You’re Welcome,
Mike,
I did not make
my point clearly - While its clear exhaust augmentation can work , the
question is - will the benefit be worth the cost in most cases?. In
effect, adding band aids trying to solve say - a fundamentally cooling
challenged installation is likely to be unproductive in the long run,
than say - investment in fixing and/or “tuning” your
cooling system
I am not against
exhaust augmentation – I just question the return on invest for MOST
cases.
I am all for
experimentation – just think that expectations should be realistic (on the
other hand that requirement will exclude most of us from this “Hobby”
{:>)).
Best
Regards
Ed
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Mike Wills
Sent: Sunday, May 02, 2010 12:43
AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Augmentors (was
Re: 20B RV-8 cooling results)
Thanks for the input - always
valued. I question whether the augmentor would help at all on the ground given
the relatively low power settings. Low exhaust flow trying to drag exit air
through a constricted tube would possibly cool worse on the ground than a
conventional exit.
You said "there's no question
that the exhaust augmentation can work if done properly...". If you mean there
is no question that effective cooling can be had in an installation using
augmentors, I agree. Again, C-310 and T-34 are examples. But if you mean that
there is no question that exhaust augmentation can provide effective cooling,
reduce cooling drag with a consequent increase in airplane performance, and
also reduce, not increase noise and weight, I'd ask you to cite examples. I
know of none.
Sent:
Saturday, May 01, 2010 6:05 AM
Subject:
[FlyRotary] Re: 20B RV-8 cooling
results
Hi
Mike,
The theory indicates
that once you get the inlet/diffuser combination worked out – the inlet size
should be in the range of from 25 – 40% of core frontal area. After
than, it’s actually the exit conditions that dominated the air flow through
the core/cowl. A streamline duct diffuser (K&W) is the most
efficient practical diffuser (at least that I have come across) – the theory
behind it is to keep the air flow energy (velocity) high until just before you
expand the duct area in front of the core to convert the dynamic energy to a
localized pressure increase. From what I have read, it appears that
smoothness and preventing air flow separation from the duct walls near the
inlet is of major importance. Any disturbance there creates an expanding
“shadow” of disturbed air which impinges on the core and reduces the
effectiveness of that area. So prevent flow separation is one of the key
challenges.
I developed my
“Pinched” ducts for short run ducts (the streamline duct requires something
like 16” for cores our nominal size for optimum performance). The ideal
behind my “Pinched” ducts is to speed up the airflow through the pinched area
giving the boundary layer of the flow more energy to stay attached to the duct
walls as it makes the curve just before the core. Any separation that
does happened is much closer to the core and generally up near the edges and
corners – where the core is not particularly effective in the first
place. Been flying with them for over 5 years and they do the job for my
installation – however, just about any, smooth flowing duct will add to
cooling effectiveness – sharp discontinuities generally do not
help.
There is no question
that the exhaust augmentation can work if done properly, the question in my
mind is whether there are easier ways to accomplish the desired results.
It’s my opinion (have not tried one) is that if it were easy to achieve
success and there were major benefits, we would be seeing them on many more
installations. The one’s I have read about that seem to be successful
were not what I would call simply installations. Most have a long tunnel
of some sort in which the exhaust is directed out the end causing airflow
inside the tunnel to be accelerated and “dragged” along and out from
under the cowl. In many cowls that presents a considerable challenge to
fabricate - as such things as motor mounts and hardware have a way of getting
in the way.{:>)
I can see that
Exhaust augmentation might be advantageous for promoting airflow during taxi
and other low speed operations such as ground run up – if other ways can not
be found to do the job. But, if an installation is not cooling at cruise
– then the cooling system needs work
Just an
opinion
Ed.
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Mike Wills
Sent: Friday, April 30, 2010 10:39
PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: 20B RV-8 cooling
results
Should have mentioned in my
previous comment about augmentors, and related to Ed's comments here regarding
positive pressure within the cowl impacting differential across the core.
Cooling is one problem I have not experienced. And I think part of the reason
is that unlike most of the guys currently flying tractor installations, with
my radiator under the engine, exit air has an unobstructed
flow.
I think I actually did a pretty
lousy job of building an inlet diffuser. I've never instrumented it and taken
any measurements, but with some tweaking I bet I could reduce the inlet size
some without a negative impact on cooling. I think it works pretty well as is
in spite of the inlet because the exit is
good.
Sent:
Wednesday, April 28, 2010 5:36 AM
Subject:
[FlyRotary] Re: 20B RV-8 cooling
results
Hi
George,
As you know, taking
heat away from your radiator cores requires sufficient air mass flow – a
number of factors affect this – one of the principle factors is pressure
differential across your core. No pressure differential = no flow.
The primary positive pressure on the front side of the core comes from
converting dynamic energy of the moving air into a local static pressure
increase in front of the core. This is basically limited by your
airspeed and efficiency of your duct/diffuser. The back side of your
core air flow (in most installations) exits inside the cowl. Therefore
any positive pressure above ambient under the cowl is going to reduce the
pressure differential across your core. So once you have the best
duct/diffuser you can achieve on the front side of the core – the only thing
left to increase the pressure differential is to reduce the pressure under the
cowl.
An extreme example is
someone who flies with an opening (such as one of the typical inlet holes
beside the prop) exposed to the air flow. In effect this hole with
little/no resistance to airflow can “pressurize” the cowl and raise the air
pressure behind the radiator cores reducing the pressure differential and
therefore the cooling. Exhaust augmentation is theoretically a way to
reduce the under the cowl pressure by using the exhaust pulse to “pump” air
from under the cowl, thereby improving the Dp across the core and
therefore your cooling.
While exhaust
augmentation can apparently work – there was a KITPLANE issue back several
years ago on the topic showing several installations where this was
used. However, from what I read (and think I understand), it takes some
carefully planning to get an installation to work correct and the effort is
not trivial. Give the challenges you may encounter (such as motor mount
struts, etc), fabrication of the augmentation exit, the need to have the
exhaust pulse exit at or inside the cowl (or construct an extended bottom cowl
tunnel) means you would have the bark of a rotary in front of your feet.
Also, to gain maximum advantage of these techniques, it is desirable to have
the exhaust velocity at the maximum – which implies little/no muffling.
Having had my muffler back out one time (at the cowl exit), I can tell you
that you do not want to position the pilot behind the exhaust outlet (in my
opinion). It is much quieter when you have the exhaust exit behind the
position of the pilot {:>).
Some few people seem
to have been able to achieve some degree of success, but even in aircraft
where you have an engine without the aggressive bark of the rotary, you seldom
see it used. The basic reason (in my opinion), is that it offers few
advantages (cooling wise) that can not be achieved easier and more reliability
by other methods. For an all out racer where noise and discomfort is
secondary, it may have some benefit.
Having said that,
it’s clear that in some installations it appears to work well (see KITPLANE
issue), but if it were the magic solution, I think many more folks would be
employing it – but, again, just my opinion.
Ed
From:
Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of George Lendich
Sent: Tuesday, April 27, 2010 9:41
PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: 20B RV-8 cooling
results
Can't say as I understand
Tracy's set-
up completely, other than it's toward the lower end of Rad sizes. I was
thinking to myself how I could create a -ve pressure in the rad outlet to
create a suction on the Rad. We all know how the exhaust augmentation works
and I was wondering why we can't do the same thing with the rad outlets by
running the rad outlets inro a larger outlet fed by outside air. At idle the
air is fed by the prop air stream and at level fight it is fed by outside air
stream.
The outside air could
be could controlled by a butterfly - simple enough. I know there emphasis
on using shutter /flaps to control the cowl outlet and I believe their good at
restricting air flow, but I don't know if this equates to a good -ve pressure
behind the Rad. This presupposes the Rads are completely enclosed for both
inlet and outlet air.
75% of my cooling problems were solved with the oil
cooler change I did but still needed more margin for hot weather climbs.
Made the decision to not change or enlarge the cooling outlet (that
adds drag) so went ahead and butchered the pretty inlets I made.
Ed Anderson's spreadsheet on BTUs & CFM cooling air required was
instrumental in deciding to go this way. It showed that without
negative pressure on the back side of the rads, there would never be enough
cfm to do the job during climb at full throttle. Negative pressure is
what I had when I flew without the cowl on but oh what a draggy condition
that was.
The old inlets were 4.5" diameter for the radiator and
4.125" diameter for oil cooler.
New inlets
are 5.190" for the rad, and
4.875" dia for the oil.
This may not sound like a lot but it
represents a 36% increase in inlet area.
Results were
excellent. Oil temp went down 19 degrees at the test speed (130) and
water temp dropped 9 degrees. On 80 degree day and 500 ft msl the oil
temp maxed out at 194F at 210 mph which is way faster than I would normally
go at this altitude. Temp was around 175 at 130. Oil Temp
in climb remained below redline (210) but the temperature lapse rate today
made results not very meaningful. OAT was dropping 14 degrees a minute
at 3000 fpm climb rate.
now back to that nasty composite work to
pretty up the inlets again. They look like large stubby pitot tubes
now.
I hadn't thought of a good name for the RV-8 but a friend in
California
recently came up with the winning idea which fit it well. "Euphoriac"
It's a term from a Sci Fi book (Vintage Season) meaning
something which induces euphoria.