dejo un muy buen artículo que nos hizo llegar Tandy Walker por
intermedio de Alfredo Herbón, como exclusividad para nuestra
página y constructores Argentinos, en donde nos muestra como
fué solucinando el tema de refrigeranción de su motor en
el Sailplane, al disponer este modelo de carenado (Cowl) de motor
Tandy es Ingeniero Aeronáutico (retirado actualmente) dentro de
la SAM integra el "Comité de Revisión de Diseños"
( Design Review Committee ) y además un constructor muy
detallista, meticuloso, prolíjo y estudioso de los pasos que va
siguiendo en sus construcciones. En todos los números de SAM
Speaks, hay una columna fija dedicada a ese Comité y sale con la
firma de los 4
Nunca duda en consultar con sus pares antes de realizar cualquier
movimiento dentro de sus construcciones y nuestro "Cónsul"
Herbón, ha colaborado con él en varias ocasiones en
Tandy Walker y su "Fubar" de Vuelo Libre
Disculpen si no he traducido lo escrito por Tandy, pero es muy sencillo de entender y llevar "el hilo".
Que lo disfruten !!!!
Edgardo A. Schneider.-
Before I leave the issue cowl exit
air venting, I want to discuss the rationale to my approach on the Sailplane's
cowl summarize some of my thoughts on this subject. Engine cooling inside a
model's cowl is probably the most least understood facet of modeling. I think
that maybe back in the early days of control line speed, the die hard
speed modelers figured out how to build a cowl with venting to keep a
racing engine at satisfactory operating temperature for many revolutions around
the speed circle. However, that skill/art has been pretty much lost
most part, very few SAM or free flight models have completely cowled engines. For those that do
have, the engine's cylinder is exposed as shown below and engine cooling simply
does not have to be addressed.
There is one thing in our
favor, our engine run times are comparatively short (less than one
Probably none of us know very much
about engine cooling inside of a cowl except that "more exit area is more
cooling". As you know I have been faced with addressing the issue of engine
cooling inside the Sailplane's cowl as shown below.
Having no real guide lines on how
to go about this, I decided to use the modeler's age old approach of "Gut Feel".
I selected a hole diameter of .8" because that provides a hole area of 1/2 sq.
in. I also selected a location for the holes well aft of the engine, but about a
fourth of the way forward from the cowl's aft bulkhead in order not to weaken
the back portion of the cowl too much. I tried to put as many evenly spaced
holes as possible in a ring around the cowl, but without cutting through any of
its internal structure. Therefore, the hole spacing ended up some what irregular
as is shown below.
This resulted in nine 1/2 sq. in.
holes as shown below. These nine holes provide a total area of 4.5 sq. in., not
counting the additional area around the cylinder head or exhaust.
The key question of course is this
amount of exit airflow area sufficient to keep the engine from over heating
during the relatively short power climb out? Since I did not know, I decided to
ask for the opinions of several life long experienced model builders. Their
responses to my question were most interesting and I think it is instructive to
share their various views with you, which are presented below.
Looks workable to me Tandy. The only thing I might have done
differently by "gut feel" would have been
place them almost all the way back inside the cowl to eliminate possible
stagnant air behind the vents.
should have plenty of venting. The only way to really tell is in the air.
to the McCoy 60 powered speed ships, I'd guess you've got a lot
You are verging on overkill and should not be
worried about proper ventilation. Check out some of the old speed
models that used McCoy 60s. You'll find that
you probably have three or four times as much cooling as they had.
Didn't most get run for short FF motor runs
with no vents with no ill effects? Probably not an issue.
Only time will tell
for sure, but they look adequate to
All I know is that it looks good. Should be good enough to get the model
almost out of site.
I thats plenty of exit holes and you
wont have any cooling problems especially with the exhaust ducted
I think you have more than enough
venting. Don’t worry any more.
I must say I'm not qualified to
answer this quandary. It appears to me that
you have more than adequate venting. As I recall most enclosed cowls in the old
days have few, if any additional vent holes. It seems the old-timers left the
cyl. head out in the breeze but the cylinder itself was pretty well enclosed in
a tight cowl.Yours looks good to me. Plenty
of air exits!
Ed Shilen used to say, the exiting area should meet or exceed
the air coming in. I don't
know how big the front of the cowl is but I would say it is
reduced by the size of the engine.
Many moons ago I was told or read that 4 times the
intake is what to use for exhaust when cooling engines and batteries. Not sure
of validity of this info?
My gut feeling, nothing
scientific, says you have plenty of airflow in and around the engine. Smart
move on the irregularly spaced vent holes. I see no reason to believe there will
be heat build up of any significance in the cowl. 4.5 sq. inches of vent should
more than handle venting of all input air. Airflow across the McCoy should be
An old rule of thumb is
that the area of the cooling air exit should be about
4 times the inlet hole.
I've used that for 60 years and never had a problem.
I didn't comment on your
cowl cooling holes because I have no basis on which to judge them. But it sure
looks like enough to me! Any way to instrument it? There are some little
stick-on gadgets I have seen that have heat ranges on the. I'll try to get
more info on them.
I do know that most Sailplanes I
have seen flying do so without the cowl. I suspect that Goldberg had the cowl to
make the ship look more attractive and for selling purposes was not concerned
either. All I can say is try it. If the engine seizes up from over heating, you
will know that id did not work. Personally, I would probably put the cowl on for
show and take it off for flying. I think the cowl is less attractive
with all of those holes in it.
I didn't want to make any comment to
you, because my experience with cowled engines is really poor and it is a very
complex subject. I suppose the outlet area is enough, but my unique concern with
those holes distribution at some distance forward ( I don't know how much ),
from the firewall, is the possibility of the generation of a
localized air vortex that could alter the air inlet into the venturi and alter
the smooth running of the McCoy when the model gains speed, but please take this
just as an "imaginative suspect", just that. As English men says : "The proof of
the pudding .... " Surely your flying test will have
the last word, and in the worst case of a malfunction with the cowl on, you
could try some elongation backwards in a pair of holes ...
In all the responses I found it unusual that no one mentioned
fuel. In the old days 3 to 1 gas/oil mix was a given. In this day and age, in
the LMR events, most contestants use a mix of methanol and oil or Klotz 100, in
which the engines running methanol based fuel runs much cooler, which would
require less venting. So, the type of events the model flies in could also have
some effect on the amount of venting required.
Some Sailplane flyers you may not
Bob Oslan (Dooling 61)
Larry Davidson (K&B)
Furthermore, all of our comments seem to be starting with the
presumption that the cowl impedes cooling which may or may not be true. When
getting my private license, I saw some mechanics testing an engine with the cowl
off and assuring all that this would be limited to avoid overheating. Their
thought was that a properly designed cowl would channel the air and result in
more cooling. Probably a lot more going on there than here.
I read through a couple of my
college propulsion books on engine cooling the other evening. As it turns
out, aircraft designers configure cowl exits to generate a pressure drop
relative to the incoming air. This results in a venturi effect that draws or
sucks the air through the cowl and out into the free stream as illustrated in
the figure below. Therefore, in hind sight it might have been better on the
Sailplane to have put a radius around the outside edge of the firewall in
conjunction with an opening all the way around the rear of the cowl structure.
The cowl attachment to the firewall would have had to be different for this type
of an implementation however. At any rate, I think this approach is something to
consider on future projects.
The technique of how to cool a fully cowled engine is to force the
cooling air to go through the cooling fins of the engine and not around
them. This is done by packing the flow area inside the cowling with baffles
that fit tightly against the sides of the cooling fins, but not the front or
back. This forces the air through the fins instead of taking the easy way
around them. This is the same approach used by C/L speed fliers.
full sized aircraft this approach has two favorable results. In addition to
cooling the engine, it also lowers the drag by keeping the airflow through the
cowling at a practical minimum. It requires HP to pump this air thrrough the
cowling. This expenditure of power shows up as increased cooling drag. This
effect is much less important in our models at their lower airspeeds, but it is
have a Pacer C powered by a Torp .29 and it has, as you know, a fully cowled
engine. My cowling is split along the vertical center plane of the model. On
the inside surfaces of the cowling I have built up shapes of balsa which bear
against the sides of the engine. Also, there is an exhaust stack which carries
the exhaust out the side of the cowling.
This rig has never shown any signs of overheating. I first flew it at
the 1984 Bong SAM Champs.
As you can see, the opinions
varied quite a bit, which was to be expected. However, taken collective, one
gets a feeling that there is probably enough exit airflow area to keep the
engine from over heating. So I will leave the cowl as it is for the present
time. However, after I get the new McCoy 60
broken in on the bench, I will start the Sailplane's initial flight testing
without the cowl. Then once consistently good runs are achieved during the 35
second climb out, I will install the cowl and evaluate the engine's performance
with the cowl. As Gene Wallock said, "The only way to really tell
is in the air." I realize that
this discussion has little to do with the Sailplane's construction, but maybe it
will be of some help to you if you ever completely cowl in an engine on one of