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RV-4 CHT cooling

S

Steve Sampson

Well Known Member
I would like to hear of other RV4 cooling experience.

I hardly have a problem but am trying to operate with the CHT not more than 400F in the climb. At about about 135/140mph near gross I can make about 1400fpm with everything forward. (160hp O-320 MT 2blade c/s prop, p-mags). After around 3000' I then have to change something or I break my self imposed 400F limit. I would however like to be able to climb with everything forward to the stratosphere, or at least 10000'.

One solution is to get the speed above 150mph which of course cuts the climb rate. Another is to reduce to around 25 square and keep the same speed. The second is the more effective because I suspect the p-mags retard a little which more than compensates for the reduced fuel flow.

I dont have an oil temp problem, in fact next time the top cowl is off I will reduce the air flow to the oil cooler to get the oil temp up a bit leaving more air for the CHT.

I am wondering if I should cut some of the bottom cowl away. It is clear to me that with the 4-pipe exhaust there is less air outlet area than inlet area. This may well be the real issue but once its cut its gone for ever.

Thoughts?

PS I should add that the UK is not like Texas. Ambient today was typical at 65F at low altitudes and 45F at 8500'
 
Hi Steve....

....I'd resist cutting the lower cowl just yet. Years ago, the Cessna 188 Agwagons and Agtruks added a 1" lip to the lower cowl and dropped temps by 40 degreesF or thereabouts, since it added a low pressure area there. It'd be easy to do with a few pop-rivets. The Cessna lip was almost perpendicular to the airflow, and flush with the rearmost edge.

Regards,
 
From everything I've read, and people I've talked to, exit area vs. inlet area should be about 3:1, you want lots of low pressure in the bottom side to draw the air through. If you are not worried about cooling drag that much, the lip is as good an idea as any. Another highly sucessful solution is the addition of louvers to the bottom cowl. I know Van's sells a couple different ones that can be added to lower pressure.
 
osxuser said:
From everything I've read, and people I've talked to, exit area vs. inlet area should be about 3:1, ...
Click to expand...

Are you sure? I understood the ratio was more like 1 inlet : 1.05 to 1.25 exit.
1:3 will give a huge hole for the exit.

Steve, I believe Greg Scott used a lip to bring down his CHTs, I think he duct taped the lip on to start with until he was happy with the size. I think I will be adding louvres over the winter - my CHTs can get often get to 400 in the climb.

Also see this recent thread http://www.vansairforce.com/community/showthread.php?t=33369

Pete
 
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I have quite a bit of time in RV4s and consider it to be one of the best of all the cooling systems. Before you start making modifications to the exit air check the basics first: ignition timing, fuel flow, obstructions to the flow through the cylinder fins, and for leaks in your baffles, particularily around the inlets. Make sure that your inlets are properly shaped and formed without any sharp edges that could trip airflow. Opening the exit area and adding louvers may help with your cooling but they will also add cooling drag. Look for all other potential problems before you give up speed!
 
Cowl flap.....

.....is also an option, probably the best solution. Cut the lower center outlet section 6" forward, along each side and then re-attach it with a hinge and a cable control. like Bonanzas and such do. Open for climb and closed for cruise...really the better solution.

Regards,
 
Adding a 'lip' to the lower cowl could induce some burbling.
One of my Citabrias would really growl at certain speeds and the cause was the lip. Reduction of the lip cured the burbling.

My -4, BTW actually runs very cool-- too cool in temps below 40 deg F. I do try to maintain 125kts in climb and that usually gets me ~1600'/min.
(Lyc O360)
 
Steve
I had a look at your post again and I would like to state again that i do not feel your problem is in the outlet air area. The four pipe exhaust system has been used effectively on many RV4s so I would not blame it for reducing the exit area size. As your oil is cool then the problem may not be airflow. Check your fuel flow at full power and see what it is. Higher is better than lower as you can always adjust the mixture yourself. These airplanes are cooled with fuel as much as with the air and oil, especially in the climb phase.
 
RV4 CHTs. Some pictures of the outlet air exit.

Thanks for all the replies. No I wont cut it yet but I still think it is the way to go.....but I might be wrong! I guess I was hoping to see the bottom of other RV4 cowls to see just how they were cut. Mine looks like this.

Pierre/Skyhook, I was interested in your idea of a cowl flap. It comes up in the RVator, 24 years. I doubt I will do it though because I like simplicity. It set me thinking at what temp would you shut it. I know 400F is getting hot, but when are the cylinders starting to get too cool?

Tom, full power fuel flow is right on the Lyco number for an O-320. I have checked several times in the past. Yes, my oil does run cool until I spend any time in the circuit below 100mph. Then it noticeably climbs. In fact you can see it here. Its the blue line at the top. At the end of the flight you see 4 touch and goes. I think I have already done the basics. P-mag timing is different though. It advances at high power so will be more demanding on the cooling. You have obviously more experience than I have of -4s. Because VANS give no guidance on where to cut the cowl I wondered if my basic exit was smaller before the 4 pipes go in?

Pete, I certainly could put a lip on but it shouldn't be necessary and I worry I will loose a knot. I don't really have a problem, its just -4 cool well in general and I am wondering what is different, since it could do better.

I will continue to poke at the issue and get more hours on the engine, but more thoughts are very welcome.
 
Fuel Flow, more info

Tom, full power fuel flow is right on the Lyco number for an O-320. I have checked several times in the past.
I will continue to poke at the issue and get more hours on the engine, but more thoughts are very welcome.
Click to expand...

Steve,
Where are you getting your Lyco Fuel Flow numbers?
I see you have a FF number in your graph, but I don't know how to put a gph number to it. I do see it follows the MP pretty closely.
I ask because when I look at my Lyco user manual there is a chart for FF but it doesn't say what carb, just the MA 4-5 or what airplane, ie, induction system it is installed in.
Precision gave me some PPH or flow numbers on the phone one day that were about what the #'s Lyco chart showed. But they couldn't tell me what my carb was flowing in my RV-6.
From what I understand major manufactures like Cessna ask the carb maker to give them a carb like a MA 4-5 with a certian flow number to match their intented induction system. Hence we see the 10-3878 or the 10-4164 etc.
Vans has never done that, given that each RV can be set up differently. So we are guessing at the proper FF by installing what we think will work based on what seems to be working for someone else. As an example, Vans use to sell their 0-360's with the leaner 10-3878 and now they sell them with the richer 10-4164. Why the change? well I think there was some feed back from Precision prompting them to go with the richer carb. Anyway, we are kind of on our own for figuring out the proper fuel flow for our airplanes with our setups. I used my EGT/cht's as a determining factor as to when my carb was rich enough. My -6 would pass the flying Lean/Rich test with no problem, but would over heat on the hard climbout. I found good information on AVWeb, John Deakin, look for articles called.......here are the links
http://www.avweb.com/news/pelican/182179-1.html
http://www.avweb.com/news/pelican/182176-1.html
http://www.avweb.com/news/pelican/182583-1.html
Good luck.
 
Bob Martin said:
Steve,
Where are you getting your Lyco Fuel Flow numbers?....
Click to expand...

Bob, I was reading off the operator's handbook for the 160hp O-320. It gave (from memory) 13.26USGal/h.

I get up to be 13.39 USGal/h (or 50 litres/hr) at full flow fuel, or 101%.

Perhaps I dont understand this but I would have thought the engine requires an amount of fuel, shown in Lyco's curves, and the carb is just a way of achieving that?

Thanks,
 
Fuel for thought

Steve, to long ago to remember numbers I had my servo on my IO320 flow tested and adjusted buy an authorized repair station. He had to do it 3 times to get EGT?s correct at full throttle. One time it was leaned by 2 gph at full throttle, when I test flew the 4 the EGT?s went past 1600deg within seconds after take off, pulled the throttle back, landed and had it readjusted to a little fatter than what Bedix called for. Oil temps never went past 200 at anytime. What I see similar here is oil temp is unaffected but EGT was, I didn?t have CHT, but would think there might be a correlation between CHT and EGT. Although you have a carburetor, this is just an example of mixture vs EGT?s at full throttle not leaned. This whole process began with a mag drop, after mags overhauled, wound up to be a little piece of **** in one injector that caused a 200rpm drop on only one mag. Go figure!
 
more air

Steve,
Think more air!
Van's FAB and K&N air filter seem to breathe very well.
Apparently better than Factory Certified systems.
So if we take Standard Chart numbers and then add more air.......
we get leaner mixtures.
Hope it helps.
 
Steve, the first thing to check is for flashing obstructing the cooling air flow around the spark plugs. Filing them smooth made a 30 degree difference for me.
 
The Ram Air up there...

I had a Ram air/filter setup on both my RV4 and now my Rocket. Having flown many FAB setups I can assure you they are adequate for most users. However comma, if you are a gear-cranium like myself, 1" increase of MP at 11,500 feet is alot, so therefore, RAM Air! My airbox has a flapper valve that I close on the ground and open on takeoff with a K&N filter on the closed intake.

As far as cooling goes on the RV4, I rarely disagree with Pierre, but a cowl flap isn't necessary on the RV4 or the Rocket. On my RV4 and Rocket I cut 2" off the bottom lip, didn't lose one knot of speed and lowered all of my temps 20-30 degrees. Hot day climb temps on my -4 were max of 355F. The Rocket hits 390F on the rear cyllinders only on an extended climb and inflight I see as low as 288F running LOP. You can always put a restrictor over the oil cooler inlet as you stated earlier and increase the air over the cyllinders. I have seen this as a common problem with many of the new cowlings on RV4's I have inspected for customers. Your mileage may vary...

Smokey
HR2
 
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Fuel Flow Check

I asked Don Rivera of Airflow Performance about fuel flow on a Lycoming and this was his response:

One of the fallacies of setting a fuel flow number at takeoff is that is not how an engine operates. Correctly the engine requires a fuel air ratio to operate correctly. That being said the fuel air ratio for max power is determined by the engine manufacture and then tweaked by the airframe manufacture during certification flight tests. So the fuel flow number you see at take-off when using a Bendix/ Precision, or Airflow Performance fuel injection will be dependant on the horsepower the engine is making. This is because these type of fuel injection systems measure the amount of air the engine is consuming then deliver the appropriate amount of fuel for the calibrated fuel air ratio the system was set up for. Unlike a Continental system that delivers a fuel flow based on RPM and throttle angle. With this type of system (probably what Deakins is talking about) the fuel flow is the same regardless of how much power the engine is making, it only knows the RPM and throttle position.

He also suggested how to check for lean verus rich:

What your looking for is that the engine is getting the correct fuel air ratio not a fuel flow amount. You can do this by looking at EGT change from full rich to peak. Set a power level at around 3000' of 2400 RPM 24" MAP. Lean the engine until the first cylinder peaks, record the number then put the mixture full rich, record the full rich EGT for that cylinder. If the change is 175-250 degrees F then the fuel air ratio being delivered by your fuel injection system is correct. If it's less than 175 it's too lean , if it's more than 250 it's too rich. This can only be adjusted by a jet change on a flow bench.

Not so sure this is related to the cooling issues but was informative and relevant to the other posts on this thread.
 
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