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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