Flying and climbing faster on the same power - reducing cooling drag

About the Benefits
  • Overview
  • Getting rid of the heat
  • Heat - the top end killer
  • More power per cubic inch
  • Better performance and fuel economy
  • Radiator technology - past and present
  • Shock cooling - Problem solved!
  • The secret to achieving longer TBOs
  • Flying faster on the same power - reducing cooling drag
  • Side benefits - Safer cabin heat
  • Dispelling the Myths

  • The title of this section says it all. There are two ways we can fly and climb faster, either develop more power or reduce the airframe drag. Today's air-cooled engines require a substantial amount of air pressure in order to move enough volume of air over and through a large and complex engine structure. This creates a lot of drag and that drag adds to the overall parasitic drag of the airframe. This type of drag is called cooling drag. In addition it is very difficult to get the airflow over the engine to pass evenly by each of the cylinders. The result is uneven cooling of the cylinders that frequently results in cylinder to cylinder temperatures varying as much as 50F from each other. This substantial variation of cylinder temperatures makes fuel efficiency a much more difficult problem since the hottest cylinder will typically limit how lean the other cooler cylinders can be run. Using an air-cooled engine means that there are few options on how to pickup cooling air and where the heated waste air can be dumped.

    A water cooled engine offers several benefits to reduce cooling drag. The first benefit is a sleek, tight cowl since little space is required to be maintained between the engine and the cowling. We can now completely control the cooling airflow requirements in a separate radiator design that is not constrained by the engine location, envelope and surface area. Such a radiator can be designed to provide substantially more thermal conductivity/efficiency than would ever be possible with direct air cooling of cylinders. A radiator can be located in more physically suitable locations on the airframe. Air pickup can also be located in more aerodynamically optimum locations and internal ducting offers the opportunity to create a far more efficient air stream to the radiator. Slowing the incoming cooling air and allowing it to expand towards the radiator surface allows much more efficient cooling. More heat is dissipated to a smaller air volume over a compact and lightweight radiator surface.

    Cooling air pressure = Drag

    We all want to minimize the drag on our aircraft to achieve maximum performance and their are a myriad of modifications on the market to help reduce drag. However, the one major source of drag that nobody can really do much about is cooling drag. Generally, cooling air pressure drop is expressed in terms of Inches of Water ("H2O) - a much more sensitive scale than the inches of mercury ("Hg) scale we use to express atmospheric pressure. The typical 4 or 6 cylinder air cooled aircraft engine typically requires a minimum of 6-8"H2O of air pressure in order to force sufficient air through the engine to keep it cool. The typical GA aircraft engine installation generates anywhere from 8-12"H2O of pressure - well more than needed in level flight.

    In short - high cooling air pressure translates directly to cooling drag. Wind tunnel testing has shown that as much as 10% of the mechanical horsepower generated by an air cooled aircraft engine is lost to cooling drag - just to keep the engine cool! This is a huge waste of energy. What makes this worse is that in addition to the parasitic cooling drag that is the result of the cooling air pressure drop across the engine, there is also additional airframe drag that is created by the turbulence in front of the engine cooling inlets that are typically located near the top of the cowling. This turbulence creates airflow separation over the airframe - especially over the top and sides of the cowling and airframe which are low pressure areas and that adds even more parasitic drag over the entire length of the airframe.

    Lower cooling drag =
    better performance =
    better fuel economy =
    better range.

    It isn't so much about the volume of air required to cool the engine but the pressure needed to force the airstream through the heat exchanging surface. A radiator can be designed to present far less drag on the passing air flow than does a complex structure such as an air cooled engine with baffles. Reducing the air pressure needed to allow air to flow through the radiator translates directly into an overall reduction in cooling drag.

    Better still - ducted radiator air inlets can be located in more aerodynamically efficient locations. On our 1971 PA28-180, we located the intake cooling air inlet on the lower part of the cowling. The single air inlet provides cooling for both the radiator and oil cooler as well as engine induction air. The flow separated and slanted inlet design we used is derived from the P51. The design minimizes airflow disturbance around the inlet and any spillover from the inlet is directed underneath the airframe into the high pressure region that contributes little to the overall parasitic drag of the airframe especially when considering that it is mixing with the already turbulent exit air from the cowling just a few feet further back.

    The result of this approach is that a far smaller air inlet is required and that the airstream carries off far more waste heat. A smaller volume of air with less pressure is handling a higher heat load. It also means that the cooling air inlet can be located and shaped in such a manner as to take advantage of the pressure dome that will be created at the inlet. This pressure dome can be blended into the surrounding airframe structure to produce a smooth aerodynamic contour. The overall result is substantially reduced cooling and parasitic drag.

    We have found that very little research has gone into radiator design for light GA type aircraft. At Liquid Cooled Air Power, we have spent a substantial amount of time and effort not only designing our CoolJugs system but also the airframe installations as well. We have learned how to design small, very sturdy, lightweight radiators for use in the tight confines of light GA aircraft that provide amazingly high thermal efficiencies with minimal cooling drag. On our 1971 PA28-180 our radiator installation creates about 3"H2O of air pressure drop at 167 mph - about 1/3 that of the air-cooled installation on the same aircraft. The resulting increase in top speed performance has been an astounding 10% (from 154 mph to 169 mph) and climb rate improvement of nearly 60% (from 900 fpm to >1500 fpm). No, our airspeed indicator isn't bent. We verified everything on multiple test flights using a precision calibrated airspeed indicator and cross referenced readings with winds aloft and GPS based ground speed.

    Just to make certain we ran a race head-to-head with a 1984 Piper PA28R-200 Arrow IV! Our 1971, 180HP, fixed gear, hershey bar winged, PA28-180 out climbed and flew faster than a 200HP, retractable gear, semi tapered winged, 1984 PA28R-200 Arrow IV! Let us assure you that based on precision engine dynamometer testing we verified that the water cooled engine in our 1971 PA28-180 was producing the same 180HP as the original air-cooled installation! If you don't believe that this level of performance increase is really possible merely by converting to a water cooled engine installation you can watch the video of the head-to-head air race on our website.

    The much more efficient radiator also means that far less air pressure is required for cooling on the ground - eliminating the need for supplemental boost fans in both of our installations. In fact we have been able to idle the PA28-180 Cherokee indefinitely with as much as a 15 knot tailwind on the ground - something that would overheat the typical air-cooled installation! We can run at full takeoff power, leaned to best power mixture (about 25F rich of peak) at sea level on the ground on a 95F day for well over 10 minutes with only a 10F rise in coolant temperature!

    In our LongEZ installation - a pusher configuration with major ground cooling challenges in its original air cooled configuration we were able to perform the same ground run tests at the Mojave airport in California on days when the outside air temperature was >120F! Simply stated - there were no ground cooling problems at all and we did not need a supplemental electric cooling fan for the radiator.

    As for flying - both the Cherokee and LongEZ were able to climb out at maximum effort (Best Angle, minimum airspeed) on the hottest days all the way up to 10,000 feet without any cooling problems! Imagine how hot the cylinders would get on your current, air cooled engine on a Best Angle climb from sea level to 10,000 feet holding Vx when the outside air temperature is 120F!

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    1413 Prospect Ave., Hermosa Beach, CA 90254
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    Proudly MADE in the U.S.A.