David_Moen

Here's one that's got me a bit puzzled....

I have a 1.60 on an Ultra Stick, the engine is open to the air moving around it. I have a velocity stack on the carb, maily to keep the airplane clean. The velocity stack is cut at an angle, and I have it installed so that the oening faces forward into the airstream. The engine runs like a champ, tons of power for any kind of manuever except one, left to right knife edge passes. Going right to left, everything is great, I've attached a photo for "reference", no intention of showing off of course . The engine runs great in this orientation, but flip it over and fly left to right and it starts to sag almost immediatley. I've recently removed the choke from this engine as it was not used, but I can't recall if this problem existed last season before I took it out.

Would re-orienting the opening on the velocity stack change things?

DarZeelon

David,


I would doubt this would have such a profound influence.

The stack on your Walbro is is close proximity to the prop.

When flying in a knife-edge, the blades has no incidence difference, when passing over, or under, unlike the incidence difference, which influences pitch, when flying at a high angle of attack.

In light planes you must feed some rudder to keep it on track, when in a climb. This is toward the blades that go down, at a higher pitch than up-going blades, relative to the direction of the airflow.


Try removing the velocity stack and see if this still happens.

My standard glow 1.60 model (in the same model) does nothing of the sort.

pe reivers

The phenomenon is known from other gassers as well, mostly with knife edge flight with the large angles of attack. There are multiple posts here.
you have the engine side mounted, and right to left the regulating membrane is verical. The regulating membrane air hole is in the still air behind the carb.
left to richt, the membrane is horizontal, and pulling the gas needle shut by it's weight. This is further aggravated by the airflow now blowing full force over the membrane plate hole, probably creating just a bit more vacuum. These two factors combined tend to lean out the engine too much. Look closely at that hole. if it faces forward in L>>R knife edge the reverse may be true, and ram air pushes the needle fully open, causing the engine to flood.


First try to re-arrange the cover plate, and see if matters improve in knife edge L>>R. If it does, my reasoning is OK.
Then add a pressure compensating tube to the cover plate and lead it to the carb throat or into the fuselage to at least eliminate the adverse airflow effects on the regulating membrane.
I am about to make a connection to my muffler, and connect the tubing there, but that is for a different set of reasons.

David_Moen

Thanks for the input, I think I'll try fashioning a temporary shroud of some sort to shield the carb and negate any aerodynamic influences on the back of the carb. I'll let you know what I find!

pe reivers

David,
It may be sufficient to rotate the steel cover plate to a different position.
On my plane the vent hole is at right angles to the carb center line, and away from the prop blast.

David_Moen

I was looking at that last night Pe, on this engine the hole on the cover plate is facing the right side of the engine, so in level flight it is facing the ground. I will try rotating the plate so the hole faces the bottom of the engine, away from the prop blast.

David_Moen

To report back, I turned the velocity stack 180 degrees to see ehat effect that had. I can now knife-edge in both directions, but I have a rich mid-range condition which is not manifest on the ground during static testing. Before changing the orientation of the velocity stack mixing was spot on at all throttle settings. I think I will put the velocity stack back where it was and re-orient the cover-plate as per Pe's reccomendation.

DarZeelon

David,


This sounds like a good experiment.
Good luck!

Although a 'buffeting' effect is usually evident (i.e. actually changes the ambient pressure - compressibility) at super-sonic speeds, I would guess this may have an effect, as an open window in a car does.

pe reivers

It all stems from the origin of diaphragm carburettors, which were never meant for airplanes in the first place, but feel at home in the still air around hand-held appliances.
Drilling a small hole that ends <above> the choke valve, and connecting the diaphargm pressure to that hole relieves the problems somewhat. It equalizes diaphragm pressure and carb intake pressure.
Beware: If the hole enters the carb air duct below the choke valve, the choke will cease to function.

The Walbro is like my dearly beloved. The size of her wrath in non-proportional to the wrongs I do.[&o]