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  #1  
Old 12-05-2020, 06:42 AM
bjdecker's Avatar
bjdecker bjdecker is offline
 
Join Date: Jun 2005
Location: Georgetown, TX
Posts: 895
Default Lower Cowl Exit - Firewall Bump

First off, thanks to CRABANDY for his original work 5+ years ago.

This past week, I decided to add the firewall bump to my RV-7 (as depicted in builders manual, section 12, page 12-2, figures 12-1 and 12-2.)

I've been gathering test data for over 2 years now, and have a pretty solid data set for a variety of operating conditions, power settings, etc.

With the "bump" installed, there appears to be an increase in TAS of 3 kts. This was measured at 6500MSL, 2500RPM, 24.1" MAP, 9.7GPH - 50°lean of peak, 5°C OAT, 30.20"inHg. TAS was 168kts.

CHT's and OT was a few degrees warmer, but all within limits and nothing really concerning:
No Bump - With Bump
285 287
314 314
290 293
276 279
OT:
185 187

I attribute the increase in airspeed to a reduction in cooling drag due to "shrinking the exit".

Next steps -
Change to stainless steel and extend from 15" to ~18" to cover more of the area above the cowl exit.

Cheers!
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Last edited by bjdecker : 12-05-2020 at 06:59 AM. Reason: Reordered CHT values - was 2314, now is 1234.
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  #2  
Old 12-05-2020, 07:06 AM
crabandy crabandy is online now
 
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Default

Nice!

Curious if you noticed a sound/tone change with the bump installed.
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  #3  
Old 12-05-2020, 09:40 AM
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bjdecker bjdecker is offline
 
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Andy,

I think it's a bit "smoother" -- Unfortunately, I don't have a frequency plot before/after to know for certain.

B
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Old 12-05-2020, 09:53 AM
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Quote:
Originally Posted by bjdecker View Post
I attribute the increase in airspeed to a reduction in cooling drag due to "shrinking the exit".
Due to increased exit velocity.

Now think about getting all those tubes and hoses out of the exit flow.
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Last edited by DanH : 12-05-2020 at 09:57 AM.
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Old 12-05-2020, 03:20 PM
David Paule David Paule is offline
 
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The OP referenced this post in Crabandy's blog.

Dave
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  #6  
Old 12-06-2020, 08:59 AM
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Quote:
Originally Posted by DanH View Post
Due to increased exit velocity.

Now think about getting all those tubes and hoses out of the exit flow.
Nice Dan -- where did you end up putting the oil cooler exit diffuser?
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Old 12-06-2020, 12:32 PM
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Quote:
Originally Posted by bjdecker View Post
Nice Dan -- where did you end up putting the oil cooler exit diffuser?
In that early construction photo, it is not cut in yet.

First duct was 4" sceet with glass transitions. Later I built a glass duct with a little larger exit. Been flying it a long time now.
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  #8  
Old 10-03-2021, 01:55 PM
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bjdecker bjdecker is offline
 
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Default The BUMP...

Since I had the cowling off to do surgery on the alternator, I took the opportunity to upgrade the cowl exit ramp/bump from the aluminum version to stainless steel.

I made it a bit wider, taller and included some adel clamps (WDG21019-DG12's) to attach to the engine mount.

Thanks again crabandy, DanH for your work here...
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Old 10-03-2021, 06:27 PM
scsmith scsmith is offline
 
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Making it wider is good. You would like to span the full width of the cowl exit opening.

Making it 'longer' is not really necessary. You can accelerate flow happily around a fairly tight radius. Extra length adds more surface skin friction drag without really doing anything beneficial.

The reverse bend that you now have to facilitate mounting with the Adel clamps is not so good. The goal is to create a smooth ramp for acceleration, like a trumpet bell. If anything, you would like to roll a small lip on the edge that goes back the other way. Can you cut the portion out in between the mounting points, or notch it just inboard of the mounting points and roll that middle portion back? In doing so, you may lose some strength, so you might want to put some doublers locally at the mounting points.

Your speed increase is gratifying. You have increased the effective exit area by getting good high-velocity flow across the whole cross section of the exit (assuming the bump spans across the whole exit). You have eliminated the vena contracta in the exit flow that is caused by flow around the sharp corner at the bottom of the firewall. I am surprised you saw slight increases in CHT, since if anything, you have more cooling flow. Perhaps just variation from slightly different run time or other test condition.
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Old 10-03-2021, 07:15 PM
Kyle Boatright Kyle Boatright is offline
 
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There is a picture partway down the first page of Vetterman's website showing an alternative method of securing the exhaust pipes from side to side movement. The alternate method gets the horizontal rubber hose out of the airflow path near the cowl exit. Here's a link to the page:

http://www.vettermanexhaust.com/

It might be possible to combine the alternate exhaust mount and a firewall bump with a little metal creativity. Might be worth another knot or two and a little better cooling.
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