Hello all,

I was just wondering about these. My uncle is a tool and die caster at GM Lordstown, he's responsible for designing parts to fit in the cars, as well as designing (and producing a prototype) the machines that will be put onto the assembly line to put his new piece into the car itself.

Anyways, he was looking over my plane and said that he could easily produce aluminum props (for free) that would be perfectly balanced. I dont know how he'd manage that, but he said all he needs is a prop to go from and he'll put it into the computer and he could have the prop in no time. How does this sound to some of the more experienced pilots? Think it would be safe to give it a shot? I dont see why not, it'll probably go on my spad anyways.

Any input would be great

Thanks,

Ben

Metal props can fatigue. Hairline cracks that you cannot see will develop, with a possible catosphic failure! Use plastic or wood. Besides they are illegal the AMA has outlawed them for the above reasons.

How do they fatigue? If they're perfectly balanced throughout, then it being basically a 2-force member, the force acts through the ends. I'm not arguing with you, you definately know more about this than I do, I am just curious. Do they bend from the rpm?

I'm not also an expert but I guess the stress is due to the force that act ove the prop that bend it forward the aiplane direction. This force changes all the time from idle to full throttle and this back and forth causes fatique.
There should be however a techinque to build props with metal, maybe multiple layers or other stuff that make it safer, since real planes uses metal props, am I right?

Nilo

The root of the blade, the line perpendicular to the length of the blade is carrying two major forces. The first is tension, each blade is trying to break at the hub and fly off in a separate direction. Added to this the back of the blade, which is now scratched from being forced into the thrust flange on the engine is under further surface tension from the air loads trying to bend the blades forward. The scratches on the back face are the flaws that become the origin of a fatigue crack. These two forces combined with the very high loads at the ends of the scratches add up to a tension load that the metal cannot handle and a little crack forms in the end of the scratch. Each time the propeller is accelerated and loaded that crack grows another little bit. At some point we reach critical crack length and the crack runs completely through the root of the hub between the mounting hole and one edge of the blade. We just doubled the loads on the remaining half of the hub, which will succumb to the same failures in even less time.

Fatigue in highly loaded components is very very unpredictable due to all the variations we put propellers through.

Second factor is that the resilient woods and plastics we use for propellers are a little weaker than the strength of human flesh and bone. The wood and plastic blades usually break on contact with enough human parts. That new aluminum blade will proceed to meat grind your body part to nothing.

Thirdly once flung off the hub wood and plastic have pretty low mass and slow down relatively quickly. Metal on the other hand doesn't. I once investigated an Air Force mishap in which a CH-53 taxied into a parked helicopter, and they grazed rotor tips. A FOUR OUNCE blade counterbalance left the end of the slow spinning (ground idle) rotor blade, and flew horizontally for 1/8 th of a mile and went clean through the fuselage of a KC-135. It then went over base operations and broke the front windscreen of a C-130 that was another 400 yards away!

If the aluminum propeller did come off of your model it would go through you in less than a heartbeat.

HTH
Former Chief Structural Engineer for the C/KC-135 and Presidental aircraft fleets.

Real planes use metal props, but turn them at much lower RPM. They have log books and strict inspection and overhaul points based on number of hours of operation, which is basically the number of revolutions of the prop.

How many guys log exactly the run time of their engines?

The forces on a prop that matter are fore-aft, not along the blade. If the engine were always pointed directly in to the wind, the force would be constant, but it isn't. If the engine is even a fraction of a degree off from the direction of flight, then the blade at one point of a revolution has a higher angle of attack than at another. That cause more lift and more force at one point than another. This cause the blade to flex a little as the force increases and decreases. That's like bending it back and forth with every revolution of the engine.

There's a world of difference between 1,500rpm and 15,000rpm when it comes to blade life.

Whoa!!!

That means I should stay away from full scale aircraft!!!

At least stay away from the props!

Hi Smal Fry,I'll loan you my deer rifle if you are in a hurry to hurt yourself or someone at your flying field,would think are pro die caster would know better.[&o]

So everyone is saying that the metal will break because it's too rigid?

TerrellFlyer, I fly out of my yard (36 acres in the back) and I'm not in a big hurry to take that path yet. I'll keep you in mind though if I ever get the urge lol.

Also the torsional vibration as it accelerates and deccelerates twice for every full turn of the crank on a single cyl. engine.

Is it possible to design one that wont though? I mean, this is what this guy does for a living. He can build a car in his sleep, he's not a metalurgist, but has a grasp of engineering physics (mechanical engineer by degree). He did a lot of the upgrading in the engine of the new Cobalt made by GM. Maybe it's not possible, but man, it'd be great if it was!

Small Fry,
Think about this. Everyone is trying to say don't use metal props. Also think about this. There are no commercial metal props for models. PERIOD. There is a reason. They are not safe. You can't spot the hairline fractures before they become big problems. Please don't do it.
John

Gotta add my two cents worth. Most general aviation props are made of steel, not aluminum, as steel is more flexible than aluminum. Also, the props are highly polished to remove any imperfections that can cause stress risers. Most modern larger aircraft (at least the ones I've dealth with E-2/C-2, P-3). Also, I would think that a nylon or wood prop would actually absorb some vibrations, due to flexing of the blades, where an aluminum prop would magnify them.

Oh, and somewhat off topic, on most turbine powered helicopters, the rotor head turns at 100% of flight speed all the time, the gas generator section of the engine will vary RPM to keep the rotor speed at 100% as loads are induced. Sounds strange but it works

Andy

Why? I can't think of any advantage it would have, but there are several obvious disadvantages.

JNorton, I'm not arguing. I'm just askin questions because I'm curious. geeez!

MikeL, aside from the obvious disadvantages, read my first post: It'd be great because I'd get them for free.

Props only cost a few bucks a piece, and with some practice you won't break them very often. I've only broken one in the last two years, which is likely more a testament to my luck as opposed to my landing consistency!

Most GA props are made of Al. NOT STEEL. I just finished school at east coast aerotech and my fingers are still sore from blending out nicks and scratches with a riffle file on Al props. Steel would be too heavy.
Also, a metal prop for our RC engines would be too much rotating mass and would degrade performance greatly, IMHO

sean

well, regardless now. It's a no no, and I got all my questions answered. Thanks to all for the quick replies.

MikeL, I'm a college student working and paying my way through engineering school. More often than not, the "few bucks" you mentioned go towards gas lol.

The steel props are hollow.

My bad. That's what happens when you talk to a pilot about mechanic stuff..... I always figured they were aluminum, but one day while talking to above mentioned flyboy about this and that he said nickel steel.... maybee he meant the erosion strips.

Small Fry
omg...i remember the first summer i start flying, i had 2 replace the prop every couples of flights because of bad landings.
Oh yes thats a lot of props, i used about 3 to 5 props a week. Even if you can have them for free, i think it would a lot more
expensive to produce them than to buy plastic or wood props. The other think you should consider, i always carry differents size/pitch
of props when flying new plane or new engine to make some testing. I have many engine size, .40,.46,.50,and .68, imagine how many different props size i have in my shop, i must have about 20 different props. I don't think your buddy would like to make you new props every time you change your plane or engine. The last and the most important, man, a metal prop would be too Fu**** dangerous, imagine if you get your fingers in there ! or even worse loose control and hit someone !!! this thing will not break like a plastic prop ! Plz don't use metal prop.

My initial reaction would be "why not just go out and cut your fingers off with a saw?" The wood and composite blades are dangerous enough but start turning that metal prop and you've got a great little circular saw going. Stay away from the metal prop idea is my best answer.

Speaking as a former Machinist, I can see where your Uncle is coming from. There were many times when I was tempted to make a prop, not to use, but just because it is such an intriquiging project.

It is also a LOT of work, and something you would do as a "Labor of Love".

Believe me, even if he WERE to make one for you, it wouldn't be something he'd want to do more than once or twice.

And don't think for a minute that metal will hold up better than plastic. A minor scrape will bend either one, but Plastic will return to it's original shape. Metal won't.

Metal props are just wrong in more ways than we can list.

I'm not going to hammer you like some others will for asking a simple question. Next time your out with your plane get someone to hold your plane while you stand in front of it. Point your plane into the sun or a good light, so you can really watch the prop. Run the engine up and down and watch the flex in the prop. You can really see the prop flexing. We were at the field a couple of weeks ago, and a friend had a YS he couldn't get running. He would get it about 1/2 to 3/4 throttle and the engine would stop abruptly, man I thought the prop was going to come apart. Oh well, just thought you might find that interesting, and could see how a perfectly balanced prop would still undergo a lot of stress. Just my $.02