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Alternate air source??

chris mitchell

Well Known Member
I have an IO-320 with a Superior cold air sump installed in my (90% done) RV-4. I decided to move the cowl air intake forward rather than deal with the snorkel arrangement I had on my RV-8. I was planning to use the Flyboys Accessories intake arrangement with the cone and filter connected to the adapter ring on the front of the injection servo using a short length of scat tubing.

Before doing any installation I checked a few measurements. Ooops. Basically, the filter and cone will just fit if I connect the filter direct to the adapter ring on the servo. But that means I cannot fit any sort of alternate air arrangement between the filter and the servo.

So my question is - is an alternate air source essential? I've never had to make use of the source in a few of hundred hours behind injected engines of various flavors.

If the answer from the cognoscenti is that Alt AIr is essential - any good ideas about how I resolve the problem?

AS ever - many thanks in anticipation of advice.

Chris
 
Chris - do a search for the topic and you will find an equal amount of debate as primers and such. I would guess it is pretty close to 50/50.
 
After reading the other threads on this topic, I opted not to install the SJ intake alt air option. No worries about ingesting that hardware down the road. Still considering better options to add alt air, though.
 
So my question is - is an alternate air source essential? I've never had to make use of the source in a few of hundred hours behind injected engines of various flavors.


Chris

It is only essential if a bird, ice or other object blocks the normal flow of intake combustion air.

Have a look at some of the earlier F1 rocket intake / air boxes. They used a flat rectangular Honda Civic filter. There is plenty of space for a flat leaf spring / reed valve on top of the filter box that will seal well , but allow the engine to run if the filter or intake get blocked.
 
You only need it when you need it

I ran setup which is very similar to yours for many hours without an alternate air source but it always bugged me. My biggest concern was a plastic bag sucked into the intake on takeoff so I decided to make my own setup. I molded my own cone, purchased a smaller K&N filter, and then machined a simple ?no pilot action required? alternate air source. Granted this took a bit of doing and required both a 3D printer and a bit of metal work but it sure gives me a peace of mind.

http://www.darinanderson.com/2018/09/still-working-on-9.html
 
Personally, I don't think I'd be comfortable flying behind an engine that didn't have alternate air of some sort.

I realize that the chances of needing it are remote, but I know of one case on a certificated airplane where it would have ended badly if there hadn't been an alternate air solution. This isn't a friend of a friend urban legend, it's a real situation that happened to a multi-thousand hour gold seal flight instructor through no fault of his own.
 
So my question is - is an alternate air source essential? I've never had to make use of the source in a few of hundred hours behind injected engines of various flavors.

If the answer from the cognoscenti is that Alt AIr is essential - any good ideas about how I resolve the problem?

AS ever - many thanks in anticipation of advice.

Chris

Entirely your choice - I have it on mine and never considered not doing it...... until I later read on here about guys not doing it. Hmmm. Points either way.
 
No need for one.

Almost 1,000 hours and no bird, leafs, or plastic bags have tried to commit suicide by jumping into the intake.

The thought of something happening on take off, the pilot realizing the intake is blocked, and opening the alt air before a forced landing is probably a stretch.

I wouldn't bother adding the complexity. I suspect there has been more problems caused by engines ingesting parts of the alt air contraptions than caused by the lack of alt air.
 
In making this decision you may want to check with your DAR if you are using one for your AW inspection. An alternate air source was on the list provided by my DAR that he wanted to see when I built my RV-10. I would want one regardless,,,,something I hope I never need to use but its there if I do.
 
In making this decision you may want to check with your DAR if you are using one for your AW inspection. An alternate air source was on the list provided by my DAR that he wanted to see when I built my RV-10. I would want one regardless,,,,something I hope I never need to use but its there if I do.

Hmmm. To me that seems like something thats outside of the DARs purview to decide.....where exactly is the line between a DARs responsibility and a design decision?
 
No need for one.

Almost 1,000 hours and no bird, leafs, or plastic bags have tried to commit suicide by jumping into the intake.

The thought of something happening on take off, the pilot realizing the intake is blocked, and opening the alt air before a forced landing is probably a stretch.

I wouldn't bother adding the complexity. I suspect there has been more problems caused by engines ingesting parts of the alt air contraptions than caused by the lack of alt air.

Hence the ?no pilot action required? option I used. You only need it when you need it. Not before. It?s purely a risk tolerance issue. Many don?t feel the need because of their experience, others do... personal decision and only one person can make that call.
 
Hmmm. To me that seems like something thats outside of the DARs purview to decide.....where exactly is the line between a DARs responsibility and a design decision?

Its what he wanted to see, so that's what I made sure was there. Its HIS decision on weather or not to sign the AW certificate.
 
Hence the ?no pilot action required? option I used. You only need it when you need it. Not before. It?s purely a risk tolerance issue. Many don?t feel the need because of their experience, others do... personal decision and only one person can make that call.

And many of those have come apart with rivets, bolts, etc. getting sucked up into the engine.

No thanks, I'll take my chances with a plastic bag.
 
Well, it's interesting to see the range of responses. I absolutely see the " its only essential if you need it" view. I thought maybe I should have said mandatory - but even that seems to be contentious.

I guess on balance I will feel a bit uncomfortable about not having one. I'll see if I can engineer the space I need to get a side arm on the length of tubing between filter and adapter. Maybe shorten the nose cone a little, use a shorter filter. THe two things that are fixed and cannot be re-engineered are the location of the adapter on the front of the fuel servo and the location of the intake on the front of the cowling - the former can't move back and the latter can't move forward.

I'll see where I get to and post some photos.

Many thanks for all the comments.

Chris
 
IIRC, an RV 7 went down in a snow storm attempting to cross the Sierra mountains when it is believed to have ingested enough snow to clog the air filter. Two died. Perhaps other can fill in the details.
 
Well, it's interesting to see the range of responses. I absolutely see the " its only essential if you need it" view. I thought maybe I should have said mandatory - but even that seems to be contentious.

I guess on balance I will feel a bit uncomfortable about not having one. I'll see if I can engineer the space I need to get a side arm on the length of tubing between filter and adapter. Maybe shorten the nose cone a little, use a shorter filter. THe two things that are fixed and cannot be re-engineered are the location of the adapter on the front of the fuel servo and the location of the intake on the front of the cowling - the former can't move back and the latter can't move forward.

I'll see where I get to and post some photos.

Many thanks for all the comments.

Chris

Howdy Chris,
This is how I did it:

20160922_193431%201_zpsygptqsyj.jpg


20160920_193503%201_zps1nfo8o1f.jpg


20160920_193403%201_zpsmo3wh0jo.jpg


I made an airbox out of fiberglass. I believe the filter is a 0995. The alt air door hinge/gasket is sheet silicone rubber stock. It is manually actuated. I had to put a spring steel strap with a ramped block on it to make it seal properly.

Ed Holyoke
 
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