What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

The Shrinking Exit

RV8RIVETER said:
Dan beat me to it. ;)

To tweak it a little. Good cooling is efficient mass air flow thru the cylinder fins. The major driver of that is pressure differential and making the air go where you want it to.
Click to expand...

If good cooling is efficient mass air flow, how is that efficiency defined?

There's plenty of air available in front of an air plane to affect cooling, so the amount used, as it is free, can't be the answer. It has to be the amount of drag created as the air enters and moves through the engine compartment. The basic premise here is that increasing air flow through the RV engine compartment results in more drag. That may not be true at the speeds we fly at and the compromised shape of the cowl itself to accommodate the engine.

Seems like if inlet air is restricted because of internal pressure caused by a small exit area, drag is created in front of the engine compartment as oncoming air creates a wall of pressure at the inlets and spills over the top, bottom and sides of the exterior of the cowl. It has no where to go so that's what happens. The front of the cowl is not continuously smooth with its inlet holes, there is a radical cowl shape change from near vertical to near horizontal, and there is a spinning prop stirring things up to boot. This is not a drag free environment and there may be more drag with less air entering the engine compartment.

If pressure within the cowl is low due to a large exit area, air enters the cowl, removes heat, and passes on through. It is assumed this creates more drag than oncoming air deflected around the cowl due to a restricted exit area and I question it. I have a hard time believing it makes much difference at the speeds we fly.

The increased drag (if any) created by a 219% exit area is not significant. The airplane goes as fast as the published numbers at Vans and the up side is very good cooling. It would be most disappointing to close the cowl to original specs, not fly any faster, and maybe have cooling issues. Seems like a not brainer to leave it alone as is.
 
I'm lost!

DanH said:
.....Pierre, the first thing to do is measure pressures. Run one leg of a manometer to the upper plenum. Run the other to a few different locations; the cooler outlet face, a spot near the cowl exit (where your duct might end), and perhaps just outside the cowl exit. Make a few flights and record the differential pressures at your usual climb speed and at cruise. A cooler exit duct would be a waste of time if there is no significant difference between the three locations.
Click to expand...

How in the world is this done? If a manometer is a piece of tubing with fluid in it, it'll all run out if one end is at the plenum and the other down at the oil cooler exit??:confused: How do I record pressures in flight if the manometer is under the cowl? I used to think that I'm not mechanically challenged but this is confusing.

Thanks,
 
Airspeed indicator

An ASI is just a differential pressure gauge. You can measure the pressures in knots and then convert that back to an engineering unit if needed. And not spill water on the interior!!


pierre smith said:
How in the world is this done? If a manometer is a piece of tubing with fluid in it, it'll all run out if one end is at the plenum and the other down at the oil cooler exit??:confused: How do I record pressures in flight if the manometer is under the cowl? I used to think that I'm not mechanically challenged but this is confusing.

Thanks,
Click to expand...
 
David-aviator said:
The increased drag (if any) created by a 219% exit area is not significant. The airplane goes as fast as the published numbers at Vans and the up side is very good cooling. It would be most disappointing to close the cowl to original specs, not fly any faster, and maybe have cooling issues. Seems like a not brainer to leave it alone as is.
Click to expand...

I do not mean to say you should alter your airplane. Clearly you are happy with the way it performs, which is what we all strive for.

I just think you under estimate the effect of drag on this configuration. Dan's experiments, Bonanza's with cowl flaps open/closed, ect show there is clearly a measurable difference.

By efficient I merely mean get the highest cooling amount for the minimal amount of air. Air has mass and the more air you take in the more mass you are decelerating, which is drag. If we could take the air in and then send it out at the same speed and direction then we would not have drag. But alas, that is not the case. An efficient cooling system is one that is taken as a whole. It is not one thing, exit area, or inlet area, ect.
 
pierre smith said:
How in the world is this done? If a manometer is a piece of tubing with fluid in it, it'll all run out if one end is at the plenum and the other down at the oil cooler exit??:confused: How do I record pressures in flight if the manometer is under the cowl? I used to think that I'm not mechanically challenged but this is confusing.

Thanks,
Click to expand...

Pierre, imagine a piece of clear tubing oriented as in the drawing below. Fluid (water usually, I like adding food coloring to make it easier to see) is added such that the upward facing "U" portion of the tubing is half full up each leg. One end is routed to a point in the plenum, the other to one of the other locations Dan mentions (for example). Obviously, in order to see this in flight, the tubing has to penetrate the firewall somewhere. Also, the tubing has to be fastened to something (piece of wood, aluminum sheet, hard plastic, whatever) so it doesn't just flop around, and the U has to be oriented vertically as in the drawing, and you have to be able to see it while you are flying.

Without going into the nuts and bolts of the installation, now imagine flying with this. If there is a pressure differential at the points of interest, then one leg of the fluid is exposed to a higher pressure than the other, and so the fluid columns in each leg will be at different heights. The difference in height is the pressure difference. Assuming you use water, the difference would typically be denoted in inches (or millimeters if you prefer) of water.

Note that it's good to have an idea of what pressure differential you might expect to see prior to setting this up. If the differential is greater than the water column height, you can wind up sucking the water out of the manometer.

i-pF9hxnQ-M.jpg
 
Thanks..

Lars said:
Pierre, imagine a piece of clear tubing oriented as in the drawing below. Fluid (water usually, I like adding food coloring to make it easier to see) is added such that the upward facing "U" portion of the tubing is half full up each leg. One end is routed to a point in the plenum, the other to one of the other locations Dan mentions (for example). Obviously, in order to see this in flight, the tubing has to penetrate the firewall somewhere. Also, the tubing has to be fastened to something (piece of wood, aluminum sheet, hard plastic, whatever) so it doesn't just flop around, and the U has to be oriented vertically as in the drawing, and you have to be able to see it while you are flying.

Without going into the nuts and bolts of the installation, now imagine flying with this. If there is a pressure differential at the points of interest, then one leg of the fluid is exposed to a higher pressure than the other, and so the fluid columns in each leg will be at different heights. The difference in height is the pressure difference. Assuming you use water, the difference would typically be denoted in inches (or millimeters if you prefer) of water.

Note that it's good to have an idea of what pressure differential you might expect to see prior to setting this up. If the differential is greater than the water column height, you can wind up sucking the water out of the manometer.

i-pF9hxnQ-M.jpg
Click to expand...

I've been mulling it over and fits what I thought..thanks.

I'll have to figure out how to penetrate the firewall....perhaps through the hot air valve with the heater hose disconnected?

Best,
 
gasman said:
No........... your not.

Mine measures as low as .05 in-Wg.......... http://instrumentation2000.com/ueiem150digitalmanometer0-20wcrange.aspx
Click to expand...

It is nice to know I'm not the only one missing out on flying around with a tube filled with water, tacked to a piece of plywood :). I've had many hours of engineering 'fun' with mine. Direct differential measurement capability is useful. http://www.omega.com/pptst/HHP-90.html

Maybe this thread should be renamed "I'll show you mine if you show me yours."
 
We have used analog manometers from the HVAC industry for years to do this stuff. Cheap, steady, accurate. You'll learn a lot!
 
Sorry, been out of town all day.

Looked at digital manometers, but I'm really cheap....had lots of vinyl tube and a yardstick.

You can't spill water in the cabin....the ends of the tubes are in the engine compartment.

The yardstick sits upright in the right passenger footwell of the RV8, taped to the canopy rail. No problem, so surely there is enough room in an RV10.

I made miniature "firewall fittings" by chucking some 3/16" all-thread rod in the lathe, drilling the center, and removing the threads from each end of a 2" length. Use an AN3 check nut on each side of the firewall:

Bulkhead%20Fittings.jpg


The yellow tubing is tygon, sold as "small engine fuel line" at the NAPA store. Tygon handles engine compartment heat a little better than vinyl.

As I recall, Tom Martin ran his manometer tubes through the heater box.

FWIW, I also installed six "blank" wires in the wiring bundle through the firewall back when I was rigging the engine. They can be used for any experimental purpose. Right now they are hooked to temperature probes.

As Lars said, you should be aware of possible pressures, but if you screw up all you'll lose is water. The maximum possible pressure is probably around 26" H2O, but that would require 100% of dynamic pressure at 200 knots, at sea level. 100% dynamic pressure at RV cruise speed is more like 15" at 6000 ft, and you gotta hit 18" to lose water from a yardstick manometer. And you won't get 100% recovery with a cooling plenum no matter what you do.

Fly tests in the AM before it gets bumpy. Variable G makes the water column bob up and down.

I would love to hear about your differential pressures as you move one leg closer and closer to the cowl exit.
 
Last edited:
Direct differential measurement capability is useful.
Click to expand...

Which you can do (of course) with the colored water and tube manometer. Just put each tube end where you want the differential measurement. :)
 
David-aviator said:
If good cooling is efficient mass air flow, how is that efficiency defined?
Click to expand...

A serious question, so I dug around on my shelf for a reputable one-page reference. This from Aerodynamics, Aeronautics, and Flight Mechanics, Barnes McCormick...the fundamentals:

Cooling%20Drag%20Raymer.jpg

Cooling%20Drag%202.jpg


Eq 4.45 spells out the most basic concept...mass x momentum loss = cooling drag.

Note the concept in the first paragraph about energy removed and energy added.

For any given power setting the quantity of heat you must carry away is fixed. The best way to decrease required mass flow is to increase the quantity of heat transfered to the mass, ie heat the air as much as possible during its pass through the engine compartment. Doing so requires less mass to carry away the same quantity of heat. Measuring the air temperature increase following a pass through the system (or an individual cylinder baffle, or an oil cooler) is one very real yardstick for cooling efficiency. Heat exchanger efficiency = cooling air temp rise / (CHT-OAT)

Seems like if inlet air is restricted because of internal pressure caused by a small exit area, drag is created in front of the engine compartment as oncoming air creates a wall of pressure at the inlets and spills over the top, bottom and sides of the exterior of the cowl.
Click to expand...

Cowl and inlet shape can be done badly of course, but do realize that external diffusion is in itself nearly frictionless.
 
Last edited:
Thanks for that bit of information, Dan. Very interesting.

I have a Dwyer magnehelic instrument and should be able to measure the differential from the inlet to the exit. Next would be a couple temperature measurements to see how much heat is being extracted. Then a moveable cowl flap could be installed to see how much things would change, including air speed.

One thing leads to another. :)

Thanks again for the enlightened response to the original question, learn something new every day.
 
Duct Flow Analysis - How To

pierre smith said:
I'd like to build an exit duct for my -10's oil cooler as well. In case you don't know, they're mounted on a firewall box that's angled about 30 deg downward, with the air coming in the top.

How do I determine the exit area? The cooler is fed by a 4" duct off the back of # 6 cylinder.

Thanks,
Click to expand...

Pierre and Dan, I'll give you the short course in how to size duct area to work for a given mass flow and pressure drop. Its not hard to do, if you know the pressures and temps before and after the cooler interface.

Cooling drag is analyzed as a momentum loss across the system. The "system" is composed of an inlet; a heat exchanger; and an exit:
Cooling%252520Drag%252520Diagram.jpg

Flow conditions are denoted in the schematic. Ahead of the inlet plenum, we have free stream pressure and velocity: P0 and V0. At the heat exchanger interface, there will be (most likely) a temperature rise and pressure increase: Pb and Tb. Heat is added by the exchanger: Q in. Flow across the cooler interface will lose pressure due to friction loss through the cooling fins. At the back side of the cooler, we thus have an incremental loss of pressure and an increase in temp in the flow: Pb-dP and Tb+dT. The "dP" and "dT" represent delta-P and delta-T across the cooler. At the duct exit, we have velocity Ve and pressure Pe which will vary with duct design and exit area. Down stream of the exit, we once again reach free stream conditions: Vinf and P0 (free stream velocity and static pressure).

The hard part in sizing the cooling duct system is knowing the flow conditions at the cooler face Pb and Tb, and knowing how much heat is added to the flow and how much pressure is lost across the cooler. But these can be measured with a little work.

The mass flow through the system can be found when we know the density of the flow, its pressure, and the area across the cooler. More mass flow equals more cooling. The total drag of the system can be found using compressible flow analysis (momentum loss). Inlet and outlet sizes can be optimized for the mass flow needed.

Years ago, I had a spread sheet worked up that did the analysis, and sized the cowl system for Kestrel Aircraft's KL-1C airplane. It worked great. I could do the spread sheet again, seeing there's probably some interest in knowing how to size inlet and exit areas for optimum performance.
 
digital manometers

To you guys who are using digital manometers: How do you damp out the readings? I have one, but it is useless because the displayed value does not settle down. I have tried using a fish tank air stone on the end of the line and also adding some water to the tubing to no avail. The model I have is simple and has no filtering options.
 
Bill Wightman said:
Dan, I see you have a source on how to do it. We must have posted at the same time!
Click to expand...

Looks like it!

The McCormick text can be an example of a perfect duct, as I spoke of in post #102. It doesn't address frictional issues due to duct shape or construction. As you say, with enough data you can back into the best duct dimensions. I'm suggesting (to Pierre) that the first thing to do is gather enough pressure data to determine if a cooler outlet duct would even be worth doing in the standard RV-10 cowl.

My own current duct experiment is a reversion to the cut-and-try tradition. It should be pretty easy to tell if duct friction and area was cutting into cooler mass flow.
 
Kinnerhatz said:
To you guys who are using digital manometers: How do you damp out the readings? I have one, but it is useless because the displayed value does not settle down. I have tried using a fish tank air stone on the end of the line and also adding some water to the tubing to no avail. The model I have is simple and has no filtering options.
Click to expand...

Sorry, mine has a "smooth" function.
 
Kinnerhatz said:
The unit I'm using is the UEI EM201 Digital Manometer, but again, I need to figure out how to damp its readings.
Click to expand...

According to the manual description you have a min/max function that will hold until you reset. You may want to try that, set up your test condition and record max values only or record both and average for a particular test.
 
Got in a flight today to check the new cooler exit duct. Low layers and an overcast at 5000 so I was a bit limited; I'll get more Tuesday when I fly to an auction in Chattanooga.

Looks like an incremental improvement, as expected. I climbed out of Wetumpka at 110 knots to 5000 then dived into Alex City for cheap fuel (note to self: $4.75 is cheap...keep repeating, $4.75 is cheap..). I just wanted to get everything hot. The real test was a departure at 185F oil temp on the runway before going forward with the throttle. A 110 knot climb from 600 had the oil temp stabilized at 205F at 5000 with OAT at 79F. It came right back to 185 when level.

OSH in two weeks....I dropped down to 1500 and dragged the 20 miles back to Wetumpka at 80 knots, just like the Ripon approach. Prop forward and 1950-2000 RPM did the trick; oil temp stabilized at 195F about 5 miles out. Yep, we're ready!
 
Last edited:
Next experiment.

Cyl #3 has always been about 25F warmer than the coldest cylinder. I figure it's mostly a case of lower local pressure in the right rear corner of the plenum; there's a 4" hole back there to feed the oil cooler. Take a look at the pressure maps in CR3405.

In addition, the oil cooler air supply temperature has risen 15-20F (compared to OAT) by the time it gets to the oil cooler face. No big surprise; the air passes lots of hot cylinder fins in the upper plenum on its trip to the cooler inlet duct.

We all know what a difference 15-20 degrees reduction in OAT can make, so I started thinking about how to get some unheated air back there for cyl #3 and the oil cooler supply. I'm going to try separating the air into two parallel paths. One is conventional (the usual movement inside the plenum enclosure). The other is via a new duct attached to the underside of the plenum roof. The duct does not extend all the way to the rear plenum wall, but rather just to the rear edge of the #3 fins.

Single%20Duct1.JPG


The goal is to deliver cool air, so the duct is insulated by laminating a sheet of ordinary 1/16" fiberfrax felt into the surface. I did not want the fiberfrax to soak up resin, as that would both make it heavy and defeat some of its insulation properties. So, to stick it in place I laid a thin coat of micro on the duct surface, then pressed the felt into it. It then got a cover sheet of very light crowfoot glass.

Single%20Duct2.JPG




Working on my annual now and hope to be back in the air in another week or so. The goal is to maintain low oil temperature while running the very smallest exit at 100F OAT. I may not have any more really hot test days until next spring, but you never can tell about fall weather in south Alabama.
 
Last edited:
Hi guys-
My first VAF post, other than the buying/selling on the classifieds. I'm a newbie so mostly I just sit back and absorb information.
Accidentally wandered onto this thread (too easy to do on VAF... link to link was carbon fiber oil pan - Tom Aberle - vernatherms - here...). Anyway, I think any of you interested in this topic would find Kays and London's Compact Heat Exchangers an excellent resource. Expensive, but highly recommended.
Eric
 
The above experiment to direct cold air to the area of the oil cooler duct entrance reduced air temperature at the oil cooler face by about 10 degrees F, with the expected effect on oil temperature.

There was no attempt to seal out hot plenum air. No point; the single duct only had a cross section area of about 7 sg in, not large enough to supply all the required air. So next experiment; let's take all the oil cooler air from just inside the cowl inlets, and send it to the cooler with a sealed system.

Recently in another thread I wrote about converting to two-part urethane for many of my mold and forming tasks. Here's a pretty good example; a few minutes with some paper and tape for dams, mix, pour, and a half hour later start carving and sanding. This form would have been very difficult with clay or block foam.

Double%20Duct%201.jpg


Duct%20Layup.jpg


Should be flying again in a week or so.
 
Last edited:
Dan

Dan-I am not an RV builder/owner and probably never will be. With the loss of Paul Lipps, you are at the top of the list of people generating ideas that are of interest to me. There is a lot of stuff here that is interesting and sometimes entertaining but you are an idea guy and that is my primary focus. Do you remember the guy from nowhere, Oklahoma a few years back that posted a lot of stuff on cowling/cooling. He had a controllable, external inlet for the oil cooler. (RV6)
 
That would be Alan Judy.

Alan is another great innovator....a Naca type air inlet for his oil cooler and replacable inlet rings for summer or winter cooling and so on.

Best,
 
Cooler

Thanks Pierre, I couldn't remember the name. He is on the OK panhandle, not the end of the world but if you stand on a ladder you can see the end of the world from there.
 
Dan, how much space between the duct and top of cyl fins? Any thought to insulating the duct? ooking forward to your results.
 
RV8R999 said:
Dan, how much space between the duct and top of cyl fins? Any thought to insulating the duct?
Click to expand...

About 1-1/2" ~ 2" at the intakes and more towards the rear. I'll insulate like the previous duct.

Do you remember the guy from nowhere, Oklahoma a few years back that posted a lot of stuff on cowling/cooling. He had a controllable, external inlet for the oil cooler.
Click to expand...

Yes, Alan Judy, as Pierre said. Beautiful work.

Air flowing through the cooler when the vernatherm is open is just drag, so flow control is smart. Others are doing it with a butterfly or a shutter.
 
Ok, done, off to the airport. The black section with the flange gets riveted into the rear baffle wall. A slip fit (at the arrows) allows plenum lid removal. The duct is insulated with 1/16" fiberfrax and Vans stick-on aluminum reflector material.

Now if I can just arrange for some hot weather ;)

Finished%20Duct.jpg


Revised%20Plenum%20Lid.jpg
 
Last edited:
An artisan..

Very interested in your results. what area ratio did you use from duct inlet to entrance to the rear baffle slip joint? Were the two duct inlets of equal area or did you try to balance based on CHT deltas in flight?

are you planning a full pressure survey of the ducting as well?
 
RV8R999 said:
what area ratio did you use from duct inlet to entrance to the rear baffle slip joint? Were the two duct inlets of equal area or did you try to balance based on CHT deltas in flight? are you planning a full pressure survey of the ducting as well?
Click to expand...

The design approach is entirely TLAR. I did try to make the area total equal the 4" SCEET down to the cooler, a little over 12 sq in. I recorded some pressures previously, so I'll no doubt make a comparison.
 
Duct work looks great Dan! Couple pressure/temp related questions:

From the mention of 12 sq" of mini-duct inlet and outlet (to the oil cooler), I take it you're just trying to keep the pressure the same throughout the duct, and you're just trying to get lower-temp cooling inlet air directly to the oil cooler, versus air that has passed over the cylinders on its way to the oil cooler...correct?

IIRC, your CHTs have been great, but your oil T has been hotter than desired (correct?) so do you think you will be trading a bit of higher CHT for cooler oil T? (Makes sense, just wondering if that is the logic.)

I know testing will reveal more, but do you have any concerns that the duct is eating up too much upper plenum volume? I mention this because I am about to remake my baffles and install a new plenum, and one of my mentors gave me a bit of a friendly lecture on how well the standard baffles (no plenum) cool if installed correctly. His assertion was that installed plenums reduce the upper "plenum" volume normally provided by the cowl by too much. I discussed Paeser a bit, and will stick with my plan of making a well-sealed plenum, but I wanted to ask if you feel there is a point where your upper plenum volume will get too small.

If CHTs are impacted too negatively in the experiment, have you also considered a NACA duct on the cowl to feed the cooler directly? Is that a later experiment, or is that too draggy to be beneficial?

I've been fortunate to have good cylinder cooling and low oil Ts, but I have a cavernous, draggy exit (2.25:1) and hope to reduce that as you have. As I start down that road, this oil cooler work of yours is important to remember, along with your exit work. Thanks for sharing it!

Cheers,
Bob
 
rvmills said:
I take it you're just trying to keep the pressure the same throughout the duct,..
Click to expand...

Yes. And on the practical side, larger takes up valuable space and smaller won't flow enough air.

.. and you're just trying to get lower-temp cooling inlet air directly to the oil cooler, versus air that has passed over the cylinders on its way to the oil cooler...correct?
Click to expand...

Correct. Taking oil cooler air from the baffle wall behind #3 resulted in a temperature at the oil cooler face much higher than OAT.

IIRC, your CHTs have been great, but your oil T has been hotter than desired (correct?)...
Click to expand...

Yes. I think of them as not properly balanced...the system needs less cylinder cooling and more oil cooling.

... so do you think you will be trading a bit of higher CHT for cooler oil T?
Click to expand...

I don't expect this experiment to change CHT at all (we'll see!). Lower oil temperature will allow another exit area reduction later, which will increase CHT.

...do you have any concerns that the duct is eating up too much upper plenum volume?......... His assertion was that installed plenums reduce the upper "plenum" volume normally provided by the cowl by too much.
Click to expand...

This plenum was built as large as possible. My humble student's understanding is aligned with your friend; larger is better. However, I don't know of any published test data and remain open minded.

Realize I've not changed total cooling air quantity. And the exchange of dynamic for static is external in this scheme; what I do up inside the entrance shouldn't matter very much. All I've done is segregate the oil cooler air so it doesn't pick up so much heat before arriving at the cooler.

... have you also considered a NACA duct on the cowl to feed the cooler directly? Is that a later experiment, or is that too draggy to be beneficial?
Click to expand...

If this doesn't do it the next step is probably a much larger oil cooler. I don't know if an external oil cooler intake would add much drag, but it has to be more than not having one ;)
 
Regrouped the replies a bit, some good notes there...thanks!

DanH said:
And on the practical side, larger takes up valuable space and smaller won't flow enough air.

I think of them as not properly balanced...the system needs less cylinder cooling and more oil cooling.

Realize I've not changed total cooling air quantity. And the exchange of dynamic for static is external in this scheme; what I do up inside the entrance shouldn't matter very much. All I've done is segregate the oil cooler air so it doesn't pick up so much heat before arriving at the cooler.

I don't expect this experiment to change CHT at all (we'll see!). Lower oil temperature will allow another exit area reduction later, which will increase CHT.
Click to expand...

All makes sense, and the last note provided a bit of an ah-ha moment! Good thinking! So plenum + duct volume and flow should remain equal to previous plenum flow and volume (or close, given the space occupied by the duct material), and the air previously lost from the plenum back at the opening to the oil cooler, will now be lost at the inlet. It'll be interesting to see if 1 & 3 creep up slightly, and 2 & 4 decrease slightly. Cool experiment (bad pun! ;))

DanH said:
Taking oil cooler air from the baffle wall behind #3 resulted in a temperature at the oil cooler face much higher than OAT.

If this doesn't do it the next step is probably a much larger oil cooler. I don't know if an external oil cooler intake would add much drag, but it has to be more than not having one ;)
Click to expand...

I figured you had measured temps into and out of the oil cooler (believe I've read it here), and that will be a telling measurement in this experiment. When I purchased my plane, it took fresh air for the cockpit vents from the back of the baffle (next to the oil cooler), into a mix valve, then into the cockpit vents (no naca scoops on the plane at all). I didn't like that as the only source of fresh air, so I changed it, and blocked that opening in the back of the baffle. The interesting thing is that the air was quite cool, even coming from the upper plenum. Pretty large plenum on the 540 in the six cowl though, so the smaller plenum required for an 8 cowl may be impacting (and sounds like your measurements bear that out). I didn't take measurements back then, so its just offered as a data point of interest.

On the cooler, I have a baffle-mounted "double-wide", and rarely see over 185, and sometimes have to work to get it up to that. I cover 1/2 in the winter, and it just cools and cools. Wreaks havoc on my aft baffle (another reason they are getting replaced this winter...with more structure!), but I don't think I have room for a remote cooler of that size...especially as I try to reduce firewall clutter for smoother exit flow. With that big cooler, the only times I've seen over 200 is a multiple hop scenario on hot days (once in Las Vegas on a form hop, following a X-C there, and then on the Mythbusters shoot day, on hops 6-9 wih OAT 100+). The big cooler really works well (another data point).

DanH said:
This plenum was built as large as possible. My humble student's understanding is aligned with your friend; larger is better. However, I don't know of any published test data and remain open minded.
Click to expand...

Mine as well, though reading Paeser and others, and listening to many racers, the recommendation is "plenum...just do it!". I know Mark (F1 Boss) is trying to cypher up a plenum for his 550 Rocket, and has had heat issues, so that argument still seems to hold water. I've seen a lot of standard baffles leak (though some do very well), so I'm guessing it's all about plenum sealing. As I jump into baffle/plenum work this winter, I'll try to go as large as I can...thanks for the nudge in that direction!

Nice work, as always sir!

Cheers,
Bob
 
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!!
 
Last edited:
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
 
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
 
rvmills said:
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.
Click to expand...

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.
 
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.....

Variable%20Exit%20Closed.jpg


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

Variable%20Exit%20Open.jpg


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.

Variable%20Exit%20Ver1.jpg


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....

EFIS%20Variable%20Exit%20first%20test.jpg


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.
 
Last edited:
Congrats!!

DanH said:
Now the good part.....thumbed the switch to close the door, and picked up four knots....
Click to expand...

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

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

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.
 
"... 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
 
You must log in or register to reply here.
Back
Top