October 2015 Questions...

Has the corvair had issues concerning carburetor icing and what are your recommendations ?

Carburetor icing occurs when the temperature inside the carburetor drops to a point at which the water vapor in the airflow freezes and attaches to the throttle plate and reduces the air flowing into the intake, resulting in a loss of power.  This condition is usually noticed when a reduction in power has occured from a cruise configuration (in aircraft) to a decent configuration.  However, this is not always the case.  I have had incidences where it occured in full power flight as well.  The conditions just have to be perfect.    That being said I have never experienced carb icing or loss of power due to icing in any of my engines that are currently set up with the Aero-Injector carb from Sonex or expect any on my new carb design.  I will go through my thoughts and  recomendations on carb setups below...

    MA3 Series Carburetors...  A good, aviation style carb (actually tractors use as well) that has been around a long time.  Very prone to icing due to the fact the throttle plate is below the venturi and fuel spray.  The throttle plate provides a place for ice to attach and form.  A carb heat box is absolutely necessary with this style of carburetor.  Issues :  in the corvair configuration it has been necessary at times to send the carb in to have it rejetted for individual applications because the corvair does not draw air the same as an O-200 does.  Usually I have seen that it needs to be jetted richer.  The cost of the carb is typically around the 900.00 mark plus any rebuild changes.  The bolt-on flange is standard.  The carb can only be located in one orientation and uses up a large amount of room in the cowling for controls and carb heat boxes.  Performance on the carb once set up is reliable and efficient.  The A series has a accelerator pump (not necessary for safe orperation). 

    Ellison Injector (2 - 3A) Series....This aviation focused slide body carb does not have a throttle plate in the original sense.  It has a sliding "door" that exposes more of the intake as it uncovers more of the fuel tube or rail.  The fuel rail (as I call it) is semi fixed and has a series of holes drilled in it that are drilled in a progressive manner to allow more fuel as the slide opens.  The tube or rail can be rotated by cockpit control as a mixture adjustment devise exposing the holes to a lower pressure - drawing more fuel.   It does not have a direct idle-cut off but relies on a diaphram valve to shut the fuel off when the low pressure in the venturi area is gone.   This type carb does have a slight venturi on it and I can see occasion where ice may be able to form on the spray bar but I believe the occasion to be rare.  Ellison does recommend carb heat.  Issues:  The installation allows for differing orientation but can be somewhat complicated because of the design of the control connections and lack of support for control cables.  Brackets have to be made to secure the controls adequately for proper use.  The carb is not cheap @800 last look.. Our issues have mostly dealt with vapor lock in our installations.  The diaphram will heat soak and cause bad idle and engine shut downs if not dealt with properly.   A fuel pump can help reduce vapor lock to a point since the diaphram can regulate fuel pressure.   Our best advise is to run cool filtered air into the carb from outside and we also run a blast tube the surrounds the fule line and ends up at the gascolator to cool that as well.   Once you overcome the vapor lock issues it seems to work well.
    ROTEC 40mm... Actually very similar to the Ellison in installation and use.  Tends to be set a little on the rich side.  Not very adjustable but useable.

    Aero-Injector 35mm....An aviation focused slide body carburetor from Sonex.   It also has a sliding door to open the intake but draws fuel in a very different way than the Ellison.  The Ellison relies on the low pressure around the fuel rail to draw fuel out of the holes in the rail as well as fuel pressure on the system.  The Aerocarb uses air flow around a tapered needle to draw fuel from a hole that changes in size as the needle moves in and out.  Fuel always exits out of the same place no matter the position of the slide.   The fuel leaves the hole and is instantly in the intake with nothing in its way until it reaches the head valves.  If icing were to occur there isnt anything to attach to.  A larger orifice for fuel to exit means the carb is less prone to having a hole plug up due to debris or icing.  The Aerocarb also does not have a venturi in the normal sense.  Issues:  The Aerocarb requires some time to set up and dial in but once set up in pretty much hands off and very reliable.  One of the main complaints over many years was a "sticky" throttle control.  This is due to vacuum differencial when at partial throttle that holds the "door" against the throttle body.  However, if you are patient and let the pressures normalize on both sides of the door, it slides easily again.   Pilots would try to jam the controls and could bend the contro cables.  Sonex redesigned the carb to reverse the controls (now the aeroinjector) to alleviate this issue but it now requires a control reverser.  The company does recommend carb heat but does state that it is carb ice resistant.  I like that is it simple to install and the aerocarb used standard controls from Aircraft Spruce.  It does not like higher pressure fuel pumps but works well with gravity feed systems.  Cost is around $450 (new). 

    Recommendations:   First, know your system - regardless of what you install.  Know its benefits and problems.  Understand its function, use, maintenance and care.  Be informed in order to use it properly.  
EXAMPLE:   We had a customer that had an Aerocarb on his Zenith 701 that complained about setting up and dialing it in.  It took him about 5 engine hours to understand it and get it working properly for his flying.  When he finished his Zenith 650 he chose to install a MA3 thinking it would just bolt-on and work perfectly.   He spent a lot of time finding out that the carb was set too lean - shipping it to a carb guy - having it rejetted - reinstalling and more testing.  He said, "If I had had the Aerocarb I could have just adjusted the mixture here and not had so much down time.."  Lesson learned.  Not all things are plug and play.  Know your system.

In the majority of our aircraft we have been using the Aerocarb/Aeroinjector.  We use a spigot mount (simple), two controls from Aircraft spruce, 3/8 fuel lines with firesleeve, gravity feed, and a simple K&N airfilter. No carb heat.  We also pull all our intake air directly from the bottom cowl.   I have used this installation since 2003 in most of my aircraft and found the engines easy to start, no primer needed, simple to install, no carb icing, no fuel pump and easy to clean.   Our developmental objective is to focus on simplicity and reliability.  This carb has the least number of parts and creates the least problems.   We will post more about our carb installations in the near future.


    Pictured above is the installation in our W8/10 Wittman Tailwind.  Notice the simplicity of the installation.  Currently at 100 hrs in this airframe.

    Moving Forward.....We are currently developing our own carburetor for our engine installations due to the fact that the Aeroinjector only comes in one size plus we wanted to have a carburetor that was as easy to install as possible.   One of our biggest concerns has been the installation errors and control problems.  Our new design will improve this as well as give us options for sizing and turbocharging.  We also want a carb that is on the shelf for us all the time.  Look for more information in the next few months.