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Airflow Straighteners

rv8ch

Well Known Member
Patron
My FM-200 is running a bit rich, and Don speculates that it's due to the very turbulent air coming in the FM-200, which is causing the "sensor" to feel that there's more air coming in than reality.

IMG_1766.jpg

This seems to also be a common problem with MAF sensors on cars, and the solution to that seems to be something called an "airflow straightener". It seems one of the popular ways to do this is with an aluminum honeycomb structure just before the sensor.

s-l500.jpg

Seems companies like Bosch include this feature in some of their MAF sensors.

vauxhall-2-0-petrol-maf-mass-air-flow-meter-bosch-0280217106-at-autovaux-genuine-vauxhall-suppliers-bfe.jpg


Does anyone have any experience using these on a Lycoming engine? Another option is of course to make the air coming into the throttle less turbulent, but that would require a lot of work on my intake, filter, cowl, etc. that I'm not really interested in taking on now. Another option is to get a leaner main jet in the FM-200 which I've ordered, but seems like another workaround. Thanks for any tips.
 

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Not exactly, but

I have applied a lot of flow conditioning devices for flow measurement; usually industrial gas turbine fuel flow related. Being that you are interested in a "permanent" installation downstream of any filtration, I'd be careful. Any failure scenario would probably dramatically block air flow; possibly worse. You can calculate and model it all day long but without the new device being instrumented for (mainly flow induced) vibrations, you would be rolling the dice a bit (English phrase about risk, sorry) regarding potential metal fatigue.

What kind of MAF sensor is it, hot wire anemometer? Other?
 
My FM-200 is running a bit rich, and Don speculates that it's due to the very turbulent air coming in the FM-200, which is causing the "sensor" to feel that there's more air coming in than reality.

How rich is "a bit" rich? What is your full power fuel flow (and field elevation)?

Skylor
 
19 GPH and 1421 ft

How rich is "a bit" rich? What is your full power fuel flow (and field elevation)?

At full rich I see 19 GPH at 1421 ft. I was thinking it should be closer to 16 or 17 GPH.
 
How rich is "a bit" rich? What is your full power fuel flow (and field elevation)?

"A bit rich" is likely only a symptom of the problem, sort of an average end result. Turbulence means instability, nearly instant, moment to moment variations in the dynamic and venturi pressure at the sample points. That pressure drives the movement of the air diaphragm, thus the metering ball.

I don't know for sure, but the AFP design may be a bit more sensitive to turbulent inlet flow given its single pitot opening and small, high gain venturi. The Bendix style would tend to average out pressure variations across the diameter of the inlet, as it has four pitot tubes and a large diameter venturi. Good design is artful compromise. AFP's small high gain venturi provides better signal at low flow without choking high flow.
.
 

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turbulence

"A bit rich" is likely only a symptom of the problem, sort of an average end result. Turbulence means instability, nearly instant, moment to moment variations in the dynamic and venturi pressure at the sample points. That pressure drives the movement of the air diaphragm, thus the metering ball.

I don't know for sure, but the AFP design may be a bit more sensitive to turbulent inlet flow given its single pitot opening and small, high gain venturi. The Bendix style would tend to average out pressure variations across the diameter of the inlet, as it has four pitot tubes and a large diameter venturi. Good design is artful compromise. AFP's small high gain venturi provides better signal at low flow without choking high flow.
It's starting to make more sense to me now. I think this was what Don was telling me but I wasn't getting it.

I guess the best solution would be a new air intake, but that's going to take weeks of fiberglass work.

Another option is the "straightener", obviously installed in a way that will stay in place - not 100% sure that this can be done without more fiberglass work, but much less than a new intake.

And the short term workaround is a leaner main jet, which Don is sending me. He didn't sound very optimistic that this would "solve" my problem of too rich at full rich mixture setting. I can of course just move the red lever, which is what I'm doing now, but it feels weird to not be at full rich on takeoff and landing at such a low altitude (1400 ft).
 
Mickey, is your FF calibrated?

Are the numbers of burned fuel correlate with the amount you need to top off the fuel tank?
 
Frankensnorkel

Not necessarily. Got some photos, lower end of your snorkel?

I'm kind of embarrassed to show this, but this is my snorkel.

http://www.rv8.ch/frankensnorkel/

IMG_9120-1024x768.jpeg


IMG_1633-1024x768.jpeg


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It's hard to tell, but it's not as smooth as the Van's snorkel. I feel I have a lot of room to make it better. At least there is no place in there that has a smaller cross section as the FM-200 intake. But it is far from smooth with laminar flow.
 
The actual shape might be more of a concern than the interior surface finish. While developing the new snorkel for the RV 14 we discovered that the shape of the turn into the airflow performance servo is very sensitive. The explanation that Dan gave a few posts back about the differences in how the RSA versus airflow performance servos meter inlet air is I think a likely explanation. Because of this, before going to far, you may want to send your home made one to Don and ask him to flow test it for you
 
Flow straightener I made for a Rotec TBI. Works perfectly.
 

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Correct

Exactly where I "borrowed" the idea. That was an inspirational thread, but I should have listened to Dan and just put in a straight pipe.

Perhaps in round two.

I'd like to find a way to get a straightener in there without massive amounts of work - that will be a rainy wintertime project.

I also had the idea of designing something in CAD and 3D printing something better, but not sure my skills are up to that challenge.
 
don´t worry!!

don´t worry about the looks....it will disappear underneath the cowling anyway!

The cirrus pilot won´t notice when you overtake him/her!!:D

The look is just for you!!



Nice Valve covers though……..
 
intake shape

The actual shape might be more of a concern than the interior surface finish. While developing the new snorkel for the RV 14 we discovered that the shape of the turn into the airflow performance servo is very sensitive. The explanation that Dan gave a few posts back about the differences in how the RSA versus airflow performance servos meter inlet air is I think a likely explanation. Because of this, before going to far, you may want to send your home made one to Don and ask him to flow test it for you
Thanks for sharing this Scott. Don mentioned this to me when I called him. I guess I have a few options:

  1. find a way to install a straightener
  2. remove the snorkel and change to a straight intake
  3. live with it (it's really not that bad)
  4. change the servo to something less sensitive to turbulent airflow at the intake

As much as I like building, I'm really going to struggle to take the aircraft out of service for any length of time - flying it is really enjoyable, and I'm only in phase 1 - I can only imagine how great it is to travel in it. :)
 
Thanks for sharing this Scott. Don mentioned this to me when I called him. I guess I have a few options:

  1. find a way to install a straightener
  2. remove the snorkel and change to a straight intake
  3. live with it (it's really not that bad)
  4. change the servo to something less sensitive to turbulent airflow at the intake

As much as I like building, I'm really going to struggle to take the aircraft out of service for any length of time - flying it is really enjoyable, and I'm only in phase 1 - I can only imagine how great it is to travel in it. :)

The reason I mentioned it is that during our "experimenting" phase, we test flew a design that at certain power conditions, the engine would actually die for a couple seconds, come back for a couple seconds, die for another couple seconds,...... repeat, for as long as you left it in that condition. Caused a few instances of increased blood pressure, until we finally discovered what was causing it.

For those that make their own home made snorkel, having Don test it to tell you whether it seems to flow at a steady state through the entire throttle range could be worth while. If you are already flying yours and not having any problems then it probably isn't necessary.
 
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stop/start

The reason I mentioned it is that during our "experimenting" phase, we test flew a design that at certain power conditions, the engine would actually die for a couple seconds, come back for a couple seconds, die for another couple seconds,...... repeat, for as long as you left it in that condition. Caused a few instances of increased blood pressure, until we finally discovered what was causing it.

For those that make their own home made snorkel, having Don test it to tell you whether it seems to flow at a steady state through the entire throttle range could be worth while. If you are already flying yours and not having any problems then it probably isn't necessary.
Very interesting. I reduced the main jet size by a half size, and it has been much smoother since - perhaps that's a similar thing - spikes of either no flow or too much. Don was not too excited about doing this, but thought it would be worth a try. I have another jet one more half size smaller which I might try.

I described my snorkel and Don said it's probably extremely turbulent, and I'm sure he's right. I had no idea that it would even be an issue when creating it, but it's clear why now.

I might just cut a hole in the cowl try one of these.

https://airflowperformance.com/index.php/product/fm-200-alternate-air-duct-2-valve/

FM-200-Alt-Air-2-Valve.jpg
 
Velocity Stack??

I've been following this closely since I'm molesting my snorkel too. I cut a hole in the front of my snorkel and added a ram air inlet. I'm currently working on the shutoff valve. I've been thinking about adding a short velocity stack to the servo inlet. Internet snooping shows different lengths, and of course they all add 'lots of horsepower'. I hate to disregard their results, but have you seen some of the aerodynamic doodads? Holy cow! Bolting a bathtub on there would be an improvement to some of the stuff I've seen.

Anyway, I'm curious to know if anyone else has tried the stack, and what the results were.

Regards, Lance
 
Add a scoop and the MK3 filtered ram air kit with the backplate that mates up to the FM200's optional connection sleeve.

AIRBOX_2.jpg


P1010677.jpg
 
An aside for casual readers..

The "RSA-5 class" fuel controls (Bendix RSA-5 and subsequent Precision Silver Hawk, AFP FM-150, and Avstar equivalents) are physically shorter than the FM-200 seen here. The FM200 also has a round, no-flange inlet rather than a 4-bolt flange. The additional length in particular makes good modification of a stock RV-8 snorkel more challenging. Note the very small (actually non-existent) radius where Mickey's snorkel turns into the mouth of the fuel control.

Mickey, this just isn't hard to fix. The minimum effort approach is a simple reshaping of your current snorkel, something like the attached. And as you've no doubt heard, the new IO-390-119 effort for the RV-14 resulted in a good snorkel shape for the FM-200, by test, not guess. It will be an available part eventually, and you may be able to adapt it to the -8 cowl, or the cowl to it.
 

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RV-14 snorkel

...Mickey, this just isn't hard to fix. The minimum effort approach is a simple reshaping of your current snorkel, something like the attached. And as you've no doubt heard, the new IO-390-119 effort for the RV-14 resulted in a good snorkel shape for the FM-200, by test, not guess. It will be an available part eventually, and you may be able to adapt it to the -8 cowl, or the cowl to it.
Thanks Dan - I didn't know about the RV-14 snorkel - will have to keep an eye out for that. You are absolutely right - I need to fix this. I like the idea of buying a solution I can (almost) bolt on - my fiberglass skills are so bad that I can see this taking me months, and I can't stop flying that long!

That said, I need to pull the snorkel off and have a look at what it would take to do what you suggest - might also be possible to safely glass in the flow straightener honeycomb I picked up off of ebay. Rainy weekend here so perfect timing. :)
 
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...my fiberglass skills are so bad that I can see this taking me months, and I can't stop flying that long!

Mickey, this fix should be easy, a few 20 minute work sessions.

Start with a short length of tube, about 1.5" or 2" diameter, and a rectangle of scrap aluminum. Duct tape them together with the scrap tangent to the circumference. Smear a little wax on the duct tape, then lay up three plies of 9 oz plain glass using fast hardener. Set aside to cure.

Make two saw cuts per the black line in the sketch. Remove the light green chunk. Now wrap 80 grit paper around another short length of the same size tube as the previous form, and sand a semicircle per the dark green.

When the fast hardener has the original layup firm enough, pop it off the ad hoc form. It should fit right into the sawed out gap in your snorkel. Reach inside with your fingertip and smear a filet of flox/epoxy in the 90 degree junction all along where the new part meets the snorkel. Again set it aside to cure.

When cured, cut and sand to remove all the excess exterior flange from the new portion. Round off the junction, exposing the flox filet, and lay up one exterior ply over the seam. You can also touch up the inside with some micro if desired. Sand inside and outside smooth after cure. Mount and go test fly.

Will it fix your problem? Can't guarantee it, but I'd bet a cold beer ;)
 

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Actually, I think that one has been discarded in favor of a better one.

Dan is correct
The snorkel shown in the photo was the early prototype / hybrid made by modifying the original that had been used with the RSA servo that was on the original 390A engine.
It was used for the initial flight testing of the new engine installation until the work was done to tweak the design for making production tooling.

It was at that point things started getting interesting.

We have have a reasonably good understanding of what makes good shapes when efficient movement of air is involved.
Very small tweaks were made to the shape (when compared to the original hybrid version that was used initially) while the final tooling was being made.

That snorkel was installed and everything was great, until we discovered a specific power range/flight condition where the engine was not happy at all.
This shape had very little difference from the current part that everyone is flying on the RSA style injection systems, but it was not working with the AFP.

Don and engineering at Lycoming worked hard to help but but were sure we had some type of ignition problem based on how it was acting. Long story short, we eventually came around to the realization that it was the snorkel. Don tested if for us on his flow bench and said it was horrible :eek:

Here is the interesting part..... at some point I discovered that if the (new) cowl flap was open, the engine ran fine. Close the cowl flap and the engine would start its die, run, die, run, cycle.
So just the small amount of pressure change across the inlet filter because of the change in upper plenum pressure with the cowl flap movement, was apparently having a noticeable effect on the inlet flow to the servo.
Redesigned the snorkel to have a very generous turn radius in the inlet run to the servo, and the problem was solved. We had Don test the final version as well and he said it flows very efficiently.

A side note top this (that I almost hate to mention so keep in mind it is just a single data point that was obtained during some other flight testing without any structured test plan).
While doing some flight testing at a dry lake bed (think miles and miles of smooth flat landing area) we did an impromptu test of the induction system by noting the MP value on the EFIS at ground level and then doing a flat out WOT run over the desert at 10 ' AGL. The in flight MP indication was a delta of +0.9 inches (0.9 " of ram air MP boost).
We are not prepared to officially promote this as part of the new design and we have no data for the old version to compare it too, but it is conceivable that closing down the outlet flow on the cowl in high speed flight will help with induction pressure since the inlet filter is inside the pressure plenum.

Here is a couple of photos of the final version prototype of the AFP snorkel.
Note the generous turn radius (about the same radius as the inlet to the servo). It may be difficult to copy this because the new RV-14 bottom cowl was reshaped ahead of the servo to give some additional room, but you should try and copy this to what every degree is possible. Note the photos are of the snorkel taped together after it had been cut in two to use it as a tool for making production tooling. Hopefully they will be useful.
The first is a view straight down from above showing the turn radius. The second looking directly fwd. from the servo side (but upside down). Hopefully they will be useful to those that are doing a DIY snorkel.
 

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I have a snorkel that I purchased from Vans a couple months ago and in checking it out found the outlet flange diameter to be 2 1/2 inches. My Superior IO360 has the FM 150 servo with forward facing cold air sump.

I spoke with Don at AFP when ordering another item and he mentioned that I needed a 3 inch opening mated to the FM 150 or I would lose around 5 hp.

Is there another version of the snorkel that Vans sells or do I have to do surgery on this one?
 
I have a snorkel that I purchased from Vans a couple months ago and in checking it out found the outlet flange diameter to be 2 1/2 inches. My Superior IO360 has the FM 150 servo with forward facing cold air sump.

I spoke with Don at AFP when ordering another item and he mentioned that I needed a 3 inch opening mated to the FM 150 or I would lose around 5 hp.

Is there another version of the snorkel that Vans sells or do I have to do surgery on this one?

The one you have as all that is currently available from Van's (that is why all of the discussion about DIY snorkels).
There will be a AFP version available soon but it will be of no help to you because it is for the 200 series servo which is larger in diameter than the 150 series.
 
snorkel

The one you have as all that is currently available from Van's (that is why all of the discussion about DIY snorkels).
There will be a AFP version available soon but it will be of no help to you because it is for the 200 series servo which is larger in diameter than the 150 series.
Do you think this snorkel will work on an RV-8? I modified mine to add the airflow straightener, and it had a dramatic effect. The idle is very smooth down to about 600 RPM. Everything feels much smoother. I believe I have an induction leak, which I hope to address with Ross's kit which I ordered, but will need to get the induction tubes welded since I have the Superior cold air induction which has fatter induction tubes.

Here's a picture while the epoxy/flox was drying, before cleanup.

Snorkel with Airflow Straightener before cleanup.png
 
Follow-up

Just a follow-up on this thread - the airflow straightener is still installed and working well.
 
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