Propeller deflection.

Thanks. Otherwise these are my favorits on 049 size COX engines and they have never snapped. Unlike the black 5x3 props that once were on the market with visible micro cracks building on the blades and snapping if bent a little by hand.

I've seen this too. It may be due to shrinkage.. which I think would tend to "pull'' the blades in the direction of their twist.. rather than the opposite.

Here's a pair of Cox black 6 x 2 L/H's... The one on top is NOS.. and the one on the bottom is well-used and shows this deflection.



The backward-deflection probably corrects itself somewhat when running at speed on an engine. A straight (new) prop likely deflects forward.. when grabbing air.. especially the more flexible sport-props.

I have used a Cox 5x4 prop on a Norvel .061RC, but one can visibly see the plane slowing down after a extended full throttle run, the engine loves it though...

Afterwards the prop is visibly bent forward. Never seen one bent backwards, perhaps after a rough landing in tall grass?

Carefully examine those for pitch. I have a hypothesis that they have permanently lost pitch so when relaxed the prop has a different shape.

Yes, a few bad landings will make a different thing happen that causes that shape. Good supple nylon props can take it to a certain extent..

Phil

I don't know about grabbing the air but; have seen this and as the RRRss get up they straighten out and also flex back as the RRRs slow ,,,, i figure its a good flexible prop and don't consider it dangerous >> if they're not damaged or old.

Funny post. I have had a couple propellers like this myself as well and was just thinking that it was caused by the way they were stored. I never thought or looked into a logical reason why.

The heavier mass of the prop is in the leading edge versus the trailing edge. One may want to take a caliper and measure the hub front lengthwise along the prop blade axis and perpendicular to the prop blade axis. I'm thinking that the hub may be stretching from the centrifugal blade loading. If not the hub, might be the root portion of blade that is undergoing plastic deformation (permanent stretching). It would not surprise me that you may have a slight change in prop pitch as a result, too. Fiber reinforced props would have less tendency to do this, IMO.

Yep, they do that.

Cox props are not really the best props available. The grey competition ones are OK, they hold thier pitch anyway. APC are probably the best performing props for 1/2A that I have come across.

I will say that those Cox flexy props can handle a ton of abuse and they do what they are designed to do.

Cribbs74 wrote:I will say that those Cox flexy props can handle a ton of abuse and they do what they are designed to do.

You certainly have an understatement there, Ron. I learned that lesson back in the 1970's. I was a fan of Top Flite white nylon props on my .020 and .049 reedies because of their cost effectiveness and durability. I tried the gray Cox competition props. Problem I ran into is they were brittle enough, that a less than perfect landing broke a prop blade. Ditto with wood props. Both were more expensive.

Most the time I'd clean the grass stains off the nylon prop and continue flying. The worst that could happen to a nylon prop was a set of nicked tips from ground strikes. That evening I could trim the blade tips, rebalance and continue flying.

It is really unnerving to run out of props because of the high rate of breakage. Sport flying is supposed to be fun, and that I had with the more durable nylon props.