Griff,
I've been down the road on Corvairs, and I still have to say that they
are wonderful engines. Six smooth cylinders, and if one quits, you can
still take off and fly the plane home (just ask Steve Makish). Try that
with ANY four cylinder aircraft engine. Now that we've fixed the broken
crank issues with fifth bearings from vendors like Sport Performance
Aviation (just as Revmaster and GPASC solved the VW crank breakage with
a front bearing), not a single Corvair crank has broken. Parts are
relatively inexpensive, and certainly more plentiful than O-200's.
Corvairs are dirt simple to rebuild...even easier than the four cylinder
VW engine! I would stick with the stock 2700cc size....the 3100cc is a
noticeable increase, but not really worth the time and expense to implement.
As for O-200 performance vs Corvair on a KR2S, they are almost exactly
the same. I've "raced" Larry Flesner in his KR2S with N56ML, and I was
a bit faster in speed and climb, but you could credit that to the "new
airfoil", and probably some penalty to Larry's plane for being a little
larger all the way around, plus just the pilot weight, maybe prop
diameter....so call it even. Fuel consumption in the O-200 is
considerably higher though, at least partially due to lower compression
and a "stone-age" carburetor on it.
Mininum 150 mph in a KR2 or KR2S is a given, with any kind of reasonable
engine. N891JF with a 2180 will do 165 mph wide open, and with the
right prop, 170 mph. But I don't like wasting fuel like that, so I
usually do 150 mph true airspeed. At 150 I usually burn about 3.3 gph!
Even in Corvair powered N56ML, I would fly throttleed back at something
like 165 mph most of the time, burning around 4 gph. What's not to like
about that? And that's on auto-fuel....a considerable operational cost
dropper, not to mention environmental issues like more lead in the air.
In short, they all work, but the 4 cylinder VW is more maintenance with
more frequent valve jobs. Corvairs just don't have valve issues. I
think Joe Horton' splane has over 1100 hours on the same set of heads
and valves that he started with. I doubt that happens on VWs, at least
not if you've every seen one suck a valve after the head breaks off
(been there, done that too).
As for KR2 length, I can tell you it makes a real difference, especially
in landing a tail dragger. I've owned and flown both, and it's not
trivial. But with a wide runway, there's nothing wrong with a KR2 in
landing. It's possible to make it longer....Jim Hill did it and so did
Stefan in the Netherlands, and it's pretty easy to lengthen the
horizontal stabilizer and rudder a few inches to help stability. Some
folks say there's no problem with the KR2 stability, but they've
probably never flown a KR2S either.....
Mark Langford
m...@n56ml.com
http://www.n56ml.com
Huntsville, AL
On 10/18/2021 12:40 AM, G R Pickett wrote:
I know this has been hashed out before, but when it comes to a KR2 original
length, what are some opinions ?
I want to complete a plane that's definitely faster than 150 mph. Really,
closer to 175 kts would be even better. I know that some of my old
acquaintances in the '90s were getting those speeds (safely) with O-200's, but
that was before the Corvair engines became popular. And yes, I understand that
the KR2S came about to calm down the twitchiness of the short KR's, but I have
a short one, so I'm going to build retro. Anyway, I have two of them.
Corvair Pro's: Cheaper to build and get replacement parts for. More modern
design. 6 cylinders run smoother. Designed for unleaded fuel. Easier and
cheaper to upgrade HP. Capable of more HP at altitude than a VW. Narrower deck
than a Continental, potentially less drag. Seems about as easy to work on as a
VW.
Continental Pro's: proven aviation engine. Lots of parts, mechanical
knowledge, and mechanics out there. Most likely easier to fix at more
airports, when necessary. Dual ignition system. Probably in more KRs, so most
known problems have more solutions.
Corvair Cons: Limited number of sources for aviation-grade parts/knowledge.
Fewer sources for high grade parts. More expensive than a VW engine. Heavier
than a Continental. Not an aircraft engine, so longer flight testing required.
Continental Con's: It's an aircraft engine, so parts are always higher priced.
Intended for leaded fuel, so when that goes away, it's still more expensive to
operate, even if the fuel burn is lower (sort of a weak argument, since so many
are run on MoGas anyhow).
I'm sure I left something out. Too old for racing, just want to keep up with
all the RV's around me, or at least not fall so far behind.
Griff Pickett
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