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-   -   Alt Air? (https://vansairforce.net/community/showthread.php?t=148746)

bkervaski 04-18-2017 03:29 PM

Alt Air?
 
Guys --

What's the purpose of the Alt Air control I see on some of the panels .. it's not alt static but something to do with the carb .. trying to figure out why some have it and others don't and is this something that would be needed on the IO-390.

Thanks!

-- Bill

BGS 04-18-2017 03:41 PM

These are two separate things. The alternate air is a backup for the air intake into the engine, in the case that the filter gets plugged and engine cant get enough air for some reason.

There is also the alternate static in the case that the static ports get blocked.

Both should be incorporated into the build.

Hope this helps.

bkervaski 04-18-2017 03:46 PM

Ahh, okay, that, and a bit of searching, it makes way more sense now. I've never seen an "Alt Air" port in any of the stuff I've been flying.

(thanks)

So much to learn and loving every minute!

BobTurner 04-18-2017 03:49 PM

You most likely have been flying airplanes with 'carb heat'. Carb heat not only supplies warm air to the carb, it also bypasses the air filter in case it's clogged.
Most fuel injected engines do not have a heated air option, just a filter bypass called alternate air.
If you choose to install a carbureted engine you'll want carb heat instead of ALT air.

bkervaski 04-18-2017 03:50 PM

Bob, that brings it home, perfect, thank you.

N941WR 04-18-2017 03:54 PM

It depends on how you fly. In my VFR plane I have neither Alt Air or Alt Static Air.

If I were to add IFR capability to my plane, you can bet I would add both.

SMO 04-18-2017 05:35 PM

Quote:

Originally Posted by N941WR (Post 1166578)
It depends on how you fly........

And the level of risk you are willing to accept. A plastic bag blocking the air intake can stop your engine, an alternate air source can keep it running.

YellowJacket RV9 04-18-2017 05:41 PM

Quote:

Originally Posted by BobTurner (Post 1166575)
You most likely have been flying airplanes with 'carb heat'. Carb heat not only supplies warm air to the carb, it also bypasses the air filter in case it's clogged.
Most fuel injected engines do not have a heated air option, just a filter bypass called alternate air.
If you choose to install a carbureted engine you'll want carb heat instead of ALT air.

On the RVs (at least those that I know of) carb heat does not bypass the filter, so alternate air is still often recommended. It is introduced by means of a trap door in the base of the filtered air box.

Chris

BobTurner 04-18-2017 05:48 PM

Quote:

Originally Posted by YellowJacket RV9 (Post 1166602)
On the RVs (at least those that I know of) carb heat does not bypass the filter, so alternate air is still often recommended. It is introduced by means of a trap door in the base of the filtered air box.

Chris

You are correct. I should have said "You have most likely been flying CERTIFIED carbureted...."

rockwoodrv9 04-18-2017 06:12 PM

Is it opinion to have the alt air intake in addition to carb heat or recommendation? My understanding was it was a choice and not really needed if carb heat is installed. It isn't a difficult thing to add, not nearly as tricky as the carb heat.

I did not plan to have the alt air. Should I?

N941WR 04-18-2017 06:23 PM

Quote:

Originally Posted by SMO (Post 1166601)
Quote:

Originally Posted by N941WR (Post 1166578)
It depends on how you fly......

And the level of risk you are willing to accept. A plastic bag blocking the air intake can stop your engine, an alternate air source can keep it running.

You are 100% correct. However, many (most?) airplanes do not have an alternate air path. So, apparently many people, myself included, are willing to take the risk. Although I'm sure it has happened, I am yet to see a plastic bag blowing across any of the airports I have visited or at altitude.

I'm just thankful you didn't not mention having a bird go in the intake. If you are worried about that, then you should cover the canopy with a fence of some type because there is a better chance of taking a bird through the windscreen than in the air intake.

SMO 04-18-2017 06:59 PM

Quote:

Originally Posted by N941WR (Post 1166615)
Although I'm sure it has happened, I am yet to see a plastic bag blowing across any of the airports I have visited or at altitude.

I bring it up because I believe that was my experience as related in this thread: http://www.vansairforce.com/communit...hlight=plastic

BobTurner 04-18-2017 07:21 PM

Quote:

Originally Posted by N941WR (Post 1166615)
You are 100% correct. However, many (most?) airplanes do not have an alternate air path.

Virtually all normally certified planes do have an alternate air path. It's a certification requirement, IIRC.
As an EAB airplane you are of course free to do what you please. But given how relatively easy and inexpensive Vans' alternate air design is, it seems penny wise but pound foolish not to install it.

N941WR 04-18-2017 07:36 PM

Quote:

Originally Posted by BobTurner (Post 1166630)
Virtually all normally certified planes do have an alternate air path. It's a certification requirement, IIRC.
As an EAB airplane you are of course free to do what you please. But given how relatively easy and inexpensive Vans' alternate air design is, it seems penny wise but pound foolish not to install it.

I do not know what the certification requirements for a new plane are but regarding your "penny wise but pound foolish" comment, I disagree. When I was building a lot of people put in the trap door that was held shut with a magnet. Some of these were suckered into engines. Thus Van's came out with the sliding door. If I remember correctly, there were similar issues with the slider. Thus, rather than risking my engine with a known failure mode, I elected to leave it off.

Robert Anglin 04-18-2017 08:03 PM

The question.
 
If I were to be building with the installation of an IO-XXX "take your choice", I would as I have done, install Alt. air for the engine. We have it on our bird for all the good reasons that a Part 23 spam can dose. Just my two cents on the question. Yours, R.E.A. III #80888

rockwoodrv9 04-18-2017 08:18 PM

Quote:

Originally Posted by Robert Anglin (Post 1166648)
If I were to be building with the installation of an IO-XXX "take your choice", I would as I have done, install Alt. air for the engine. We have it on our bird for all the good reasons that a Part 23 spam can dose. Just my two cents on the question. Yours, R.E.A. III #80888

What about a carb engine with carb heat installed?

BobTurner 04-18-2017 08:55 PM

Quote:

Originally Posted by rockwoodrv9 (Post 1166652)
What about a carb engine with carb heat installed?

If you use the stock design where the heated air is introduced before the filter, I'd put in the ALT air. The current pivoting plate door seems pretty robust.

N941WR 04-18-2017 08:59 PM

Quote:

Originally Posted by rockwoodrv9 (Post 1166652)
What about a carb engine with carb heat installed?

I have carb heat and didn't bother with the alt door. I also avoid taking off from shopping center parking lots.

I did replace the almost useless juice can heat muff for this from Wicks and actually see a noticeable RPM drop now.

It really comes down to your risk tolerance. Ask yourself this, "What are the odds of an intake blockage?" If you don't like your answer, install it

rockwoodrv9 04-18-2017 09:23 PM

This is the exhaust system I have and the heat muff. I do not think it is the best option for exhaust or how the FAB fits to the exhaust. I would not do it this way again.
https://www.vansaircraft.com/pdf/rev...ther/OP-54.pdf

I believe the carb heat I installed is acceptable risk for me and will not be installing the alt air.

Dbro172 04-18-2017 09:38 PM

Video
 
Here is a video of my alt air door in action. This is looking up at the bottom of the filtered air box, controlled from cable in cockpit. https://youtu.be/YhsEx4_Z9v0


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