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Posted

during pre-flight tests, while checking the engine anti-ice

system, the manual says to look for changes in the

GG (high gas-generator) RPM.

 

where's that gauge?

 

also, that's a part the manual leaves me guessing so much!

:(

Posted

Hi Acedy,

 

Technically speaking, the free power turbine RPM is not the same as the rotor RPM, since the gearbox converts the very high revolutions of the free power turbine to something that works for the rotor blades, right?

Posted

aledmb,

 

As I understand it, we have a compressor RPM gauge, which is on the Right Forward Panel, below the EGT gauge, and we have a rotor RPM gauge, which is on the Left Forward Panel.

 

The compressor (or "engine" to be generic) RPM gauge measures the percentage of RPM of the compressor stages. The rotor RPM measures percentage of main rotor blade RPM. The two are not directly related, as the engine could be spinning at near max RPM, when you have a relatively low rotor RPM due to maneuvering/collective use. This is due to the fact that the gearbox, which drives the main rotors, is connected to the free power turbine. This turbine is called "free" because it is not coupled to the compressors. It is spun by the hot gases that came out of the combustion chamber.

 

In some turboprop aircraft, you can start the engine while holding down the propeller. You will more than likely ruin the engine, but you'll be able to keep it stationary, as the turbine is not mechanically driven. Does that make sense?

 

Best regards,

Posted

I was about to ask something related but upon thinking about it I think I figured it out, let me know if I'm wrong. I was wondering why low rotor rpm kills the electric to the ka-50 even if engines are at or near max rpm. Is it because of the free power turbine being connected to both the generators and the main rotors, so if one slows it slows the other?

Posted (edited)
Hi Acedy,

 

Technically speaking, the free power turbine RPM is not the same as the rotor RPM, since the gearbox converts the very high revolutions of the free power turbine to something that works for the rotor blades, right?

 

That's correct of course, but since there is a mechanical link between both, their RPM are proportional. Because the FT/rotor RPM gauge is calibrated in percent of max. RPM, it measures both values.

Edited by Acedy
Posted
I was about to ask something related but upon thinking about it I think I figured it out, let me know if I'm wrong. I was wondering why low rotor rpm kills the electric to the ka-50 even if engines are at or near max rpm. Is it because of the free power turbine being connected to both the generators and the main rotors, so if one slows it slows the other?

 

The generators are attached the the main gearbox, thus are directly tied to rotor RPM.

Posted

On western helicopters (Bell-407 for example) there is one pointer for PT RPM and one for rotor RPM on the same gauge. In powered flight (clutch engaged and PT transfering torque to the rotor) the two pointers match. In autorotation (free-wheel clutch disconencted) you can see the PT pointer falling behind. Russian helicopters use only rotor RPM gauge concept. There is no direct indication whether the free-wheel is engaged or not.

"See, to me that's a stupid instrument. It tells what your angle of attack is. If you don't know you shouldn't be flying." - Chuck Yeager, from the back seat of F-15D at age 89.

=RvE=

Posted
The compressor (or "engine" to be generic) RPM gauge measures the percentage of RPM of the compressor stages.

 

Very good post overall, I just jave one correction.

 

It's not compressor RPM, is "gas generator" RPM. It's a technicality, since they are both physically connected and therefore have no choice but to spin at the same speed. The gas generator (GG) turbines are immedaitely behind the combustion chamber. Many people will also refer to engine RPM as "N1" and rotor RPM as "N2".

 

The only other thing I would mention is that there is a clutch between the free turbines on the engines and the main gearbox. This allows the engines to drive the main gearbox, but prevents the main gearbox from driving the free turbines. This is important if your free turbine can no longer spin because of damage.

Posted
That's correct of course, but since there is a mechanical link between both, their RPM are proportional. Because the FT/rotor RPM gauge is calibrated in percent of max. RPM, it measures both values.

 

Acedy,

 

Great point, percentage works for both!

Posted
Very good post overall, I just jave one correction.

 

It's not compressor RPM, is "gas generator" RPM. It's a technicality, since they are both physically connected and therefore have no choice but to spin at the same speed. The gas generator (GG) turbines are immedaitely behind the combustion chamber. Many people will also refer to engine RPM as "N1" and rotor RPM as "N2".

 

Hi AlphaOneSix,

 

Thanks for the correction, very warranted!

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