QuickTalk 12 - QUICKIE HINTS
- Category: Q-Talk Articles
- Published: Monday, 31 October 1983 06:11
- Written by Jim Masal
- Hits: 943
/QBA has received several favorable comments on the report by Ray Anderson and Harold Little concerning new heads and graphite gaskets for the Onan engine (QUICKTALK Issue #9). As a result, we are printing an additional section of their original report on engine modifications in the coming months. -Ed./
The success of the head/gasket test program has produced an engine with excellent cross-country serviceability, to be used with great confidence. Ray then decided to return the engine to its original Onan configuration. He purchased a new block to rebuild his engine to essentially the B-48G-GA020, 20 HP configuration. Ray chose not to purchase, at this time, three items which technically convert the 18 HP to the 20 HP set-up, but to just test the easily retrofittable parts (heads, gaskets and head bolts). The items not obtained were the carburetor, pistons and oil sump.
The new block was built up with the original 18 HP parts, except for replacement of those that Ray observed to be unserviceable. He also put rotators on all the valves; even through wear was not a problem. The crankshaft, alternator, rods, piston pins and pistons were all professionally balanced prior to buildup. The initial run-in was done with carefully monitored ground running in several start, run and cool-down cycles. The heads were affixed using the standard Onan head bolts, although it would be an added safety measure to obtain SAE Grade 6 or 8 bolts of a length to engage the entire tapped length of the holes n the block. The bolts were torqued to 14 foot-pounds and checked prior to each restart. The relaxation of the torque ceased upon seating of the graphite composition head gasket. Final torque was limited to a maximum of 15 foot-pounds.
As the Onan B-48G-GA020 engine has an identical block, except for a small cast-in depression in the piston crown, it was decided to adjust the timing to the 20 degree BTDC specified for the 20 HP engine. This is important, for with the new heads and the flat crowned 18 HP pistons, the compression ratio will be slightly higher than the Onan 20 HP engine, which is to our advantage.
To simplify timing, a timing mark should be inscribed on the flywheel and engine mount when the engine is out of the aircraft. First, lightly sand the engine mount below the timing mark on the gear cover and on the face and edge of the flywheel about the timing mark. Then paint those areas with good quality flat white paint. After the paint has cured, carefully scribe a line down the engine mount that is an extension of the 20-degree timing line and do the same above the timing mark on the face of the flywheel, carefully extending the line over the edge. With a ballpoint pen, fill the scribe line with black or blue ink, which will allow for a simple timing adjustment after mounting the engine.
The carburetor can now be modified to allow better airflow, as the B-48 engine is a carburetor-restricted design. Remove the two screws from the throttle butterfly shaft, remove the butterfly from the slot and then remove the shaft. Cut the shaft at the ends of the slot into the slot to remove that part of the shaft, which has the butterfly retaining screw clearance holes, NOT the side with the tapped holes. File the cut ends of the shaft to a smooth radius, then reassemble the butterfly to the shaft. File the protruding ends of the screws to the same contour as the shaft, and also file the screw heads to a smooth profile to the airflow. Disassemble the throttle, reinstall and reassemble it in the carburetor using RTV silicone rubber on the screws to lock them in place. Also, flush seal the choke shaft holes with silicone rubber to enhance the smooth flow of air into the carburetor. Allow the rubber to cure at least 24 hours prior to running the engine.
After adjusting the timing and modifying the carburetor, the mixture should receive careful attention. N1V required a slightly richer mixture to achieve maximum RPM, but that adjustment was done in careful increments. This is a necessity, as the adjustments on the ground, which will seem minor, will have a very dramatic difference in flight as you climb. Therefore, fly the aircraft at various altitudes and speeds, noting the OAT, RPM and pressure altitude, then adjust for an RPM rise on the ground. Re-fly the test series, correcting speed and altitude for OAT and pressure and note RPM. After flying several test series, you will be able to tweak in the mixture for the speed and altitude that are important to you (while staying within CHT and oil temp limits).
The Onan B-48 block is designed for continuous running at 3600 RPM at a maximum output of 25 HP, so these modifications will not cause the engine to be operated in the high wear, short service life area. However, the power curve begins to fall off, as does propeller efficiency over 3600 RPM, so it is best to choose your propeller to absorb all of the engine power that you require in the flight regime that is most important to you, so as not to exceed 3600 RPM.
To achieve the highest RPM with the new installation, all of these adjustments were necessary. Upon obtaining the maximum RPM, final flight testing was begun. In over fifteen hours of flight testing, many improvements have been noted, but most significant is that no head bolt torque relaxation has been found.
Although not directly related to performance, an item related to high time engines concerns vapor coming from the oil breather to the extent that it fouls the fuselage. Clamp a 1/4" stainless tube to the left exhaust pipe under a stainless steel hose clamp and insert the lower end through a hole drilled tangentially into the bottom curve of the exhaust pipe. Bend the upper end of the tube well clear of the exhaust pipe and connect it to the oil breather tube. The oil mist will be well vaporized by passage through the tube in contact with the exhaust pipe, and the exhaust will then entirely disperse the vapor into the slipstream with no further fuselage accumulation.
The B-48 engine is volume cooled, not pressure cooled, as are most aircraft engines, the plans constructed baffles require modification and additional baffling to provide the correct air flow for adequate cooling. Volume cooling requires that the ram air velocity be maintained through the engine cooling jacket while absorbing the engine heat and then be exhausted from the engine compartment to expand. With the plans baffling, the ram air enters the cowl plenum, reducing its velocity, then turbulently flows down through the engine cooling fins, extracting engine heat and expanding within the engine compartment, effecting a pressure drop across the engine. As this air flow is not adequate, it must be converted to velocity cooling.
Two steps must be taken prior to baffle improvement to obtain the best airflow. First, remove all mold casting flash from the engine fins. The flash is the jagged thin aluminum between fins in the area between the tappet box and cylinder top. The flash should be carefully removed and the area dressed smooth with a file. Inspect the entire engine for flash or any other obstruction or areas that would unnecessarily turbulate the cooling air flow. It is important to ensure the smoothest flow of the cooling air for volume cooling.
The second step is to remove the dipstick, measure the oil fill tube length, and cut off the oil fill tube approximately 1" above the oil sump/block joint. Then cut the tube approximately 1/2" below where the tube narrows below the dipstick cap enlargement. Tightly pack a clean cloth into the stub tube in the oil sump and then deburr the cut edge. With a hacksaw, slot the short tube cutoff on the diameter to just below where it widens for the dipstick cap enlargement. Deburr all cut edges and radius the outside of the cutoff. Sand away the paint on the narrow end of the cutoff and remove all the sanding residue. Wipe away the oil that is inside the tube in the oil sump and then clean the inside with a good solvent that will leave no residue. Coat lightly approximately 1/2" down the inside of the oil sump tube with hardening Permatex, coat the outside of the short fill tube to above the hacksaw slots, then carefully drive the short fill tube into the oil sump stub, using a wooden block, a mallet and very light blows. Drill two holes in the assembly as far apart that will still allow insertion of 1/8" steel pop rivets. Deburr the holes on the inside of the tube, wet two closed-end pop rivets with the Permatex and set the rivets. Wipe a smooth radius in the Permatex at the bottom of the driven stub and around both the inside and the outside of the pop rivets. Allow the Permatex to cure, then carefully vacuum the chips out of the tube and finally, very carefully, remove the cloth from the base of the tube such that any debris not removed by the vacuum will be carried out by the cloth.
Measure the installed length of the stub oil fill tube, taking into consideration the amount of overlap of the two pieces that you have put together, then cut the dipstick so that it is the correct length for the new installation. If you want to retain the markings from the original dipstick, cut off the marked end, and using steel pop rivets, attach it in the correct position on the shortened dipstick. This new installation is primarily to increase the undisturbed airflow over the cylinder, but it also allows you to check the oil level by reaching in through the cowl air inlet to remove and read the shortened dipstick.
Now that the ram air has the least impeded flow into the engine that is possible, a new baffle is required, in addition to modifications of the existing baffling. The new two-piece baffle is in gasketed contact with the cowling immediately above the cowl air inlet opening, extending across the cowl to join tightly the tops of the cylinder head baffles, and then it extends back to join tightly to the rear vertical engine baffle. Attach a piece of air inlet hose of the type that is used on the carburetor heat box to the carburetor air inlet and connect the end of the hose to a hole in the new baffling in a convenient place above the cowl air inlet opening. The baffle is to be as close to the engine as possible; preferably going under the intake manifold, but over is acceptable to prevent removal of the manifold. It must be cut to fit tightly around the point housing, and the carburetor flange, if it goes over the manifold. To ensure a snug fit, slit lengths of inexpensive rubber tubing, such as windshield wiper tubing, to be placed around the edges of cut openings for the obstacles. The object is to maintain the baffles tight to the cowl air inlet and to the engine, with as little leakage as can be obtained.
The engine bottom baffle is now moved up to close gasketed contact with the base of the cowl air inlet, maintaining tight contact to the cylinder head baffles and the front of the block. The baffle is then extended, in tight contact, up around the back of the cylinders to the middle of the cylinders, using the cylinder-casting mold parting line as a reference. The baffle will only direct air around the cylinders, it will not direct or block any airflow to or from the heads. As the curved cylinder baffle extension will be unsupported, it may be necessary to safety wire it, around the cylinder, to be certain that it will remain tightly in place without vibration. As an added touch, which will theoretically enhance the exit air flow, the ends of the curved cylinder baffle extensions may be rolled, on a quarter-inch radius, but they must not be allowed to drop below the cylinder centerline. The roll will help the stiffness and will also help resist fatigue cracking due to safety wire holes.
The head baffles will have to be notched to allow a bend at the bottom to hold the baffle tight to the head. It will also be necessary to put a hole in the baffle for the spark plug. This hole should be contoured such that there is adequate plug clearance in the baffle with the minimum air leakage. Baffle gasket material in this area will help greatly. The front of the head baffles must be tightly gasketed to the cowl and tightly attached to the top and bottom baffles.
With the baffles tight to the engine and tightly gasketed to the cowl, only a very small ram air box is open to the cooling air entering the cowl opening. The ram air is directed immediately to the engine, where the tight baffling causes the air to flow with the greatest velocity just over the areas to be cooled. With minimum leakage to the void areas around the front of the engine under the cowling, the highest velocity air will be most effectively directed to cooling the engine, which is exactly how Onan designed the engine to be velocity cooled. It cannot be overstressed to maintain tight baffling with no leakage to achieve excellent engine cooling.
The cowl cooling air exits have been inadequate on several Quickies, including N1V. However, a simple enlargement of the gills did not provide a commensurate improvement in the cooling, so the exit cooling air path was reviewed. The cooling air exits and expands, as it has been raised to a higher temperature by extracting engine heat. Unfortunately, this air exits into the rising air flow over the center of the canard and these impinging flows interfere with each other, reducing the efficiency of both. An examination of popular low wing production aircraft that cool well reveals that the engine cooling air exits below the firewall either behind a small fixed cowl lip or through extendable cowl flaps. Therefore, it was decided to install cowl bottom cooling air exits.
To prevent loss of the lower cowl attach points, two exits were made, after removing the lower cool, by cutting in 1-1/2" from the edge of the cowl approximately 1" from the center lower cowl attach on each side of the attach. Another cut was made 6" outboard of the original cuts and heat was applied to the area between the forward ends of the cuts by using a heat gun. The tabs between the cuts were then formed downward when the cowl had softened enough to allow the bending. The tabs were held at approximately 45 degrees to the cowl bottom surface, then the heat was removed and the tabs allowed to harden at that angle. The flight test results of the newly installed bottom cowl cooling air outlets has shown that the engine will cool very well in all normal aircraft operating configurations, with the CHT staying below 350 degrees and the oil temperature averaging 200 degrees.
/The final installment in the next QUICKTALK issue will review engine vibration and propeller balancing. Below are several questions from members regarding Anderson and Little's original report./
Q: What are the part numbers to be ordered from Onan?
A: Onan 20 HP heads (left 110-2877, right 110-2878) and graphite gaskets (110-3181). The correct plugs are Champion NR-RBN-13Y. Be sure to order by part number. There seems to be some confusion in this area by builders and Onan dealers.
Q: What about seating the gaskets?
A: Torque the bolts to 14 foot-pounds, then run five ground runs from full cold, checking the torque between each run. There should be no torque release after the third or fourth run, indicating gasket seating. After the fifth run, torque to 15 foot-pounds, reassemble the baffles and cowling, go fly and forget the heads. That is what Ray did. After 20 or 30 flight hours he checked the heads and found absolutely no torque release.
Q: How should the new heads be installed?
A: Install per the Onan service manuals (18 or 20 HP). There is to be absolutely no applications of coatings to the heads. The bolts (not studs) must be clean, shiny and dry, to be installed with the conical washers used by Onan. Do not use Helicoils, as they retard the heat transfer across the bolt/block interface.
You can order a PDF or printed copy of QuickTalk #12 by using the Q-talk Back Issue Order Page.