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Author Topic: Prop Safety (Safe tip speeds for component props Freewing, Flight Line, FMS)  (Read 624 times)
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Konrad
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« on: March 15, 2017, 10:11:32 AM »

I'm having grave concerns, with what is a safe speed for the "Scale Props" we now see on the new generation of "Foam Warbirds". I just witnessed a prop failure at my field. The propeller was of a construction that is new to me. It is where the individual blades are held on to a plastic disk (spinner back plate) with small sheet metal like screws. I’d like to learn what is the safe operating speed for this style of propeller?

I did a search on the AMA’s site and only came up with restrictions for a propeller made with metal blades and some topics addressing tip speed and noise. I fully understand the restriction on metal blade as metal tends to work harden and crack when subjected to cycle loads.

I think we all as safety conscious modelers would want to know what is the safe tip speed for this type of prop. I’m looking for information like what Graupner supplies with their Super Nylon prop, that is the tip speed must be kept below 180 meters per second. They also supply a formula to derive this limit K/D=RPM. Where “K” = 3438 and “D” is the diameter of the prop in meters. Landing Products (APC Props) offer a site that address this issue for the various types of props they manufacture http://www.apcprop.com/Articles.asp?ID=255 . As the distributors, I’m thinking MotionRC through their upgrade page and their forum Hobby Squawk, often post performance enhancing upgrades to the base product they sell, knowing the safety limits of the prop is critical to a safe and happy experience for all at the flying field.

I’d like to know what is the safe speed (tip speeds) for these component props that come with the Freewing, FMS and Flight Line models? I’ve asked MotionRC for this data and they don’t seem to have the engineering breadth to answer this fundamental safety question. Not that they should but the OEMs should provide the importer and distributors this information. I’d like to ask does anybody here have a link or know what the safe limits are for this type of prop?

BTW, I’ve asked for the AMA’s stance on this subject. I’ll let you all know what if anything the AMA says on the subject.

All the best,
Konrad
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Bill G
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« Reply #1 on: March 17, 2017, 02:24:35 AM »

I believe it was MPJ props, which had plug-in blades, that I used some years ago.  They seemed to hold up, but were potentially dangerous, as if there was any unseen damage due to a rough landing, etc, the blade securing tabs could develop cracks.  I question any of these props with individually attached blades.  Actually came across one in my junk pile recently, were the base was all glued up at one of these crack points.  I was thinking "what the Shocked was wrong with you back then, doing hokey stuff like that".
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Konrad
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« Reply #2 on: March 21, 2017, 01:39:33 PM »

I was thinking "what the Shocked was wrong with you back then, doing hokey stuff like that".
Yep, that is why I said I didn't know the history of the prop I witnessed fail. I just learned that  FMA has a new prop out for there 1.5m warbirds.
And I'm shocked stock FMS prop only has one fastener per blade to absorb the lead lag impulse from the motor torque impulses. This will result in the blade acting like the pry bar trying the shatter the hub. I have to ask the FMS engineers the same question you did of yourself.

See this video at about the 11:30 point.
https://www.motionrc.com/collections...089p#video-tab

Just a note the single fastener orientation of the classic folding prop allows the blade to move in the plane of the prop disk from the motor torque impulses without damage (force) to the hub.
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Konrad
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« Reply #3 on: May 28, 2017, 12:39:36 AM »

Another data point!
https://www.hobbysquawk.com/forum/rc-airplanes/propeller-airplanes/77820-1700-fms-p-47-propeller-separation

Come on all you OEMs, please state what are the safe operating parameters for these godawful component props! APC, Graupner and most reputable firms have publish safe operating parameters for their props.

The eye you save might just be mine!!!!
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Bill G
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« Reply #4 on: May 28, 2017, 02:46:58 PM »

Outside of inherent stress concentration problems with most any of these removable blade designs, one key point I noticed in a discussion was the mention that FMS claimed to have changed the "carbon content" of the plastic. I'm sure that any plastic injection facility they would use is a top end, ISO compliant facility with consistent quality and strict adherence to material composition specs.  There's no substitute for good old "keep it simple" designs, that don't have a number of potential issues, specific assembly/maintenance procedures, also not requiring top end materials.
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Konrad
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« Reply #5 on: May 28, 2017, 09:34:58 PM »

Outside of inherent stress concentration problems with most any of these removable blade designs, one key point I noticed in a discussion was the mention that FMS claimed to have changed the "carbon content" of the plastic. I'm sure that any plastic injection facility they would use is a top end, ISO compliant facility with consistent quality and strict adherence to material composition specs.  There's no substitute for good old "keep it simple" designs, that don't have a number of potential issues, specific assembly/maintenance procedures, also not requiring top end materials.
That's precious, "Changed the carbon content"!

Did they add more of less coal, graphite or diamonds? Was the form, short fibers or continuous fibers tip to tip? Were we to think that carbon adds some desirable property to the plastic? And if so which ones?

I too like the KISS principal of design. Unfortunately most organization are only able to comply with one of the "S"s!

Crying in my beer,
Konrad
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Bill G
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« Reply #6 on: May 31, 2017, 12:12:06 AM »

Those were my thoughts also.  The didn't specifically say that they are now running fiber reinforcement ran lengthwise, or what the change in the fiber reinforcement was.  Nothing said that anyone could hold them to.
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Konrad
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« Reply #7 on: May 31, 2017, 05:17:15 PM »

My english teacher is pointing out that it should read principle. It looks like corporations and I have a lot in common! Embarrassed the shame of it!
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