The work has been done before by others
N91CZ said:
Bryan, Two key features to consider are the size and location of the inlet. The Bonanza has a very large inlet which produces a very low inlet velocity not unlike many GA designs. This causes the air to be slowed before passing through the inlet and produces what is known as external pressure recovery. The engine compartment can only flow so much air no matter how large one makes the inlet. If the pressure across the inlet were uniform, air velocity would be also. Since the inlet on the Bonanza extends all the way in and behind the spinner a large pressure gradient is formed. The area near the spinner will actually be slightly negative (Like a RV). Farther out along the propeller diameter the pressure rises and becomes positive. (See Pic below)
This sets up the scenario Walter described above. Excess air enters outboard in the inlet and that excess air then exits the inlet inboard near the spinner. (Originally seen by Ref. [1])
On the opposite end of the spectrum is a small inlet placed farther outboard as seen on many home builts. If the inlet is small enough, entering air will flow at aircraft speed. In this case, air does not back up in front of the inlet and the gradient across the face becomes much more uniform. (See Pic below)
All pressure recovery then takes place when the air slows inside the plenum. Tufting of this configuration showed that all air that came in stayed in just as would be expected. RVs should fall somewhere in between.
Some are modified with round inlets and some bring the inlets into the spinner. Depending on the particular configuration being discussed you will see vastly different behavior in airflow, both internally and externally.
[1] Ref: "An Experimental Investigation of the Aerodynamics and Cooling of a Horizontally-Opposed Air-Cooled Aircraft Engine Installation", NASA report 3405, Stan J. Miley and Ernest J. Cross, Jr., Texas A&M University; John K. Owens, Mississippi State University, David L. Lawrence; 1981. (note: this is a summary report of research and reports done in late 1970 by the above authors.)
Bottom line there is a big hole (cowl inlet) and not all of the hole area is doing something, at least useful. In fact from the reverse airflow (near spinner you see above) you might lose air, so why have it? RV builders have tuffed their cowls long ago and have seen the same thing. This is not new.
Making the "hole" smaller (cowl inlet), more aerodynamic, while its placed in better air flow (further out along the prop blade) you gain efficiency. Also using a sealed plenum means less leaks and less loss, allowing further reduction in cowl inlet size.
The round shape of the inlet is convenient for sealing and gets the most of the inlet area as far outboard in "good air" while still fitting form or profile of the cowl (no top secret about round, just practical and it fits).
The size of the hole in your cowl is a mix of requirements for volume and balance with speed of air entering. Too slow (big hole) you get stagnation and spillage air flow and drag around the inlet, especially during high speed cruise. Too fast (small hole) you get turbulent flow inside the duct and plenum and may be not enough volume at slow speeds.
The whole "package" from the inlet, diffuser (part that joins the inlet to the plenum) and plenum is critical. The weak link in the chain analogy fits here. With out all parts working together you lose efficiency. Some guys just cut a round hole in the cowl (including some major certified and kit plane makers) and dump the air into a cavity with out any diffuser. The inlet and diffuser (a continuation of the inlet) is 3 dimensional deal, not just a 2D hole in the face of the cowl. The inlet shape and diffuser is part black magic, art and physics. The problem we have is there is no room for them and the prop is beating the air to death right next to it.
One thing everyone does is make the inlets symmetric. Really they should be shaped and angled differently to account for prop flow. In high speed level cruise the prop effect is smaller, but minor shape optimizing could be done to the inlets left to right. However people like to see symmetry, if for no other reason aesthetics.
Soft seals against the cowl is marginal, as discoverd by Ref [1]. Some make nice decent seals, but over time it usually degrades. The more air you have leaking from the high pressure area above the cylinders to the low pressure area below the cylinders (with out going through the cylinders), is in a word, wasteful. The more you reduce the differential pressure the less cooling you get, a lose-lose situation.
The so called Lopresti speed merchant cowls, Barnard "Holy Cowl" or "Sam James" cowl's are really just the off shoot the basic principles Miley and company at Mississippi State showed in the late 70's. However hats off to all those Gents for making the theory into hardware, espcially for RV's. Not to take away from GAMI's video or research, but giving credit where credit is due, Miley and company where first.
A crude picture I made showing the pressure distribution from the prop the
further you go from the spinner and general flow of stock RV and modified
cowl with widely spaced round inlets, sealed to a less leak prone plenum.
