The Meredith Effect

On Tue, 31 Aug 2004 22:32:29 -0400, "Stan Kap"
wrote:

Hey Corky,

Forget about the engine, just add a large pipe to your mouth and jet that
out the back of the aircraft. There's so much hot air coming out, you
should break the sound barrier. The amount of thrust created by this affect
will be negligible at best. Get out your thermodynamics book and see for
yourself.

Stan Kapushinski

Stan, I know that the thrust from such a setup will be negligable, I
said as much below.

Do I expect this to waft me through the skys at 200 mph while burning
4 gallons per hour? No. You can't make a silk purse out of a sow's
ear. The Christavia will not cruise beyond 130 mph (if that) at any
engine setting below full throttle because it's a four seat fabric
covered STOL type. In addition, the less the power available (heat)
and the slower the speed, the less the effect.

In addition I wrote:

The catch? Meredith's calculated effect got progressively more
powerful the faster the airplane went. In the Mustang, it's maximum
effect occured at around 400 mph and at high (above 20,000 feet)
altitude. These are speeds and altitudes which are out of reach of
almost all homebuilts.

Did you read this?

What I expect I will get from this setup is augmented cooling by use
of the exhaust system. There will be a minor thrust affect, but I'm
not counting on that to boost the cruise or top end. The important
thing to me is adaquate cooling under all conditions. Using exhaust
augmentation is one way, but not the only way, to achieve that.

Are you ok?

Corky Scott

Corky Scott

Let's see .001 hp added instead of 25 hp loss sounds good
It might not give you any additional 'thrust' but if it reduces the drag the
net affect could be the same.
Let us know how it works out
And oh yeah Stan take a chill pill ;-)
John



Stan Kap wrote:

Hey Corky,

Forget about the engine, just add a large pipe to your mouth and jet that
out the back of the aircraft. There's so much hot air coming out, you
should break the sound barrier. The amount of thrust created by this
affect will be negligible at best. Get out your thermodynamics book and
see for yourself.

Stan Kapushinski

Corky Scott
The concept as outlined by Meredith, was that the air to cool the
engine should be directed to the radiator via a duct that expands at
the face of the radiator (which slows the velocity down and increases
pressure), then reduced behind it (which re accelerates the air). The
idea was to slow the air down such that it passed slowly enough
through the radiator to actually do some work (remove heat from the
fins), then be accelerated again to exit parallel to the slipstream.

I believe that both Hoerner and Kuchemann & Weber both published some
research on that very subject. Essentially, one wants to have trumpet bells
facing the front and back of the radiator. The trumpet bells serve convert
the dynamic pressure to static pressure (and back again). The downside for
us homebuilders (ok, "you homebuilders" :-) is that the length of the
trumpet needs to be a substantial length. IIRC, it is something like 2x the
maximum dimension of the radiator. So for a 20" x 20" x 1" radiator, one
would need something like a 40" long trumpet under the cowl. I don't think
I'm alone here in saying that ain't gonna happen.

Then along came two engineers, Kays & London, who researched ducting for
radiators ["Compact Heat Exchangers", Kays & London]. Their research
indicates that a simple wedge duct on the front face feeding the radiator
was only slightly less effective at recovering the dynamic pressure than the
Hoerner trumpet. And for our purposes, a wedge duct is completely doable.

What Kays & London also discovered was that clear space behind the radiator,
and (relatively) gentle bends in the ducting all around, were critical to
good airflow. IIRC, the clear space behind the radiator needed to be a few
multiples of the thickness of the radiator. So a 2" radiator might need 6"
to 10" of unobstructed clear space behind it, and even then it couldn't end
with a perpendicular flat plane like the firewall.

Some builders/pundits have proposed dumping the radiator air directly into
the slipstream by mounting the radiator so that the back face is pointed
straight out the side or bottom of the cowl (with a hole in the cowl,
obviously). The thought process goes: the radiator wants an unobstructed
exit, the exit air is slow, the slipstream is slow, why not just dump it
overboard immediately? The jury is still out on this line of thought.

Here's an intriguing addendum that never got utilized by North
American in the Mustang, or by any other fighters: Meredith wrote that
the jet effect would be greatly enhanced if the exhaust system could
be piped to discharge within the exhaust ductwork that carried the
heated air from the radiator. This is because any additional heat
would expand the air, increasing the velocity of the discharge and
therefore the thrust attained.

Atwood described this is being a quasi jet engine. However, the
problems of routing the exhaust back to the exit duct were considered
insurmountable and doing so was never seriously considered.

That was then, this is now. Routing the exhaust tubes into the
radiators exhaust duct is exactly what I am doing with my V6
installation in the Christavia Mk4.

Others have researched and even implemented exhaust augmentors in homebuilt
aircraft with some success. They aren't a free lunch, however. For
starters, exhaust systems are notorious for developing leaks at the most
inopportune times & places. The longer and more complex the plumbing, the
greater the opportunity for Murphy to make his presence known. Exhaust
systems get HOT. Do not underestimate the potential for the exhaust
augmentor to radiate significantly inconvenient quantites of heat into the
backside of the radiator.

With the radiator just in front of and at the bottom of the firewall,
there is room to bring the exhaust system in behind the radiator and
have it discharge facing out the exit duct.

The radiator wants an unobstructed path for the air leaving. Facing the
firewall is non-optimal. Adding an exhaust pipe makes it worse. Also, be
certain the exhaust pipe cannot radiate heat into the radiator.

The idea is to have the pipes terminate inside the duct so
that the exhaust pulses not only heat the air, they accelerate the air.

Great in theory. Good luck on your execution.

Do I expect this to waft me through the skys at 200 mph while burning
4 gallons per hour? No. You can't make a silk purse out of a sow's
ear. The Christavia will not cruise beyond 130 mph (if that) at any
engine setting below full throttle because it's a four seat fabric
covered STOL type. In addition, the less the power available (heat)
and the slower the speed, the less the effect.

What it will (should) do is make sure that the radiator/cooling system
functions properly by pulling the air through the system at all times.
The neat thing here is even during climb, when traditionally the
airspeed, and therefore air through the cooling system, is low and the
heat produced in the engine high, the air flow through the system
increases automatically because of the increased exhaust flow. More
power, more airflow, less power, less airflow.

Great in theory. I cannot help but wonder, however, if you could get more
"bang" for your effort if we instead concentrated on ensuring the propering
ducting of the radiator(s). Never forget that the oncoming airstream has
only about 0.1 to 0.2 psi available to shove the air through the radiator.
That isn't much, and every little obstruction hurts.

Also, try not to make the classic mistake of using a "NACA duct" to feed
your radiator. In almost every case it doesn't work. Even the writers of
the NACA papers describing the ducts said not to use them to feed radiators
(the ducts were designed to feed jet engine intakes).

At this point I'm just about finished with both headers. I have to
weld up the rightside flange that bolts the exhaust tubes to the
collectors so that I can unbolt the exhaust system and remove the
engine, then the exhaust system is finished and I can tighten
everything up, adjust the carburator float level and see how/if it
runs.

I have to make adjustments to the PSRU during the initial run to make
sure the belt is properly tensioned, then the prop goes on and I begin
a long series of engine runs during which I'll be keeping a record of
engine coolant temps, oil pressure, oil temp and coolant pressure.
The record will be available for the FAA once the airplane is ready
for it's inspection... sometime down the road.

Once you're to the point of taxiing (or even flight testing), one can
fashion a simple multi-point water manometer system using clear drip
irrigation tubing & fittings, cut up kitchen sponges (for diffusers), and
colored water. It might be a useful instrument in determining where the
airflow is and how to make the engine cool better.

Good luck!

Russell Kent

"Russell Kent" wrote

Great in theory. I cannot help but wonder, however, if you could get more
"bang" for your effort if we instead concentrated on ensuring the
propering
ducting of the radiator(s). Never forget that the oncoming airstream has
only about 0.1 to 0.2 psi available to shove the air through the radiator.
That isn't much, and every little obstruction hurts.

Russell Kent

I have to agree, about the exhaust augmenter. Why not make the ducts right,
and put an electric fan in behind the radiator, ala many/all front wheel
drive cars. at your speeds, the drag should be low.
--
Jim in NC

On Wed, 1 Sep 2004 20:56:39 -0400, "Morgans"
wrote:

I have to agree, about the exhaust augmenter. Why not make the ducts right,
and put an electric fan in behind the radiator, ala many/all front wheel
drive cars. at your speeds, the drag should be low.
--
Jim in NC

That's plan B.

Corky Scott

Corky Scott wrote in message . ..
On Wed, 1 Sep 2004 20:56:39 -0400, "Morgans"
wrote:

I have to agree, about the exhaust augmenter. Why not make the ducts right,
and put an electric fan in behind the radiator, ala many/all front wheel
drive cars. at your speeds, the drag should be low.
--
Jim in NC

That's plan B.

Corky Scott

A fan is more weight and complexity and another failure point.
None of those are welcome in a light airplane, especially with all the
propeller blast that can be employed with minimal drag if enough
informed thought goes into the design. I find that allowing the ideas
to churn around in my head for a couple of weeks often points out
problems before any material is cut, and avoids the endless fiddling
that often follows hasty construction.
Some people enjoy fiddling. I'd rather get something to work
properly first time and use the free time to come up with another
far-out idea.
You might consider some device to control rad airflow, as there is
usually too much flow at cruise and it just costs speed. Some sort of
cowl flap, though with the exhaust pipes the shapes might be
complicated. I'd like to see an automatic cowl flap, with a manual
override, using an actuator like the VW Beetle had on its fan intake.
When the sensor bellows got warm (it was in the cooling outflow) it
expanded and opened the fan duct. I wonder if the
thermostatically-controlled radiator coolant flow could be employed to
operate a cylinder to open cowl flaps? More complexity and another
failure point...

Dan

"Dan Thomas" wrote

A fan is more weight and complexity and another failure point.

Weight, I doubt it, if you calculated the trade off in extra weight of the
exhaust system. Complexity? A Fan is complex compared to a unknown system
using exhaust to get good airflow for ground operations? Pleeze. Failure
point. If the damn fan does not work, you overheat while you are on the
ground, and don't take to the air. How often do those fans fail? The
junkyards are full of working ones on wrecked cars, that have not failed.

None of those are welcome in a light airplane, especially with all the
propeller blast that can be employed with minimal drag if enough
informed thought goes into the design.

Exzacary. Using airspeed and prop blast while at high power settings, with
the design optimized for minimal drag at flight speeds, means something else
for ground operations. Corky favors exhaust augmentations, while I think a
fan is a better idea. No covincing me that it is any more complex than
that.

--
Jim in NC

On 2 Sep 2004 13:38:49 -0700, (Dan Thomas)
wrote:

You might consider some device to control rad airflow, as there is
usually too much flow at cruise and it just costs speed.

You just described the anomaly that has dogged aircraft cooling
systems ever since they were first designed. If you size the cooler
and ductwork for effective cooling on the ground and in high
temperature situations, you end up with a system that has more cooling
capacity than necessary for high speed or cruise operations.

Variable outlets seemed to be the best solution but many certified GA
planes design for the worst case scenario (large inlets and no
variable outlets) and pay the cost of relatively high cooling drag at
cruise.

In my case, it really wouldn't matter much if I had the entire
airplane plumbed for underskin radiators that contributed no drag at
all, the thing just isn't going to be a fast cruiser because of the
initial design. It's a strut braced, high wing monoplane with a
really big, high lift wing.

While the designer of the Christavia (Ron Mason) worked very hard to
produce an airfoil that has both high lift and high speed cruise, the
"high speed cruise" should be considered relative. High speed to Ron
meant over 120 mph. The airfoil has a bit of concavity along the
bottom. It isn't flat, it's beyond flat, slightly. You don't see
airfoils like that on high speed airplanes, but it produces a LOT of
lift. One of those many compromises that go into airplane design.

Even though I won't see much in cooling drag reduction (or rather even
though I should see some cooling drag reduction, it likely will not
manifest itself with much increase in cruise speed), it's still worth
it to me to procede with the design for reasons of cooling efficiency.
If this works as planned, I should never have to worry about
overheating on the ground during long holds at any temperature.
Climbing for long periods in hot weather also should not bother
because the higher the power output from the engine, the greater the
volume of exhaust pulses. The greater the volume of exhaust pulses,
the more the air flows through the system... in theory.

In keeping with my building maxim of not attempting anything that has
not already been tried and proven, so that I don't have to rebuild if
it does not work (and besides I'm not an engineer), this is one of
those things that **should not** cause much anxiety. The radiator,
while seemingly small, is large enough to cool the engine (180 to 200
hp) all by itself, given proper duct design. I'm also utilizing a
Modine oil cooler (common with the Ford V6 conversion), which helps
with the cooling chores. The Modine cooler is more properly called an
oil temperature conditioner in that the coolant lines are plumbed
through it, rather than air flow.

My initial engine runs will be conducted using the radiator that the
engine used while in Ford cars, unfortunately. This is because I feel
I have to test the engine extensively on the ground, and fabricated a
mobile engine test stand with which to do this. I don't want to have
to fabricate two cooling cooling systems as just doing the one for the
Christavia will take time enough.

For the engine testing, I just need to make sure that the engine is
viable and will handle full and cruise power for extended periods. I
will need to be able to winch the test stand into the back of my
pickup and drive up into the woods for the extended running periods so
I don't disturb the neighbors. I do have neighbors, even though I
live in rural Vermont surrounded by hills and woods. Well, one or two
anyway. ;-)

Corky Scott

On Wed, 1 Sep 2004 15:52:22 -0500, "Russell Kent"
wrote:

The radiator wants an unobstructed path for the air leaving. Facing the
firewall is non-optimal. Adding an exhaust pipe makes it worse. Also, be
certain the exhaust pipe cannot radiate heat into the radiator.

Poor description of the location on my part, my apologies. The
radiator will be set in a cut out at the bottom of the firewall. This
is a Christavia, which is a big airplane. The firewall extends down
below the fuselage tubing so that there is a section underneath the
fuselage tubing that is unused space. I have cut out a section of the
firewall where the radiator's duct will pass through and still have
the firewall protecting the foot well. The area that forms the exit
duct will be made from the same firewall material so that the
firewall, in effect, continues as the exit duct.

The exhaust system by the time it reaches this duct, consists of two S
shaped 2 inch tubes. They sweep in from the side and then turn and
face the rear about a foot behind the radiator. They terminate about
6 to 8 inches from the end of the radiator exit duct. This gives the
exhaust outflow a chance to accelerate the air behind the radiator.

This is nothing new. Exhaust augmentation has worked on a number of
military airplanes and continues to work on civilian airplanes. It's
just more complicated to arrange than simply sticking the exhaust pipe
out the cowling.

Corky Scott

Hi Corky and all,

Most interesting posting about the Merdith effect. To improve the cooling
drag of our Rotax 914 liquid cololed engine I made some research on the
subject.
After building a small wind tunnel I was able to determine a suitable
"trumpet" diffuser and nozzle combination with encouraging pressure and flow
characteristics.
The airplane is nearing completion with still some fairings and fillets to
adapt to the bottom of the cowling and fuselage.
First flight hopefully within the next few weeks.
Hoerner and Kuchemann's books are "must reads", as are quite a few of the
NACA reports of the 1938-1940 period.

Regards,
Gilles Thesee,
Grenoble, France
"Corky Scott" a écrit dans le message de
...
In the supplements section of the online version of Air&Space Magazine
is an article entitled "The Meredith Effect". There are two parts to
the article, the first is a synopsis by F.W. Meredith, B.A. of his so
named cooling system design which came from his "Note on the cooling
of aircraft engines with special reference to ethylene glycol
radiators enclosed in ducts." The second is a commentary written by
Lee Atwood who was lead engineer for North American when they designed
the P-51 Mustang.

Atwood goes on at considerable length about how they managed to
utilize Meredith's design in the Mustang which was the major factor
that accounted for the Mustang's high speed, not the use of a laminar
flow airfoil.

The concept as outlined by Meredith, was that the air to cool the
engine should be directed to the radiator via a duct that expands at
the face of the radiator (which slows the velocity down and increases
pressure), then reduced behind it (which re accelerates the air). The
idea was to slow the air down such that it passed slowly enough
through the radiator to actually do some work (remove heat from the
fins), then be accelerated again to exit parallel to the slipstream.
Meredith calculated that the accelerated and heated air could be
speeded up such that it actually added to the thrust of the airplane
in addition to that provided by the propeller. In the case of the
Mustang, this jet of heated cooling air reduced cooling drag to almost
nothing. It did not eliminate it entirely, but it reduced it to the
point where cooling drag was merely "3% of the thrust of the
propeller."

The catch? Meredith's calculated effect got progressively more
powerful the faster the airplane went. In the Mustang, it's maximum
effect occured at around 400 mph and at high (above 20,000 feet)
altitude. These are speeds and altitudes which are out of reach of
almost all homebuilts.

The actual amount of thrust garnered by this system was and remains
extremely difficult to quantify because at the time there were no wind
tunnels big enough to hold the full scale airplane and accelerate the
wind in the tunnel to the necessary 400 mph at which the effect is
greatest.

Here's an intriguing addendum that never got utilized by North
American in the Mustang, or by any other fighters: Meredith wrote that
the jet effect would be greatly enhanced if the exhaust system could
be piped to discharge within the exhaust ductwork that carried the
heated air from the radiator. This is because any additional heat
would expand the air, increasing the velocity of the discharge and
therefore the thrust attained.

Atwood described this is being a quasi jet engine. However, the
problems of routing the exhaust back to the exit duct were considered
insurmountable and doing so was never seriously considered.

That was then, this is now. Routing the exhaust tubes into the
radiators exhaust duct is exactly what I am doing with my V6
installation in the Christavia Mk4.

With the radiator just in front of and at the bottom of the firewall,
there is room to bring the exhaust system in behind the radiator and
have it discharge facing out the exit duct. The idea is to have the
pipes terminate inside the duct so that the exhaust pulses not only
heat the air, they accelerate the air.

This does two things, 1. It accelerates the air through the ductwork.
2. It creates a negative pressure behind the radiator which sounds
like the same thing as 1, but really isn't. 3. It can produce
positive flow through the cooling ductwork even sitting on the ground
with tail to the wind. Ok, that's three things.

Do I expect this to waft me through the skys at 200 mph while burning
4 gallons per hour? No. You can't make a silk purse out of a sow's
ear. The Christavia will not cruise beyond 130 mph (if that) at any
engine setting below full throttle because it's a four seat fabric
covered STOL type. In addition, the less the power available (heat)
and the slower the speed, the less the effect.

What it will (should) do is make sure that the radiator/cooling system
functions properly by pulling the air through the system at all times.
The neat thing here is even during climb, when traditionally the
airspeed, and therefore air through the cooling system, is low and the
heat produced in the engine high, the air flow through the system
increases automatically because of the increased exhaust flow. More
power, more airflow, less power, less airflow.

At this point I'm just about finished with both headers. I have to
weld up the rightside flange that bolts the exhaust tubes to the
collectors so that I can unbolt the exhaust system and remove the
engine, then the exhaust system is finished and I can tighten
everything up, adjust the carburator float level and see how/if it
runs.

I have to make adjustments to the PSRU during the initial run to make
sure the belt is properly tensioned, then the prop goes on and I begin
a long series of engine runs during which I'll be keeping a record of
engine coolant temps, oil pressure, oil temp and coolant pressure.
The record will be available for the FAA once the airplane is ready
for it's inspection... sometime down the road.

Corky Scott