Cowling Negative Pressure Slot

[GraemeSmith]

Sorry - trawled the search function - can't find it. Though I know I've seen it somewhere......

Want to pull MORE air through the cowling to allow me to use the most conservative CHT's I can achieve. I've been chasing even the most minor gaps in the baffling to get the air to go down over the cooling fins. Now I need to encourage the air to move through more.

I'm sure I've seen a thread/discussion/document about widening / enlarging the negative pressure lip at the bottom of the cowl.

Can someone please point me at it or suggest workable dimensions.

There must be a point at which enlarging the slot does NOT help pull air through - right?

Shape of the metal lip. Anyone tried different profiles and compared drag/effectiveness?

TIA!

[WesRagle]

Hi Graeme,

You have probably seen this, but page 67 and 68 of the present revision of the AeroVee assembly manual covers cowl openings on Sonex aircraft.

Wes

[Brett]

I tried the recommended size,,,didn’t work.

[GraemeSmith]

Hi Graeme,

You have probably seen this, but page 67 and 68 of the present revision of the AeroVee assembly manual covers cowl openings on Sonex aircraft.

Wes

See - I just needed pointing - thanks Wes....

[WesRagle]

I built #664 before Sonex LLC had developed the fence baffling. The original baffling design didn't work very well and I had high CHTs. Since I was impatient to solve the problem I wanted to quickly isolate the problem to either inlet or outlet. So, I built a ridiculously large outlet "scoop" and talked my friend and test pilot into taking it for a spin. How large was the scoop? I can still recall the radio call: "It's getting hot. It won't accelerate. I'm coming down."

The first thing I noticed when he taxied up after the flight was oil running out the back of the cowl (the old baffle design had the prop hub in the low pressure area of the cowl and the prop hub is not sealed). So, I had achieved my objective of lowering the pressure in the low pressure area of the cowl but to no avail. BTW, the scoop was so large that the plane would not accelerate past 100 mph. Long story short, it eventually became evident that a new baffle design was needed and Sonex LLC came through about three months later.

By the time all the dust had settled and the new baffling was installed I already had a small, cockpit adjustable flap on the outlet. I never used the flap much. It did help some during extended climbs. I think that was mainly due to extending downward during high angles of attack and so preventing the outlet from being blanked. Not sure.

The forces on the flap could be felt through the button lock bowden cable. It wanted to open up some if closed down, and close some if opened all the way. I usually adjusted the flap so that the force was neutral. That seemed to work best overall. Climb CHT stayed below 400 F and cruise CHT stayed below 350. After what I had been through I was ecstatic with those numbers.

I would still like to know if a tunnel outlet (think RV) would work well on a Sonex/Onex. That type of outlet wouldn't be effected by AOA. Working well would mean reducing pressure in the low pressure area without inducing too much drag.

Anyway, enough rambling, past my bedtime :-)

Wes

[GraemeSmith]

There is an idea - a cowl flap like on bigger complex aircraft. Open for take off, landing and IF hot. Closed and streamlined when level in cruise. I'll think on that a bit.

[WesRagle]

The flap I had was only about 4 inches from front to back. If I were to try it again, I would make it deeper such that when opened It would form an tunnel and add to the aft facing area of the opening. In fact, before I even tried an adjustable flap I would build it fixed in place and verify it worked well in climb before I went to the trouble to make it adjustable.

BTW, in your quest for the cool VW, did you take a look between the cylinder head cooling fins and see how much flashing was blocking the cooling air?

Wes

[GraemeSmith]

BTW, in your quest for the cool VW, did you take a look between the cylinder head cooling fins and see how much flashing was blocking the cooling air?
Wes

Going to be revisiting that after I just saw the before and after pictures.