Cowl flap thoughts

[crabandy]

I think I’ve seen/googled most of the ideas out there when it comes to cooling the lycoming in the summertime. I’m sure there are many more out there that I haven’t seen. I’ve done a decent amount of work with inlets/plenum/sealing and lastly ignition timing for temp control. Most people seem to be concerned with climb out CHT’s. I currently have no issues climbing out on a hot day, I have issues taxiing after a quick turn on a hot day and I’m carbureted. CHT’s and oil temps are fine but fuel seems to be boiling, erratic chugging during taxi. Idle mixture is correct and an excessively lean mixture and RPM’s above 1000 help.

With my stock cowling I used to see heat radiating out of the upper inlets while fueling. I don’t see it with my sealed inlet boots. During ground operations the pressure diff between the upper cowl and lower cowl is very small. How many people pop the oil door open after they land in hot temps?

Most cowl flaps are installed in conjunction with the lower outlet, I’ve stared at this space on my cowling for months trying to come up with ideas. I’ve shrunk and contoured my exit to approximately 35 sq inches, not a lot of room for a cowl flap with a vertical induction carb and 2 exhaust pipes.

I’ve also been staring at the rear of my upper cowling, what about an upper cowl flap, similar to the old 30’s cars/trucks had in front of the windshield? With the low pressure diffs on the ground it would take advantage of convection in addition to a larger exit area during climb out. I kept hoping for an easy test without months of work and downtime.

I’m currently finishing an oil change and making room behind the upper engine mount to possibly add an upper cowl flap when it dawned on me, why don’t I spend 2 minutes and remove the oil door and go fly several quick turns? On 4 cylinder lycomings the oil door is behind the upper plenum and part of the lower cowling volume/pressure. Roughly 5x5 (25 sq in) in the upper cowling that is convection friendly on the ground, also pretty easy for anyone else to test fly and and see what kind of cooling the extra exit area gives them.

[Plummit]

Just removing the door may not work. I'm pretty sure that part of what makes a cowl flap work is the flap only allowing exit air, which you won't have with just a hole. But by all means try it and let us know what happens!

-Marc

[Taltruda]

I used to fly a friends glasair 2rg that had two slots on the upper cowl with a spring loaded door. It souls open and vent off the heat when on the ground but would close itself when ramair in the upper cowl would build up.

[MikeyDale]

When I started flying my 7, I always thought something was wrong with my engine. After landing on taxi it would pop and chug! I kept thinking my plugs were fouling! I even bought a set of fine wire plugs to solve the problem (well worth the money by the way but didn't help my problem). I was at 52F one day where Walt was inspecting my Transponder having lunch outside the diner and lo and behold, every RV that came taxiing by was popping and chugging just like mine! Boost pump helps but she's gonna pop when that fuel boils in those injector lines!

[rocketman1988]

Many years ago, in one of the experimental aircraft magazine (can't remember which one), there was an article on automatic shutters for the top side of a cowl.

The basis of the design was quite elegant. A set of shutters with moveable doors was installed in the top of the cowl. The activation of the shutter doors was accomplished using the charge cylinder from an automotive thermostat. The plug inside of the charge cylinder expands as it is heated past it's critical temperature and moved the shutter doors to the open position. As the charge cylinder cools, it contracts and closes the shutter doors...

Of course, with modern tech, you could accomplish the same thing using a linear actuator and a temp sensor connected to a small arduino unit...and it would be programmable for any given temperature or time...

These shutters are remade and would do the trick:

https://antisplataero.com/products/o...er-air-shutter

[Mike S]

Originally Posted by rocketman1988

Many years ago, in one of the experimental aircraft magazine (can't remember which one), there was an article on automatic shutters for the top side of a cowl.

The basis of the design was quite elegant. A set of shutters with moveable doors was installed in the top of the cowl. The activation of the shutter doors was accomplished using the charge cylinder from an automotive thermostat. The plug inside of the charge cylinder expands as it is heated past it's critical temperature and moved the shutter doors to the open position. As the charge cylinder cools, it contracts and closes the shutter doors...

Yep, remember that one.

A simpler setup is to use a magnet to hold the door in the up/closed position, and you just give the door a push down to open it when you land. Air pressure will close it when you take off.

[crabandy]

Originally Posted by MikeyDale

When I started flying my 7, I always thought something was wrong with my engine. After landing on taxi it would pop and chug! I kept thinking my plugs were fouling! I even bought a set of fine wire plugs to solve the problem (well worth the money by the way but didn't help my problem). I was at 52F one day where Walt was inspecting my Transponder having lunch outside the diner and lo and behold, every RV that came taxiing by was popping and chugging just like mine! Boost pump helps but she's gonna pop when that fuel boils in those injector lines!

Mikey,
I’m carbureted with dual electronic ignition and auto plugs, I think the fuel is boiling in my carb bowl....

[crabandy]

Thanks for all the other upper cowl flap suggestions, if I make an upper cowl flap I would like it to be controlled from the cockpit. Still thinking and waiting.

Pretty sure a couple flights without the oil door will tell me what the extra exit area on the upper cowl will or won’t do.

[Gliderguy89]

I'm in the habit of opening the oil access door after lending. Amazing amount of hot air exits and I suspect goes a long way to keeping fuel lines and the carb cooler for departure. 5psi pressure from the aux pump on the fuel lines also helps delay any perking.

[Raymo]

Originally Posted by crabandy

Mikey,
I’m carbureted with dual electronic ignition and auto plugs, I think the fuel is boiling in my carb bowl....

I think the fuel is getting hot before it gets to the carb, where it gets hotter. You might try putting a phenolic block or similar insulator between the carb and sump to insulate it from the block heat.

You haven't mentioned if turning on the boost pump helps. If not, your "boiling in the bowl" theory sounds more feasible. Also, that gascolator is likely part of the problem, heating the fuel before it gets to the carb.