How do you cut down props?

I just compare the distance to the tips with a sharp pointed engineering compass. I scribe it where I want to cut with one point in the screw hole and draw the compass over the tip to mark it, and then cut it with scissors or fingernail clippers, round it with sand paper and double/triple check the blade length with the compass.

Andrews method is probably more precise than mine, but mine usually balance fine.

Both good ideas, I will try both and see what works best for me. Thanks Gents!

Ron, here is a more detailed post about my method I wrote last week:
https://www.coxengineforum.com/t2973-yet-another-prop-balancing-thread

Ron, I'm not sure if your familiar with the tip shape. A square cut tip just like the Cox rubber ducky is not very efficient. The reason however why the Cox prop works so well is that it has a blade area that remains constant from hub to tip. The APC's are very different as some of them really radically taper down. After I trim a APC for instance, I go back and round the tip from the leading edge of the prop to the trailing edge. Doing this makes the prop more efficient instead of just cutting and leaving the tips squared off. I don't always balance my props. In combat, the props are usually short lived aside from a Cox rubber ducky. I will balance a prop that has been trimmed down though. My findings with APC is quite positive as far as balance is concerned. I will sand but this does take a bit of time. I take a single edge razor blade and scrape the heavy blade of needed which is quite faster. Ken

Ken Cook wrote: Ron, I'm not sure if your familiar with the tip shape. A square cut tip just like the Cox rubber ducky is not very efficient. The reason however why the Cox prop works so well is that it has a blade area that remains constant from hub to tip. The APC's are very different as some of them really radically taper down. After I trim a APC for instance, I go back and round the tip from the leading edge of the prop to the trailing edge. Doing this makes the prop more efficient instead of just cutting and leaving the tips squared off. I don't always balance my props. In combat, the props are usually short lived aside from a Cox rubber ducky. I will balance a prop that has been trimmed down though. My findings with APC is quite positive as far as balance is concerned. I will sand but this does take a bit of time. I take a single edge razor blade and scrape the heavy blade of needed which is quite faster. Ken

Ken, I hear the term "Rubber Ducky" being thrown around, but I am not sure what that is. Are we talking about the safety tip ones? Like Bernie's yellow prop?
Or just the regular flexible Cox standard ones? I like the competition ones the most the problem is they are very brittle and like to snap on less than stellar landings.

Thanks for the good tips! No pun intended.

Ron

cribbs74 wrote:

Ken Cook wrote: Ron, I'm not sure if your familiar with the tip shape. A square cut tip just like the Cox rubber ducky is not very efficient. The reason however why the Cox prop works so well is that it has a blade area that remains constant from hub to tip. The APC's are very different as some of them really radically taper down. After I trim a APC for instance, I go back and round the tip from the leading edge of the prop to the trailing edge. Doing this makes the prop more efficient instead of just cutting and leaving the tips squared off. I don't always balance my props. In combat, the props are usually short lived aside from a Cox rubber ducky. I will balance a prop that has been trimmed down though. My findings with APC is quite positive as far as balance is concerned. I will sand but this does take a bit of time. I take a single edge razor blade and scrape the heavy blade of needed which is quite faster. Ken

Ken, I hear the term "Rubber Ducky" being thrown around, but I am not sure what that is. Are we talking about the safety tip ones? Like Bernie's yellow prop?
Or just the regular flexible Cox standard ones? I like the competition ones the most the problem is they are very brittle and like to snap on less than stellar landings.

Thanks for the good tips! No pun intended.

Ron

The yellow reallly flexy and tough ones Bernie does sell them.

Funny, while they are flexible I see signs of stress cracks after an impact. So they usually only last one rough landing. They are also a pain to balance.

So you are saying those are the Rubber Duckies?

Ron, that term is used for the soft flexible props. I personally don't use the safety tip props. I'm sure they would work. I prefer the straight tipped black flexible props. I certainly agree with your findings as well with the grey. They do work more efficiently though. There are limitations to any prop. We use the Cox props in the 26k-30k+ range. We flex the props prior to flying and check for cracks at the hub. APC props are like the Cox grey, a landing with the prop hanging down even on soft grass results in a broken prop. I don't want to sound cheap, but I broke about 5 props one after noon and some of the APC props cost about $3.95 a piece. This would be the APC 5x3 and 4.6 x 3 size. The Cox black 5x3 works quite well on a Tee Dee and trimmed down versions work well on the Norvel's. I had positive results clipping the prop for the Tee Dee, I just found that there's a point where the prop just doesn't work effectively which was around 4 3/8". If for instance you have a model that maybe just flying too fast for your liking, this technique could certainly help. Ken

We have a few beginners here on the forum, so...
Somebody needs to mention that any propeller should be "clocked" on the drive hub so that it will lay in the horizontal position when the engine comes up on compression. This way when the engine quits, the air will push the prop to lie flat when gliding in for a landing. This works like a charm to avoid prop breakage upon landing.

lousyflyer wrote:We have a few beginners here on the forum, so...
Somebody needs to mention that any propeller should be "clocked" on the drive hub so that it will lay in the horizontal position when the engine comes up on compression. This way when the engine quits, the air will push the prop to lie flat when gliding in for a landing. This works like a charm to avoid prop breakage upon landing.

I've done this as it's what the engine instruction sheets say to do however, I had no idea why. Now I know

Usually though I am coming in at full power when I break the props!

Yeah, you might want to work on that part.

lousyflyer wrote:Yeah, you might want to work on that part.

I'm trying! That's why I'm breaking props.

lousyflyer wrote:We have a few beginners here on the forum, so...
Somebody needs to mention that any propeller should be "clocked" on the drive hub so that it will lay in the horizontal position when the engine comes up on compression. This way when the engine quits, the air will push the prop to lie flat when gliding in for a landing. This works like a charm to avoid prop breakage upon landing.

I tried that one when I heard about it, but the prop would bounce off compression and the blade would hang straight down. I would Put the plane in a dive to gain airspeed and push it level,,, still no dice. Now I just clock them to give me a few more milliseconds to get my finger out of the way if I am hand flipping,, which is basically the same.

Flying off grass helps a lot.

The only attention I pay to it is to place it so the piston covers the exhaust when the blade is where I hook the spring, and then it's closed when I prime it too(that ends up being pretty close to where y'all are recommending). The MA props are almost as tough as the black rubber duckies, I've never broken one. Looks like we all have different ways of treating it.

Bare at RCG claims the usually heavy sided rubber ducky hubs should be positioned to add to the inadequate(so he says) counterbalance on the Cox crank. In other words point the heavy side of the hub down when the piston is at BDC.(The heavy side is always the side that has the molding divot on it)

Rubber duckies are all of the standard black Cox props. I don't know what the yellow ones are called, except I cannot seem to balance them. Maybe it's the front holes position causing me to have that problem, as I've never tried reaming one and adding a spacer in the backside.

We all have had our shares of what Ron has been experiencing. Breaking props under full power coming into the ground which made me chuckle a bit is something he's going to need to work on. The flexible props certainly will work better for you in that case. With the engine just coming up on the compression stroke, the prop should be level with the wing. This in turn should windmill the prop into that position when power quits. It will just rotate until it bumps into the compression stroke. This is theoretical as it may work 75% of the time. Ken

Ken Cook wrote: With the engine just coming up on the compression stroke, the prop should be level with the wing. This in turn should windmill the prop into that position when power quits. It will just rotate until it bumps into the compression stroke.
Ken

This is the timing that I use for all displacement engines. Like Ken said, it doesn't aways work, but it's the best positioning I've found.

COX black and MA seem to be more durable; COX gray and APC less so.

Sometimes it depends on what engine you are running. I would much rather break a prop than bend the crankshaft on one of my Space Bug, Space Bug Junior, or Thermal Hopper engines.

It was so nice when the Babe Bee came out with a prop screw that could be replaced.

Most other engines had a threaded shaft with prop nut...except OK Cubs, and perhaps one or two others I can't remember.

Of course most modern engines have a prop screw.

George

I think my MP Jet .061, still in the box, has the threaded shaft and nut. I'm interested to see how that engine runs. That'll be my next shop project.

Hi Ron,

Remember when you trim a larger prop down, the prop's pitch goes up as the pitch is a based on a average angle, compare the tip angle with the angle 1'' in and you'll see what I mean.

Also, a fingernail clipper works great for trimming damaged tips or reducing diameters on nylon 1/2a props.

take care,

Mark