. According to Pirke, this should not suppose to happen, but he isn't crazy enough to test this in the RL... ;)

 

 

I know that with model helicopters it is indeed quite possible to stop the rotors in autorotation, and even start them again, given sufficient altitude. But then, the rotors on a model have considerably less mass, considerably lower moment arms, and considerably higher ranges of collective pitch.

 

On a full-scale helicopter, i would not be the least bit surprised to find out that it is impossible to drop the rotor RPM to a point where no control authority exists.

Even if it were aerodynamically possible, there is so much energy stored in a full-size helicopter's rotor system that i doubt you could get it high enough to bleed the RPM down to zero on drag alone before hitting the ground.

Given the way full-size heli rotors are washed out (different pitch at root than at tip) i would not be shocked to learn that it is impossible for them to stop completely so long as air is flowing through them.

 

In fact, i'd bet dollars to donuts that even in a straight-down autorotation with the collective pitch at maximum, the rotors would not slow down lower than about 30% max RPM due to vortex ring state.

[Griffin]

I don't think anyone has been saying about completely stopping them.

But if pilot doesn't act correctly, there is a point after which the RPM can not be restored and is fatal in real life. Helicopter professionals here have said that the DCS Ka-50 is way too forgiving.

[AlphaOneSix]

Vortex ring state is not possible in unpowered flight, such as during an autorotation.

[Dejjvid]

Feels like you drop way faster now. Could be a consequence of the pitch angles too. Gonna test autorotations this weekend and see how it performs.

[ShuRugal]

Vortex ring state is not possible in unpowered flight, such as during an autorotation.

 

my Raptor has a dent in the tailboom which begs to differ.

[AlphaOneSix]

my Raptor has a dent in the tailboom which begs to differ.

 

Okay. I'm just saying that "autorotation" and "vortex ring state" are mutually exclusive.

 

Vortex ring state requires that some minimum amount of power available (between 20% and 30%) is being used to drive the rotor.

 

Autorotation, by definition, implies that there is 0% power available.

 

You simply cannot have both at the same time.

[Yo-Yo]

Okay. I'm just saying that "autorotation" and "vortex ring state" are mutually exclusive.

 

Vortex ring state requires that some minimum amount of power available (between 20% and 30%) is being used to drive the rotor.

 

Autorotation, by definition, implies that there is 0% power available.

 

You simply cannot have both at the same time.

You are definetily right. I guess that VR state could appear only at the moment you apply collective to flare - the power source would be rotor inertia. But it is uncertain because VR requires some time to stabilise and the time rotor has enough energy can be insufficient to start VR.

[Yo-Yo]

Didn't Yo-Yo write in another thread that rotor had to much weight and that it is adjusted in 1.2.0?

 

I did not. THe rotor was heavier than necessary but it means that the initial blade pitch when collective is at zero was higher than necessary. The rotor mass or weight was not changed at all because it right.

[Dejjvid]

I did not. THe rotor was heavier than necessary but it means that the initial blade pitch when collective is at zero was higher than necessary. The rotor mass or weight was not changed at all because it right.

 

Ok, then i misunderstood your post.

 

While we're talking rotors i have another question you might be able to answer Yo-Yo.

 

I closed both cut-off valves and applied the rotor brake instantly, rotor rpm was around 80%. At first the rotors started to slow down, but after a few seconds the brake stopped working. And after 2min my rotors were still rotating slowly, and it didn't matter if i engaged or disengaged the rotor brake. Is it modelled that you can destroy the brake?

 

(I know it isn't good to engage the brake over 20%)

Edited July 27, 2012 by Dejjvid

[Yo-Yo]

Yes, you burnt the brake

[Kenta]

(I know it isn't good to engage the brake over 20%)

 

Actually... the break can be used when rotors are at 30% rotation and below.

[Mikoyan89]

@Yo-Yo

I'm a bit confused...you said that rotor was heavier than necessary,then you stated that rotor weight wasn't changed because it's right...

But you also said:

THe rotor was heavier than necessary but it means that the initial blade pitch when collective is at zero was higher than necessary

so i assume that heavier doesn't mean "more weight" but "higher blade pitch"....is it correct?

If so,to fix this,developers adjusted the blade pitch resulting in more collective input needed for every maneuvre from take off to landing (noticed by the blade pitch indicator),but nothin else has been touched in the FM,is it correct?

[ShuRugal]

Okay. I'm just saying that "autorotation" and "vortex ring state" are mutually exclusive.

 

Vortex ring state requires that some minimum amount of power available (between 20% and 30%) is being used to drive the rotor.

 

Autorotation, by definition, implies that there is 0% power available.

 

You simply cannot have both at the same time.

 

 

I haven't the numbers to do the math to back it up, but i should think that the energy stored in the turning rotors would be more than up to the task of providing 30% power output long enough to enter VRS.

 

Strictly speaking, the air being moved by the rotors shouldn't give a rat's ass whether they are under power from the engine or not; If they are rotating within the operational range, have sufficient angle of attack, and have significant upflow, a central vortex will form. The air doesn't know whether the rotors are turning at X RPM because the engine is running or because there is enough inertial energy in the blades that they haven't dropped below X yet. If there's enough energy in the rotors to lift the helicopter, there is enough to enter VRS, regardless of the source of that energy.

 

Now, whether or not VRS would cease after the rotor RPM dropped, i have not experimented with and cannot say for certain, but as VRS is a self-perpetuating condition, i would be surprised if it did.

[AlphaOneSix]

The rotor blades don't continue to turn during autorotation because of blade inertia, they continue to move because the total aerodynamic force points forward of vertical. Also, during autorotation, all of the airflow comes from beneath the rotor, and travels up through the rotors. It is impossible for vortices to "stick" to the rotor during an autorotation do to this upward flow.

 

During vortex ring state, the downwash flies right back up through the rotors, but is then pulled back down through the rotors. This does not occur during autorotation.