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Helicopters and Twin Turbines

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To answer a recent question that was posed in the forums, with only one engine working (referred to as One Engine Inoperative or OEI), the 412 can achieve a 300fpm climb carrying its Max Gross Weight (this is at or near sea-level, and at 60kts airspeed, and 15-25 degree celsius outside air temp). So it is quite capable even with one engine out...HOWEVER, that capability is only used to get it back on the ground in short order, and even then typically a run-on landing is used, and not a hover. (a failed engine would always be an emergency, never normal operation) Because with that little power to spare, a hover could be very dangerous. Especially if you were at or near max gross weight. And to put this into a real-world context, these 412s are operated at Max Gross Weight a lot. Especially when they are making trips back and forth to Oil Rigs. Because the customers do not like paying for extra trips.


The great thing about aviation engineering, especially in helicopters, is that a complex function might actually be represented by a simple mechanism. What I mean is, if you take a component, what it "does" might be incredibly complex, but "how" it does it is kept as simple as possible because of the various benefits. Such as weight, reliability, serviceability. So to the casual observer, you might look at some mechanical component of the system, and think, that doesn't look so complicated, but not realize that it is indeed complicated, and must have taken a lot of research and design effort to simplify it to the structure it currently has.

In this regard, engineers probably feel under-appreciated. Because the better they do, and the harder they work, the more simple things look! (being a software programmer is similar)

When I first started with the 412 project, I kept thinking, ok if 2 turbines provide 100% of the power used for flight, then of course, if one fails, you would only have 50% of that power for flight. I was pretty misguided.

The goal of power on a helicopter is to keep the rotors turning at a given RPM. To be simple, anything that causes that rotor RPM to decrease, is going to cause the engines(s) to work harder to keep that rotor RPM at its desired level. So simply put, if one engine dies, the other just starts ramping up, and working harder, to provide the necessary power to keep the end result (the rotor rpm) the same. So with only one engine working, it is going to take more power than normal to produce the same amount of rotor rpm, and that is where all of the limiting factors come in. Sure the engine can spin faster, but how much faster before failure, how much hotter?, how much more force? (torque). The engine has no emotion, it will keep increasing its RPM as long as you keep adding more fuel, so it doesn't care if you keep flying like you have 2 engines sharing the work, it will just work as hard as it can, until it fails as well.

Think of how different this is to something like a King Air, which coincidently, has VERY similar engines, but that are separated, and operate separately. Just because one engine fails, doesn't mean that you now can move the throttle lever for the other twice as far forward, its still going to have the same limits that it did when both engines were running. It won't produce the same airspeed, or climbing potential for the aircraft, but at least its nature is basically the same whether the other engine is running or not.

It would be like having a King Air with no limit to the amount of throttle you could apply, and the only thing the pilot had was a desired airspeed selector and an AutoThrottle with no limits. He would dial in let's say 200kts and fly along. Then one engine fails, so the other just starts steadily increasing to keep the airspeed at 200kts. It doesn't care how much it has to increase, it ONLY knows that the aircraft has to be doing 200kts.

In the above example, the airspeed is similar to your rotor RPM. You are telling the helicopter's system that you want the the rotor rpm to be 100% (or whatever), and therefore, do whatever it takes to keep it there.

Of course there is a LOT more to this type of system, lots of modes, and choices, and limits, but I think you get the basic concept.

The other day, one of our members, Punker (currently in his turbine helicopter training) had a post here that was mentioning that he had his first experience with "Manual" mode in his helicopter training, and that it was quite an experience. Basically, this mode meant that amongst many other things, it was now HIS job to make sure that the engines were turning fast enough to maintain the desired rotor RPM. In a complex helicopter, this is actually a fairly difficult task! Because Every limb on your body is now doing something different, and every limb has to be doing its particular job correctly. Oh yes, and the big limb, of course I mean the brain, is trying to manage it all. I would say that those that master flying on manual mode under a broad range of circumstances, have a very big limb...once again, I mean brain.

Remember, when the throttle and fuel system is in automatic mode (the "normal" mode), the sytem is keeping your rotor rpm at 100% for you. So it tends to be steady, and predictable. Also keep in mind that every irregularity in your main rotor rpm, is typically represented in your tail rotor rpm as well. Because they are linked by a fixed gear ratio. So every time punker adjusts his throttle grip, and is a little low, a little high, a little low, etc... He is working his pedals to componsenate for the airframe torque, and tail rotor effectiveness changes he is inducing...all the while he is keep the helicopter level with the cyclic...AND he probably has an instructor watching and judging him....THIS has got to be a lot of fun.

Now imagine doing this (manual mode) with two engines, and two throttle grips, and where one engine is behaving strangely for whatever reason....In a helicopter like the 412 where the control responsiveness is very sensitive. I wonder how long one of us would last? I'm guessing, not long at all. Not me any way.

Manual Mode = Throttle grips Explicitly controlling the amount of fuel being fed to the turbine. Like the gas pedal in your car.

Automatic Mode = Throttle grips are Telling the system what rpm you desire, and it takes care of the fuel. Like cruise control in your car.

Oh yes, guess I better take the opportunity to say that my 412 will not have a manual mode on the fuel system available, because as of yet, I haven't found a way to make FS represent this. Which is ok, because in a helicopter like the 412, you would RARELY operate the engines this way, and if you didn't know just what you were doing, you can literally blow them off the aircraft. (think, loud terrible explosion, and then realizing your engine is sitting on the tarmac in a smoking heap). Oh yes, and then no-one will hire you when you are looking for a new job!





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