May be a bigger cooler before more duct area?
Alex wrote:
I had a thought about the scat tubing, which went something like this: Larger diameter is better, because reducing the air speed through the tube for a given mass flow rate will reduce the pressure loss in the tubing.
True, good idea. I think there is only so much the cooler can flow and increasing the diameter is not going to increase the delta-P with any significance, because it is determined by the system. By system I mean how much air pressure is going in the tube in the first place, which is based on the aircraft cowl, upper plenum, exit pressure and forward speed.
Alex wrote:
"So the larger the cross-sectional area of the tubing prior to the hard shell "plenum", the better. Given this, has anyone used 2 or more parallel runs of scat tubing, rather than just one wide tube? It seems that two 3.5 inch tubes will have the same cross sectional area as a single 5 inch tube, although they will have 41% more surface area. This might be an easy way to increase the cross-sectional area of the tubing run."
Another good idea, but is it needed. I know the fire breathing 200HP IO-360's tend to shed more of their heat thru the oil and run high oil temps, especially in RV-8's (which I think Van addressed with a larger cowl outlet). My answer would be not only a larger duct dia but also a bigger oil cooler, really big cooler (see picture). The little coolers we run are OK on the 150-180HP, but when you get to the 200HP you may start thinking about the next size up before a bigger hose. Note the large cooler on this 200HP turbo RV. The cooler vents direct out the side of the cowl thru louvers, efficient but much more drag. (click to enlarge)
Alex wrote:
"The air velocity through the tube is roughly inversely proportional to the cross-section area, and the drag on a given section of tube is roughly proportional to the square of the velocity. That would suggest that drag through the scat tubing goes down with the 4thpower of diameter. I don't think it falls off quite that quickly because the surface area goes up, but it should be significant nonetheless."
Oh great, I started everyone thinking like engineers.
Again good idea, but my whole philosophy is balanced components, the right size duct based on the pressure, flow and oil cooler limitations and requirements. Going bigger with a hose will not necessary make it better. You got to get the air thru the cooler. There is only so much CFM's the oil cooler will pass for a given Delta-P and only so much BTU?s it can dissipate. The most efficient duct, hose diffuser will not help if the oil cooler is not up to the job (meaning bigger). For most of us (in order of good better best) 8406R, 10599R or 8432R are good enough. However the high compression engines (IO-360-200HP) in hot climates may need a bigger heat exchanger. I think the 10610R or 10611R are the next size up but are about 0.5-1.0 inch wider. The bigger cooler is not a lot more efficient but has the capacity to flow more air and oil, so in the case of a bigger cooler to get the advantage you need the greater air flow because they are designed to use it. I did not calculate the correct size duct for the big coolers, but it is more than the 4" for the smaller 8432R sized cooler.
Also a BIG factor is getting the delta-P up, which is hard to do, because you are dealing with the dynamic q (pressure) from the plane moving thru the air and what you can pick -up off the back of the baffle. The best way to improve source of air for the cooler (without adding drag with an external scoop) is make your two front cowl inlets better (smooth, no leaks and away from the prop hub), seal your upper plenum to be more efficient (less leaks, soft seal which are not as good as the solid top plenum) and improve the lower plenum of restrictions to promote free smooth flow out the cowl exit. The cowl exit air should enter back into the free airstream parallel with it.
That is why those aftermarket round cowls and plenums work. Sealed upper plenums with a tight seal to the round cowl inlets gives the same cooling air with less inlet area, they just have less losses. They get cleaner air away from the dirty air off the prop hub and it does not rely on soft seals and friction to seal against the cowl (one moving the other not). The idea is get the required flow with less inlet area (cooling drag), and it works. If you have cooling problems make sure the baffle seals are as good as you can get. Test by NASA of the GA fleet show cowls have leaks of 50% or more. The GA planes share the same cowl design and baffle as a stock RV uses.
There is only so much total Pressure you have to use. The only way make efficient use of what you have is with components matched in size for the environment and have min losses. Don't forget the oil cooler exit as I mentioned. Local airflow and pressures under the cowl can impede the flow out the cooler. Guide and shield the coolers exit towards the low pressure area in the cowl (exit).
Great discussion, George