Carb Heat

[trwolf]

I will be installing a Zenith carb on my VW engine, so carb heat is in order. As I approached the build I noticed that the AL flanges were the greatest cost element in the build. Anywhere from $16 to $30 each! Ouch, cause I needed 5 of them. Had to be a better way. I finally settled on some downspout adapters from Home Depot at $1.50 ea, these have a bit of a taper and a pressed lip/rim on one end. They are slightly tapered but long enough to nip off an inch and then slit and fold back the edges to make a pretty symmetrical 2" nipple. Total investment including some 1/16" AL for the diverter was $10.

[mike.smith]

I made mine of aluminum the first time. It didn't last long. The heat and vibration wore fatigued the aluminum in short order and it failed. Made the next one from stainless steel. So keep an eye on on it.

[peter anson]

The spun aluminium flanges from Spruce are from $8.75 to $13.55. I have used them and they look really good and are about 0.060 wall thickness. No failures in 7+ years of flying.

https://www.aircraftspruce.com/catalog/appages/ductflanges.php

Peter

[SonAl]

Is it required, or a good idea, to dump the carb heat air overboard when not in use? The Sonex plans version doesn’t show the carb heat air going anywhere when not used.

Should the carb heat air always be flowing through the muff/scat tubing when not being used?

Thanks.

[peter anson]

Not sure what arrangement you have for your hot air/cool air valve but on mine the valve opens one inlet and closes off the other so when you are in the normal cool air mode the hot air inlet is blocked off. No air flows through scat tube, which means the hot air from around that exhaust pipe just passes out the cooling system outlet like all the other hot air.

Peter

[SonAl]

I don’t have a system yet, still in the building phase (wings right now). I’m in Canada so am required to have carb heat with the AeroInjector. I was planning to follow the stock air box on the plans, but then I read somewhere about having an outlet for the carb heat air (or cabin heat air) so it could always ‘flow’ when not in use. Apparently to opinion was the heat buildup in the muff around to exhaust pipe would be too much otherwise.

Also, is there any reason that you (or anyone) knows of for carb heat bypassing the air filter with the plans arrangement? Most builders sites seem to show this as well.

BTW, looking forward to installing your cockpit vents, when I finally get there. Would any of the other NACA vents you have be suitable to supply fresh air to the engine filter?

Thanks,
Al Arbuckle
Sonex (now B) 617

[peter anson]

Hi Al, I have had two types of heat muff on my Sonex. The first was a fairly conventional heat muff but a while back I fitted a pair of aluminium absorption mufflers and was a bit tight for space so just made the front part of one of the muffler tubes into the heat muff. I haven't had any problem with parts for either of them overheating.

My previous aircraft had both carb heat and cabin heat from heat muffs and the scat tubes seemed to tolerate the heat OK.

The heated air bypassing the air filter is an odd one. It seems to be a common practice but I don't know the reason. My Sonex is fitted with a Jabiru filter box and the heated air goes through the filter on that.

The 3D printed vents would definitely NOT be suitable but I have a larger vacuum formed ABS vent which might be suitable although at this time I don't have a suitable outlet for it. I was a bit skeptical but at 100 knots it should flow about 55 litres of air per second which is roughly what a 2.2 litre engine should draw at 3000 rpm. It's still one of those things where you probably won't be able to tell if it has a good, bad or no effect.

Peter

[inventor]

That is a good looking muffler. Will you tell where you got it?

[GraemeSmith]

The heated air bypassing the air filter is an odd one. It seems to be a common practice but I don't know the reason.

I believe it is because in high humidity situations where ice might form on the carburetor - then if moist enough (or raining) at low temperatures - ice might form on and close off the air filter. I've seen pictures of iced up aircraft where this was definitely the case.

For the carburetor planes - the Carb Heat bypassing the air filter saves the day.

On fuel injected planes I've flown - there is a sprung flap in the air induction inside the cowl - if the air filter ices over - then the induction vacuum opens the lightly sprung flap allowing cowling air into the system to keep the engine running.

--

In either event - the air is unfiltered and use of Carb Heat while on the ground should be minimal, consistent with conditions or testing function so as not to draw dusty air into the engine.

[peter anson]

I believe it is because in high humidity situations where ice might form on the carburetor - then if moist enough (or raining) at low temperatures - ice might form on and close off the air filter. I've seen pictures of iced up aircraft where this was definitely the case.

Sounds like the correct explanation Graeme. On the Jabiru filter box, the hot air going through the filter would presumably melt any ice forming but it wouldn't have the instant effect of bypassing the filter completely. Incidentally, I think I have only ever experienced carburetor icing once and it was at cruise, not particularly cold but very humid; we were dodging rain squalls. The symptom was an odd periodic misfire accompanied by a thump that could be felt through the floor. I opened the throttle and turned back to the departure airfield and the problem disappeared. We turned back to the destination and had no re-occurrence. My guess is that the misfires and accompanying bangs were caused by lumps of ice going through the engine and that opening the throttle was enough to clear the problem. Next time I did the same trip my passenger opted for a 2 1/2 hour drive rather than a 45 minute flight.

That is a good looking muffler. Will you tell where you got it?

Home made:

Don't feel envious. They don't seem to have much effect other than adding a bit of maintenance time.

Peter