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  #1  
Old 08-22-2017, 04:04 PM
13brv3's Avatar
13brv3 13brv3 is offline
 
Join Date: Nov 2005
Location: Tellico Plains, TN
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Default Unfiltered Ram Air?

Greetings,

Does anyone fly with unfiltered intake air? I've owned a few experimental planes and have never considered such a thing before now. The RV-3B I purchased recently has horz FI with a selection for unfiltered RAM Air, or filtered air. I understand the idea is to open the RAM air on climb out, and then close it again on landing approach. Assuming you're not flying through visible dust clouds, just how bad is that for the engine? The whole idea rubs me the wrong way, but maybe it's not really a problem?

I'd be tempted to use the filtered air all the time, but it's not really made for that. The filter is located inside the cowl where it's not cool or pressurized as much as it could be, so there's a distinct loss of power.

I guess I'm trying to decide whether I should leave well enough alone, or change the system for a better, full time filtered air setup.

Thanks,
Rusty
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RV-8, SN-80587, built, flown, sold.
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  #2  
Old 08-22-2017, 04:29 PM
johnthecontractor johnthecontractor is offline
 
Join Date: Aug 2016
Location: Anderson, SC
Posts: 8
Default

yes,

not an RV, but I currently fly a Laser 200 - no air filter. Also, ever look at the front of an Extra 300, Walter doesn't put a filter.

I did put fuel injection on a 3B several years ago and the fuel servo was slightly taller than the carb pushing the air box down into the cowl. I spent alot of time finding a different ht air filter and cutting down the air box.

Im not saying I dont like air filters, I do. Im just saying it does not bother me not to have one.

Mutter
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  #3  
Old 08-22-2017, 04:33 PM
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Mel Mel is offline
 
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Location: Dallas area
Posts: 10,768
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This was originally the standard configuration on the RV-3 and -4.
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  #4  
Old 08-23-2017, 07:43 AM
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13brv3 13brv3 is offline
 
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Location: Tellico Plains, TN
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Thanks for the comments. This picture shows the current filter arrangement, which is certainly not ideal for full time use.

https://www.amazon.com/photos/share/...rijSjtI52clht9

I may have to fly it as the builder originally intended for now, but once I get a hangar closer to home, I'm going to probably rework this to a better full time filter configuration.

Cheers,
Rusty
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RV-8, SN-80587, built, flown, sold.
RV-3B, SN-10751, rotary engine, built, flown, sold
RV-8, SN-82470, built, flown, sold.
RV-3B, SN-11351, purchased, , flown, sold
A&P - 2018
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  #5  
Old 08-23-2017, 07:56 AM
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rleffler rleffler is offline
 
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I have a Rod Bower RAM air mod for my RV-10

http://www.ramairforhomebuilts.com/

I typically don't open it until I get about 5,000 ft. Although, I usually don't close it on the descent. It yields about a 1" MP increase.
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  #6  
Old 08-23-2017, 08:27 AM
rmarshall234 rmarshall234 is offline
 
Join Date: Jun 2008
Location: San Diego, CA
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My RV-3 did not have a filter. I put about 500hrs on it and it had 700 SMOH when I sold it. Although I think air filtration _on the ground_ is a very good idea, my results were excellent over a 7 year period. Compressions and oil burn were still quite good after 700 hrs on a pumped up O-320. Mine was w/o an oil filter so oil changes were every 25hrs. Maybe that helped.

Anecdotally, I once flew it wide open at sea level on a standard day and the MP gage showed almost 2" of ram air "boost". Mine had an Ellison TB on it.

The plane I'm building now will be set up the same way or may have "filtered air" as an alternate source.
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  #7  
Old 08-23-2017, 09:42 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by rmarshall234 View Post
My RV-3 did not have a filter...Anecdotally, I once flew it wide open at sea level on a standard day and the MP gage showed almost 2" of ram air "boost".
I flew mine upside down across the Atlantic.

(Note: Both statements are nonsense)
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  #8  
Old 08-23-2017, 11:32 AM
SJordan SJordan is offline
 
Join Date: Sep 2016
Location: Scottsdale, AZ
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I'm currently building a -4 with horizontal induction using the Air Flow Performance Y for filtered or ram air. Its going to work but adds weight and complexity. If I could go back I would just use ram unfiltered air 24/7. I live in the AZ desert and have rebuild a handful of off road engines that have seen pounds and pounds of dirt go through them. All of them showed little problems dirt related. Our planes will see little to no dirt, I say run it the way you got it (ram air) and don't think twice about it.
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  #9  
Old 08-23-2017, 11:36 AM
SJordan SJordan is offline
 
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Location: Scottsdale, AZ
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Quote:
Originally Posted by rmarshall234 View Post
My RV-3 did not have a filter. I put about 500hrs on it and it had 700 SMOH when I sold it. Although I think air filtration _on the ground_ is a very good idea, my results were excellent over a 7 year period. Compressions and oil burn were still quite good after 700 hrs on a pumped up O-320. Mine was w/o an oil filter so oil changes were every 25hrs. Maybe that helped.

Anecdotally, I once flew it wide open at sea level on a standard day and the MP gage showed almost 2" of ram air "boost". Mine had an Ellison TB on it.

The plane I'm building now will be set up the same way or may have "filtered air" as an alternate source.
All the charts I have seen say a 2" rise in MP would required something like 400kts. I think 1" rise is MP on an RV is about all you can hope for, but 1" is 10hp so I will gladly take it.
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  #10  
Old 08-23-2017, 12:45 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by SJordan View Post
All the charts I have seen say a 2" rise in MP would required something like 400kts. I think 1" rise is MP on an RV is about all you can hope for, but 1" is 10hp so I will gladly take it.
Maximum available dynamic pressure, standard day, is approximately 1.55" Hg at 180 KTAS at sea level. 200 KTAS at 1000 feet is 1.86". Even if your RV will run 200 KTAS in level flight, it is not possible to convert all the 1.86" available dynamic pressure to increased manifold pressure; an engine intake is not a closed end system like an airspeed pitot.

Given an IO-360 at 2700, 3" intake diameter, 1000 feet standard day, and 200 KTAS, MP rise would be about 0.9559" Hg, for a total MP of 29.812".

Returning to the OP's question, there isn't any good reason to run filterless on an everyday airplane, if the system can be built with enough filter media area. By measurement, the pressure drop for my entire airbox, including filter, is 0.19" Hg, as compared to an FM-200 with a standard test bellmouth. For perspective, Rod Bower reported a bit more than that for the open butterfly in his ram air system. With no butterfly, the drop might be half as much, but not zero. We're talking about giving up filtration in return for less than 0.1" Hg, max. Doesn't make sense.

Reno racers? Sure, take no prisoners.
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