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Cowl engine intake design question

Larry DeCamp

Well Known Member
I have been reading archives all day that support the "hole" in the cowl should protrude into the air stream to avoid boundary layer on the cowl surface. I would prefer to keep the protrusion minimum for appearance. Is a circular orifice perpendicular to the airstream suffice, or does it need to stick way out like the stock Vans cowls ??
 
I have been reading archives all day that support the "hole" in the cowl should protrude into the air stream to avoid boundary layer on the cowl surface. I would prefer to keep the protrusion minimum for appearance. Is a circular orifice perpendicular to the airstream suffice, or does it need to stick way out like the stock Vans cowls ??
So this is for the carb or FI from the bottom of the engine I assume.... Are you planning on making your own?

I will give you my very opinionated biased suggestion. Van's FAB (air-box) and scoop setup is a very good design, for a carb or updraft FI. I got 1" of MP rise at altitude in cruise with my RV-4 (vs Alt air on). To save time and effort I would buy Van's off the shelf induction system (cowl scoop/box).... If you have time to kill and want the challenge make your own scoop and box (and alt/carb heat air), it will take longer and likely end up costing more $ as well. In the end it might not work that great.

Here are some ideas if you want to go custom:

Link below looks sexy but has two big issues as installed in this plane. The filter is right at the high velocity inlet, draggy and little to no pressure recovery. Second is the scoop is near the prop hub where the air get's beat to death.... However getting close to the prop closer to the prop tip gives you more ram pressure. There is an excellent book called "Speed with Economy". I recommend it. His engine inlet scoop was LONG and ran to a fraction of an inch from the prop... (Look at some of the reno formula racers.) However you have to get closer to the prop tip not the hub. Prop hub area near spinner is bad air...

http://www.painttheweb.com/rv-10/SjCowl.aspx

This shows a SJ cowl grafting on a Van's inlet on. I recommend this.

http //romeolima.com/rv8/Cowl.htm

You can use van's inlet (which is not round but kind of 'D' shape) and stick it on the lower cowl like this. How you filter it and get pressure plenum to recover some RAM air (and alternate or heated air) will take some creativity. This is clearly smaller than the full Van scoop, which is also an area for the FAB airbox to sit in and cowl air outlet....

https://glasair-owners.com/uncategorized/use-of-vans-induction-air-scoop-in-a-glastar/

This to me is an ideal inlet. Also the shape of the inlet should be like an inside out airfoil... It is not too close to hub but close to prop, and does not stick out like Van's which is much lower on the cowl. However as I first said the Van's intake set up and is easy and works well.

andy_sideview.jpg
 
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Thanks Jet Pilot

Appreciate the effort to post links. I am not a performance or speed addict, but lots of food for thought.
The Vans FAB was a real bump in the SJ cowl bottom so I sent it back and fab'd a custom with oval filter (K&N 60 SI surface) to narrow the GUPPY fish look. Still not happy with looks. AFP says their FM-200 cone is more than adequate and that agrees with K&N tech criteria. So, the dilemma persists, use a cone up high or big box (ugly cowl) down low.:confused:
 
Perpendicular is generally fine. No, it does not need to stick way out.

However, a circular orifice in a surface that is inclined to the flow, such as the cowl lower surface, will not produce full ram pressure. You must project out some distance in order to get out into the undisturbed onset flow, or else your opening will have flow across the opening that is trying to turn to match the shape of the lower cowl.

The alternative is to make the inlet shape non-circular and/or oversized so that it is able to accommodate some substantial angle to the onset flow without separating, and create external diffusion to help achieve full ram pressure. Also, the "corner" flow below the spinner where the flow has to turn through an angle to match the front of the lower cowl has pretty high pressure to start with. That is why the P-51 inlet is there. And the intake on the F-1 Rocket.
 
Would the most efficient inlet be aligned with the spiral flow from the propeller rather than the oncoming airstream ahead of the aircraft? I've been wondering about this, and this thread has posts from those who can give an enlightened answer.

Cheers, David
RV-6A KBTF
 
However, a circular orifice in a surface that is inclined to the flow, such as the cowl lower surface, will not produce full ram pressure. You must project out some distance in order to get out into the undisturbed onset flow, or else your opening will have flow across the opening that is trying to turn to match the shape of the lower cowl.

Excellent point. The question becomes "How much is some distance?"

This requires a picture!

Intake%20Length.jpg


First example projects out to a point very near the blade path, and is at a significant radius from the hub. Example 2 is located at the same blade radius, but doesn't project forward as much. Example 3 is near the blade path, but at much less radius and higher, near the spinner.

Steve, your thoughts please. I'll venture that #1 and #2 would result in pretty much the same intake pressure, climb or cruise, thus my previous note saying it doesn't need to be way out there. However, as you say, if dimension "A" becomes too short, flow following the cowl face will reduce available dynamic pressure. So, returning to Larry's question, is there a rule of thumb for a minimum dimension "A" ?

Also, the "corner" flow below the spinner where the flow has to turn through an angle to match the front of the lower cowl has pretty high pressure to start with. That is why the P-51 inlet is there. And the intake on the F-1 Rocket.

Agree, Example #3...decent pressure in cruise, although I don't think it will do as well in climb. In cruise, that location exhibits relatively high pressure, but with low forward airspeed, it's too inboard to harvest any significant dynamic pressure found in propeller outflow.

When working with cooling inlets at maximized prop radius, low speed, high power climb coefficients of pressure are much higher than the available freestream dynamic.
 
Spinner high pressure / attached flow ?

Thanks Steve and Dan. Your comments raise the question:
If the flow off the spinner is high pressure, and ?stuck? to the surface, why not continue the surface into the intake airway like Rocket / P51 ? The bottom of the airway could be perpendicular like dan?s example #3.
 
All sorts of compromises must be addressed here.

If he bottom of the "D" inlet is properly shaped, it might be advantageous to tilt the inlet slightly downward - seeking to obtain the best Delta P at lower speed and high power demand (climb out).

FWIW
 
Still not happy with looks. AFP says their FM-200 cone is more than adequate and that agrees with K&N tech criteria. So, the dilemma persists, use a cone up high or big box (ugly cowl) down low.:confused:
I get it, this is a looks thing. You are not too concerned about efficiency.

This is what you want:
https://www.jamesaircraft.com/products/air-filter-kits/

You want the FI no bottom hump look? Sam James will allow you to have no or little bottom hump with carb... SJ has a right angle snorkel that transitions to forward facing, duct to the inline air-box. However with the O-360 that is a BIG carb and the engine is taller than than an O-320... Good luck, shows Pics when done. Love to see the solution.

There is nothing really magical about round unless you are going to have a venturi shape (curved surface) in the throat of your round intake... and then an efficient plenum right behind that. Round, "D", rectangle... Up to you and your taste. Just have generous radius on the lip of induction inlet and sufficient area... with nice smooth transitions preferably to larger area plenum to convert ram air (velocity) to pressure before going through filter. This is what the Van FAB box does well. Placement of your induction scoop relative to prop was mentioned before.

There are only so many solutions. There are aerodynamic research papers, books on the topic of scoops and cowl inlets.... Since you are going for look do what looks authentically pleasings, it is not that critical... If you have a Carb (especially O-360) the Vans FAB and scoop is easy and works well. It is a big hump however on the bottom of the cowl. I don't find it objectionable. I like it, but the Fwd facing FI cowl and vans air-filter in the engine cooling cowl opening is clean looking... You could change your engine to FWD FI but that is big bucks.
 
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Excellent point. The question becomes "How much is some distance?"

This requires a picture!

Intake%20Length.jpg


First example projects out to a point very near the blade path, and is at a significant radius from the hub. Example 2 is located at the same blade radius, but doesn't project forward as much. Example 3 is near the blade path, but at much less radius and higher, near the spinner.

Steve, your thoughts please. I'll venture that #1 and #2 would result in pretty much the same intake pressure, climb or cruise, thus my previous note saying it doesn't need to be way out there. However, as you say, if dimension "A" becomes too short, flow following the cowl face will reduce available dynamic pressure. So, returning to Larry's question, is there a rule of thumb for a minimum dimension "A" ?



Agree, Example #3...decent pressure in cruise, although I don't think it will do as well in climb. In cruise, that location exhibits relatively high pressure, but with low forward airspeed, it's too inboard to harvest any significant dynamic pressure found in propeller outflow.

When working with cooling inlets at maximized prop radius, low speed, high power climb coefficients of pressure are much higher than the available freestream dynamic.

I don't know of any rule of thumb, except for pitot tubes, they typically say 8--10 diameters. But the flow around the cowl will influence the flow angle even in example 1. Certainly much less so than example 2.

Assuming the inlet is oversized somewhat compared to the throttle bore, it may be relatively insensitive to flow angle. I've seen data for pitot tubes that show something like 1% error at 10 degree flow angle, whereas an oversized shroud surrounding the actual pitot tube can reduce error to 1% at 38 degrees. (ref 1.) Again it is important to have a nice leading edge lip on the inlet, both inside and out. The inside lip will help prevent separation as the flow turns into the duct from any angle misalignment, and the outside lip will help prevent separation that would be spillage drag.

Classical propeller theory would say that the increase in stagnation pressure behind the prop is uniform over the propeller disc if the propeller is optimally loaded (uniform induced inflow). The theory also says that the pressure increase exists in the propeller stream tube far downstream. Certainly practical propellers have some hub loss that reduces the inflow velocity near the center, especially C/S props that usually have a round root area that extends just outside the spinner. So there may be some measurable benefit to moving out some. But as for downstream distance behind the prop, there is not much opportunity for any dissipation between example 1 and 2 -- I would expect the measured stagnation pressure to be the same for both in the absence of the cowl flow.

And Dan did make an important distinction between climb and cruise -- at high speed, the pressure rise due to the prop gets fairly small, whereas the dynamic pressure from forward flight gets big. During climb, the opposite is true.

Reference 1. Ernest O. Doebelin, "Measurement Systems -- Application and Design." McGraw-Hill, 1975.
 
I don't know of any rule of thumb, except for pitot tubes, they typically say 8--10 diameters.

Thank you sir...I'll take it! Useful to know.

Assuming the inlet is oversized somewhat compared to the throttle bore, it may be relatively insensitive to flow angle. I've seen data for pitot tubes that show something like 1% error at 10 degree flow angle, whereas an oversized shroud surrounding the actual pitot tube can reduce error to 1% at 38 degrees. (ref 1.) Again it is important to have a nice leading edge lip on the inlet, both inside and out. The inside lip will help prevent separation as the flow turns into the duct from any angle misalignment, and the outside lip will help prevent separation that would be spillage drag.

That's a mistake I suspect with mine...it's relatively low Vi/Vo like the cooling inlets, but I didn't provide enough exterior radius around the combustion intake.

Classical propeller theory would say that the increase in stagnation pressure behind the prop is uniform over the propeller disc if the propeller is optimally loaded (uniform induced inflow). The theory also says that the pressure increase exists in the propeller stream tube far downstream. Certainly practical propellers have some hub loss that reduces the inflow velocity near the center, especially C/S props that usually have a round root area that extends just outside the spinner. So there may be some measurable benefit to moving out some.

This from CR3405. Seems to suggest a definite benefit to locating an inlet at the largest practical prop radius.

Prop%20Rake.jpg


Prop%20Outflow%20Text.jpg


Prop%20Outflow%20Cp.jpg


Observations based on measured upper plenum pressure during a 100 KIAS climb at WOT:

http://www.vansairforce.com/community/showpost.php?p=1177277&postcount=198
 
Again it is important to have a nice leading edge lip on the inlet, both inside and out. The inside lip will help prevent separation as the flow turns into the duct from any angle misalignment, and the outside lip will help prevent separation that would be spillage drag.

Can we expand on this a little? I?m working on my inlet now, converting to 3? round and moving it 2.5? forward. What is a nice shape for the lip?


Don Broussard
RV9 Rebuild in Progress
IO-320-B1A, Hartzell
Slick/ElectroAir
57 Pacer
 
One of our RV-8 builders has been gathering cowl pressure data, so i asked him to do a WOT climb and record. The result is interesting in the context of this discussion.

His cowl is stock RV-8. Mine has round, low Vi/Vo inlets relocated outboard and slightly upward. This photo from preliminary work circa 2008, showing both inlet types:

Inlet%20Prelim2.jpg


This is pressure data, mine circa 2012 or so, and the stock cowl yesterday. Note difference in Cp:

Climb%20Plenum%20Pressures.jpg


I do not wish to treat one data sample as an absolute truth. However, it matches the CR3405 indications, and I would be delighted if other builders would make the same measurements to expand the sample base.

What is a nice shape for the lip?

Steve once suggested looking at the shape of the inlet ring on current jet engine cowls. There is also old NACA test data for round engine cowls, and other sources.
 
Pitot angle to free air-stream (normal flight regimes) has little effect on the accuracy of airspeed. The static port is however much more effected and where the AS error comes. The static port on fuselage of SE prop tractor planes is a huge compromise and accurate like a broken clock, correct twice a day.

I love any discussion where the 1970's Mississippi State U, NACA paper on cooling of Horz opposed air-cooled GA planes is brought up. I wish they continued their research, but the aftermarket and experimental community have continued. If you look at the Reno Formula and Sport class you will see some interesting things. The Red Bull Air racers also have very well engineered solutions for reducing airframe and cooling drag, which does not allow engine mods, aero mods are OK. Talk about OCD picky. The devil is in the details. They are shaving fractions of a second from course times.

I would not dismiss the pressure pulse from prop adding MP to induction even in cruise. The book Speed With Economy did a lot of experimenting. You have to get induction inlet (away from hub) close to prop, I mean very close. So close most would not risk it for daily flyer. It makes sense at high cruise the advantage is diminished. Looks do matter and it might add more drag from long scoop might negate the power gain.


This is a summary of Speed with Economy, by Ken Paser, from EAA chapter (free)
http://www.eaa393.org/Presentations/ScotS_rev_SwEconomyKentPaser2006.pdf

I have a copy and it's $90 used on Amazon!
https://www.amazon.com/Speed-economy-Experimental-performance-improvement/dp/B0006F5HSY
 
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This is a summary of Speed with Economy, by Ken Paser, from EAA chapter

I too have a copy. Ken obviously added speed to his airplane, but that doesn't make the mods optimal, only better than before. It also pays to remember some things make airplanes faster, but not better overall.
 
Some of the fast F1 guys locate the induction inlet close to the prop and a fair ways out to recover max pressure at the carb. Found gains by doing it, confirming theory.

In some of my rad testing with pressure sensors, pressure fell off rapidly with increasing distance from the prop disc. Pressure was low near the prop shank as well, higher once I got out to about half blade radius.

For best ground cooling at idle, where pressure is low, I located my rad inlet as far forward as practical given the HX location and at about half blade span position.
 
This from CR3405. Seems to suggest a definite benefit to locating an inlet at the largest practical prop radius.

Prop%20Rake.jpg


Prop%20Outflow%20Text.jpg


Prop%20Outflow%20Cp.jpg


Observations based on measured upper plenum pressure during a 100 KIAS climb at WOT:

http://www.vansairforce.com/community/showpost.php?p=1177277&postcount=198

This is interesting, because the distribution of stagnation pressure fairly closely matches the distribution of bound circulation on the blade. This probably makes sense, since it is the bound circulation that is doing all the work.

One practical challenge of trying to position the intake farther from the hub arises for horizontal induction engines. I made my inlet come straight forward, and it looks like Dan's example #3. Folks with vertical induction sumps and an elbow will end up with the inlet positioned much lower, as in #1, #2. In order to try to lower my inlet, I would have to build in an S duct, which unfortunately would have to be rather short because of my ram-air/filtered air box. If the flow in the S duct separates, the result would be lower MAP than what I have now for sure. (Any old B-727 pilots out there? You know what it sounds like when an S-duct separates in front of a fan jet engine!) It would take some clever design work to try to integrate a gentle S turn into my ram/filtered air box. Making more of a plenum would enable that, at the risk of losses in a short diffuser into the plenum. Hmmmm. Trade-offs.
 
I too have a copy. Ken obviously added speed to his airplane, but that doesn't make the mods optimal, only better than before. It also pays to remember some things make airplanes faster, but not better overall.
100% spot on. I agree. However it shows systematic approach and methodology in making changes and measuring results. It is an old book and not everything he did worked or would I consider. However many things he did are now common speed tricks like hiding antennas in cockpit or reducing drag with cowl and fairing changes. Not sure he invented anything, but he put it to practice and quantified the changes. The end result was after many small changes he had much more speed for same fuel burn.
 
How is Horsepower established ?.

Thanks to all for the great response re: intake location/ geometry.
Much was focused on MP improvement potential due to location and geometry.
I assume that the rated horsepower of an engine is done at standard pressure (or corrected equivalent ). This leads me to conclude that if all I care about is 180 HP to fly the airplane, I just need to assure the intake supplies standard pressure or more on takeoff climb? This raises the question; did the certification engine have a filter, elbows, turbulent separations or a perfect tube into the engine.
Kyle at AFP offers to bench test my intake system. Test for what ? How is good defined ?
 
This raises the question; did the certification engine have a filter, elbows, turbulent separations or a perfect tube into the engine.

I've seen 'em on dynos with and without filters, but never with a duct.

Kyle at AFP offers to bench test my intake system. Test for what ? How is good defined ?

The common term is "carb loss", the pressure drop due to intake restriction. It applies to any flow restriction, not just carbs.

Is it a worthwhile service? Yes indeed. One measurement can banish a lot of BS.

Airbox%20on%20Flow%20Bench.jpg
 
Here is some of that banishment.

Early construction photo, a diffuser-type airbox with a K&N RU-3120:

Cone%20Filter%20Airbox.JPG


On the flow bench at AFP, it was not great; in addition to merely being restrictive, it also drove the FM-200 slightly rich.

RU3120%20results.jpg


And test results for a new airbox with a large area, deep pleat K&N 33-2124...a loss of 2" H20 (0.147 Hg) in return for full time filtration.

33-2124%20results.jpg


P1010010.JPG
 
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That big bell mouth on the intake will produce essentially ambient stagnation pressure in that air box.

Is that intake diameter what will be on the airplane? Or is that just the airbox intake, and there would be a smaller intake tube in front of that with a diffusing transition?

If there is a smaller intake tube in front of that and they had modeled it, with a small bell mouth on the inlet, they would have captured any potential loss due to separation of the diffuser.

There are some folks that strongly believe in having a plenum just in front of the servo/carb, with a "velocity stack" bell mouth inside. They supply this plenum with a small intake tube that transitions (diffuses) into the plenum. They seem to think that this will somehow create a higher stagnation pressure than what is available at the intake. I try to tell them the best they can hope for is to break even, and the potential is there to lose quite a bit.

Of course, a straight tube from the intake to the servo/carb does also have some viscous skin friction loss. This can be reduced by making the intake oversized (lower velocity) and then accelerating by tapering gradually or right at the servo with a nice radius on the intake flange of the servo.
 
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Dan, could you make a sketch of what the internals of that airbox looks like?

It looks like that is the test article pictured prior? It looks like the test has the same inlet diameter that the airplane has, with just a small diffusion in the front third of the box?

And although you cut the loss in half compared to the original diffusing inlet with the conical filter, that one only added 4" H2O loss, which still seems pretty small.
 
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Bell mouths when testing articles on a flow bench are standard ware. You can end up with some nasty turbulence otherwise which can nix the accuracy and validity of the tests.

Good to see actual numbers here Dan. You have a very efficient filter system.
 
Is that intake diameter what will be on the airplane? Or is that just the airbox intake, and there would be a smaller intake tube in front of that with a diffusing transition?

4" ring leading directly into the airbox. The idea was to go low Vi/Vo, like the cooling inlets, reducing risk of separation.

Dan, could you make a sketch of what the internals of that airbox looks like?

It looks like that is the test article pictured prior? It looks like the test has the same inlet diameter that the airplane has, with just a small diffusion in the front third of the box?

Best photo showing both halves of the box. The filter is trapped between the rails. Yes, the same as pictured earlier. 4" into 4", short internal diffusion, which I really wasn't depending on for much.

Airbox%20halves.JPG


P5240004.JPG


P5050005.JPG


Airbox.JPG


And although you cut the loss in half compared to the original diffusing inlet with the conical filter, that one only added 4" H2O loss, which still seems pretty small.

It is small, but the goal was to get filter loss down to near nothing, like an open, unfiltered intake. As it turns out, the filter loss here is just a wee bit less than Rod Bower posted for the open butterfly on his intake.

The test surprise (illustrating why a trip to the flow bench has value) was how the "bad" intake was driving the FM-200 rich by 11% or so. A/F would have been about 10.2
 
Bell mouths when testing articles on a flow bench are standard ware. You can end up with some nasty turbulence otherwise which can nix the accuracy and validity of the tests.

Yes of course. Drawing from still air, a bell mouth needed in either case.
 
4" ring leading directly into the airbox. The idea was to go low Vi/Vo, like the cooling inlets, reducing risk of separation.



Best photo showing both halves of the box. The filter is trapped between the rails. Yes, the same as pictured earlier. 4" into 4", short internal diffusion, which I really wasn't depending on for much.

Airbox%20halves.JPG


P5240004.JPG


P5050005.JPG


Airbox.JPG




It is small, but the goal was to get filter loss down to near nothing, like an open, unfiltered intake. As it turns out, the filter loss here is just a wee bit less than Rod Bower posted for the open butterfly on his intake.

The test surprise (illustrating why a trip to the flow bench has value) was how the "bad" intake was driving the FM-200 rich by 11% or so. A/F would have been about 10.2


Really nice. Yes, as with cooling intakes, external diffusion is always best. (low Vi/Vo)

I'm a bit puzzled -- the Bower intake, like mine, does not go through any filter when the butterfly is opened. Would not expect any loss at all, except for a little viscous skin friction on the tube. I see about 0.25 in. hg. loss on mine going through the conical filter compared to straight ram. My air supply for the filter box comes from the #2 intake cooling ramp through a 2.5" scat tube with no inlet lip at the ramp. But with the butterfly open and full ram, I would not expect any noticeable loss.

The richening is surprising. Isn't that something that would get adjusted for on installation, adjusting the base mixture screw to get 30--50 rpm rise at idle from rich to ICO?
 
I'm a bit puzzled -- the Bower intake, like mine, does not go through any filter when the butterfly is opened. Would not expect any loss at all, except for a little viscous skin friction on the tube.

Screen capture, circa 2014.

Rod%20Bower%20IO-580%20Dyno%20Numbers.JPG


The butterfly drop...difference between (1) a bare FM-300 with an Airflow Performance supplied velocity stack and (2) the open 3.5" valve is 29.29 less 29.10, or 0.19Hg, or 2.56" H2O. Conceivably the drop is not specifically due to the butterfly, but rather due to selecting a 3.5" diameter for a design feeding a 580. On the other hand, I would not make that design choice.

Sucking through the big reed valves, the loss was 29.29 less 27.88, or 1.41" Hg, or 19" H2O.

We should note that these measurements appear to have been made on Lycon's test stand, which is a prop dyno...it supplies the intake with ram pressure from the prop outflow, not static pressure like most dyno installations.

I see about 0.25 in. hg. loss on mine going through the conical filter compared to straight ram. My air supply for the filter box comes from the #2 intake cooling ramp through a 2.5" scat tube with no inlet lip at the ramp. But with the butterfly open and full ram, I would not expect any noticeable loss.

Now I'm surprised, as you're describing something I didn't know existed, a Bower system with the filter canister fed from a ram air source. Are there no reed valves in your installation?

The richening is surprising. Isn't that something that would get adjusted for on installation, adjusting the base mixture screw to get 30--50 rpm rise at idle from rich to ICO?

Idle mixture screw adjustment does not affect WOT mixture.
 
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Screen capture, circa 2014.



We should note that these measurements appear to have been made on Lycon's test stand, which is a prop dyno...it supplies the intake with ram pressure from the prop outflow, not static pressure like most dyno installations.



Now I'm surprised, as you're describing something I didn't know existed, a Bower system with the filter canister fed from a ram air source. Are there no reed valves in your installation?



Idle mixture screw adjustment does not affect WOT mixture.

Most dyno installations indraft from nominally quiescent air in the enclosure, which, with a good bell mouth, should be the same as stagnation pressure. Albiet not boosted stagnation from a propeller.

Yes, I made my canister myself with a ram air source. No reed valves. It just didn't seem to make sense to indraft the warmer, lower pressure air in the lower cowl plenum. I've also been concerned that those reed valves may not seal all that well.


Hmm. I was never at all clear on how the mixture adjustment works on a Bendix RSA-5, but leaning it did seem to reduce my WOT fuel flow - perhaps my imagination.
 
Yes, I made my canister myself with a ram air source. No reed valves. It just didn't seem to make sense to indraft the warmer, lower pressure air in the lower cowl plenum. I've also been concerned that those reed valves may not seal all that well.

Ahhhh.

Agree, sucking hot, low pressure air through reed valves makes no sense.

Got pictures?
 
See this old post:
http://www.vansairforce.com/community/showthread.php?t=49556&highlight=ram+air

In that post I mis-stated the pressure drop through the filter. In previous and later posts I stated it correctly. Filtered = 0.25" hg loss at cruise power (2400/24) and 0.3" hg loss at take-off power.

I remember now, thanks.

No shutoff for the ducted supply when the butterfly is open, which means both intakes are providing pressure. That's clever. It's possible the cooling intake has more available dynamic pressure than the ram inlet.

I have upper plenum pressure plots for three RV8's. Assuming stock exit size and decent baffles, upper plenum pressure will ballpark around 0.95 Hg at 170 KTAS and 3500 ft. You've seen the piccolo tube setup. They don't measure right there in the inlet, so we can't be super sure you have the 0.95 at that specific location...but its a fair assumption.

What is the area of the ram intake under the spinner?
 
I remember now, thanks.

No shutoff for the ducted supply when the butterfly is open, which means both intakes are providing pressure. That's clever. It's possible the cooling intake has more available dynamic pressure than the ram inlet.

I have upper plenum pressure plots for three RV8's. Assuming stock exit size and decent baffles, upper plenum pressure will ballpark around 0.95 Hg at 170 KTAS and 3500 ft. You've seen the piccolo tube setup. They don't measure right there in the inlet, so we can't be super sure you have the 0.95 at that specific location...but its a fair assumption.

What is the area of the ram intake under the spinner?

Interesting thoughts. I always figured the cooling intake probably ran slightly lower pressure than the induction inlet, and that I was probably leaking a little bit of my induction pressure with back flow through the filter and into the upper cowl plenum. But I don't know. It doesn't seem to be much flow either way.

It is hard to assign a hard number for the induction inlet area, because the lip is such a generous radius, and then it starts to transition to round right away. It is customary in the jet engine inlet world to use the "hightlight" area, which is to the leading edge point of the lip. If I count it that way, I'm guessing something like 12 sq. in. If instead, I take the area to the inside tangent point where the lip ends and the duct starts, subjectively, it is probably about 9 sq. in. The cylindrical tube with the butterfly valve that runs into the filter can is 3" dia, so 7 sq. in.

I have the picollo's installed, I just haven't gone out to fly the test yet. I have to go down to the sacramento valley to get down to 3000 msl and I've been waiting for cooler weather, which is now. I'll try to do it this week.
 
Interesting thoughts. I always figured the cooling intake probably ran slightly lower pressure than the induction inlet, and that I was probably leaking a little bit of my induction pressure with back flow through the filter and into the upper cowl plenum. But I don't know. It doesn't seem to be much flow either way.

It is hard to assign a hard number for the induction inlet area, because the lip is such a generous radius, and then it starts to transition to round right away. It is customary in the jet engine inlet world to use the "hightlight" area, which is to the leading edge point of the lip. If I count it that way, I'm guessing something like 12 sq. in. If instead, I take the area to the inside tangent point where the lip ends and the duct starts, subjectively, it is probably about 9 sq. in. The cylindrical tube with the butterfly valve that runs into the filter can is 3" dia, so 7 sq. in.

Assume 12 sq inches, 170 KTAS, 3500 ft, std day, and 2400 RPM. Available ram would be 0.85" Hg, a little less than what appears to be available at the cooling intake. However, throw in filter and duct loss, and it would probably be a wash with the butterfly open

if we use the 9 sq in value, available ram drops to 0.67" H, and the cooling air route would probably have positive flow.
 
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