The Shrinking Exit

[gereed75]

Bob, My hangar mate just made a very nice plenum top by molding it up to the top of his Van's cowl (RV-7). This will yield a shape that fits right under the cowl line and will maximize the plenum volume.

You could lay it up inside the cowl, but because of the bowl shape, you will get epoxy pooling in the center low spot and this would be a pain to deal with.

Obviously the trick is protecting your cowl from epoxy. We used packing tape and sandwiched the glass and epoxy betweem sheets of release plastic (maybe Visqueen will work).

This plenum top was then mated to Van's stock baffles. Came out nice.

He is now working on mating the stock Van's inlets to this plenum. We are using a "snorkel" kind of idea to get complete sealing between the inlets and and the plenum.

The "snorkels" were laid up "female" inside the stock openings. As the cowls are taken off, the snorkels will remain attached to the plenum via neoprene boots.

I think it will be pretty slick.

Glad to hear you got my outlet fairings and hope you get some use out of them. Focus boy focus, you have a full plate going there!!

[rvmills]

Gary,

Right you are. Really need to focus on the baffles. The glass work will come one project at a time.

Hopefully I can get a layup or layer done on the glass projects each day (as they come up), then get baffled the rest of each day.

Really glad I have a heater...it's gonna be a bit of a long winter in the hangar (lots to learn!!)

Thanks again for the loaner parts, and I'd love to see pics of the plenum and snorkels if or when ya got 'em!

Cheers,
Bob

[DanH]

Out flying around tonight. Works fine. No CHT change, spread is about 20 degrees across all four. Oil temperature stayed on the vernatherm set point, hardly a surprise at 55F OAT. Not much else to say until the return of hot weather. Until then I'll devote my experimental time to basic pressure and temperature measurement

[DanH]

Originally Posted by rvmills

I figured you had measured temps into and out of the oil cooler......When I purchased my plane, it took fresh air for the cockpit vents from the back of the baffle (next to the oil cooler).....The interesting thing is that the air was quite cool, even coming from the upper plenum.

Thought about that one Bob. It is interesting. Perhaps those parallel valve cylinder fins truly don't heat the plenum air very much. Perception of temperature (as compared to measurement) can fool you; rigging a temperature probe or two for moving around under the cowl is cheap, useful and fun. Add it to your winter list.

Here is a paste from my notes, temperatures recorded on a day trip to Jackson last summer, after settling into cruise.

Morning OAT 64F @ 8500 ft
Oil cooler inlet probe 81F
81-64= 17 degree rise

Afternoon OAT 86F @ 2500 ft
Oil cooler inlet probe 101F
101-86= 16 degree rise

The OAT and inlet temp probes agreed within 1 degree in the hangar before pullout. And the inlet probe is truly isolated within the fiberglass duct at the cooler face....no radiant heating from some other source.

I'll get some new measurements shortly. A smaller temperature rise is pretty much a no-brainer.....that's not really the experiment. The extended duct length surely adds some friction, which acting alone would reduce oil cooler mass flow. I think the friction will be offset by the higher pressure at the cowl inlet source; see the plenum pressure map in 3405, 0.3F inlet, page 112.

[DanH]

Exit panel Ver.4 got its shakedown cruise last night. This one extends 4" further aft than previous, meaning 4" aft of the firewall. The slope in the RV-8's inset exhaust ramp means the primary exit is again progressively smaller than previous exit panels.....



....possible because this one incorporates additional exit area on demand.



The door is hinged just forward of its center, so the forward edge raises up into the cowl as it opens. The result is two additional exit areas with the frontal area addition of one. Having eliminated the entire exit chute from the bottom of the RV-8's cowl there just wasn't much virtue in frontal area addition, even in slow speed operation.

The center-hinged door also reduces mechanical loads on the operating mechanism, a linear actuator with a linkage rigged to be over-center when the door is closed.



Ok, preliminary data, a shakedown cruise. I launched with the door open and climbed from 200 feet to 10,500, WOT, 2700 RPM, leaning in the climb, at 105 knots indicated, arriving with oil temperature on 197F and CHTs of 347, 330, 355, and 334. OAT was 70F on the ground and 49F at 10,500.

Pushed over and established the usual settings for fast cruise, WOT, set RPM, lean to 100-125 ROP for best power, trim, altitude hold on. Indicated TAS settled in at the usual 181 knots, more or less, which is what I had with the previous shorter exit.

Now the good part.....thumbed the switch to close the door, and picked up four knots....



Me like.

I'll now move into an extension of the cowl pressure and temperature data gathering that Ken, Sonny, and myself have been playing with since last winter. In the photos above you can see the brackets for an exit pitot-static and a temperature probe taped and riveted into the exit. We'll publish here on VAF in due course. Probably rig a video camera too. This one may be worth some yarn and tape.

[jarvis]

Simply awesome, Dan. Is everything glass--except the pushrods, bolts, nuts, etc.?

[DanH]

Is everything glass--except the pushrods, bolts, nuts, etc.?

Yes. Fabrication details in this thread, post 67, 68, and 82:

http://www.vansairforce.com/communit...ad.php?t=22931

[Mike S]

Originally Posted by DanH

Now the good part.....thumbed the switch to close the door, and picked up four knots....

Dan, congratulations on this new setup, Expiremental aviation at its best

Bob A.----you watching this one???

[zav6a]

Man, that is pretty.

Hard to believe there was any drag to wring out of that configuration let alone 4kts.

Looking forward to seeing your data. I won't be surprise if someone buys the rights. Great R&D work.

[erich weaver]

"... a linear actuator with a linkage rigged to be over-center when the door is closed."

Cool stuff.. Minor detail: if it's truly over center, that means it would be jammed, right? I'm confused on why you describe it this way.

Regards,
Erich