Wooden vs. Metal Props

[ROB-x38]

G'day all

As the topic implies I'm interested in the pros and cons of a wooden prop vs. a metal prop.

Simple as that...

Cheers. :ok:

[stiknruda]

Wooden props tend to vibrate less ( they absorb more vibration)

Wooden props tend to be a lot lighter

Wooden props abrade in rain and are generally less resistant to minor stone damage.


Stik

[Dusty_B]

Wooden props shatter on a propstrike, thus reducing shockloading to the crankshaft (ie, a good idea on a taildragger).

[LowNSlow]

Agree with all the above but the pros for a metal prop are:

More inertia which helps keep the engine turning around while you try to discover why it stopped which is important if you don't have a starter motor. Also helps in hand starting.

You can dress dings out of the leading edge which you can't do with wood!

Less susceptable to damage due to people mishandling your wunderplane in the hangar.

Varnished wooden props look lovely.

GT Props are an Italian company that produce a range of composite props (PFA types only I think) which I have heard people, Kingy for one, rave about.

[c-bert]

Can you get composite props?

[stiknruda]

Many of the major manufacturers make composite blades.

Stik

[bar shaker]

LowNSlow

From experience, the leading edge of wooden props can be repaired quite easily with epoxy resin. The process would be similar to filling a dink on a metal blade. It will still need to be rebalanced as soon as poss in both cases. I don't know what prop repairs, if any, are allowed on CoA machines.

As for inertia, I doubt it would keep rotating for more than a few seconds if it was made from depleted uranium.

A wooden prop is a thing of beauty in my book. Until its covered in epoxy resin repairs, that is. Then its time for a new one and the old one becomes dining room wall art.:}

[Flik Roll]

wood repairs are ok until it gets to the inner 1/3rd, then it requires a bit more than epoxy resin IIRC?

[Kingy]

Chaps,

I as L'nS as indicated I have now replaced the props on all three of my aircraft with Italian G.T. props. They are made from ash with a carbon fibre leading edge. The Cub has now done over 250 hours with the GT and it's pretty much as good as new despite having been caught out by some pretty heavy rain on occasion!

Performance benefits are really quite stunning over both Lodge and Sensenich and they are beautifully made. GT claim that there is a slight variable pitch effect due to the blades flexing at high RPM - not sure about this, but on my Sky scooter for example the climb rate has more than doubled!

Anyway, back to the question - I am always a little nervous about seeing a metal prop on an the sorts of taildraggers I get to fly, nose over and you are looking at a shock loaded engine. As already stated they are heavier, which will increase gyroscopic precession. They are generally they are thought to have performance benefits over wood, but I not so sure of this anymore - certainly thre GTs use much more advanced aerofoils than the average metal prop, which may have been certified perhaps 50 years ago - things move on!

BTW, Im not connected with GT in any way – just a pleased customer.

Kingy

[djpil]

A rough performance comparison on a 200 hp aerobatic machine:

1. First prop was a wooden Sensenich. Static RPM only about 2250. Max speed S&L about 138 kts. Made lots of noise in a dive. Seemed as if it twisted and became finer pitch at higher airspeeds.

2. Second was a Hoffman. Static RPM about 2350. Max speed S&L about 147 kts. Nice all round.

3. Third was a metal Sensenich. Even more static RPM. Max speed S&L 155 kts at about 3050 RPM. Best performance all round.

(figures subject to change when I wake up and refer to my notes)

[stiknruda]

Max speed S&L 155 kts at about 3050 RPM. Best performance all round.


That'll be one of those special angled valve O-360's then? :)

Special because you are allowed to rev it past the redline of 2700 S&L ;)

In the States a couple of months ago, an airshow performer of some note, told me that, "Yeah 2700 is just a guide not a limit. Hell, Stik, I don't never (sic) take her above 3325 - cos that's when the valve train starts to bounce!"

Not pi55ing on your parade, Dave, but to me 2700 is a limit.

Stik

[djpil]

No worries, Stik.
3000 was my limit in that aeroplane. +7/-5g.
There are many others who pushed their Pitts S-1's and monoplanes a lot harder. Of course, things break the harder you push them or, at best, the life is significantly reduced.
We kept our S-2A purely per the book.
Decathlons are even less robust so well inside book figures.

[LowNSlow]

bar shaker regarding inertia I was referring to when one forgets to change tanks and the engine stops firing rather than when it throws a rod and stops due to mechanical failure. In these circumstances the metal prop will give you an important few seconds to switch tanks while the prop continues to rotate in the airflow. The wooden prop on my Cub wasn't very good at this....

[bar shaker]

LNS

I would think that the prop with the widest blades (assuming same dia) would be most likely to windmill and inertia would be a negligable consideration, especially on a component that is designed to weigh as little as possible.

However, I accept that your experience proves otherwise.

[Kingy]

BS,

I've got to say I'm with L'n'S. The difference in inertia is really marked on small motors like the A65 especially with 6ft props. My Cub prop will stop pretty much straight away in the air, but the same engine with a metal Mc McCauley as fitted in my old Taylorcraft would windmill all the way down. It's really noticeable when you swing them too - A good heave with the metal prop would spin the engine over two compressions!

Kingy

[LowNSlow]

Bar shaker the steel Fairey-Reed prop on my Auster was definitely NOT designed to be as "light as possible". It would chop your toes off if you let it fall on them! Classic British over-engineering like the rest of the aeroplane except for the supermarket shopping trolley free castoring tailwheel (thankfully replaced by a previous owner with a decent sized Scott item).

[bar shaker]

Gents

I think you miss read me. There is a world of difference between wind milling and inertia keeping a propellor rotating.

Kingy, is this different props on the same engine? Same compression ratio?

I once trailored my aircraft, in its pre hangar days, without tying off the prop. At anything above 50mph road speed, it would wind mill. This is a wooden prop. I am 100% sure that a composite or metal prop would have wind milled almost as well, subject to sufficient airflow over their narrower blades. This has nothing to do with inertia.

When parked up after a flight and you switch off the mags. The time it takes the prop to stop has everything to do with inertia. Its not normally very long, certainly not long enough to mess about changing tanks.


:)

[Kingy]

BarShaker,

A 65 in both aircraft - obviously different actual engines, but the same type, and both in good heath (comps in the 70's). We are talking about a small engine with a 5.4-1 compression ratio here - the effect of a heavy metal prop over a light wooden one is really obvious. The inertia makes the compressions seem less when you swing (due to the flywheel effect), the throttle response is much slower and yes I really do believe the extra inertia keeps the prop turning in the air for longer than the wooden alternative.

Once stopped, my Cub, even a dive to VNE won't spin the prop due to airflow... (don't ask!)

Kingy

[bar shaker]

Kingy

So given that the metal prop is much heavier and won't wind mill in a Vne dive, which do you prefer?

bs

[Kingy]

Chaps,

WR is right, once stopped the prop on my Cub won't spin from a VNE dive (which is only 100mph). If I had stuck the nose down in the first place, rather than pulling back to convert energy into height perhaps the prop would have continued turning... don't know!
What I do know is, with the much heavier metal prop on the T-craft it was pretty much impossible to stop the prop whatever you did speedwise.

Oh the fun you can have with your own planes and a deserted airstrip..! ;)

Kingy

[DubTrub]

Kingy: I know you don't have the T any more, but it can be done...but one must almost stall the wings. After stabilising the engine after the climb, throttle must be shut, so as to create the greatest "suction" within the engine, and get back close to the stall, switch the mags off (airframe shakes a bit then).

Once stopped, 60mph glides are great...especially in the summer, with the thermals under those big wings.

To re-start, one "smartly" goes into a dive...no need to wait for the prop to spin, just wait for a "twitch" and then apply a little aft stick. The change in direction (precession?) will cause the prop to ovecome the compression, and the rpm will very quickly go to almost red-line.

Re-establish the glide at 60mph, and let the engine thermally stabilise after the dive for a few minutes, and only then switch the mags on. Warm the engine up slowly, just the same as one would on the ground.

One used to be able to air start with 700' loss. Needless to say, one only ever carried this out at a safe height and over a big runway! and one got quite used to dead-sticking all the way down, but I prefer to air start now.