Snorkel geometry question?

Larry DeCamp · #1 08-03-2019, 10:32 AM

I have followed Dan and Steve on airflow topics but still do not find a post for my dilemma. I would prefer to avoid a large bump on the bottom of my SJ cowl for filter box and legacy snorkel design. My question is, is it a big deal to simply have openings in the front of the cowl that do not protrude like a snorkel? Some entry radius could be incorporated of course. Does "protusion" beyond the cowl surface matter ?

scsmith · #2 08-03-2019, 12:37 PM

Hmmm.

First, you do need to have a sealed passage, so air doesn't come in through the opening and flood into the lower cowl area. That probably goes without saying.

The idea of extending a tube out in front of the cowl surface a short distance is that it captures flow fairly straight on, before the flow turns to follow the sloped surface of the cowl. Depending on how far in front of the cowl surface the inlet extends, you get a fairly high fraction of the available ram pressure rise.

If you have just a hole, with a radius lip feeding into a tube, you will still get some 'ram' pressure, but because of the slope of the cowl and the flow around it, you probably won't develop full stagnation pressure.

At sea level at 170 kts, the pressure rise to stagnation from free-stream static is about 1.4 in" hg. There is probably also a slight additional rise from the propeller velocity addition, but that is pretty small at cruise conditions. If I were to make a WAG, I would say that a flush hole on the forward face of the cowl where an intake tube would go that is the same diameter as the fuel servo might get something like 1/4 --1/3 of the stagnation rise. So something like .3 --.4 in. hg. So you would be giving up about 1" hg. MAP. Again, this is really just a guess. If you make the hole bigger and feed into the servo with a smooth transition, you will get more of the available ram pressure.

There might be an in-between solution that would appeal to you. If you can build up an area that is locally perpendicular to the flow rather than sloping, and blend that into the general lines of the cowl, you might get a shape you are satisfied with, and get something closer to 80% of stagnation, and not have a full tubular "pitot tube" sticking out.
You could also think about a more tailored inlet shape, like the "smile" inlet on a F-1 Rocket or the P-51. That's what I did. Mine does not extend forward of the cowl surface as far as the Rocket, but it does extend forward a little with a lip. The P-51 doesn't have any lip at all along the top edge. With a supercharger, I think you get the manifold pressure you want, whether you start out a little higher or not from the inlet design.

scsmith · #3 08-03-2019, 12:57 PM

The other choice, of course, is to pull induction air from the cooling inlet, with a filter in the cooling ramp and a duct down to the fuel servo. This can achieve very good MAP if the area around the filter has a radiused inlet lip and the duct is well designed. I think Dan Checkoway found that he got MAP that was very close what you would get with a ram intake and a filter.

With a SJ cowl, you would probably want to use a bigger intake ring so you are sure you capture enough air for cooling and induction.

Larry DeCamp · #4 08-03-2019, 05:35 PM

Thanks Steve,
You raise the question of adequate supply for the SJ to feed the intake.
The inlet is 4?dia. The 2/4 side is the only option with enough real estate for the servo air intake supply.
So, does anyone know if a 4?dia cowl hole can supply an 0360 the combustion air, cylinder cooling AND the oil cooler ?