Another dumb question?

ok THAT MAKES SENSE BUT WHY does A BACKFIRE cause loosening of the screw?

crs1945 wrote:ok THAT MAKES SENSE BUT WHY does A BACKFIRE cause loosening of the screw?

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.

Great Thanks for the quick replies>

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

crs1945 wrote:ok THAT MAKES SENSE BUT WHY does A BACKFIRE cause loosening of the screw?

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.

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.

George

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.

Thanks for all the answers. Now I understand!