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Alternate air questions

Charles in SC

Well Known Member
I am wondering who has and does not have an alternate air source for their intake and why. If you have it what type is it? Cable operated, magnetic or other.
I have a James cowl and intake. Does the Van's intake have an alternate air source? If so does anyone have a picture?
 
Yes, the stock cowling configuration for the RV-7 has an alternate air source operated by pulling a Bowden cable. It's just a cutaway on the side of the filtered air box with a round plate that pivots on a bolt when the cable is pulled. According to Vans instructions it is advisable to install it if operating in snow or ice conditions.
Tom.
 
Thread with Many Pictures

I am wondering who has and does not have an alternate air source for their intake and why. If you have it what type is it? Cable operated, magnetic or other.
I have a James cowl and intake. Does the Van's intake have an alternate air source? If so does anyone have a picture?

Mine is cable operated, all carbon fiber, and can be opened and closed from the cockpit. 3 years and 270 hours - and it's working fine. Here (http://www.vansairforce.com/community/showthread.php?t=85559) is a link.
 
Am I nuts thinking the carb heat inlet works as alt air? I suppose it depends on the setup. I have the James cowl, down draft carb, and modified Vans FAB. With the exhaust with the heat muff in front, it has been a PIA!
 
Rv6, vans airbox with no alternate air. I am in a very warm climate, so never gave it a second thought.
 
Am I nuts thinking the carb heat inlet works as alt air? I suppose it depends on the setup. I have the James cowl, down draft carb, and modified Vans FAB. With the exhaust with the heat muff in front, it has been a PIA!

No You're not, Alt air is usually only found in Fuel Injected setups since they don't have carb heat.
 
Am I nuts thinking the carb heat inlet works as alt air? I suppose it depends on the setup. I have the James cowl, down draft carb, and modified Vans FAB. With the exhaust with the heat muff in front, it has been a PIA!

Carb heat air comes in ahead of the filter, like normal intake air. Alternate air bypasses the filter.... at least in my setup, vans standard issue.
 
Carb heat air comes in ahead of the filter, like normal intake air. Alternate air bypasses the filter.... at least in my setup, vans standard issue.

I see that on my FAB plans. I suppose there could be a situation where that would be advantageous since the parts are included in the kit. My understanding was it was needed for the FI systems as Z-EDD pointed out.

I love Dans system - lots of work to do it, but it looks very good. I do live in snow and ice conditions, but I have no plans to fly in that type of weather.

I will check with the guys around here and see what they did. I am considering FI as a winter project so I will have to modify things then and could make the change. I need to get this plane in the air first! I hope to have the DAR out by the end of the month.

Thanks for the info.
 
Carb heat air comes in ahead of the filter, like normal intake air. Alternate air bypasses the filter.... at least in my setup, vans standard issue.

If thats the case then I am wrong and apologise. My answer was based on experience across a large number of piston engine types where Alt air was only ever found in FI types and was taught never to check carb heat on dirt/gravel as the carb heat bypassed the filter. If Van designed RV's so that the carb heat air is filtered then Alt air is indeed a good idea. My aircraft will have FI so I have only been researching that setup.
 
I'm on the other side of the box, my donor engine in a former life swallowed a screw and got rebuilt because of it. I see alt air is a good idea for IFR ops but I decided to not install one because of all the stories I read on here about pieces getting ingested and causing internal engine damage, and the fact that I am just a VFR guy for now. the odds may be equal to an alt air door falling apart or ingesting a bag or bird without one :rolleyes: in either case we have a heavy glider to get us down safe to a road or field.
 
I built the intake, filter, and alt air exactly as the Sam James 'structions called it out. (vertical intake FI). Yes, it is an alternate intake path, but as pointed out above it does not bypass the filter. But there are advantages, in that the heated alt air coming in from the heat muff will melt frost or packed snow that might be clogging the filter. I perceived that was really the intent of the Sam James design.
 
After reading about loose bits getting sucked into the engine, cracking etc I encapsulated a .040 aluminum ring on the bottom of the air box. The clevis is R/C model vintage. So far so good @ 370 hrs...



 
Yes to Alt air door, no to following install plans

My first build suffered FOD damage as described by Brett, so agree with his concerns. But I still see the value of installing this safety device on filter housings. The valve was installed originally per plans with Proseal and rivets attaching the base plate to the fiberglass housing, at about 700hrs the base plate vibrated loose (?) and wore the rivet holes & rivets enough that 3 of them were sucked up thru the engine causing overhauls on 2 cylinders.

My first attempted repair was to re-rivet the base to the fiberglass with large diameter washers inside the housing for the rivets to squeeze against, all again Prosealed. 100hrs later I noticed these rivets loose again! Third round I tried a different repair method.

My method now consists of drilling the base plate for AN509-8 screws, dimpled (not countersunk!) and mounting to fiberglass housing with Proseal as before. Inside the housing I create a .040 donut shaped plate with nutplates for the #8 screws, this plate also Prosealed. This system has the screw fasteners engaged with metal parts on both sides, not allowing any fastener to fiberglass movement that may eventually wear loose.

I have incorporated this mod on probably 6 aircraft so far with no further issues showing up. Although I have only seen this damage on updraft installations, I do the same for horizontal injected air boxes too.


Ron- I have also seen the hinge screws come loose, on one plane the valve door was hanging at the bottom of the cowling. I suggest you replace the hinge screw with something that is drilled & safety wired.
 
mine is a normally aspirated O360, no alternate air. Since I don't fly in icing conditions the only case where I can see needing it would be if I had a bird strike directly in the intake snorkel. I considered it but decided that the maintenance and the possibility of issues with unfiltered air entering the engine was not something I wanted to deal with. K.I.S.S.
 
I'm on the other side of the box, my donor engine in a former life swallowed a screw and got rebuilt because of it. I see alt air is a good idea for IFR ops but I decided to not install one because of all the stories I read on here about pieces getting ingested and causing internal engine damage, and the fact that I am just a VFR guy for now. the odds may be equal to an alt air door falling apart or ingesting a bag or bird without one :rolleyes: in either case we have a heavy glider to get us down safe to a road or field.
That's my reasoning as well. :)
 
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