Why is the main rotor slowing down and the helicopter stalling in the ground sometimes?

[AeriaGloria]

1 hour ago, AlphaOneSix said:

I have a theory.

 

You say that the rotor stops (or rather, that the rotor speed goes fully down?), but in the picture the rotor is spinning. Does the rotor stop spinning? Or are you assuming that the rotor rpm gauge is reading zero? Because the rotor rpm gauge appears to be pegged at 110%, not zero.

 

Funny enough the exterior view control bar shows 3% rpm, I assume it’s showing engine rpm.

Jetcat, the pictures help but ultimately we can’t tell anything from the pictures unless there’s a video or track of yours along with it so we know what conditions are happening.

in addition I added a picture of the same engine from the Black Shark manual, it’s the same exact variant of engine, the teal/light blue part on the far right is the turbine and driveshaft going to the gearbox that powers the rotors, as you can see in the color coded diagram it is not connected to the turbines of the engine, but spins freely in the engine exhaust

Check if you have birds on in settings, make sure dust protection is on, and record a track or video when you have a chance 

[AlphaOneSix]

Well the text he wrote and the pictures don't jive, in my opinion. I know exactly why and how this happened, but only if it's true that the rotor did not stop turning, at least not initially. I'm looking for a rise in rotor rpm that pegs the needle at 110% following a reduction in collective pitch as well as raising the nose of the aircraft with the cyclic. If that's the case, I have the answer, I just need that confirmation because without it I could be wrong.

[admiki]

Good catch on 110%. Looking at his pictures, rotor seems to be spinning.

I'm interested why engines would flame out?

 

Edited November 9, 2021 by admiki

[sLYFa]

8 hours ago, admiki said:

'm interested why engines would flame out?

 

 

FT overspeed protection

[AlphaOneSix]

1 hour ago, sLYFa said:

FT overspeed protection

Yes, this.

[admiki]

What is FT?

[admiki]

OK, so I did some testing. VRS (I don't think OP was in VRS since it's absolutely hard to control from all the bucking it does) and 200 kgs over MGW autorotation both pegged RRPM at 110% and I did not  get flameouts. I don't think we will solve this without track file.

[sLYFa]

4 hours ago, admiki said:

What is FT?

FT stands for free turbine. Its the last two turbine stages that are connected to the gear box. The idea behind FT overspeed protection is that if for some reason the mechanical link between FT and gearbox breaks, the FT will start to start spinning uncontrollably fast and desintigrate, sending turbine blades through the whole engine compartment. To prevent that, each engine has two FT overspeed sensors that trigger above a certain rpm threshold and shut off fuel supply to the affected engine. Now you can also trigger that with insanely high rotor rpm, since FT and rotor are mechincally linked through the gearbox. OPs rotor rpm was pegged at 110% but the actual rotor rpm could've been even higher, triggering the fuel shut off. 

[admiki]

Yeah, have no idea why I didn't figure that one on my own.

Speaking of connection between rotor and free turbine, aren't those connected via sprag clutch? Hence rotor overspeed would not affect free turbine?

[AlphaOneSix]

1 hour ago, sLYFa said:

each engine has two FT overspeed sensors that trigger above a certain rpm threshold

116% plus or minus 2%

 

42 minutes ago, admiki said:

Speaking of connection between rotor and free turbine, aren't those connected via sprag clutch? Hence rotor overspeed would not affect free turbine?

Yes there is a clutch, but that is to prevent driving the N2 turbine when the engine isn't running. When the engine is running, the N1 RPM (nearly 20,000 rpm at 100% N1) is way higher than the N2 RPM (15,000 rpm at 95% NR), so if the rotor speeds up, the N1 will push the N2 higher to keep up. 

In other words, the main gearbox isn't driving the N2 RPM, the engines still are. The engines are always trying to get the N2 to match their N1, and if the rotor speeds up too much, they will succeed (or at least succeed enough to trigger an FT overspeed condition).

Edited November 10, 2021 by AlphaOneSix

[Funkysak]

I think it is safe to say the issues are  an operator malfunction. And no fault of the Mi-24 its self.

[AlphaOneSix]

Yep, lowering the collective and bringing the nose up are a sure fire way to spool up the rotor. 

[admiki]

But, pitch gauge is showing 10 degrees of pitch

[AlphaOneSix]

3 hours ago, admiki said:

But, pitch gauge is showing 10 degrees of pitch

This doesn't seem relevant without knowing what it was before the problem occurred. He said he lowered the collective and that's all that's required to have a rise in rotor speed.

[Gunnar81]

On 11/6/2021 at 3:02 PM, AeriaGloria said:

I fly level with full collective all the time. It just takes almost 10 degrees nose down pitch. Doppler never has a chance to work. 

, Poor Mi-24!! Would you give the beast a break!! Have some compassion!@!

Edited November 11, 2021 by Gunnar81

[Funkysak]

This line of discussion is why I love DCS so much, the level of realism put into the official modules is so good. Helicopters can be very "frustrating" in DCS until you learn about ground effect and transitional flight and effects like vortex ring state and how torque and weight effect your virtual helicopter. Chucks Guides at Mudspike are a good place to start. Learn what your gauges mean and what they are telling you. There are also great videos on YouTube from DCS players that also fly or have flown real helicopters that explain the physics going on.

 

[AlphaOneSix]

Here is a picture of the FT overspeed sensor, with the white ceramic potting compound. The second one is right above it but that part is cutaway so they only have one visible. There are two more on the other side of the shaft but they are spares and an electrical connector must be moved to enable those two.

[AeriaGloria]

9 hours ago, AlphaOneSix said:

Here is a picture of the FT overspeed sensor, with the white ceramic potting compound. The second one is right above it but that part is cutaway so they only have one visible. There are two more on the other side of the shaft but they are spares and an electrical connector must be moved to enable those two.

not to derail this thread but until we get response from OP what are the gears sending power vertically for? Driving the oil cooler turbine or accessory gearbox? 
 

EDIT: Nevermind. Looking at your excellent pdf on Mi-8 systems that also has a section for TV3-117 engines, I see it is the external accessory gearbox And FT speed governor with PTIT sensor? I know this is VM variant whereas Mi-24 has VMA, but this is Very interesting 

Edited November 13, 2021 by AeriaGloria

[AlphaOneSix]

1 hour ago, AeriaGloria said:

what are the gears sending power vertically for?

The gears are used to transmit the Free Turbine rpm to the engine fuel control. In this manner, the engine fuel control knows the rotor speed, and thereby knows if it needs to increase fuel flow to increase engine power and therefore rotor rpm, or decrease fuel flow to reduce engine power and therefore rotor rpm.

The accessory gearbox is on the front of the engine and is not visible in this picture.

The PTIT probes are not really visible in this picture either. There are several (12 or 14?) PTIT probes that are installed right between the combustion can and the compressor turbine.

There is very little difference between a VM and a VMA. It's basically just better cooling that allows more power to be produced at the top end.

[rayrayblues]

Still no track.     

[b1n4ry]

Its sounds to me, that he was experiencing rotor stall. By Pulling up the nose, and descending to fast, he increased the AoA on the blades so high, the he entered an uncontrolled rotor stall.

I am not an expert on this, so take it with a grain of salt.

[erniedaoage]

I don't know as much technical details as most of you guys, but nobody mentioned the speed gauge between 400-450 in those 2 cockpit screens he posted and i think thats quite too high. I got the same experience in 2011 when i got the ka50 and was doing some high speed low level flying and overspeed the aircraft till i fell out of the sky quite quickly. 

[Threeps]

On 11/7/2021 at 9:58 AM, Sabre_Ewan said:

Earlier in the thread you mentioned having full collective (rotated). The collective control isn't the rotation, it's the up & down movement.

The rotating part is the throttle, and that should (in all normal conditions) be left at full while you only interact with the collective.

You aren't somehow getting the 2 mixed up and trying to fly with the throttle are you?

This is what I suspect is happening as well.    The throttle should be at 100% during flight  for most conditions.  I think he's either mapped the two together on the collective up / down axis or  he's twisting the throttle as a part of in-flight controls.  

[itstimmeh]

On 11/7/2021 at 11:01 AM, admiki said:

No. VRS will not cause engine flameouts.

Hi

This is slightly OT but it is possible (although I highly doubt it is modelled in DCS) for aircraft with high-mount engines (like the Mi-24) to suffer compressor stalls during VRS incidents; the oxygen-free exhaust gas is recirculated around the rotor disc and fed back into the intakes.

It's rare and almost always unrecoverable.

I agree with others that an overspeed is the most likely candidate for engine shutdown, if it's not a control issue. Again, OP needs to provide a track for us to look at.

 

All the best

Timmeh

 

[itstimmeh]

On 12/21/2021 at 3:33 PM, erniedaoage said:

... when i got the ka50 and was doing some high speed low level flying and overspeed the aircraft till i fell out of the sky quite quickly. 

 

Hi

Exceeding max IAS in flight will absolutely cause a retreating blade stall; bad for any helicopter, however coaxial systems are even more susceptible to it.

In the real world you'd also be (potentially) destroying your blades as the tips reached their mach limit.

 

All the best

Timmeh