VansAirForceForums  
Home > VansAirForceForums

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

- Today's Posts | Insert Pics


Go Back   VAF Forums > Main > RV General Discussion/News
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
  #1  
Old 09-28-2019, 01:37 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 882
Default Cowl engine intake design question

I have been reading archives all day that support the "hole" in the cowl should protrude into the air stream to avoid boundary layer on the cowl surface. I would prefer to keep the protrusion minimum for appearance. Is a circular orifice perpendicular to the airstream suffice, or does it need to stick way out like the stock Vans cowls ??
__________________
Larry DeCamp
RV-3B flying w/ carb / Pmags / Catto 2b / Steam
RV-4 fastback w/ Superior XP360/AFP/G3X/CPI/Catto3b
Clinton, IN
Reply With Quote
  #2  
Old 09-28-2019, 03:01 PM
gmcjetpilot's Avatar
gmcjetpilot gmcjetpilot is offline
 
Join Date: Jan 2005
Posts: 3,987
Default

Quote:
Originally Posted by Larry DeCamp View Post
I have been reading archives all day that support the "hole" in the cowl should protrude into the air stream to avoid boundary layer on the cowl surface. I would prefer to keep the protrusion minimum for appearance. Is a circular orifice perpendicular to the airstream suffice, or does it need to stick way out like the stock Vans cowls ??
So this is for the carb or FI from the bottom of the engine I assume.... Are you planning on making your own?

I will give you my very opinionated biased suggestion. Van's FAB (air-box) and scoop setup is a very good design, for a carb or updraft FI. I got 1" of MP rise at altitude in cruise with my RV-4 (vs Alt air on). To save time and effort I would buy Van's off the shelf induction system (cowl scoop/box).... If you have time to kill and want the challenge make your own scoop and box (and alt/carb heat air), it will take longer and likely end up costing more $ as well. In the end it might not work that great.

Here are some ideas if you want to go custom:

Link below looks sexy but has two big issues as installed in this plane. The filter is right at the high velocity inlet, draggy and little to no pressure recovery. Second is the scoop is near the prop hub where the air get's beat to death.... However getting close to the prop closer to the prop tip gives you more ram pressure. There is an excellent book called "Speed with Economy". I recommend it. His engine inlet scoop was LONG and ran to a fraction of an inch from the prop... (Look at some of the reno formula racers.) However you have to get closer to the prop tip not the hub. Prop hub area near spinner is bad air...

http://www.painttheweb.com/rv-10/SjCowl.aspx

This shows a SJ cowl grafting on a Van's inlet on. I recommend this.

http //romeolima.com/rv8/Cowl.htm

You can use van's inlet (which is not round but kind of 'D' shape) and stick it on the lower cowl like this. How you filter it and get pressure plenum to recover some RAM air (and alternate or heated air) will take some creativity. This is clearly smaller than the full Van scoop, which is also an area for the FAB airbox to sit in and cowl air outlet....

https://glasair-owners.com/uncategor...-in-a-glastar/

This to me is an ideal inlet. Also the shape of the inlet should be like an inside out airfoil... It is not too close to hub but close to prop, and does not stick out like Van's which is much lower on the cowl. However as I first said the Van's intake set up and is easy and works well.

__________________
George
Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767

Last edited by gmcjetpilot : 09-28-2019 at 03:40 PM.
Reply With Quote
  #3  
Old 09-28-2019, 04:19 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 882
Default Thanks Jet Pilot

Appreciate the effort to post links. I am not a performance or speed addict, but lots of food for thought.
The Vans FAB was a real bump in the SJ cowl bottom so I sent it back and fab'd a custom with oval filter (K&N 60 SI surface) to narrow the GUPPY fish look. Still not happy with looks. AFP says their FM-200 cone is more than adequate and that agrees with K&N tech criteria. So, the dilemma persists, use a cone up high or big box (ugly cowl) down low.
__________________
Larry DeCamp
RV-3B flying w/ carb / Pmags / Catto 2b / Steam
RV-4 fastback w/ Superior XP360/AFP/G3X/CPI/Catto3b
Clinton, IN
Reply With Quote
  #4  
Old 09-28-2019, 10:09 PM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 8,892
Default

Quote:
Originally Posted by Larry DeCamp View Post
Is a circular orifice perpendicular to the airstream suffice, or does it need to stick way out like the stock Vans cowls ??
Perpendicular is generally fine. No, it does not need to stick way out.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #5  
Old 09-28-2019, 11:30 PM
scsmith scsmith is offline
 
Join Date: Jan 2008
Location: Ashland, OR
Posts: 2,225
Default

Quote:
Originally Posted by DanH View Post
Perpendicular is generally fine. No, it does not need to stick way out.
However, a circular orifice in a surface that is inclined to the flow, such as the cowl lower surface, will not produce full ram pressure. You must project out some distance in order to get out into the undisturbed onset flow, or else your opening will have flow across the opening that is trying to turn to match the shape of the lower cowl.

The alternative is to make the inlet shape non-circular and/or oversized so that it is able to accommodate some substantial angle to the onset flow without separating, and create external diffusion to help achieve full ram pressure. Also, the "corner" flow below the spinner where the flow has to turn through an angle to match the front of the lower cowl has pretty high pressure to start with. That is why the P-51 inlet is there. And the intake on the F-1 Rocket.
__________________
Steve Smith
Aeronautical Engineer
RV-8 N825RV
IO-360 A1A
WW 200RV
"The Magic Carpet"
Hobbs 600
also LS-6-15/18 sailplane
VAF donation Jan 2019
Reply With Quote
  #6  
Old 09-29-2019, 12:40 AM
koupster's Avatar
koupster koupster is offline
 
Join Date: Dec 2013
Location: SLC, UT (KBTF)
Posts: 196
Default

Would the most efficient inlet be aligned with the spiral flow from the propeller rather than the oncoming airstream ahead of the aircraft? I've been wondering about this, and this thread has posts from those who can give an enlightened answer.

Cheers, David
RV-6A KBTF
Reply With Quote
  #7  
Old 09-29-2019, 07:34 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 8,892
Default

Quote:
Originally Posted by scsmith View Post
However, a circular orifice in a surface that is inclined to the flow, such as the cowl lower surface, will not produce full ram pressure. You must project out some distance in order to get out into the undisturbed onset flow, or else your opening will have flow across the opening that is trying to turn to match the shape of the lower cowl.
Excellent point. The question becomes "How much is some distance?"

This requires a picture!



First example projects out to a point very near the blade path, and is at a significant radius from the hub. Example 2 is located at the same blade radius, but doesn't project forward as much. Example 3 is near the blade path, but at much less radius and higher, near the spinner.

Steve, your thoughts please. I'll venture that #1 and #2 would result in pretty much the same intake pressure, climb or cruise, thus my previous note saying it doesn't need to be way out there. However, as you say, if dimension "A" becomes too short, flow following the cowl face will reduce available dynamic pressure. So, returning to Larry's question, is there a rule of thumb for a minimum dimension "A" ?

Quote:
Also, the "corner" flow below the spinner where the flow has to turn through an angle to match the front of the lower cowl has pretty high pressure to start with. That is why the P-51 inlet is there. And the intake on the F-1 Rocket.
Agree, Example #3...decent pressure in cruise, although I don't think it will do as well in climb. In cruise, that location exhibits relatively high pressure, but with low forward airspeed, it's too inboard to harvest any significant dynamic pressure found in propeller outflow.

When working with cooling inlets at maximized prop radius, low speed, high power climb coefficients of pressure are much higher than the available freestream dynamic.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #8  
Old 09-29-2019, 12:11 PM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 882
Default Spinner high pressure / attached flow ?

Thanks Steve and Dan. Your comments raise the question:
If the flow off the spinner is high pressure, and “stuck” to the surface, why not continue the surface into the intake airway like Rocket / P51 ? The bottom of the airway could be perpendicular like dan’s example #3.
__________________
Larry DeCamp
RV-3B flying w/ carb / Pmags / Catto 2b / Steam
RV-4 fastback w/ Superior XP360/AFP/G3X/CPI/Catto3b
Clinton, IN
Reply With Quote
  #9  
Old 09-29-2019, 06:29 PM
Marc Bourget Marc Bourget is offline
 
Join Date: Apr 2009
Location: Stockton, California
Posts: 224
Default

All sorts of compromises must be addressed here.

If he bottom of the "D" inlet is properly shaped, it might be advantageous to tilt the inlet slightly downward - seeking to obtain the best Delta P at lower speed and high power demand (climb out).

FWIW
Reply With Quote
  #10  
Old 10-02-2019, 09:51 AM
gmcjetpilot's Avatar
gmcjetpilot gmcjetpilot is offline
 
Join Date: Jan 2005
Posts: 3,987
Default

Quote:
Originally Posted by Larry DeCamp View Post
Still not happy with looks. AFP says their FM-200 cone is more than adequate and that agrees with K&N tech criteria. So, the dilemma persists, use a cone up high or big box (ugly cowl) down low.
I get it, this is a looks thing. You are not too concerned about efficiency.

This is what you want:
https://www.jamesaircraft.com/products/air-filter-kits/

You want the FI no bottom hump look? Sam James will allow you to have no or little bottom hump with carb... SJ has a right angle snorkel that transitions to forward facing, duct to the inline air-box. However with the O-360 that is a BIG carb and the engine is taller than than an O-320... Good luck, shows Pics when done. Love to see the solution.

There is nothing really magical about round unless you are going to have a venturi shape (curved surface) in the throat of your round intake... and then an efficient plenum right behind that. Round, "D", rectangle... Up to you and your taste. Just have generous radius on the lip of induction inlet and sufficient area... with nice smooth transitions preferably to larger area plenum to convert ram air (velocity) to pressure before going through filter. This is what the Van FAB box does well. Placement of your induction scoop relative to prop was mentioned before.

There are only so many solutions. There are aerodynamic research papers, books on the topic of scoops and cowl inlets.... Since you are going for look do what looks authentically pleasings, it is not that critical... If you have a Carb (especially O-360) the Vans FAB and scoop is easy and works well. It is a big hump however on the bottom of the cowl. I don't find it objectionable. I like it, but the Fwd facing FI cowl and vans air-filter in the engine cooling cowl opening is clean looking... You could change your engine to FWD FI but that is big bucks.
__________________
George
Raleigh, NC Area
RV-4, RV-7, ATP, CFII, MEI, 737/757/767

Last edited by gmcjetpilot : 10-02-2019 at 10:28 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 10:50 PM.


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.