Deadeye

I'm on the final stages of my Byron CAP 21 with a converted 45cc Homelite. My carb is about 3/4 inch from the side of the cowl. My velocity stack is twice that length. Instead of cutting a big ugly hole in the cowl, can I just leave the stack off? If I have to put it on, would it hurt to cut it down so it fits inside? Which direction should it face? Angled part forward or backward? Thanks guys.

w8ye

My Feedback: (16)

Randy, you've opened up a gray area. You may just try it like you want to and see what happens?

The velocity stack's one function is to keep the pressure wave of fuel that stands just outside the carb entrance from blowing down the side of the plane and have it go through the engine.

The other function -- often, the air blowing by the carb entrance creates a vacuum at the entrance of the carb. This messes up your mixture sometimes and will cause the engine to surge.

The venturi is sometimes cut at a angle. This angle is turned toward the front, but sometimes, you may find the engine may run best with the venturi in most any other angle?

There is a lot of air swirling around the area and it just depends.

Some venturi are not cut off towards the front.

There will be some different opinions here.

The bottom line is that there are a lot of variables and you may need to try several different methods before your engine runs the way you want?

You are the one that needs to be pleased?

Enjoy,

Jim

Deadeye

Thanks for the reply. This would explain why I couldn't find a definitive answer with my searches. Bad thing, is it is on a Byron's CAP 21, and I really would rather not have a deadstick on this snap-happy plane. I guess I will be doing some major ground testing.

blikseme300

My Feedback: (118)

Deadeye

The main problem is that the air flow will be much different in the air than on the ground. Get the engine to run as well as possible on the ground and try flying after. Plan your testflight well, think of each eventuality and what you will do. If needed, get an experienced pilot to either call for you or to do the testflight for you.

Safe flying!

smallplanes

My Feedback: (3)

I have a 29% edge 540 with a FPE 3.2 (52cc) on it and i did not put the stack on it for the same reason you don't want to put it on. I have been flying it for a while now with out the stack and it works perfect. I have never had a dead stick or anything bad happen form not running the stack. You will have a cowl on the plane and that will help the carb. I think you will be fine to fly the plane with out the stack, you just need to get the motor running a little rich the first flight or two then lean it down a little at a time.


smallplanes

Deadeye

Thanks you blikseme300 and smallplanes. I feel a bit better not running a stack for the first flight or two. Now if somebody could say "Don't worry about the plane snapping all over the sky", I might not need to wear a diaper for the maiden.

blikseme300

My Feedback: (118)

Deadeye

All aerobatic planes can cause you to require a change of clothes if they are not set up right. Get the CG and lateral balance right, use the recommended surface throws - dial in expo to soften them too. If you are nervous, you will be jerky on the sticks. I know it sounds like a contradiction but you need to relax even if you are nervous.

The most important single thing is power/speed management. Don't glide land them. Do a proper procedure landing, decreasing throttle gradually. Don't just chop power. When on final be gentle on the sticks. Use your rudder to line up, not ailerons! Adjust sink-rate using throttle - gently, don't chop! Just before touchdown you should be very near idle.

CAP's can be a pleasure to fly. Even though they are not my favorites I still keep one around because they can do the most awesome snap-type aerobatics. Thats why they also have a bad reputation. Plan ahead for your manuevers and don't just bang sticks. You will get to love the precision and the CAP will make you a better pilot.

Let us know how it goes.

Safe flying!

buck1856

My Feedback: (49)

personly i dont have any stacks on my engines yet because of the cowls,and theres no problems.second,if you want to,take a peice of 1/4 inch tri-stock and tape it to your leading edge of the wing before you fly about 1/3 of the way from your fuse to the tip.this causes a sharper angle of attack on this section of the wing and it will stall first reducing tip stall.or,set both ailerons at neutral to the trailing edge as always.after this,(RAISE BOTH AILERONS) 1/8 of an inch eqauly on both sides.this creates washout and also reduces tip stall.after your familiar with your plane you can remove these two cures as neccesary or slowly.if you raise the ailerons make sure there the same.

Deadeye

Thank you guys for the replies and suggestions. I'm feeling better all the time. I will be test running the CAP on the ground sometime this week. She's just about done.

Jimmbbo

Agree that the best path is try it as is, get the engine running well statically then start a test sequence with slowly increasing aerobatic maneuver intensity to see if the flow through the cowl is adversely affected... (with deadstick altitude always available )

With any luck, the cowl will act as a plenum, providing positive pressure to the carb inlet with most maneuvers .. with bad luck, the flow will be so fouled up that the carb will see negative pressure, and the engine will respond accordingly...

Keep us posted!!

BillS

If you are concerned that the cowl might not act as an air box then a small air box or plenum chamber could be fabricated. All you would be doing is preventing air from rushing across the face of the carburetor and raising or lowering the pressure seen by the carb. Many get by without a cowl and without a velocity stack.

Bill S