The Care and Feeding of Your Round Engine.
...specifically the Jacobs R755
I will be compiling information and links on and about the
technical aspects of owning and maintaining a radial engined
If you should have any information or a link that you would like to share with me I'd be
more than happy to posted them on this page.
YIKES ! I've been looking for radial engine information and haven't had any luck in my search.
BUT.... I've been hearing things about rotary engines on the biplane forum that have been looking mighty fine. A VERY
inexpensive automotive engine that weighs hardly anything that puts out a lot of horse power. Hmmmm.... makes me wonder
if I should go the rotary route. Wouldn't be very nostalgic though, would it. Food for thought, but I'm not eating it.
Back to looking for more information about radials !
The infamous.... Hydraulic Lock...
One sure way to break a radial engine
All radial engines have at least a few cylinders pointing down from the crakcase. Problem is... oil wants to accumulate in one or more of these
cylinders between your flights.
Problem: When you go to start the engine the next time, some of those cylinders may hve enough oil lying in them to block the movement of the piston
in the cylinder when it attempts to go to top dead center. You may be unlucky enough when the engine begins to crank over that one of the top cylinders fires off first.
This will cause the piston in an oil-blocked cylinder to crash into the oil with tremendous force. When this happens it's usually the connecting rod that will bend or
How Oil Collects in the Lower Cylinders:
By design, radial engines are prone to hydraulic lock. Oil is drawn, by gravity, into the two lower cylinders, number 4 and 5 on the seven cylinder Jacob, where it pools in the combustion chamber.
If the quantity is small
then it won't affect the engine on start up. It will either be burned in start up or expelled; either smoke or drops of oil. If there is a sufficient amount of oil in the
cylinder the piston will contact it upon reaching top center. The piston will try to compress the oil, which won't compress, and a hydraulic
lock will occur, and it's at this time that an engine part will break. There will either be a bent link rod, a piston pin could bend or a cylinder head can crack.
Reading an article written by Radial Engines LTD, and I quote: "The Jacobs engine, however, with its thick cross section aluminum rod will often
bend the link rod but continue running. It is common to diassemble a running R755 at TBO to find a #4 or #5 rod bent so badly that it is striking
the cylinder skirt with each revolution.
I don't know if this is a good or bad thing.
How to Prevent Hydraulic Lock from Occuring:
Always pull the propeller through at least two complete revolutions prior to starting the engine. Even if the engine has only run for a short period of time.
By pulling the prop through two complete revolutions the crankshaft will have taken each cylinder through all of its cycles and you will ensure that
that all cylinders are clear of excess oil.
Note: If you're one of thoses that are always in a hurry to get started and get going somewhere... hang an IO-540 on the front of your biplane. Starting the radial
is an art and a joy in and of itself.
If you find resistance while pulling it through, remove the spark plugs from cylinders 4 and 5 to drain whatever oil has accumulated in these
cylinders. NEVER pull the engine backwards to clear a hydraulic lock. This procedure pulls the oil into the combustion chamber and will most likely
be drawn back into the cylinder, by vacuum, when the engine starts. You are guaranteed to bend or break a link rod then.