shrinking exit area

[thunder669]

So I have spent many many hours on here reading posts about inlet vs exit area and all the different theories on best way to accomplish it. based on that and Dave anders articles in kitplanes I have been working to modify my RV4 cowl. I shaped it more like a James cowl but am using their large rings. this seems to be the standard for all the RV cowls except the 4 where they use their medium rings. I obviously want more speed but most importantly I want good cooling even on a long hard climb on a hot day. Doing the math these appear to be the same size inlets that Dave used as he published he has 34 sqin of inlet area. but based on all the reading it appears I have way too much exit area and that could cost me speed. I see lots of posts on shrinking that area but not a lot of info or pics on how. any and all ideas would be great and pics regardless of which model of RV you have could definitely help inspire some good ideas.

Thank you

 

[wawrzynskivp]

No pics on hand

I walked this ground.

I started by fixing blocks of foam in my exit chute until I found an exit area that worked for me. It's not a direct relationship because it doesn't profile the form of the final area, but it gets you in the ballpark for your cooling needs.

My exhaust is two to one, so I have two tubes in the exit area. If you can imagine that portion of the fiberglass as very soft and pliable: I pressed the cowl exit area between the pipes so that rather than one large box it now conforms to the pipes and is nearly even with the fuselage in the middle. In order to actually do that glass work I removed a piece of glass nearly as wide as the cowl exit and long enough to meet up with where the cowl exit first begins to form (transition away from the rest of the cowl bottom) further forward on the cowl. So the net effect is that the only protrusion left from the original cowl exit box is enough glass to fair over the exhaust pipes with enough room for them to wiggle a bit.

I also built a transition ramp behind the cowl exit ala Vetterman's blogs. So my experience will not be exactly the same as others.

Even on normal summer days the fixed exit area is sufficient for climbs/cruise. To account for climbs on really hot days I added two independent electric cowl flaps. Rather than buying anything purpose built I just made very fine cutouts in the cowl bottom and used those cutouts as the cowl flaps.

An advantage of that method is you can integrate your oil cooler air outlet near one of your cowl flaps and target oil temperature in relationship to additional engine cooling.

PM me if you want to see pics.

 

[Marc Bourget]

Duck,

Interesting!

Did you ever compare the Delta P between upper and lower with aux cowl flaps stowed vs deployed?

 

[wawrzynskivp]

Follow up testing

My effort was really just about the temperatures, so I never followed up with delta Ps

Happy though. Never seen any higher than 360 CHT and very consistently at the 340 range.

 

[wawrzynskivp]

Found a pic

Pic Attached

Exhaust tubes are extended to keep hot gasses from entering an air conditioning condenser further aft on the belly.

 

[Toobuilder]

Seems there is a lot more to the success of a given I/O scheme than just a rule of thumb ratio. Considering the conventional rule of thumb where the outlet area is some factor larger than total inlet area, I'm way off base with my 44 inches of inlet area and a hair over 31 on my (fixed) outet. I cool fine on long, slow climbs, and am just where I want to be in cruise. My CHT spread is pretty tight between all 6 cylinders and the absolute value is a touch cooler than Id like.

Where I diverge from even the more radical of the "outlet reshapers" among you is rather than try form the outlet tunnel into a shape that gathers, organizes and accelerates the outflow to match free stream velocity, I just cut to the chase and built 20 inch long augmentor tubes. Pretty hard NOT to get airflow organized and accelerated when you are shooting it down a straight tube. Made a believer out of me and I think at some point we are going to apply the concept to a buddy's RV-4 project.

All this to say that the aerodynamic solution at the outlet takes some work, but if done right will pay off big.

 

[DanH]

Fixed exits, Schmidtbauer (left) and Anders (right).
.

 

[thunder669]

Thanks for the pics. I also have the dual exhaust so will definitely be a bit more of a challenge then the single exhaust in the middle. I look forward to getting it a bit smaller for the speed and if necessary i can always add a cowl flap in the middle on the bottom of the cowl for a long climb, but inlet air should never be a problem