VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics


Go Back   VAF Forums > Model Specific > RV-7/7A
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 08-19-2014, 09:51 AM
turbosaaber turbosaaber is offline
 
Join Date: Aug 2006
Location: boynton beach fl
Posts: 210
Default exhaust exit fairing dimensions

Made a fairing, took the pressure recover wheel pant and used it as a basic outline for the fairing but it looks way longer than Larry Vetterman's. Does anyone have any dimensions out there? Just don't want to make it too long or short. Mine is 24".
__________________
Carl Bridges
RV7 N278RV
NOW FLYING
Reply With Quote
  #2  
Old 08-19-2014, 12:08 PM
crabandy crabandy is offline
 
Join Date: Jan 2010
Location: Ottawa, Ks
Posts: 2,188
Default

I've contemplated the same, but haven't made one yet. If I make one I would use the oil/exhaust pattern on my belly to size the fairing.
__________________
RV 7 400 hours and counting
19 donation done
Reply With Quote
  #3  
Old 08-19-2014, 12:19 PM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 9,500
Default

Instead of trying to fair a lump, why not just get rid of the lump?
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #4  
Old 08-19-2014, 02:26 PM
turbosaaber turbosaaber is offline
 
Join Date: Aug 2006
Location: boynton beach fl
Posts: 210
Default Increase exit air velocity and smooth airflow

Dan, not sure what you mean. Bob axom's testing showed marginal gains from side skirts. Vettermans design seems to smooth out the airflow. I'll post a pic showing turbulence.
__________________
Carl Bridges
RV7 N278RV
NOW FLYING

Last edited by turbosaaber : 08-21-2014 at 04:57 PM. Reason: image upload
Reply With Quote
  #5  
Old 08-20-2014, 06:13 AM
turbosaaber turbosaaber is offline
 
Join Date: Aug 2006
Location: boynton beach fl
Posts: 210
Default sorry for the large pic....moderators feel free to resize

sorry about the size of pic...i thought medium would be fine but it is too large.
__________________
Carl Bridges
RV7 N278RV
NOW FLYING
Reply With Quote
  #6  
Old 08-20-2014, 06:39 AM
turbosaaber turbosaaber is offline
 
Join Date: Aug 2006
Location: boynton beach fl
Posts: 210
Default which would be better

a teardrop fairing without a lump or one with one. I guess it would be best to just make two and test. The one without the lump would be different than Bob's as its sides would taper and curve in a teardrop shape. In theory which would provide greater speed?
__________________
Carl Bridges
RV7 N278RV
NOW FLYING
Reply With Quote
  #7  
Old 08-20-2014, 07:56 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 9,500
Default

Carl,

That's not "turbulence", at least not per a textbook definition. Your photo (somebody re-size it please) shows one half of a classic Von Karmen vortex street, the unsteady separation of flow behind a blunt body.

Adding a tail can reduce or eliminate the alternating vortices. For example, in the case of the cylinder shown in the illustration, adding material to form the classic teardrop, or even just adding a single center fin with a length of more than the diameter of the cylinder will kill the alternate shedding.

The blunt body in your case is the giant laundry chute used as a cowl exit. It has a supply of air into the wake which is independent of the external flow, which offers another way to kill the alternate shedding.

Consider a bit of accounting. Separate form drag (a function of frontal area and wake) from cooling drag (the energy necessary to re-accelerate the exit air to freestream velocity).

Reducing the size of the blunt body reduces frontal area. It can also reduce the exit area, which increases exit velocity, which reduces cooling drag. A smaller blunt body with no great velocity difference between freestream and exit flow produces no vortices, i.e. a reduction in wake (pressure) drag. These three facts say the first modification to consider is a reduction in the size of the blunt body, with a corresponding reduction in exit area.

Note that fairing the tail end of the body for wake reduction only improved one of the three drag sources.
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 08-20-2014 at 04:56 PM.
Reply With Quote
  #8  
Old 08-20-2014, 08:24 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 9,500
Default

Carl,

Below is a link to a short video taken to check operation of a variable exit area door. Note that when frontal area is large and exit velocity is low, the yarn tufts are quite unorganized. However, at 14 seconds I close the door (partially, the system needed more structural work to get full sealing, search "The Shrinking Exit"). Even with just partial closure, the messy wake disappears, both at 120 knots, and later at 180 knots.

http://youtu.be/nA5PY7PYBsU

The door structure was later stiffened, so now it closes pretty well. There is ~4 knots between open and closed, in cruise.

http://youtu.be/aIBXAE2Ezn4
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 08-20-2014 at 08:28 AM.
Reply With Quote
  #9  
Old 08-21-2014, 02:38 PM
turbosaaber turbosaaber is offline
 
Join Date: Aug 2006
Location: boynton beach fl
Posts: 210
Default thank you Dan

Thank you for your explanation as I understand what is happening now. I was wondering too if the two exhaust pipes are also creating von karmen vortex streets as well and wondered if creating a fairing around the exhaust pipes in the lower portion of the cowl would increase exit velocity. I'm going to take a look at my exhaust and see how I can reduce my cowl laundry chute shape and reduce my outlet size. Look at a cowl flap too. Initially seems like the easiest thing to do would be to add a teardrop but overall this is just mitigating the problem and not fixing the root problem. I think this is going to be a fun lengthy project.
__________________
Carl Bridges
RV7 N278RV
NOW FLYING
Reply With Quote
  #10  
Old 08-21-2014, 11:11 PM
rvmills's Avatar
rvmills rvmills is offline
 
Join Date: Nov 2007
Location: Reno, NV
Posts: 2,125
Default

Carl,

I'm a Shrinking Exit disciple too, and have some thoughts for ya. Concur with Dan on the multi-faceted (multi-drag-type reduction) benefits of shrinking the exit. Larry Vetterman's after cowl was effective for him, but in reading his site, and comparing his pics, it seems it was a good solution for an A-model, where he may have had limited ability to reduce the cowl exit dimensions, due to the nose gear components. His pics show two variations, one with wide exhaust pipes exiting the cowl via individual bluf bodies, and one with the ex pipes closer together and exiting via the after cowl. In both cases, he used louvers to get the exit airflow needed for cooling. So the design of the teardrop after body requires much thought and experimentation with size, shape and exit air control (louvers, etc) to find a solution. Pics of Larry's work, for ref:





Garry Reed and others used an after fairing of a different design, to straighten the flow aft of the exit. Bob Axsom used that concept as part of his experiment in this area…what you called side skirts. The reverse flow seen in your pic with the oil could also be partially due to chaotic flow at the edge of your exit, but its hard to see that in the pic. The blunt body sure does seem to have an effect on the flow as well, as pointed out by Dan. Here is a picture of Gary's exit fairing, which he said increased cooling and increased speed…meaning he increased mass flow and still gained speed (typically they work inversely, so he may have really increased the efficiency of his flow with this fairing). I still have Gary's patterns, and can pass them along if desired.



Even with Gary's good results, and Bob Ax's extensive and meticulous testing of side skirts and a bit of a venturi-creating bump on the cowl tunnel bottom (documented in the thread "Forming Aluminum for a Cooling Outlet Fairing"), these measures seem to have lower return on investment than shrinking the exit and increasing the efficiency of the exit flow. Since you have a 7, not a 7A, you should be able to do both of those last two things, though its definitely work, and a gut-check to cut into the cowl of a flying airplane (especially if its painted…AMHIK!).

I studied "The Shrinking Exit" thread quite a bit, and consulted with Steve Smith quite a bit along the way (thanks mucho fellas!). Steve told me one of the most effective measures to increase exit flow efficiency is to add a firewall radius, like the radius that the 8 has. This is also described in a thread called "RV-7 Cowling Exit Bump", with lots of pics. I chose to make mine look like an 8s radius, only slightly larger. Others have made large-radius shields further up into the engine mount area. I also took Dan's advice and made my radius of SS and continued it onto the belly as a heat/fire shield:



Then I reshaped my 6 into 2 exhaust pipes to bring them closer together, to allow the shrinking of the exit. Here are a few pics that compare before and after. Per Steve's great advice, the goal was to make the new tunnel smaller, with all faces more inline with the free stream airflow than before, and have it extend slightly behind the firewall (a bit of the side skirt concept, which I still intend to extend a bit in future experiments).






New pipes, old cowl:


New pipes, new cowl:


I picked up good speed, and my temps did increase, as expected when reducing cooling mass airflow (and cooling drag!). I'm at the point where cooling is adequate in cruise and in racing, but in lower airspeed regimes (slow climbs or formation flying), cooling is not quite adequate. Solutions range from installing a cowl flap to installing a louver, which could be exchanged for a flat plate plug for racing. Likely my winter project! Alan's (Anti-Splat Aero's) EZ-Cool Flap looks like a nice solution. A concern I have with a cowl flap is the impact that the side walls and actuator might have on the exit airflow when the door is closed. When open, the mass airflow is large, and increasing cooling is the concern, rather than cooling drag or speed. When closed however, I wonder if those sidewalls and the actuator, when raised up into the cowling, might become impediments to exit airflow, and cost speed. That is why I'm thinking of trying the removable louvers first. Its also why I would think that the internal exhaust pipe fairing that you mused on in your last post might not be an effective tool…but I'm unsure about that. I'd be very interested in other's thoughts on this concept. If it wasn't for the obsession with tip-top speed, I'd go for the cowl flap…I'm studying more on that in prep for the mod. Dan's cowl flap is very elegant and solved the sidewall issue…and would require more exhaust and cowl changes...it remains grail-like!

Good luck with the project…you're right about it being fun and challenging!

Cheers,
Bob
__________________
Bob Mills
RV-6 "Rocket Six" N49VM
Reno-Stead, NV (KRTS)
President/Sport 47/49, Sport Class Air Racing
President, Formation Flying Inc (FFI)
Flight Lead, Lightning Formation Airshows

Last edited by rvmills : 08-22-2014 at 10:29 AM.
Reply With Quote
Reply


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 02:06 AM.


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.