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Why don't we use metal propellers?

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Paul E15/01/2010 07:50:04
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hi, i am doing an Alevel presentation about aluminium and why it is preferred over wood for full scale aircraft. but as an rc flyer, i was also going to turn the idea around and explain why we use wood over metal for modelling. could i have a couple of ideas of why we do so (safety, lower rpm because more load etc) and if any of you are full scale pilots, why you do not use wood but an aluminium alloy for your props. thanks, Ryan.
Doug Ireland15/01/2010 08:28:36
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Despite being illegal (BMFA rules), I'd have thought the answer was obvious. They are downright dangerous!
Peter Miller15/01/2010 08:33:22
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To my knowledge the SMAE (BMFA now) banned the use of metal propellers back in the early 50s or even earlier. I know they were banned when I started in 53
 
Not sure why but I suspect that they could shatter too easily as they would probably have been cast in those days.
 
In the early days of laser engines Neil Tidy said that a four stroke could cause a nylon propeller to fail through fatigue. The acceleration and deceleratio in the speed of rotation of the engine would make the prop flex back and forth until it failed..
00115/01/2010 09:09:53
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As model aircraft are occasionally involved in 'untidy' arrivals where the propeller is in risk of contact with the ground or other objects a disposable plastic or wood unit that will easily fracture is definitely preferable.
 
A metal propeller would not only be extremely dangerous to the operator and bystanders, but would also be more expensive. The possibility of damage to the internal parts of the engine when it suddenly stopped would also be a consideration if the propeller stayed intact.
I remember that some control line speed records were set by models with metal propellers, but it may have been under special exemption from the rules, or was abroad. (Russia?) where the rules were not in force.
I do not fly full size aircraft, but surely a lot of light aircraft have wood props ?
 
 
 
 
Doug Ireland15/01/2010 09:19:55
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A lot of home-builts have them Richard, but your average "Spam Can" has a metal prop.
Martin Harris15/01/2010 13:28:41
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It's actually rather a good question. 
 
Is a metal prop inherently more dangerous than, say, a thin carbon fibre one which is extremely resistant to bending but will shatter into a jagged edged cutting tool if subjected to an impact?
 
Having witnessed he effects of the tip of a thin APC prop on a Moki 180 running along a set of fingers at tickover I don't think arguments about sharpness have much validity either.
 
I'd guess that fatigue causing unforseen blade shedding, coupled with greater kinetic energy in the blade might be the most likely danger or is it a case of "we've never allowed them so they must be dangerous" ?
 
Is there anyone around that remembers exactly why the rule was introduced?
James4015/01/2010 14:02:30
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Alloy although light weight still has more mass than a wooden prop, the gyroscopic effect of the prop would be quite nasty, you'd be learnng to fly all over again taking gyroscopic procession in to account.
Bert15/01/2010 14:24:43
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Posted by Martin Harris on 15/01/2010 13:28:41:

 
 
Is there anyone around that remembers exactly why the rule was introduced?

 It was to outlaw the Albon Bambi

Paul E15/01/2010 16:31:54
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thanks people for your replies, i am going to be talking about both the pros and cons for a metal propeller for full-scale and model aircraft.  i know richard bond hinted at full-scale aircraft using wood props over metal, and i few websites say this is because of cost, weight, less vibration etc. Do you have any other ideas of why they do so?
Ernie15/01/2010 16:57:57
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Hi Paul,
1  Sometimes, Its better for things to break, rather than transmit forces through the airframe.
2  Timber and plastic props are (usually)  quite cheap
3  A metal prop. can bend, and if it's slightly out of line, can cause excess vibration
 
Finally, I prefer wood, because I can rework them , easily balance them, and get a great finish if I need it. ( and they are rediculously cheap
 
ernie
Romeo Whisky15/01/2010 17:08:50
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My wife has a metal prop inside her Magimix.  It's made by Sabatier and after seeing what it does to carrots I wouldn't want my fingers anywhere near a metal prop!
Peter Miller15/01/2010 18:13:07
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The ban was in force before the Bambi was on sale.
Sparks15/01/2010 19:05:42
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Aluminium has defined fatigue life when subject to fluctuating stress - it WILL fracture in time, depending on magnitude and number of stress cycles. The cyclic-speed nature of a single-cylinder two-stroke would certainly induce fluctuating stress into the root of an aluminium prop blade. One 'twang/rev', @ 10 krpm, will rack up 10^7 reversals in abt 17 hours. Small nicks and material imperfections considerably reduce the fatigue life of aluminium. 
 
Aluminium fans are successfully used in many industrial/automotive applications, where the stress factors are known and catered for (mind you, I have seen quite a few  fans shed their blades where the sums hadn't been done - quite spectacular on a 16-blade 2m dia  fan!).    
 
In a uncontrolled application, such as R/C planes, I certainly wouldn't wish to be in the path of a aluminium blade that is nearing the end of it's fatigue life.
 
As my old Chief Engineer used to say, 'Lad, if in doubt, make it stout, of stuff that you know about!' 
 
 
 
 
    
Peter Miller15/01/2010 19:20:11
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You just reminded me of a description in Aeromodeller from the 50s or 60s. Someone testing an engine with an aluminium fan.
 
The fan shed a blade which buried its self in the bench. The writer had to use a pair of pliers and a lot of strength to get the blade out of the bench.
 
 
Doug Ireland15/01/2010 20:04:55
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Posted by James40 on 15/01/2010 14:02:30:
Alloy although light weight still has more mass than a wooden prop, the gyroscopic effect of the prop would be quite nasty, you'd be learning to fly all over again taking gyroscopic procession in to account.

 When I was a student pilot the instructor demonstrated the differences in the aircraft's  flight attitude by increasing and reducing the engine power. Increase would climb to the right and decrease would descend to the left. So gyroscopic action is apparent regardless of the composition of the prop.

kiwi g15/01/2010 20:24:19
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Its probably more to do with manufacturing costs.
I see lander ducted fans have gone to alloy .
I have to agree about whats being said . stress factor , weight , damage to engines..
I guess lander believe its inside a fan shroud, so if it was to explode it would be contained?
Interesting tho is that the fan spins alot faster than a prop.
Or am I wrong.
Cheers Kiwi G
 
mal brewer15/01/2010 20:29:40
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  The early Phil Smith designed 'Veron' range of ducted fan models,the La 17, Sabre & FD2 used an aluminium fan,mind you,they were only using a .5c.c. diesel.There was also a semi-scale design in the range,I can't recall the name of that one.Veron used a different name for the ducted fan system,can anybody remember  what it was ?..........Mal
Bert15/01/2010 21:07:30
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Posted by Peter Miller on 15/01/2010 18:13:07:
The ban was in force before the Bambi was on sale.

 Peter - it's a joke mate

Bert15/01/2010 21:15:14
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Posted by mal brewer on 15/01/2010 20:29:40:

  The early Phil Smith designed 'Veron' range of ducted fan models,the La 17, Sabre & FD2 used an aluminium fan,mind you,they were only using a .5c.c. diesel.There was also a semi-scale design in the range,I can't recall the name of that one.Veron used a different name for the ducted fan system,can anybody remember  what it was ?..........Mal
 
 
Phil Smith stopped making the fans out of aluminium because of metal fatigue problems. The fans were made with plywood hubs with some kind of fibre material for the blades.
 
Bert
Mark Lubbock15/01/2010 21:44:37
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I suspect a metal prop would be fine if subject to similar use to full size. i.e If you damage a full size prop it is replaced, model props get bent on a regular basis & if straigtened would eventually be weakened & fail. Also it is very easy & cheap to mould props in plastic in our model size & they are pretty much proven unless abused.

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