Increased My Cowl Exit Area

[robcaldwell]

My original exit opening was minimal. Following Mark Goldberg's advice, I increased the exit opening by 80 square inches. I now enjoy lower CHT's by as much as 13 degrees F.




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[Nev]

robcaldwell Thanks Rob. Any chance you could give some hard numbers for reference? Something like ISA +5 (f) 2000ft, 90kts climb, Cylinder head temps 360F.
Lots of good detail in those pics btw.

[robcaldwell]

I have no idea what you are asking me Nev ... LOL. I just know I don't get as hot as quickly anymore and at cruise I am slightly cooler. Keep in mind I am still performing engine break in ops (yes at 35 hours... because of nitride plated cylinders....). I have only seen 360F (or barely less) on maybe 2 cylinders during this time. #6 is 410 in cruise. All others are 370F to 395F. Again, engine break in ops at WOT, 2500 RPM, full rich. Won't know much else until engine settles in. I use Savvy Aviation for engine diagnostic analytics and they tell me all is going well and to keep it up.

I might be a little ahead of myself with this information as I planned to release the results of my Phase 1 flight testing with these anomaly’s in detail later. Just thought I would share the benefit of the increased bottom cowl exit.

[AZBearhawk272]

Rob,

You may be proving a point without realizing it. If the upper deck air has a path to the outlet without going through fins the outlet can be made infinite in size without
improving cooling. The cooling air must be forced through the fins. The increased outlet and the very steep angle of the lip will not create a bigger suck once the air is cavitated turbulent.

I have seen most of these perturbations and modifications and the biggest improvements was to make sure that all the air absolutely had no way out other than through the fins.
Getting the air leaks closed down resulted in an Arizona Bearhawk dropping temperatures in cruise about 25 to 30 deg. And then lower lip was then reduced back to something that looked
" Normal ".

Just thinking you may have some big leaks somewhere.

Kevin D
#272 Chandler AZ

[robcaldwell]

I don't believe the upper deck has an alternative path to the outlet as I am using a plenum and everything I can see, including pin holes, are sealed up. However, I do believe that my prop governor is a major obstacle on the left side and in front of #2, 4, & 6. I fabricated an aluminum box that covers the governor and splits the air coming through the inlet.

Keep in mind that the 13F reduction I indicated was during break in settings (WOT, 2500 RPM, Full Rich). Yesterday I reduced the power to WOT, 2300 RPM and LOP. With that I saw 388F on the #6 and all others were well below that. (engine still not fully broke-in with piston ring and cylinder wall friction events still occurring).

[AZBearhawk272]

I was impressed with your plenum as I watched your build and that should be a pretty efficient setup.
Time will tell, we chased high temps on Scotts BH for a year and finally found the leak was baffle seals between the engine and the nose bowl. losing seal in flight.
The best is yet to come, there is a whole country west of you that needs exploring.

[robcaldwell]

Thank you, Kevin! Yes I will be heading west beginning next week with our trip to Oshkosh! My machine just itches to fly cross country. The FAA gave me a 50nm radius to conduct my phase 1 which has been challenge to stay within that border with the speeds I am seeing. Feeling pretty good about the trip. The engine is settling in, but still not fully broke in. Will probably fly most of the trip WOT, 2300 RPM, and ROP. I'm being advised to stay away from LOP until after 50 hours on the engine.

[Battson]

Did you measure any airspeed reduction? I don't expect any. I have been unable to measure any difference with the cowl flaps open and closed, although she is persnickety about getting up onto the 'step'. They are good for around 15*C (30*F).

That temperature difference is smaller than I expected considering the generous increase in opening area.
I tend to agree with Kevin, I think you might have proved the lip design is more important than the cowl exit area.

[swpilot3]

Kevin, where exactly was the leak on Scotts nose bowl? On the outboard or inboard baffle?

[AZBearhawk272]

They all added up but the illusive leak, Scott chased down to the front inlet baffle below the prop governor where the rubber should have been sealing against the nose bowl
down below the inlet opening. Difficult to find, almost impossible to see the contact fit in the area.

Kevin D

[robcaldwell]

Battson I did not think to check for airspeed reduction. I don't think I lost any speed. But I continue to add fairings and such so maybe it's a wash.

I'm also not thinking I have any sealing loss at the inlet as I fabbed aluminum baffles there. (Shown in this video):

https://youtu.be/oYMVXIajP6Q?t=499