45 Horse Half VW Prop Weight

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Rockiedog2

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There have been some crankshaft failures in the Casler 45 horse motors. According to a letter put out by Scott the 45 hammers the crank much harder than the smaller halves and of course the 4 cylinder ones(that makes sense). Having recently become aware of the problem, one of my first thoughts was about prop weight. I've run a number of props on my 45 and weighed all of them. I don't have the weights in front of me but they vary widely(particularly as a %) from like 2#12oz or so to nearly 4#. So I'm wondering if the lighter or heavier is "best" in regards to crankshaft failure. I can see it being either way...the heavier prop dampens the hammering of the 45 but puts more load on the crank and the lighter the opposite. I expect there must be some minimum prop weight for a 45 but don't know that anybody knows what it is...my current prop is 2#xoz and it does hit very hard...very little flywheel effect. So what's best to avoid crank failure, heavier or lighter and where is the xover point?
 

Dana

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A lighter prop will always mean less stress on the crankshaft.

Dana
 

Pops

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A lighter prop will always mean less stress on the crankshaft.

Dana
Correct. I used a Culver 60"x26" prop on my 1835 , 4 cylinder VW engine that weighed 3 lbs and a 7.55 to 1 compression ratio on the engine. At cruise it is producing about 33 HP from the rated 60 hp at 3400 rpm. So its lightly stressed and is very smooth. Also used a stock high quantity German crankshaft of 69 mm.
 

bmcj

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Not an engine person at all, but couldn't you soften the blow by upping the octane to slow the burn? Or maybe retard the timing a bit (would probably cause a reduction in HP too).

Feel free to correct me or call me for excessive BS if I am wrong.
 

BBerson

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The answer isn't that simple.
First some basics:
Intermittent firing piston engines need a flywheel of some sort, either in the crank itself, an external flywheel or the prop itself. Or all three.
An engine with low total flywheel inertia has large variation in rpm between explosions.
An engine with high total flywheel inertia has minimal variation in rpm between explosions.
Which is better for the crankshaft?

My Limbach (VW based) engine has a 23 pound Hoffmann three position prop installed and is certified.
More to say. But that is a start.
 

N8053H

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I spoke with Scott for some time yesterday about this issue. The cranks used on the stroked 45 hp engines are breaking between the throws in the webbing. This is a stock part of the crank that is breaking. There has been four failures so far. Three of these have been in ultracruisers and on in a legal eagle.

These engines are being operated at 3000 rpm's and above. But like I mentioned to Scott this really tells us nothing. What we or he needs to know is how hard the engine are being pushed. I asked him if anyone is using a MP gauge and he said no. But he wished they would. He would like to see these engines operated bellow 25 mp in cruise flight.

He is also offering a decompression kit to lower the compression to 7.1. This kit consists of two .60 rings that go between the head and cylinder, and adjustable push rods. The kit cost 50 bucks. Scott will be out of the office and the shop closed for a week. All next week until the 4th of September. Just a heads up if you would like to order one of these kits. Do it now or wait a week or so.

As to fuel only use 91+ octane fuel. Also try not to use fuel with ethanol in it.

We spoke in length about engine load and RPM's. All but one of these failure's have happened in UltraCruisers. He as do I believe the engines are being pushed to hard. He really wishes everyone would follow my lead and install a MP. Keep this gauge under 25 in cruise flight. A MP gauge is really the only way to know how hard you are pushing your engine. Scott agree's with this statement.

I used a flywheel on my engine for the reason stated above. I noticed it soften the blows or made the engine run smoother. I am not using one now for I did not have the bolts to match with my new prop, but it may go back on for the reasons stated above by BBerson. I agree with his statement on this.

He is seeing issue's with engines using dual carbs, the big heads, and lifted cam or hi lift cam. I am lucky and have a stock cam, stock heads and single carb. Scott informed me I have the 42 hp version of this engine. He has not had any issue's with this engine or the 38 HP version. None of these engines have come back with any failures as of today.

I operate my engine by engine load. I watch that MP gauge. I only go above 25 mp on climbout. At about 200' altitude I am reducing mp to around 25. Once I am at altitude I will never again see rpm's or engine load above 25mp. MP will stay between 23-24 in all phases of flight after climbout. If I want a higher altitude I do not go above 25 MP to get to this altitude.

Scott wishes more would use this gauge and fly like I just mentioned. If one did have an issue with their engine, they can give Scott some info to work with. Saying I run my engine at 3000 RPM's tells scott very little as to how hard you are running the engine. I had one prop and at 21 mp I was turning 3100 rpm's. The prop I use now at 21 MP I am lucky to turn 2600 rpm's. RPM's do not tell you how hard one is working an engine. It just tells you how fast its turning.

These engines that failed where all at or above 130 hrs. The ultralcruisers had between 150-300 hrs on them. The legal Eagle had 130 hrs on it.

A little info on this issue. I hope this helps you with your 1/2 VW.

Tony
 

N8053H

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This thread should have been put in the auto Conversation area of this site under 1/2 vw for others to follow. If this could be moved to that area it would be a great help to others in the future. IMHO.

Tony
 

Rockiedog2

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Here's the letter Scott put out. It was on the Half VW site. I assume it's ok to run it here since he's trying to get the word out.

Hi All
The cranks I have been using are stroked German VW cores. I have been using them for about 16 years. For both the 37hp and 45hp engines. The 37hp

engine(78mm) has never had a crank breakage. The 45hp engines (86mm with 2.00"Chevy journals) have had some breakages through the web between the main journal and the rod journal. After that I changed crank manufactures and beefied up the web area. Now, last week I had one of these new cranks break. Not where it was beefied up but in the web between the two rods. It is a full .750" thick. It is part of the crank that is not modified. It is the stock VW part. As of now I am thinking the stock VW crank is pushed beyond its limits. These cranks are used in the four cylinder configuration on high rpm, high compression engines without any problems. The two cylinder engine hammers the crank much harder because of the firing impulses

being 360 degrees apart. So, as of now I am de-stroking the 45hp engine down to a 82mm VW journal size which is a 1138cc instead of a 1194cc.

It should produce around 42hp. I am hoping this solves the problem.

Scott

Hummel Engines



BBerson touched on what I've been wondering about....prop weight that's a compromise between crank stress and flywheel effect. But I don't know if that's really of concern here; like I said it was one of my initial thoughts. Tony did Scott say if the motors continue to run or did they sieze or what? Did they all behave the same? My plan would obviously be influenced by what happened when they broke. I got no idea what MP mine is pulling I just know I run it high rpms routinely and I'm running the dual carbs Scott doesn't like; the prop is so thin it may not be working that hard. Think I'll put an MP on it and see. When I was tuning mine for max output I dyno tested it and it made 37 horsepower with the Zenith updraft carb; it couldn't breathe and was a dog behaved just like the 37 I had on it before. Another "45" with the updraft Zenith made 38 HP on the same dyno. I installed the dual Mikunis; got it tuned for 1350 egt and it made 43 hp w/o being broken in. It's been run more time since then, has loosened up and probably is making the 45 horses I paid for. I need more info from Scott...I'm not really too inclined to pull the throttle back on the thing; that's what mine is all about. Think I'll get back on the Half Vdub site it's probably all abuzz. More comments on this welcome
Spencer
 
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N8053H

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They all seized or locked up and stopped running in flight. No one has been hurt. The legal eagle was up around 6000' doing stalls.

IMHO and Scott agree's, you want to keep the rpm's bellow 3000 in cruise flight. I was using a prop and at 21 mp the engine was turning 3100 rpm's. Once you install a MP gauge you will find 21 mp is just above an idle setting. Idle is something like 15-17 mp. On the ground my airplane won't even move at a setting of 21 mp. So this prop was under loading the engine. This can cause the same problems Scott is seeing " broken Crank " as will over working the engine. Over reving an engine is not good. If she is turning 3100 at 21mp. She will be screaming at WOT of around 32MP. I was seeing with this prop around 3650-3700 at WOT. That is over revving. The engine will not last running it like this. I will call this running it hard but really you are not running it or loading the engine enough. She is over revving.

Again IMHO no dual carbs on these engines. When speaking of carbs when using an updraft, don't use the same updraft carb on the 37 hp as you do on the 45 hp. You will need a larger carb. Scott mentioned this to me. I am using the stock 37hp carb that my engine had on it before the upgraded engine was installed. Scott told me if I changed this to the next size up I would get more power. I am happy with what I have now and will not change it.

Make sure you set your timing correctly. Pre-ignition could cause this problem of broken cranks.

After this flying season I am sending Scott my engine. He is going to pull her apart and check everything. This engine can then be used as a base. I hope to have 150 hrs on it by then. If all is going good and no issue's, I would fly this setup. Change nothing. You get what you get for power. Trying to up the power can cause premature engine failure.

Now this kit Scott is offering to reduce the compression could be the answer to using dual carbs, high lift cams and bigger heads. But it will take many hrs of flight testing by us who use these engines before we know. I myself will keep my engine detuned and see what happens with this engine.

To be continued...
 

Tiger Tim

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What about contacting a company like SCAT to make a suitable crank? Won't be cheap but cheaper than what might be inevitable hospital bills and airplane replacement, no?
 

Rockiedog2

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Thanks for that Tony
I was running the Zenith that Scott sent with the motor which was in fact a larger carb than the one that was on my 37. At that time Scott would build the motor with the dual Mikunis and some were buying that setup...rather than send it back to him for that upgrade I just did it myself which wasn't that big of a deal. According to the dyno it was a 37-38 horse motor with the Zenith so I didn't over power mine just got it up near advertised HP rating. I don't know what cam it's got in it...whatever Scott was installing in them then and the cylinders are Nikisil with whatever heads/valves he was using then. Yes, the rpm limit is 3600 per the manual...the unspoken word at that time was that the bottom end was good for 6K, that looks to have been disproven by the developing service history. Good we found all this out w/o anybody getting hurt and can adjust as required to fix things. I don't know what I'm gonna do yet. I got no prob running 3K cruise but would like to run full throttle on TO/CO just need more info yet. I'll keep the dual carbs and change up how I fly it for the time being. According to the experience with the dyno there's no reason to buy a 45 with the updraft just buy a 37; mine ran great with it. Duals oughta make it really sing. Around all these close in obstacles here at the strip performance changes really showup and there just was no noticeable diff in the 37 and the 45 with the Zenith. I like the MP idea got one back there on the shelf. Thx again Tony
 

RJW

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I realize this might not be totally relevant to this 1/2 VW discussion, but here is a recent Materials Laboratory Factual Report regarding a failure of a stroked German-core VW crankshaft in a 4 cylinder engine.

http://dms.ntsb.gov/public/58500-58999/58722/591778.pdf
Looks to me like the fillet wasn't ground properly. This is a common cause of crank failure on many types of motors. Stroked cranks are particularly susceptible to this kind of failure since stroking them reduces the longitudinal stiffness of the crank. Large, properly ground fillet radii are crucial.

Rob
 

Pops

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A lot of problems with VW engines are the result of the relatively soft magnesium main bearing saddles being pounded out from an unbalanced engine. Broken cranks, rod bearing from lost of oil pressure from between the center main bearing and saddle, etc. There is an aluminum case but its 17 lbs heavier and I understand there are other problems with these.
 

BBerson

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A lightweight prop is only needed if the front prop shaft or bearing is small and can't handle the gyroscopic loads of an aviation flight, especially aerobatics. So using a car crankshaft is a concern.
Now, Scott said the crank broke between the two rods, or in the center of engine, so not a problem of gyroscopic loads.
Most likely resonating torsional vibration.

Scott also said the stroked crank was made from "used welded German cores". The used cranks could have thousands of fatiguing hours from 50 years, who knows? Or the welding, who knows? I don't know. Used welded cranks might be fine at 3500 rpm. No crank exotic alloy can correct the problem if the crank is resonating with extreme shock reversals. Something else must be changed.

I think it is probable that it happened to resonate with this particular stroke. Like Scott said, the stroke might be beyond the core strength limit. Need to start the crank design over for that stroke. Changing anything would change the resonating. It really is just hit or miss guesswork without a full vibration analysis. And that is reported to be around $70,000 or something, so unlikely.
Running the engine derated with low manifold pressure seems absurd to me. And may or may not help.
 

Hot Wings

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I realize this might not be totally relevant to this 1/2 VW discussion,
Of course it's not! Presenting information that was gathered in a scientific way has no bearing on the discussion at hand. :gig:

This is a common problem with stroked VW cranks and even cranks that have been forged and ground as strokers from the start of their lives. If you look at an original stock stroke VW crank you will find that the radius is actually undercut. This allows regrinding without the problem of reduced radius when the work is done at a machine shop that doesn't understand how critical the little details can be.
 

Rockiedog2

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BBerson
the counterweights that Scott welds on...would it be an option to change the weight on those slightly to move the vibration out of the critical range? I know he balances the things pretty close...there is a tab goes on 2 of the prop bolts that he drills a variable size/depth hole in to get the final balance. Any change there might make things worse, make it run rough? This is above my pay grade...
 

Pete Plumb

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I realize this might not be totally relevant to this 1/2 VW discussion, but here is a recent Materials Laboratory Factual Report regarding a failure of a stroked German-core VW crankshaft in a 4 cylinder engine.

http://dms.ntsb.gov/public/58500-58999/58722/591778.pdf
Now THIS is what was needed for this discussion. I would have been interested to know the alloy of the "plain carbon steel" and the Rockwell C hardness of it. This is of great interest to me since my larger displacement, DP-1, O-100 twin experiences the same hammering loads as its "half VW" little brothers. We have figured the "peaks and valleys" of a twin's power impulses to be an order of magnitude (10 times) different. So, flywheel effect is VERY important and since most of us don't run huge starter-gear flywheels, we have to rely on heavier props for that. With that in mind, Dana's statement "A lighter prop will always mean less stress on the crankshaft" is true for the bending loads the crank experiences due to gyroscopic processions during hard maneuvering but not for the "hammering" loads it takes from having a small mass moment of inertia. I'm very concerned about guys using these light weight carbon props on twins. A large, heavy flywheel on either end would help a lot but we can't afford the weight or the room for that.

A friend of mine, Dave Massey (of F1/Cassutt raceplane builder fame), tells a story of dynoing one of his race O-200s without a flywheel at all. I guess it EXPLODED the "U-joint" coupling in a matter of seconds! When he called the manufacturer of the dyno to tell of the event, they asked "are you testing an aircraft engine?" He told them "yes" and without hesitation he said "you don't have a flywheel on it, do you?"

The relatively small fillet sizes look suspect. Just for comparison, fillet size on the DP-1 is .160" r.
Food for thought.
 
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