Air box exhaust (Air Maze type)

[7ACDriver]

Just installed this new aeronca air box from Univair (U4-601-1)and it just does look right ...Most champs have a pipe from underneath the air box and sticks out of the cowling. Did I order the wrong airbox or is there an additionnal part to be installed?

Otherwise the installation went well and the airbox seemed to be of a better quality than my previous one.

[Tim Juhl]

The picture in the Univair catalog of PN U4-601-1 shows the exhaust off the bottom of the airbox. I think I would pose the question to them.

Tim

[Holcombe]

There is an exhaust pipe down although mine has never had one installed. There is a hole in the coweling for it.
I can't see what difference it could make except possibly as a safer place to get rid of raw gas if you really over prime. Three full pumps on my primer and I sometimes see a little raw gas coming out the bottom of the air cleaner. May be more inside. With a backfire to ignite that it could get ugly.
Don't overprime, and don't worry.

[Tim Juhl]

I am not an engineer or an A&P mechanic but I won't let that stop me from sharing my opinion on your situation

My take on the original airbox design is that the hole in the bottom serves as an escape route for the hot carb heat air that is directed into the airbox continuously while flying. While it is true that dumping the hot air into into the cowling is probably fine, extending a tube from the flange through the cowling hole into the slipstream may create a little suction to help draw the heat out.

If your airbox lacks an exhaust port, I guess I wonder where the hot air goes when you're not using carb heat. I suspect some may leak around the flapper and be drawn into the carb while in flight. This warmer air could A.) reduce engine performance B.) Under certain conditions of temperature and humidity promote carb icing.

If it were me, I'd be ordering a flange from AC spruce or the like and riveting it to the underside of the box (cut a hole too!). Get a piece of tubing of the same OD and a piece of hose or scat tubing (and hose clamps) to join the tube to the flange and fabricate your exhaust tube. Be sure the flange is positioned on the airbox to clear the flapper and line up with the hole in the lower cowling.

Tim

[Captgrumps]

Tim
Just in case.

Don't forget to get a one time approval from the aviation God..

But on the other hand I doubt if any could pass the test.

Doug

[David Johnson]

Guys, I always felt the hole and tube were there to funnel gas out of the cowling when the engine was flooded. Otherwise, it would dump inside the cowling and really get a fire going. Trust me, I have experience setting planes on fire! David J.

[Nathan K. Hammond]

I've thought the same thing as David; it keeps the flame of a backfire from igniting fuel/oil inside the cowl.

For what it's worth, my airbox doesn't have the blast tube.

nkh

[7ACDriver]

Don't know who's plane this is but this type of air box exhaust looks pretty neat,

http://s600.photobucket.com/albums/tt90 ... t=5152.jpg

[Tim Juhl]

Not to beat the subject to death, but....

I've had occasion to flood my Champ, particularly back in my hand-propping days. The fuel always dripped out from under the filter assembly. If you look at the airbox with the champ in a three point attitude you will note that the bottom of the airbox angles down towards the filter with the result that any fuel from flooding or a leaking carb will run down the flapper and exit out through or around the filter, not the exhaust tube. As to backfires, the exhaust tube we're describing is located behind the flapper so unless the carb heat is on during startup any backfire will be directed primarily towards the filter.

As far as getting the FAA's blessing to install the flange, since the flange is part of the original Aeronca airbox design it would not require a field approval. Indeed, I would think you'd have a bigger problem if it wasn't there. Installing a new airbox would only require the appropriate logbook entry by an A&P.

As to "why" the tube exists, let me quote from the service manual: "A butterfly valve connected to the carburetor heater control cable directs the airflow to the carburetor, when heat is desired, or through the outlet at the bottom of the air scoop assembly when not in use."

Probably 2/3rds of the Aeroncas I've seen don't have a tube poking out through the lower cowl. I suspect they just became lost sometime over the past 60+ years. They certainly came installed from the factory.

Tim

[flyingfool]

To further "complicate" this subject. The picture of that airplane looks really cool. But I think it is not installed correctly. In fact because of it's angle, I think it will do more harm than good as it appears to be angled such that ram air is forced INTO it. Rather than sucking heat (or anything else like gas) out of it.

I agree. In order to prop my plane, I must prime it until just a few drips of fuel drain out of the bottom front of the carb box. Then 6 blades and it starts on the first blade 95% of the time and 99% of the time with a 2nd blade.

I understand that there are two different types of carb box mountings. The first type, has the bottom of the carb box level to the ground in a three point attitude. And therefore when in level flight, the bottom of the carb air box is sloped towards the filter. This type of box the tube the extends down is suppose to be a 90 degree (straight) cut. With the straight cut and the angle rearward, the air exhaust pipe is pointing slightly backwards which helps create suction to help suck out the hot unused carb heat.

The second type the bottom of the carb box is sloped upward towards the filter in the 3 point attitude. So while in flight the carb box is approximately in a level condition. This type of carb box is supposed to have a air exhaust extension pipe that is cut at an angle. So that the longer edge points to the front. This is so the pipe itself causes the suction help to pull out the hot carb heat air. I'm not sure what the angle is supposed to be. But it probably is on an Aeronca drawing somewhere.

At least that is what I've been told in my attempts to research this topic a bit.

I personally have the first type that is level in the 3 point attitude. I do NOT currently have a pipe. And I personally think the angled cut pipe looks a whole lot better which is not what I'm supposed to install. Getting the pipe installed is on my things to do list. I'd say half if not the majority of the champs/chiefs/taylorcrafat's etc that I've seen do not have this pipe. Not saying it is not correct. Just saying I don't think the mandatory nature of having it installed is well enforced. Maybe it should be???

Just trying to pass on what I've heard.

[Tim Juhl]

Flyingfool,
I am only aware of one design of airbox on early champs and chiefs. The airbox on the fuel injected L16's was different but many of those have since been changed back to carburetors which required a change in airbox as well. That said, some people have been known to adapt a generic airbox like the one used on the Cessna 150 (especially if they went to a Marvel-Schebler carb) which is like the second box you described. The sloped bottom of the Aeronca airbox results in the carb heat exhaust tube being angled backwards. Airflow over the tube helps draw heated air from the airbox. I did put an angled cut on the end of mine for appearance as much as anything.

As others have said, this tube is often missing, probably with negligible effect on the engine's operation. As to whether you could get in trouble for not having it, there are very few FAA inspectors with an intimate knowledge of old aircraft like the Champ or Chief. Unfortunately the same can be said of many younger A&P's. At most you might get the question "what's that extra hole in the cowling for?"

Whether 7ACDriver's new airbox has a tube or not is less important than whether or not it has a hole in the back to vent the heated air when carb heat is not being used.

Tim

[7ACDriver]

I will probably add an exhaust on my new air box, one that is angled with the longer tube in front to avoid getting RAM air...I was thinking to extend a pipe directly from the airbox in a way so that it sticks out of the cowling but If I remember correctly there is only about half an inch of pipe sticking out of the new air box. Not much to grab on plus not really sure I want to start drilling holes in my new 500$ air box! Maybe the scat hose and flange would work out better, although it might be a bit of work getting the cowling on and off with the scat hose since the hose would have to be direct without any bends or curves so fuel doesn't accumulate in there...

[7ACDriver]

Another option might be to weld the pipe on the airbox or would that eventually break off with the vibration?
I thought of attaching the exhaust pipe with a few srews but that might also come undone with the vibs and this might also lead to cracks...

[Tim Juhl]

I had to replace the flange on my airbox because the air buffeting the tube caused the screw holes to elongate and eventually crack. I don't think welding is a solution because if you try welding on aluminum that thin you will probably distort the airbox sheet metal. My choice of using a short piece of scat tubing to join the flange to the exhaust tube is designed to prevent stressing the airbox flange.

Tim

[7ACDriver]

I was working on my primer line that runs to my spider and I overprimed the engine, excess fuel drips to the front of the airbox and comes out under the airmaze filter. So besides air coming out from the bottom of the air box I don't think fuel will??