The amazing exploding KA-50

[deltatango]

I am getting [seemingly] random crashing of the helicopter. One minute I'm flying along happily, the next minute - BANG! An explosion, followed by uncontrollable roll and soon after, a crash into the ground. I've checked that nothing is around to shoot at me and a couple of times, I paused it after the bang, for a look at the outside. All I see is the Rotor blades seem to be half their original length.

 

I thought I might have been overstressing the aircraft, as the first time it happened, I had been following the other chopper on the Campaign mission #1 for ages and doing heaps of overspeed and rotor RPM warnings. But it has since happened without ever exceeding any limits.

 

Any ideas?

[GGTharos]

Fly slower. It's not a bug, it's your flying ;)

[Panzertard]

Thats what the warnings are designed to warn you about- getting outside the envelope. Don't :)

And dont follow the AI choppers - they "cheat" a little with the physics. :)

[EtherealN]

Right turns and right rudder especially can cause rotor disc intersection even if you are not at overspeed.

 

If you want to check it out, fly the chopper to a good speed (but not overspeed, 250-280km/h will do) and then switch to outside view. You can then see the discs being real close to each other. Now punch right rudder hard and watch yourself fall out of the sky.

 

Repeat it but punch left rudder - you should still be okey.

 

This is one of those things that takes a little extra getting used to with co-axial designs like the Kamov buerou is so fond of.

 

Though, Panzertard, on following other choppers: in that mission your mission is explicitly to follow that chopper. However, it's an Mi-24 (so no co-axial problems for it) and the highest speed it requests if memory serves is about 250km/h. If he ends up getting RPM and overspeed warnings on that mission he's doing something real strange and unecessary.

Edited June 7, 2009 by EtherealN
fixed a missing letter

[Distiler]

mhh wasn't left turn or right rudder that made higher chance of blades collide?

[EtherealN]

Right turn and right rudder is the dangerous one, since it causes the blades to come closer from the differential inputs to achieve those moments.

 

Generally speaking, when in high speed or performing very drastic maneuvers (or stupid suicidal aerobatics like wingovers), left turn is to be preferred, since left turns cause the discs to separate further.

[deltatango]

Thats what the warnings are designed to warn you about- getting outside the envelope. Don't :)

And dont follow the AI choppers - they "cheat" a little with the physics. :)

 

No, That's what I'm saying. There was NO warnings and NO limits exceeded. No high G, High roll rate or high alpha-dot. It just broke.

 

As someone else was saying, the blades do come awfully close on the right side in high speed cruise. I took a coupla screens during one mission that show my blades doing this, but my 3xAI flight member's blades were parallel. As someones else said - the AI seem to cheat a little.

 

Maybe this was what happened to me. Is there any warnings in the flight manual that advise against certain parts of the flight envelope?

 

I'm off to do some test flying to see if I can replicate the problem reliably. I'll report back findings...

[Panzertard]

Ah, sorry about that - a bit tired last night. I mistook the "I had been following the other chopper on the Campaign mission #1 for ages and doing heaps of overspeed and rotor RPM warnings" as the norm for how you fly and flew that night :)

[deltatango]

test results.

 

Ok after extensive (well 20 minutes and about 25 flights) the results are in:

-Flights conducted with full fuel, Vikrs outboard and S-8s inboard. Altitude sea level to 1000m.

-As mentioned above, right turns are the danger.

-A right turn with low rotor rpm (anywhere < 85%) will cause the blades to clip every time.

 

Scenarios that cause this.

1. High speed right turn with collective in same position as cruise. So if you are crusing along near Vne and you break right: BANG.

 

2.Any sharp right turn, anywhere from hover to Vne, while pulling max (or just high) collective, especially if it results in the low rotor RPM warning going off. At higher speeds just right cyclic will do it, but as the speed decreases, added right pedal helps the blades meet. This exactly what you do anyway. Ie: low speed maneuvers require more pedal. More pedal and less right cyclic will also do it.

 

3. A right turn during a descent, as you are leveling off (and so pulling on more collective). An example of this is when you descend towards a valley floor, then level off and turn to follow the valley. You must keep your rate of descent low to avoid low rpm. This can be a problem if the valley walls are steep and you are flying NOE. Vortex ring is not a problem if you have forward speed.

 

EDIT: I forgot to mention about the Flare dispenser. By default, flares are dispensed from the left side only. So a couple of times when I have seen a missile launch, I have dumped flares and Broken right (away from the flare). I was wondering how I still got hit so quickly. Of course this was the blades hitting - not me getting hit by the missile. So from now on, I shall set the flare dispenser to the right side first and break left.

 

cheers,

DT

Edited June 7, 2009 by deltatango
added note about flare dispensers

[EtherealN]

No, That's what I'm saying. There was NO warnings and NO limits exceeded. No high G, High roll rate or high alpha-dot. It just broke.

 

Yeah, this is the specific danger of co-axial helicopter designs. If you were to set the warnings to go off early enough to cover every possible scenario where a rotor disc intersection can happen, you would be having such a high margin that the specific advantages of the co-axial design would be nullified. For this reason there is a heavy weight put upon pilot training.

 

And in case you are wondering, the specific gains are in fuel efficiency - a conventional helicopter "wastes" about 30% (if memory serves) of it's engine output purely on the tailrotor, whereas a coaxial can use 100% of it's engines towards gaining lift, therefore being able to use less power in cruise and gaining a longer action radius. There is also a theoretical benefit in top speed since you do not have the problem of the retreating side of your rotor disc giving less lift due to your airspeed. Someone with better knowledge of specific aerodynamics would have to chime in on that though, since my knowledge of aerodynamics do not allow me to pass a judgement on the design compared to - f.ex - the Lynx's design which is also a very high-speed solution.

 

But a very simplified version of it is that if you are moving forward at 300km/h airspeed and your blades are rotating clockwise, and (number out of my arse just for illustration) the airspeed of the blades at the tip being 600km/h, you will then have an effective aerofoil speed on the left side of 900km/h, and on the right side 300km/h. The differential there would require a huge cyclic input or trim to keep the lift on each side even. The co-axial design, having two rotors on the same axis moving in opposite directions, will have the same differentials but with opposite force, meaning the lift on each side will be equal. (There's some real difficult maths to do with the distance from centre-of-gravity of the two discs on that front, and I cannot master those equations.)

 

However, since there is differential lift on each side from each rotor disc, they will start moving towards each other as airspeed increases. In the case of the Ka-50, this happens on the right side.

 

And this is where it begins to turn dangerous. On a conventional helicopter you will induce yaw (the movement of a tailrudder on a fixed-wing, or turning on the horisontal) through adding or reducing the force of the tailrotor. However, the coaxial doesn't have one. If it had one, the point of the coaxial would be lost. So what it does is that it changes the collective setting of the two discs, reducing one and increasing the other. This causes the total resistance to their movements through the air to become differential, which induces a yawing motion. But in the case of a right turn, it also causes the discs to move closer, since the lower rotor is at a higher angle on the right side (more lift) and the upper rotor is at a lower angle (less lift).

 

So because of all that, even at relatively low speeds a sudden and sharp right rudder input can cause the rotors to intersect even though there is no previous warning from pure overspeed - technically it's not overspeed, it's pilot error in the control inputs. For this reason, I have been made to understand that the russian Navy (the traditional client of Kamov) and the russian armed forces operating the Ka-50 and Ka-52 only put very seasoned helicopter pilots in charge of the coaxial helicopters.

 

An interesting note, by the way, is that the design also allows the helicopter to be smaller, since you do not need a tailboom long enough to separate the non-existant tailrotor from the main rotor hub. This might explain why, in the soviet and later russian navies, the Kamov buerou was so popular while in the army Kamov had a much harder time.

 

As someones else said - the AI seem to cheat a little.

 

Yeah, the AI uses a simplified flight model. While it would have been desirable to have the same detail in the AI flight model, it just isn't feasible on todays computers. Imagine running all those computations, constantly, for every single Kamov in the air in the scenario. No normal consumer would be able to afford the computing power required for that.

 

Couple of years down the line, though, who know. :)

 

Maybe this was what happened to me. Is there any warnings in the flight manual that advise against certain parts of the flight envelope?

 

In the absense of tracks, I believe it is what happened to you. It fits descriptions I've seen before receiving tracks from other people.

 

I am however unsure of what the manuals state about the topic. I never checked it out due to being familiar with it already. (The Ka-50 has been my baby since the mid-90's, and the Hormone has been a love of mine as well.)

[Panzertard]

As Ethereal put it - if you need to break, break to the left.

If you intend to barrel roll - do it to the left. All in the sake of keeping the blades separated.

Edited June 7, 2009 by Panzertard

[EtherealN]

On the note of barrel rolls, btw:

 

Get 220-240km/h airspeed (NOT groundspeed, ignore HUD). Gently pull back to gain 30 degrees nose-up attitude, at this point apply gentle left cyclic until you have roughly 45 degree bank. Now apply full left cyclic, pull gently back, and give it a gentle left rudder to help keep it clean.

 

Maintain until the barrel roll is complete.

 

A note: if you gain more airspeed than that, you are liable to get rotor clash when inverted unles you are really good. If you get less airspeed than that, getting inverted will cause you to fall out of the sky - inverted. That is not the time where you can rescue yourself with the ejection seat. ;)

[deltatango]

it's still doin' it

 

Ok, I've just successfully completed the epic campaign mission that involves flying over the snowy mountains to take out enemy supply depot (artillery and cargo) and then flying back. As I was climbing away from the target area on the way home - BANG. Blades julienne...

 

I immediately paused the sim to take down the parameters, which were:

-Climbing LEFT turn. Not aggressive.

-High collective setting (at upper detent on Saitek X52 Pro)

-Airspeed 200km/hr (no warning tones heard)

-Groundspeed (in HUD) +251

-Altitude 2797m

-Nr ~98% (no warning tones heard)

-Blade angle 11.5

-Fuel Aft 3.2, Fwd 3.8. No bullets, no Vikrs, about half S-8s fired.

-Pitch, Bank & Heading buttons engaged. FD AP disengaged. Route mode not engaged.

 

So why is it still doing it?

[Panzertard]

Hm, %98 rotor rpm - is that what you are saying?

When you throttle up - do you PAGEUP twice, or three times when you start the bird? Just thinking - maybe you are using max throttle setting, which may cause it?

 

I've never used max except in a couple of emergiencies .. and always been flying slow at that moment.

[159th_Viper]

Ok, I've just successfully completed the epic campaign mission that involves flying over the snowy mountains to take out enemy supply depot (artillery and cargo) and then flying back. As I was climbing away from the target area on the way home - BANG. Blades julienne...

 

I immediately paused the sim to take down the parameters, which were:

-Climbing LEFT turn. Not aggressive.

-High collective setting (at upper detent on Saitek X52 Pro)

-Airspeed 200km/hr (no warning tones heard)

-Groundspeed (in HUD) +251

-Altitude 2797m

-Nr ~98% (no warning tones heard)

-Blade angle 11.5

-Fuel Aft 3.2, Fwd 3.8. No bullets, no Vikrs, about half S-8s fired.

-Pitch, Bank & Heading buttons engaged. FD AP disengaged. Route mode not engaged.

 

So why is it still doing it?

 

 

Post a Track for a detailed/accurate autopsy :)

 

That said - having regard to your above text, the 'High Collective Setting', 'Groundspeed' and 'Blade Angle 11.5' are all indicative of a Helo that has just about had enough and decided that you're better off walking home.......

[deltatango]

Hm, %98 rotor rpm - is that what you are saying?

Yep. On the Rotor RPM gauge near the landing gear indicator lights. So 98% = between the two red lines on the gauge.

 

 

When you throttle up - do you PAGEUP twice, or three times when you start the bird?

No. I always have the throttles to auto. I have them mapped to 2 axes on my Saitek Pro Throttle quadrant (I use an X52 pro for the collective).

 

I will post a track soon.

[Panzertard]

I would say 98% sounds a bit high - 85% - 90% IMO is more normal in that configuration you were flying, but I could be wrong.

 

And what Viper said - a .trk file would be nice. You should find the latest .trk files in your game \Eagle Dynamics\Ka-50\Temp folder. Not sure if it cleans out after you end the missions - worth checking.

[deltatango]

That said - having regard to your above text, the 'High Collective Setting', 'Groundspeed' and 'Blade Angle 11.5' are all indicative of a Helo that has just about had enough and decided that you're better off walking home.......

 

That 11.5 was about a second after the BANG, when I had the presence of mind to hit PAUSE. So the engine power indicators were also indicating zero at that time. I don't know if the blade angle would have also instantly changed in response to the blades being all of a sudden half their original length.

[GGTharos]

No, the blade angle responds to the collective alone, IIRC - that gauge is the measure of your collective setting.

 

Your airspeed is too fast. 130kph is the best airspeed for a climb. The faster you go, the closer the discs get...the thinner the air, the closer the discs get ... etc.

[EtherealN]

Wait, am I getting something backgwards here? Shouldn't thinner air mean there is less lift on the blades, thereby causing the lower disc to rise less and thus they shouldn't get as close?

[GGTharos]

Less lift means more collective ;) - I left that assumption out though, sorry.

[EtherealN]

Oh, yes, of course. I should have made the connection.

[GGTharos]

Climbing leads to thin air.

Thin air, leads to less lift.

Less lift leads to using more collective.

More collective ... to the dark side it leads.

[159th_Viper]

More collective ... to the dark side it leads.

 

:megalol::megalol:

[deltatango]

Ok,

I've uploaded a TRK file. It's recorded in the same location that I had the problem with left turn before. Its shows a couple of left and right turns, before the blades meet in a nose up manueuvre. It was recorded with TrackIR and I have noticed that if you manually change the view, it doesn't come back to the right spot in the cockpit.

 

What would be good is some kind of graph in the Flight manual that shows areas of the flight envelope to be avoided. The graph would show airspeed vs altitude or collective position. Because there must be parts of the flight envelope where full or at least high collective settings can be used.

 

This links shows such a chart for a KingAir (U-21G) http://www.tpub.com/content/aviationmaintandmisc/TM-55-1510-215-10/TM-55-1510-215-100159.htm

 

Although this one is for a fixed wing aircraft, similar charts exist for all rotary wing aircraft, usually to show the 'deadman's curve' - an area which recovery from autorotation is not possible.

high alt crash.trk