pe reivers

Hey guys/gals
Recently I aquired a 1/4 scale J3 cub model, which I planned to use for glider tow. The plane had it's share of problems, I got it cheap, so took time to solve all.
Regarding the engine (58cc MVVS standard with side muffler), I never got it to run consistently, and was tweaking needles all the time.
Conditions:
rear carb
cowl extracts air in a slit between fuse and upper front deck
I have made a cooling duct to ectract the hot cooling air
Carb got hot, so I blocked air from side mufflet into the rear of cowl
Added cool air scoop louvres on cowl top

Still the needles were not consistent, and sometimes the engine went lean, and the other moment overly rich, with differences in ground handling and flight.
So I planned a few cures like:
- reference pressure into the fuselage
- using balsa box system

To be continued

pe reivers

Since the conditions changed so much under the cowl, no carb could have provided a constant mixture. So I decided to test an idea that I had for a loooong time, but never had reason to test.
Instead of the in-fuse balso box, I wanted to use the static pressure tube approach, like in a pitot tube, but then only half of it, disregarding the dynamic pressure part for speed readings.
So I made a simple brass tube, bent it to shape, pinched one end shut, and made two opposite saw cuts in the closed end.
The other end was connected to the carb refernce pressure cover.
The pictures say all.
The staic pressure tube runs straight forward into the airflow, and about 12mm (1/2") out from the cowl

pe reivers

test run results so far, usng 3W 22x10 and Mejzlik 23x10:
Main needle could be closed two full turns
idle needle could be closed 1/2 turn
stable running and throttle response (plane static) after prolonged idle or prolonged 1/2 throttle
No difference or needle tweaking with cowl on, or cowl off.
Test flights are planned tomorrow.

Rfabbre

My Feedback: (6)

Pe,

This is really starting to sound interesting. I am anxious to hear the end results as I have been experimenting with cooling baffels and air dam extractors (louvers) on my cowl.

pe reivers

The engine in my cowl is fully dammed in baffles and jackets. The carb gets cool air from forward facing louvres on top of the cowl. They should provide pressurised air under the cowl to prevent hot air from the exhaust from flowing to the carb. There is no way to do this halfway. It's all or nothing.

Rfabbre

My Feedback: (6)

Pe,

Anxious to hear the results. Can you send photos of the top of the cowl and the inside baffels?

Thanx

pe reivers

I made aluminum jackets and ducts, and stuffed the openings with steel wool and newspaper+cyano+tape. Have to find the pictures I took of the jackets.

bob_nj

My Feedback: (62)

Nice going Pete!
Let's get that thing in the air for the "proof in the pudding" test.

Why did you face the tube forward and opt to cut a slot as opposed to just facing it rearward
and leaving it alone, or just facing up and leaving it alone?

Just curious...

pe reivers

Search internet for "pitot tube", and you will see the general construction and orientation into the airflow. Normal position is underneath the wing, well clear of the prop arc. I hope I will get away with it operating in the prop wash.

I discard the dynamic pressure part, and just use the static pressure part, and it needs to run in undisturbed air as much as possible, with holes opening up in the tube side.

soarrich

My Feedback: (98)

Doesn't the 3W snorkel give the best reading of the pressure at the carb opening, whether it's pointed forwards or backwards?

raston

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pe reivers

Not realy. When the pressure under the cowl changes, the rise/fall in pressure has less influence on fuel flow, than on amount of air (by weight) entering the engine, and mixture strength will be disturbed.
Look at it this way: If you take your engine into high altitudes, you have to lean the needles, even though carb mouth and membrane pressure change equally.

BTW,
I test flew the plane this afternoon through the whole speed range using a 23x10 prop. The 24lbs Cub now has unlimited vertical. Normal on the wing flight is at about 1/4 throttle.
There was one full speed lean hickup, so the main needle needed opening a wee bit (~8 minutes), and that was all. Opening the main did not cause static rpm to drop, so I guess I was slightly lean on the high needle to begin with.
Even with the static tube in the prop blast disturbance, did not prevent it from working as it should. A better place would be under the wing at half span, but that requires some extra plumbing.