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Related, but not the same: What happens when you apply full cyclic + collective?

If the pilot suddenly applies full collective pitch on a turbine powered helicopter, will the engine stall? It seems from the other question, that this is not typically mechanically limited, so it is possible to force this onto the helicopter. However, does it cause the engine to stall? Any other problems?

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  • $\begingroup$ I'm guessing that the rotor RPM would drop and if left to deteriorate the blades would stall. Are you asking about piston helicopters or turbine? $\endgroup$ – Ron Beyer Apr 5 at 3:17
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I'll assume you're talking about typical turbine helicopters. The fuel controller on a helicopter's turboshaft is used to regulate rotor RPM via its gas generator's (the compressor/burner/turbine section) speed and torque. The RPM governor setting is coarsely set with throttle, then fine tuned with a trimming function controlled by a toggle (blip) switch on the collective to set the precise rotor RPM the governor will maintain.

The fuel controller will now govern rotor RPM automatically at that setting as you load the rotor with collective, by increasing fuel flow to increase the torque output of the gas generator. So torque, fuel flow and Turbine Outlet Temperature (TOT) will all rise as you pull collective. Both torque and TOT have operating maximum limits, so how much collective you can pull is therefore limited by a torque and/or TOT red line.

Trying to take off in marginal conditions due to load, temperature or altitude, you will find yourself pulling pitch until you are at a TOT or torque limit before you can even hover, and if you really really had to get airborne (like say you were an overloaded medivac Huey in Vietnam), you'd have to get the machine sliding along the ground first to get some translational lift in order to take off (one big advantage of wheeled landing gear).

Anyway, it's a hydromechanical fuel controller/speed governor that just does what it is told, so if you just haul the collective all the way up just sitting there, it'll depend on how much fuel the controller has to dump in to maintain RPM as you shoot off the ground straight into the sky. You will probably exceed the TOT or torque limit or both, making it a very expensive takeoff.

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  • $\begingroup$ Exceeding these limits severely, wouldn't that cause the turbine to flameout? $\endgroup$ – AlphaCentauri Apr 5 at 22:46
  • $\begingroup$ That shouldn't happen. The fuel controller regulates fuel flow to keep the correct air/fuel ratio in the burner can so it shoudn't cause an overrich mixture or anything like that. In very early jet engines the thrust lever was not much more than a needle valve so if you applied too much it would dump fuel in and flame out or let the flame grow too big and burn up the turbine. Hydromech fuel controllers respond to your power command but manage the fuel flow relative to mass air flow while they do it. But if you command too much power they won't stop you. FADEC engines will however. $\endgroup$ – John K Apr 6 at 0:42
  • $\begingroup$ No I get that, in a plane, if you suddenly give full thrust, without a closed loop system to control fuel you could flameout. But in a plane, there isn't anything that will literally act as a sudden, huge mechanical load on the engine. If you give full collective pitch, doesn't that literally slow down the turbine drastically? If you slow it down below a certain RPM, surely it will just flameout? Or do I fail to understand the mechanics even on a very superficial level here? $\endgroup$ – AlphaCentauri Apr 6 at 11:20
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    $\begingroup$ Well there is going to be some lag between detection of RPM decay and responding by dumping fuel into the engine, so if you yank the collective up as hard as you can, you will probably get both; some decay in RPM plus temp going through the roof as the governor tries to stop the decay. On a free turbine turboshaft like a PT-6 the power turbine is just sitting in the gas generator's exhaust and the governor just makes the gas generator blow harder on it when you call for more torque by pulling collective. $\endgroup$ – John K Apr 7 at 1:18

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