Smooth bottom cowl system???

[Bill Dicus]

Disclaimer 1st! I'm not a engineer and know little about flow dynamics. The Van's snorkel has a large increase in x-sectional area just before narrowing to the throttle body entry. It always seemed paradoxical to me that pressure is low in a constricted area such as that (and flow velocity high). Downstream from the narrow area the pressure goes up and flow velocity down. Might this yield an increase in MP? As long as flow rate remains high enough, that sounds good. Van considered the snorkel lost minimal MP compared to the standard scoop intake but was a little cleaner, so about a wash overall. Anybody knowledgeable please comment/correct. Bill

[Bill Dicus]

Disclaimer 1st! I'm not a engineer and know little about flow dynamics. The Van's snorkel has a large increase in x-sectional area just before narrowing to the throttle body entry. It always seemed paradoxical to me that pressure is low in a constricted area such as that (and flow velocity high). Downstream from the narrow area the pressure goes up and flow velocity down. Might this yield an increase in MP? As long as flow rate remains high enough, that sounds good. Van considered the snorkel lost minimal MP compared to the standard scoop intake but was a little cleaner, so about a wash overall. His design surely does not have a linear decrease in cross-sectional area. Anybody knowledgeable please comment/correct. Bill

[David-aviator]

Originally Posted by DanH

Dan C's MP data is interesting. Theory says a good snorkel from the cooling air intake should get pretty close to the same MP as a straight front intake. The theory assumes good intake pressure recovery, a correctly sized filter, and no adverse twist and bends.

Dan's "snorkel" looks like it should work better than Van's. As I recall the stock fiberglass duct has a pretty sharp radius right before the throttle body. Dan used an alternate air duct from AFP which brings air to the front of the TB at less of an acute angle.

If you crank K&N's equation for filter size, the filter supplied with the Van's snorkel is undersized. Still, the pressure drop may not be significant.

I'm near enough to building a combustion air intake to be thinking about this stuff. At the moment (ha!) I'm inclined to take air from the left cooling intake, pipe it to a filter can in the lower left front cowl, and turn it into the AFP FM-200 with a 3" radius silicone hose. The filter can would accommodate a conical filter with more area than the standard flat filter, and would allow a water drain. The silicone hose would require a bump in the lower cowl; there's just not much room to work with in front of AFP's longer TB.

Ah, but there's one caveat with his MP numbers. His "y" has one butter fly valve. When the ram valve is opened, air flow is direct to the FM200 but also flows back up the plenum to the filter via the "y" connection. This would make it almost impossible to see much increase in manifold pressure and explains why those with the 2 butter fly set up see a jump in MP. Air flow is blocked to the filter.

When I talked with Don at AFP about buying this device, I did not know it came with 1 or 2 valves. Don said the one valve system will work and not only that, the filter will get cleaned by reverse air flow when the ram valve is open. I opted for the 2 valve deal and that's why there is a jump in MP when the ram air knob is pulled.

[DanH]

<<I opted for the 2 valve deal and that's why there is a jump in MP when the ram air knob is pulled.>>

(breathless with anticipation he asks <g>): How much does it jump?

[David-aviator]

Originally Posted by DanH

<<I opted for the 2 valve deal and that's why there is a jump in MP when the ram air knob is pulled.>>

(breathless with anticipation he asks <g>): How much does it jump?

It was about 1.5". The rpm went up from 2250 to 2400, I could hear and feel it.

(Sure will be glad when this winter is history. Several days ago we had 4" of snow on the runway, then it melted and there was mud, today it was frozen hard as a rock and rain is in the forecast this week end. It will be days until the frost is gone and things dry out. That is the down side of living with a grass runway.)

[DanH]

<<It was about 1.5". The rpm went up from 2250 to 2400, I could hear and feel it.>>

Holy cow. Is your filtered air like Dan C's (from the left cooling air inlet) or just from inside the lower cowl?

[David-aviator]

Originally Posted by DanH

<<It was about 1.5". The rpm went up from 2250 to 2400, I could hear and feel it.>>

Holy cow. Is your filtered air like Dan C's (from the left cooling air inlet) or just from inside the lower cowl?

It was from inside the cowl and too small. A second version uses an intake from the left baffle area and is better but needs improving. I did not read about the Dan C install until just a few days ago. I am finding out lots of other guys have done it similar and it works but each is a little different.

None of this is rocket science. Van has been down this road and what is in the plan is good stuff all based on Lycoming, Precision Air or a carb, and their FAB. The problem is it was not designed around AFP and some adjustments have to me made to use this very good system. I came away from OSH last year thinking the ram air inlet for AFP was a good idea and that's what's going on here. In a brief discussion, Van himself seemed to agree . They know what is offered in the kit will not fit that system.

[f1rocket]

I'm curious because I am considering a ram air system. When do you pull the lever for ram air? I assume it is when you are out of the "bug" zone? What is the logic for when you go to ram?

In considering a ram system, I've made the assumption that you want to open it up once you are reasonably safe from ingesting bugs. However, I've still seen bugs as high as 5-7K in the summer in the Midwest. If I fly at lower altitudes typically, I'm afraid I would never be able to open it. I must be missing something.

[David-aviator]

Originally Posted by f1rocket

I'm curious because I am considering a ram air system. When do you pull the lever for ram air? I assume it is when you are out of the "bug" zone? What is the logic for when you go to ram?

In considering a ram system, I've made the assumption that you want to open it up once you are reasonably safe from ingesting bugs. However, I've still seen bugs as high as 5-7K in the summer in the Midwest. If I fly at lower altitudes typically, I'm afraid I would never be able to open it. I must be missing something.

All very good questions and I do not have the answers. In fact I will add one more to the list - could a big fat high flying bug shut down the engine if it got into the intake of the FM200? From the AFP manual on the air section of the controller "A measure of the air flow consumption of the engine is accomplished by sensing inlet air pressure and venture throat pressure in the throttle body." Could these sensors be jammed by an ingested bug? Maybe Don Rivera could shed some light on the issue if he is tuned in.

I read Dan C's report on ram air one more time.
http://www.rvproject.com/20050426.html
He has proven it may not be worth the weight and complexity if one can get the air flow issues of filtered air worked out. He calculates ram air boosts MP .3". The MP comparisons at 5500' with 3 other airplanes seemed to confirm it. His filtered air MP was as good as ram air with his flying friends.

Obviously there is considerable pressure at the baffle inlet that makes this possible and it appears he achieved it even with the use of scat tubing. This may be why Van decided ram air was not worth it and leads one to believe the FAB unit with its smooth interior is a very good way to go with a Precision fuel controller. But it does not resolve the issue of doing it with AFP. Each pilot is coming up with his solution and until some vendor comes up with a dedicated AFP FAB, they will all be a little different.

With regard to opening the ram valve, I've been doing it at about 2000' AGL. The Dan C. report does address this issue as there is considerable dust to contend with in Southern California.

[Bill Wightman]

FWIW:

The "ram rise" we speak of here refers to the dynamic pressure imparted to the flow due to the speed of the airplane. This is simple to calculate:

Pressure = 1/2 * air density * velocity^2

At 8000 MSL, and standard conditions atmospheric air density is .0018678 slugs/ft^3.

Lets calculate the dynamic pressure (Q) for 200 mph, which is 293 ft/sec.

These conditions yield a theoretical maximum ram pressure of 80.17 lbf/ft^2 or .557 lbf/in^2.

My RV4 would get about a 1/2" rise in MAP when I switched from the alternate source in the lower cowl area to the ram source at the upper rear baffle. At my normal cruise speed/altitude, I found that I was getting 80-85% of the theoretical max ram recovery. Not too bad.