newbie question re prop overspeed with constant speed prop

[blane.c]

Looking at the downside, depending on the "wobble" at the time of separation a propeller can travel in a wide variety of directions most contain just air but some contain "your airplane" so if you are unlucky you can have additional problems to the weight and balance issue and sudden change of sound issues you will be experiencing.

 

[wsimpso1]

Flushing the box is not a good way to get rid of crud. An immediate scrupulous cleaning and inspection of the box might have been a good idea. Here is why: Bearing and gear debris is usually high hardness. Bearing debris is then imbedded in soft surfaces and then wears away the hard surface running against it. Bearing and gear debris does not imbed in other bearings or gear teeth, but it microscopically dents these rolling contact surfaces. This looks like etching of the surfaces, which accelerates wear and shortens life of the surfaces by increasing the rate of surface fatigue (wears the hard layer down faster), opening clearances and making more hard debris which accelerates the process. If the dust you describe is aluminum, a view under a microscope with show these tiny chips to be either bright or dull. The bright ones are new and likely quite soft, which means the box is still making these chips. Dull means it has oxidized, and it is now a nice hard polishing abrasive, which will embed in soft surfaces and/or wear other surfaces it rubs on.

From the sounds of things, this box ran more than 10 hours with debris circulating and maybe making more debris. Even if you can clean the box, replace the parts that appear to be damaged, and run it some more, the hard debris most likely shortened the life of the gears, bearings, seals, etc in the box, governor, and prop. Any loosening of bearings takes the gears out of conjugate action, and then shortens gear, bearing, and shaft lives. Trying to fly it fixed pitch sounds to this transmission engineer like a seriously short term end game.

Given my view of the gearbox, if someone I know just had to fly it, I would advise flights to remain within the glide cone to an airport.

If it were mine, I would do four things:

• Remove the existing gearbox and any plumbing or hoses, install a new gearbox with new plumbing;
• Send the prop and governor to a prop shop, and have them take samples of oil, contamination, and silt, then clean and IRAN both;
• Find a way to filter the oil as it is run - 25 micron is not too fine, we filter log splitter hydraulics at 5 micron;
• Inspect oil frequently. That includes looking at the dipstick, doing oil analyses, inspecting your filter, you-name-it.

Why replace the plumbing? In the automotive transmission world, we learned the hard way that significant tranny damage deposits debris in the cooler and lines that is almost impossible to flush out, but that goes back into circulation later and precipitates a repeat tranny failure. Starting in the mid 1990's, my employer instituted cooler and line replacement if ANY metallic fines were found in a failed gearbox - repeat failures just about vanished once that became standard. So, yeah, replace any plumbing in gearbox, governor, and prop circuit.

I would also not be able to resist tearing down the removed gear box. Take samples of the silt in the box, governor, and prop for analysis. Gear faces, bearing surfaces, splines, and seals would get the look with magnification and the sun or strong light over my shoulder on to the parts, and maybe under a binocular microscope too. Metal surfaces imbedded or damaged by hard debris will look etched or roughened and may sparkle in the sun or a single point light source. Seals or journal bearings may have debris imbedded or a lot of wear. If the particles are very fine and left over from earlier issues, the parts may look fine, but I would still be very skeptical of their acceptability.

I would also make note of the materials in all damaged surfaces and then send oil and contamination off for analysis. Wear metals will reflect the surfaces damaged. On top of steel and aluminum, if there are journal bearings, expect aluminum, tin, zinc, copper, lead. Other materials might be dust from cleaning abrasives. If you can reassemble with all known good parts, go ahead. I dearly hope you will not need it.

Billski

 

[Toobuilder]

The governor on my Titan T-51 seems to be a definite weak point, probably because it shares oil with the gearbox and gets gummed up with metal dust.

Anyway, my tentative solution is to treat it like a ground adjustable prop and set the pitch at a midpoint pitch.
My question is whether my thinking is correct, that being IF I reinstall the governor, the fastest I should fly is the speed at which the prop will not overspeed if it suddenly reverts to fine pitch.

In that situation the governor will just be to allow lower rpm and thus less fuel consumption while it is working. When it fails I'll just have to slow down.

Does this make sense? I'm new to aviation so I'm likely missing something.

Thanks
Jim

I re read the above and can offer some thoughts - hope they are helpful.

Since you are self identified as new to aviation, you might not be as aware of the operation of a CS prop as you should be. From an operational standpoint, the black knob sets the MP (power) by defining the throttle blade angle, and the blue knob sets the RPM of the propeller through the governor flyweights. The blue knob on a CS prop does NOT define a set blade angle - it sets RPM. If you set 2200 RPM on the ground tied to a tree, the blade angle will change to constrain the prop to 2200 based on air density, humidity, temperature and how much power is being thrown at it - in other words, it will do whatever it has to do to maintain that RPM. Throw 200 knots of airspeed into the mix and the prop will respond the same way - except now the pitch will be much more course than the tied to a tree condition. With all that said, even with the blue knob full forward as with a TO condition, the governor is still active almost immediately. As soon as the forward speed of the aircraft starts to overrun the fixed blade angle of the low pitch mechanical stops, the governor steps in to add more and more angle. In most airplanes, the prop is OFF the mechanical stops and ON the governor by 20 knots or so. This is why you will often hear/feel the governor "surge" early in the TO roll - that's the governor taking over. If this same governor fails 30 seconds later at 100 knots as you're passing the airport fence, the combination of power and airsped will drive an overspeed when the blades drop to the low pitch stops. So with your proposed scheme, the only way to save the engine from an overspeed is to adjust the fine pitch stop to a course angle that matches the desired airspeed and power setting. But the governor will really only start working ABOVE this speed and power, so you are really not buying any safety net if you feel the governor is going to fail. Trust me, you are NOT going to be able to move the throttle fast enough to save a runaway governor, and if you are at a high airspeed, it wont matter anyway. BTDT. On top of that, the new low pitch setting will not allow the propeller to even meet the desired RPM untill you meet that speed. This means the TO RPM will be low, and the resulting power output of the engine will also be much lower. You are in for a much longer TO roll and slower climb.

Back to "normal" operation: in the typical, NA aircraft engine, power drops with every foot of altitude gain. So a typical crosscountry mission in a CS equipped airplane will see the throttle advanced to full at the beginning of takeoff and left against the stop through the climb, cruise, and only retarded at the TOD some hours later. Its the propeller that is manipulated for RPM, not MP - sometimes its pulled back in the climb, and ultimately set at TOC for the cruise segment. Of course, the mixture is leaned during the climb to maintain best power all the way up, and many of us then go LOP for the cruise segment (I'm typically LOP during the last half of the climb).

If you are THAT worried about your new gearbox, set it up to run in a loaded condition on the bench for 50 hours or until you are comfortable its not making metal. Flying a questionable gearbox is nuts.

 

[Dan Thomas]

A big enough overspeed can fail the prop. There are many tons of force pulling on each blade, and the prop hub can only hold so much. Props aren't designed with a 3.8 load factor, and are run much closer to their structural limits than an airframe. An overspeed of 20% increases the forces on the blades by 44%. A 30% overspeed increases them by 69%. This is bad. If that hub fails and lets go of a blade, that engine will rip itself off the airplane and it won't even glide after losing all that weight off the nose. The CG shifts way aft.

The prop manufacturers have some tight limits for overspeeds before requiring removal and teardown and NDI to find cracks.

 

[pantdino]

any competent mechanic aircraft or automotive should be able to remove the cowl, prop, and gearbox , take it apart, inspect and clean....from your other thread, I can not remember weather you have given up on Titan or not, but it does appear you hire out these services rather than doing it yourself. See if there is a decent race car mechanic in your area, around my home there are very few professional aircraft mechanics

The gearbox was disassembled and the bad, cheap bearing replaced by Titan's professional aircraft mechanic.

 

[Toobuilder]

...and the torsional resonance issues can be epic. Like backing out prop mounting bolts so forecefully it breaks the lockwire, brinels the hub/flange interface, and (in my case) breaks the crankshaft dynamic counterwieght ears. All with about 2 seconds of overspeed.

Dramatic video of a massive prop overspeed:

 

[pantdino]

A big enough overspeed can fail the prop. There are many tons of force pulling on each blade, and the prop hub can only hold so much. Props aren't designed with a 3.8 load factor, and are run much closer to their structural limits than an airframe. An overspeed of 20% increases the forces on the blades by 44%. A 30% overspeed increases them by 69%. This is bad. If that hub fails and lets go of a blade, that engine will rip itself off the airplane and it won't even glide after losing all that weight off the nose. The CG shifts way aft.

The prop manufacturers have some tight limits for overspeeds before requiring removal and teardown and NDI to find cracks.

Yes, the prop manufacturer states the degrees of overspeed and the steps that must be taken if those occur.
The whole point of my removing the governor is to reduce the probability of overspeed to zero.

Mine is not the only airplane that has experienced governor failure, and at least one of those have experienced prop overspeed. The reasons remain unknown. Most likely fines in the oil, but no one knows for sure.

My working hypothesis is that the cheap bearing failed, filled the gearbox with fines, and those fines were not adequately cleaned out when the bearing was replaced. So the multiple (over)fill and empty steps will be to create enough swirl and splash to wash out the fines.

If there is still debris after 10 fills and empties, then there is something in the gearbox still making metal. If not, then it is worth doing a short flight to see if the oil stays clean.

Running on the ground under load for 2 hrs is not an option with this plane. Like a real P-51, the cooling system was designed to have air moving thru the belly scoop. Prop wash does not provide enough, and the engine will overheat.

One thing that confuses me is that according to these comments a plane needs a CS prop to fly. Tens of thousands of airplanes flying around out there with fixed props would seem to contradict this.

I share the opinion of you all that a fixed pitch prop will leave a lot of performance on the table with this plane, but I really don't see I have any choice if I don't want to experience governor failure and prop overspeed.

EDIT- It seems to me the system which involves the gearbox and governor sharing oil is fatally flawed because the governor tolerates zero debris in the oil and gearboxes always make some. Note that when I took the governor apart there was zero visible debris but the governor was stuck nonetheless. I think the only real cure here would be to install a separate oil pump and filter on the gearbox.

 

[Toobuilder]

My recommendation was not to run the "airplane" on the ground, but to remove the gearbox from the airplane and "bench run" it. Bolt it to a big electric motor or a junkyard Vortec engine and run the hell out of it on the safety of the ground. But to that point, it would be relatively easy to cool your airplane for extended ground ops. Plenty of liquid cooled engines are stationary and run pumps and generators all day long. Might be as simple as a big fan shooting down the scoop.

And yes, plenty of airplanes run fixed pitch props - including early WWII fighters. It CAN be done, but your POH is out the window, and so (probably) is your AWC. "Locking out" your prop is a major operational change and would likely not look to favorable to the Feds if something happened and it wasn't documented. Just something else to consider.

 

[rv6ejguy]

Due to the automotive engine having a much wider rpm range than a Lycoming, you leave a lot more power on the table with a prop pitched for cruise flight on the auto engine during TO and climb. You can fly but that probably isn't why you have a V8 in there.

A good GB won't make any significant fines once the gears are worn in. Nothing I can see in my oil or magnetic plug at annual. 441.8 hours and the GB has never been opened.

 

[wsimpso1]

The gearbox was disassembled and the bad, cheap bearing replaced by Titan's professional aircraft mechanic.

That's nice.

Listen - You still have "dust" coming from someplace. If it is leftover, it is still contributing to an early demise of the gearbox and anything else sharing that oil. If it is new, you still have some other parts wearing and contributing crud. Either way, the crud in there is most likely contributing to creating more crud... Your gearbox is trying to tell you it is going to fail soon.

If you IRAN the gearbox, and it is squeaky clean and makes no more "dust", and you still wanted to run your prop fixed pitch, you can find the pitch that let's you do everything and clamp it there with the stops. That will let you prove to your self that the gearbox is working. the rest of use would fix the prop and governor and put it back on, if not immediately, then shortly after the gearbox shows it is good.

Back to the top. We do not care what has already been fixed. As long as you have "dust" in the oil, you have a gearbox that is telling you it is going to fail soon. Please listen to it.

Billski

 

[TFF]

I believe Toobuilder has the edge on all of us having a gov fail and trashing a new engine and prop.
Your assumption on a fixed prop with other fixed props has one issue, performance. The performance range of a C172 vs your plane is different ball parks. Ok the first Spitfires and Hurricanes has fixed pitch props. It has been done, they sucked taking off or going around to the point they were crashing them if they were not on the ball. They needed the CS prop and got them as soon as they could source it, it was actually a protected secret at the beginning of WW2.
You cant just remove the gov. The prop is now random; it can do anything it wants. You will have to break the prop open and lock the blades from inside; fix the pitch. Some props fail at fine pitch, some fail at course. Some are just going to be random.
Down to it , you have an expensive toy that might break every flight. Like owning a competitive race car. It is not a solid low maintenance commuter car. It suck you are having problems, but your going to tear the plane up monkeying with it. One thing if it was the 1930s and you were inventing a CS prop and you are willing to loose a couple of planes got the advancement of technology.
Are you the pilot on this thing? At some point a rational pilot will say too much to risk my neck.

 

[Turd Ferguson]

My question is whether my thinking is correct, that being IF I reinstall the governor, the fastest I should fly is the speed at which the prop will not overspeed if it suddenly reverts to fine pitch.

That's going to be a very low power setting which won't be practical at all.

 

[Dana]

The gearbox was disassembled and the bad, cheap bearing replaced by Titan's professional aircraft mechanic.

Yeah, and it's still making metal?

My working hypothesis is that the cheap bearing failed, filled the gearbox with fines, and those fines were not adequately cleaned out when the bearing was replaced. So the multiple (over)fill and empty steps will be to create enough swirl and splash to wash out the fines.

Meanwhile, the abrasive crud is chewing up other parts of the gearbox... and generating new crud...

One thing that confuses me is that according to these comments a plane needs a CS prop to fly. Tens of thousands of airplanes flying around out there with fixed props would seem to contradict this.

Airplanes with a much narrower speed range, yes. Golf carts don't need multi speed transmissions either, but cars do. But even so, a fixed pitch propeller is different from, and a lot less failure prone than, a constant speed prop that's been hacked to lock it to one set pitch.

 

[MadProfessor8138]

Am I the only person that has recognized these facts in this thread ?

1. The OP admits to being poorly versed in aviation but has a very expensive toy to play with.........
Luckily it's not a Bonanza....because,well,we all know how that story works itself out in these situations.

2. The OP has asked for help but seems to know more than several very experienced people offering him advice on the subject.
Anybody wanna place your bets on which side the coin is going to land on this issue......think "in breaking news" & "film at 11:00".

3. The factory messed up a very expensive gearbox with a cheap bearing......so why isn't that gearbox setting at the factory,either being repaired or replaced on their dime ?
If you can afford a T-51.....my bet is you can afford a lawyer to take care of the issue for you.
Or,if you're more of a hands on person...that same lawyer can post your bond for you when the judge gets there in the morning.

Dunno.....maybe I just see things different than others...

Kevin

 

[pantdino]

OK, you guys have convinced me. I'll pull the gearbox, take it apart, and have a professional look at the innards.

I can mount the new one, run it a bit, check the oil, and go from there. If it looks clean I'll mount the governor.

The plumbing is very minimal and large bore, allowing inspection scope access and washing out with gasoline by repeat agitation and dumping until spotless.

What kind of professional should I google in my area?

 

[pantdino]

Am I the only person that has recognized these facts in this thread ?

1. The OP admits to being poorly versed in aviation but has a very expensive toy to play with.........
Luckily it's not a Bonanza....because,well,we all know how that story works itself out in these situations.

2. The OP has asked for help but seems to know more than several very experienced people offering him advice on the subject.
Anybody wanna place your bets on which side the coin is going to land on this issue......think "in breaking news" & "film at 11:00".

3. The factory messed up a very expensive gearbox with a cheap bearing......so why isn't that gearbox setting at the factory,either being repaired or replaced on their dime ?
If you can afford a T-51.....my bet is you can afford a lawyer to take care of the issue for you.
Or,if you're more of a hands on person...that same lawyer can post your bond for you when the judge gets there in the morning.

Dunno.....maybe I just see things different than others...

Kevin

I tried to say I was expressing my thoughts and was open to those thoughts being corrected. I guess I should have said it more often or more clearly.

I'm not a pilot and will be hiring professional pilots to fly for me. Maybe if one is a CFI he can teach me to fly in it, but I think it unlikely that at the age of 66 I would have enough years ahead of me during which I would be a competent pilot to make it worth the expense. I read Kathryn's report and "elderly" or "low hours" pilot seems to be a recurring theme in flights that end badly.

I haven't made any comments about the situation of the factory installing a cheap bearing so people can draw their own conclusions. They did sell me a new gearbox at 60% of the normal price, so I can't say they didn't do anything to compensate me.

 

[wsimpso1]

Smart man!

Auto racing shop that specializes in transmissions and axles.

Industrial machine shop that specializes in transmissions and gearboxes.

Bring all available instruction on disassembly/reassembly of this box and fits/clearance information on bearings, shafts, etc.

 

[BBerson]

What kind of professional should I google in my area?

Should have a professional aviation mechanic/inspector with experience in the aircraft type and all the manuals and service letters.

 

[MadProfessor8138]

pantdino,
A few things........

1. I'm glad you're having the gearbox looked at and evaluated by a professional.
I would dare say that they are going to find issues that you or myself wouldn't have.

2. Why is the factory not taking care of this issue on their dime?
The problem originated with them and they just decided to pass the issue & expense onto the customer ???!!!
Don't understand this.....

3. You think 66 is too old to become a competent pilot.........
You are mistaken on that point sir....
Many of the pilots I hang around with are in their 80' & 90's.....and they are the best pilots you will ever find.
Now.....start getting some hours in the air young man.

Kevin

 

[BJC]

3. You think 66 is too old to become a competent pilot.........
You are mistaken on that point sir....
Many of the pilots I hang around with are in their 80' & 90's.....and they are the best pilots you will ever find.
Now.....start getting some hours in the air young man.

BJC