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Post  crs1945 Mon Nov 09, 2015 7:11 am

Cox engines can run in either direction, correct. When turning clockwise ( when standing in front of the plane ) this is the same direction that you turn a screw to tighten it, loosen it is to the left or counterclockwise.Then why doesn't the props screw come loose and the prop fly off into the wild blue yonder when the prop is running in a counterclockwise direction 9 again when standing in front of the planex & looking towards the tail?? RC Plane
Chuck
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Post  fredvon4 Mon Nov 09, 2015 7:53 am

The reed valve Cox engines can run either direction and the front rotary valve engine only run one way depending on what crank shaft is installed (they do make a Left hand crank)

99% of the time if you tighten the prop screw, Nut, or bolt (standard clock wise tightening) it has enough force to keep the propeller fixed even though the running rotational force is in the direction of loosening the fastener.

That said, we all have experienced the prop coming loose at one time or another. Most of the time it is from a back fire or we just failed to check and tighten it up after several runs/flights.

Bottom line is, we all should always test and tighten the prop screw frequently because they do come loose

PS I gave you a greenie because it is a good (not dumb) question and everybody needs to think about these things for safety reasons.


Last edited by fredvon4 on Mon Nov 09, 2015 7:56 am; edited 1 time in total (Reason for editing : add the PS)
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Post  Marleysky Mon Nov 09, 2015 8:11 am

Hey, Fred popped in with the correct response while I was reading the question.

My answer would have been something like "torque"  is the friction between the threads of the prop screw and the threads of the crankshaft. Don't know what the torque values of those small threads would be and I don't have torque wrench that small! The friction of the threads exceeds the force of the rotation and keeps the prop on. If NOT tightened enough the prop will fly off and as Fred said they should be checked before each flight as a safety precaution.
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Post  crs1945 Mon Nov 09, 2015 8:38 am

ok THAT MAKES SENSE BUT WHY does A BACKFIRE cause loosening of the screw? I Love This Forum!
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Post  Marleysky Mon Nov 09, 2015 9:02 am

crs1945 wrote:ok THAT MAKES SENSE BUT WHY does A BACKFIRE cause loosening of the screw? I Love This Forum!

A- a frontfire keeps the motor running in the same direction and the prop stays on.
B- a backfire is a sudden and forceful reversal of direction and the prop flys off.

Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel ( propeller) For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.
When the engine backfires it produces a turning force that exceeds the holding force of the prop screw and off she goes!
I don't want to get to scientific, it hurts to think that much. Just tighten your screws and have some fun.
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Post  crs1945 Mon Nov 09, 2015 9:30 am

Great I Love This Forum! Thanks for the quick replies>
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Post  batjac Mon Nov 09, 2015 2:47 pm

Chuck, here's how I like to explain it:

You are right about the righty-tighty, left-loosy for the prop screw.  So, to tighten the screw you turn it clockwise.  Or, from the screw's perspective, the screw is stationary and the crankshaft is rotating counterclockwise.  So, counterclockwise rotation of the crank = tighter prop screw.  On the great majority of the Cox engines out there, and for all rotary valve engines that do not have the special left hand crank, the prop is turning counterclockwise, which means the crankshaft is turning counterclockwise.  So, other than the initial force turning the prop to get the engine started, all rotational forces during the engine run are acting to keep the prop screw tight.  In fact, the constant pulses as the cylinder fires are like a little super-tiny impact wrench tightening the prop screw.  When the engine backfires, it is a rapid, violent clockwise rotation of the crankshaft.  Or, from the crankshaft's perspective, the mass of the prop and prop screw just made a violent jerk counterclockwise in the "loosen" direction.  That's why the prop comes off on a backfire.

For an engine running clockwise, or with what's commonly called a "left handed prop", or a "pusher prop", the rotational forces are acting in a "loosening" fashion.  That's where the friction of a tight prop screw is all that's holding the prop on.  On the bright side, when the engine backfires on a pusher setup, it only serves to tighten the prop screw.

The Perfessor Mark
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Post  GallopingGhostler Mon Nov 09, 2015 3:12 pm

crs1945 wrote:ok THAT MAKES SENSE BUT WHY does A BACKFIRE cause loosening of the screw? I Love This Forum!
Combustion is a controlled explosion, which is a violent impulse. At speed these combustion explosions occur quick enough so that the sensation appears as a mild high frequency vibration. It is why one should never use electric props on a glow, diesel or ignition engine, unless they are rated for internal combustion use. Electric motors don't have continuous hammering impulses like internal combustion engines.

Think of it as a jack hammer. As others state, it doesn't hurt to check prop screw for tightness before first flight of the day and after any event that stops prop from turning like touching the ground or vegetation.
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Post  gcb Mon Nov 09, 2015 3:18 pm

Back in "Ancient Times" when we used wooden props, it was common for the wood to compress if left tight on the engine between flying sessions. You would need to tighten it as you started a flying session. Some of us would loosen the prop a bit after each flying session so the prop would not get crushed.

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Post  balogh Mon Nov 09, 2015 4:48 pm

Just check the backside of the hub of nylon props and you will understand why the prop stays on. When you tighten the nut/screw whatever, it pushes the prop tight on the prop drive plate that has a recessed surface that bites into the prop hub. You will see the bite-marks of the drive plate on the prop hub. This surface unevenness creates such a friction that even if the nut/screw is not tightened very much the prop will rotate together with the drive plate.

In case a back-fire loosens the nut/screw whatever, the recessed surfaces of the prop hub and the drive plate are allowed to detach from each other, the friction ceases to exist, and the torque of the crankshaft wil no longer drive the prop.

A metal prop would get loose much sooner than a nylon or wooden prop because the latter are softer and allow the formation of the bite marks on their hub backside.
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Post  crs1945 Tue Nov 10, 2015 6:19 am

Thanks for all the answers. Now I understand! This Site Rocks!
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