johngoodman
Well Known Member
Has anybody done the RVBits Plenum? how does the oil door work? Is it already baffled?
John
John
You could do it again but I don't think the SJ cowl is available for the -10 any more...
Guys, let's not get confused. A plenum lid is just a sealing device. If it seals well, it has done its job, and it would be a very poor lid indeed that didn't seal better than conventional flap seals. Air which bypasses the hot engine fins is mass flow with no purpose, meaning it's just pure drag. In reality, a plenum lid is simply one component of a drag reduction system; good sealing allows a mass flow reduction, which is a function of exit area.
Exactly correct. If done well, a plenum will make the most of the pounds of cooling air coming into the engine such that you can reduce the total pounds of air and still have the same heat rejection as a standard cowl with baffles. This is the approach with the James Cowl.
There is no free lunch. Reduce the engine cooling drag and you have a faster airplane for the same power. Using a well designed plenum allows you do to this while maintaining adequate engine cooling. Note however this is for your design flight target. If your target is a fire breathing climb at 3000 fpm plane, a plenum is not for you. If you respect the engine, provide reasonable speed during a climb and want superior cruise efficiency, a plenum with an appropriate cowl is the path.
Carl
Why would that make any difference? The plenum just forces all the air through the cylinders. It doesn't control the ins & outs; that's a separate choice, right?
If your target is a fire breathing climb at 3000 fpm plane, a plenum is not for you.
With all due respect Carl, that's nonsense.
Again, a plenum lid is just a sealing device. There is no "design point" speed, and it does not control how much mass passes through the system. It only ensures none of the mass can bypass the engine and thus fail to pick up heat. As such, a plenum lid benefits every part of the performance envelope, but low velocity climb most of all, as that's when the least total pressure is available. When starting with very little, there is none to waste.
I should have been clearer. My design target was high efficiency cruise, not rate of climb competition. With that target, total engine cooling drag can be reduced (as compared to a stock cowl) while still providing adequate cooling. With a good plenum, you can do more engine cooling drag reduction and still retain adequate cooling at your target operational envelope as compared to leaky baffles.
If rate of climb is the target, open up to let as much air in and out as you can. The addition of a plenum would help at the margins.Carl
If you are not using the James Cowl there is little to be gained with a plenum.
RV-8 in progress (will use stock cowl and homemade plenum)
Thanks, Jan. I've pretty well figured it out. I finally figured out how the oil door is handled. The real reason I thought about it is the problems with putting the top cowling on without folding a rubber baffle the wrong way.
John