Props - Wood vs. Metal

[Bill Greenwood]

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.

[RetroAcro]

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.

[Hal Bryan]

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.

[Bob Dingley]

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

[Bill Greenwood]

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.

[Bill Greenwood]

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.

[RetroAcro]

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.

[Bill Greenwood]

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.

[Bill]

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.

[WLIU]

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

[RetroAcro]

OK, didn't have the time or motivation to get all scientific, but made some notes about the propellor swap-out on the 85hp stock Reed Clipped J-3 Cub that I fly. Sensenich 72GK44 wood prop was replaced with a metal Macauley CF71-44 . First thing you'll notice is that the heavier metal prop takes longer to spin up to the RPM you've got the throttle set to when starting, which gives you even more time to get your hand on the throttle as the engine "spins up" after propping it from behind. J-3's were made for this. Another nice thing about metal is that you can prop it with less effort if you bounce it a little against compression and get the prop moving downward before pulling it through...the momentum does a lot of the work.

I can't say I noticed any obvious increase in takeoff/climb performance with the metal prop. Hard to say without doing comparison tests. Not sure if I'll get around to it like I did with the Pitts, but it would be interesting to get some good climb numbers flying the two props back-to-back so that density altitude conditions are similar. It turns up about 50 RPM less on climbout (engine developing less power), and leveling off, setting 2,350 RPM, it indicated 79, which is about 5 MPH faster than the wood prop at that RPM. Nope, Clipwing Cubs are NOT faster than the longwing J-3. Not sure yet if fuel flow is going to be the same at it was before at this RPM.

As expected, the stick feels heavier in flight with the metal prop due to the slight move forward of the CG, as well as the gyroscopic (pitch) resistance of the metal prop vs. wood. I went through the reverse of this when changing from a metal prop to composite on the Pitts...it's quite a bit lighter in pitch now.

Idle RPM can be set lower with a metal prop - more flywheel effect, which allows for a lower steady idle RPM. The difference shows up during landing - it seems to be a little less "floaty" due to less residual thrust at the lower idle. It also seems to drop altitude more dramatically in a slip than it did with the wood prop...I assume for the same reason. I consider these to be good things.

Due to the CG move, I noticed that I must now pull the stick just a little further aft and put it on the stop to get a true 3-point touchdown power off. Before, you could touch down very slightly tailwheel first if you pulled the stick to the stop just before touchdown.

So a very slight performance advantage (speed, not climb) with the metal prop, but it very much changes the entire "feel" of the airplane. I actually prefer the lighter feel of the airplane with the wood prop, but there's enough good about the metal prop that I think it'll stay.

[WeaverJ3Cub]

Nice "science" and explanation, RetroAcro. I don't have a dog in this fight :), but it's very interesting nonetheless.

[Bob Dingley]

Wood vs. metal? There's another kind. Way back when, I came back from globetrotting with USMC and made another run at my PPL. My instructor mentioned that all that stuff we practiced was to satisfy the CAA. We now had the FAA and they had other ideas for what a PPL should know. For a J-3 to haul a battery, generator, radios,etc the A-65 was upgraded to 75HP and a Beech R-003 controllable pitch prop was mounted. At that time it was called a Beech-Roby. It sure made the Cub climb. It was controlled by crank in the cockpit. Looked a lot like a window crank from a 52 Desoto.

Bob

[Bob H]

Wood blades are generally used for lower power engines ( <100 hp) because they are lighter, absorb energy pulses better than aluminum (damping) and are frangible in case of a prop strike. But drawbacks include shape control in manufacturing, moisture absorbtion which can effect balance and mass and contour changes. They don't have the same stiffness as metal so can't maintain resistance to bending for given loading and that limits power capability. Cost is usually lower than other materials and esthetics are pleasing. I have a GSC 2-blade, ground adjustable wooden prop on Rotax 912 with good performance and smooth operation.

[Bill Greenwood]

Bob, the Spiteful, with wooden blades had a Griffon with 2500 horsepower.
But , then again that was just Supermarine, and they probably didn't know that wood was only for low power engines under 100 hp.

[Bob H]

Mulitblade wood props were made during the war to conserve metals, but I assumed this thread was about GA aircraft with relatively low power ( <100 hp) Cub class engines as initially stated. If you try hard enough, you can find exceptions to any normal application. For consistent shape control, balance, strength, low rotating mass and relatively good damping, a composite blade is hard to beat. If you fly in rough places where a prop strike is possible, wood props will disintegrate like an energy fuse, protecting the engine from damage. I had that happen and was thankful for a $300 wood blade replacement cost instead of thousands for a metal blade and the associated thousands more cost of engine teardown and repairs.

[rosiejerryrosie]

Where did you get a wood blade for $300.00?

[Bob H]

GSC in Canada makes a ground adjustable wood prop; wood blades and an aluminum hub for around $500 delivered. If blades are broken, a new pair runs around $300.
I fly from a high altitude airport and need to be able to tweak blades for density altitude operations so an adjustable blade system is necessary. I also have a 3-blade Warp Drive in reserve but the esthetics of the wood blades are appealing and should there be a prop strike, the wood blades disintegrate and save the engine from damage.

[rosiejerryrosie]

Great. The last wood prop I bought cost me $1300.00. I'll have to research GSC. Are they good at making recommendations as to what size prop to use?

[Bob H]

If you Google GSC Systems, will get their website. Can probably send a note and get answers from technical man Rick. Remember, their props are intended for lower HP engines so a Lyc 360 would be too powerful for their products.
I live/fly my Pulsar at 7000' so density altitude is a factor and having the gnd adj prop works well.

[rosiejerryrosie]

Thanks for the info. Their props should be able to handle a Continental A-65.:)