• Welcome aboard HomebuiltAirplanes.com, your destination for connecting with a thriving community of more than 10,000 active members, all passionate about home-built aviation. Dive into our comprehensive repository of knowledge, exchange technical insights, arrange get-togethers, and trade aircrafts/parts with like-minded enthusiasts. Unearth a wide-ranging collection of general and kit plane aviation subjects, enriched with engaging imagery, in-depth technical manuals, and rare archives.

    For a nominal fee of $99.99/year or $12.99/month, you can immerse yourself in this dynamic community and unparalleled treasure-trove of aviation knowledge.

    Embark on your journey now!

    Click Here to Become a Premium Member and Experience Homebuilt Airplanes to the Fullest!

Thrust bearings, 5th bearings, and crank failure

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

mcrae0104

Well-Known Member
Supporting Member
Joined
Oct 27, 2009
Messages
5,006
Location
KEIK (CO)
Continuing from another thread...

Hello Andy. What I think you need to consider here is that the thrust bearing in each of these engine types is usually located at the business end of the engine. An exception to this is the LS Chevy which has its thrust bearing on the middle journal of the crankshaft which demonstrates that in an automotive engine, the thrust loads may be somewhat different. The usual train of thought is that the thrust bearing surface is best located near the source of the axially applied thrust. In a V8, since the diameter of the rear of the crankshaft is both larger and enclosed rather than smaller and protruding like the front of a V8 automotive crankshaft...this seems the better choice for locating the thrust bearing. Further, it then becomes logical to reverse the automotive engine end for end and make the rear of the engine face the front of the airplane. In effect, the rear is now the front of the engine. A further benefit of this is that the engine should now be rotating in a direction that a conventional airplane propeller will work correctly. If you mounted most auto engines with the crankshaft snout pointing forward, the rotation of the prop would be in the opposite direction. I'm not sure which direction a Corvair is designed to rotate, but I would speculate that which ever end of the engine is being used, it was chosen because it was the end that rotated in the same direction that most airplane propellors would be usuable.


In the case of the Corvair, the business end is the front of the engine (as installed in the car, just like a VW) and the thrust bearing is on the opposite (distributor) end. In a flight application, the orientation remains the same (unlike a VW, which is sometimes run with the prop on the pulley end and sometimes off the flywheel end like Pops did). The Corvair rotates opposite from "normal" aircraft engines but reverse cams are available.

Thats why I'm wondering why the WW fifth bearing isn't more popular, since unlike the others it serves as the thrust bearing as well.

If I end up using a Corvair, I want to remove as many crank failure risk factors as possible, and I think that would mean:
  • Probably not going with the 3400cc stroker that SPA plans to market. I anticipate that a 110-120hp, 3100cc engine will be plenty for my LSA design (although even 3100cc may be pushing the limits a bit). Another option might be to use the 3400cc but derate it and run either at lower RPM or lower compression ratio.
  • Locating the thrust bearing at the prop end to eliminate axial loads close to their source. Pops--how did you accomplish this in your half VW?
  • Using a fifth bearing to reduce bending across the throws. I'd like to accomplish this with a combined fifth bearing/thrust bearing, but maybe the case can be machined to accommodate the standard thrust bearing at the #4 (front) instead of #1 (rear).
  • Using a front-mounted starter (and maybe a front alternator, although I'm not sure this is as critical as the starter location). The aesthetics of a super sleek cowl will have to be second to safety.
  • Indexing the prop 90deg to the #6 throw (the one nearest the prop that deals with the most bending stress). If that means I can't hand prop, I can live with that. Proper maintenance should minimize the need for hand propping, and I'd much rather be stuck somewhere with a starting problem than have a broken crank.
  • Using the best crankshaft available. Right now I think the best candidate is the Weseman 4340 billet crank, with:
    • Large journal radii, so that this area has less stress concentration in bending and is less likely to see fatigue cracking
    • A stronger material (4340 steel)
    • Stress relief
    • Nitriding
The only downside, if indeed it is one, is that this crank is not forged. Of course, forged is superior to cast due to the grain alignment, but I do not know how a CNC-machined billet crank compares to a forged one. Anybody know? Fortunately, by the time I am ready to build an engine, there should be a healthy number of hours on this type of crank since it's installed in the Panther prototype now.

Andy
 
Back
Top