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Engine RPM decrease at collective release?


D4n

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Hi, why would the collective idle decrease engine RPM? Wouldn't it be way smarter if collective only controls pitch-angle, and engine stays at min. 95% RPM the entire time? That way rotors would instantly get the necessary power to not lose too much rotor RPM. (currently, when slowing down really fast, engine RPM goes WAYYY down.)

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You're probably exceeding the engine governor's limits by pulling excessive collective (and thus increasing the rotor blade AoA) and the RPM then decreases, because that's just what it does due to the increased aerodynamic load from the rotor.

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He's talking about engine RPM, not rotor RPM.

 

DanielNL, the engines don't have a way to dump their power overboard if it's not needed, all of the power generated by the engines must, by design, be converted into rotor speed. If you reduce the power required by the rotor, the engine power must also be reduced, or else the rotor speed would quickly exceed its limits. This is managed automatically by the engine fuel control unit. Each engine's fuel control has an internal, mechanical governor that regulates engine power in order to maintain rotor speed at a certain value. An increase in rotor RPM results in a reduction of fuel introduced into the engine, and a corresponding decrease of engine RPM, just as a decrease in rotor RPM results in an increase of fuel introduced into the engine, and a corresponding increase in engine RPM. All in order to maintain rotor RPM at or near a prescribed value.

 

Virtually all turboshaft engines work this way. There is no way to maintain a high engine RPM without that RPM being converted into excess rotor RPM.

 

In fact, I'm at a loss to think of any engine at all that does not reduce its power output when the demand for power decreases, be it a car, a jet, or a helicopter.


Edited by AlphaOneSix
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The engine RPM depends on the load. (See the AplhaOneSix comment.) Mind the engine has independent drive turbine. Than means it is not connected with compressor. The compressor is driven by its own turbine.

 

So, the engine core turbine that drives compressor is changing its speed to keep the rotor turbine in steady RPM.

 

Check this video:


Edited by AJaromir
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And since all mass is slow, the one system will always be lagging behind the other system, e.g. Engine RPM and main rotor RPM will always be lagging. Change one, the other will react but slower.

 

E.g. pulling power (increasing blade AOA) will slow down the main rotor due to drag, the engine governors detect this and increase fuel flow to compensate but before the engine can properly react, it takes time to get the fuel to the combustion chamber, burn it and use the power to increase main rotor RPM via the power shaft.

 

Decrease power and the opposite happen, main rotor RPM increases due to decreased resistance, the governors will correct by reducing fuel flow to the engines. In the mean time that it takes for the engine RPM to drop, the excess power is transferred to the main rotor, it hasn't got any other way to go! Main rotor RPM increases until, due to less fuel being burned, the engine RPM decreases which will lead to the main rotor rpm to decrease as well.

 

As you can see in the video, the gas turbine is basically two counter rotating shafts that are not directly connected to each other so for one to influence the other, a change in pressure or hot gas needs to take place which needs time to influence the other since the medium power tranfers is air.

 

Yank the collective too hard (remember the manual, no more than 3 degrees per second) and the system is unable to keep up, leading to a too rapid deceleration of the main rotor which, in turn, leads to loss of generators which leads to loss of dampening channels and so on.

 

The other way around isn't good either, drop collective too fast and you risk overspeeding the main rotor which has its problems too.

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He's talking about engine RPM, not rotor RPM.

 

I know, but shouldn't the governor try and hold the engine RPM even when the PIC pulls excessive collective? And its authority then just runs out when the aerodynamical load from the rotor gets high enough, causing the engine RPM to drop. EDIT: Or rather the governor throttles the fuel flow even when the engine RPM starts to drop in order to avoid excessive torque?


Edited by msalama

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the mi-8 has two engines and a power coupler.

which means the govenor has a much more complex job to govern the throttles than in the single engine helicopters.

 

this means more delay in how fast you can move the collective.

move it to fast and one engine will have more work to do . because it will be driving the other engine which is trying to catch up. as well as driving the output shaft.

move the collective slowly and the engines remain in balance giving more power to the drive shaft. because they are not driving each other. they are driving the shaft.

you can see this in the EPR when one engine is much higher than the other. that is the driver engine. the lower is the driven engine. and it can take a while for the governor to balance it out.

and you can exceed the engine limits on the driver engine. for max power etc.

you should avoid that state.

 

so from a braking rotor pitch of 1-3 degrees it should take 5 seconds for you to pull the collective too takeoff power and stop.

to allow all the automatic stuff time to do its thing.

 

and 5 seconds can feel like a lifetime when someone is shooting at you :)


Edited by Quadg

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I know, but shouldn't the governor try and hold the engine RPM even when the PIC pulls excessive collective? And its authority then just runs out when the aerodynamical load from the rotor gets high enough, causing the engine RPM to drop. EDIT: Or rather the governor throttles the fuel flow even when the engine RPM starts to drop in order to avoid excessive torque?

 

How so? maybe when engine would have infinite power than governor would be able to keep rpm stable.

Higher load on main rotor is increasing gas temp which can damage the turbine blades if temp is exceeded.

When collective is pulled fast both engine and rotor rpm will drop.

And when collective is reduced to 0 engine rpm will drop to lower power level for saving fuel, extending engine life time etc


Edited by grafspee

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How so?

 

I said "try and hold", meaning it tries to hold the RPM stable up to a +/- limit, or within its control authority. And that limit is definitely exceeded when someone pulls excessive collective. So I didn't actually contradict anything you said.

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I said "try and hold", meaning it tries to hold the RPM stable up to a +/- limit, or within its control authority. And that limit is definitely exceeded when someone pulls excessive collective. So I didn't actually contradict anything you said.

Ok, than im backing off.

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I know, but shouldn't the governor try and hold the engine RPM even when the PIC pulls excessive collective? And its authority then just runs out when the aerodynamical load from the rotor gets high enough, causing the engine RPM to drop. EDIT: Or rather the governor throttles the fuel flow even when the engine RPM starts to drop in order to avoid excessive torque?

 

The governor doesn't try to hold the engine RPM, no. The governor tries to hold rotor RPM. (This is true when the throttle is set to its full right position). If the rotor RPM starts to drop (typically due to an increase in collective pitch), more fuel is introduced, and the engine RPM increases. If the rotor RPM starts to increase (typically due to a decrease in collective pitch), less fuel is introduced, and the engine RPM decreases. If the engine RPM reaches the limit for the electronic governor (101.15% for dual engine operation, 102.5% for single engine operation), then no additional fuel can be introduced, and the rotor RPM will begin to drop. If the pilot pulls excessive collective, the rotor RPM will drop because the engines cannot spool up fast enough to maintain the rotor RPM, but the engine RPM will definitely increase quite rapidly.

 

the mi-8 has two engines and a power coupler.

 

I'm assuming that you're referring to the power synchronizer on the engine fuel controls. In which case each engine has one. Both engines connect directly to the main gearbox, so maybe you are referring to the main gearbox itself as the power coupler. While it's true that one engine is the driving engine and one engine is the driven engine, I think you're overestimating the difference between the two.


Edited by AlphaOneSix
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Maybe OT but back in the days when I flew 90" R/C helicopters with .61cu 2-Stroke engines the hardest part in tuning the mechanics wasn't the washplate linkage or tail rotor, it was tuning the carburator so that your RPM would not decrease or increase but stay as stable as it can be when you worked either the collective or cyclic controls, or tailrotor to yaw, all of those feed or get fed from the main engine and change the load on the engine shaft. You can draw parallels between a .61 1.6BHP 2-Stroke and a full size RL turbine, the goal remains the same, keep the RPM on the rotor stable to avoid another force to change your attitude.

Things got easier when RPM-governors got introduced in the late 90's but it wasn't the sole solution, you still needed to obey the law of not changing things to quickly. The carb can open or close in a fraction of a second but the engine cannot response faster as it can, no matter how fast you open or close the carb, you still needed to move it wisely to avoid sudden change of attidude, or worse, a stalling engine. In times of BL-Motors and LiPo batteries this has changed, with 10x or more of the power available compared to when I started you can slam the cyclic and collective as fast as you can and the shaft will maintain it's RPM, no wonder...with 15kW compared to my mere 1.2ish kW's back then.

Apperently, RL turbines are nowhere near the capabilities and reaction time of BL-Motors fed by LiPo batteries at 300Ampere and the bigger the mass ( The Mi-8 is a flying Milk Truck ) the slower you have to work the controls to avoid accidents.

 

If you haven't seen R/C helicopters of the precent years doing acrobatics, go YT and do it, it's an eye opener what helicopters can do, if mass and power are in a certain relation, granted you have sharp skills, eagles eye and some guts to risk a few grand.

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The governor tries to hold rotor RPM.

 

OK, thanks for the education. I always thought it tries to hold the engine RPM directly. S!


Edited by msalama

The DCS Mi-8MTV2. The best aviational BBW experience you could ever dream of.

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  • 1 year later...

One more thing. 😉

The whatever engine is not possible to instantly increase its thrust. If the system puts too much fuel into combustion chamber, the engine may start burn. Just like the old ME-262 did because it did not have any hi-tech engine control system. The pilot was controling the fuel valve directly. Too fast increase caused low pressure compressor stall, catching the fire by overfilling engine by fuel and very often it was instakill of your engine due to very high temperature. Too fast decrease caused engine flameout (flame was literally blown out) or again, the engine burning caused by high pressure compressor stall. In those era no aircraft did have surge or spill doors, variable blade angles e.t.c...   🙂


Edited by AJaromir
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  • ED Team
On 12/27/2019 at 9:40 PM, AlphaOneSix said:

 

The governor doesn't try to hold the engine RPM, no. The governor tries to hold rotor RPM. (This is true when the throttle is set to its full right position). If the rotor RPM starts to drop (typically due to an increase in collective pitch), more fuel is introduced, and the engine RPM increases. If the rotor RPM starts to increase (typically due to a decrease in collective pitch), less fuel is introduced, and the engine RPM decreases. If the engine RPM reaches the limit for the electronic governor (101.15% for dual engine operation, 102.5% for single engine operation), then no additional fuel can be introduced, and the rotor RPM will begin to drop. If the pilot pulls excessive collective, the rotor RPM will drop because the engines cannot spool up fast enough to maintain the rotor RPM, but the engine RPM will definitely increase quite rapidly.

 

 

 

I'm assuming that you're referring to the power synchronizer on the engine fuel controls. In which case each engine has one. Both engines connect directly to the main gearbox, so maybe you are referring to the main gearbox itself as the power coupler. While it's true that one engine is the driving engine and one engine is the driven engine, I think you're overestimating the difference between the two.

 

Actually is even more complicated.
There are two governors in the fuel governor of TV3-117 - the engine rpm governor and the rotor (power turbine) rpm governor. Additionally, a synchronizer coordinates both engines working on common load.
Initially the fuel is metering maintaining engine rpm, so the engine can hold required rpm. This area is from idle to so called "RIGHT CORRECTON" when engine rpm is set at the level where the power is enough to achieve 95% of the rotor. As the rotor gets 95% the second feedback loop starts to limit fuel less than the first regulator metered for its rpm.
The real thing is even more complicated because it combines feedforward and feedback control, synchronisation of two engines, spool-up limiters etc... Collective lever changes engine rpm settings, etc.

 

Ніщо так сильно не ранить мозок, як уламки скла від розбитих рожевих окулярів

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Ничто так сильно не ранит мозг, как осколки стекла от разбитых розовых очков (С) Me

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On 12/26/2019 at 2:25 PM, AlphaOneSix said:

He's talking about engine RPM, not rotor RPM.

 

DanielNL, the engines don't have a way to dump their power overboard if it's not needed, all of the power generated by the engines must, by design, be converted into rotor speed. If you reduce the power required by the rotor, the engine power must also be reduced, or else the rotor speed would quickly exceed its limits. This is managed automatically by the engine fuel control unit. Each engine's fuel control has an internal, mechanical governor that regulates engine power in order to maintain rotor speed at a certain value. An increase in rotor RPM results in a reduction of fuel introduced into the engine, and a corresponding decrease of engine RPM, just as a decrease in rotor RPM results in an increase of fuel introduced into the engine, and a corresponding increase in engine RPM. All in order to maintain rotor RPM at or near a prescribed value.

 

Virtually all turboshaft engines work this way. There is no way to maintain a high engine RPM without that RPM being converted into excess rotor RPM.

 

In fact, I'm at a loss to think of any engine at all that does not reduce its power output when the demand for power decreases, be it a car, a jet, or a helicopter.

 

 

I guess technically, if you wanted to, you could design a planetary gearbox (just as the real thing has), where you could have the engine run at maximum RPM and the cog within the planetary gearbox be used to "throw over" the excessive energy. That by giving the engine a different set of cogs to work with to maintain stable rotor RPM given the pitch of the blades and load. However, I´m also seeing only negative effects of such a configuration. Engine at max rpm would mean higher fuel consumption, even when not needed to, as well as reduced service life for the engines as they operate on the maximum all the time.


Edited by zerO_crash

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