RE: Do I need a velocity stack?
 

No, you indeed did not state other that what you used. Having the cut forward prevents many problems with stacks out of the cowl.
I just commented on the ram effect,

RE: Do I need a velocity stack?
 

Well done Grosbeak

RE: Do I need a velocity stack?
 

Tough crowd to please, isn't it? ;) [:o]

RE: Do I need a velocity stack?
 

:DYes it is, but it nice to learn and see what other people have done/tried and believe in.

I think Pe, Antique, you (Truckracer) and some others are very knowledgeable on our gas engines.:D

RE: Do I need a velocity stack?
 

Over the years I have run gas engines, mostly Zenoah 23s and 38s. I found that in most cases I had to add some type of velocity stack on the carburetor. Also I soldered a brass tube to run tubing into the fuselage. So when I finished my 1/4 scale Balsa USA Fokker D-VII and run it up for the first time, it didn't produce full power with the cowl on. I found by placing my finger behind the carburetor. the power would smooth out and increase. I knew stack was needed and a tube into the fuselage would help. Also I decided to close the hole in the choke. I made a stack out of sheet brass shaped to force air into the carburetor. First I ran the engine without the cowl-good power- put the cowl on ran engine same problem low power. Then I began trimming the shape of the stack reducing the height. Trial and error, trimming, running the engine, the engine began to run at full power with the cowl on. The D-VII flew very well, the Zenoah 38 pulled the 24 lb. airplane plenty of power. A mishap caused the plane to crash, but I built another Fokker D-VII and installed the same carburetor on another Zenoah 38 and the new D-VII is flying great. Velocity stacks do help if the carburetor is out side the fuselage or gets direct air over the carburetor opening(as you can in the last picture). But you may have to experiment a little to get satisfactory results. On cowled engines, the tube soldered to the hole of the carburetor plate and fuel tubing run into the fuselage does wonders for the power run. Here are some picture of the fix on the Fokker D VII.


Rich

RE: Do I need a velocity stack?
 

I don't think the engines need a really nice fancy looking tapered intake stack. The simple flanged tube works really well.  The main thing is to help protect the main jet on the Walbro carbs from the air flowing past it when the plane is in the air. With the main jet located about 1/4 inch down from the top of the carb, air flow past it at flying speed can cause all sorts of weird problems to occur.

Here is a simple intake stack on my SV-17cc engine as shown on my Escapade 60 plane.
http://i248.photobucket.com/albums/g..._engine_04.jpg

Now this example of a Walbro carb on this little NGH engine was a good demonstration of the air pressure causing the fuel pressure regulator to stop feeding fuel to the engine. Note the spinner and its close proximity to the carb regulator cover. If I took the airplane up high and did a big long power dive, right as I leveled the plane out at the bottom of the dive, the engine would just quit.  I surmised that a pressure zone built up there in front of the carb, behind the spinner, causing the regulator to cut off the fuel flow.  Putting on a tube onto the regulator cover and routing a tube back into the fuselage solved the issue.
http://i248.photobucket.com/albums/g...irplane_02.jpg
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RE: Do I need a velocity stack?
 

I have also found that, in most cases, you do not want a beveled stack to face into the airstream; i.e. no forced input to the stack. Usually works best if the bevel faces aft but, if in cowled area, experiment for best position as airflow within the cowl can be tricky.