cowls, plenums, drag reduction

I've spent hours going through the archives and all I can say is man some of you guys are artists with composites and huge wealth of knowledge. so I just recently bought an RV-4 with an O-360 A1A in it. as we all do I am putting a bunch of time and work into personalizing it and making it mine. If everything goes to plan my goal is to completely repaint and fix up some of the fiberglass components. I was planning to do a sam james cowl but they have a 6 month back log and that would make it spring so that would push the paint off for another year.

Over the weekend me and a good friend who has flown an RV4 for several years got into a conversation about drag reduction. His thoughts are the primary drag reduction from the Sam james cowl is due to the plenum design and reduction in cooling drag and that the parasitic form drag is probably nearly the same. This makes sense to me since it is common knowledge that cooling drag is a very high percentage of overall drag. so this conversation and reading all of your posts on plenum building has me thinking about making my own plenum and fitting it to the factory cowl. the cowl will also need some work because the original builder cut it too short so there is a large gap between it and the spinner. and speaking of spinners this one is also not cut or fitted properly and will need to be replaced, but it is one of the early 12" ones that vans no longer supplies. needless to say I have big plans for this bird, just hope i have the skills to make it all happen.

We all have plenums. It is simply the interior cowl volume above the engine. That may seem pendantic, but it's important to realize a plenum lid is nothing more than a sealing device to replace flap seals. Better sealing (with a lid or flap seals) can result is less total mass flow for the same cooling, because it can eliminate a lot of mass flow which was simply bypassing the hot parts. Cooling drag = mass x loss of velocity, so less mass means less drag.

It is relatively easy to make a lid which seals to the engine sheet metal. The trick with plenums is how they are coupled to the inlets, and how well the inlet/plenum system converts available dynamic pressure to increased static pressure above the engine. Unfortunately, some are significantly worse than a stock Vans cowl with flap seals. Low upper plenum pressure means after the pressure drop across the engine, there is none remaining in the lower cowl plenum to accelerate the mass out the exit. The loss of velocity is drag.

Read these:

https://www.danhorton.net/Articles/Flap Seals vs Plenum.docx

https://www.danhorton.net/Articles/Cooling Efficiency.docx

while going through the archives your responses are definitely some of the ones that stuck out and your composite work is absolutely amazing. I do understand and agree that they are all plenums in one way or another but was just going with the common theme that the sealed lid is referred to as a plenum VS baffles or rubber flaps. I've only had the plane a very short time but it did have some huge holes in the baffles that I immediately plugged and used lots of red RTV as a temporary fix with the goal being a better more permanent solution in the coming months. I also agree that sealing the intake to the cowl is the biggest challenge here. With the Sam James cowl you have the ready made rings on the cowl and front of the plenum to connect, where as to do this with the stock cowl I would have to figure out how to manufacture some sort of oval shaped rings to seal into the cowl openings and then flex tubing to connect to the box. I saw a post of I believe it was an 8 where he had done just that and it was beautiful, but didn't find any post on how he made the rings and stuff, just the finished product. once I have the plenum and intake cooling air figured out I look forward to the challenge of figuring out the best way to modify the exit air opening for max speed and efficiency, one step at a time I guess

thunder669 said:

With the Sam James cowl you have the ready made rings on the cowl and front of the plenum to connect, where as to do this with the stock cowl I would have to figure out how to manufacture some sort of oval shaped rings.....

Click to expand...

There is a lot more to it than round rings.

The three plots below were all generated with round inlets, but note the significant differences in upper plenum Cp.



Seriously, download a copy of CR3405 and read it a few times before you cut glass. You'll be glad you did.

https://www.danhorton.net/Articles/NASA_CR3405 Cooling.pdf

Dan,

I get a 404 error when I click on that link.

scsmith said:

Dan,

I get a 404 error when I click on that link.

Click to expand...

Fixed.....