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Prop Safety (Safe tip speeds for component props Freewing, Flight line, FMS)

Konrad

Posting Elsewhere
#1
Greetings All,

I’m new the this forum but have been in the hobby for many decades, read since 1969 ouch!

My concern is that 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.


With that out of the way 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
 

JimCR120

Got Lobstah?
Site Moderator
#3
Konrad, I just wanted to say hello and ask if you would tell us more about yourself. Have you been in the hobby the whole time? Do you have a particular airframe you prefer? Where in the world are you? Thank you sir and welcome to the forum.
—Jim
 

Konrad

Posting Elsewhere
#4
Jim,
Yes, I've been fighting gravity pretty constantly since then. I have been in just about all aspects of the hobby at one time or another, with the exception of FAI F1A type models and Giant scale. My 15 minutes of fame in this hobby is that I build the FAI F3D Pylon engines that won the 1989 USA NATS.

Today my love is gliders with an emphasis on slope gliders. Being as I live on the West Coast in San Francisco, you might understand the draw of the slope. In the last 3 or so years I've changed from a rabid anti foam, anti ARF modelers to one that now has over 2 dozen flyable EPO type foam models. Most of these sport/pattern types.

And thank you for the warm welcome.

LIAMNAVE,
Hack?

There may be some truth in that. But I strive to make my next model better than my previous model. It is all part of ones growth in the hobby. I hope to learn from you all here and to help those that ask for it here at Flite Test with technically solid advice.

BTW; I make my living as an aerospace engineer having, brought a few transport aircraft to market, getting them approved with the regulatory agencies.

Back to issue at hand, has anybody had experience with finding the safety limits of these component props?

All the best,
Konrad
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#5
Finding the limits? NO. Exceeding the limits? Yes and just last weekend when a plastic prop of generic brand threw a blade. It was a 6x5 prop on a 2850Kv motor running on a 3S. The prop let go at maximum pull as the plane was accelerating in a vertical climb. The blade broke at the hub or disk and the prop had spent its entire life in a spinner.

The resultant imbalance destroyed the firewall and ripped the motor mounting bolts through the firewall. the motor was shut down with a second or two of the explosion but that was far too slow to save the firewall.

The prop was not able to be located and if it had hit someone I fear that the injury could have been major.

The prop supplier, HK, also does not give any of the information you are chasing either. Looking for a better prop before I replace the firewall of do any other repairs.

Later!
 

Konrad

Posting Elsewhere
#6
Finding the limits? NO. Exceeding the limits? Yes and just last weekend when a plastic prop of generic brand threw a blade. It was a 6x5 prop on a 2850Kv motor running on a 3S. The prop let go at maximum pull as the plane was accelerating in a vertical climb. The blade broke at the hub or disk and the prop had spent its entire life in a spinner.

The resultant imbalance destroyed the firewall and ripped the motor mounting bolts through the firewall. the motor was shut down with a second or two of the explosion but that was far too slow to save the firewall.

The prop was not able to be located and if it had hit someone I fear that the injury could have been major.

The prop supplier, HK, also does not give any of the information you are chasing either. Looking for a better prop before I replace the firewall of do any other repairs.

Later!
I have to ask did the prop show evidence of spinner rub?

Now a quick back of the envelope calculation it looks like you were around 29K rpm (3 x 4.2v x 2850kv x .8 efficiency) multiplied by the circumference of your prop 1.57 feet (.5 x 3.14) equals a tip speed of about 45k feet per minute. Which is 752 feet per second. That comes to 229 meters per second way over the 180 m/s safe limit described by Graupner!

At those speeds you might want to look at props made with continuous fibers running tip to tip. These are not 75 cent props but closer to the 15 dollar range

To find the safe limits for a prop, you would want to run a statistically valid number of props to ultimate failure. Maybe 5 props below freezing. Another 5 at room temp. And another 5 at 45 C degrees. And add a safety factor depending on the spread. Using your prop it looks like Graupner was using a safety factor of 80%.


So what is the safe speed for the large scale looking props, with small screws holding on the blades, we see on these foamy warbirds from Freewing, FSM, Flight Line etc.?

All the best,
Konrad
 
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Hai-Lee

Old and Bold RC PILOT
Mentor
#7
As for evidence of prop contacting the spinner, I had just cleaned balanced and recoloured the spinner, The recolouring was with a paint that barely binds to the plastic and so flakes off it there is the slightest impact, (hence the work done on the spinner),

There were absolutely no marks around the clearance hole for the blade and the break line was around 10mm from the outside of the spinner.

I will definitely be sourcing a better prop for this installation before I attempt the repair. As a side concern would you know if the effects of the combination of thrust and centrifugal force would have caused a fatigue type failure or a possible structural failure due to manufacturing defect.

Also the prop had almost paddle like blades, (I liked the static thrust on test), should I be seeking a "Speed" prop blade design?

Never fly too high!!! (Drugs and planes don't mix:rolleyes:).
 

Konrad

Posting Elsewhere
#8
Cycle (high or low) failures are a possibility. I don't think you suffered a failure from a manufacturing defect. It looks like you were just way beyond the design limits of the material and prop design. We would need to look at the fracture face under a microscope to get a good idea as to what really happened. But 220 m/s with a non reinforced prop is telling me, you were just spinning it too fast.

Matching the prop's power profile to the airframe is another subject all its own. I place NO value on static thrust numbers, as the wing needs velocity to produce lift. Some guys make a big deal about static thrust but this really is only because they can measure it. It has little or no bearing on how the prop and plane will perform at speed.

Not knowing what you have, a Graupner 4.7 x 4.7 speed prop may be what you need?

All the best,
Konrad
 
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