Prop Hubs for VW AeroConversions
A recent kerfluffle on the Sonex newsgroup compels me to take up this matter again.
This post:
VW Cranks (Was Re: Sonex Incident - 9/29/2007)
Frank,
I have sat back too and read this thread on the
VW problems ....part of me just said to keep
quiet .....but the other parts says to share what
you know .....I am no expert but 20+ years of aero
vw use has taught me some stuff ......so here goes;
You hit the nail on the head here. However the VW
with a stock crank and shrink-fit hub works very
well on engines up to 1915cc .....I know because
I have flown behind them for 20+ years, the added
torque of the 2180 increases the risk of failure,
and many of the guys I call experts agree.
I have an Aerovee 2180, I like the looks of it and
I like the package as a whole. When I bought mine
I knew I was going to discard the AeroVee crankshaft
and shrink-fit prop hub!
I immediately sent the block to Steve at Great Plains
and had the block machined for the Force One Prop Hub,
I also purchased his aircraft purpose-built 82mm forged SCAT
crankshaft. It was designed to have the extra
metal in the area of failure, and they have never
had one fail to date. They use it on 2300cc VW's.
There is nothing wrong with the VW, but building a
2180cc engine begins to exceed the strength of the base
engine it's built from. My last 2180cc VW engine had over
1400 hours on it when I sold it ....and it was built by me
from parts I purchased from CB Peformance with the
exception of the crankshaft and prop hub, these came
from GreatPlains!
All my VW engined planes to date were single seat aircraft,
but my SONEX will carry passengers (wife and children) so I
spent the extra money to insure my engine stays together.
Nothing against Sonex / Aerovee, but the VW engine was NOT
designed by them, so they are NOT the last word on anything
VW. They put together a nice package and it's a really
nice engine, but they know of the possible issues with
the VW crankshaft design with a shrink-fit prop hub, they
are aware ....that's why they stress correct prop hub installation,
the Aerovee is not their first use of the aero-vw engine!
Even when installed correctly as per Aerovee, the design is
marginal ...NOT bad ...but marginal! There is room for error
that can cause failure. One of the established installation
procedures and setup is the have the front 2 cylinders at
TDC ....(pistons out towards the cylinder heads) and install
the prop in a horizontal postion, plus insure the prop
tacks true. This will increase the survival rate of the
prop hub. Also it has been found that keeping the prop
length 55" and below is safer. These are just common VW
engine practices by others ......the big practice is to
use wooden props only! This one is obvious but I felt it
needed said as not everyone here will buy a prop from Sonex.
While on the subject there is a SERVICE Bulletin on
all VW Supercases, the #4 bearing oil feed has been found
to be improperly sized / installed on 1 out of every 4 new
cases. The #4 bearing is the small bearing behind the
prop hub .....this has not been attributed to the prop
hub / crankshaft failures but needs resolved for long
engine service. Contact Steve Bennett at GreatPlains if you
want a better explanation.
It's a simple fix, can be done with the engine in the plane. You
drill out an oil plug, internally remove the restrictor
plug and install a screw in plug. Steve at Great Plains
reminded me of this .....and "my Aerovee block" was one of the
defective ones. He fixed it while he had it in his shop.
While I love my Sonex and the Aerovee, it's NOT a perfect
powerplant as received from Sonex. The Aerovee 2180 is just
assembled from off-the-shelf components internally. The
Aerovee specific stuff is outside of the engine, all the
pretty red anodized parts, the Aerovee Carb and there sort
of neat ignition system .......all nice stuff!
I have flown behind VW engines for a long time, and I have
lost a prop hub on an early Monnett engine (before Aerovee)!
I don't plan to loose another ....of anybodys manufacture!!!
I don't worry about warranty, there really is none once
the engine is flying ....I am currently NOT using the AeroCarb
either, I use an EFI system from SDS (Simple Digital Systems).
I don't have cooling problems and have never had any, the
EFI precisely controls the mixture .....it starts on the
second rotation of the prop, even at 20 degrees .....I have
run it on Ethanol ...just a programming change to switch
between fuels. Yes, EFI is more complicated, yes it has an
electric fuel pump ..(I use redundant pairs)...but it is as
reliable as anything out there, I have been using this system on
some other aero engines as well as the VW for 4 years .....
the SDS system is aircraft quality ...and is not cheap, but
I prefer EFI!
Not going to argue any of this, just sharing what I know
on the subject! You have a question I'll be glad to answer
it ....just too old to debate stuff that I have learned the
hard way!
My 2 cents!
Les
Sonex #1094
Generated this response:
Hello again Sonextalk Group-
As promised, we wanted to address some of the comments made the past couple
of days on the AeroVee Design. We feel these comments need to be responded
to, especially those that are misleading and provide limited background
information. We provided our responses after the quoted text excerpts from
these postings.
>You hit the nail on the head here. However the VW
>with a stock crank and shrink-fit hub works very
>well on engines up to 1915cc .....I know because
>I have flown behind them for 20+ years, the added
>torque of the 2180 increases the risk of failure,
>and many of the guys I call experts agree.
=As addressed in the past, there is very little "stock" about an AeroVee
Engine Core. Nearly 100% of the component parts are aftermarket component
parts. The Crankshaft is forged -and Nitrtrided- 4340 Steel. A common
misunderstanding (even from those that claim to be vw experts) are the real
loads that need to be addressed in designing conversion components for the
VW Core. The Modern AeroVee Conversion exists today because of the many
shortcomings of the existing conversions that were offered by other
manufacturers in the late 1990's. The AeroVee has been challenged in
rigorous flight testing under the highest stress flight
situations...including Aerobatics. Most pilots do not engage in this kind of
flying. We insist on testing our installations in these most rigorous
conditions.
>All my VW engined planes to date were single seat aircraft,
>but my SONEX will carry passengers (wife and children) so I
>spent the extra money to insure my engine stays together.
=There have been many thousands of rides given in AeroVee Powered Aircraft
of all types. There is no need to speculate that the AeroVee Prop Hub design
somehow makes the aircraft unsafe. This is an irresponsible comment and an
irrational concern not based on fact, but purely speculation and opinion. An
AeroVee Shrink Fit Prop Hub installed and maintained properly yields the
highest strength and simplest installation on any VW Engine Conversion.
>While on the subject there is a SERVICE Bulletin on
>all VW Supercases, the #4 bearing oil feed has been found
>to be improperly sized / installed on 1 out of every 4 new
>cases.
=This Service Bulletin on the #4 Bearing is many years old. We have not
encountered any cases in the many hundreds that have come through our
factory that have this issue. If this was a true service issue, there would
be many vw vehicles stuck on the side of the road that you would be passing
on a routine basis.
>While I love my Sonex and the Aerovee, it's NOT a perfect
>powerplant as received from Sonex.
=There is no manufactured product or Aircraft Conversion that does represent
the perfect powerplant or the perfect product. However, the great success of
the AeroVee can be attributed to its combination of simplicity,
reliability,and light weight in an extremely robust and well proven package.
It's the pursuit of perfection that most distinctly separates
AeroConversions and Sonex products from the competition. We promise that as
opportunities present themselves, we will continue to improve and expand
our product lines as any reputable and successful aircraft manufacturer
would.
> about warranty, there really is none once
>the engine is flying
=I'm not sure where the information about a warranty expiring once an
AeroVee Flies was obtained. This is completely false. If a part is found to
be defective in an AeroVee whether it is flying or not, there most
certainly is warranty coverage. Obviously, if you modify the component parts
or if the builder is found to be the one that caused the problem, then the
replacement cost on parts are your responsibility.
Spending $1500+ on a Fuel Injection System for your AeroVee makes absolutely
no sense to us whatsoever....especially if it's put on a Sonex, Waiex, or
Xenos set up for Gravity Feed. We have many thousands of hours flying with a
smooth running, low fuel burn, no carb heat, no mixture problems and no
overheating using the standard AeroCarb. For our factory aircraft located in
Oshkosh, WI we make two carb adjustments per year. One approx. 1/4 turn
needle adjustment in the spring as it warms up and one 1/4 turn needle
adjustment in the fall as the air becomes cooler and denser... without the
complication of vapor-lock prone fuel pumps, which do have a tendency to
fail even if they are run in a redundant configuration. Gravity hasn't
failed us yet...it if does, we certainly have bigger things to worry about.
The reason the AeroVee is sold as a kit is that we back the system as a
whole. If a builder makes modifications to the engine (i.e. Prop hub,
induction system, etc), unfortunately the AeroVee Name is still on the
engine package. If and when an individual has a failure and these
modifications are not revealed on an accident report, then its the AeroVee
Name that loses credibility. This is the same reason that AeroVee
Components are not sold individually to be used on hodge-podge engine
conversions usually done by individuals who have no flight or design
experience with auto conversion engines.
We look forward to continuing to hear flight success stories and supporting
all of our customers in completing their dreams of building and flying their
own aircraft.
As always, please feel free to contact Betty, Heather, Mark, Kerry, John,
Jason, Kristy or myself at the numbers or e-mail below with questions or
comments.
Regards,
-jeremy
--
Jeremy Monnett
CEO
Sonex Aircraft, LLC
www.sonexaircraft.com
Notice carefully the selective quoting of the original post, and the essential failure to address the key issue, the reliability of the shrink fit hub, and the exactitude it requires.
R.S. Hoover , known to a generation of Volkswagen enthusiasts as Veeduber@, has put together this little history of putting props on VW engines. The Aerovee hub is, in Mr. Hoover's lexicon, Thick (or heavy) Walled Shrink Fit hub. I believe it is long, per his example figure 9. Mr Hoover suggests some possible failure modes. I'd like to suggest another.
Using this calculator, and a lot of assumptions and estimates, I found that the allowable fit tolerance between the nose of the crank and the ID of the hub was uncomfortably small. The new tolerance on the nose of the crank, from VW back in the day was about 9 ten thousandths of an inch, I.E. the #4 bearing journal could be as small as 1.5739 inch or as big as 1.5748. For the shrink fit to work, the frictional forces between the hub and the shaft have to be high enough to withstand the torque to be transmitted, AND the hoop stresses induced have to be low enough to keep the hub from failing due to fatique. Again, using fishy dimensions I SWAG'd from pictures of the Aerovee hub, known dimensions of VW cranks, allowable stresses for common grades of steel, I found that the allowable tolerance on the fit was LESS than the tolerance on the crank itself. And aftermarket VW parts are not known for holding close tolerances.
I encourage others, particularly someone with an AeroVee hub handy to measure to an accuracy of .0001", to work through the calculations themselves, and report the results to the rest of us. An undergraduate education in mechanical engineering MAY help in deciphering the calculations, but those not burdened by such can probably puzzle their way through it ;-)
To wrap it up, I believe the AeroVee long and thick shrink fit hub CAN work IF the crank and the hub are measured and matched to a high degree of accuracy. There are at least 100 AeroVees that ARE flying, some with hundreds of hours. I've chosen to drive my prop from the flywheel end of my engine. It's not flown yet, and there are very few converted VW engines of this configuration that have.
Labels: aviation
posted by flybynightkarmarepair @ 8:24 AM
2 Comments
2 Comments:
Hello,
Since the original Post was mine I wanted to add a few points that need to be clarified and may NOT have been brought out in what you
posted. I have a mechanical engineering degree ...so math is something I can do easily.
What this all comes down to is how much margin for failure are you comfortable with. If you prefer, what odds of failure can you live with? 1 in 50? 1 in 500? 1 in 10,000? I want the max safety margin I can afford, but that's
just me.
First, it's NOT the prop hub that fails. It's the crankshaft, so let's call it what it is, a "crankshaft failure".
The shrink fit hub is NOT new or an Aerovee design, it has been used on airplanes for a many many years .....the shrink fit hub is just NOT a new item. There have been many attempts to improve on the simple concept, but it is still basically the same as it was 20 years ago.
Again, the reason for most failures has NOT been the actual failure of the prop hub, but the failure is the stock VW crankshaft design and there are several key issues that all contribute to a failure and ultimate separation of the hub / prop from the engine.
#1 The prop hub only slides onto the crankshaft just less than 1 inch. That creates a great deal of mass when the prop is installed hanging out in front of the crankshaft unsupported. Think about it for a minute ....!
#2 The prop hub end of the stock VW crankshaft has a rather large threaded hole bored down the middle of it to retain a bolt for a pulley on the VW engine in the automotive configuration. This hole
goes fairly deep into the crankshaft ....on the outside of the crankshaft (same end) is a keyway groove cut into the OD of the crankshaft .....there is an area of this end of the crankshaft where the threaded hole down the center and the keyway groove almost meet, There is approx 1/8th
of an inch of crankshaft metal between the keyway groove and the threaded hole. The prop hub is slipped right over crankshaft but
does NOT cover this weakened area,
it basically stops right even with this weakened area of the crankshaft. The area I am talking about is just in front of the crankshaft oil slinger disc ....when you install the prop hub you press it right up against the oil slinger disc .....a nice
clean place to break is provided, and that is where they break,
approx. 1/8th of an inch in front of the oil slinger. The crankshaft is almost designed to break here, but this is by accident, NOT intentional, it's just an automotive part not designed to carry this kind of load. The good
thing is that the metal the crankshaft is made from is very rugged ...and you have to create
a situation for it to crack ...
and we do just that ...read on.
If all things are perfect, hub pressed on correctly, propeller perfectly balanced and tracking true ....the #4 bearing getting good oil for cooling and lubrication, most of the time there
is no issue, everything works in harmony, but .....if there is a flaw in the crankshaft casting, the hub is not installed perfectly,
the propeller is NOT pefectly balanced and tracking true, the #4 bearing is not getting adequate lubrication.... a slight
vibration starts to fatigue the
crankshaft ...a crack forms in the area described earlier between the keyway slot and the threaded hole bored down the center of the crankshaft .....once the crack grows long enough to bridge the area between the keyway slot and the threaded crankshaft hole ...the crank fails
and the prop hub comes off in flight.
It is believed that bigger engines have accelerated this failure somewhat due to the increased torque and strength of the firing pulses of the engine ...and failures have or will increased in numbers. 20 years ago an 1835 cc VW was big, today the 2180cc and 2300cc VW's are common place. We have not mentioned the junk cast crankshafts that once flooded the market ....we are just talking about the current design of the market's shrink fit prop hub and a stock VW crankshaft design.
Some VW prop hub crank failures don't happen until the engine has several hundred hours on them. So
there are a large number of VW based engines that have not reached this point yet ......so don't think it won't or cannot happen ....time will tell. I prefer
to be pro-active, if there is a problem ...it will happen to me, I am one of those guys ....so I acted, but when it comes to safety I have no problem spending some extra money.
I mentioned a safety margin in my original post, it is here that I feel there is room for improvement.
If everything is not exactly perfect with the crankshaft from
the manufacturer, if the prop hub
is not fully heated and installed within the first try at installing it, if the prop is not perfectly balanced and running true ....the risk of failure of the crankshaft is increased many times over ......and we all know that these are experimental aircraft and engines we are talking about .....most folks do not have the nice shop equipment, shop ovens, IR temp guns, hydraulic presses ....most are do-it-yourself folks with often just common hand tools ...so the design needs to be goof proof,
with a large margin for safety, in case something less than ideal is
encountered during the building process...and heaven forbid goes undetected. Maybe the engine kit seller should build this assembly for the kit builder since it's so critical ....that has got to be an improvement, right? In my simple mind, if a step or process has serious consequences, steps should be taken to insure success and safety, and the old shrink fit prop hub on a VW crankshaft of stock dimensions has possible failure written all over it, it did 20 years ago ...and even thought it's better today than it was it's still an issue. Just a simple comment but my local FAA rep hates VW engines with shrink fit hubs ....the guy has seen plenty of props come off ....enough said!
The shrink fit hub on a standard dimension VW crankshaft is not fool proof. Time has proven this to be the case. There is an after market crankshaft and prop hub design that solves all of these problems and gives you a very large safety margin with regards to failure of the crankshaft.
#3 On the VW type 1 crankshaft
there are 3 large bearings and one small one. Any guess where the smaller bearing is? It's the bearing right behind the prop hub.
An area where a bearing with the most force is placed on it is where the smallest bearing is. Does that make sense? The number 4 bearing carries a great deal of the radial prop load. There has been an issue for years with all new VW engine cases where the oiling to this bearing has been severly limited, case in point, my Aerovee engine case, the #4 bearing oil port was completely blocked. I was told by Aerovee that this was not the norm, and my case somehow slipped through their inspection process, want to guess how many others have slipped through ....I know of 8 confirmed
Aerovee cases where the oil to the #4 bearing was either blocked or partially blocked. The only oil my #4 bearing would have gotten was what was splashed onto it by the camshaft gears. In an automobile this is sufficient most of the time
as the #4 bearing only carries a alternator pulley load ....in an aero engine it carries the prop radial loads. This is a big issue,
don't take this sitting down, if you are building a VW engine, have this checked.
So what does this mean? Well, the #4 bearing will wear very quicly, maybe worn out in say ....75 hours .....this is a guess, but based on a few engines I have been exposed to in the past 4 months it is close, once the bearing is worn past it's limits the crankshaft is not unsupported and begins to flop around .....(okay, a stretch but done so to illustrate a point)and this adds to the already imperfect situation I described earlier ....now you have an added problem that possibly accelerates
the crankshaft failure. This is
just common sense .....! The whole VW crankshaft design and use of a shrink fit hub on a big stroker VW engine has issues. I guess the question is ...if only 1 in 300 fail is it worth doing something about it. What about 1 in 500? I
prefer to prevent it in the first place ....this flying hobby is dangerous enough without the prop
falling off in flight!
This is NOT speculation either, this is actually happening. This is NOT an Aerovee problem ....it's a VW aero engine problem.
Aerovee has probably the best of the shrink fit assemblies I have seen, but it's the over-all design of the VW crankshaft and use of a shrink fit prop hub that has it's limitations.
I spoke of a purpose built prop hub and crankshaft earlier, guess
what, it has a very large #4bearing that goes with the crankshaft. The bearing surface is almost 1.5 inches wide and has an oil seal that is similar to the oil seal at the flwheel end of the engine. The whole crankshaft and hub package gives you a very large safety margin for crankshaft failures, plus you can't make a mistake putting this system together that will get you in trouble, if you can tighten a bolt, install a cotter pit add some loctite ...then you can put this system in your engine. A small amount of machine work is required of the engine case for the larger #4 bearing, the seller will even do that for you ...for a small fee.
Let's recap; the failures of the crankshaft with shrink fit prop hubs comes from a single or combination of these following.
a) Crack forming within the crankshaft, between the keyway slot and the threaded hole down the center of the crankshaft.
b) Shrink fit hub not installed correctly.
c) Propeller balance issue or tracking problem.
d) the #4 bearing failing due to some issue with lubrication.
e) a prop strike in the past,
this can initiate the cracking process.
I noticed you mentioned driving your prop from the flwheel end of the VW .....that is the best way
known, but the Force One crankshaft and hub are the next best thing! It's so simple I cannot believe it was not invented years ago .......it gives you confidence just by looking at it.
There is another advantage to the Force One Prop Hub system, you can easily remove the prop hub ...it uses a taper fit to grip the crankshaft, this is a very strong
old school way of driving devices
and it works well, and this conncetion will yield if the connection is over loaded ....as in
a prop strike ...and NOT break the crankshaft. It might slip ...but should get you home .....
I want to make it clear that I am in no way bad mouthing Aerovee over
this, even though they are taking it that way .....they have been very easy to work with and I like their product .....I had a concern with their crankshaft and hub, they told me NOT to worry about it ...it was reliable, that was NOT the answer I was looking for, so I made a change to improve the only issue I had with my Aerovee.....because I prefer bigger saftey margins when and where possible, and this was an easy way to address a known problem.
It was a simple upgrade to my Aerovee, and it made flying this engine safer, there is no down-side to this modification, it's a win-win improvement. Okay, $900 is sort of a down side, but it only stings for s few minutes!!!!!
All of the other Aerovee parts bolts right into place. It would really say something if Aerovee would at least offer this upgrade as an option ......give the builder
the added safety margin, to me that
would really say something about the seller ......that would impresse me!
Since I was quoted here .....I wanted to make sure my side was fully understood. The Aero VW is a nice engine, and reliable, but as we move forward we need to embrace improvements to old ideas. If you understand the engines limitations you can find ways to work around them. There is no reason I should not get 1500+ hours out of this engine.
For the record I would have driven
my engine from the flywheel end, I always did it that way in my Gyroplane days with VW's, but the Sonex has a small engine compartment ....I did not want the extra engineering challenge, I wanted this aircraft project to remain simple for a change!
Thank you,
Les
Gentlemen, Please investigate the Revmaster crankshaft configuration. It uses no keyway and is a taper fit. The best out there bar none, in my opinion. It has a large bearing surface and also uses the 4340 nitided shaft. Call them for good information.
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