Forming aluminum for a cooling outlet fairing

[rzbill]

Originally Posted by 160kt

I'm wondering if it's some kind of urban legend perpetuated by the Internet....
The author was Todd Parker and his article was posted in Sport Aviation March 2006

So if anyone knows where the exit ramp design development for the NACA ducts I sure would appreciate the info
Thanks
Nec

There is plenty of exit geometry testing by NACA. I cannot remember seeing an instance of them testing a reverse of the curved sidewall inlet. Maybe they did, I just did not see it. It is not a sidewall shape I would automatically try for an exit, due to my belief that the sidewall geometry is directional for generation of edge vortices spilling into the inlet. I don't see that working in reverse. There are some BASICS that can be drawn from theory and testing. A converging ramp, such as the RV8 exit floor has a large range of functional angles because it is in a positive pressure gradient (my assumption by inspection, not data) which keeps the flow attached. The backside of Bob Axsoms coanda bump was an expansion area with negative pressure gradient. The exhaust/oil pattern showed separation, a small piece of data. The NACA INLET is designed to minimize separation in an expanding environment. The 7 degree data is VALID for this narrow application (expansion) as suggested. I had additionally suggested convergence of the cowl at 7 degrees (trying to minimise separation on the cowl exterior and matching flows to reduce the disruption at the rear edge of the cowl) in order to reduce the volume expansion past Bobs coanda bump inside the cowl. As Mills said, that could be counterproductive. I'm not sure. The 7 degree common aero knowledge is hardly an urban legend

EDIT REF: "It is empirically observed that the best divergence angle for a two dimensional subsonic diffuser of a given area ratio is about 7 degrees" Hill, Peterson - MECHANICS AND THERMODYNAMICS OF PROPULSION. Addison Wesley Pub 1965.

[160kt]

Page 3 of Dave Anders notes
http://sacrvators.com/Aircraft%20Efficiency%20N230A.pdf

3) shape - NACA studies
- straight sided rectangular shallow angle converging ramp - ramp as parallel to free stream flow as possible at end
- width to depth ratio of ramp
a) 7 to 1 for cooling systems
b) 1 to 1 for exhausts (augmenter

Evidently Anders knows about the NACA studies?

The RV8 outlet and the Grumman Tiger/Cheetah outlets have straight sided exit ramps. The Grummans have some of the ramp covered. It would be great to know how to design it.

If there is a study that NACA did then I'm thinking a whole lot of experimentation has already been done. Why repeat that? I want to experiment with the benefit of the science already concluded guiding my efforts

[160kt]

I'm curious why RV's have the "exit ramp" out in the slip stream. The Grummans picked up quite a bit of speed when the "exit ramp" was placed internally. The Grumman Traveler exit ramp looks like an RV 6 until 1975 when they placed it "inside" the cowl. It picked up 6 knts. Bob Axsom have you got room to move all that "stuff" inside the lower cowl skin and make the skin even with the fuselage skin, not hanging below?

Dave Anders believed whatever NACA data he had enough to put the ramp inside the cowl AND to get the ramp slope down to 7:1 he modified the bottom of the fuselage to continue the ramp way back.......

Dan Horton picked up more speed the more he removed the hanging down part......

[DanH]

The RV8's exit slot (after trimming the cowl) is about 7 to 1.

Version 4 of my own cowl exit will again reduce fixed area by extending the exit 4" aft of the firewall. However, it will also incorporate some variable exit area for low airspeed use.

[160kt]

Correctly assessing or quantifying improvements I have found to be very difficult. I discovered it is extremely hard to get repeatable results with flight testing for a particular mod. So I understand Bob Axsom's efforts in this thread. He is an extremely hard worker. And I don't want to take away from all he is doing and has done. But here are so many variables it makes it really hard to determine if what's done really is what causes the speed increase. For example, in this thread you one time mention that CHT's went up a little. To me this says cooling air went down. The logic flows that the stuff put at the exit may have caused lower cooling airflow which in turn may be contributing to the speed increase. So how much speed increase is do to that versus the effect of the changes with the air exit? Well that's the whole problem. And until you properly instrument the cowl you just won't know whether all your work actually increased the speed by itself or it just affected another variable that actually caused the speed increase.

[Bob Axsom]

I worked in aerospace for 50 years from the lowest level to the project office and I recognize wisdom were I see it I think and you guys have it. I just flew down to Abilene, TX yesterday and worked most of the night with changing winds aloft forecasts and race course changes including one added turn that was published after most people went to bed with the wrong latitude value. What a great group - We worked together and and got by what could have been an awkward situation. I woke at 00:50 CDT this morning and saw the e-mail announcing the added turn and started working it into my race flight plan and altitude strategy (winds). I developed a crude Excel spread sheet last year for helping me plan what altitude I will fly in each leg. The race is a real race not one of those compromise things with handicaps or fuel burn estimates, etc. The fastest wins and that tends to stimulate speed modification action. MANY speed modifications do not work even though the experimenter's intuition says it has to work. I had 3.4 hours sleep last night (yes I maintain a sleep log) and a very stressful day including some serious scud run challenges through the Ozarks to get home here in northwest Arkansas so bear with my lack of clarity - I'm tired but I'm trying.

When you try to exploit a concept I believe you must work with it until you succeed or you are convinced that it will not work and have some rational thoughts about why it failed. If you learn something from the failure it was a good experiment. The cooling drag concern is popular with the technically elite so it must be worth looking at and trying to reduce it must be a good thing. I have a significant stack of aluminum, rubber and stainless steel from my failures. Not what I have read or been taught in the classroom but what I have fabricated installed and tested. As I have gone through this growing and satisfying experience I have accepted intellectual input on test methodology from some of our best contributors to this forum and from lessons learned when I used to be a useful member of the aerospace community but in every case I control my experiments and I do not bow to technical arrogance and I judge the results. If you don't do that you will be defeated early in you life as an experimenter.

My initial thought on cooling drag was the cooling air should have easy passage through the cowl to the fixed outlet. I made a curved aluminum panel from the bottom of the engine to the bottom of the firewall so the air would follow the curved surface to bottom of the fuselage. THIS IS NOT A SIMPLE THING TO CREATE! All of the things that transition from the airframe to the engine have be accommodated by this thing you are constructing from many pieces and it has to be serviceable and the penetrating wires, tubes engine mounts, etc. have to be accessible for maintenance. OK ... ? Anyway, I did that and as I was doing it I said to myself, if I just let the air follow this cowl wide curved panel to the bottom of the firewall/fuselage the air outboard of the outlet goes into those lower corners and gets trapped there or at least does not have a equal shot at the outlet. So, I decided to make vertical panels that slant from the inner side walls of the lower cowl inboard at an angle determined by the rearmost location of cylinders 3 and 4 (of course they are staggered so the angles are different) to the width of the outlet. When all of this work was done and tested I found the the top speed of the airplane had dropped below my base line speed of 170.67kts by 2 knots. I made many modifications and nothing worked until I sealed the lower cowl with two additional horizontal baffles below the cowl split line preventing a back flow path from the lower cowl up past the valve covers and back to the area between the engine and the firewall. When those baffles were installed the speed jumped up 6 kts for a 4 kt gain over the baseline for my specific airplane.

I have made many changes since that was achieved but NONE did anything but lower the speed and when the change was removed the speed came back. I believe that a near optimum combination of upper plenum and lower cowl configuration has been achieved for my specific airplane with significant additions of aluminum and rubber to my sacrifices to the God of Speed. Some new idea may arise in this area that would make the plane faster but I personally have tried enough that I do not believe they will.

After some time I thought the outlet configuration was not effective based on tuft testing by others that showed back flow. A fellow named Geer, from Pennsylvania I believe, offered in this forum and experiment that he had tried with the outlet that he reported increased the speed of his plane. I did not want to just copy his shroud design and gain limited knowledge - basically GO/NOGO. I saw several areas to work on but I'm getting tired and I suspect you have long since stopped reading so I will jump ahead to the final configuration.

The bumps are now stainless steel panels that are attached to the curved baffle from the engine to the bottom of the fuselage so now the panel extends from the engine past the bottom firewall/fuselage interface and gently curves back to the fuselage with a narrow gap at the trailing edge to pass the air that comes through the oil cooler, blast tubes and heater dump from zone 3. There is a center fin that keeps the exhaust pipe outputs separated, outer vertical panels to keep the the cooling air and the ambient air separated and a horizontal panel that does not let the outlet air travel vertically downward into the ambient air under the fuselage without starting to travel aft toward the tail. The vertical and horizontal panels evolved through many trials and are now actually one piece. This all made sense to me but the real breakthrough came when someone in this forum suggested that I should try parabolic like cutouts between the the horizontal panel and the two outboard panels. After I did that the speed jumped over 1.7 kts. The cutout is not unlike the plume pattern of the exhaust on earlier versions of this outlet shroud. The original wide coverage of the bottom of the fuselage with oil and exhaust products is now defined by straight bands the width of the outlet all the way to the tail - no evidence of turbulent flow at all. Refinements in the implementation can be made but I do not believe there is anymore speed there. Wow that was tiring - now the system will probably say I used too many words in a post.

If you don't try to test because you say it's is too difficult to get consistent results that is too bad. I wish you would try harder to control the testing otherwise you will always be stuck with hand me down knowledge which is good but not the best kind. You have to work at it to really know the facts on the best level.

Bob Axsom

[hydroguy2]

Plenty of us are still reading Bob. I've seen the improvements you've made and continue to make. You are a true Experimenter. Keep up the good work

[rockwoodrv9]

Im another one who reads every post you make Bob. I am not building a race plane, but I sure enjoy how you keep searching for that one more knot! I would love to see pictures of your modifications, but since you have spent so much effort making them, they are your "advantage" in your racing!

Keep it up Bob, many of us here and one upstairs, are surely following your progress and racing successes.

[humptybump]

Thanks Bob. I will likely never get the racing bug but your test methodology has been incredibly valuable for me. I too read your technical posts carefully and completely.

[Jerry Cochran]

Hi Bob,

I too read every one of your posts even tho I can't fly right now because of a panel overhaul. This question has prob been asked before but a quick search gave no joy.

How do you assure accuracy of one speed test to another since you're looking for a knot or two at a time? I've tried to repeat testing using the same altitude/rpm/MAP even on the same flight using the 3 or 4 heading GPS method and have not been consistent to within a knot.

What am I doing wrong?

Thanks, no hurry with a reply...

Jerry