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Yukon: I'm thinking this is pretty good evidence (as well as the great number of RV's displaying marginal cooling on hot days) that the exit area on Van's cowling is too small. What do you guys think? I'm considering a set of these.
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Some RV's (especially RV-4's) are over cooled, while the same engine prop in a RV-6 is more toasty? Go figure same/similar areas.
Tom, I'd avoid any cowl louvers until you fly. I think you have a O320 or O235? Rarely if ever have I heard of 320's (150/160HP) reporting high CHT issues. 150HP always run cool. It makes sense, less hp, less heat. I'd assume a O235 would run cool as well (in a cowling basically made for 180hp or more).
Per the time honored tradition, if CHT/OT temps are hot in climb, you can always lower nose and increase speed. I can see if you can't get temps down in cruise you might think about more cooling.
Oil temp should not be an issue if you use a SW (South Wind / Stewart Warner) oil cooler installed properly. The IO360 angle valve (200HP) is more of a challenge to cool apparently, needing a larger oil cooler capacity. More HP more heat, but also the 200HP angle valve uses oil more to cool the engine than parellel valve 360/320/235's.
Increasing the exit air will cool the engine, no doubt,
but it will slow your cruise and top speed. I'd like to get data on the before & after louver speeds. Even after installed you could block it and flight test the delta in speed and temps. I could see making small louvers that pop open for climb only and keep the a smaller fixed exit? hummmm
From Dave Anders notes:
4) inlet to outlet ratio: what?s recommended, what works & why:
a) Stock Van?s: RV4 ratio ~ 39sq? X 60sq? = 150%
RV6 ratio ~ 44sq? X 56sq? = 127%
Designed for full throttle climbs at 90-100 mph
(gmc note: not sure but assume 100F day? Airspeed Vy?)
Results = increased drag at higher speeds
b) what works better:
Inlet 34sq" ( decreased To 30sq" no change)
Outlet 26sq" (excludes exhaust area)
Ratio 76%, could be smaller w/same ratio, CHT?s 350 max.
Dave clearly feels the exit should be 24% smaller than inlet, which is probably true in cruise. Clearly Van has larger outlet ratio, even larger than the 1.20 that RV8N Karl posted from another reference. It's clearly a moving target dependant on OAT, power, phase of flight and goal (max speed or more cooling).
A pilot controlled cowl flap is nice, but it adds complexity, weight and pilot chores. If you can do with out a cowl flap, it fits the KISS principle of the RV.
RV's have low drag and high climb/cruise speeds, so we benefit by having more cooling air to work with, than a C-172 for example, even in climb. We can climb at high speed and still out climb a C-172. In the summer you may have to lower the nose and increase airspeed sometimes. Climb rates are so good with RV's it's not a big sacrifice. This is true of many planes with out cowl flaps.
I remember flying the trusty C-172's w/ 4 people in the summer, crawling to altitude with intermediate level offs to keep OT green. Funny thing, there was no CHT gauge, so ignorance and rental planes are bliss?
No I did not abuse the above rental plane, in fact I was a CFI showing pilots how not to overheat in the summer. Most GA planes have no CHT gauge. May be they are over cooled so CHT is not an issue? May be oil temp is a good enough secondary indicator. For certification they have do a climb test and maintain temps in green. Of course green CHT per Lyc is hotter than what I want to run at. I like to keep it below 400F with out exception.
These new multi data channel engine monitors tells you if paint is flaking off the valve cover make me laugh.
May be we have too much info? Kidding, I'm all for multi channel CHT/EGT, especially for experimentals. However we may worry a bit much, but CHT=400F or less is key to long engine life.
As far as oil temp, my theory about high oil temps has to do more from poor efficiency oil coolers (SW clones) and poor installation (using vans airbox kit). There is plenty of air in stock cowls, it is just how you use it.
If you have a hot IO360/200HP and operate out of death valley, louvers may be justified. I see a plus with the louvers. You can block them off in the winter. I know of a few RV-8, 200HP IO360's guys felt the urge to resort to louvers for hot days. However I think they had other issues, like using poor quality or small oil coolers for the job and/or not supplying the oil cooler with enough air.
Look at RV969WF (Alan Judy's) hot red RV-6. His exit is about 25 sq-in (not sure if its total area with or without exhaust). His RV-6's IO360/200HP inlet was cut down to nothing. I think he went with two 2.123 or 2.75" dia inlets or about 10" sq-in! This is less 1/4 of Van's stock cowl inlets. However Alan has a separate cowl scoop for his oil cooler which he can open/close from the cockpit. He stated his cooler inlet area is about 12.5 sq-in. So the total area is 22.5" and exit 25" so the ratio is 1.11. Angle valve IO360's reject more heat through its oil than parallel valve engines. Angle valve Lycs have oil squirt'ers on the piston backs to carry the heat away for one example. So his approx 10 sq-in cowl inlet area would be too small for any engine with out a separate oil cooler scoop, especially for a parallel valve engine, which needs air over the jugs.
I have 4" rings for a plan-jane O360/180hp. That's a bit over 25" sq-in. I've not decided what to do about the cowl exit area, but its going to get reduced some what. Less than 25"? Probably not. If I use Dave's 26" plus exhaust, it would be about 30"-32" total exit area. So my ratio would be about 1.24. A simple cowl flap would be cool. ANY IDEAS?
Exit Area Too Small ??????
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