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Aurelflying

Naca for EZ

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Here you go, see attached.

That's a P-51 style, Jon.

 

I can't say what NACA dimensions work best on a Varieze but I don't think the dimensions are  critical.   Aurélien, do a Google Images search for "NACA scoop varieze" or "NACA scoop" and will see lots of them.  Just build one similar to what you see.  Also see the NACA scoop that Klaus sells for the Long-EZ.   He is pretty careful about testing what he uses.    http://lightspeedengineering.com/Services/NACAduct.htm

 

The original submerged scoop came from inlet research for supersonic aircraft and was just adopted by EZ builders without much experimentation.   It works OK on EZs but I think the best cooling comes from a P-51 style forward-facing scoop.  

 

I have experimented on my Cozy's NACA scoop by putting a pitot tube in the center of the opening.  I found that the airflow would skip over the scoop.  The flow improved with vortex generators mounted ahead of the scoop.   You might get good or better flow into a submerged scoop with parallel sides versus diverging sides.  There is lots of room for experimentation here.

 

As for the opening size, I would suggest taking some measurements off of flying EZs with decent cooling and scaling the opening to your own horsepower.  For example, the Cozy IV plans with 180 HP engine call for a NACA opening at the firewall of 3.1" x 15" or .258 sq inches per hp.  

Edited by kent ashton

-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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That's a P-51 style, Jon.

 

I know... but a NACA duct was not in the original VariEze plans, so I went with "big airflow intake" instead.

 

Here's a good spreadsheet (assuming it's accurate) for calculating the depth and width at various points.


Jon Matcho :busy:
Canard Zone Developer & Builder
Rebuilding Quickie Tri-Q200 N479E
Building Cozy Mark IV+ (widened rear)

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Yep, If he will google "NACA duct"  he will also come up with pics and coordinates that can be scaled to make various lengths of NACA duct.  For example:  http://www.rcgroups.com/forums/showthread.php?t=127858

 

Some interesting oil flow pics here:

http://wiki.velocityoba.com/index.php?title=NACA_Ducts

 

Always easiest to start by copying someone who has a good working system.  However, I tried using small NACA scoops for downdraft intakes and they didn't work for squat.  I thought I was copying some good ones from Wayne Blacker's nice Long-ez but I see now he is not using them either.

 

The main lesson I learned out of that was to pay attention to square inches of intake and exit area.  I think there are some good ideas on that here:  see "Downdraft cooling design" here  http://www.longezpilot.com/N6PC%20page8.htm

 

 

Rule of thumb through experience and many iterations of test flights of several aircraft. A 4 inch high by 6 inch opening or 24 square inches is the minimum inlet size to cool 160 hp. That equates to 12 square inches per side of cowling for inlet opening. Add an additional 2 square inches per each 20 hp.  You can go smaller but you will struggle with CHT’s. I’m not telling you how to design your inlet size, I’m telling you what works for those of us that have done the testing and modification to downdraft cooling. You can build whatever you choose. Keep this in mind; the inlets should not interfere with airflow to the propeller. They need to be designed and positioned in a manner that allows efficiency yet does not impede on prop performance.

 

Next you have to allow for the hot air to exit the rear of the cowling. Sometimes this is difficult due to the cowling opening being small. The propeller acts as a big air buffer at the back of the cowlings. The exhaust air has difficulty getting past the pressure pulse that the prop creates between it and the cowlings. You need to make sure you have adequate venting at the back side of the aircraft. Do not close up the cowlings to tight or you’ll have engine CHT issues. So allow for the cooling air and exhaust air to exit the rear of the plane. You’ll need 2 to 2.5 times the exhaust area as the intake (Engine inductions system, oil cooler and cylinder cooling).

 

 

-Kent
Cozy IV N13AM-750 hrs, Long-EZ-85 hrs and sold

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