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Thread: Props - Wood vs. Metal

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    Props - Wood vs. Metal

    Retro, rather than debate wood vs metal props with you, since you passed over my part about the high performance Spits and Spitefuls, how about selling me your used wood prop for your Cub when you "upgrade" it to a metal prop? If so give me the lenght and pitch , and if it is in good condition I'll make you an offer.
    And yes I have flown Cubs with both wood props as God and William Piper intended, and also with tin can props. I have also flown other plane that used both kinds of props. I still like wood. I have not flown a Pitts. I don't know if this is a positive factor for acro or not, it may be , but one thing I noticed was the wood prop with less weight and inertia would both rev faster upon adding power and especially slow down faster when closing the throttle, as compared to the metal prop.

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    RetroAcro's Avatar
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    Quote Originally Posted by Bill Greenwood View Post
    Retro, rather than debate wood vs metal props with you, since you passed over my part about the high performance Spits and Spitefuls, how about selling me your used wood prop for your Cub when you "upgrade" it to a metal prop? If so give me the lenght and pitch , and if it is in good condition I'll make you an offer.
    And yes I have flown Cubs with both wood props as God and William Piper intended, and also with tin can props. I have also flown other plane that used both kinds of props. I still like wood. I have not flown a Pitts. I don't know if this is a positive factor for acro or not, it may be , but one thing I noticed was the wood prop with less weight and inertia would both rev faster upon adding power and especially slow down faster when closing the throttle, as compared to the metal prop.
    Bill, as you partly mention, wood and wood/composite props have many advantages over metal, which is why they are preferred in many cases - less vibration, lighter weight, lower cost, looks (on antique/classics), less stress on the crank flange/main bearing due to much lower gyroscopic forces due to lower mass, and less torque...again due to less mass, and also throttle response, as you mention. The vastly reduced gyro/torque issues are the biggies for acro pilots. Makes the airplane easier to fly at low airspeed/high RPM, and makes your engine happier. Broken crank flanges and departing props have happened with metal props. But all-wood props are rarely used on high performance aerobatic airplanes due to integrity issues and performance disadvantages. There is one maker who makes a high-lamination wood prop that some folks use on experimental Pitts S-1/One Design types. But wood/composite is much more favored due to better performance and blade integrity.

    Unfortunately, the laws of physics still rule, and compromises DO exist...in the form of lost blade efficiency for wood. You simply cannot build a wooden blade with the same efficient airfoil shape, with the same rigidity and shape maintaining properties (under load) of metal. I base this on performance data that I've observed on my own and read from others who have done scientific comparison tests in various aircraft types. You may be the first I've heard imply that that wood is generally equal to or better performing than metal, all things being equal. But it's probably true that the differences are least noticeable on an airplane like a J-3. And you have to be careful to compare applies-to-apples, since pitch differences between props most influence performance impressions. I will get some comparison numbers when the new prop is on the Cub. I'll give you wood prop for free if the metal does not perform better overall...it's still in good shape :-) Regarding your mention of Spitfires, I don't see how that non-scientific, anecdotal reference is attributable to wood blades somehow being more efficient than metal. I'm more interested in scientific comparison data.

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    EAA Staff / Moderator Hal Bryan's Avatar
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    Guys, I split these off so that the original thread can stay on the topic of celebrating a new tailwheel (read: real! ) pilot, while the prop debate can continue here.

    And...go.

    Hal Bryan
    EAA #638979
    Online Community Manager
    EAA—The Spirit of Aviation

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    Just looking for opinions, I've formed my own. It stands to reason that wood props resonate less.
    What if some clumsy oaf were to push or pull on a prop while moving an aircraft on the ground. Pilots are taught not to do this. A metal prop could be bent enough to displace a tip so that its out of plane and no longer tracks. An out of track prop will produce a medium freq vibration. This is not likely with a wood prop. I check props by measuring the distance from each tip to a convenient point on the airframe. Does this make sense? A metal prop can be fixed by any prop shop. I don't know if a wood prop can.

    Bob

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    Retro, I don't want you to give me your wood prop , I was offering to buy it if, as you said, you were going to "upgrade" to a metal prop. That is if your's is the size and pitch I need for my J3 to replace one that is cracked.
    I have no doubt that you'll like the new metal prop, that is pretty much human nature.

    The climb and top speed figures that I gave for Spit and Spiteful are a little more than anecdotal, not like some guys sitting around the bar saying "what'll she do". They are official flight test figures from Boscome Down, the British flight test facility equivalent to Wright-Patterson. They are not just possibly pumped up figures from a manufacturer, Boscome Down was not in the business of trying to sell any brand or type of airplanes or parts. But, of course, anyone may put more credence in the figures given by Pitts or other acro owners.
    Last edited by Bill Greenwood; 07-13-2012 at 10:06 PM.

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    Bob, aircraft makers almost always say not to pull or push on props to move a plane, and it is considered the wrong procedure. But I have never actually seen a prop damaged by this way of moving a plane which is all too common. Think of this;there is no way a person pushing or pulling on a prop is going to develop the kind of force that the engine does when running. and flying the plane does not bend the props beyond the distance where they cannot flex back to normal.
    So it is best not to push/pull on the blades, but probably not a real concern as a practical matter. I would push equally on the blades, not just all on one side of the prop, when you push or pull on one.

  7. #7
    RetroAcro's Avatar
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    Quote Originally Posted by Bill Greenwood View Post
    The climb and top speed figures that I gave for Spit and Spiteful are a little more than anecdotal, not like some guys sitting around the bar saying "what'll she do". They are official flight test figures from Boscome Down, the British flight test facility equivalent to Wright-Patterson. They are not just possibly pumped up figures from a manufacturer, Boscome Down was not in the business of trying to sell any brand or type of airplanes or parts. But, of course, anyone may put more credence in the figures given by Pitts or other acro owners.
    I still fail to see the significance of the Spitfire reference in this discussion of wood vs. metal. How did the Spitfire perform with a comparable metal prop? I know, I know. And BTW, the Griffon Mk XII Spitfire blades are made of compressed wood impregnated with resin at high pressure. Not exactly the type of "wood" prop we use on our airplanes. But in any case, lots of airplanes go fast on wood props. Like I said, I'm more interested in scientific comparison data. To say one airplane does "x" with a "wood" prop is meaningless to this discussion...which is about comparison. Comparison data means figures for the same airplane with a metal prop are provided, and analyzed scientifically...meaning the tests are conducted identically, at the same W&B and density altitude conditions.

    My conclusions on metal are based on the comparision tests I've done, as well as what I've read from others in the aerobatic community, as well as test data I've read from RV builders/pilots. RVs guys care a lot about speed and performance. Not nearly as many choosing all-wood props these days. And if they do, it's mostly due to cost. Modern technology has largely replaced wood with composite...for folks who would otherwise have run wood props in the past. Composite is a step up in efficiency and performance (and blade integrity/strength). Look at the Formula One and Biplane class flightline at Reno these days. Should have some numbers on the Cub soon.
    Last edited by RetroAcro; 07-14-2012 at 06:23 AM.

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    Supermarine built about 23,000 Spitfires. A few of the early ones had metal props. But from then on they all had wooden props, mostly of laminatined spruce with a resin coating, Jablo.
    I don't have test figures of Spit with both types of props. But I assume the designer, manufacter, and test pilots had such data, and they had some reason to settle on a standard of wooden props for all Spits, even the latter models with over 2000 hp and speeds over 440 mph. And so even for the 2500 hp Spiteful, the fastest actual production combat piston prop fighter.
    Perhaps if the had the benefit of all the data from all the Reno formula and biplane racers they would do it differently, like Cessna or Cirrus does.
    Of course a metal prop can work well and perform well on an airplane, Strega has lapped Reno at an average of over 490 mph with a metal prop.
    But your idea that only metal or maybe plastic props can have high performance is nonsense.

    Sort of a corollary, if all the new stuff, compostite props and all is so special, and the guys that advocate it are so much smarter than they were in the old days; it should be so easy and simple to design and build an all new, all the lastest plastic plane to easily beat Strega, with all its old parts and technology, and win the unlimited prize at Reno. Perhaps they could use a twin fuselage and powered by auto engines which everyone knows have so much more modern tecnology.
    Last edited by Bill Greenwood; 07-15-2012 at 03:54 PM.

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    The brits built Mosquitos out of mostly wood to free up metal for more urgent uses. Could the same philosophy also have applied to props? How can we know, 70 years later, what drove their choices of prop materials; engineering or expediency? War distorts many things.
    Bill

  10. #10

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    One big issue with switching between metal props and composite (wood or carbon fiber) props is torsional resonance. The acro folks found that replacing a aluminum constant speed propeller with a new carbon fiber blades propeller cause the internal gears in the engine to self destruct due to the reduction in propeller mass that was damping some of the vibrations created each time a cylinder lit off. The solution was to use a crankshaft that had 6th order counter weights installed (Examples: IO-360-A1D or IO-390).

    Lower horsepower engines don't have this problem but as the compression ration and/or horsepower goes up, this becomes a much larger issue. I think that I understand that one large technical advance that made the RR Merlin a success was figuring out how to build a gear box that could transmit that much horsepower to the light weight prop that they hung out front.

    So prop swaps should not be done casually or you might find yourself demonstrating your off field landing skills.

    Best of luck,

    Wes
    N78PS

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