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  #31  
Old 08-24-2017, 11:28 AM
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RV7A Flyer RV7A Flyer is offline
 
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Quote:
Originally Posted by DanH View Post
But a bug? Allow an example. A Bendix-style dual diaphragm fuel control balances dynamic (ram) and venturi pressure against a fuel pressure delta:



Plug the ram opening with a fat bug, or snow, or ice, and theory says mixture is going lean.
Wouldn't it stay the same as long as you're at a fixed/steady altitude and power combination? If it was completely blocked (i.e., one port), wouldn't that mimic what a pitot system does if it gets blocked (unchanged until a change in altitude)?
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  #32  
Old 08-24-2017, 12:22 PM
vic syracuse vic syracuse is offline
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I see I've been mentioned a couple of times, so I guess I should chime in.

Yes, the early models did not have filters. My RV-4 was that way, and while I didn't like it, it didn't seem to be a problem. I did run safety wire back and forth across the air duct to catch the big stuff, kind of like chicken wire. I was in Cleveland, Ohio at the time, and 99.9% of the time flying off of paved strips. I don't think I would like that today when flying off of grass strips, which I have been doing since 1997. I am amazed at how dirty the K&N filter gets, with all kinds of bugs, grass, and other stuff. And even for the customers who do the majority of their flying off of pavement, the filters do capture a lot of junk.

It was a carburator version of the O0320 on the RV-4, but as Dan pointed out, with an injected system there is no way I would run unfiltered air. It seems silly to me to risk an expensive airplane and engine for an almost imperceptable amount of MP increase. It's not like the RV's are underpowered.

For racing, it's a whole different story, and usually there is a nice runway right underneath you.

With regards the Rod Bower systems, yes they may be improved now, but on the two I removed (which were done according to plans) the engines were really choked at full throttle. As a demonstration, on the takeoff rollI would engage the bypass and the engine would immediately surge, dramatically improving the takeoff roll and climb performance. Once the owners saw this, we went ahead and installed the stock FAB and air filter. I assure you that it's not because I like building and installing the FAB's.

Vic
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Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES
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  #33  
Old 08-24-2017, 03:08 PM
Bicyclops Bicyclops is offline
 
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Default Filtered ram air

I had the Superior cold air sump and an updraft cowl, so I took a deep breath and cut all the protrusions off the cowl and started over.

[IMG][/IMG]

This how it wound up.

[IMG][/IMG]

This is the back side of my airbox. The alternate air door is cable actuated with a pushbutton lock cable.

[IMG][/IMG]

[IMG][/IMG]

The alt air door open. I used silicone rubber sheet as both gasket and hinge. The spring steel strap is to insure a good seal. I can pop it open past that with a good hard pull on the control. Should be enough air to get me to an airport.

Ed Holyoke
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  #34  
Old 08-25-2017, 08:49 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by RV7A Flyer View Post
Wouldn't it stay the same as long as you're at a fixed/steady altitude and power combination? If it was completely blocked (i.e., one port), wouldn't that mimic what a pitot system does if it gets blocked (unchanged until a change in altitude)?
Joe, that's a good catch, thanks. Did some checking last night; neither AFP or Bendix air diaphragms have bleed hole (I assumed they did), thus you're correct. Assuming a good solid plug in the ram opening, trapped ram pressure would not bleed away, thus the ball valve would not move if nothing else changed. (Bonus trivia; the AFP fuel diaphragm does have a bleeder, Bendix does not.)

Given a plugged ram opening, ball valve position would go awry with changes in density or throttle. Open throttle, lean, closed throttle, rich, etc. Again, I have no idea exactly how much, but my vote remains with keeping the trash out.
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  #35  
Old 08-25-2017, 09:02 AM
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DanH DanH is offline
 
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Originally Posted by Bicyclops View Post
This is the back side of my airbox. The alternate air door is cable actuated with a pushbutton lock cable.....I can pop it open past that with a good hard pull on the control.
Ed, you probably have, but be sure to test or calc how much knob force would be required to pull it open against perhaps 20" Hg (a partial filter blockage). An alternate plan might be a trigger release on a door that pulls inward (no significant pilot force required), or a spring loaded door requiring no pilot action at all.

Airbox with a spring-loaded automatic door. A microswitch flashes a cockpit LED in the event the door opens. http://www.vansairforce.com/communit...t=44856&page=2

There are some certified airplanes with similar alt air schemes. This one makes the FM-200 go slightly rich, and it would reduce airflow somewhat, so I expect some power loss if I suck up a plastic bag, block the filter, and the door opens. On the other hand, I have a 390, and expect to have enough power remaining to make it a non-event. In return it is truly automatic, no pilot action required, no diagnosis while playing deer-in-the-headlights as the runway end goes by.

Note to readers...Ed and I spoke at OSH. He may have interesting data to share later, after more flight test.
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Last edited by DanH : 08-25-2017 at 11:26 AM.
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  #36  
Old 08-25-2017, 09:31 AM
HSANTIBANEZ HSANTIBANEZ is offline
 
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I have the Rod Bower and read the same MP with another RV at the same speed and altitude WOT.

SO IT DOES NOT WORTH Use the Snorkel and the filter and get the same results.

BDW I Have the Hartzell Prop, may be with other works beter because the root of the prop does not push air to the scoop in my installation.
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  #37  
Old 08-25-2017, 03:04 PM
Bicyclops Bicyclops is offline
 
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Howdy Dan,

Thanks for pointing that out. Something perhaps I didn't think hard enough about. I'll probably replace my pushbutton locking cable with a locking Tee handle so I can get more oomph on it if I ever need to. With a good grip and adrenaline, I'll get it open if I have to, I hope.

I have limited space available on the airbox to make it inward opening. I think that I might have trouble making room for the air to get past that in quantity, as well. It would be hard to make the inner face of my very rough homemade airbox as flat as it needs to be to seal, too. As it is, the opening is smaller than the normal intake. I haven't tried to calculate how much that might limit power, but it's got to be better than none at all.

Ed Holyoke

Quote:
Originally Posted by DanH View Post
Ed, you probably have, but be sure to test or calc how much knob force would be required to pull it open against perhaps 20" Hg (a partial filter blockage). An alternate plan might be a trigger release on a door that pulls inward (no significant pilot force required), or a spring loaded door requiring no pilot action at all.

Airbox with a spring-loaded automatic door. A microswitch flashes a cockpit LED in the event the door opens. http://www.vansairforce.com/communit...t=44856&page=2

There are some certified airplanes with similar alt air schemes. This one makes the FM-200 go slightly rich, and it would reduce airflow somewhat, so I expect some power loss if I suck up a plastic bag, block the filter, and the door opens. On the other hand, I have a 390, and expect to have enough power remaining to make it a non-event. In return it is truly automatic, no pilot action required, no diagnosis while playing deer-in-the-headlights as the runway end goes by.

Note to readers...Ed and I spoke at OSH. He may have interesting data to share later, after more flight test.
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  #38  
Old 08-25-2017, 03:22 PM
Bicyclops Bicyclops is offline
 
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Default MAP at WOT - ram air effect?

Using online density altitude calculators, it appears that I am seeing something like an inch or two more MAP than the expected absolute pressure. I have a homemade filtered ram air induction as seen in an earlier post. I need to do more testing to validate that claim, and I'm curious what other people's numbers are at various altitudes and temperatures.

The density alt calculators want: Altitude, OAT, altimeter setting, and dew point. The one I used returns: Density altitude and absolute pressure, among other things. I didn't record dew point for that day, but various numbers make little difference in the output.

https://wahiduddin.net/calc/calc_da.htm

So, for my flight at 13500', 74degree, 30.07 alt setting , I see a calculated absolute pressure of just over 18" and my MAP gauge was reading 20.5 at wide open throttle.

Anybody have numbers to share?

Ed Holyoke

[IMG][/IMG]
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  #39  
Old 08-25-2017, 03:39 PM
TShort TShort is offline
 
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Some numbers from my stock -10 with FAB per plans:

Just flew home from KDDC yesterday, at 13,500' OAT 41F alt 30.09 WOT MAP was reading 17.4"
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  #40  
Old 08-25-2017, 05:08 PM
vic syracuse vic syracuse is offline
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You can check your digital MP gauge on the ground for accuracy by moving the BARO setting on your altimeter until the Altitude reads Zero. Your MP indication at that point (with the engine off of course) should be within a couple of tenths of the new BARO setting.

Another way to check your actual RAM air increase is to note your MP prior to starting your engine and then fly donw the runway at 50' or so with wide open throttle and full RPM and check the MP pressure indication. It's better to do this with someone else on board looking at the gauge. . The difference in indications will show that you have a gain or a loss.

Vic
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Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES
Kitplanes "Unairworthy" monthly feature
EAA Sport Aviation "Checkpoints" column
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Author "Pre-Buy Guide for Amateur-Built Aircraft"
www.Baselegaviation.com
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