From: enns@waterloo.hp.com (Roger Enns)
Newsgroups: rec.aviation.homebuilt
Subject: Re: VW engine conversion (by HAPI) & a few questions
Date: 16 Oct 92 17:03:52 GMT
Article-I.D.: waterloo.Bw84qH.6IM
Organization: HP Panacom Div Waterloo ON Canada
X-Newsreader: TIN [version 1.1.4 PL6]

thomas.e.lester (tel@cbnews.cb.att.com) wrote:

: >You can use any of a number of carbs on your engine.  Two basic
: >varieties are the throttle body injector type, or constant
: >velocity carb, and the regular venturi type carb.  If your intake
: >is set up as per std. VW aero practise, with the carb below the
: >engine, then carb ice is a very real concern.  The intake and
: >carb can collect ice very easily, so be sure to set up a very
: >ample source of carb heat.
: 
:     Probably good advice, but most VW powered craft I know of have no
:     carb heat and do alright.  I only have a few inches of exhaust in the
:     cowl and so would be hard pressed to add carb heat.  Also, no air filter.
:     (using a modified POSA)

I wouldn't fly any aircraft without carb heat where I live.  Carb
ice is a very real concern in my climate.  I had carb ice so bad
that I couldn't restart the engine for a good ten minutes until
the ice melted from the heat of the engine.  This was during
taxi-tests with the Tillotson carb.  The ice formed so fast, that
unless carb heat was applied at the instant ice was detected, it
would be too late!!  A switch to a CV type carb (Ellison) has
alleviated this problem.  Many people say that carb heat is not
needed with a CV carb, but the Ellison manual very emphatically
disagrees with this opinion.  Whether they are just covering
their collective buts, or they have experienced icing, I do not
know.  A couple of weeks back at about 3000ft ASL, in near freezing
temps I noticed a slight drop in rpms.  After carb heat, the
problem was gone, but reoccurred once a couple of minutes later.
No problems for the rest of the flight.  I cannot say definitely
that this was carb ice, but I sure do like to have that 'carb
heat' lever to pull on if I suspect anything wierd.
slight drop in rpm 


: 
: >Using a CV carb will minimize carb icing problems, but not
: >eliminate them (I had an incident of carb ice on my Ellison 2
: >weeks ago).  If you are flying in areas where carb ice is not
: >generally a concern (Arizona, for instance), a venturi carb would
: >do the trick for you.  If you plan to fly in areas prone to carb
: >ice, then I strongly recommend a CV type carb, as a safety
: >precaution only.
: >
: >CV carbs for VW's come in two major types: 1) POSA and variants,
: >2) Ellison TBI.
: >They both operate on the same principle, with
: >the Ellison being the 'Cadillac' IMHO, and the POSA being a
: >somewhat crude but workable alternative.  You could probably get
: >a used POSA for 50-100 bucks, Ellison new is ~$400.         
: >
: >
: >Fuel pressure requirements vary from carb to carb.  I believe the
: >POSA requires a fuel pump and regulator.  I am running an Ellison
: 
: 
:     No.  The POSA is a gravity feed system and floods out under
:     the pressures of a fuel pump.

Why would it flood out if you have the pressure regulated to
approx. same pressure as head pressure from full tank?
I have heard of problems with POSA in regards to the mixture
leaning itself out as the fuel level in the tank (and hence fuel
pressure) decreases.  Many people run a small FACET type fuel
pump, and regulate pressure to .5 - .75 psi range.  This ensures
consistent fuel metering regardless of tank level.

:     If anyone can tell me how to add a fuel pump to a POSA, I would
:     like to do so for inverted flight possibilities.

Feel free to add a fuel pump+regulator to your aircraft if you
notice the above-mentioned problems, but to fly inverted is not
recommended IMHO.
Unless you have some type of dry-sump system for your VW, the
oil pressure will approach zero in seconds.  But even before that
becomes a problem, the oil that spills in around the piston
bottoms will be forced past the rings in huge quantities, foil
the plugs, and the engine will stop cold.  There was a warning to
this effect in Sport Aviation about ten years ago, when VW
powered aircraft were at their peak.

: >(filter changes, gascolator checks, etc).  As long as the
: >compression is set to some wildly high value, you should be able
: 
: 	I think you meant "NOT set to wildly high" ??

My english teacher always told me to proof read ... :-)

: >to keep the prop windmilling without trouble.  A friend did some
: >power-out glide tests in his Dragonfly, and said that the prop
: >stops at about 80mph, but it takes 140mph to get it spinning
: >again!
: 
:     I have not yet had the courage to test the restart capability,
:     but I did do a dead stick landing on second flight when my 
:     throttle cable broke   8-{  .

I bet that was scary, no go-around option available that time!

I have not yet had the pleasure of dead stick landing our DF.
The above-mentioned Dragonflyer was doing glide tests at various
airspeeds.  He climbed to more than 8000 ft before shutting down
the engine.  He graphed the results in our newsletter.  Judging
by the number of data points, he must has spent days repeating
this test varying the airspeed and gross wt!  He said the engine
always started right up when he turned the mag back on.

:     And I can tell you the prop came to a very decisive stop with no
:     hint of even wanting to windmill at 90 mph indicated.  Of course this
:     is a new 2180 cc engine and is still very tight.
:     8 to 1 compression.
: >
: >Also, Great Plains is another VW conversion company, sorry no
: >address on hand for this one.
: 
: 	Great plains aircraft supply Steve Bennett 708-464-4178
: 	PO box 304 St Charles Ill.  60174
: 
: 	-----------------------------------------------------------------
: 	Tom Lester				att!cblph!tel
: 	Sonerai IIL (under TEST)		(614) 860-3259
: 	N4LX 					(614) 927-8106

Roger Enns (enns@waterloo.hp.com) Panacom Automation Division, HP
C-GIIV Dragonfly, 1979 RX7 13B, 1985 Colt