Home > VansAirForceForums

-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

Go Back   VAF Forums > Main > RV General Discussion/News
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Thread Tools Search this Thread Display Modes
Old 04-19-2012, 05:21 PM
Join Date: Sep 2005
Location: 1T7, Kestrel Airpark , Texas
Posts: 782

Originally Posted by flyboy1963 View Post
I may be ill-qualified to comment, but this is the part of home-building that excites me, too!
...and if it's too much work to re-profile the exit ramp, can we cheat a bit by using vortex generators at the right spot, to keep the flow attached, with little drag penalty?
Actually, a properly shaped coanda surface will keep the flow well attached. You do not want vortex generators on the exit, they decrease the flow velocity and at the outlet you want to increase velocity as much as possible.

Now a good place for them would be on the inlet wall, that would allow a greater than the 7deg divergence angle.
Wade Lively
-8, Flying!
formerly IO-360A3B6D, Dual SDS CPI now, WW 200RV
Reply With Quote
Old 04-19-2012, 09:31 PM
Bob Axsom Bob Axsom is offline
Join Date: Mar 2005
Posts: 5,685
Default 4-19-12 work and observations

I went back to the airport today with the intention of patching the cutouts on the development cover but it looked liks a very messy job with the location of the mounting interface and a previous patch so I spent 6 hours making a new cover.

When I pulled the development cover off, the right Channel was free of any mark but the left channel had a elongated brown smudge on the high part of the bump.

This is from the breather which is directed onto the left exhaust pipe. On the back side of the bump following the line of the streak there is a swirl to the left (counter clockwise) from near the center to the inside of the left flow fence (sorry I had to reverse the photo direction to get the picture):

This indicates to me that the flow is not linear and may be influenced by the exhaust cutouts. I have no reason to believe the the flow in the right channel was not similar.

When I finally got the new cover fabricated it was near dark so I could not do a proper speed test.

It sounded loud as you would think an exhaust pipe stuck in an enclosed chamber would sound. I only got up to 4,500 ft MSL but it would not surprise me to find that it was near 6,000 ft D alt. The highest ground speed I saw was in the 180s and the TAS on the AI was around 170. I was not impressed.

Tomorrow I have to fly to Llano, TX for a race on Saturday. I cannot do a formal speed test enroute because I do not have enough fuel. Maybe I can do one down there. I don't want to run the race with a suspect configuration. I have to try to remember to take the thin triangular flow fences that I can swap in tomorrow at Llano it the speed test is bad.

I am interested in the results of the covered test (gut feel - it's gonna be bad) and the open channels with the bumps (hopefully better). I have my thoughts on why but I'll let the results speak for themselves.

Bob Axsom

Last edited by Bob Axsom : 04-19-2012 at 10:51 PM. Reason: typo - isn't it always
Reply With Quote
Old 04-19-2012, 10:17 PM
johnny stick johnny stick is offline
Join Date: Dec 2008
Location: Los Angeles, CA
Posts: 301
Default looks interesting

Looks to me like the bumps are working. The exhaust and airflow are staying attached to the bumps. If they were not staying attached, I think the swirl would not be there and you may see some reverse flow; I do not see this in the picture. I am curious to see the speed results, but I bet you are right. Maybe all it needs are some generous fillets on the inside?? Just guessing.

Thanks for all these experiments. I suspect there will be a new "stock" exit coming from all of this. I cannot question the speed increase, it seems very real.
Reply With Quote
Old 04-19-2012, 10:58 PM
Bob Axsom Bob Axsom is offline
Join Date: Mar 2005
Posts: 5,685
Default I don't want you to get the wrong impression

I will continue working on this because I think there is something to be gained but I don't claim one microknot yet. My best speed to date is 184.4 KTAS. Until I pass that number I'm just thrashing around looking for speed.

Bob Axsom
Reply With Quote
Old 04-20-2012, 01:41 AM
rvmills's Avatar
rvmills rvmills is offline
Join Date: Nov 2007
Location: Georgetown, TX
Posts: 2,161

Some very thought provoking discussion! I know Bob is gearing up for the Hill Country Air Race tonight, but this will give him some great post-race reading bounce his results off of!

Comments below:

Originally Posted by rzbill View Post
<snip> One item that has been included in my thoughts is inclusion of the 7 degree ramp angle used on the "floor" of a NACA duct. It is designed to allow velocity change (expansion) with minimal separation turbulence and I really don't want to do the math to redesign it so 7 degrees is it.

I think the coanda bump can be doing a reasonable job of accelerating the exit air via area reduction and possibly redirecting the exit air to be more parallel to the belly (if it is not separating on the rearward expansion ramp) Needs tuft test.

I postulate that a cover might work better if is at a 7 degree angle to the free stream. (Narrowing the exit)
The purpose is twofold:
1) Reduce the expansion area (slowing of velocity) on the backside of the coanda bump2)

Purposely expand the freesteam and slow it in the local region of the exit to better match the two stream velocities. <snip>
Paeser postulated the best angle for exit air was at 10 degrees to the free stream...not far from the NACA 7 degrees...though it may be a slightly different concept at work (not sure).

There may be some acceleration of the air on the first half of the bumps, but I would be concerned about separation on the aft side (as you and others have mentioned). There may be some coanda effect, but I doubt laminar flow can hack that angle, and it sort of has a venturi-like look to it. Only down side is the back half of the venturi would expand and slow the exit air...opposite the goal. (more on that below).

Also, I think slowing the freestream air locally would be counterproductive...I know you're talking about makiing the flows more compatible, but it would seem slowing anything in this area would be drag-producing. Just a comment for discussion...this is all facinating stuff!

Originally Posted by DanH View Post
<snip> I don't see much area reduction.

Hard to tell if the exhaust jet is attaching to the curved surface, a proper Coanda Effect. Bob, are there now exhaust stains on the bumps?

If the tailpipes were pointed more directly at the curved surfaces and the belly cutouts were filled, I would then expect the exhaust flow to attach, and to entrain cooling outlet flow through a reduced area, increasing it's velocity.
Dan, I think there is probably some area least compared to Bob's previous configuration, without the bump. I do see your point about re-filling those cutouts (though I'm concerned about blocking the exhaust directly and causing odd flow patterns downstream), and I also wondered about exhaust stains on the bump. Looks like it is there, but the swirl pattern may indicate separation or turbulent flow.

My pea-brained effort at a similar concept to what your last paragraph says is pictured below (a mod of a picture I did earlier):

From discussions with Paul Lipps, the coanda radius at the base of the firewall might do the lion's share of redirecting and attaching the flow exiting the cowl. Then the continuation of that curve...a softened version of Bob's current bump, as others have suggested) might maintain laminar flow and continue that coanda effect further aft. Opposite that upper bump, on the lower surface, another bump that forms the front half of a venturi could add to the area reduction and exit flow acceleration. Tuning of the system would mean trying to direct the flow aft without reopening of the venturi prior to the end of the ramp (I think)...which I don't depict well in this picture. I do agree with you that decreasing the down-angle of the pipes might increase Bob's current coanda effect, and might allow the aft part of the bumps in my picture to be more aligned with the belly and the freestream.

My kluge of a bunch of stolen ideas, somewhat applied to what Bob is currently working...all FWIW!

And great work always! Good luck in the race!!

Bob Mills
RV-S6 "Rocket Six" N49VM
Cross Country-Marshall Field (07TS)
Georgetown, TX
President/Sport 49, Sport Class Air Racing
Trustee, Formation Flying Inc (FFI)

Last edited by rvmills : 04-20-2012 at 01:49 AM.
Reply With Quote
Old 04-20-2012, 05:31 AM
Bob Axsom Bob Axsom is offline
Join Date: Mar 2005
Posts: 5,685
Default Thanks for looking at my results and providing observations

Thanks for looking at my results and providing observations. I just woke up in a sweat totally unprepared for the race tomorrow with a configuration that I think is slow and many thousands of Arkansas and Oklahoma but mostly Texas bug guts coating all the exposed forward surfaces of the Blue Bird. I have to bring: bucket, sponge, rags, tools, triangular light weight flow fences, RTV, computer, changing course info, ... It's all for points to stay in the season SARL National Championship hunt in this race I'm afraid. After I get there and refuel I need to fly the course as I understand it, do a proper speed test, start changing the configutation I suspect, wash the plane, look at and replan the course in my preparation for the race on Saturday, study the winds and determine my altitude strategy and perhaps eat and sleep in the Annual Crawfish Festival envirement. I'm hoping for VFR weather tomorrow so I can fly direct 385 nm instead of the 448 Victor airway route around Dallas Class B and Bush's Ranch. It's a fuel thing (I had red lights for both tanks when I arrived at Taylor last week against the 50-60 kt winds). My race speed was 208 plus a little for the first two races, I would like to break 210 tomorrow.

If I get to run the speed test in either or both configurations I will share the info when I get back.

Then there is the Big Country Air Fest Race at Dyess AFB next weekend in Abilene, TX that has to be prepared for.

Bob Axsom

P.S. I am already planning an easy mod the will allow extending the post bump configuration. I wanted a little reflex in this configuration but I may have brought it on too abruptly or maybe the shape should be convex to sustain the attachment? The work will continue and the results reported.


Last edited by Bob Axsom : 04-20-2012 at 05:46 AM.
Reply With Quote
Old 04-20-2012, 06:37 AM
logansc's Avatar
logansc logansc is offline
Join Date: Feb 2005
Posts: 573

Bob: I for one, consider your efforts here virtually 'heroic'! Good on you for picking a target challenge and going after it with such gusto and determination. Good luck---we're all pulling for you!

Lee Logan
Ridgeland, SC (3J1)
F1 Rocket #160 flying
Reply With Quote
Old 04-20-2012, 02:08 PM
DanH's Avatar
DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 10,695

Listen to Mr. Axsom, who is being realistic....he is getting speed changes, but so far none of it is actually making it go faster.

The latest photos pretty much confirm a lack of any actual Coanda Effect; little or no exhaust staining (exhaust jet not attached), and the oil swirl on the backside (separation).

Consider these sketches.

This is more or less what Bob has now. Red is the exhaust jet and pink is cooling exit flow.

Realign the exhaust jet so it impinges on the curved surface and you see the Coanda effect....the attached flow turns the corner. Close the exit area by moving the hump forward and adding a bit to the lower cowl (cyan). The result is an entrained, exhaust-augmented cooling flow. Yes, you might lose a little power, as Bob noted earlier, because local pressure at the pipe mouth is higher than external static. After all, the engine is an air pump.

Now consider the nursery rhyme.."All the kings horses and all the kings men...."

The theme here seems to be putting the air back together after you shoved it apart with the huge cattle chute cowl exit.....but as you've seen, it's not so easy to put Humpty Dumpty back together. So why not just shrink the cattle chute and do an augmented exhaust without all the humps and bumps?

One possible "why not" is airframe noise; exhaust augmentation seems to generate reports of vibrating tin. Perhaps we should just think conventional. So far most of you are running a fixed outlet area based on the worst case cooling problem, which is hot day, slow speed climb. The required exit area for slow speed is way too large for high speed cooling, where it flows perhaps twice the necessary mass. We need a variable exit area, large for climb, small for cruise.

We have one cowl flap experiment in this thread. As noted, closing off the exit to the cattle chute with a flap doesn't work; it's the Humpty Dumpty problem again. So instead of trying to put the air back together behind the cattle chute, just cut the chute down to the cruise size, which is what you wanted for speed anyway:

Now your only problem is flowing enough mass for slow speed put a hinge in floor of the chute and make it variable. Yep, the low exit velocity will mean all kinds of vortex stuff behind the large, open exit.....but in slow speed climb, who cares?

This is nothing new. Take a look under some production GA birds. Ken Kopp showed you a nice variation on the idea in a recent thread, and plotted the actual exit velocity, mass flow and drag reduction.

Sorry, but this thread has been painful to watch.
Dan Horton
Barrett IO-390

Last edited by DanH : 04-20-2012 at 02:15 PM.
Reply With Quote
Old 04-20-2012, 04:06 PM
David Paule David Paule is offline
Join Date: Dec 2009
Location: Boulder, CO
Posts: 5,253

DanH's final two sketches are similar to what my 1955 Cessna 180 has, stock from the factory. The cowl-flaps-closed air exit area is remarkably small, especially considering the inlet area, which is huge.

Overall performance of the plane is excellent.

The cowl flaps are stoutly-build and have side fences built-in for when open. They have openings to clear the exhaust pipes (3 into 1) and there's not a lot of clearance around them. The pipes are oriented down about 40 degrees, though, so there's lost energy there.

The firewall does not have the rounded bottom to aid in attaching the flow. That would probably be an improvement, were there room, and assuming it could get approved.

Reply With Quote
Old 04-20-2012, 05:49 PM
scsmith scsmith is offline
Join Date: Jan 2008
Location: Ashland, OR
Posts: 3,059
Default Some insights

I've been following this discussion and useful test results, but resisted chiming in. At this point, the discussion has narrowed and gotten distilled down to the point where I think I can add a couple of contructive observations.

The sketches in DanH's last post show a very important distinction from the tested design with the bumps. The goal is to accelerate the exit flow. The bumps accelerate the flow, but then, BEST CASE, if the flow doesn't separate, they slow the flow back down again, giving back what they just did. Worst case, they separate, and although the flow then streams back from the separation point at a higher velocity, you pay for the pressure drag on the rear-facing surface. the only way that this bump makes any sense is to fair it out onto the belly of the airplane over a very long length. That will help some (as some results have shown) -- but it doesn't do anything to reduce the overall airplane frontal area -- a missed opportunity.

It is FAR better to do as Dan sketched, accelerate the flow by contouring the outer boundary to a smaller exit that is aligned well with the outer flow. This accelerates the exit flow and keeps it that way. Also you get to reduce the airplane frontal area at the same time. Hard to decide how to book-keep that as airplane form drag or cooling drag, but the benefit is the same either way.

Second: Less important, but worth thinking about -- A truism of fluid dynamics is that you can accelerate flow over a very short distance without incurring very much, if any, loss. This is because the accelerating flow is reducing static pressure (Bernouli's eq.) and so the boundary layer sees a strong "favorable pressure gradient" that guarantees that the flow won't separate, and suppresses turbulence.
The corollary is that you must be very careful about slowing flow down, this must be done very gently, or it will separate, because the boundary layer has to fight its way into the increasing pressure region.

So, the purpose for mentioning this here has to do with the size and radius of the curved fairing that covers the square corner at the bottom of the firewall. This fairing, BTW, is the SINGLE MOST IMPORTANT THING you can do to improve your cooling situation. But my point here is, it can be rather small, such as on the stock RV-8. As long as the flow doesn't separate, there is not much benefit from making this fairing more gradual, and in fact, there is a point where it will actually hurt. Guiding the flow to accelerate more gradually with big, smoothly curved surfaces means that the flow is scrubbing on those surfaces over more area, and the skin friction drag from that will at some point cancel any benefit from the more gradual acceleration. I don't think the stock RV-8 radius separates, but it might, its a pretty tight radius. (what happens if it is too tight a radius is that the flow accelerates too much around the tight curve, and then slows back down some) It might pay to make the radius a little bit bigger (say, 2.5" radius instead of 1.5" radius, for example). But I would avoid going to so much extra work to create a large gentle ramp that will have lots of scrubbing drag on it. Without some detailed study, it is impossible to say what the exact optimum is, but I can assure you that the smaller radius, like the stock RV-8, is not significantly worse than the optimum.

Third: A very minor symantic issue. Lots of people here are refering to the flow staying attached over a hump or curve as a coanda flow or coanda effect.
This is technically incorrect. A coanda flow is a very specific case where there is a high-energy jet flow added on the hump or curve that helps the outer flow stay attached around the curve. The jet is normally introduced through a thin slot just upstream of where the surface would curve dramatically away from the flow, and the jet "sticks" to the surface where a normal flow would just separate. In the whole discussion here, the only thing that remotely approaches a coanda jet would be if the exhaust flows were purposefully aimed to impinge on the upstream side of the bumps to try to help the flow stay attached over the back side of the bumps.

In our efforts to accelerate the exhaust flow, we are dealing with a stream that is deficient in velocity, not a high-energy jet that has exess velocity.

This is a small point - it's just a name after all, but it does add confusion when a term is used that only applies to a different situation.
Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet" Flying since Sept. 2009
Hobbs 700
1/4 share in 1959 C-182B (tow plane)
LS6-15/18W sailplane SOLD
bought my old LS6-A back!!
VAF donation Dec 2021

Last edited by scsmith : 04-20-2012 at 05:54 PM.
Reply With Quote

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

All times are GMT -6. The time now is 07:03 PM.

The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.