Before we get too carried away
by Ed running with a lot less intake area than the ‘rule-of-thumb’ suggests;
keep in mind that there are other variables, and there are some simple laws of
physics. The ‘rules of thumb’ assume ability for sustained high power
with outside air temps of 90 – 100F.
It also assumes an air temp
increase through the rad of 40-50F. If you can achieve higher temp
increase, then you can adjust down the flow. The law of physics is that
the air has a known specific heat; i.e., the amount of heat it will absorb per
degree of heat up. Knowing how much heat you have to dissipate, it tells
you directly how much flow you need, and knowing how fast you are going tells
you directly how big an opening you need to get that much air. When the
intakes are up front behind the prop you get some benefit from ‘external
diffusion’ in front of the cowl, and perhaps some improved flow from the prop
wash.
Having more outlet area then the
1.4 factor may also reduce require inlet area a bit, but at the expense of
drag. Ideally you would like to use some remaining pressure to
accelerate the exit air to near free stream velocity, the larger the
outlet the slower the exit air, the more drag.
I would suggest
that with 28 sq. in. of intake a.) Ed could not do sustained full power climb
from low altitude on a 95F day, b.) he is getting greater than 50F air temp
increase through the rads, and c.) he is actually generating less than 180 hp
because fuel flow estimate will give you a max, which is not actually achieved
in a rotary because of unburned fuel burning/going out the exhaust
port.
I expect he may
agree with a. and b. but never c. J; right Ed?
Clearly the
‘rules-of-thumb’ give you more inlet area than needed at cruise on an average
day. That’s why cowl flaps are good.
Al
> Al Gietzen wrote:
>
> > Doug;
> >
> > I agree with your ‘rule-of-thumb’ numbers.
My analysis came up with
> > coolant inlet area in sq. in. of 1/3 the HP
(.33) for climb out on a
> > 90F day. It assumes a 120kt climb speed for
my Velocity. I used 45% of
> > that additional for the oil cooler. Assumes
scoop efficiencies of 85%
> > or better.
> >
> > Al
> >
>
> > Second, even if cooling can get out, if it
can't get in, it can't be
> > there to cool the heat exchangers. Rule of
thumb: 0.3 sq. in. of
> > cowling inlet air opening per HP. 200 HP x
.3 = 60 sq. in. Note: This
> > assumes a reasonably shaped inlet cowl which
has been discussed online
> > often. IMHO: Berni's plane inlet shape and
inlet cowl is fine, but I
> > question his inlet opening
_area_.
> >
> snip
>
> > Don't mean to start another stream of
threads on an old subject, but
> > we sweated over this one for 3 months and 3
systems and one might save
> > a lot of time by comparing ones system to
these simple "works great"
> > rules of thumb which are the result of LOTS
of technical and
> > experimental work.
> >
> > Doug Dempsey
> >
> > N6415Q and RV7 in process
> >
> > Colorado, USA
> >
>
> Don't won't do demean or dismiss your
experimental work in any way, but
> Ed is running with half the inlet area, and
unless something has changed
> with his new found power, he'd doing just fine.
Just to be sure that
> we're all talking apples, I can confidently quote
him at 28 in^2 inlet
> for coolant, which I believe is half of what you
recommend above.
> Reality isn't meeting theory at eye level here,
and everyone will be
> much better off if we know why.
>
> --
> This is by far the hardest lesson about freedom.
It goes against
> instinct, and morality, to just sit back and
watch people make
> mistakes. We want to help them, which means
control them and their
> decisions, but in doing so we actually hurt them
(and ourselves)."
>
I
am running with 28 sq inches of total inlet area, much more outlet
area
than Doug mentions and not producing 200HP
continuos. My best estimate
based on fuel flow is I produce around 180HP perhaps a
bit more on a cooler
morning.
I agree apples and oranges get compared
frequently. But, rules of thumb
are just that - generally a place to get
started. I don't think anyone
would say that a rule of thumb means the "optimum" for
a specific
installation. Just good enough. I have a
rule of thumb that says given
enough surface area and airflow you WILL cool.
Not too helpful though and
certainly does not address the cooling drag you may
impose. I mean if your
rule of thumb says you have to have sufficient area to
let the hot air out
and sufficient inlet to let the cold air in - well,
OK, I can buy that - but
not too useful. If you put numbers to it like
Doug has done that becomes
more useful but is not the final
answer.
For some of us, rules of thumb are simply a gore to
understand what's behind
them. How did they come about, what do they
mean? To others they are a
heaven - sent- answer that does not require listening
to or reading this
sort of stuff {:>).
So I am not certain we need to necessarily even
attempt to explain Rules
of Thumb, they are simply a starting point that
experience has shown will
work (most of the time).
Ed A
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