The Meredith Effect

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