Compulsion Posted June 17, 2012 Share Posted June 17, 2012 While flying a week or so ago my instructor told me to reduce revs due to the risk of damage to the timber prop in the rain. While taxing and running up you have to be very careful of stone damage. Why do we use timber props and not an alloy or carbon fibre prop? Link to comment Share on other sites More sharing options...
jetjr Posted June 17, 2012 Share Posted June 17, 2012 Good at absorbing vibration, light, cheap, repairable, no "life" on the ones I know of. Set pitch too which is of comfort to engine makers like Jabiru Vibration damping is of critical importance in 4cyl Jabiru. CF is excellent but more expensive in most cases. Better in rain and for minor damage but can transfer vibration. More often produced as ground adjustable which is a good thing but adds complexity and possible under/over loading engine Alloy are much heavier 4 Link to comment Share on other sites More sharing options...
Compulsion Posted June 17, 2012 Author Share Posted June 17, 2012 Great answer thanks for that. Link to comment Share on other sites More sharing options...
Owi Posted June 17, 2012 Share Posted June 17, 2012 In a word - cost. By the way, the fancier, more expensive alloy or carbon fibre props are also susceptible to stone damage but are more durable. Cheers, Link to comment Share on other sites More sharing options...
Fatman1238 Posted July 17, 2012 Share Posted July 17, 2012 While flying a week or so ago my instructor told me to reduce revs due to the risk of damage to the timber prop in the rain. While taxing and running up you have to be very careful of stone damage. Why do we use timber props and not an alloy or carbon fibre prop? Wooden props absorb more vibration than metal & composite ones, which in turn create less vibration to the engine components. Link to comment Share on other sites More sharing options...
facthunter Posted July 17, 2012 Share Posted July 17, 2012 Scratches and nicks in metal props cause cracks to form . Also If they are not certified for the particular engine they have no guarantee that they are safe. They may have harmonics that make them resonate. You don't pick this up externally. The wood prop is the one that most forgives torsional vibration, which all engines have, so it's the safest. Nev Link to comment Share on other sites More sharing options...
eightyknots Posted July 17, 2012 Share Posted July 17, 2012 Scratches and nicks in metal props cause cracks to form . Also If they are not certified for the particular engine they have no guarantee that they are safe. They may have harmonics that make them resonate. You don't pick this up externally. The wood prop is the one that most forgives torsional vibration, which all engines have, so it's the safest. Nev What are the major drawbacks to having a composite prop? Link to comment Share on other sites More sharing options...
Guest Andys@coffs Posted July 17, 2012 Share Posted July 17, 2012 What are the major drawbacks to having a composite prop? engine RPM's 100% and 1 side of the prop feels the need to leave..............May sound like a joke but it isnt. There have been a number of composite props do just that on direct drive engines. When it occurs the qwuestion is, can you turn it off, or will the engine fail, before it tears itself or the engine mount of the firewall. If the engine stops before departing then you can probably do the usual deadstick landing, if the engine, prop and perhaps mount all exit, then you are about to meet your maker and there isnt a single thing you can do to prevent it, other than try and talk your passenger into emulating an engine weight out on the nose.... There are things in our aircraft type that you can play with and try alternates.Im of a view that in many cases props arent something you want to play with, and if you do, then it better be a case of "Im just doing what all those others that went before me have done", rather than "gee I'll bet no one has every tried one of those on one of these....." Andy Link to comment Share on other sites More sharing options...
eightyknots Posted July 17, 2012 Share Posted July 17, 2012 Andys@coffs' date=' post: 228981, member: 94[/email']]... When it occurs the qwuestion is, can you turn it off, or will the engine fail, before it tears itself or the engine mount of the firewall. If the engine stops before departing then you can probably do the usual deadstick landing, if the engine, prop and perhaps mount all exit, then you are about to meet your maker and there isnt a single thing you can do to prevent it, other than try and talk your passenger into emulating an engine weight out on the nose.... ... Thanks A@C. It would be very interesting to see some 'case studies' on this phenomenon: I have never heard of it until today. These sorts of props look interesting: http://www.whirlwindpropellers.com/ga/index.html Link to comment Share on other sites More sharing options...
M61A1 Posted July 17, 2012 Share Posted July 17, 2012 engine RPM's 100% and 1 side of the prop feels the need to leave..............May sound like a joke but it isnt. There have been a number of composite props do just that on direct drive engines.When it occurs the qwuestion is, can you turn it off, or will the engine fail, before it tears itself or the engine mount of the firewall. If the engine stops before departing then you can probably do the usual deadstick landing, if the engine, prop and perhaps mount all exit, then you are about to meet your maker and there isnt a single thing you can do to prevent it, other than try and talk your passenger into emulating an engine weight out on the nose.... There are things in our aircraft type that you can play with and try alternates.Im of a view that in many cases props arent something you want to play with, and if you do, then it better be a case of "Im just doing what all those others that went before me have done", rather than "gee I'll bet no one has every tried one of those on one of these....." Andy Absolutely.......If I remember correctly, there are composite prop manufacturers that warn specifically against using them on direct drive engines for that very reason. The power pulses and associated resonance they can create, can quickly destroy the type of prop that has it's blades sandwiched between two halves of a hub. It can happen in an instant with flight loads that you may not experience with ground running. It really wouldn't take much to turn a blade failure into a fatality. Link to comment Share on other sites More sharing options...
Guest Andys@coffs Posted July 17, 2012 Share Posted July 17, 2012 I recall Les Bolenhagen (previous owner of Bolly Props) in SA tried some of his ground adjustable props on a 4cyl jabiru that was flying from Truro Flats probably 5-7years back the aircraft shed a blade over the airfield. As I recall the engine stayed attached because the vibrations were so bad it threw the carby off the inlet manifold, thus immediately starving the engine before the mounts gave up. The prop design and blades it used have been very successful in other gear reduction scenario's. Les then baned them from use on Jabs, not sure what happened after he sold up. The direct drive 4cyl apparently is more punishing on a prop than the 6cyl. Now that was some years ago and the original owner, I have no idea if the current owner and current technology have the same problem or not...... I'll bet if you google "jabiru bolly prop failure" (without the quotes) your bound to find something of interest to read, like this http://www.raa.asn.au/documents/airworthiness/AN061009-1-3_bolly.pdf Andy Link to comment Share on other sites More sharing options...
boingk Posted July 19, 2012 Share Posted July 19, 2012 4cyl engines are inherently not as well balanced as a flat six, which incidentally has almost perfect balance characteristics if set out properly. Flying in the rain with timber props is inherently dangerous as the prop is effectively being water blasted - this will at best mar the finish and at worst delaminate it and cause structural failure. Modern urethane treatments generally negate this but classic laquered / painted props are highly prone to damage. On wooden props, my Minicab has one on it and it performs well. It was also fitted in 1969... whilst 'on condition'! Since then its done 900 hours. If thats an endorsement for the long life and reliability of a wooden prop then feel free to take it. Cheers! PS: Although the current prop appears fine... I'm replacing it with a new 72" x 48" in laminated German Beech. I think 43 years and 900 hours is more than enough for one prop. Link to comment Share on other sites More sharing options...
jetjr Posted July 19, 2012 Share Posted July 19, 2012 Yep, Jab 4 cyl have given many prop makers problems, I know of a couple of others too in Europe. Not all handled the problem as well as Bolly.........These french guys kept selling them. 6 cyl Jabiru have no such problem. I have a new 2B bolly and it really is very impressive performance. I also did a dynamic prop balance and the improvement is excellent. I cant believe some of the vibration I put up with using wooden props. Balancing will be added to my annual service list. CF dont loose track either which was a problem with the 3 wooden versions I used before I went to CF My old Jabiru wooden prop has been nearly wrecked twice by the new owner flying in the rain. 1 Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now