Engine RPM drop?

[Jafferson]

During a landing aproach by lowering the collective, the engine RPM drops from 90% to 75%

At the transition to a hover, sometimes the engine is to slow to back up to 90%

ending up in a vortex ring state. Feels like a lack of horse power?

I try to prevent this by holding the RPM at 90%

But in some situations its unfavorabel and its necessary to max down collective for descend.

 

How you deal with the RPM drop?

Have the mi8 a governor switch like the huey?

Whats better with the MI-8 left or right turn landing?

How to not smash the cow down in to the dirt?

 

----

 

A nice landing from the manual is not a problem, i want to talk about combat landings, the aerobatic kind.

For exemple, yesterday i did a landing with a 300kph descend max down collective full left ruder, near ground pull stick full right ruder, to do a 180, in to a left sideslip right turn 360 landing.

Some times the RPM drop is messing everything up at such maneuvers.

 

I want to master this...

Edited September 28, 2017 by Jafferson

[msalama]

I want to master this...

 

Maybe you're too heavy? You usually have to be well below the MTOW in order to handle her successfully in all flight phases.

 

But anyway. I'm not experiencing this, altough I usually travel pretty lightish it has to be said :)

[Esac_mirmidon]

Try the same with 50% internal fuel. People sometimes did not

realize the difference in handling from a fully loaded Mi-8 and another lighter.

 

If after that is doing the same you must take in consideración other factors

Edited September 28, 2017 by Esac_mirmidon

[Jafferson]

Yes, weight is a huge factor, but i want do the best results with 100% fuel, because when i learned the huey and he is much more forgivin, i loaded 50% and later when i started my first coupel mission on blueflag and 104th with a overloded chopper i failed.

[gospadin]

At the transition to a hover, sometimes the engine is to slow to back up to 90%

ending up in a vortex ring state. Feels like a lack of horse power?

 

It's not a lack of raw horsepower. Heavy blades can only be accelerated so quickly by the engine governor.

 

Make sure you don't change the collective pitch by more than 1 degree per second, and you should be fine.

[Art-J]

Jafferson, it seems you're trying to make this crate do things it was never designed or intended for. Sure it has governors (which are on all the time by the way) and gobs of power, but still, it's a flying bus with even empty weight three times higher than Huey and it's much bigger too.

 

By the book you're supposed not to do vertical descends with a rate higher than 3-4 m/s and collective pitch change rate of at least 1 deg/sec while reducing and at least 5 secs from 1 deg to take-off angle while increasing. Anything more excessive is doable, but means asking for trouble.

[Flamin_Squirrel]

Helicopter performance is related to rotor RPM not engine RPM. Remember, the engine is simply there to keep the rotor spinning. Only if you raise the collective and the engine can't keep the rotor RPM above ~90% are you power limited.

[Jafferson]

I did some tests past days, the mi-8 can fly with one engine and closed throttle on 100% fuel. Horse power should be enough, but sometimes its still a mysterie why 2 engines and open throttle cant spool up?

How ever, i did practice on one engine flying and autorotation.

Im getting better...

[Flamin_Squirrel]

sometimes its still a mysterie why 2 engines and open throttle cant spool up?

 

Because the engine is a free turbine turboshaft, meaning the rotor isn't directly connected to the main drive shaft of the engine, so RPM of one isn't directly proportional to the other.

Edited September 30, 2017 by Flamin_Squirrel

[Jafferson]

sometimes its still a mysterie why 2 engines and open throttle cant spool up?/QUOTE]

 

Because the engine is a free turbine turboshaft, meaning the rotor isn't directly connected to the main drive shaft of the engine, so RPM of one isn't directly proportional to the other.

 

 

 

I know, what i mean is... sometimes the engine is not responding from rising the collective, even on slow changes.

Edited September 30, 2017 by Jafferson

[javelina1]

from what I remember, if the fuel pump switches aren't enabled (at start up), the engine(s) spool up very slowly. But I think that also triggers an alert. I'm at a loss for the moment.

[msalama]

the engine is not responding from rising the collective

 

Not catching your drift now. Are you talking about the main rotor's responsiveness? Because the engine RPMs don't follow collective changes per se.

[Jafferson]

Not catching your drift now. Are you talking about the main rotor's responsiveness? Because the engine RPMs don't follow collective changes per se.

 

 

 

I talk about the engine is not kicking in to Keep spin the rotor.

[Flamin_Squirrel]

I talk about the engine is not kicking in to Keep spin the rotor.

 

Is your rotor RPM actually dropping below ~90% though? If not, you've nothing to worry about.

[Jester986]

I've never flown an Mi-8, but some types of turbines have a lot of lag. The original 206L was pretty notorious for it. I'd suggest getting your power in earlier then you need it. Also horsepower is proportional to rpm, so if your engine rpm droops it may not be able to make the horsepower necessary to bring it back up even if it would normally be fine with that load. You can fix this by quickly bumping the collective down and back up and or adding some aft cyclic. When you bump it down it gives the rpm a chance to catch up to the point where it can make its full horsepower.

[FragBum]

During a landing aproach by lowering the collective, the engine RPM drops from 90% to 75%

At the transition to a hover, sometimes the engine is to slow to back up to 90%

ending up in a vortex ring state. Feels like a lack of horse power?

I try to prevent this by holding the RPM at 90%

But in some situations its unfavorabel and its necessary to max down collective for descend.

 

How you deal with the RPM drop?

Have the mi8 a governor switch like the huey?

Whats better with the MI-8 left or right turn landing?

How to not smash the cow down in to the dirt?

 

----

 

A nice landing from the manual is not a problem, i want to talk about combat landings, the aerobatic kind.

For exemple, yesterday i did a landing with a 300kph descend max down collective full left ruder, near ground pull stick full right ruder, to do a 180, in to a left sideslip right turn 360 landing.

Some times the RPM drop is messing everything up at such maneuvers.

 

I want to master this...

 

Ah! Your likely requiring more power then you have available. I did a similar thing last night and just didn't have enough momentum left in the rotors and I had already pushed the turbines too hard so engine speed had dropped and rotor speed dropped and I dropped it! :D

 

Flying the Mi8 like that you need to consider mass and speed.

 

Mi8 about 10,000Kg max ~13 Tonnes (corrected typ weight)

Huey about 4000Kg max ~ 4.3 Tonnes

Gazelle about 2000Kg that's about it.

 

300Klm/H :cry:

 

Best to set up the Mi8 with say 500ltrs of fuel that will give you some wiggle room and better thrust vs weight. It's surprisingly agile but is still about 9000Kg. (corrected typ weight)

Edited October 5, 2017 by FragBum
(corrected typ weight)

[FragBum]

Okay try this load out the Mi8 with about 70% fuel and no weapons the Mi8 will be about 10,000kg. (5 Gazelles or 2 and a half Hueys)

 

**Clearly my Russian is rubbish,.. that is the main rotor AoA not torque. Duh!

 

Bring the Mi8 up to about 2.5 mtrs and sit in a stable hover so you don't need any further collective input, this will be about 6.7deg AoA. **I guess this value would change slightly depending on air temp and pressure however for the exercise it's still usable.

In the following the turn rate is about 12 seconds.

 

Keeping the mi8 level apply a small amount of left torque, without any collective input change the Mi8 will turn left and sink perhaps to the point the nose wheel will touch the ground.

 

Reestablish a stable hover as above in the Mi8 and now apply a small amount of right torque pedal and keep the turn rate about the same with no collective input change and the mi8 will rise, be aware when you stop turning the Mi8 will sink and you will need to account for this or it may sink to the ground.

 

During this there should be no and I don't want to say noticeable effects on turbine and rotor as I notice the turbine and rotor noise change but engine and rotor RPM should be fairly constant.

 

Reestablish the hover as above now push the cyclic forward and with careful torque and cyclic input leaving the collective input where it is you should be able to reach a 200Kph plus speed and you should have a climb rate of about 1 plus meters per second for quite some time.

 

If you can hover you can fly. :thumbup:

Edited October 6, 2017 by FragBum
<typo> **added