Debra
12-21-2007, 04:20 PM
Greetings Clarence,
I have been flying R/C since 1983 and have enjoyed your columns for
years. Other than one abortive attempt in the early 1960's with a Cox P-40 I
have no control line experience. A couple of months ago a club member
brought out an old Jr. Flightstreak and I helped him get it going. Now I have
been bitten by the Control Line Bug. I have a Super Clown under construction and rather than buy a C/L engine or wire open an R/C throttle I want to convert an R/C engine to CL. I have made a venturi for an old ASP .25 and low and behold it works, but I am wondering about if I did it right. I used the max carb opening for the ASP as a size for the venturi and I used an old Nelson pylon .40 needle valve and enlarged the carb screw holes on the case of the engine so the needle valve holds the venturi in place. The case shifted while drilling out the holes (not clamped tight enough) so the needle is off center somewhat. I also have an old Enya 40 I want to try next. I plan to leave the engine case alone and mount the needle through the venturi. The old Pylon engines I have with a venturi have the needle behind the intake hole and a small hole entering the venturi. Is that a better
arrangement than through the center of the opening?
Are there any parameters or rules of thumb such as length, venturi
opening size, needle valve placement for engine displacement? I have a bench top lathe and mill and I don't consider myself a machinist, just a chip
maker but I enjoy the challenge of making things and see them work. Any
comments will be greatly appreciated.
Dave B.
Dear Mr. B,
For control-line stunt and pattern flying it is best to use a standard spray bar type needlevalve assembly. Whether it is centered or not will not make any difference. The jet type needle assemblies are intended for racing purposes and pressure feed and do not depend on venturi vacuum to draw the fuel. Using the same intake diameter as the R/C carburetor was correct.
The venturi sizes used for R/C are generally a little smaller than for control-line due to the use of larger fuel tanks and flight runs. There are no set parameters for venturi length. dia. etc., as too many variables are involved. Some engines have better fuel draw ability than others, i.e., whether muffler pressure is to be used, etc. Manufacturers usually start with an intake dia. that is usually used for their particular displacement size, flight test, and go from there, i.e., if the engine sags in maneuvers, close the venturi down. If it holds it's setting the whole tank run, try opening it up slightly.
Hope this helps.
Clarence Lee
I have been flying R/C since 1983 and have enjoyed your columns for
years. Other than one abortive attempt in the early 1960's with a Cox P-40 I
have no control line experience. A couple of months ago a club member
brought out an old Jr. Flightstreak and I helped him get it going. Now I have
been bitten by the Control Line Bug. I have a Super Clown under construction and rather than buy a C/L engine or wire open an R/C throttle I want to convert an R/C engine to CL. I have made a venturi for an old ASP .25 and low and behold it works, but I am wondering about if I did it right. I used the max carb opening for the ASP as a size for the venturi and I used an old Nelson pylon .40 needle valve and enlarged the carb screw holes on the case of the engine so the needle valve holds the venturi in place. The case shifted while drilling out the holes (not clamped tight enough) so the needle is off center somewhat. I also have an old Enya 40 I want to try next. I plan to leave the engine case alone and mount the needle through the venturi. The old Pylon engines I have with a venturi have the needle behind the intake hole and a small hole entering the venturi. Is that a better
arrangement than through the center of the opening?
Are there any parameters or rules of thumb such as length, venturi
opening size, needle valve placement for engine displacement? I have a bench top lathe and mill and I don't consider myself a machinist, just a chip
maker but I enjoy the challenge of making things and see them work. Any
comments will be greatly appreciated.
Dave B.
Dear Mr. B,
For control-line stunt and pattern flying it is best to use a standard spray bar type needlevalve assembly. Whether it is centered or not will not make any difference. The jet type needle assemblies are intended for racing purposes and pressure feed and do not depend on venturi vacuum to draw the fuel. Using the same intake diameter as the R/C carburetor was correct.
The venturi sizes used for R/C are generally a little smaller than for control-line due to the use of larger fuel tanks and flight runs. There are no set parameters for venturi length. dia. etc., as too many variables are involved. Some engines have better fuel draw ability than others, i.e., whether muffler pressure is to be used, etc. Manufacturers usually start with an intake dia. that is usually used for their particular displacement size, flight test, and go from there, i.e., if the engine sags in maneuvers, close the venturi down. If it holds it's setting the whole tank run, try opening it up slightly.
Hope this helps.
Clarence Lee