RV-7 N117TR

[crabandy]

As long as I measured correctly it should work, I was surprised how far forward I was able to bring the inlets. I temporarily mounted the prop to check clearances, the inboard blade trailing edge is about 1 1/8 from the flat face of the cowling behind the spinner.



More clearance outboard due to the blade shape of the Catto.

[dave4754]

to have your little girl to help you.

Nice look on the cowl

[PilotRPI]

That drywall saw looks awesome for pumpkin carving.

[Jim Frisbie]

by enlarging the inlet are you also enlarging the exit area?
Most of the RV-s that have cooling problems do so because the exit
area of the cowl is smaller than the inlet, creating back pressure.

The appropriate ratio I believe is 1.17 to one in favor of the exit.
Thoughts?
Jim Frisbie
RV-9A, 450+ hrs.

[crabandy]

Originally Posted by Jim Frisbie

by enlarging the inlet are you also enlarging the exit area?
Most of the RV-s that have cooling problems do so because the exit
area of the cowl is smaller than the inlet, creating back pressure.

The appropriate ratio I believe is 1.17 to one in favor of the exit.
Thoughts?
Jim Frisbie
RV-9A, 450+ hrs.

Well........shrinking my exit to create more pressure will be part 2.

I'm hoping the larger lower velocity round inlets in addition to the extra sealing of said inlets and the plenum will keep CHT's in the low 400's on a heat soaked takeoff at 90* and a full power 110 knot climb to altitude. At cruise I should have more pressure differential than needed to cool the engine, I'll need to shrink the exit area to create more pressure in the lower cowling to squirt the air out the exit with less cooling drag.

I measured my original inlet area at roughly 43 sq inches, 43 x 1.17 = 50.31 sq I inches. My original exit subtracting the exhaust pipe area was about 49 sq inches, I nibbled a bit off the end and made it 52 sq inches, both are in the ballpark of your ratio. I don't think cutting a larger opening would yield much of a result as my lower cowling pressures are already quite low.

After reading (several times and still not exactly understanding) NASA CR 3405, AIAA80-1242R I've gleaned these summaries and I'm hoping to gain from:

-Lower Vi/Vo (larger) more forgiving inlets, drag penalty from lower Vi/Vo is very small
-Annular inlets are more forgiving at higher Angles of Attack
-Better diffuser shape aft of the inlets
-Moving the inlets outboard and closer to the prop
-Better sealing with a plenum vs baffle material
-Better sealing of the inlets to the cowl/plenum
-Easier cowling removal and installation

But then again I'm an amateur learning by doing (messing up)!

[DanH]

Originally Posted by Jim Frisbie

Most of the RV-s that have cooling problems do so because the exit area of the cowl is smaller than the inlet, creating back pressure.
The appropriate ratio I believe is 1.17 to one in favor of the exit.
Thoughts?

Yes.

There is no "correct" inlet-outlet ratio.

Cooling is a function of mass flow through the cylinder fins and oil cooler fins. That mass flow is directly related to pressure difference between the upper and lower cowl volumes. It doesn't matter what those pressures might be, as long as the difference exists. For example, assume a particular engine model needs 6 potatoes pressure delta across the fins to cool a particular power setting at a particular air density (i.e. altitude). Airplane A measures 10 potatoes in the upper cowl and 4 in the lower cowl. Airplane B measures 7 potatoes in the upper cowl and 1 in lower cowl. They will cool exactly the same. Airplane A will be faster, as exit velocity will be higher (less momentum loss), because it had a better intake (i.e. better conversion of the available dynamic pressure to static pressure).

Note that "backpressure" can be highly desirable.

[crabandy]

Good news is the prop flange bushing that I staked and used red loctite on seems to be holding, I torqued a socket over the bushing to 50 ft/lbs twice as a crude test.

Bad news is I didn't care for the way I originally blended the rings into the front of the cowl. I basically ran dental floss over the front of the ring and back along the cowl to find the intersection where blended in ring met cowling, the results were to sharp/square/pointy for my liking and didn't jive with the rest of the cowling. I sanded all the foam back off and ran some more tape to try to come up with a more pleasing shape, back to the mind rambling drawing board.







I also made a little progress on my center baffling piece, I made my first real airplane part with my ancient slip roll. You can see the RTV line where the stock baffling was, I tried to extend it back for a longer diffuser. Now to dream up some attach brackets and electronic ignition flywheel sensor wiring armor. I read someone else's thread on protecting the wiring, mine could be better.

[krw5927]

Andy, the next weekend day with decent weather around here I'm coming to OWI and taking you for a much-needed RV flight. You've been down far too long!

[crabandy]

Originally Posted by krw5927

Andy, the next weekend day with decent weather around here I'm coming to OWI and taking you for a much-needed RV flight. You've been down far too long!

Sounds good to me!

[crabandy]

I made a center line down the cowl, it doesn't look right but the inside lines are parallel to the center line. My eye likes the lines pointing toward the outside.



In my first go I used the wife's hot glue gun to glue foam to the cowl, It OK but as the foam was sanded down the little parts without any glue would lift and flex. I think the hot glue gun would still work good to hold blocks of foam together as long as the glue didn't protrude into the area being sanded.

I cut up a Coors Light box and a Dr. Pepper box and used packing tape to stick it on the cowling inside out to hold my pour foam. The Coors Light box was slightly waxier/slicker than the Dr. Pepper and released from the foam just fine, I waxed one of them but couldn't tell the difference when I pulled them off. The pour foam is something else, mix it as fast as you can and it's growing in 20-30 seconds! It's ready to sand in about 30 minutes, too bad I have to go to work!