Turning rotor backwards - damage to hydraulic system
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Turning rotor backwards - damage to hydraulic system
I found a warning in an obscure part of the Maintenance Manual which said "Do not turn the rotor backwards on the ground as damage to the hydraulic system may occur"
It didn't elaborate and I have been told the same thing on a previous type but no one has been able to explain what the damage is. The hydraulic pumps were in both cases of the rotary multi piston constant flow variable pressure type with a swash plate (not THE swash plate) controlling the stroke of the pistons according to demand. The hydraulic systems were pretty similar between the types with an integrated hydraulic unit with integral pressurised reservoir.
Anyone got a good explanation for what damage would happen if you effectively ran the pump and therefore the system in reverse by rotating the rotor backwards (eg by turning the tail rotor/fenestron to position the main head)
It didn't elaborate and I have been told the same thing on a previous type but no one has been able to explain what the damage is. The hydraulic pumps were in both cases of the rotary multi piston constant flow variable pressure type with a swash plate (not THE swash plate) controlling the stroke of the pistons according to demand. The hydraulic systems were pretty similar between the types with an integrated hydraulic unit with integral pressurised reservoir.
Anyone got a good explanation for what damage would happen if you effectively ran the pump and therefore the system in reverse by rotating the rotor backwards (eg by turning the tail rotor/fenestron to position the main head)
As a first guess, running a pump backwards means flow in the system is backwards. In the forward operation there are filters in place to prevent contamination from the reservoir from reaching tiny clearances and either gouging them or preventing them from closing when expected. Going backwards would tend to push any trapped contamination back out of the filter into the tank to be picked up.
It might be possible to evacuate/cavitate a portion of the system which might lead to damage during subsequent restart.
Offhand I cannot imagine that there are any components that are particularly vulnerable when being operated backwards. It would take a look at the schematic to make a guess about that.
There is a concept "Chesterton's Fence" that the best thing to do when encountering a fence with no obvious purpose is to leave it alone. Someone felt there was a purpose and went out of their way to spend time and effort and money to build that fence. Of the two possibilities, unless the fence is a significant problem, either taking down the fence will be meaningless or it will lead to a possibly very unwanted result.
In short - don't turn the rotor backwards.
It might be possible to evacuate/cavitate a portion of the system which might lead to damage during subsequent restart.
Offhand I cannot imagine that there are any components that are particularly vulnerable when being operated backwards. It would take a look at the schematic to make a guess about that.
There is a concept "Chesterton's Fence" that the best thing to do when encountering a fence with no obvious purpose is to leave it alone. Someone felt there was a purpose and went out of their way to spend time and effort and money to build that fence. Of the two possibilities, unless the fence is a significant problem, either taking down the fence will be meaningless or it will lead to a possibly very unwanted result.
In short - don't turn the rotor backwards.
Just because I had to look it up, the reverse flow would because each piston would be moving up when the bottom of its cylinder was aligned with the discharge port, and down when it was aligned with the inlet port.
Just to clarify - I don't mean that the system will tolerate full power or even a large fraction of full power in reverse - seals are designed with pressure from one direction and won't last long with pressure from the other side. This is part of that fence analogy. Maybe it's worse that there isn't mechanical resistance in that case.
Like wrench1 I would be interested to know the helicopter model, and are you a pilot, technician? As an ex-Puma/Chinook technician I would frequently rotate the trasnsmission slowly in either direction for maintenance, and to facilitate ground handling.
Thread Starter
I find it curious that a " ... warning ...... damage to the hydraulic system may occur" is found in " ... an obscure part of the MM".
Like wrench1 I would be interested to know the helicopter model, and are you a pilot, technician? As an ex-Puma/Chinook technician I would frequently rotate the trasnsmission slowly in either direction for maintenance, and to facilitate ground handling.
Like wrench1 I would be interested to know the helicopter model, and are you a pilot, technician? As an ex-Puma/Chinook technician I would frequently rotate the trasnsmission slowly in either direction for maintenance, and to facilitate ground handling.
The middle warning
Found in the Flight Control Computer functional check on Bell 429. Other aircraft is EC135. The warning is not present in the towing / tie down sections where you have to routinely rotate the main head to line up the main rotors for tie down. In case you're wondering, the aircraft is in exactly the same state mechically for the FCC check as for parking/tie down.
IIRC, which isn't often these days, the EC135 preflight included turning the freewheeling shafts independently to check operation as the rotor couldn't be pulled back.
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Out of interest, is the FCC check done with electrical power on, whereas parking/tie down with it off? Could imagine there may be solenoids open/closed in the system when powered up that aren't when powered down. Maybe?.
The middle warning
Found in the Flight Control Computer functional check on Bell 429. Other aircraft is EC135. The warning is not present in the towing / tie down sections where you have to routinely rotate the main head to line up the main rotors for tie down. In case you're wondering, the aircraft is in exactly the same state mechically for the FCC check as for parking/tie down.
You'll find in newer models they use more electro-mechanical controls which require specific conditions to check various systems. In some cases its the opposite where electrical power must be on prior to manually moving a control input. I believe this is one and is not a general rule but have seen similar warnings with other specific mx tests. If it were a general limitation it would be in the RFM so pilots wont break stuff.
Reservoir Check Valves
As may have already been mentioned, turning the rotor backwards will cause gearbox driven hydraulic pumps to backflush the filters. This will push debris into the hydraulic system, and flight control actuators.
On some helicopters like Bell 429, there is a check valve between the reservoir and return filter to prevent back flow. When turning the rotor backwards on a 429, the check valve can cause the pump to become damaged from cavitation.
On some helicopters like Bell 429, there is a check valve between the reservoir and return filter to prevent back flow. When turning the rotor backwards on a 429, the check valve can cause the pump to become damaged from cavitation.
Thread Starter
You'll find in newer models they use more electro-mechanical controls which require specific conditions to check various systems. In some cases its the opposite where electrical power must be on prior to manually moving a control input. I believe this is one and is not a general rule but have seen similar warnings with other specific mx tests. If it were a general limitation it would be in the RFM so pilots wont break stuff.
From that response I am assuming I was correct in that it is nothing special about the FCC check that makes damage possible, but simply turning the rotor backwards - I think one suggestion (thank you whoever that was) was inadvertently getting all the crap back off the filter into the pump. That seems possible with the hydraulic diagram from the Systems Description.
Extract from Bell 429 hydraulics diagram
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When installing main rotor tie down, AFM for Bell 505 indicates rotating rotors in the opposite direction. Not sure why...
"Rotate main rotor blades in opposite direction of rotation until main rotor blades are aligned with tail boom."
"Rotate main rotor blades in opposite direction of rotation until main rotor blades are aligned with tail boom."
Bell have come back to me already and agree the warning should be more widely used (specifically in the towing and tie down sections). Nice to see some manufacturers get back to you quickly.
From that response I am assuming I was correct in that it is nothing special about the FCC check that makes damage possible, but simply turning the rotor backwards - I think one suggestion (thank you whoever that was) was inadvertently getting all the crap back off the filter into the pump. That seems possible with the hydraulic diagram from the Systems Description.
Extract from Bell 429 hydraulics diagram
From that response I am assuming I was correct in that it is nothing special about the FCC check that makes damage possible, but simply turning the rotor backwards - I think one suggestion (thank you whoever that was) was inadvertently getting all the crap back off the filter into the pump. That seems possible with the hydraulic diagram from the Systems Description.
Extract from Bell 429 hydraulics diagram
Last edited by CTR; 19th Jul 2023 at 19:05.
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Just curious but how much pressure / suction would rotating the rotor system backwards at 2-3 rpm max generate?
Never saw any crap on a hydraulic filter except once when the fluid in a system somehow got contaminated…we never did figure out how the contamination occurred. Discovered by the engineer during his evening DI. fluid was an opaque yellow colour instead of the expected clear Cherry Jello red.
System flushed, filter replaced ect. never happened again. Strange.
Never saw any crap on a hydraulic filter except once when the fluid in a system somehow got contaminated…we never did figure out how the contamination occurred. Discovered by the engineer during his evening DI. fluid was an opaque yellow colour instead of the expected clear Cherry Jello red.
System flushed, filter replaced ect. never happened again. Strange.
Last edited by albatross; 20th Jul 2023 at 16:00.
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As may have already been mentioned, turning the rotor backwards will cause gearbox driven hydraulic pumps to backflush the filters. This will push debris into the hydraulic system, and flight control actuators.
On some helicopters like Bell 429, there is a check valve between the reservoir and return filter to prevent back flow. When turning the rotor backwards on a 429, the check valve can cause the pump to become damaged from cavitation.
On some helicopters like Bell 429, there is a check valve between the reservoir and return filter to prevent back flow. When turning the rotor backwards on a 429, the check valve can cause the pump to become damaged from cavitation.
Take a look at the schematic for reference. For the return filter, you are mostly correct. But for the pressure filter, it will suck debris through the pump and push it into the return system bypassing any filters.
Just curious but how much pressure / suction would rotating the rotor system backwards at 2-3 rpm max generate?
Never saw any crap on a hydraulic filter except once when the fluid in a system somehow got contaminated…we never did figure out how the contamination occurred. Discovered by the engineer during his evening DI. fluid was an opaque yellow colour instead of the expected clear Cherry Jello red.
System flushed, filter replaced ect. never happened again. Strange.
Never saw any crap on a hydraulic filter except once when the fluid in a system somehow got contaminated…we never did figure out how the contamination occurred. Discovered by the engineer during his evening DI. fluid was an opaque yellow colour instead of the expected clear Cherry Jello red.
System flushed, filter replaced ect. never happened again. Strange.