Pete Plumb
R.I.P.
Hi Everyone! By far, the most-asked-question to me about my DESIGN of the DP-1 has been my decision to use a cast crank. The following is one of many inquiries about that decision I received on my website (Pegasus O-100 Model: DP-1 Engine Kit) and I though I would copy and paste (with slight editing to make my response clearer) the Q&A here - not only to spark a little discussion with, what I consider to be a VERY intelligent bunch of guys, but to answer that probing question some of you may have had as well. It is a fairly long dissertation and still just summarizes the subject matter. Enjoy:
To Pegasus Pete:
Hi
I’ve been looking at this design from the get go, looks like a great idea….with one flaw that I could see. You went with a cast crankshaft? If history (Volkswagon conversions come to mind), cast cranks are a recipe for disaster in aircraft engines. I see you are using a special heat treated version, but it will still pale in comparison to say a 4000 series forged crank, which is very well proven in aircraft conversions. I’m just wondering where your proof is in saying that a cast crank can be as strong, handle prop loads, and will generally be a good replacement for a proven(forged) design. By the way, I’m not attacking you here, just wondering why you didn’t go with tried and true over completely unknown on an otherwise well thought out design. Thanks.Log in to Reply
To Pegasus Pete:
Hi
I’ve been looking at this design from the get go, looks like a great idea….with one flaw that I could see. You went with a cast crankshaft? If history (Volkswagon conversions come to mind), cast cranks are a recipe for disaster in aircraft engines. I see you are using a special heat treated version, but it will still pale in comparison to say a 4000 series forged crank, which is very well proven in aircraft conversions. I’m just wondering where your proof is in saying that a cast crank can be as strong, handle prop loads, and will generally be a good replacement for a proven(forged) design. By the way, I’m not attacking you here, just wondering why you didn’t go with tried and true over completely unknown on an otherwise well thought out design. Thanks.Log in to Reply-
Pegasus Pete says:
December 2, 2015 at 10:58 PM
Hello XXXXXXXXXXXXX! Sorry for the late response.
Very good questions and believe me, A LOT of engineering thought went into that decision! To answer the main question as to “WHY” I went cast instead of forged, the decision was many-fold but primarily I wanted a final shape that does not lend itself to forging easily. Secondarily, I like a couple of characteristics of ADI better than forged 4140 or 4340 steel – those being 1) strength to weight ratio, 2) its frequency dampening and 3) its natural wear resistance with plain bearings.
Now, as to the VW cast crank comparison, I’m at a bit of a loss to comment because I don’t know what type of cast iron they used. To be able to compare apples to apples, I would need to know the dynamic properties of the iron (or steel), the cross-sectional shape, the loads that were applied and the moment, the fillet size (a HUGE factor and generally too small on VWs) and finally, the type of failure they experienced and where. I know NONE of those things so any kind of practical analysis is out the window.
What I DO know is that Continental’s engineers made it very easy for me to engineer a [theoretically] strong crank for the DP-1 from cast ADI. The fillet sizes are HUGE in comparison to the VW, the crankpin widths are huge, the diameters are all reasonable and the basic design is sound. I used C.F. Taylor’s book “The Internal Combustion Engine in Theory and Practice”, Volume 2 as my “bible” on this crank design. There is a wealth of information for the engineer in that book including empirical data and tests from many contributors. The crank design I used for the DP-1 is compared with others on page 495 and is shown to be the strongest cross-section tested! He also mentions on page 501 that cast cranks lend themselves better to large counterbalances than do forged.
Opinions are worthless to me. Engineering data that can be calculated then TESTED is EVERYTHING to me. During my R&D stages, I had “experts” – who, by the way, didn’t know a single load, a single moment, a single size or cross section – tell me a cast crank would break. When I asked them how they knew that, they just said “because I know”. In an engineer’s world, that is just pure bull****! Give me empirical data! Give me NUMBERS!
Here, allow me to dispel a few misconceptions many people have about cast vs forged cranks. We HAVE to stop thinking about cast iron in the same way we have since the turn of the century or more. An ADI crank is NOT “yer basic gray iron crank”. Modern metallurgy in ductile irons and the heat treating of same has come A LONG WAY since the days of the Romans. Austempering is a modern process for achieving superior structural characteristics of ductile iron. It opened up the door to creating strong, light-weight structures with less expense and more environmentally friendly methods than forging. It is a wonderfully versatile material with MANY applications. One thing to consider is just because a crank is made from 4340 and forged DOES NOT mean it won’t break. There have been many aircraft crankshaft failures over the years. In forging, you are taking an extruded chunk of round stock, heating it up to extreme temperatures and then mashing it, bending it, and pushing it into a shape. Forging steel into a crankshaft is a very violent and extreme procedure and cracking CAN occur. While the grain structure of a cast crank is much different than that of a forged crank, it is not necessarily inferior.
The data I used to determine whether or not I could design a structurally sound ADI crank for the DP-1 originated from research done by PhDs in metallurgy, engineers at Ford, Chrysler and Mercedes, John Deer and many other people WAY smarter than me. Then, what I did is calculate the worse-case bending stress (Fb= Mr/I), the torsional stress (T= Tr/I) and combinations of loads that the crank is likely to see from the prop, piston and rod inertial loads (from C.F. Taylor's book and others) and then made sure that the cross sectional area of the stressed sections could handle the loads based on the information I had on Grade 2 ADI. Just so you know (assuming the data I have on ADI is correct), Grade 2 ADI has the same, or better, qualities than 4140 steel in Fu, Fy Fbr and similar elongation. Its Fatigue strength in Rotational Bending (@10 x 10^6 cycles) is listed at 70,000 psi. To put that into context, the maximum, worst-case-scenario combined loads on this little engine’s crank putting out 105 ft. lbs of torque is about 15,000 psi. Now, according to the the S-N curves (Stress and Number of Cycles) for ADI, combined stress that is kept below 33% of Fu shows no failures – or 49,900 psi in this case. At 15,000 psi, we are at about 10% of Fu.
The true test will be the actual duty cycle that we experience. But my numbers for the cross sectional areas and the loads imposed on those structures show that the Grade 2 ADI was a good choice for this crankshaft. If you have data to the contrary, please, by all means, tell me about it so I can investigate it further.
If you are interested in learning more about ADI you can find a wealth of information atDuctile Iron Data - Section 4. Also, Applied Process, our Country’s premier austempering house (and ours) has great info at http://www.appliedprocess.com.
Thank you for your inquiry! I would never consider it “beating up on me” to ask an intelligent question about such an important part!
Thanks,
Pete
