Cowl Inlet Ramps - needed?

[rvbuilder2002]

Originally Posted by Veetail88

Don't air plenums restrict the intake in the same way? Maybe more so? Just wonderin......

They can, but they don't have too! It all depends on the design.
I know of RV's that cooled better once the builder designed plenum was removed and switch to traditional baffling.

[Kyle Boatright]

Originally Posted by Veetail88

With all due respect to Dan, who is obviously one of the smarter guys on this site, I'm not sure I'm buying into that whole shorter nostril trimming makes the airflow more efficient thing. On it's face, sure, looks good, but isn't the intake volume pretty much dependent on the size and efficiency of the cowl outlets? Don't air plenums restrict the intake in the same way? Maybe more so? Just wonderin......

You're mixing issues. A plenum, by itself, does not restrict airflow. It hopefully reduces or eliminates the air leakage which occurs around typical baffle installations.

That said, IF your plenum eliminates wasting of (say) 10% of your cooling air compared to stock baffles, you can make the inlets and outlet 10% smaller without losing cooling capacity, and will have a lower drag cooling setup.

[rvsxer]

[/quote]

What kind of clearance between the #1 cylinder and the inlet ramp (at the "pinch" point, measuring from the cylinder head or barrel) is good to have? What do some of you have on yours? My #1 and #3 both run a little hotter and I don't have that much space there.

[rightrudder]

Hey Mike,
A lot of folks suggest that you should be able to get your fist between the leading cylinder and inlet ramp. In stock form, the left side ramp was just about perfect, but I cut and re-glassed the right side (left in picture), moving it forward to approximately match the cylinder stagger.

Temps are quite even now in cruise, though #1 is a little hot still in climb. I've cut the air dam in front of it a little more, and will see if that helps on my next flight.

[brad walton]

As a data point, the ramp to #1 cylinder space on mine is about a fist. The ramp to #2 cylinder space is several inches larger. #1 and 3 cht's are evenly cooler than #2 and 4 cht temps.which are evenly hotter by about 20 degrees. In retrospect, I probably should have installed the ramps to better reflect the stagger of the cylinders.

[Doug]

Originally Posted by Mel

... the incoming air becomes turbulent and will not cool the engine properly.

I have always wondered about this assumption since I read an article where John Thorp was quoted as saying "The optimum airflow over the engine is opposite to what you want over the outside of the airplane. You want it as turbulent as you can make it so you don't have any stagnation. What you want is the air to mix to give maximum heat exchange."

It is certainly counter-intuitive and I wonder if there is any science to support either stance. NACA papers perhaps?

How I remember this stuff I don't know - the interview was published in the winter edition of "Air Racing" 1973.

[rzbill]

Originally Posted by Doug

I have always wondered about this assumption since I read an article where John Thorp was quoted as saying "The optimum airflow over the engine is opposite to what you want over the outside of the airplane. You want it as turbulent as you can make it so you don't have any stagnation. What you want is the air to mix to give maximum heat exchange."

It is certainly counter-intuitive and I wonder if there is any science to support either stance. NACA papers perhaps?

How I remember this stuff I don't know - the interview was published in the winter edition of "Air Racing" 1973.

Thorp is correct when reffering to the air in immediate proximity to the cylinder fins. Between the fins I mean. Turbulent flow is better for local heat transfer. Same could be said for the oil inside the oil cooler. Turbulent is better.

HOWEVER, this should NOT be construed as "turbulent is better at the cowl entrance". Totally different physical process at the inlet. Laminar is better for turning velocity into pressure.

[ILikePike]

IF not installing all Vans parts is bad I guess I am in trouble. I didn't install the little fillets in the front outside corners of the baffle. The seals on the front of the cowl fill this area very well. I found that it would make the air filter removal on the left side difficult. I may put a finger sized fillet of RTV in the corners just to fill in the last little bit. Anybody else leave them off?

[rzbill]

Originally Posted by ILikePike

IF not installing all Vans parts is bad I guess I am in trouble. I didn't install the little fillets in the front outside corners of the baffle. The seals on the front of the cowl fill this area very well. I found that it would make the air filter removal on the left side difficult. I may put a finger sized fillet of RTV in the corners just to fill in the last little bit. Anybody else leave them off?

You talking about here on a horizontal injection cowl Jeff?

I don't think the plans call for it next to the filter. Its a bit fiddly to get one mounted. I was not sure this "mod" would stick. It has.
FYI the upside down flush rivet does not go through the filter mount. It only goes through the "L" bracket directly below so the side panel is loose when the 3 screws are removed for filter maintenance.

[rvsxer]

Originally Posted by rightrudder

Hey Mike,
A lot of folks suggest that you should be able to get your fist between the leading cylinder and inlet ramp. In stock form, the left side ramp was just about perfect, but I cut and re-glassed the right side (left in picture), moving it forward to approximately match the cylinder stagger.

Temps are quite even now in cruise, though #1 is a little hot still in climb. I've cut the air dam in front of it a little more, and will see if that helps on my next flight.

Thanks, Doug. Good picture. My RH ramp is about the same size as your old one. I'll probably open that up as you did.