Propeller Centrifugal Force

[kyleservicetech]

Everyone who flies model airplanes knows (or should know) it is never a good idea to stand in line with the models propeller when the power system is running at full power. This is especially true with the higher powered electric models we are now running, not to mention the big gasser models.

I ran across this web page for calculating centrifugal force, and applied it to a 19X12 APC-E Wide blade prop. Taking a guess at the effective weight and span of one of the propeller blades, this program gave a centrifugal force of 850 POUNDS at the hub of that propeller, while turning at 7200 RPM.

Note that this program has drop downs that allow data entry in either metric or the "old USA format".

http://www.calctool.org/CALC/phys/newtonian/centrifugal

[hayofstacks]

Even though there is a lot of speed, there isn't much enertia. Not saying I would want to get hit by a prop that came off at wot.

The prop will lose a lot of airspeed with distance, not sure how you would calculate it though. Since a prop is ment to move air, it will move a lot of air when it comes flying off, giving you a ton of drag and slowing it fairly quickly.

These props really are dangerous though. I remember smashing my finger and cutting up my arm after I accidently shut down my controller with the plane still armed. Now I double check that my bind is at 0% throttle.

My dad hates plastic and nylon props. He said back in the day if you stuck your finger in a wood prop it would cut ya up but break off. With nylon you could lose your finger.

[kyleservicetech]

Originally Posted by hayofstacks

Even though there is a lot of speed, there isn't much enertia. Not saying I would want to get hit by a prop that came off at wot.

The prop will lose a lot of airspeed with distance, not sure how you would calculate it though. Since a prop is ment to move air, it will move a lot of air when it comes flying off, giving you a ton of drag and slowing it fairly quickly.

These props really are dangerous though. I remember smashing my finger and cutting up my arm after I accidently shut down my controller with the plane still armed. Now I double check that my bind is at 0% throttle.

My dad hates plastic and nylon props. He said back in the day if you stuck your finger in a wood prop it would cut ya up but break off. With nylon you could lose your finger.

Yeah
Many years ago, I put a 12 inch prop on a very high powered universal type one and a half horsepower motor from work. The motor was outside, securely mounted. The motor was driven by a 120 VAC Variac (Variable Transformer) After switching off the power, that prop unscrewed itself, and shot forward some 30 feet.

The thing to watch for is any damage to the prop after one of those "Nose Overs".

Back in the mid 1990's, a blade came off of one of my models with a geared Astroflight 40 motor. The resulting unbalanced blade completely ripped the entire front end off of that model, until the wires pulled loose killing the power.

The damage was so significant, that model never flew again.

If you've looked at those APC-E props, those blades are a pair of rotating knives. You better believe those 1KW and more props are treated with a LOT of respect.

[hayofstacks]

Watching my dad do touch and goes with a nitro is a blast. He brings it in low and fast, just touches the main gear and then imediately pulls up. Anytime he thinks his prop might have hit, he immediately kills the engine incase the prop becomes unballanced and rips itself apart. He keeps his speed up so that he can bring it down perfect with it dead stick.

He told me stories about busting props on his old .049 race motors that didn't have a throttle. He would plow the plane into the ground then run over to it and start stuffing the motor into the ground trying to kill the engine before it ripped itself apart.

[JetPlaneFlyer]

Originally Posted by hayofstacks

The prop will lose a lot of airspeed with distance, not sure how you would calculate it though. Since a prop is ment to move air, it will move a lot of air when it comes flying off, giving you a ton of drag and slowing it fairly quickly.

Not necessarily.. They have a lot of drag if moving sideways through the air but centrifugal force pulls the blade out radially. If the prop fails at the hub then the blade would tend to travel outward, moving lengthways like a spear, with very little drag.

Best to keep out of the area in front of and to the side of a fast running prop.

PS.. In my 'throtte-less' i.c. flying days the recommended way to kill a running motor in an emergency (if you couldn't get your finger over the carburator inlet) was to throw a rag in the prop. I'd very much not recommend intentionally jamming the nose of the plane into the dirt to stop a motor

[rcers]

I have seen a prop fail at the hub, at least about in inch away. It was not super high power and IIRC was about 11 or 12" or so.

As pointed out above it was after a nose over. It was spectacular and I agree with above you don't want to be in-line to forward that it where the blade went. I would note that it is not aerodynamically stable and the energy twisted the blade and it did not fly for long or very far.

It did DESTROY the model (on the ground). The fragments of the nose flew about as far as the propeller blade!

Again I would have not wanted to be in the path but if you were a few feet away I don't think the blade would get to you in most cases.

The odd thing is one of the Heli guys "touched" while mowing the grass (inverted blade touch) and one blade half made it from the near edge of the flight line to the pits - it was VERY alarming.

Mike

[pizzano]

Having been associated with electric and nitro heli's, one of the most common safety issues constantly ignored especially by the newer pilots while trying adjust something on the bird while powered-up (it only takes one event to wake them up) is the damage that can be done by a prop failure.

Whether its' loose mounts, unbalanced blades/shaft, worn bearings, improper size, or blade material.......prop damage can be associated to more injuries around heli's than any other issue.

Back in the day (not so long ago) many of the 200-450 sized heli's came stock with wooden/fiber reinforced blades. They were inexpensive, a pain to balance and heavy. But (since blades) are the most frequently replaced item on a heli, much like props on planks, vendors started offering several composite types, carbon fiber, fiberglass...ect....lighter, less balance issues and nearly as cheap as wood.

The problem became durability....when one of these fail (or hit the ground), it's not just a scattering of pieces moving through the air more than 100mph.......but an explosion...!......and not just a splinter or two of wood chunks....but multiple pieces of razor sharp shrapnel.

Obviously, a wooden blade can take a finger or arm off...not as clean as a fiberglass or carbon fiber blade (which is easier for the surgeon to mend)....Just the same, caution, respect and common sense is your best friend around whirling air movers....!

I've had very good experiences with the APC UN reinforced Nylon props.......I've tried wooden (XOAR, Pro Tech) plastic (GWS, Great Planes) and fiberglass (aero naut Glas) on three of my electric driven planes.....The APC (at least in my experience) has been by far the most durable and nick forgiving......although, none of them have been larger than 10x6.

[pd1]

Last fall a pilot got disorientated while flying, the plane made a sharp turn and hit at the edge of the pit area.
It shed a blade and the blade traveled about 6 feet before the blade hit another person in the lower abdomen.
My friend hurt so badly he was off his feet for a couple of days.
A departing blade can cause a lot of damage.

You should never knowingly stand in the prop arc.

[kyleservicetech]

Originally Posted by hayofstacks

Even though there is a lot of speed, there isn't much enertia. Not saying I would want to get hit by a prop that came off at wot.

Don't know why I thought of this, an arrow also has a lot of speed, and not a lot of inertia. Guess it all depends on whether the flying prop blade is flying flat, tumbling through the air, or like flying like that arrow.

[kyleservicetech]

Originally Posted by pizzano

The APC (at least in my experience) has been by far the most durable and nick forgiving......although, none of them have been larger than 10x6.

Agreed, my APC-E props have been excellent in their operation. But, having done the numbers on that 19X12 prop, methinks after three years of flying with it, time to buy new ones!

[Dr Kiwi]

Just to be unutterably pedantic, centripetal force (or lack thereof) is the key here:

According to Newton's third law of motion, for every action there is an equal and opposite reaction. The centripetal force, the action, is balanced by a reaction force, the centrifugal ("center-fleeing") force. The two forces are equal in magnitude and opposite in direction. The centrifugal force does not act on the body in motion; the only force acting on the body in motion is the centripetal force. The centrifugal force acts on the source of the centripetal force to displace it radially from the center of the path. Thus, in twirling a mass on a string, the centripetal force transmitted by the string pulls in on the mass to keep it in its circular path, while the centrifugal force transmitted by the string pulls outward on its point of attachment at the center of the path. The centrifugal force is often mistakenly thought to cause a body to fly out of its circular path when it is released; rather, it is the removal of the centripetal force that allows the body to travel in a straight line as required by Newton's first law. If there were in fact a force acting to force the body out of its circular path, its path when released would not be the straight tangential course that is always observed.

[kyleservicetech]

Originally Posted by Dr Kiwi

Just to be unutterably pedantic, centripetal force (or lack thereof) is the key here:

According to Newton's third law of motion, for every action there is an equal and opposite reaction. The centripetal force, the action, is balanced by a reaction force, the centrifugal ("center-fleeing") force. The two forces are equal in magnitude and opposite in direction. The centrifugal force does not act on the body in motion; the only force acting on the body in motion is the centripetal force. The centrifugal force acts on the source of the centripetal force to displace it radially from the center of the path. Thus, in twirling a mass on a string, the centripetal force transmitted by the string pulls in on the mass to keep it in its circular path, while the centrifugal force transmitted by the string pulls outward on its point of attachment at the center of the path. The centrifugal force is often mistakenly thought to cause a body to fly out of its circular path when it is released; rather, it is the removal of the centripetal force that allows the body to travel in a straight line as required by Newton's first law. If there were in fact a force acting to force the body out of its circular path, its path when released would not be the straight tangential course that is always observed.

Yeah, I'm aware of the centripetal force definition, and in fact debated on using it for this thread.

But, while just about everyone knows that centrifugal force is involved in something spinning, but maybe not everyone knows about the centripetal terminology.