Model Airplane News

Engine Clinic

oil content fuels will usually give a small increase in fuel economy.

Boxer Engine Cooling U Since air-cooled opposing four-cylinder (boxer) engines are commonplace in both full-scale and model aviation, overheating of the rear cylinders is obviously not a problem. I am wondering why not, since in addition to blocking airflow to the rear cylinders, the front cylinders also pass waste heat to the rear. Is it necessary to have duct work to direct airflow from the top to bottom of the cylinders instead of from the front to the rear? Have opposing four-cylinder engines ever been designed with an offset between the front and rear cylinders in order to provide better frontal airflow to the rear cylinders (a flat V)? Ron Struss

The world’s first quantity- produced “gas model” engine—the Brown Jr. Model B. Designed by Bill Brown in conjunction with Walter Hurleman, the engine had an initial estimated production run of 1,000 engines that each sold for $21.50. Shown here is engine number B516 from my collection.

Overheating of the rear cylinders has always been a problem, Ron, but usually just ignored with our model engines. Super Tigre uses a restriction in the crankshaft of their larger in-line twins, restricting fuel mixture to the front cylinder so that the rear cylinder runs richer. Full-scale aircraft use sheet metal baffles to direct the incoming air through the rear cylinder fins. Many modelers also incorporate baffles to do this. As far as I know, no “flat four” type engines have been produced with the rear cylinders off-set, although it may have been done. I seem to recall having seen pictures of an engine many years ago with the front cylinders incorporating a shallow inverted V and the rear cylinders an upright shallow V.

Magnum Fix

U I just read my question concern-
ing the Magnum 70 and its “eating glow
plugs” problem in the January 2013 issue
of Model Airplane News and don’t think I
ever got back to you with the solution to
the problem. I had sent the engine back to
Global for “troubleshooting” twice with
the same results each time. No improve-
ment. Then a colleague told me the hobby
shop where I purchased the motor had
received a bad batch of these engines and
all the guys were experiencing the same
problem of inconsistent performance and
frequent glow plug changes for the quick
fix. Seems the solution was to replace the
valve springs, the consensus being that the
motors were put together with Magnum
52 valve springs instead of those for the
larger 70. I sent the engine back with
specific instructions to replace the valve
springs. Global’s solution was to ship me a
brand-new engine along with the old one I
had sent in. This new engine has performed
perfectly and on par with the Magnum
52 and 90 I have been running on other
planes, and I have some spare parts from
the old engine if needed. The bottom line
is, I finally have a reliable Magnum 70. I
don’t know if it really was a valve spring
problem and can’t explain why that would
have caused successive glow plug problems
and inconsistent performance, but I finally
have an engine I can trust from one flying
session to the next. Perhaps you can pass
this along to other modelers who may be
experiencing the same problem.
Bob Davis

Thanks for updating us on your glow plug problem, Bob, but I sure have a hard time swallowing that the valve springs could have had anything to do with it. Weak valve springs or a sticking valve can cause inconsistent running, but shouldn’t cause an engine to eat glow plugs. However, strange things do happen on occasion.

4-Stroke Venting

U I have a question on crankcase vent-
ing on 4-strokes. On most of the Saitos I
have, the crankcase vent is in the center
of the backplate. I acquired a biplane with
an inverted Saito 1.15. The vent is on the
bottom of the backplate when the engine
is upright but when inverted, the vent is
at the very top of the backplate. In this
instance, the builder had connected the
crankcase vent to the pressure line, run-
ning from the muffler to the fuel tank. I
am guessing that the muffler was also pres-
surizing the crankcase, as a lot of oil was
coming out of the exhaust, but it ran OK.
I disconnected the crankcase line from the
pressure line and am now just venting the
case out of the bottom of the cowl. Do you
see any particular problem running the
engine in either configuration or is there an
advantage to either of the configurations?
I assume that if the vent is vented out of
the plane in the normal manner, the pres-
sures in the crankcase, when the engine
is running, will blow the excess oil out of
the vent regardless of if it is on top when
inverted or on the bottom when upright.
Don Morse, Carson City, NV

Don, I have never tried connecting the crankcase vent to the muffler pressure line, so I cannot say for sure what the result would be. However, logic tells me that the crankcase pressure and muffler pressure may cancel each other out, mostly depending on which is stronger. One thing is for sure, you would not want the crankcase slop going back into the fuel tank. Just vent the crankcase out of the cowl as you have been doing.