Counter/non-counter weighted engines?

OK,
I am going to admit my own lack of knowledge here.
Aside from the obvious, just what exactly is the difference between a counterweighted and non-counterweighted engine? Under what conditions would you want one or the other? What are the relative advantages and disadvantages to either and any other info you think is relevant.

This is somewhat of a semantics issue so I will try to give more info for your understanding.

Engines have multiple vibration orders (meaning combined sets of frequency and amplitude) that are not all simply the same frequency as the crank RPM. For instance, since a 4 stroke fires every other rotation, there is a 1/2 order vibration (1/2 RPM) and it is followed by basic balance of the crank or prop that would be 1st order (same as RPM).
The whole vibration formula has multiple order terms starting with the primary order vibrations (above) and then the secondary order etc etc and the influence gets smaller and smaller as the terms go off to the right side of the equation as they describe smaller and smaller physical influences.

All aircraft engines are counterweighted for the first few modes (sorry I don't remember how many). The term "counterweighted" as generally applied to Lycomings is in reference to a specific crankshaft design that includes extra weights that actually move in relation to the crank. They are in a kind of pendulum arrangement on the crank webs that allows slight movement of the weight. The movement is needed to address higher order (further to the right in the equation) and more complex vibration modes. A quick google search says 6th and 8th order.

My understanding of history is that the "counterweighted" crank was introduced with the angle valve Lycomings and it was signified by a "6" in the engine suffix. With the advent of so many EXP Lycoming clones, both angle and parallel valve, I don't think one can tell what is counterweighted and what is not without knowing the engine details from the builder.

Since I am shooting from memory, I welcome any corrections or clarifications to the above from other members here.

To add to Bill's (excellent) description, a possibly more direct / simple explanation.
An engine is not built with a counter weighted crankshaft to make it better, it is generally done to make it work.
The vibration modes induced into a crankshaft are a function of the crankshafts size, stiffness, the compression ratio of the engine, stroke of the pistons, ignition timing, what propeller is installed.... and on and on....

Most of these influences are fixed by design. But some of them we as builders can influence, such as installing a previously untested propeller, or an ignition system that is able to advance the ignition timing beyond what is typical with magnetos. When that is done, it is possible to take any engine (counter weighted or not) outside of the safe zone of propeller/crankshaft vibration interaction.

My parallel valve 180HP 0-360 (0-360-A1F6) has a counterweighted crank as built by Lycoming. So this is not just for angle valve engines.

Bevan

The O-360-A1F6D on my RV-4 has a counter-weighted crank.

I was concerned about any issues with the counter-weighed crank and using a prop that was different than what the engine and crank were designed for, so I called and talked to a tech at Lycoming. I told him that I was using a wood prop and he said I should have no problems.

I don't know what his answer would have been if I had been using a metal prop.

rzbill said:

This is somewhat of a semantics issue so I will try to give more info for your understanding.

Engines have multiple vibration orders (meaning combined sets of frequency and amplitude).
The whole vibration formula has multiple order terms starting with the primary order vibration and then the secondary order etc etc and the influence gets smaller and smaller as the terms go off to the right side of the equation as they describe smaller and smaller physical influences.

All aircraft engines are counterweighted for the first few modes (sorry I don't remember how many). The term "counterweighted" as generally applied to Lycomings is in reference to a specific crankshaft design that includes extra weights that actually move in relation to the crank. They are in a kind of pendulum arrangement on the crank webs that allows slight movement of the weight. The movement is needed to address a higher order (further to the right in the equation) and more complex vibration mode. Memory says 4th order but I'm not sure.

My understanding of history is that the "counterweighted" crank was introduced with the angle valve Lycomings and it was signified by a "6" in the engine suffix. With the advent of so many EXP Lycoming clones, both angle and parallel valve, I don't think one can tell what is counterweighted and what is not without knowing the engine details from the builder.

Since I am shooting from memory, I welcome any corrections or clarifications to the above from other members here.

Click to expand...

As Bill said, the engine model number will have a number, typically a "6" in the suffix, such as IO-360A1B6. However, it is much more than semantics. A counterweighted crank has moveable weights that do as Bill said, but the crankshaft is about 6 pounds heavier and is more costly. The advantage is that the engine will have none or lesser RPM restrictions depending on the prop.

I have an IO-360A1A that does not have a counterweighted crank with a Hartzell HC C2YK prop. This engine/prop combo has 2000-2350 RPM restriction, but it's an rpm range that is rarely used so it's of no consequence. To be sure about any rpm restrictions for your proposed engine/prop combo you must refer to the prop type certificate data sheet. The 200hp angle valve engines currently sold by Vans have the counter weighted crank.


I hope this helps.

Jerry Esquenazi
RV-8 N84JE

I would think, if you had a metal prop, you would contact the prop manufacturer to ask if they had done a vibration survey with that particular prop and engine combination. If they did, they would give you the resulting recommendation (restrictions). If they did not test that combination, they may say "don't know".

I might surmise that a counterweight crank would be an effort to make a minor change in the vibration survey result so as to make a particular engine/prop combo more practical. ie move the RPM restricted range away from where one wants to cruise at.

Bevan

RV8Squaz said:

As Bill said, the engine model number will have a number, typically a "6" in the suffix, such as IO-360A1B6. However, it is much more than semantics. A counterweighted crank has moveable weights that do as Bill said, but the crankshaft is about 6 pounds heavier and is more costly. The advantage is that the engine will have none or lesser RPM restrictions depending on the prop.

I have an IO-360A1A that does not have a counterweighted crank with a Hartzell HC C2YK prop. This engine/prop combo has 2000-2350 RPM restriction, but it's an rpm range that is rarely used so it's of no consequence. To be sure about any rpm restrictions for your proposed engine/prop combo you must refer to the prop type certificate data sheet. The 200hp angle valve engines currently sold by Vans have the counter weighted crank.


I hope this helps.

Jerry Esquenazi
RV-8 N84JE

Click to expand...

Thank you for that. I know the "counterweighted" -6 crank will have less vibration issues but I had not internalized the practical aspects of it.(meaning getting rid of the prop restrictions). I remember those restrictions on the IO-360-A1A in my dads Mooney. Opening a can of worms here but I am going to say that being able to run in the 2000 to 2350 RPM range is important to me because I have signed on to running oversquare LOP at low altitude as is the current rage . I was lucky to learn from Les Dowd that my non-counterweighted engine and Hartzell prop don't have any restrictions.

If it were not for that particular combination, I think I would think very hard about doing what is needed to clear the restrictions, even buying the -6 crank.

All,
great stuff.
Thanks to all for the education.
I am going to assume (foolish I know) that if you call an engine shop and say I want X engine and it will swing Y prop that they might be able to advise which kind of crank is better suited for you chosen combination?

Torsional Dampers

And just to add fuel to the fire, the "counterweights" aren't really counterweights at all.

They are torsional vibration dampers.

A true counterweight is firmly bolted to the crankshaft, and is there to balance the spinning mass.

A torsional damper is added to absorb the twisting motions in the crank due to firing pulses. See: http://en.wikipedia.org/wiki/Torsional_vibration

A great read is how the engineers solved the torsional vibration issues on the Pratt & Whitney R-2800 during WWII. See: http://www.enginehistory.org/NoShortDays/Development of the R-2800 Crankshaft.pdf

More than you wanted to know!

PCHunt said:

And just to add fuel to the fire, the "counterweights" aren't really counterweights at all.

They are torsional vibration dampers.

Click to expand...

Actually pendulum absorbers, not dampers, but bless you Pete. Labels are important if readers are expected to really understand.

Folks, study the location of the pendulum absorbers on a Lyc crank and you'll see they are 90 degrees to the crank throws, not opposing them. They "counterweight" nothing but themselves. However, other engines can and do use the pendulum mass as a counterweight for piston and rod mass. In all cases the effective order is determined by pin and bushing dimensions.

The Chilton patent:

http://pdfpiw.uspto.gov/.piw?PageNu...1=2112984.PN.%26OS=PN/2112984%26RS=PN/2112984

I am going to assume (foolish I know) that if you call an engine shop and say I want X engine and it will swing Y prop that they might be able to advise which kind of crank is better suited for you chosen combination?

Click to expand...

I think you'll find a lot of shops know about the parts, and not much about how they actually work, or how effective they are. The engineers who measure propeller stress have the answers to your question.

And some more info From Lycoming SIL 1012G:

Crankshaft counterweights are calibrated to absorb torsional (twisting) vibrations on the crankshaft at critical frequencies between power strokes during engine operation. Each crankshaft has its torsional frequency which is a function of crankshaft length, crankshaft stiffness, stroke, mass, and moments of inertia driven by the engine. A crescent-shaped counterweight is attached to top and bottom lugs on the crankshaft by a pair of rollers. The diameter of the holes on each counterweight is a specified size that corresponds to the pair of rollers as a matched set. The rollers rock back and forth inside the holes of the counterweight to absorb torsional vibrations as the crankshaft rotates. This dynamic causes dampening of the resonant frequency of the engine/propeller combination. The counterweights turn opposite the crankshaft’s vibratory torque energy to decrease torsional vibration.

There are also many notes from Lycoming about abrupt throttle movements possibly causing damage to or detuning counterweights, something to consider if you operations call for this (aerobatics, formation come to mind or if you're just a ham fisted operator).

rzbill said:

Thank you for that. I know the "counterweighted" -6 crank will have less vibration issues but I had not internalized the practical aspects of it.(meaning getting rid of the prop restrictions). I remember those restrictions on the IO-360-A1A in my dads Mooney. Opening a can of worms here but I am going to say that being able to run in the 2000 to 2350 RPM range is important to me because I have signed on to running oversquare LOP at low altitude as is the current rage . I was lucky to learn from Les Dowd that my non-counterweighted engine and Hartzell prop don't have any restrictions.

If it were not for that particular combination, I think I would think very hard about doing what is needed to clear the restrictions, even buying the -6 crank.

Click to expand...

Here is a copy of the aforementioned restriction/recommendation from Hartzell.



It seems that if your engine has modifications like electronic ignition or FADEC, the restriction is still in place.

So for someone like me with an ECi engine and electronic ignition without a counterweighted crank... How would this all apply?

I dare say that it is just like it but different! That is the experiment in experimental category!

CJ

DanH said:

Actually pendulum absorbers, not dampers, but bless you Pete. Labels are important if readers are expected to really understand.

Folks, study the location of the pendulum absorbers on a Lyc crank and you'll see they are 90 degrees to the crank throws, not opposing them. They "counterweight" nothing but themselves. However, other engines can and do use the pendulum mass as a counterweight for piston and rod mass. In all cases the effective order is determined by pin and bushing dimensions.

The Chilton patent:

http://pdfpiw.uspto.gov/.piw?PageNu...1=2112984.PN.%26OS=PN/2112984%26RS=PN/2112984



I think you'll find a lot of shops know about the parts, and not much about how they actually work, or how effective they are. The engineers who measure propeller stress have the answers to your question.

Click to expand...

Dan,

In the Hartzell document they refer to the design as an "undamped engine".

What is in a name? I am seriously inquiring what the difference is between a damper versus an absorber.

...just curious.

CJ

Looking at the original poster's signature line I think he may be wanting some practical help to decide which engine to buy. I've noticed there are a number of counterweighted and non counterweighted engines appearing over at Aerosport Power and Titan Engines.

Counterweighted engines are likely to run more smoothly and reduce torsional vibrations on the engine and prop. This might be a good thing if you intend on departing from tested and approved engine/prop combinations.

On the other hand, the heavier crank and counterweights themselves add additional weight to the engine (I would guess 7 lbs total). As others have said they can be detuned by rapid throttle changes and generally one more thing to go wrong and cost money at overhaul.

The only sure way to know if a given propeller is compatible with a given prop is to test it and analyse the vibrations. Although I have never heard of a prop departing an RV due to electronic ignition or high compression, that doesn't mean it is impossible.

In the past Experimental engines were built from PMA'd parts to established (certified) design. In the last few years the experimental engine market has expanded and there is a lot more choice. It seems like it is going to be impossible to test all of them with every possible propeller. Is that an issue? I guess we will find out in time.

Captain_John said:

Dan,
In the Hartzell document they refer to the design as an "undamped engine".
What is in a name? I am seriously inquiring what the difference is between a damper versus an absorber.

Click to expand...

Bad nomenclature is common, even with factory folk. Consider "shock absorber", which is really a damper, or "radiator", which is really a heat exchanger.

To your question; a damper is usually considered to be a device that removes energy from the system, most often as heat. A pendulum absorber removes no energy. Instead, it changes the length of the pendulum when the mass rocks on the pin/bushing.

rwtalbot said:

The only sure way to know if a given propeller is compatible with a given prop is to test it and analyse the vibrations......It seems like it is going to be impossible to test all of them with every possible propeller. Is that an issue? I guess we will find out in time.

Click to expand...

Well said Richard.

A standard eci crank costs a little less than 4K, 6K for Lycoming. A counterweight crank is about triple that new.

Sigh....Semantics.

Engineering terms getting in the way of teaching.

I can make an equal case that the bifilar mounted masses on the -6 crank are counterweights in that they are weights and their action is counter to the loads they are intended for.

Counterweighted crankshaft

ccsmith51 said:

The O-360-A1F6D on my RV-4 has a counter-weighted crank.

I was concerned about any issues with the counter-weighed crank and using a prop that was different than what the engine and crank were designed for, so I called and talked to a tech at Lycoming. I told him that I was using a wood prop and he said I should have no problems.

I don't know what his answer would have been if I had been using a metal prop.

Click to expand...

I have put a wood/composite prop on a IO-360C1D6 and have often wondered about the wisdom of this. My concern relates to propellor Moment of Inertia which for a light wood/composite prop is much lower than a metal prop, and the potential to damage the hangers/rollers of the counterweights at low rpm (starting) & by changes in rpm in higher regions.

Never had anybody provide a technical/scientific answer to this but any empirical conclusions could be equally valuable.

Dan, thank you for the description.

Bill, I hear you! I teach electricity and you must choose words wisely!

All of the guys on this forum compose an incredible resource!

How did we ever get by without the internet!?!

CJ

Captain_John said:

All of the guys on this forum compose an incredible resource!

Click to expand...

You can say that again.
Fantastic data provided. Thanks to all who had input. Great stuff.

FWIW the FAA claims that rapid opening and closing of the throttle can detune a counterweighted crank. How that applies to formation flying I'm not sure. I never had an issue with it.

GO300's have multiple weights and will detune easily on rapid throttle reduction. More than a few skylark owners have thought there was somethin wrong when there wasn't. They can be super smooth in cruise though.

Counterweight

walkman said:

FWIW the FAA claims that rapid opening and closing of the throttle can detune a counterweighted crank. How that applies to formation flying I'm not sure. I never had an issue with it.

Click to expand...

The geared Lycoming engines were very prone to "detuning", mostly caused by pulling the power to idle at too high an airspeed on final. I think the guideline was not to pull the power to idle above 120 statute. Also the geared Lycomings were turning 3400 r/m or so on takeoff.
The counterweighted engines in the Pitts survive for 1400 hours of throttle abuse, no problems.

Spectrum Analysis

My 2 cents,
When having your engine/propeller balanced ask the service if they can perform a Spectrum Analysis, it will provide a report of each mode/node by ips and RPM.
The report should show each ? node amplitude. Example I have a o-540 with a 5th and 6th order counter weight, it at one time had the counter weights changed to heaver weights per SB which will accommodate a compact hub propeller design, similar to what Vans sells. It might detect other lurking issues.

rvator10 said:

My 2 cents,
When having your engine/propeller balanced ask the service if they can perform a Spectrum Analysis, it will provide a report of each mode/node by ips and RPM.
The report should show each ? node amplitude. Example I have a o-540 with a 5th and 6th order counter weight, it at one time had the counter weights changed to heaver weights per SB which will accommodate a compact hub propeller design, similar to what Vans sells. It might detect other lurking issues.

Click to expand...

Let's not confuse X-Y axis vibration with torsional vibration.

rvator10 said:

My 2 cents,
When having your engine/propeller balanced ask the service if they can perform a Spectrum Analysis, it will provide a report of each mode/node by ips and RPM.
The report should show each ? node amplitude. Example I have a o-540 with a 5th and 6th order counter weight, it at one time had the counter weights changed to heaver weights per SB which will accommodate a compact hub propeller design, similar to what Vans sells. It might detect other lurking issues.

Click to expand...

Here's an IO390 Spectrum Analysis

Lycoming 4-cyl crank with pendulums, pins, and bushings. They're between the throws for cyls 3 and 4, and positioned 90 degrees to those throws. As such, they cannot counter recip forces. Some other engines do use torsional pendulums in a dual role, recip counterweight and torsional absorber.



From ECI's notes on dynamic crankshaft balancing:



A dynamic propeller balancer like Walt's uses a single sensor (here in green) mounted to a case bolt just behind the ring gear. It detects acceleration in two axis (X and Y). If the sensor could be mounted precisely on the crankshaft rotational axis, that is all it would detect, just the linear up-and-down and side-to-side of rotating imbalance (cyan arrows).

However, such a mounting is physically impossible; best we can do is a case split line bolt. That means there is a distance (an arm) between the crank axis and the sensor (here in orange).

The crankshaft/propeller system also vibrates torsionally. Here the system is simplified as two flywheels, one representing the prop and one representing the crank. The propeller mass moment of inertia is many times the inertia of the crankshaft assembly, thus the relative size difference.

Torsional vibration is a twisting of the shaft (the connecting stiffness) between the inertias. The two inertias oscillate in opposite directions, twisting the shaft one way, then reversing and twisting it the other way. We would not even notice torsional vibration in our engines except for one detail; the oscillation of the crank is transmitted to the engine block by the piston sidewalls. The engine block thus vibrates (rocks) around the crankshaft axis as well as in the X and Y axis.

Here's the important part. If the sensor could be magically mounted on the crank center, it would not detect any of the block movements with a torsional source. However, since it is mounted with a arm between the crank center and mount point, its X axis detects both linear and torsional movements, i.e. prop imbalance and torsional block reaction. That is why the balancer can show orders other than the 1st....even though the 1st is the only one that can be balanced with weights on the ring gear or spinner. The detection of other orders is useful, and they may change as rotating balance is adjusted with weights, but they're not a rotating imbalance. They are ghosts of the torsional vibration superimposed on the X axis detection.

Postscript 11/6: Order sensing description is subject to modification; consulting in progress.

Follow up on the above.

Previously I wrote:

A dynamic propeller balancer like Walt's uses a single sensor (here in green) mounted to a case bolt just behind the ring gear. It detects acceleration in two axis (X and Y).

Click to expand...

I've since learned that the DSS MicroVib system (used by Walt) senses in a single axis only...the vertical, or Y axis in the above sketch. As such it cannot sense block rocking in response to torsional vibration or cylinder firing.

Turns out the series of half order multiples in the spectrum plot (0.5, 1.5, 2, 2.5, etc) are mostly harmonics of the firing order, and are transmitted to the sensor by conduction, not any actual movement of the engine. Think about a ringing bell, a sonar pulse in water, or placing you ear to the railroad track, and you have the right idea.

Bottom line, in the context of this discussion, is that the prop balance system tells nothing useful about torsional vibration, not even a ghost, including the effect of pendulums.

prize

Well guess I win the prize for the only RV to sling a prop! I'm going to have to change the name of my plane (six gun) I was taking some acro from Patty after it happened and she said to call it (prop slinger) but I think I will call it 6 gun still. Anyway I went flying on a Thursday for a short flight and returned to land pulled power back in pattern to land and the plane sounded like it was running really rough landed done a run up everything checked good shut down and done a few things at the hangar.Another RV showed up and wanted to fly around a bit so off we went another run up all fine so off we go for a few minutes and we part company back to each others airport 3 miles out I pull power back to land I'm at 1200 agl and bam there goes my prop I think because I can't see out the windshield call my buddy and let him know I'm going to try to make it back to airport.Long story short landed with out a problem and shut down and still didn't believe prop was gone got out and looked and it wasn't there got down and kissed the ground and said thanks Lord for letting me live. 6 Gun had a io-360 angle valve with a Hartzell BA and a Sabra 2.5 spacer all of which was no longer there and the ringgear also gone.The engine we built did not have counter weights so when I had it yellow tagged I had new weights installed and crank balanced. 51 hours on the plane when it happened so far I have not been able to find the prop so I'm not a 100% sure what happened only that I have six broken bolts left of the assembly in the crank lugs.I'm pleased with the outcome because I lived to fly again and one day I'll get it put back together so 6 Gun will fly again.
Bob

YIKES!!



Glad it turned out so well!

New name for your ride: "Gunslinger"

6 Gun said:

Well guess I win the prize for the only RV to sling a prop! I'm going to have to change the name of my plane (six gun) I was taking some acro from Patty after it happened and she said to call it (prop slinger) but I think I will call it 6 gun still. Anyway I went flying on a Thursday for a short flight and returned to land pulled power back in pattern to land and the plane sounded like it was running really rough landed done a run up everything checked good shut down and done a few things at the hangar.Another RV showed up and wanted to fly around a bit so off we went another run up all fine so off we go for a few minutes and we part company back to each others airport 3 miles out I pull power back to land I'm at 1200 agl and bam there goes my prop I think because I can't see out the windshield call my buddy and let him know I'm going to try to make it back to airport.Long story short landed with out a problem and shut down and still didn't believe prop was gone got out and looked and it wasn't there got down and kissed the ground and said thanks Lord for letting me live. 6 Gun had a io-360 angle valve with a Hartzell BA and a Sabra 2.5 spacer all of which was no longer there and the ringgear also gone.The engine we built did not have counter weights so when I had it yellow tagged I had new weights installed and crank balanced. 51 hours on the plane when it happened so far I have not been able to find the prop so I'm not a 100% sure what happened only that I have six broken bolts left of the assembly in the crank lugs.I'm pleased with the outcome because I lived to fly again and one day I'll get it put back together so 6 Gun will fly again.
Bob

Click to expand...

WOW Bob, I'm glad you got down in one piece, Can you post some photos?Why couldn't you see? What happened to the counter weights? Was the cowl damaged when the prop assembly departed in flight?When was the Crank yellow tagged before or after the departure?Great job on making the airport.
RHill

prop

A few answers to questions the counter weights were installed at o/h 51 hrs TT on them the engine I purchased to o/h did not have them installed I wanted them installed because I was running high compression pistons .I used a SJ cowling so the spacer was needed with the Hartzell BA prop .The nose of the cowling was destroyed as the prop contacted the front of it and the ring gear came out the front and went under between the landing gear.Oil covered the windscreen so forward visablity was limited only able to see out side of Canopy .The spacer had its on set of bolts and the prop bolts into lugs built into the spacer the spacer bolts were broken off in the crank flange .A couple hrs before the prop departed I noticed a slight vibration in the wing tips on roll out after landing so I took the tailwheel off and checked it. On the flight before it came off when I pulled power back to land it sounded rough like fouled plugs so I done a run up after landing and checked good on the next flight the prop departed when I pulled back to idle to decend . It was talking to me before it happened I just did not know what it was saying.Why it happened I have a few ideas but I can't say for sure unless the prop is found.
Bob

add on topic drift

Just to drift a bit... my engine builder claims that the way most pilots are taught to cycle their prop during run up is hard on counterweights.
A DEEP, then repeated cycle of the prop is not good practice according to Jay. He has built overhauled engines most of his long career.
A quick pull of the prop control to hear a slight change, and you are done.
That and the advice to check mags before top of descent, are both counter to common CFI lore. But, they seem to be coming from those who have thought it through.

flightlogic said:

Just to drift a bit... my engine builder claims that the way most pilots are taught to cycle their prop during run up is hard on counterweights.
A DEEP, then repeated cycle of the prop is not good practice according to Jay. He has built overhauled engines most of his long career.
A quick pull of the prop control to hear a slight change, and you are done.
That and the advice to check mags before top of descent, are both counter to common CFI lore. But, they seem to be coming from those who have thought it through.

Click to expand...

To add to that unless you are in an unfamiliar aircraft, troubleshooting, have had the prop off or something similar I recommend folks don't cycle the prop at all, serves no real purpose and is hard on the engine.

What would the prop cycling recommendation be for a new build with a new engine and new prop? (in this case, Lycoming factory O-360-A1F6 counterweighted engine, Hartzell C/S compact hub prop with 7666 blades)

Canadian_JOY said:

What would the prop cycling recommendation be for a new build with a new engine and new prop? (in this case, Lycoming factory O-360-A1F6 counterweighted engine, Hartzell C/S compact hub prop with 7666 blades)

Click to expand...

My opinion, cycle it a couple of times just to be sure it's working and to bleed some of the air out, 100 rpm drop is enough. It will probably still surge on TO so throttle up slowly (which is also good practice), rapid throttle movement is hard on the engine even without counter weights.

Back to the prop exiting the aircraft, great job flying the aircraft!!!! You are certainly a very lucky guy. This is one reason I like to stay away from "non-standard" installations, no desire to be a test pilot. A sabre prop extension with a CS just looks like trouble to me. If you need the extension the Hartzell extended hub would be the "preferred" way to go.

6 Gun said:

A few answers to questions the counter weights were installed at o/h 51 hrs TT on them the engine I purchased to o/h did not have them installed I wanted them installed because I was running high compression pistons .I used a SJ cowling so the spacer was needed with the Hartzell BA prop .The nose of the cowling was destroyed as the prop contacted the front of it and the ring gear came out the front and went under between the landing gear.Oil covered the windscreen so forward visablity was limited only able to see out side of Canopy .The spacer had its on set of bolts and the prop bolts into lugs built into the spacer the spacer bolts were broken off in the crank flange .A couple hrs before the prop departed I noticed a slight vibration in the wing tips on roll out after landing so I took the tailwheel off and checked it. On the flight before it came off when I pulled power back to land it sounded rough like fouled plugs so I done a run up after landing and checked good on the next flight the prop departed when I pulled back to idle to decend . It was talking to me before it happened I just did not know what it was saying.Why it happened I have a few ideas but I can't say for sure unless the prop is found.
Bob

Click to expand...

I hope you will come back and start a new thread with your information. Some pictures of the failed parts would be helpful. It will get lost in this thread and is possibly off topic.

I have the Hartzell composite prop + the 2.5" Saber space (not flying yet). I have mine hand tight on the crank and it would be easy to forget to torque the bolts. One indicator of torsional slippage on the crank flange would be fretting between the aluminum hub and the steel crank. In determining the safety of this extension, I was referenced the many many extensions used with heavier props, longer extensions and high g loading in aerobatic and racing applications. I have not been on VAF forever, but this is the first failure noted on this extension.

Sharing your experience would be greatly appreciated.