Cooling and RAM Air

[David-aviator]

This thread is in the FWIT category. Some guys are having cooling issues, some are not.

I seem to have stumbled on a method of providing excellent cooling for the cylinders and oil and not appreciably increasing drag. Looking at the set up would make one suspicious of that claim but the numbers seem to support the conclusion. The machine cranked up to 202 mph yesterday at 8500'.

The next subject is RAM air into the fuel injection system. It is without doubt worth considering. It took some cowl adjustment to provide for a connection to a filter by-pass in the left intake area and one would think messing with Vans cowl is asking for trouble with regard to drag, but that does not seem to be the case.

I am convinced the key to good cooling is air flow across the oil cooler. The type of cooler may be a factor also, but air flow comes first. A RV-4 guy reported poor cooling with a James inlet arrangement with upper cowl pressure at 10:1 with outside air. That's a lot air packed into the upper cowl area. That ratio would seem important but I believe what is more important is the lower cowl ratio to outside air pressure. If the ratio in the lower cowl is the same as outside air, there has to be tremendous flow from the upper high pressure area down and out and cooling ought to be very good. But that is not the result. Cooling is not good and it can only mean air is not flowing down and out because the pressure down there is higher than outside air. And that is caused by a restricted exit area.

Vans has cut the inlet to exit area ratio very close out of a consideration of cooling necessity and drag imposed by exit area dimension. As near as I can calculate, that ratio is about 108% exit to inlet area when considering the exit area occupied by exhaust pipes and a NG leg with -A models. My exit area is about 226% to inlet with the 5x15 bottom exit and 2 Bonanza side vents at 14 inches of area each. That is overkill on exit area but may explain way I have not seen an oil temp above 180 even with a climb to 12.5 yesterday with heat soaked engine at take off and the OAT at 92F. The gross exit area is left over from the Subby adventure trying to get a couple too small radiators to work.



Beyond cooling issues, I believe the RAM air inlet to the AFP fuel injection system is worth considering. It provides for squeezing max performance out of the engine. There is a manifold pressure penalty pulling air through a filter.



The Subaru cooling set up was a compromise from the get-go. I did some informal tuft testing to confirm my suspicion that air was not going through the radiators as much as it should and thought the tuft test proved it. Some other guys asserted the test was not valid because the airplane was not inflight. I believe to this day the cooling set up did not work efficiently because the back side of the rads was within an inch of the engine block and air flow in that area was total chaos. Air should be plenum ducted to and from the radiators with gradual area diffusion as was proven with a number of WWII fighters.

The lack of performance of my airplane with the H6 led to some very black days for me in concluding there was really something wrong with my machine and how I built it. But switching to the IO360 has pretty well vindicated the matter. The same airplane is now performing about as well as can be expected and is at or over Vans numbers. I have not had any black days with the Barrett Lycoming IO360X engine and the Catto prop.

That's not to conclude the alternative engine effort is for naught. It is working well for some guys but it did not work for me. I had one of the first H6 engines and its application has been much improved from 6 years ago.

[BrentHumphreys]

Good Read

One of my buddies down here has one of the earlier Egg's, and he is a big fan. He has had to modify the cowling to get his temps right.

If you are ever in the SoMo area, it would be nice to get a look at your airplane.

[Bill Wightman]

Dave, super good info here. Thx for your reporting. I think you have very accurately nailed the real cause of what for many is an elusive problem - cooling.

My current setup (hasn't flown yet) uses a full time K&N filter in front of the #2 cylinder. As you noted, I expect some loss across the filter and wonder if you might have any data for how much loss to expect.

Thx!

[RV8RIVETER]

I am glad your plane cools well and thanks for sharing the data.

I also believe that the 220% inlet/exit ratio is very excessive. Dave Anders advocates 75% based on his data/experience. And Pual Lipps cooling arrangement is as tight and minimalist in area as they come and he cools well. No change in speed means no more drag than the original configuration, which is of unknown efficiency. However, your goal appears to be better cooling with no additional penalty, which you have acheived.

Your post is a good reminder that there is no one spec to cooling design that is the ultimate or a cure all. You need to know your design goals, good cooling or lowest drag most efficient cooling. The entire system must be designed to work as a whole to meet your goal, with consideration to air flow and pressure. Just like the oil cooler debate, Positech, SW, or other, it is generally not the cooler itself but the entire installation.

[DanH]

Don't get too wrapped up in the 10:1 reference. Here's the quote from that thread:

Pressure difference in flight between plenum and outside is 10 to 1 (10in.hg to 1 in.hg).

Recall my comment at the time was "Interesting data point."

The maximum possible dynamic pressure recovery at sea level and 200 knots is around 135 lbs/sqft. The above quote says 10in.hg, which is 707.46 lbs/sqft. Impossible.

Assume he meant 10" H2O. That is within the realm of reason (52 lbs/sqft), but it wouldn't be very good pressure recovery if he was down low and moving fast when he took the measurement. More information needed.

[Bill Wightman]

David & Dan,

In my somewhat limited experience, I've found the difference in pressure across the cooling interface (cylinders, oil cooler, or whatever) is the thing to keep track of. As David said, its "airflow first" and I believe he hit it right on with that comment, after all isn't airflow the cooling medium?

So, feel free to correct what I propose here (I'm not real long on duct flow and/or cooling experience) but if we manage the problem by #1: trying to max-out dynamic pressure recovery at the cooler face and then #2: presenting the cooler exit with a nice low C_p (coefficient of pressure) then we get good m_dot (mass flow) thru the cooling interface. Correct?

And, Dan, I think you're right about the 10:1 figure. At 200 mph true, 8000' and 100% ram recovery, the numbers are (FWIW):.

Static Pressure: 1571 psf
Dynamic Press: 80 psf
Max total Press: 1651 psf
Ratio: 1.051 (total pressure / static pressure)

I don't see how units of measure matter in this calculation, so I just did it in pounds per sq foot.

10:1 might be possible if we strap a radiator to the Space Shuttle and take data on re-entry.

[David-aviator]

Originally Posted by Bill Wightman

My current setup (hasn't flown yet) uses a full time K&N filter in front of the #2 cylinder. As you noted, I expect some loss across the filter and wonder if you might have any data for how much loss to expect.

Thx!

It won't be much if the filter is large enough (and uses external air). Due to space requirements, I tried a smaller than recommended K&N and it caused a considerable loss of power right after take off. It was internal air only and I thought there was something wrong with the engine until the RAM air valve was opened. I did not notice the manifold pressure change but rpm went up 200.

At present the filter set up is somewhat like Vans except I made a metal box that is attached to the ramp forward of #2 and has a Bracket filter (I think BA-5110, I'd have to look it up). It is connected to the AFP "Y" with scat.

The pressure loss is .4 - .5" at 75% power. It's not a lot. Opening the valve right after take off with WOT throttle is more noticeable.

I wanted to use a large K&LN cone filter with internal air only but finally decided internal air is heated too much during the summer and the penalty on take off with the RAM valve closed could be significant. Some guys are doing that but I have not heard any reports on how it affects performance on a hot day.

I believe the larger the filter, the less pressure loss it will cause.

[DanH]

<<In my somewhat limited experience,>>

Heck Bill, we're all working with limited experience, so we go with fundamental theory. At least you had the formal training. My wee mind is stretched like a Cub gear bungee in a student landing......which is good. Education and recreation, right?

<< I've found the difference in pressure across the cooling interface (cylinders, oil cooler, or whatever) is the thing to keep track of.>>

Certainly the most important thing.

[LarryT]

... x the OD surface area is the way K&N calculates their air flow rating, if you want to compare filters. Confirmed in discussions w/ K&N tech support.

I posted this info on another thread when I changed from a E-3260 filter that has to be hacked up and RTV'd to a E-3322 that fits arround the projections on the bottom of an MA-4SPA carb. E-3260 = 300CFM rating E-3322 = 295.

The air flow requirements I calculated conservatively several different ways:
1. Fuel Flow for max power take-off
2. Air flow at 100% volumetric efficiency @ 2800 RPM
3. Air flow through carburetor bore at 31" pressure differential.

All three methods yielded flow rates of ~ 250 - 260 CFM, though I think the methodology was conservative enough that that flfow rate would not be achieved in the real world.

LarryT

[David-aviator]

Originally Posted by LarryT

... x the OD surface area is the way K&N calculates their air flow rating, if you want to compare filters. Confirmed in discussions w/ K&N tech support.

I posted this info on another thread when I changed from a E-3260 filter that has to be hacked up and RTV'd to a E-3322 that fits arround the projections on the bottom of an MA-4SPA carb. E-3260 = 300CFM rating E-3322 = 295.

The air flow requirements I calculated conservatively several different ways:
1. Fuel Flow for max power take-off
2. Air flow at 100% volumetric efficiency @ 2800 RPM
3. Air flow through carburetor bore at 31" pressure differential.

All three methods yielded flow rates of ~ 250 - 260 CFM, though I think the methodology was conservative enough that that flfow rate would not be achieved in the real world.

LarryT

There is also a HP method of this calculation. Ross told me it is 1.5-1.6 CFM per HP which would indicate a 180 hp engine will gulp in 270-288 CFM.