Cowling questions

[ColoCardinal]

I'm beginning the work on my Van's vertical induction cowling.
The first question / concern I have regards the alternate air system. I'm not excited by the system supplied with the kit. An alternate air source is important enough on every GA plane I've ever flown that it's tested before every flight. One that requires a partial dissasembly of the aircraft to "reset" doesn't seem safe to me. Has anyone cooked up a better system they would like to share?
A (distant) second question - has anyone used the hinge pin system also used by Lancair on the IV-P. It has a threaded end and leaves only a .20" hole in the front of the cowl. No cover plates and very simple.

 

[L.Adamson]

The alternate air system is for one of those " once in a lifetime events"....such as snow/ice packing around the filter, or a direct hit from a bird which also prevents the carb heat door from opening.

Many prefer to not install the option. I did, because the idea came about from an actual icing situation. It's just something that doesn't need to be operated before every flight. The biggest problem is getting the installation to where there are no rivets that come loose at some point. There are numerous articles in the search archives for this. Some have also built more elaborate sliding door systems.

L.Adamson --- RV6A

 

[Charles in SC]

Mine is different because I am going with injection but you might get a idea by looking at a Piper Comanche, I flew a 260-B for a few years and I remember it having an alternate air door that was held shut by what looked like a household cabinet door magnet. When the intake stopped up the engine had enough intake suction to pull the magnetic door open. I put it to use once in some ice. The engine about died before it opened but it did work. The pucker factor was high before at the time of opening.

 

[ColoCardinal]

Cowling question

Thankls guys. "Engine induction" is listed by the FAA as a major cause of engine failures so I definately want an alternate air source. I didn't find the sliding door system but like the idea. I'll look again. My Cessna Cardinal has a system that is similar to the heater box on the RV (a swinging door) and works great.

 

[Lars]

I built a sliding door for mine. It was an interesting exercise but a lot of extra work. Most of the pieces are aluminum, except for the sliding gate, which I made from a piece of 1/8" Vespel that I had lying around. I made an oval of .020 aluminum that I bonded to the inside of the airbox (you can see bubbles where I missed getting the epoxy distributed). The air filter is compressed against it, so it's not going anywhere. The fiberglass airbox is sandwiched between the oval piece above and the gate assembly below. The fasteners are #6 flat head screws with MS21042 stop nuts. I will be monitoring it closely once I get the engine running, since others have had issues with that part of the airbox self-destructing.

 

[Rupester]

Sam/Will James approach

I'm installing the Sam/Will James alternate air system as we speak. It uses a cable operated hinged flapper/door to pull air from inside the cowl if freeze-up occurs on the air filter. It's a decent, functional approach that is reasonably simple .... but not as elegant as the sliding door above.

 

[ColoCardinal]

Cowling question

Lars, Thanks for the help. Your slider is just like what I'm looking for. Extreamly robust! Not at all like the flimsy part in the kit.

 

[Lars]

Thanks for the compliments. I wish I could say I originated the idea, but it's not mine. If you root around here with the search function, you'll turn up some other similar ideas that I used for my inspiration.

Actually the only unique thing about mine is the Vespel- that little square is probably worth 20 or 30 bucks by itself. It was a sample given to me years ago when I was considering it for a company project. Sat in the drawer for 10 years until it reappeared on my radar.

Also, remember that mine hasn't flown. No service history but it looks cool

 

[DanH]

A sudden blockage will subject it to full atmospheric pressure, yes? If the door is 3"x3" you're looking at 125-135 lbs. Might want to do a quick shop test to see if bending or simple friction prevents movement or fails the assembly.

 

[ColoCardinal]

Cowling question

A sudden blockage will subject it to full atmospheric pressure, yes? If the door is 3"x3" you're looking at 125-135 lbs. Might want to do a quick shop test to see if bending or simple friction prevents movement or fails the assembly.

That's a good point. The slider appears to be more likely to work in a "worst case" situation whereas the pivot door with the kit appears that it may not, there's no substitute for a test. I'll be sure to keep that in mind.

 

[Lars]

A sudden blockage will subject it to full atmospheric pressure, yes? If the door is 3"x3" you're looking at 125-135 lbs. Might want to do a quick shop test to see if bending or simple friction prevents movement or fails the assembly.

Indeed, Dan. You are keeping me honest. The 1/8" Vespel is a lot stronger (not to mention stiffer) than the thin fiberglass it replaced. It also happens to be the SP-21 15% graphite-added variety, so it works well for bearing surfaces- in fact that's how it came into my possession. I was working on an exotic bearing project More information here, for the curious: http://www.boedeker.com/polyimide-sheet-rod.htm

I did a "thumb test" on the thing (area is roughly 2x2, about the same opening area as the Van's device) before final installation on the fuel servo. Which is to say that I set the FAB assembly upside down on my bench, and smashed my thumb into the gate while I tried to slide it with my other hand, by pulling on the bolt I added for the purpose. It slid as if there were no load being applied.

But I won't know if it's really ready for prime time until I've run the engine. I have worried about the thing- and not just because of the gate material. I added a bunch of mechanical complexity to my FAB, not to mention small parts that will probably do their best to separate themselves into individual components if they have the opportunity.