Titanium Propellers- Please help
 

Just curious- Aside from cost, does anyone know why airplane propellers aren't made out of titanium?

The lack of elasticity.

If you are at an airport and look into the front of any of the large turbofans, you'll see the giant front fan with anywhere between 32 and 42 blades. Each blade weighs about 15 lb. on large engine and is approx. 24" in length. The blade was made from a solid block of titanium in most engines and it costs well over $25,000. to make.

Titanium is a perfect material for this application.........strong and relatively ductile. The blade won't fracture if the engine hits a bird at 300 mph.

The only reason that it's not on the Cessna is cost.

What are current Cessna and or other light aircraft propellers made of? Aluminum? Carbon fiber?

Common props are aluminum, wood and more recently, carbon fiber. I don't know if c/f is certified for use in certificated aircraft yet but there are Light Sport aircraft and Experimentals with them for sure.

The pictures I see of belly landing results show prop tips bent back. I'm guessing wood doesn't bend even at those rpms so none of those props were wood. I don't imagine carbon fiber or many composites bend either. Maybe ductility is a design factor or FAA requirement.

Sixto
87 300D

Anybody see the show about the GE90 115B? http://www.geae.com/engines/commercial/ge90/ge90-115b.html
Carbonfiber fanblades, with titanium leading edges. The blades themselves are curved for more volume. Quite impressive it is!
Some more info....http://en.wikipedia.org/wiki/General_Electric_GE90
And a closer shot of those beautiful blades http://en.wikipedia.org/wiki/Image:GE90_dsc04644.jpg

Looks like a shredder!:eek::cool:

GE took a leap forward with the GE-90 that no other manufacturer would do. The risk of carbon fiber on a fan is significant. Typically, it doesn't have anywhere near the impact resistance of titanium. However, the compelling weight benefits on such a large engine were irresistible for GE and resulted in reduced weight for the entire supporting front end of the engine. With an engine that has a weight of about 17,000 lb.........every lb. is important.

To date, there have been no GE-90 fan "incidents" that I'm aware of.........a tribute to GE's design and rigorous testing.

I thought that some of you "prop heads" would enjoy this

RAF Sheppards Pilot to Safe Landing

No mention of how they plan to clean the pilot seating.

$25k for one blade.. nice

Aren't the blades investment cast? Titanium is not easy to machine.

Wow!

Brian,
That the GE-90 was a breakthrough use of fiber is a surprise! CFAN is just south of Austin and it seems like it's been there for a zillion years so I was under the impression carbon fiber had been used in turbofan applications for quite some time. Learn something new every day.

Yes, I'm sure they start with something less than a full block..........the machining of a complete block would definitely take forever.

Some of the new blades are cast with a hollow core. That's quite a feat.............

CFAN is owned by GE and is the company devoted to those blades. Whether GE got the technology from CFAN or vice-versa.........I can't say.

All I’d like to add is never confuse the application of materials used in piston driven aircraft propellers with the blades of a turbojet/fan engines. They are two totally different applications with two totally different requirements.

There is a hell of a lot more involved than cost.
Flex requirements. Vibration absorption/transfer. Gyroscopic issues. Power input/removal rates. Crankshaft power impulses. Torque/Leverage issues…..

I think titanium would be perfect for a piston driven aircraft propeller. It would flex quite a bit less than the aluminum, but that could be designed into the pitch. The weight is comparable to aluminum, so the gyroscopic issues are minimized. It's much stronger than aluminum so that it can take significantly more power and the crankshft power impulses wouldn't bother it.

I don't see any detriment to titanium for a piston driven propeller other than cost and the fact that it's more than you need to get the job done.

Early work in carbon fiber fan blades was done by GE back in the '80's with the Unducted Fan Engine.

http://www.airspacemag.com/history-of-flight/16044907.html

I have to fly this evening, so no time to get deep into this.
Maybe Shelby will later, as she has a rather boring HD tonight.

You are for the most part correct, if you’re talking about an application such as a large transport aircraft where power is run-up and set with only minor adjustments throughout the flight. But for an application such as an aerobatic aircraft, many of the reasons you feel titanium would be the right material, are actually why it would be wrong. As for all the aircraft in between… well, there is certainly the issue of cost involved. But there are also many factors involved where having the ultimate in rigidity, light weight and strength is a bad thing.

Don’t stay focused only on the issues related to how the motor will affect the propeller, but focus also on how the propeller and its properties affect the motor. Also – How the flight of the aircraft can affect the propeller, and how that in turn affects the motor.

I'd need to look it up, but IIRC, the density of Ti is very close to the density of Al. Therefore, the engine won't know whether the propeller is titanium or aluminum.

I'm listening for any other variable that might affect the engine...........the increased strength and stiffness won't..........

Sorry, but:

Like hell it wont !!

There’s a good reason why the best all around propellers are neither aluminum or titanium, but wood/composite.

Stiff/ridged is a bad thing. Just ask Piper and their Malibu issues with cracked supercharger housings. It was all caused by vibration issues from stiff aluminum propellers. To go to an even stiffer titanium propeller…. Ouch.

Aluminum and Titanium both support harmonic vibration. This is why so many propeller driven aircraft have propeller RPM restrictions (And I don’t mean the max, or red-line.). Many (if not most) failures of gyro instruments, motor mounts, engine cowlings, radio systems…. can all be traced back to the propeller and the vibrations it can not absorb and thus sends back through the engine.

Aluminum and Titanium would both exhibit typical tip fatigue failure issues. Where the aluminum props often show erosion in this respect, I might think (you're more into this than I) that the titanium may crack. “Propeller” and “Crack” are two words we don’t like to hear used together.

As far as the gyroscopic issues. Light and short are everything. The lighter and shorter the blades are, the fewer precession issues have to be dealt with, including their affects on the motor. I don’t think aluminum nor titanium are light enough to be made as short as a composite blade with the same available thrust. This is why all the serious aerobatic aircraft run MT (or some other make) composite blade propellers. And trust me, “cost” is no issue there.


- Shelby

This brings me to something I saw the other day on TV regarding boat propellers.

Maybe Hatt can jump in here. :)

A guy was talking about all these bass boats running around with pretty stainless steal props. He though that was nuts.

Being a boat that runs around in logs, stumps, and rocky waters, it made no sense not to have a more forgiving propeller.
He stated that bass boat operators would be better off with aluminum props (and a spare) that would bend (absorb) a hard strike, as compared to ridged stainless steal that transmits the strike right up the shaft to the pricey motor parts. Made sense to me. But then, I’m not a bass boat driver.
Not sure how the “sheer-pin” I’ve heard they have on the shaft comes into play here. Maybe that’s what the SS prop runners are counting on.

:confused:

- Shelby

Most, if not all metal boat props have a shear pin or a rubber hub or both. The rubber hub flexes to absorb small impacts and slips to absorb larger ones.

Another company, Comprop I think, makes cheaper plastic props designed to shear off with impact. Obviously losing a blade is not so much a problem in the water but I can imagine that the vibration would be pretty awesome spinning at 5000 RPM!

Most of the lakes around here are shallow and I hit bottom all the time. Aluminum props get pretty beat up but the stainless prop I have on one of the outboards is solid. It never bends or chips and I'd probably have to hit something metal or cement/rock to do any damage to it.

A friend of mine is building an airplane from scratch and I believe his engine/gearbox to prop incorporates a harmonic balancer and some kind of flex drive on the composite prop IIRC.

Yep, you could be right about harmonic vibrations. But, "stiffness" may be desirable or it may not be desirable depending on the frequency of the vibration and the operating frequency.

Traditionally, the stiffer the component, the higher the first order vibration frequency. If this can be well above the operating speed, the stiffer material wins.

It might be the situation, due to the shape of the propeller, that the operating range is between the first and second critical frequencies............and titanium might shift this point to an undesirable location.

In summary, titanium by itself is not an automatic reject for a propeller. It would depend on the harmonics of the material, if present, and the operating speed with respect to those harmonics.

Aluminum is more forgiving and will protect the driveline somewhat on a serious hit.

However, stainless is more durable for those operators that will hit some sand on occasion. The stainless is relatively tolerant of sand and the driveline is not harmed by a hit in sand.

So, it's strictly an operator choice. Take the more expensive stainless and don't hit any rocks..........or choose the cheaper and more forgiving aluminum

Makes good sense to me. As well as KarTeks post and the rubber mount.

Thanks for the lesson.

The only experience I have with powerboats and propellers is J & DJs Tige’ VE-24. It’s a V-Dive Inboard (I think that’s what they call it.) with a brass prop. We hit a log one time and hardly bent the tip of one of the three blades. But wow, what a vibration that made. :eek: I thought the whole shaft was going to rip out.

From what I recall, those brass props aren’t fun to buy or have fixed either. Luckily they keep a spare onboard and some funky looking puller that’s required.

- Shelby

I touched down the starboard prop on one of the ferries just briefly..........in sand...........on a Saturday afternoon (weekends are the heaviest).

It folded the very tip on one of the four blades. The wheel is all brass..........very soft..........and it takes nothing to damage it.

Hauling the 75' boat in the middle of the parking lot on Sunday morning and changing the 34" wheel was definitely not what the mechanic wanted to do that morning. But, by noon, the boat was back in the water for the Sunday afternoon rush hour (actually 5 hours stretching from 4:00 to 9:00).

I think I recall him saying something about their props being soft as well, so as not to damage the shaft, bearings or seals.

Wow. 75’ with 34” props. I think I’ll stick with the easer to operate, helicopter. :o

As I recall, we were back in the water within about 20 minutes. But that prop is more like 12” or so. :D

- S

I can drink till I can't see...........let go of the controls..........and the ferry will just continue straight ahead...........if I pull the power back to idle...........the ferry just stops and sits there...........I can pour more drinks...........

..........how'd that work for you in a helicopter............??:D

..........easier to operate............I think not.............;)

A lot of owners dive to change a prop rather than pull the boat.

IIRC, my parents run an aluminum props on their Sea Ray. Worried about breaking stuff when hitting something.

I seem to remember my dad telling a story about one of his friends running a stainless prop, hitting something, and destroying the u joint and gimbal bearing.

Zero visibility and a weight of 225 lb. for the wheel..........and the requirement to heat the hub to remove it..........make this an undesirable option for a ferry.

You operate ferrys now? I though you did custom CnC work...

I still run a charter boat on occasion. The ferries demand too much time out of the weekend.

The custom balancing tooling for jet engines is the primary business.

I can drink till I can’t see....let go of the controls....and the helicopter will just continue straight into the ground....if I pull the power back to idle....the helicopter just drops like a rock....I probably wont drink any more....

Close enough - ;)

- S

Right after finishing college, I applied for an opening they had for an illustrator. I did a little research to find out what they were and IIRC, CFAN was a consortium formed by GE, Snecma and (maybe?) RR to explore cf use in aerospace engines. It has just been there so long I figured that by now, more than one type of engine would have carbon blades.

Blades aren't their only biz according to their site. They do ducting etc...

A helicopter is like a raging bull - always trying to destroy itself and everything around it. The pilot doesn't actually fly the thing, they just keep it distracted and carefully "herd" it around and keep it from doing any damage. ;)

I heard someone say that some people are using titanium to make jewellery like wedding rings out of. If it's THAT precious, I would think it would be out of the question for ordinary air plane props.

They do it for the color. It's a cross between silver and gold as far as color is concerned. It's not a precious metal. Jet engines use quite a bit of the material up in the front. It can't take any high temperatures so anything behind the fan and LP compressor can't be produced with titanium...........inconel then becomes dominant.

I imagine a titanium wedding ring would be as light as a feather. I have a titanium bladed knife and hate it because it has no heft.

Sure ya'll aren't talking about platium? That is what our bands are made of.

I have a Breitling with a titanium case and band. It's my primary watch, as it's light enough that I can put it on and forget that I'm wearing it.

Titanium has the strength of steel and the weight of aluminum. It's expensive to purchase and very expensive to machine.

Platinum is a precious metal.........more costly than gold IIRC. It's soft as hell, however.........useless for any tools or parts.

Begging you pardon, Mistress. Did you just inherit some titanium mines? I'm perplexed as to why you would suddenly be concerned about a lack of good uses for titanium.

I've got some in my mouth.

Apparently it has the curious property of being attractive to bone cells, attractive enough so that they will bond onto its surface rather than reject it.

Which is why it's good for artificial tooth roots.

I completely understand your point, but I saw them change out an 8ft brass wheel (what, a ton?) on a 200 ft. fishing boat in Alaska while in the water. Heat, hammers and all. Putting it on a railway wasn't an option.:D

Well, I suppose that you can do just about anything if you're setup for it. With the ferry company, they can haul the boat in 30 minutes. It's more trouble to get the damn cars out of the way than to lift the boat. Their weight is not excessive..........90,000 lb...........so, the travellift can do the job without a railway.

That would have been a sight to see...........the rig to change that 8' wheel underwater and lift it back out...........

:eek:they put a ferry in slings??!! That sounds much more interesting than what I saw. There are four car ferries that cross the James River close to my home and I can't imagine then riding in a Travellift (of course the largest one I have ever operated was 35T.

I made pretty good side money my first year out of college diving on props, replacing shaft zincs, ect...b/c if the shop was backed up the yard time would make it less expensive to hire a diver.

Yep, that's what everybody said when they first did it. Never going to work on boats that large. It was the largest lift they ever built.

In reality, it works perfectly on the steel hull boats. The wooden 65 footers (now extinct) did have issues with the travellift due to stiffness problems. Their leak situation would generally worsen when the lift was used. Two points on the hull is not the best solution for a wood boat. Additionally, it was a PITA to maneuver some slave chine pieces under the boat to prevent the straps from crushing the chines.

You should see the 85' boats in the lift............but their weight is only 37 tons.........aluminum hulls.

I bought a coupla titanium crow bars from Harbor Freight. Gave them to buddies as Christmas presents. IRRC they cost about $25.