Forming aluminum for a cooling outlet fairing

[Bob Axsom]

When Jeanine and I built our RV-6A 1996-2004 our intention was to race in whatever cross country air races that came up. Usually one per year until SARL was created, now we have over 20 per year (www.sportairrace.org). With that focus during the original build, I contacted the manufacturer of the best looking "in the wingtip" antennas. When I told him that I fly IFR he told me that the antennas were intended for sport flying applications and he would not recommend them for serious IFR flying. That was a NO GO decision driver for me.

I could make cover plates for all of the external antennas, two communication, transponder and marker beacon (I already take off the navigation elements by unscrewing them), remove them for racing on site and reinstall them before flying home. The 12" stock wing tips have been replaced by a 3" streamlined tips of my own design and the performance of the com antennas placed in them for turn and finish line calls would be further compromised. I would have to route a coax to at least one wing tip and change the RF I/O connection to the race only coax someway (change connections, coax switch, etc.) disable the transponder (you don't want transmission attempts into an unloaded output, maybe some FAA problems there as well - depending on race location perhaps). Still, if I run out of more promising and less compromising opportunities the passive drag of external antennas will be kept in mind - I look at them and wonder each time I wash the belly.

The speed differential between out RV-6A and the current RV Blue race leaders is from 3 to 9 kts and I think I am going to have to deal with the thrust member of the speed equation to overcome that. Two of them will be in the AirVenture Cup Race next Sunday and we will see what we have accomplished so far. I am hoping this engine cooling air outlet mod cuts into the margin.

I completed a trial "final installation" of the new one piece cover this morning at 05:40 and there are some problems.



The 0.040" 6061 0 material I had available and used, is very soft. It is just a little harder to form than sheet lead so it shapes nicely but its softness causes mounting problems. I knew this from the 0.032" 2024-0 material used in the development cover. I planned to drill large mounting holes and use #8 hardware with dimpled washers to provide more distribution of the fastener head compression but I went too small at 1/4" dia and the washers are not all flat because of imperfect hole alignment. I plan on removing it today, increasing the hole size to 3/8" like I do on fiberglass and plastic parts installed with dimple washers and see if that works. If that is not satisfactory I will go to harder material for a new cover and cure the problem that way. Since I plan to leave for Mitchell, South Dakota for the race Friday morning there is some schedule pressure so I guess I had better eat, get cleaned up and head back to the hangar.

Bob Axsom

[Bob Axsom]

The one piece cover is reinstalled and the dimpled washers are sitting the way the should. I would have liked to see if there was any effect on speed from the cleanup but I'm out of time. I've got a lot of prep work to do before flying to Mitchell, SD Friday fot the 2012 AirVenture Cup race - one secret weapon - so no more effort on the cooling air outlet fairing. I may test it later just to get a new baseline but I think this one is done.

The secret weapon, well it is a strategy thing - tell you later Alan.

Bob Axsom


P.S. I'm calling this configuration speed 183Kts speed which is 1.4 kts less that I got one time in testing a while back so it is hard to claim an improvement in performance. I did two tests in this series (20 and 21) with no fences, no covers and no bumps which yielded 177.6 and 178. 8 knots respectively for an average of 178.2 KTAS which would indicate a 4.8 kt speed gain but the three long angle mounting rails were riveted to the fuselage already and were exposed to the air in tests 20 and 21. S-o-o-o-o, conservatively I'll take the pre-mod baseline speed as the average of the 184.4 and the 182.2 or 181.3 kts. That still yields a gain of 1.7 kts. I'm happy with that. 04:30 and I'm on my way to the hangar to do race prep.

[Alan Carroll]

Originally Posted by Bob Axsom

The secret weapon, well it is a strategy thing - tell you later Alan.

I look forward to it - we're all going to need some some new tricks to catch Brent!

[hydroguy2]

Originally Posted by Alan Carroll

I look forward to it - we're all going to need some some new tricks to catch Brent!

isn't that the truth

[160kt]

I have spent many an hour trying to locate the source study for the sugggested optimal NACA exit ramp slope to be 7 Degrees. I can't find it. Since it came up in this discussion I thought I'd ask here.

Does anyone here know the source and have a link to it?

Thanks in advance,
Ned

Originally Posted by rvmills

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 material...to bounce his results off of!

Comments below:



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!



Dan, I think there is probably some area reduction...at 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 Bob...as always! Good luck in the race!!

Cheers,
Bob

[BillL]

I have remembered the 7 deg angle since college, but never asked for reference. I searched the nasa database ( http://ntrs.nasa.gov ) and found this report: Search for this title, "NACA-TN-2888". I cannot add the link as it will yield an error. I just scanned the report, but it appears to show experimental development of pressure recovery for a 2-d diffuser. The efficiency appears to peak right at 7 degrees. ref figure 15. The report is 85 pages long so only 30 min was spent evaluating. Happy reading.

[rzbill]

I was the one that mentioned the 7 degree angle much earlier in this thread.
It is for INLETS, not exits, but the principle I was discussing at the time was the same. My original post was concerning expansion or channels with increasing volume. Depending on conditions, there is a maximum expansion rate that can be done before the air separates from the structure. Previous experimentation by NACA can help.

A poster asked for a ref document. Using the search method above, look for NACA-RM-A7I30. That is the study I based my commentary on.

[160kt]

Bill and Bill,
Thank you very much for your responses. I've read both those papers before but neither are specifically defining the design of NACA exit ramps. I remain quite curious. I would like to find the study explaining and defining the design. I've seen posters quoting the 7 degree slope and that the NACA papers say a straight side shape for exits is better than the curved shape formulated for inlets. I've spent literally days reading the NACA studies to no avail. One fellow Mark Langford http://www.n56ml.com/nacaducts/index.html experimentor made his exit ramps based on the "earlier" NACA studies and later posted that the "later" NACA studies indicated that the inlet design is not optimum for the outlet. When I ask him for references he vaguely suggested to look in an old Kitplanes. I eventually found the article he remembered and the author did respond to me with about 40 references none of which had the info on the exits. 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

[Bob Axsom]

What is your understanding of the 7 deg. optimum outlet? If you understand the concept it would be possible to begin an experiment and test process based on that.

Bob Axsom

[DanH]

A 7 degree straight-sided exit ramp inset into the belly? Sounds familiar.