I 1M

USING FANS TO ODOL AIRCRAFT ENGINES

by Molt Taylor Aerocar Associates P. O. Box1171 Longview, WA 98632 There probably is nothing more important in the installation of an aircraft engine than to be sure the engine is adequately cooled. This ranks right along with being sure the engine has lubricating oil. Since cooling is directly related to how much power an engine can develop, it is obvious that the engine must be cooled adequately if it is to deliver its "rated" power without being

damaged. There are several ways an aircraft engine can be cooled. The most common methods are air cooling and liquid cooling. Air cooling is used in most aircraft since it does not involve the weight and complexity of liquid systems. In the past, it has been more or less common for aircraft to have the engine located in the nose of the vehicle where propeller slipstream and ram air can be easily blown directly on the engine. Thus, air cooling has become routine and simple. However, some modern aircraft designs have moved the engine out of the nose in an effort to improve performance through the use of more sophisticated shapes and configurations. These designs require some other way to provide cooling airflow into the engine spaces and over the engine itself. One of the more popular ways of doing this is to use a cooling fan. This permits the designer to move the engine away from the old nose position and thus allow more efficient nose shapes, propeller placement, etc. One problem, though, has been that there is insufficient information available on the design and use of aircraft cooling fans and designers have always tended to avoid any installation they do not know much about. For that matter, even conventional nose mounted engine air cooling has often been left more or less to chance and the simple copying of some existing installations that work satisfactorily. It is the purpose of this article to give some information regarding fan installations in an effort to convey to designers some of the inherent problems and solutions offered by the use of fans and their installations. While a fan may seem to be a very simple device, it is 40 APRIL 1988

Fan installation in Coot amphibian — suction installation.

surprising how few designers have any understanding of the necessities required to get a good performing installation. There are several ways to make a fan ... and their installations and operation can be very simple or can end up being quite complicated. First, to get cooling, the fan must develop a pressure difference in the air stream that flows over the engine's cylinder fins. Without a fan, this pressure difference is normally caused by the ram of the air due to forward motion of the aircraft and/or from the propeller slipstream. Thus, the air must have a relatively positive pressure on the upwind side of the baffles and cylinder fins and a relatively negative pressure on the downwind side (backside) of the baffle line. It is this "pressure-difference" (or pressure drop) that determines the amount of airflow over the fins and the effectiveness of the cooling air. A pressure fan cooling system usually consists of a duct arrangement made of sheet metal to direct the air from where it is picked up at the front of the aircraft back to the face of the fan. The fan is usually installed in some kind of a duct-ring to help minimize tip losses (see photos and illustrations). The fan runs in the duct ring so as to effectively raise the pressure in the duct downstream from the fan which then

conveys the pressurized air to the engine cylinder baffles. Thus, if there is an air leak in the baffle system or ducting, it should be plugged. The fan itself should deliver as much air as possible to the pressure-side of the baffles. The pressure difference across the fan itself should be as great as possible and leakage around the fan blades should be kept to a minimum. This means that the tips of the fan blades should run as close to the fan duct ring as possible consistent with good design practice. Usually, 1/8 inch clearance should not be exceeded. It is usual practice to have the upwind face of the duct rounded to streamline the airflow into the fan and to mount the fan itself with only about 1/2 of the chord of the blades immersed in the duct ring, with the exit side of the blade chord extending downwind from the duct ring. This tends to unload the blades due to centrifugal force acting on the air, and can increase the airflow from the fan. The fan itself can be made from a variety of materials, and in many different arrangements. It is good practice to have an even number of blades so that every other blade can be removed, if later desired, to reduce volume. Since it must develop air pressure in a duct, the fan has to be designed a bit differently than a propeller. The usual configuration consists of a solid hub-disc

BAFFLE

COOT Fig 1

Fig 3

CYLINDER

MINI IMP Fig 4

Fig J

Some Tested Installations Figure 1 — The Coots The Coot light amphibians all use a suction fan to assist in cooling the engine during lengthy taxi operations on the water. Here the ram air enters the front of the cowl that surrounds the engine and passes up and over a deflector baffle that makes it then flow aft through the oil radiator and down through the cooling fins of the engine. At that point it flows into a plenum from which the hot air is pulled out and exhausted aftforward of the propeller. It is interesting to note that this installation not only adequately cools the engine but actu-

ally appears to assist in getting better performance from the aircraft. It is felt that this is due to the effectively increased spinner-effect from the exhausted air from the fan coming out through the center of the propeller disc. Figure 2 — Mini-Imps The Mini-Imp installations typically take their ram air in through side intake air ducts just ahead of the cylinders where it is deflected in and down over the cooling fins and oil radiator into a plenum under the engine from which the hot air is pulled aft by the suction fan mounted on the long driveshaft to

Close up view of Bullet tan.

the propeller and then exhausted up and out the top of the tail cone.

Figure 3 — The Bullet The Bullet installation takes ram air in through a flush intake duct under the engine and uses the engine cowling for a cool air plenum from which it flows up through the engine cooling fins into a hot air plenum above the engine. The heated air is pulled aft by the suction fan which then exhausts it down and aft from a short duct out the underside of the fuselage. Here, too, the fan is mounted on the long driveshaft to the propeller. with a number of short blades installed around the perimeter of the disc (see photos). These blades can be made from plastic, metal or even wood. It is not unusual to see a single casting used for the fan where the blades and hub are cast together in a single unit. Individual metal blades can be either cast or fabricated from metal sheet. Often the whole fan is made from a single piece of sheet metal with the individual blades merely twisted extensions out from the solid center. This type fan is sometimes seen in helicopter installations. Casting is generally used when plastic blades are made such as those used in some foreign automobiles. However, individual cast metal blades are most often used in homebuilt airplanes. Blades can be either left or right hand, depending on rotation direction and SPORT AVIATION 41

whether the fan is to be used as a sucker or a blower. It is important that the fan be dynamically balanced so that it will run smoothly and not cause undue vibration. This can be assured by having the fan assembly dynamically balanced by someone equipped to do it properly (such as a drive-line shop) to at least 0.2 ounce inches at expected maximum rpm. The fan should also be whirl tested if it is a new design. Whirl test to 150% expected maximum rpm. The fan can be driven by means of belts, or installed directly on the engine output flange or a driveshaft, if one is used. If a belt is used, it is advisable to drive the fan at a somewhat faster speed than the engine output. This avoids resonance between the turning engine and the fan. However, fan speed should not exceed twice the engine running speed and preferably should not be exactly twice engine speed. Fan diameters up to 24 inches are possible without too much design difficulty, and diameter will be determined by tip velocity and other considerations such as duct width, etc. Fan speeds (rpm) should be limited to a maximum speed which will not exceed the structural allowables for the fan used. The number of blades in the fan usually depends on the pressurehead desired; more blades will yield higher pressure. The pitch of the fan blades depends upon the velocity of the ram air entering the face of the fan, as well as the volume of air desired from the installation. In general, blade pitch angles will run around 45 degrees maximum for a fan turning 2500 rpm on a 200 hp engine and will develop the desired approximate two inches of water pressure-differential across the usual baffle plane on an air cooled aircraft engine. It is desirable to actually measure the pressure head across the baffles with a simple manometer which

can be made from a length of clear plastic tubing partially filled with water exposing the two open ends to the air on the two sides of the baffling under consideration. It is important to consider any pressure developed by the ramhead of air going into the face of the fan. If the ram-velocity is adequate, there should be very little pressure-drop between the entry duct opening and the front of the fan face. Ducts feeding the fan should have adequate cross-section area to assure free flowing air into the fan itself. Exits for the heated cooling air from the engine should be larger than the inlets to assure that no back pressure is being developed that would reduce the desired pressure differential and thus impede the airflow over the cooling fins. Given enough ram velocity and volume, it is interesting to note that a fan can actually put some energy back into the propulsion system. However, it is sure to take some energy to drive your fan when the forward speed is low. We should also mention that fans can also be used to push (or pull) air through a radiator to cool oil or the coolant for a liquid cooled engine. Suction fan installations are much the same as pressure installations except the air is sucked from the engine after it is heated in the process of cooling the engine. Otherwise, there is little difference between pressure and suction fan installations. Either type can be used satisfactorily. We had extensive experience with many fan installations such as in the Aerocar flying automobile (see figure 4) where ram air entered a duct fan which is located in the rear where the air is pressurized into a plenum around the engine. The pressurized air flows up and aft and then down over the engine cooling fins and out through the oil radiator into another lower plenum from

Fan Installed In Bullet. Shroud over engine removed to show suction Installation. 42 APRIL 1988

Typical fan assembly in duct ring made of fiberglass. Note how close the fan tips are to the duct ring. Blades are made of Delrin plastic mounted on aluminum discs.

which it flows back and out through the hot air exhaust outlets. The fan turns almost twice engine speed in these installations since it is mounted on the driveshaft to the front wheels between the transmission and differential of the car. We have had extensive experience with other fan cooled aircraft engines. Many of these have included engines which were equipped with fans that were part of the engine itself. Both suction and pressure type fans are common. More such engines are now appearing on the market. The new Mooney and Cessna with the Porsche powerplant is a good example of this trend. With all of these installations, it is possible to run the engines at high power, even on the ground, for long periods of time without overheating. We should mention that many fan cooled aircraft engine installations have a common problem. This is characterized by the engine getting very good cooling when it is running, but suffering inadequate cooling when initially shut down after an extended period of operation, particularly at high power. This is due to the ambient heat from the engine being confined by the necessary tight baffle system needed for a fan cooled installation. A fan cooled installation should be run at reduced power for a short time before it is shut off. If possible, they should also be equipped with some way to self-ventilate the engine spaces after shut down or use an auxiliary electric fan that is automatically shut off by a thermostat to cool the engine after running. Anyone desiring further information on fans is invited to contact the author who has a limited supply of fan blades (both hands) available for experimental installations. Advise rotation and whether pusher or puller type is desired.