I had a theory at first
I had a theory based on casual observation that if I could get the cooling air mass to flow smoothly to the outlet and keep it from passing through all of wire, tubes, hoses, engine mounts, and other items between the engine and fire wall there would be less drag and thus more speed. Instead, the plane slowed down 2 kts. I reported my results and Chris Z (Lancair builder, modifier and pilot) told me that he believed what happened is that by cleaning up the flow a greater air mass was allowed to flow through the system and this actually increases the cooling air drag. When I though about it, it made sense to me that something that was happening before the inlets in the original configuration that prevented potential cooling air mass from entering the system was no longer happening and more air was now entering the system. At that point I had been thinking about the horizontal baffles outboard of the engine in the lower cowl and I thought I would go ahead and try it before I pulled everything out. After I did that I got the 6 kt reversal to 4 kts above my baseline speed of 170.7 kts. I rationalized a few things but I do not know why the increase occurred. Certainly the volume in the cowl below the engine was reduced and alternate flow paths (back up outboard of the valve covers and into the area behind the engine and lower cowl baffles primarily) were eliminated. I worked with the inlet size and position and the outlet configuration as well as adding another large baffle below the engine to turn the air back over the filter airbox and nosegear structue but I could never improve on the speed further. Inlet modifications directly effected a rise in CHT indicating a reduction in air mass flow through the fins but there was no measurable increase in speed. A very smooth contoured extension to the lower fwd baffle which went over the nose gear inverted pyramid structure and down to the lip of the outlet was another mod the slowed the plane. I think this was due to the attached airflow being directed down into the airstream below the fuselage just aft of the cowl. I think there may be something to be gained by putting a baffle across rear of the inlets from the center baffle aft of the prop and the stock baffle sidewall; then if the baffles were holed and lined with rubber the inlets could be modified with a duct that extends through the rubber and feed the plenum without leakage backflow. Then pehaps a size could be established that would be more efficient in cooling and reduce drag. At this point I think I just have to pursue the idea until I learn whether it works or not. I also thing the exhaust configuration can be improved on the "A" models but it requires new pipes that go out more in line with the exhaust ports well outboard of the centerline induction air and nose gear systems. The cowl woul have to be modified to provide new exit faired in exit points and the centerline cowl scoop would be brought to a boat tail closure aft of the nose gear strut. I look at all of the new high performance production airplanes and I do not believe it is a coincidence that they all route the exhaust outlets this way.
On the wheel fairing bulkhead failure to increase speed, I think the configuration channeled the air around the tire in a column that exited downward infront of the tires. I think the turbulent conflict there caused drag. When I removed them the speed came back. It could be that more work with the configuration would fix the problem. The new subfairing on the nose gear definitly reduces drag and has caused no operational problems. I do hold the nose off and only land on paved strips but my hunch is this thing would not have a problem on grass.
I did not go to Taylor, Texas this past week either. I had the plane ready to go but my attitude indicator failed on Thursday. We are going to Texoma in Sherman Texas on the 19th - hopefully the weather will be nice.
Bob Axsom
