Ram air and aerocarb musings

[Bryan Cotton]

Hi all,
I have been thinking about why ram air does nor work with aerocarbs. A regular carb with a venturi will have fuel flow proportional, more or less, with airflow. If I understand correctly, an aerocarb does not, fuel flow is controlled by needle position and whatever head pressure there is. So I got to thinking that ram air would impede the flow because it would be trying to push the fuel back up the line. Does this sound right (or wrong) to anybody? If it is right, consider this - we add a super efficient inlet that raises pressure at the inlet by an inch or some amount. What if the vent line was plumbed to this inlet, so that the tank saw the same pressure increase as the inlet of the aero carb? Would it be happy? I am not saying it is a good idea, as this would mean that when fuel dumped out the vent it would go through the carb. But, I am curious.

[radfordc]

Sounds complicated to get to work right? The Aerocarb is meant to be as simple a solution as possible. I haven't touched mine since I started flying the plane in 2006. How much is gained from ram air anyway?

[fastj22]

I think if you used ram air, you would be chasing the mixture based on your airspeed. The faster you would go, IE, after takeoff, you would be leaning out as more air is rammed through the system. Likewise on landing as you slow into the flair, you would be suddenly rich. Maybe too rich for a go around.
For a happy compromise...
I built a cold air induction system that takes fresh air from a NACA duct on the side of the cowl to a box I built around my air filter. I then drilled several 1/2 inch holes in the box. Theory is the majority of the air into the filter will be cold and fresh. If there isn't enough air from the NACA, it will be made up with cowl air. If the NACA provides too much air (ram air) it will simply dump into the cowl and not pressurize the aerocarb.

Here's another musing….
I thought I had my Aerocarb basically dialed in. Yesterday I went flying with an OAT of 25F degrees. I did a normal lean of peak adjustment at run up and ran down the runway. As I lifted off, I noticed my RPMs were a bit low and the engine wasn't as smooth as usual. I turned to stay in the pattern and noticed #6 cylinder was stone cold (EGT and CHT). Ok, time for a quick landing. I pushed full rich and #6 started showing heat and the engine smoothed out. Leaning again and #6 starved out again. I guess it was the air density causing the lean condition. I hadn't flown in that cold of weather yet.

Looks like I need to adjust the needle a turn rich or so for winter flying. I'm running the stock #2 needle. I'm also going to try an air flow straightener after the carb to see if I can keep #6 from going lean before the others. #6 intake has to make the tightest turn and seems to go lean before the others anyways.

[vigilant104]

As folks work on cold air induction and ram air induction, it is good to keep in mind the potential for carb icing. While lack of a venturi may decrease the potential for formation of carb ice, it doesn't eliminate it. There's still plenty of cooling resulting from vaporization of fuel in the carb, and this can be enough to cause the moisture in the induction air to form ice and to gradually block the induction air, maybe at a point downstream of the carb.

We sometimes say that the stock Aerovee/Aerocarb/Aeroinjector setup doesn't have carb heat, but it's probably more proper to think of it as constant, mild carb heat through induction of warmed air in the cowl. Induction of cooler ambient air will undoubtedly result in better engine performance (and ram air will also lead to a more modest increase), but without the ability to add warm air to melt any induction icing I think we should recognize that it is not without risk.
The Revmaster Revflow carburetor (designed very much like the Aerocarb) is available with a controllable bypass valve before the carb. It allows warm, filtered cowl air to be used, or cooler ram air. Here's what that setup looks like (from the Revmaster web site http://revmasterautomotive.com/revmasteraviation/?p=206):
.
It wouldn't be hard to modify this to work with an Aerocarb, or build something similar.

[fastj22]

I assume that's a filter bypass from the red tube directly to the carb. So air coming from the tube doesn't get filtered. That's ok at altitude. Close the valve during takeoff and landing when you really don't want the possibility of carb ice.

I could easily modify my setup to do something similar. I use a K&N RU-1710. Could cut a hole in the top and mount a flange to it.

I'll have to think about that.

[vigilant104]

I assume that's a filter bypass from the red tube directly to the carb. So air coming from the tube doesn't get filtered. That's ok at altitude. Close the valve during takeoff and landing when you really don't want the possibility of carb ice.

Yes, you've got it right. You'd select filtered, warm air for low-altitude (TO and landing) where debris is more likely and there are fewer options if there is induction icing, and select unfiltered outside air for cruise.
I don't know how well this setup would handle induction icing once it starts--without a dedicated heat muff I'd see it more as "prevention" than "cure", but I don't have any objective data to back that up.
And, this would be one more thing to remember to do in flight.

[fastj22]

Just an item on your checklist. We always turned on carb heat during the landing checklist on my C150 and C172s I trained in.
I monitor cowl temps with my EFIS (I had an extra OAT probe I tied to my engine mount). normally it is around 120 F. Yesterday it never got above 65. But OAT was 25.
All I would really need to do to keep filtered air aways provided is to keep my filter box and just provide a valve to the NACA duct. Close it off and it will pull cowl heated air. As long as I have enough holes in the box to provide enough cowl air. It wouldn't be too difficult to fabricate a butterfly valve for the intake.

[dcstrng]

I assume that's a filter bypass from the red tube directly to the carb. So air coming from the tube doesn't get filtered. That's ok at altitude. Close the valve during takeoff and landing...

I’ve looked at that picture a couple of times and like the idea, now I wonder…

The valve notion is elegantly simple, but I wonder; once the valve is open, what prevents the ram-air over-pressure from spilling out the filter (the wrong way), and dissipating the benefit of the ram. It’s probably right in front of me, but I’m not seeing it… :?

[fastj22]

for those of us with the aerocarb, the loss in ram pressure through back flowing the filter would be preferable (Aeroconversions states not to use ram air). And any insufficient flow from the fresh intake would be made up through the filter.

[vigilant104]

Any ram air effect would be very small at any rate (dynamic pressure of about .4 psi at 160 MPH at STP), and pulses from the prop could also play havoc with fuel metering/mixture. I'm fairly sure the performance gain from using cool induction air will swamp any gains from ram air/dynamic pressure.