In a message dated 10/3/2007 3:00:41 AM Eastern Daylight Time,
europa-list@matronics.com writes:
I recall that in 1995 I asked the late Richard Cabrinha why he had changed
---From a chin intake to a pair of Lo-Presti style circular intakes on his Free
Spirit Mark II, for cooling a Continental IO 360, and he replied "because
they work!" He told me that if the diameter were too large then there would
be a reverse flow, spilling air out round the circumference of each intake.
He refined the size by putting some oil around each rim and doing a quick
circuit: inspection on landing showed whether the oil film had spread
inwards or migrated outwards, and he would then the repeat the test with
different diameters accordingly.
Hi Mike, Graham, and all,
That is essentially what I've done with my string testing, although I wasn't
aware that size had much to do with it (Typical male, huh?) I've not played
with changing the size of the intake at all. It does seem like 25 square
inches of intake should be more than adequate for cooling this little engine,
though.
.
What I've seen in my string tests is an apparent low pressure area forming
on the outboard side of the left intake as viewed from the cockpit. I've also
noticed an apparent high pressure area directly in front of the inlet when my
turbulators are not installed and this high pressure "bubble" rejects the
string as it approaches the intake from upwind. At one point it turns the
string 180 degrees and it blows towards the fan. Installing turbulators at the
lip
of the intake helped considerably. I went from about 50% of the intake
"accepting" the string, to what I'd estimate as about 80% now accepting the
string in the intake. Still not perfect, but I'm still playing with it, too.
What
I have seen in terms of engine cooling has been dramatic, though.
I need to dig up the Lo-Presti info and take a look at it. That's about the
fifth time somebody has mentioned that name to me.
An A&P friend of mine stopped by my shop yesterday and I showed him what I
was doing. He's also a glider pilot and amateur aerodynamicist. We also
discussed exit cooling flow. The trigear has a rather large triangle shaped
reinforcing piece on the nosegear leg just below and outside the line of the
cowl.
This triangle is flat to the relative wind, probably worse at climb angles,
and I would reckon that it causes some disturbance right where the cooling air
---From the cowl is trying to exit. Whether or not that is interfering with my
cooling flow remains to be determined, but it sure looks like it could use a
fairing to split the flow, hopefully accelerating it and causing a low
pressure area in that region. It stands to reason that cleaning up this area
with
a
fairing might help create a bit of suction and aid cooling flow, too.
Graham said: I wonder if your turbulator is helping keep the flow attached
over the
whole of the outside of the cowl?
I'm thinking you are correct in your assessment there, Graham. I think
"re-exciting" the flow is indeed causing it to stay attached. FWIW, I've done
some
tuft testing on my wings and I seem to have laminar flow at least as far
back as the hinge line of the aileron. The wing tip is much cleaner than I
anticipated, too. There are very few modern sailplanes that get this level of
laminar flow that far back on the wing. Shaw and Dykins are obviously genius'.
I don't have enough knowledge on the subject to really know one way or the
other how internal duct pressure is related to flow, but my tests seem to
indicate that there is either no relationship, or the relationship is inverse.
If
you think about it, restricting the flow would cause higher pressure, just
as it does when you put your finger on the end of a running garden hose. The
pressure increases, but the flow decreases. That seems to be what is happening
here.
Regards,
John Lawton
Whitwell, TN (TN89)
N245E - Flying
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