Last few weeks I was working to get as best as possible a carburettor inlet tube (velocity stack), that will increase engine power by reducing drag of inbound air, help make better mixture etc. And there it is - velocity stack for my paraglider engine :) Thanks to "pause" option, I can put this aloy handle, and inprint it. Thanks, that he ZUltrat is resistant for petrol/oil (z-abs cause to create some white layer, don't know what it was).
BTW: I wish to have option to define top and bottom layer by height. For this project I used maximum available (6 top, 6 bottom), but I had to increase layer size, to 0.19 to get bottom 2.2mm rock solid. I finished the tube by sanding and acetone bath. I flew 16hours last weekend, and didn't fall of, so it's good :D
BTW2: Hey, Zortrax Team, there is photo of you product print on top of hugest Jesus Christ monument in the World. You can use it in brochures if you want :)))))
I designed and tested 2stroke engines and carburetors for a living. You have to be careful about the length of the stack. You’re altering the frequency of the intake resonance. Since you’re running at pretty much constant speeds you will only see a performance increase if the length is perfect. So it is even possible that you have for example gained torque in the low end but have less peak power.
Also, the stack will change your air/fuel ratio. You need to rejet or readjust the H. So if you do power comparisons make sure the motor runs at the same lambda after installing the stack. Often people think they have more power after installing add ons but in reality it only made the motor leaner (which can increase power if you were too rich before).
Did you see an rpm increase at full power on the ground (more static thrust)?
Wow, great job!!
Thank you for your feedback slaweks82
I notif that there is some "magic" happens in there, because changing length by just 5mm had significant impact. I was making 10's of different shapes/lengths, inlet was base on 1/4 of ellipse, and then it was either cone, ellipse all the way down or strait as carb diameter. Seems that strait is better. My goal was to get max RPM, test was on the ground, but paraglider is so slow, so in-air-speed can be omitted I think. Is there any way I can try to calculate length of the stack?
to be honest... on 2 strokes calculations are complicated and only get you to a certain point. There are so many variables! The main problem is to be able to distinguish which of the observed changes in engine behavior are due to actually improved filling of the crankcase (good intake resonance) or simply due to a air/fuel ratio change. 2 strokes react strongly to jetting and your intake definitely will have an effect on the air/fuel ratio. So it could be that you for example see more power output with the stack installed but in reality the power increased only because the stack made the carb run leaner.
Is your carb easily adjustable?
Yes, is easy adjustable, it's Walbro 32, I'm setting it almost before every flight, and sometimes in flight as well (eg when crossing 3000ft, air is thinner, so need to lean mixture to keep it in optimum). On each velocity stack test I was setting mixture like 3 times to make sure I have accurate results. Setting is like: use H needle to find where there is drop in rpm (due lean mixture), and quickly escape, and find point to archive max RPM and then add a bit into richer side for optimal cooling, lubrication etc. I don't have EGT, so RPM counter is my only tool.
Ok, so I see, for me best method will be "print & test" :)