Why do props sometimes spin backwards after shutting down?
- Thread starter Mattio
- Start date
Firebird2XC
Well-Known Member
Re: Why do props sometimes spin backwards after shutting dow
Um.. I'm going to say they don't... and that it's a visual illusion.
Given the forces moving a prop forward, suddenly spinning in the opposite direction would seem contrary to what I know about reciprocating engines.
I may, in fact, be wrong, but that's my best guess.
Um.. I'm going to say they don't... and that it's a visual illusion.
Given the forces moving a prop forward, suddenly spinning in the opposite direction would seem contrary to what I know about reciprocating engines.
I may, in fact, be wrong, but that's my best guess.
BajtheJino
I'm looking at you.
Re: Why do props sometimes spin backwards after shutting dow
:yeahthat: If they did you engine would be toast.
:yeahthat: If they did you engine would be toast.
Joe Gremlin
Well-Known Member
Re: Why do props sometimes spin backwards after shutting dow
I've never seen a prop spin backward after shutdown. I've seen it bob back a forth a few times at the top of the compression stroke which ends up stopping it, but I've never seen one spin backward.
Useless anecdote of the day: Back when I was flying the same plane every day, the prop would stop in pretty much the exact same spot (blade at 2:00) on every shutdown and would usually display the bobbing/bouncing I mentioned above. Then one day on shutdown, it stopped dead at the 12:00 position and didn't bounce back at all. Then next shutdown, it did the same thing. I was at the MX with another plane the next morning and I mentioned this to one of the mechanics thinking that it could be a sign of something going wrong in the engine. The mechanic told me I was crazy and that it couldn't possibly mean anything. Two days later they were replacing a jug on that plane.
I've never seen a prop spin backward after shutdown. I've seen it bob back a forth a few times at the top of the compression stroke which ends up stopping it, but I've never seen one spin backward.
Useless anecdote of the day: Back when I was flying the same plane every day, the prop would stop in pretty much the exact same spot (blade at 2:00) on every shutdown and would usually display the bobbing/bouncing I mentioned above. Then one day on shutdown, it stopped dead at the 12:00 position and didn't bounce back at all. Then next shutdown, it did the same thing. I was at the MX with another plane the next morning and I mentioned this to one of the mechanics thinking that it could be a sign of something going wrong in the engine. The mechanic told me I was crazy and that it couldn't possibly mean anything. Two days later they were replacing a jug on that plane.
FlyChicaga
Vintage Restoration
Re: Why do props sometimes spin backwards after shutting dow
If I am understanding exactly what you are asking, you are seeing an optical illusion caused by the changing rotation speed of the propeller and the light upon it. As the propeller reaches a certain RPM, it will appear to quickly rotate backwards, then again forwards as it slows to near stopping speed.
If I am understanding exactly what you are asking, you are seeing an optical illusion caused by the changing rotation speed of the propeller and the light upon it. As the propeller reaches a certain RPM, it will appear to quickly rotate backwards, then again forwards as it slows to near stopping speed.
OldTownPilot
Well-Known Member
Re: Why do props sometimes spin backwards after shutting dow
Unless its a two stroke.
thats how dirt bikes and snowmobiles go in reverse, turn the engine the other way.
Unless its a two stroke.
thats how dirt bikes and snowmobiles go in reverse, turn the engine the other way.
Johnsc81
New Member
Re: Why do props sometimes spin backwards after shutting dow
No, they can and will spin backwards. the only thing stopping them from spinning backwards in the first place is the starter turning them in the correct direction and the position of the timing. but it can happen very easily if the impulse coupling is bad and it fires to early on start up, or if the engine is hot and the engine was not properly shut down, or a bad fuel servo, the fuel will self-ignite early and spin the engine backwards. essentially it is an engine run on, but firing too early. Granted if they do spin backwards, or if they start to spin backwards and then the engine decides to fire correctly then the engine is toast.
No, they can and will spin backwards. the only thing stopping them from spinning backwards in the first place is the starter turning them in the correct direction and the position of the timing. but it can happen very easily if the impulse coupling is bad and it fires to early on start up, or if the engine is hot and the engine was not properly shut down, or a bad fuel servo, the fuel will self-ignite early and spin the engine backwards. essentially it is an engine run on, but firing too early. Granted if they do spin backwards, or if they start to spin backwards and then the engine decides to fire correctly then the engine is toast.
Cessnaflyer
Wooooooooooooooooooooooooooooooo
Re: Why do props sometimes spin backwards after shutting dow
So you are saying that the valves magically work backwards now and fuel can be introduced from the exhaust valve and exhaust can be let out the intake?
So you are saying that the valves magically work backwards now and fuel can be introduced from the exhaust valve and exhaust can be let out the intake?
subpilot
Squawking 7600
Re: Why do props sometimes spin backwards after shutting dow
When you see the prop go backwards and then forward again is an optical illusion. Light has a frequency associated with it and the way you see the prop turning is related to which way the prop speed is out of synch with the lights frequency.
Thank you! I was hoping someone would bring this to light.
When you see the prop go backwards and then forward again is an optical illusion. Light has a frequency associated with it and the way you see the prop turning is related to which way the prop speed is out of synch with the lights frequency.
Johnsc81
New Member
Re: Why do props sometimes spin backwards after shutting dow
Under very certain conditions, the valves can work as advertised and still allow the engine to run backwards. It is rare and Usually only happens for a few seconds at most. Mostly on fuel injected engines.
Condition 1 (Most important): Fuel injected aviation engines are constanst flow fuel injection... the injectors never stop pumping fuel unless the mixture is in Idle cut-off. if the fuel servo is out of rig, or damaged for some reason, the injectors will continue to pump fuel, or more accuratly, not stop the fuel from flowing.
Condition 2: Valve Overlap (both valves are open at the same time during certain points of the engine rotation), Valve Lash (valves begin to open and close about 10-15 degrees before the cylinder get to TDC), Damaged Rocker, Damaged lifter, Damage pushrod, Bent valve, poor compression, dirty valve (carbon build up from running to rich, etc...) etc. etc. etc....
Conditon 3: Magneto impulse coupling dmaged/inop, or magneto improperly timed.....
you go flying, engine is hot, you shut down engine no problem, next guy comes up to fly, engine is still hot. but this time a leaking servo has allowed fuel to leak to the intake chamber on the cylinders where it vaporizes as soon as it hits the warm cylinders. in one cylinder the intake valve never completely closed so the fuel vapor has migrated into that cylinder. as he turns the starter on the engine slowly rotates thru its rotation, now the fuel vapor enters each cylinder one at a time, and then theyi expel the fuel/air mixture out the exhaust valve and fill the exhaust manifold with the Air/fuel mixture, finally one of the spark plugs ignite before TDC forcing the engine to fire backwards (destroying your ring gear and starter), now remember the fuel/air mixture that is setting in the exhaust manifold, it is now effectively the intake manifold, at least for one or two cycles, and those cylinders that have not had cooling air over them for the last few minutes are even hotter than when they were shut down (ever heard of soak-back), allowing them to self ignite the already hot fuel/air mixture..... get the picture it can and does happen. I have seen it, granted only a few times in my 10+ year career as an A&P, but it does happen, most commonly during servo trimming runs.
The optical illusion is very different than What I am talking about, the illusion of the engine turning backwards typically happens between 2200 rpm and 1500 rpm. That is caused by your eyes only being able to capture roughly 30 images per second, so as the prop blades moves in front of your eyes, your eyes catch it at a slightly different angle every 30th of a second giving the impression that is is moving slowly one direction or the other....(now I know someone is going to bring up the fact that you don't actually see the prop blade, true, your eyes can not clearly focus on something moving that fast so it appears blurred...) Because of reflection of the light off the blades, that is usually best seen when the sun is about 20 degress off of your horizontal plane and you are viewing it from an angle of about 20 degrees from the props' plane of rotation.
Under very certain conditions, the valves can work as advertised and still allow the engine to run backwards. It is rare and Usually only happens for a few seconds at most. Mostly on fuel injected engines.
Condition 1 (Most important): Fuel injected aviation engines are constanst flow fuel injection... the injectors never stop pumping fuel unless the mixture is in Idle cut-off. if the fuel servo is out of rig, or damaged for some reason, the injectors will continue to pump fuel, or more accuratly, not stop the fuel from flowing.
Condition 2: Valve Overlap (both valves are open at the same time during certain points of the engine rotation), Valve Lash (valves begin to open and close about 10-15 degrees before the cylinder get to TDC), Damaged Rocker, Damaged lifter, Damage pushrod, Bent valve, poor compression, dirty valve (carbon build up from running to rich, etc...) etc. etc. etc....
Conditon 3: Magneto impulse coupling dmaged/inop, or magneto improperly timed.....
you go flying, engine is hot, you shut down engine no problem, next guy comes up to fly, engine is still hot. but this time a leaking servo has allowed fuel to leak to the intake chamber on the cylinders where it vaporizes as soon as it hits the warm cylinders. in one cylinder the intake valve never completely closed so the fuel vapor has migrated into that cylinder. as he turns the starter on the engine slowly rotates thru its rotation, now the fuel vapor enters each cylinder one at a time, and then theyi expel the fuel/air mixture out the exhaust valve and fill the exhaust manifold with the Air/fuel mixture, finally one of the spark plugs ignite before TDC forcing the engine to fire backwards (destroying your ring gear and starter), now remember the fuel/air mixture that is setting in the exhaust manifold, it is now effectively the intake manifold, at least for one or two cycles, and those cylinders that have not had cooling air over them for the last few minutes are even hotter than when they were shut down (ever heard of soak-back), allowing them to self ignite the already hot fuel/air mixture..... get the picture it can and does happen. I have seen it, granted only a few times in my 10+ year career as an A&P, but it does happen, most commonly during servo trimming runs.
The optical illusion is very different than What I am talking about, the illusion of the engine turning backwards typically happens between 2200 rpm and 1500 rpm. That is caused by your eyes only being able to capture roughly 30 images per second, so as the prop blades moves in front of your eyes, your eyes catch it at a slightly different angle every 30th of a second giving the impression that is is moving slowly one direction or the other....(now I know someone is going to bring up the fact that you don't actually see the prop blade, true, your eyes can not clearly focus on something moving that fast so it appears blurred...) Because of reflection of the light off the blades, that is usually best seen when the sun is about 20 degress off of your horizontal plane and you are viewing it from an angle of about 20 degrees from the props' plane of rotation.
Re: Why do props sometimes spin backwards after shutting dow
-mini
Absolutely. If you don't lock it down and the wind is blowing the right direction it will.
-mini
darrenf
resident denizen
Re: Why do props sometimes spin backwards after shutting dow
I believe we spun them forward.
I believe we spun them forward.
R
Roger, Roger
Guest
Re: Why do props sometimes spin backwards after shutting dow
It's the prop reverse mechanism self-testing as the engine shuts down.
It's the prop reverse mechanism self-testing as the engine shuts down.
Lee D
Well-Known Member
Re: Why do props sometimes spin backwards after shutting dow
If the engine itself is off they can spin either way.
If the engine is running it is only going to spin one way. If it appears otherwise it is the optical illusion others have mentioned.
If the engine itself is off they can spin either way.
If the engine is running it is only going to spin one way. If it appears otherwise it is the optical illusion others have mentioned.
OhioStatePilot
Well-Known Member
Re: Why do props sometimes spin backwards after shutting dow
Huh, I was under impression that spinning a geared turboprop (like on the B100) backwards was a bad thing. Specifically what I've heard pertains to the generators wearing in to the conventional rotation and when rotated backwards it can cause components to break.
Huh, I was under impression that spinning a geared turboprop (like on the B100) backwards was a bad thing. Specifically what I've heard pertains to the generators wearing in to the conventional rotation and when rotated backwards it can cause components to break.
PhilosopherPilot
Well-Known Member
Re: Why do props sometimes spin backwards after shutting dow
As it was said in an earlier post, it can actually run backwards for a second or two upon shutdown of a hot engine. I've seen it. I've seen my dad's old POS car do it too, though you couldn't actually "see" it in that case, but it was the same thing. Hot spots and a little extra fuel...
As it was said in an earlier post, it can actually run backwards for a second or two upon shutdown of a hot engine. I've seen it. I've seen my dad's old POS car do it too, though you couldn't actually "see" it in that case, but it was the same thing. Hot spots and a little extra fuel...
Boris Badenov
Fortis Leader
Re: Why do props sometimes spin backwards after shutting dow
This is true. Spinning a TPE331 backwards is a no-no and will get your hand slapped by the CP who is training you. Add to this that the -10s on an MU-2-60 spin the opposite direction from the -6s on a -36 and then ask me how I know. "Spin it TOWARDS THE BOOTS...<under breath> idiot".
I think maybe they're talking about pistons, though...
This is true. Spinning a TPE331 backwards is a no-no and will get your hand slapped by the CP who is training you. Add to this that the -10s on an MU-2-60 spin the opposite direction from the -6s on a -36 and then ask me how I know. "Spin it TOWARDS THE BOOTS...<under breath> idiot".
I think maybe they're talking about pistons, though...