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

[JetCat]

Hi flying aces,

while enjoying the Hind I have noticed a strange behaviour: Sometimes when maneuvering or even when flying straight ahead very fast and doing (light) maneuvers with high speeds, the main rotor suddenly slows down, continues to get slower and the helicopter is crashing uncontrollable into the ground.

The engine setting in this case when it happens is always full turbine torque power (page up or mouse action on the pitch lever) plus collective pitch lever fully raised.

Is this an engine failure caused by overstressing the turbines which cannot bring enough torque on the main rotor anymore because of too much collective and therefore flame out when doing funky flight maneuvers?

Have a beautyful weekend and good flight

[MAXsenna]

Does it turn over to the right when it happens? If yes you are experiencing RBS, retreating blade stall.

Sent from my MAR-LX1A using Tapatalk

[admiki]

1 hour ago, JetCat said:

Hi flying aces,

while enjoying the Hind I have noticed a strange behaviour: Sometimes when maneuvering or even when flying straight ahead very fast and doing (light) maneuvers with high speeds, the main rotor suddenly slows down, continues to get slower and the helicopter is crashing uncontrollable into the ground.

The engine setting in this case when it happens is always full turbine torque power (page up or mouse action on the pitch lever) plus collective pitch lever fully raised.

Is this an engine failure caused by overstressing the turbines which cannot bring enough torque on the main rotor anymore because of too much collective and therefore flame out when doing funky flight maneuvers?

Have a beautyful weekend and good flight

you said you are doing high speed "light" maneuvers? What does it mean exactly? Does at any point helicopter wants to pitch up on you?

[JetCat]

No I was just trying to fly a curve (or turn) to the left or the right while 200km/h or 250km/h. Not a really sharp turn. Or when I brake the helicopter by pulling up and reducing collective pitch.

Occasionally the rotor begins to make a strange "shii shiiii shiiiiiiii shiiiiiiii" sound and is suddenly getting slower, sometimes the Hind recovers and returns to normal flight behaviour but sometimes the rotor is getting slower and slower and the helicopter is literally falling out of the sky.

This behaviour is truly interesting, a very sophisticated physics and flight dynamics engine and I assume the real Hind has done the same thing - but why is it doing this? Is the huge multi-bladed main rotor overpowering the torque of the turbines in this case when doing a maneuver while being too fast, what is happening?

[gnomechild]

A track replay or a video would be very helpful to see what the issue is

[cw4ogden]

1 hour ago, JetCat said:

No I was just trying to fly a curve (or turn) to the left or the right while 200km/h or 250km/h. Not a really sharp turn. Or when I brake the helicopter by pulling up and reducing collective pitch.

Occasionally the rotor begins to make a strange "shii shiiii shiiiiiiii shiiiiiiii" sound and is suddenly getting slower, sometimes the Hind recovers and returns to normal flight behaviour but sometimes the rotor is getting slower and slower and the helicopter is literally falling out of the sky.

This behaviour is truly interesting, a very sophisticated physics and flight dynamics engine and I assume the real Hind has done the same thing - but why is it doing this? Is the huge multi-bladed main rotor overpowering the torque of the turbines in this case when doing a maneuver while being too fast, what is happening?

If I had to guess, from description alone, I'd ask are you unloading the rotor system then loading it faster than the engines can catch up, causing rotor RPM to droop?

If you reduce power to the point the upflow is driving the rotor system, it can take a while for the engines to spool back up, when you transition back to engine driven flight.

[Quadg]

full collective? 

use the engine pressure gauge (EPR) to decide how much collective to use. It has two needles, one on either side, for each engine and three flags in the middle.

the limit for the top flag of take off power is 6 minutes. (top flag and below)

30 minutes for maximum cruise (middle flag and below)

and unlimited for normal cruise. (bottom flag and below)

use take off for take off in a hover. and landing.

use maximum cruise for climbing to altitude. (it wont take 30 minutes for you to reach max altitude..)

use normal cruise for everything else.

those are your 3  collective positions to use. The flags on the EPR will adjust for outside temp and altitude.

make sure none of the three positions exceed the Power turbine temperatures (PTT gauges for each engine) for whatever conditions you are flying in. And that you retain rotor RPM at high altitude (keep rotor RPM between 94% and 100%. reduce collective if it starts dropping, increase collective if it starts going over 100% in a descent).

Also move the collective slowly to balance the loads between the two engines. if the two needles show different settings on the EPR you are moving the collective too quickly. and overloading one engine to meet the power requirement. move the collective slower and the load will be balanced between the two engines.

that's a quick and dirty rundown on how you use the collective, EPR gauge. PTT gauges and rotor RPM gauge for keeping the hind engines sweet. you can use the blade pitch angle gauge as a quick reference. (note the three EPR positions on the blade pitch angle and just use that for quick reference, but if outside conditions change significantly refer to the EPR and check PTT's instead).

you should never exceed the take off power flag on the EPR so should never pull full collective.

 

[AeriaGloria]

6 hours ago, Quadg said:

full collective? 

use the engine pressure gauge (EPR) to decide how much collective to use. It has two needles, one on either side, for each engine and three flags in the middle.

the limit for the top flag of take off power is 6 minutes. (top flag and below)

30 minutes for maximum cruise (middle flag and below)

and unlimited for normal cruise. (bottom flag and below)

use take off for take off in a hover. and landing.

use maximum cruise for climbing to altitude. (it wont take 30 minutes for you to reach max altitude..)

use normal cruise for everything else.

those are your 3  collective positions to use. The flags on the EPR will adjust for outside temp and altitude.

make sure none of the three positions exceed the Power turbine temperatures (PTT gauges for each engine) for whatever conditions you are flying in. And that you retain rotor RPM at high altitude (keep rotor RPM between 94% and 100%. reduce collective if it starts dropping, increase collective if it starts going over 100% in a descent).

Also move the collective slowly to balance the loads between the two engines. if the two needles show different settings on the EPR you are moving the collective too quickly. and overloading one engine to meet the power requirement. move the collective slower and the load will be balanced between the two engines.

that's a quick and dirty rundown on how you use the collective, EPR gauge. PTT gauges and rotor RPM gauge for keeping the hind engines sweet. you can use the blade pitch angle gauge as a quick reference. (note the three EPR positions on the blade pitch angle and just use that for quick reference, but if outside conditions change significantly refer to the EPR and check PTT's instead).

you should never exceed the take off power flag on the EPR so should never pull full collective.

 

A lot of good info but as far as I know the time limits for different levels of power only have to do with lifetime of the engine. There are no consequences for flying the engine at take off power the entire flight other then shortening the engine lifetime after it’s done a lot more flights.

 

I also believe the fuel system will cut fuel when you reach the temperature limit, so I don’t know of a way to exceed it that’s possible, but I’m sure there’s a way. 

[JetCat]

Hey that was really interesting to read! Thanks for all the information. And yes I used full engine power (page up or rotating the pitch lever clockwise to the max) all the time plus full collective all the time during the normal cruise flight. In Syria my Hind failed two times right after taking-off when doing so, but in cold Caucasus the flights went rather good - until pulling the nose up at 200-300km/h to air-brake the helicopter, or doing turns with high speeds caused the engines to fail. I love this realism and the chance of breaking such a magnificent and complex machine with wrong piloting and piloting errors it brings so much more challenge and realism into the flights and makes actual observing the gauges necessary! In all other flight simulators absolute NOTHING ever happens, the turbines can run with 120% N2 and 1001°C ITT in the Death Valley the whole day without anything ever happening, the landling flaps can stay out 40° out till mach 0,90 and contracted and extended again and NOTHING no stupidity and piloting errors have any consequences, and any damage model on airplanes is literally not existent. In DCS the landing flaps of the Tomcat will fail when staying out for too long because of gaining too much speed too fast after the carrier launch, parts of the gear and the gear covers are blown away when extending the gear in the Hornet while flying too fast, the canopy glass is blown away when opening the canopy during flight to enjoy some fresh air - DCS has such an intense damage model and realism and so many interesting reactions to various experiments with the plane or actual piloting errors one must really think about all the physics and taking care of the planes! I absolutely love that! For the first time ever I felt like flying a real Helicopter in that Hind and being in need to carefully watch the instruments. Awesome!

Allright the manual says engine power should always stay on "nominal power" (pressing page up to maximum turbine RPM and torque) and the engine pressure rate is fully controlled with the collective pitch lever. Thanks for the info I had not studied these chapters in the handbook yet - I just tried to fly What exactly does the condition lever to the engines? It´s the two levers left of the collective pitch lever.

[Quadg]

5 hours ago, AeriaGloria said:

A lot of good info but as far as I know the time limits for different levels of power only have to do with lifetime of the engine. There are no consequences for flying the engine at take off power the entire flight other then shortening the engine lifetime after it’s done a lot more flights.

 

17 minutes ago, JetCat said:

What exactly does the condition lever to the engines? It´s the two levers left of the collective pitch lever.

the engines in real aircraft have a time between services. and they give you different power outputs depending on where they are in their service life.

in the game we get the same power every time we start. (campaign writers can simulate engine wear in the ME)

the times are to keep the engines running the same for the full period. (for as long as possible) 

those levers are there for if one of the engines is starting to produce less power. for when they are getting close to needing replacing.

in the game the engines produce identical power and are the same age. in real life you get differences in engine power and engine age. And you need to balance them to a certain degree. hence the condition levers. we don't really need them in the game.

the hind engines are not fully modelled yet. 

 

you basically only ignore the time limits if you are in the kind of war where the aircraft is not likely to survive till servicing. due to enemy action.

so world war 3. peer to peer total war is hell on equipment.

doing counter terrorist operations and low intensity warfare, you do stick to the time limits because you are likely to survive the whole thing. And are most likely to die in an accident. sticking to the official limit greatly reduces your chance of dying in an accident.

also pilots share aircraft. if the next guy has less power because you have been joy riding then he will probably tell you about it.

also the maintenance guys will give you grief for messing up their aircraft. And giving them more unnecessary work. they need to inspect the engines when you exceed the operating times.

so I tend to stick to the official limits in most campaigns. because why not?

the time limits are from the mi-8 because we don't have the official hind manual yet. (similar engines and the same gauges)

until we get the manual all this advice is unofficial, even chucks guide

[gnomechild]

3 hours ago, JetCat said:

plus full collective all the time during the normal cruise flight.

Wait what. I think that's your problem. How are you even managing level flight with full collective? You should be cruising with around 7-9 degrees rotor pitch in most cases. 

[AeriaGloria]

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. 

[JetCat]

It happened again, but this time for no reason! After having experience with the Hind for a few days now I can fly almost perfect. In this case the helicopter went high up into the air with full collective in the cockpit start and take-off training mission.

Everything went PERFECT, I lowered the collective a little bit and braked the helicopter in-air by pulling up the nose slightly and lowering it´s speed to ten knots. The helicopter with auto-pilot assist was hoovering fine with a forward-speed of only ten to twenty knots for some time, and I thought this is the ideal speed for an almost VTOL landing.

I noticed that the variometer still showed some ascend rate therefore I lowered the collective lever another tiny bit to get a 50-100 feet per minute sink rate and slowly let the Hind hoover down to the ground and land.

Suddenly the rotors begun their shiiiii shiii shiii shii sound and the Hind begun wobbling like crazy, the main rotor speed went fully down and the helicopter flew out of the skies like a dropped stone. It was the only re-build Prototype from the Nimitz and it is lost now...

Again my question: What am I doing wrong? A helicopter is VTOL-landed by lowering the air speed and slowly gently lowering the collective pitch until the variometer shows a slight sinkrate, I am doing this for 20 years in virtual Jet Ranger and EC135 and other virtual helis - why is the main rotor and the engines suddenly "switching off" while gently hoovering in mid-air when I land the Hind the same way I land any other helicopter? 

 

 

 

Edited November 7, 2021 by JetCat

[admiki]

3 hours ago, JetCat said:

It happened again, but this time for no reason! After having experience with the Hind for a few days now I can fly almost perfect. In this case the helicopter went high up into the air with full collective in the cockpit start and take-off training mission.

Everything went PERFECT, I lowered the collective a little bit and braked the helicopter in-air by pulling up the nose slightly and lowering it´s speed to ten knots. The helicopter with auto-pilot assist was hoovering fine with a forward-speed of only ten to twenty knots for some time, and I thought this is the ideal speed for an almost VTOL landing.

I noticed that the variometer still showed some ascend rate therefore I lowered the collective lever another tiny bit to get a 50-100 feet per minute sink rate and slowly let the Hind hoover down to the ground and land.

Suddenly the rotors begun their shiiiii shiii shiii shii sound and the Hind begun wobbling like crazy, the main rotor speed went fully down and the helicopter flew out of the skies like a dropped stone. It was the only re-build Prototype from the Nimitz and it is lost now...

Again my question: What am I doing wrong? A helicopter is VTOL-landed by lowering the air speed and slowly gently lowering the collective pitch until the variometer shows a slight sinkrate, I am doing this for 20 years in virtual Jet Ranger and EC135 and other virtual helis - why is the main rotor and the engines suddenly "switching off" while gently hoovering in mid-air when I land the Hind the same way I land any other helicopter? 

 

 

 

 

From what I can see, your engines shut down for some reason. Main reason why you are crashing is that you are not setting autorotation up. You cant' just leave the collective where it is. But, track file or video will be really helpful to see why your engines are shuting down.

Edited to add: are you flying cold or hot helicopter? In one of your photos I can see that external tanks valve is open, but you don't have external tanks. Never tried to see if that setup could cause engine flameout

Edited November 7, 2021 by admiki

[sirrah]

I haven't read all the above, so I'm not sure if anyone already suggested this, but @JetCat did you manage to reproduce the issue after jumping into an in flight Hind (or hot start for that matter)?

 

This could rule out a faulty startup procedure. E.g. you forgetting something during startup. (Not saying you do, but just in case)

 

I've quite a few hours in the Hind now, but can't recall experiencing anything similar to what you describe. Only times I had something similar, was after taking some battle damage.

[JetCat]

Now I think I know what the problem is!

It is called Vortex Ring which means the helicopter suddenly get´s some kind of blade stall or loosing lift force from the main rotor, begins descending with ten or twenty meters per second or more which means he is literally falling out of the sky, and giving full pitch in hot scenarios overwhelms the two turbine shafts with sudden torque request from the main rotor and makes the turbines just flame out.

 

Amazing this is what I call a SIMULATOR, all other flight dynamics in other sims are simple to handle and overall simple with no damages or piloting errors whats-so-ever and mere simple toy planes compared to the DCS realism and depth of the flight physics engine physics and more

I am going to try this:

https://www.digitalcombatsimulator.com/en/files/3317338/

...and see if the problem can be reproduced. Sorry my helicopter knowledge is very limited and basic and reduced to the very simple flight physics of a virtual Bell Jet Ranger and an Airbus EC135. Well looks like making a helicopter vertical descending with 500 or 1000 feet per minute on the VSI with slow forward movement 10 to 40 knots almost like it was some Cessna 152 before beginning to stop the descent 100 meters above the ground until almost hoovering by fully pulling the collective will not work because of vortex and blade stalling. Amazing flight physics!

 

 

Edited November 7, 2021 by JetCat

[admiki]

No. VRS will not cause engine flameouts.

The way it works in Mi24 (Huey and Mi8 are the same) is that there is a turbine wheel which is forced to move by hot gases exiting the engine. There is not hard connection between engines and rotor system.

Why don't you record a video or put up a track file, this looks like chasin our own tails.

[gnomechild]

It sounds like you're having this problem every time you reduce the collective. I would check through all your control bindings and make sure whatever you're using to lower the collective isn't also lowering the engine throttles (the individual ones, the twist grip isn't enough to shut down your engine like in the picture) or the fuel cutoffs or anything related to the engine. 

Other than that, a track file would be needed to figure out what's going wrong. 

[Sabre_Ewan]

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?

[admiki]

In his photos, rotor pitch is at about 10.5, so that looks fine. But even if he was flying with throttle, that doesn't explain engines flameout

[AeriaGloria]

On 11/6/2021 at 9:24 AM, Quadg said:

the engines in real aircraft have a time between services. and they give you different power outputs depending on where they are in their service life.

in the game we get the same power every time we start. (campaign writers can simulate engine wear in the ME)

the times are to keep the engines running the same for the full period. (for as long as possible) 

those levers are there for if one of the engines is starting to produce less power. for when they are getting close to needing replacing.

in the game the engines produce identical power and are the same age. in real life you get differences in engine power and engine age. And you need to balance them to a certain degree. hence the condition levers. we don't really need them in the game.

the hind engines are not fully modelled yet. 

 

you basically only ignore the time limits if you are in the kind of war where the aircraft is not likely to survive till servicing. due to enemy action.

so world war 3. peer to peer total war is hell on equipment.

doing counter terrorist operations and low intensity warfare, you do stick to the time limits because you are likely to survive the whole thing. And are most likely to die in an accident. sticking to the official limit greatly reduces your chance of dying in an accident.

also pilots share aircraft. if the next guy has less power because you have been joy riding then he will probably tell you about it.

also the maintenance guys will give you grief for messing up their aircraft. And giving them more unnecessary work. they need to inspect the engines when you exceed the operating times.

so I tend to stick to the official limits in most campaigns. because why not?

the time limits are from the mi-8 because we don't have the official hind manual yet. (similar engines and the same gauges)

until we get the manual all this advice is unofficial, even chucks guide

however you want to play the game is valid, but my point was that the time limits for in service engine limits would not cause an inflight failure for OP since such consequences would take multiple multiple flights.

jetcat, we really can’t do anything unless you give us a track or more info. Did you run out of fuel? Did you try a instant action to make sure it wasn’t a fault start up? If this happens in a hover, if we need it was below 35m AGL and you didn’t have dust protection on it could be dust ingestion 

Try to make the track short and start with you in the regime that causes most of your issues. As someone before said, even just recording your flight could be immensely helpful 

Edited November 7, 2021 by AeriaGloria

[JetCat]

The Hind is truly an interesting machine. Since today I am filming my flights and landings to have a better overview what happened.

When being in a too fast vertical descent (Vortex Ring state in great height caused by lowering the pitch lever and begin to sink too fast) and adding too much torque after having dropped like a stone for half a minute, the engines are overloaded while the main rotor is getting slower and slower, and the engines will flame out.

Also the Hind should not be taken off, hoovered, and flown with full collective, but with half or 3/4 - otherwise one looses tail rotor authority especially in hot areas. The engines loose 10% torque when being exposed to temperatures over 10°C (for every +10°C added to 10°C there will be 10-12% loss of torque power), that´s why the Hind can be flown in Syria only with light load and with not much fuel in the tank.

Now I know why I constantly had tail rotor authority losses after VTOL: Because the huge five-bladed rotor has so much torque that the tail rotor cannot compensate in 100% collective, especially in hot areas with limited engine power. But it flies flawless with 60 to 80% collective.

My experience with the JetRanger III and the Robin R22 are based on a simpleton and very arcady flight model because there was never any study-level helicopter not for Flight Sim 2004 and also not for Flight Sim X. Just pull the collective pitch full up 100% and fly for hours was the only challenge and the only thing to "learn". That´s why I had many strange experiences flying a REAL simulated heli for the first time instead of some simple one which simulates no failures no overstresses and absolute zero engine rotor and airframe damages at all.

[AeriaGloria]

2 hours ago, JetCat said:

The Hind is truly an interesting machine. Since today I am filming my flights and landings to have a better overview what happened.

When being in a too fast vertical descent (Vortex Ring state in great height caused by lowering the pitch lever and begin to sink too fast) and adding too much torque after having dropped like a stone for half a minute, the engines are overloaded while the main rotor is getting slower and slower, and the engines will flame out.

Also the Hind should not be taken off, hoovered, and flown with full collective, but with half or 3/4 - otherwise one looses tail rotor authority especially in hot areas. The engines loose 10% torque when being exposed to temperatures over 10°C (for every +10°C added to 10°C there will be 10-12% loss of torque power), that´s why the Hind can be flown in Syria only with light load and with not much fuel in the tank.

Now I know why I constantly had tail rotor authority losses after VTOL: Because the huge five-bladed rotor has so much torque that the tail rotor cannot compensate in 100% collective, especially in hot areas with limited engine power. But it flies flawless with 60 to 80% collective.

My experience with the JetRanger III and the Robin R22 are based on a simpleton and very arcady flight model because there was never any study-level helicopter not for Flight Sim 2004 and also not for Flight Sim X. Just pull the collective pitch full up 100% and fly for hours was the only challenge and the only thing to "learn". That´s why I had many strange experiences flying a REAL simulated heli for the first time instead of some simple one which simulates no failures no overstresses and absolute zero engine rotor and airframe damages at all.

Again, VRS no matter how severe CANNOT overload the engine or cause a flameout.

the rotor gearbox is attached to a turbine that sits in the exhaust if the engine, it’s not directly attached to the engine turbine. If you were to hold the rotor in place, the engine would run without a care unit the world. If it’s really a flameout or overload it’s something else. 
 

if you are instead referring to the loss of lift during VRS, that has nothing to do with engine but instead with airflow trying to go upwards through the rotor when the rotor is trying to push air down. Since air is trying to go the wrong way through the rotor it disrupts the airflow and causes the rotor to stall, which causes a loss of lift having nothing to do with the engine.

your engine doesn’t flameout unless you lose engine RPM specifically, and and you can’t push the engines too hard for their own good in DCS, the only way to break them in normal flight is by flying low without dust protection or turning birds on and flying so low birds get infested in the engine 

If your helicopter falls out if the sky from VRS, it’s your rotor not lifting correctly becuase it wants airflow to go down the rotor not up it. Nothing to do with the engine  

[Isegrim]

Could be birdstrike if the option is on... or Dust?!?

Or... maybe an electrical issue like Generators not on?!?

Just quesswork but.....looks strange and can have many reasons.

From the flying side possibly > rough Collective adjustments when comming to full stop > Govenor to slow to compensate> Low Engine and Rotor RPM> Low electrical power from generators > Electrical possibly switches no set properly > Not enough electrical power on FEED Pumps > Engine Flameout or simply to low Engine RPM to keep flame on.

As i have written just quess.

Edit: oh and the Variometer in the Hind is usually in meters per second. And an sinkrate of 5-10 Meters per second close to a hover stop is very fast and you very easily can find yourself in an VRS.

 

 

Edited November 8, 2021 by Isegrim

[AlphaOneSix]

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.

Edited November 9, 2021 by AlphaOneSix