Not quite
Bob Axsom said:
It would be interesting to do a test with a plate over the heater port in the rear baffle to see if there is any speed gain available from this capping this air gusher in the summer. Bob Axsom
Good thought Bob but not that easy. First blocking you oil cooler is not going to be acceptable to oil temps. Also unless you also reduce the cowl inlet area you will have the same drag (read below why). For the aircraft speed, engine-cooling requirements there is the correct cooling area. The cowl inlet is a compromise for climb and cruise requirements on say a 100F day (+41F std day). What if in fact you could adjust the inlet area in-flight like you can with exit cowl flaps; you could reduce drag and optimize the inlet for all phases of flight. It has been done but never caught on. Also think of this, a banner towing plane with the same engine, prop, oil cooler as a RV will have a different inlet / cooling design (much larger) than a +200 mph RV.
NASA's study determined or discovered the optimal ratio of air velocity in the free airstream to the speed in the cowl inlet. THIS IS WHY BLOCKING THE COOLER WILL NOT CHANGE THE DRAG, BECAUSE YOUR INLET (and exit) IS THE SAME. You can control this Velocity ratio by clever design of the cowl inlet area, shape and internal contours aft of the inlet. If the inlet area is too big you have too much drag and high internal losses from high internal velocities. If it is too small the stagnation point moves outside the inlet and will cause "spillage drag", where air comes to a stop and than spills out around the inlet. This deceleration of the air and re-mixing with the free stream air is draggy. You want free air to decelerate and go into the cowl not decelerate outside the cowl, due to a poor sized / shaped cowl inlet. By taking the cooler air away as you suggest you will not change the fact that the Van's inlet area is X, which is already more than needed. You may put more are thru the cylinder'(s) blocking the cooler, but CHT's will suffer from higher oil temps. Really air to the oil cooler is needed and will not help the cause. Using external air from another source and sumping that into the lower cowl is bad and will produce more drag, don't do that.
Also it is true Van has a cool "trick" curved surface at the top of the cowl inlet inside the cowl. I know I installed it on my RV-4. This will reduce cooling drag. However like you point out you have flat surface on the other boundaries and corners with soft seals that may leak and a diverging area and lips/edges too great to allow smooth (non-turbulent) air flow. What happens when your wing has turbulent flow? Ans: Loss of lift (pressure) and a sharp rise in drag.
Van's stock cowl takes advantage of some good details not found on the current GA planes of the day. The cowl inlets have a large outside radius and a flange that extends back a little to meet the upper cowl ramp and lower baffle ramp. However the sides are not ideal. The stock cowl is good enough as is; It just could be better, about 6-10 mph better. I think saying a SJ style cowl and plenum is a little better is fighting words to some. Vans' cowl is very good but there is room for improvement. Also Van's origninal ideas of simple fun sport planes did not see digital CRT computer monitors on the engine. Running a tighter cowl does take a little caution in pilot operations. Because the RV is so fast and climbs fast we don't get into too much problems. However in the summer, fully loaded and climbing with not pause to 10,000 feet might cause some grief, no matter what cowl you have or baffle seals.
OIL COOLERS
As far blocking the oil cooler, the mass air flow thru the cooler is not that great, and it needs about the same pressure as the engine to be efficent. This is why the SW coolers are more efficent, they are a better match and more efficent at the air flow/pressure under the cowl of a real plane not a test stand. Oil cooler installation misconceptions are pet peeve of mine. I hate the hard mount to the #4 back baffle set up because it's hard on the cooler and baffle but works kind of, but it could be better. Next to this, my pet peeve is a bad remote oil cooler installations. Remote is OK but Van's oil cooler kit is terrible. That flat little square box on the cooler in Van's kit is bad bad bad. All that applies to cowl inlets and cooling: smooth surfaces, no sharp edges, min diverging side angles, applies to the oil cooler installation. Bad oil cooler installations usually result in high oil temps not much more drag. Also the cooler exit air is rarely addressed in cooler installations. Builders just dump the cooler air out any old place. However some drag reduction can be had with cooler installation by efficiently using the available air. This in turn could allow a smaller cowl inlet. It is part of the whole cooling system. The engine and cooler needs (MUST HAVE) a certain amount of airflow and pressure to cool. The more efficient (less drag) the whole system is, the less air you need, the smaller the inlets, the less drag.
COWL EXIT
The one area not mentioned it the cowl exit. Van's little firewall rolled edge above the top of the cowl exit on the RV-8 is pretty min. The other RV's like the RV-6 and -7 don't even get this supplied. This is a place you can improve and (reduce) cooling drag. There is a lot to do in the cowl exit area. I will leave it to the reader?s imagination and research to figure out how to improve this area, which is critcal.
LEAKS ARE BAD
Side note: The less leaks you have the better. Leaks that pressurize the lower plenum do a double whammy. Leaking air into the lower cowl is air not cooling the engine and is pressurizing the lower cowl, reducing differential pressure from top to bottom across the cylinders, reducing the cooling. All this is inefficient. Making larger inlets to keep temps in line to make up for leaks is part of the cooling drag big picture. Cooling drag is not one thing like round inlets. A SJ cowl has the same temps with 1/3rd less inlet area as Van's cowl! Do the math, this is less drag.
INLET AREA
The required cowl inlet area for cooling is a matter of known formulas of air volume and pressure for a give amount of HP. The required inlet area from these equations is usually pumped up. GA planes need to account for pilot ham-handed-ness, too long climbs and inefficiencies of the system. Also most GA planes did not come with 8 channel engine monitors. Why do Pipers and Cessna's go so much slower than a RV? Many reasons including cooling drag. To be fair to Piper and Cessna they have to be conservative.
SHAPE IS KEY
Inlet shape is another matter. This is an area that has got little or no attention on GA planes (until the NASA study). On WWII birds they did all kinds of work on the cowl shapes and spinner shapes to guide the air to the engine and out, with the least drag. In the NASA sponsored research (70's/80's) for our little Horz engines, they used laminar flow "A" series NASA airfoils to shape the inlet. If you "trip" the air from the start you are screwed.
Now you are going to try to take air going 210 mph and slow it down and convert it into pressure at a certain flow over a range of air-speeds, you would want to do it smoothly with out turbulence. Turbulence is drag and loss of pressure, right. Is the current style SJ cowl the greatest? No, but it is better than Van's cowl, which is already pretty good and way better than an old Cessna / Piper.
THERE IS NO SINGLE ANSWER
Look at the research, it only gives you data and suggestions, it does not specify a design or size the hardware for a RV. Yes certain shapes and inlets where tested but not ever one. In fact NASA only worked on the inlet, sealed plenums and never test flew or optimized the rest of the ducting/diffuser. The work was left somewhat unfinished, but they did make observations and suggestions. The soft hoses that connect the cowl inlet to the plenum was unique to the origninal Barnard "Holy Cowl" was which is now made and sold by Sam James. This may not be the first time this whole system done like this, but was the first time I had seen it and no doubt the first time on a RV. I think compaired to LoPresti, Lancair and others Barnard's original design is the best application of the concepts I have discussed based on the NASA studies. I can't think of much to improve it except the actual ring profile (which I think SJ as a little of in area and shape, but not sure) and small tuning in plenum shape, cross section and volume.
In the past Piper and Cessna had openings with sharp edges, although the state of the art Piper Apache Twin (first light twin for the general public) had a pressure plenum with a fabric / felt pad seal to the cowl. In fact the Apache also had exhaust augmentor tubes to increase cooling. I owned one and could cruise +160mph on about 13 gph. Of course this is a five place plane with a cabin you could stand up in (slightly bent over) and step between the two front seats to the back seats. It is not like a little Piper Seminol. Not bad for 1958. With about 108 gal of gas you could fly a long time.
There is nothing that is perfect, but if you understand basic aerodynamic requirements and requirements for the airframe and engine you can minimize cooling drag. Cooling drag is a large part of the total drag of a RV, so it makes sense to go after it. Just blocking off the oil cooler will not do much and of course your oil temps will suffer. You still have to meet requirements of the design. What about a variable shape / area cowl inlet? Well that is too much for me. If I can pick up a sure 6-10 mph with the simple change and avoid cutting soft seals, the modification to the stock cowl is worth it to me.
G
the numbers on pinholes on the inner and outer upper cowl in 2 zillion and something. (did I do mine the way you're thinking? thought everybody did it THIS way. except the holy cowls? )
