Lower Cowl Exit - Firewall Bump

[scsmith]

Do any of you know what provides better results, the rounded 180 ramp or just adding a curved plate?

There is kind of a trade-off here. A small semicircle (like the RV-8) produces a lot of local acceleration around that tight curve. It won't separate, but it probably over-accelerates and then has to slow a bit as it blends with the main exit flow 'jet'.

A larger, more gradual plate would not have as much overshoot in the velocity, but it would have more 'wetted' area producing skin friction.

Hard to say what the real optimum is, probably somewhere in between. My gut says a 2--3" radius semicircle would be close to best, but I have nothing to support that instinct.

 

[blaplante]

As builder/pilots we would NEVER accept attaching an un-aerodynamic shape to the exterior of our planes (exception: GoPro Camera), yet we seem to forget about the airflow in & around the engine (except for DanH, ScSmith, et al)

So, yeah I think there are some opportunities in the -A models in this area, maybe tapered/triangular "after body" shape glued to the tubes, other strategic places...Hard to quantify without test data tho. :/

I 3D printed one and installed during my condition inspection. No test data to share though although anything should help. My design wraps the tube and uses a couple of through screws to join the two halves.

 

[wcalvert]

All these discussions are well and good until you get under there on your back and try to make something that fits!!!

My setup isn't the best and could be improved on. DO THIS BEFORE you get the plane together and all you plumbing etc. placed in the way. It would have been much easier to implement a good design from early on.

Keep at it!

 

[Draker]

<SNIP> I missed this step in the plans completely, and this gives me something to do this week.

I wonder: How structurally rigid does this bump have to be? Thinking about just using some leftover 0.005 titanium foil to do the job instead of finding (and fighting with) stainless steel.

EDIT: Actually, now that I look at it, I don't see how I'm going to get a decent bend in there with the new style nosegear. There is so much in the way, I don't think it's worth trying.

 

[wcalvert]

Mine is .032 Alclad sheet and with the bend ends up very rigid... don't think the SS is necessary, and you're right, would be tough to work with.

 

[Scott Hersha]

I agree with Wild Bill. I think I used .025 aluminum, and it is still very rigid. If I’m worried about fire, I have a SS firewall right behind the aluminum piece I made, which makes it at least slightly better than the rest of the firewall.

 

[abuura]

All these discussions are well and good until you get under there on your back and try to make something that fits!!!

My setup isn't the best and could be improved on. DO THIS BEFORE you get the plane together and all you plumbing etc. placed in the way. It would have been much easier to implement a good design from early on.

Well, after 1380 hours and 14 years, NOW I find out about this mod?! Thanks for all the pictures and pointers. Maybe I can get it done before SunNFun!

 

[Marc Bourget]

The "exit angle" installed on John Thorp's T-18 used a 1" radius, 90 deg, angle at the exits.

 

[David Paule]

Has anyone measured the velocity of the exit flow at any point near the exit radius?

Dave

 

[Marc Bourget]

Easy to do with a spare ASI and a handmade pitot from drawn glass tubing

(narrow the diameter and notch to break in half)

 

[Marc Bourget]

(hope this isn't a double post)

Use another ASI and construct a home-made pitot tube by drawing glass tubing so it narrows down, notch and break, polish the end with flame,

Consider using different stand-off blocks to check the velocity up to an inch off the radius.

 

[jclemens]

I 3D printed one and installed during my condition inspection. No test data to share though although anything should help. My design wraps the tube and uses a couple of through screws to join the two halves.

Care to share the file you used?

 

[DanH]

Exit velocity is actually a tricky measurement.

First, exit flow for the stock cowls tends to be unstable in terms of velocity vector. Take a look of some of the exit tuft photos and videos. Can't measure dynamic pressure accurately unless you can identify which way the wind is blowing, so to speak. This is probably the most significant problem, as the typical large exit (based on silly "exit ratios") tends to have the most chaotic flow.

Accurate velocity is the difference between local dynamic and static, so the measurement probe needs both.

A fellow much smarter than I told me that for most pitot static probes, the static port position defines the plane of measurement. See below.

Probe needs to extend forward from a mount which does not offer any obstruction to flow. I used a metal strip edge wise to flow.

Last, you'll need to measure exit air temperature in order to determine true airspeed. The temperature probe must be shielded so it does not see radiated energy from the exhaust.
.

 

[Draker]

Does anyone have a picture of successfully achieving this "bump" on an -A model using the new elastomer gear mount? The new style gear is pretty much entirely in the way of where this bump might go.

 

[PilotjohnS]

Just me

I was thinking of adding a bell mouth to the louvers on the new A style nose. Let the air nicely exit the louvers and not the tunnel.

The thought is the bell mouth should clean up the flow before it hits the outside air stream, making the merging of the air less energy negative.

And this hot air should help reduce the drag of the wing to fuselage intersection; just a thought.

 

[Ottoman]

Pictures?

I'm with Ryan on this, pictures of this on the -A model would be great! I'm at that point that it has to be done now or never.

John A

 

[Desert Rat]

Thought I would have a go at this before the engine gets in the way.

Congratulations or Criticism- both are welcome

 

[scsmith]

Thought I would have a go at this before the engine gets in the way.

Congratulations or Criticism- both are welcome

That looks good Terry

 

[scsmith]

For the -A models

One idea for the A models that are side-by-side with a flat belly would be to completely remove the 'coal shovel' outlet on the cowl and create two new outlets on either side of the nose gear. The inboard end of the opening would be determined by where the exhaust pipes need to go through, and then extend from there outboard for, oh, lets say 6 or 7 inches. Do this on either side, with the semi-circular lip added, and decide how deep the opening should be, something like 2" or so.

An additional benefit of this would be to get the main internal flows heading for the exits to be outboard of the tangle of pipes that support the nose gear, so there would be less internal loss.

 

[Desert Rat]

Thanks Steve, I know you've devoted quite a bit of time to this concept and it was reading through old posts by you and others that helped me decide what the vision should be.

Couple of questions if you don't mind;

The upper lip of this is about 1/4" from the firewall. once I get the final configuration, is there any benefit to sealing that with a bead of RTV?

This is trapezoid shaped. The upper section is 9" wide to fit between the vertical firewall tubes. Below the adel clamps it flares out to about 12" at the firewall flange. It was supposed to be 13" which is the width of the coal shovel opening, But by the time I got it trimmed and all symmetrical it had shrunk a bit. How much spill do you think I'll get off the ends? Is it enough to make it worthwhile to build another one a little fatter on either top or bottom sections?

 

[scsmith]

I wouldn't bother sealing the upper lip. The flow will attach itself to the bump somewhere out away from the firewall and flow smoothly.

The end treatments are a judgement call. One school might say that a small area of crummy flow can contaminate an otherwise smooth flow. In this case, the higher velocity of the attached flow making its way around the bump to the exit will probably blow any disturbance away with it. I would say that if it is not too hard to make a couple of extra pieces to fair the ends, go for it. Nothing says it has to be one piece. But if it is a royal chore, you could skip it and you will still have made a big improvement over what was there.

Another thing I have been meaning to comment on is the concern many folks have for the disorganized flow in the lower cowl. One has to remember that the area is large, so the average velocities are very low. So the losses from any obstructions are very small. Where obstructions start to matter is the area where the flow is starting to accelerate as the flow area is getting reduced, in the immediate few inches out in front of, and above the bump.

Brian Decker's idea to change the round rubber exhaust pipe support with a flat blade is good. I would love to find a strip of flat reinforced elastomer for that area. I may try to make one out of RTV and fiberglass.

The area around the nose gear support tubes is kind of a transition area. The flow is starting to accelerate right in that area, and it is a horrible jumble of tubes, so getting the flow to gather and accelerate somewhere else probably has some benefit.

Thanks Steve, I know you've devoted quite a bit of time to this concept and it was reading through old posts by you and others that helped me decide what the vision should be.

Couple of questions if you don't mind;

The upper lip of this is about 1/4" from the firewall. once I get the final configuration, is there any benefit to sealing that with a bead of RTV?

This is trapezoid shaped. The upper section is 9" wide to fit between the vertical firewall tubes. Below the adel clamps it flares out to about 12" at the firewall flange. It was supposed to be 13" which is the width of the coal shovel opening, But by the time I got it trimmed and all symmetrical it had shrunk a bit. How much spill do you think I'll get off the ends? Is it enough to make it worthwhile to build another one a little fatter on either top or bottom sections?

 

[Kyle Boatright]

The area around the nose gear support tubes is kind of a transition area. The flow is starting to accelerate right in that area, and it is a horrible jumble of tubes, so getting the flow to gather and accelerate somewhere else probably has some benefit.

Regarding the tangle of tubes on the nosegear aircraft. Assuming you were going to keep the stock exhaust and coal shovel, any thoughts on the relative advantages of a single fairing around that mess or whether it would be worthwhile to fabricate and fit individual airfoil shaped fairings on each tube? The single fairing might be less draggy, but would reduce the actual airflow path. The faired individual tubes would leave all the pathways open, but there's certain to be a lot of drag/disturbed flow at the intersections.

 

[bjdecker]

Brian Decker's idea to change the round rubber exhaust pipe support with a flat blade is good. I would love to find a strip of flat reinforced elastomer for that area. I may try to make one out of RTV and fiberglass.

The standard exhaust-to-engine-mounts from Vans/Vetterman for the 4->2 cross over system uses round tube joined by 3/8" rubber fuel hose. Nice, simple, and exceptionally draggy (round cross section).

After talking with Steve and Dan, I decided that the brace that holds the exhaust stacks parallel to each other could use some aerodynamic cleanup.

This cross tie sits between the exhaust stacks and connects them to each other. It is parallel to the bottom of the cowling (slightly above the "coal shovel")

Pictures are of version 1 (large holes for elastomeric bushings), Before and sort-of-after...

 

[Toobuilder]

One idea for the A models that are side-by-side with a flat belly would be to completely remove the 'coal shovel' outlet on the cowl and create two new outlets on either side of the nose gear. The inboard end of the opening would be determined by where the exhaust pipes need to go through, and then extend from there outboard for, oh, lets say 6 or 7 inches. Do this on either side, with the semi-circular lip added, and decide how deep the opening should be, something like 2" or so.

An additional benefit of this would be to get the main internal flows heading for the exits to be outboard of the tangle of pipes that support the nose gear, so there would be less internal loss.

Going a bit off script here, but I think I have implemented an extreme example of your concept on my Rocket with the augmentor tubes. Requires some fabrication, but works very, very well for me. Posted as fodder for further ideas.

 

[bjdecker]

Going a bit off script here, but I think I have implemented an extreme example of your concept on my Rocket with the augmentor tubes. Requires some fabrication, but works very, very well for me. Posted as fodder for further ideas.

Wicked pi$$ah!

 

[Webb]

Thanks Steve, I know you've devoted quite a bit of time to this concept and it was reading through old posts by you and others that helped me decide what the vision should be.

Couple of questions if you don't mind;

The upper lip of this is about 1/4" from the firewall. once I get the final configuration, is there any benefit to sealing that with a bead of RTV?

This is trapezoid shaped. The upper section is 9" wide to fit between the vertical firewall tubes. Below the adel clamps it flares out to about 12" at the firewall flange. It was supposed to be 13" which is the width of the coal shovel opening, But by the time I got it trimmed and all symmetrical it had shrunk a bit. How much spill do you think I'll get off the ends? Is it enough to make it worthwhile to build another one a little fatter on either top or bottom sections?

Im stealing with my eyes and plan to copy the your design.

CSOBeech has an article on this as a mod to improve cooling. I remember from reading it years ago. https://www.csobeech.com/Bo-Flow.html

Here are some pics from that article. You can see a small rolled edge that was added to round the sharp corner.

 

[PilotjohnS]

Cowl louvers.

Those of us with the new 7a nose gear with cowl louvers, does this really matter anymore.

I asume the louvers result in a less draggy flow since the area is larger as the flows merge.

But what can us Louvers guys and gals do to improve our hot air flow, besides talking less?

 

[Toobuilder]

Exit velocity is actually a tricky measurement.

First, exit flow for the stock cowls tends to be unstable in terms of velocity vector. Take a look of some of the exit tuft photos and videos. Can't measure dynamic pressure accurately unless you can identify which way the wind is blowing, so to speak. This is probably the most significant problem, as the typical large exit (based on silly "exit ratios") tends to have the most chaotic flow.

Accurate velocity is the difference between local dynamic and static, so the measurement probe needs both.

A fellow much smarter than I told me that for most pitot static probes, the static port position defines the plane of measurement. See below.

Probe needs to extend forward from a mount which does not offer any obstruction to flow. I used a metal strip edge wise to flow.

Last, you'll need to measure exit air temperature in order to determine true airspeed. The temperature probe must be shielded so it does not see radiated energy from the exhaust.
.

I have plans to afix a pitot probe to the belly of the Rocket to measure the augmentor tube exit velocity. Was going to do it simple and use the ship's static source and plumb the pitot to my G5 (backup to the D-180) and compare the two. Was thinking that would give me a "better/same/worse" data point WRT mass flow, but now that you mention it, the temp is going to be a valuable (and relatively easy) addition. Considering the tubes are 21 inches long, there's plenty of time to get the air organized and heading in a uniform direction and of course theres also distance from the hot pipes so the temp should be stable too.

 

[Desert Rat]

Im stealing with my eyes and plan to copy the your design.

CSOBeech has an article on this as a mod to improve cooling. I remember from reading it years ago. https://www.csobeech.com/Bo-Flow.html

Here are some pics from that article. You can see a small rolled edge that was added to round the sharp corner.

Have at it. If you have some scrap laying around, they aren't hard to make or install. Just a couple of adel clamps and 4 nutplates on the lower firewall flange. The one in my picture is just made out of .025 scrap but I think .020 would work just as well.

I've got the basic dimensions in a picture on my build log. After Steve's comments above, I think I'll make a MkII version that has parallel sides below the adel clamps about 15" wide.

The plan is to fab these before the engine is on while it's easy to fit them up, do the engine break in without them and then try a couple of variations to see if there's really an optimal shape or if minor variations don't really result in the juice being worth the squeeze.

FYI- Cessna does something similar on a couple of the piston singles but I can't remember which ones. Maybe the new 182?

 

[Webb]

Terry, pm sent. Call me please.

 

[Webb]

I replaced the large, round cross brace with a more aerodynamic shape. No flight test data yet, but I don't expect much difference.

Edit: The large holes are to fit a home brew rubber bushing.

What about making a teardrop shaped cover? Leave the leading edge intact and big enough holes to get the clamps through and around. Couple of rivets on the trailing edge. Put the clamps on it and you are ready to install.

Slide it over the hose and tighten up the clamps. Could be made and installed in about the time to make a pot of coffee and have a cup.

Removable, drag reduction of 9 times, and cheap/fast/easy to make and install.

I was thinking about this for lower engine support tubes later on when I know what I have to work around.

 

[bjdecker]

Coal Shovel

I received permission (tacit acknowledgement) from my better half to get some new cowl pieces and go to work on removing the coal shovel and shrinking down the exit on the -7.

When I last measured it, the aft opening was ~4" x ~14" (~56inSq.), less the cross section of the 2 exhaust pipes.

My intention is to shrink the exit area *and* remove/resize the coal shovel -- allow enough room for exhaust pipe shape and reduce the frontal area.

I would chop out the coal shovel (as seen in the attached pics) and then fabricate a screw-in panel affair to cover the exhaust pipes with about 3/4" or so around each one at the exit, tapering forward to flush with the nose bowl. A simple hinged cowl flap of about 6"x8" would be located between the exhaust pipes.

Has anyone else done this modification for their cross-over exhaust (4 - > 2) equipped -7 that share some pics? Any before/after performance data?

 

[rvmills]

Brian,

My 2014 cowl mod project was similar, though on a 540 with dual 3 into 1 exhaust. With Steve Smith's guidance and design assistance, I reshaped the exhaust to bring the exit pipes close together, put a SS radius on the lower edge of the firewall at the opening, and extended it back along the belly a bit. Took a breath, cut the cowl, and it was ugly...till it was pretty again. Took the exit:inlet ratio from about 1.85:1 (it was made larger than Van's stock 1.2:1 ratio by the original builder) down to Dave Anders' magic .75:1 ratio. I did not go as far as Dan Horton's cool experiment, which I believe was to .38:1

Results were a 7 knot gain at top end (Reno-tested), and, as anticipated, insufficient cooling in slow climbs or at formation training and working speeds (130 knots). Added an ASA cowl flap, and it fixed those issues. I still climb flat and fast when I can for good cooling, especially in the summer. The cowl flap, if opened in cruise, slows the plane by 4-5 knots, and cools the CHTs by about 5-10 degrees.

It was a good experiment! I know you are talking to Steve as well, which is a great way to go...the final shape of the exit, how it flows with the free stream air, and how the exit is shaped are all important details, as you know.

Sample pics below. Come by any time to take measurements or look at more photos. Good luck amigo!

Cheers,
Bob

 

[DanH]

Took the exit:inlet ratio from about 1.85:1 (it was made larger than Van's stock 1.2:1 ratio by the original builder) down to Dave Anders' magic .75:1 ratio. I did not go as far as Dan Horton's cool experiment, which I believe was to .38:1

I don't have much faith in specific inlet-exit ratios, but for the record, about 0.77:1 with the variable exit open, 0.5:1 closed. Angle valve.

The system I'd like to see on a 7A would be much like the latest RV-14 cowl, dual fixed exits outboard, a center hinged variable exit inboard.

 

[scsmith]

I don't have much faith in specific inlet-exit ratios, but for the record, about 0.77:1 with the variable exit open, 0.5:1 closed. Angle valve.

The system I'd like to see on a 7A would be much like the latest RV-14 cowl, dual fixed exits outboard, a center hinged variable exit inboard.

Ditto what Dan said. You want large inlets for good external diffusion, and that area is so decoupled from the cooling exit requirement for adequate flow but high velocity that the ratio is rather meaningless. Big inlets and big exit would cool much better, and be more draggy, than small inlets and small exit, even though the inlet/exit area ratio could be the same. So it just doesn't tell you anything.

Brian, just to clarify, the areas I told you about in emails are 'net', meaning the section area of the exhaust pipes has been subtracted. Just so you have apples to apples....

I think if you shoot for 32-34 sq in of net area by making the 3/4" clearance tunnels around the pipes and then whatever height across the middle between the pipes to get that area target, plus a big cowl flap, and you will be happy. I have 37 sq in. now and it seems I could have gone smaller.

Remember also that DanH has the oil cooler exit flow ducted all the way to the cowl cooling exit, so he is effectively decoupling his oil cooler flow from the cylinder cooling flow. This provides lower back pressure on the oil cooler so better oil cooler flow for the same net exit area. I will do this on my next one. It might take some trial and error to get the oil cooling exit 'nozzle' sized right, but that can be fiddled with independently of the cowl exit shape.

Finally, don't forget about a cowl nose seal at the crankshaft or flywheel. Not much help to raise the lower cowl pressure and then have it spill out the front.

 

[bjdecker]

The Plan...

The system I'd like to see on a 7A would be much like the latest RV-14 cowl, dual fixed exits outboard, a center hinged variable exit inboard.

I'll have something together hopefully by January for the -7. If it works as expected/desired, I'll convince one of the other -7A drivers around town to do a similar modification.

I was going to borrow heavily from the RV-14 EXP119 cowl (the -390-EXP119 with 4->2 crossover) for this project. I probably will take a mold from one to make the exhaust shrouds/lower cowl affair.

I don't plan on changing the inlets, stock-lopsided "D's" from Van's.

Steve - gotcha; my "gross" outlet (stock Van's cowl) is 56sqin, less 12.566 for the 2 stacks -=> 43.44sqin "net"..

Also, the oil cooler exit diffuser/nozzle is a project for another day -- OT's are running the 185-190F in the Texas heat, so I'm content for now.

 

[amaris]

I'm one of the nose wheel guys who has a giant piece of steel in the middle of the exit airflow. I'm not sure how much it'll help, but figured if it does any, it's worth the time. I'm wondering if those who are way more knowledgeable would weigh in on this placement. Obviously, a paper template is hard to get to curve nicely, but if it was something like this without the crease, would that work well enough?

I haven't seen anyone with the nose wheel update on any further developments on this.

Thanks for any input!

 

[scsmith]

I'm one of the nose wheel guys who has a giant piece of steel in the middle of the exit airflow. I'm not sure how much it'll help, but figured if it does any, it's worth the time. I'm wondering if those who are way more knowledgeable would weigh in on this placement. Obviously, a paper template is hard to get to curve nicely, but if it was something like this without the crease, would that work well enough?

I haven't seen anyone with the nose wheel update on any further developments on this.

Thanks for any input!

Allen, that shape looks pretty darn good considering what you have to work around in there. Please keep us posted how it works out. You are on the right track there.

 

[Kiwifly]

Another cowl outlet baffle

Did some cowl mods recently, copying ideas already thrown out on this post. My cooling was mostly acceptable, even in Moab this summer (4000'+ and 90+ oat) occasionally running warm, but there is always room for improvement. No great data collected demonstrating additional cooling or speed, but my impression is that my CHT's are running lower and dropping faster. Weather is pretty chilly up here right now, so we'll need to wait for the next warm up (June sometime!) to measure the results.

Thanks to the all forum posters for their insight and examples...

Cheers!

what's the material on your firewall ?