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To Plenum, Or Not........

At 165ktas we measured about 14.25 inH2O which is about .51 psi. The upper plenum area is about (very rough mental measurement here) 360 square inches. This results in a force of about 186lbs exerted upward on the upper cowling.

Average Lycoming plenum lid is about 32x24=768 sq in, so force is about 391 lbs. That's just for the area of the baffle walls. It must also withstand an additional ~0.25 psi over the balance of its area. The cowl puffs up quite a bit in flight.

Back to the plenum lid. Remember, 391 lbs is at cruise speed for a average RV. Q rises with airspeed. This one is withstanding ~550 lbs at 200 knots. If you compare with older photos you'll note I found it necessary to reinforce the attach methods:

3500ys2.jpg


I often see lids with no fasteners at all across the front behind the spinner, and widely spaced fasteners elsewhere. We've all looked out the windshield and seen how the cowl edge blows open between fasteners in flight. That's what happens with 0.25 psi. Consider what the edge of a poorly designed or fastened plenum lid is doing at twice or three times the pressure. Moral of the story? The mere presence of a plenum lid does not guarantee great sealing.
 
If I could offer my experience/advice.
#1 Be happy you have as much room in your RV cowl to build your plenum. I am jealous.
#2 Sure baffle seals work, but inefficient is an understatement.
#3 Cooler than normal CHT's is a bad thing. As said before, you're paying for that with speed.
I have 400 cubic inches cooled by less than 20 square inches of inlet area. Two 3.5" inlets. At race power (3,000 rpm) during the air races, I barely get over 400 CHT. Standard baffles could never do this.

Here is my current baffle/plenum I am running.

baffle_30.jpg


baffle_32.jpg


Read about it being built here:
http://www.n54sg.com/baffle-kit-diffusers-and-plenum/
 
+1

I officially +1 all of Dan and Ken's posts.

Also I like my plenum. By taking extreme measures of sealing and inlet trasition co struction to the plenum I feel we have gained speed without sacrificing engine life.

Thanks for all the information you guys have posted!!!
 
Another area for efficiency gain is how you are routing the air thru the cyl fins. With stock baffles you force the air thru the fins only at the mid point baffle. The rest is covered by turbulence and chaotic chance.

To get the most heat transfer the air needs to be forced to flow thru the fins for the entire circumference of the cylinder. The best way to do that is with individual cylinder baffles like the many studies on radial engine cooling. The same principles apply to our Lycomings.

I have baffles that does not let the air leave the cylinder fins until it gets to the bottom side. You can see some of that in my first post on page 1.
I scrounged 2 red tagged angle valve cylinders for free from ECi and Mattituck and then built a half engine mock-up to make my baffles. Plenty of red tagged cylinders out there.

 
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We were hoping to get a few more folks with stock installations to participate, specifically parallel valve 0-360 with stock inlets and baffles. Any takers? Between Dan and I we can supply you with probes, test equipment, our test methodology and data analysis - FREE :)

My RV-7 engine installation fits the above description. I'm interested in participating. I can PM you information.

No one responded to my post about CHT probe accuracy (post #37). I'm still surprised that sensor accuracy/repeatability isn't part of this discussion.
 
Here is a shot of Wade's inter cyl baffle setup.

Used to be common setup on earlier engines, Continental for one.

13july12005.jpg
 
My RV-7 engine installation fits the above description. I'm interested in participating. I can PM you information.

No one responded to my post about CHT probe accuracy (post #37). I'm still surprised that sensor accuracy/repeatability isn't part of this discussion.

Yes, please PM me. Thx!
 
Here is a shot of Wade's inter cyl baffle setup.

Used to be common setup on earlier engines, Continental for one.

13july12005.jpg

Yes, agree. Wade is spot on here. I have a similar inter-cyl baffle. I'd like to go one step further though - measure the exit areas of the lower cyl baffles (both the inner and cyc heads). Assuming a virtually leak free upper plenum and accounting for oil cooler, heater box, blast tubes, all the remaining air in the upper plenum MUST go through these four little exit areas (1 for each cylinder). The assumption is the air in the upper plenum is unorganized and turbulent so lets create upper cylinder baffles with an exit area about 1.5 that of the lower cylinders and place the opening near the top of each cylinder. In theory this should force the cooling air through all of the fins in a more uniform manner. Lots or room for experimentation with the size and location of the Upper Cylinder Baffle Inlet (UCBI) in relation to the Lower Cylinder Baffle Exit (LCBE). My thought was to pick 1 cylinder and start with the inner cylinder portion first because it would be easy to fabricate and install and see if anything changes. I'd be surprised if someone hasn't done this already on and RV? Pretty sure Dan and I discussed the Canard guys doing this as a matter of routine?
 
I often see lids with no fasteners at all across the front behind the spinner, and widely spaced fasteners elsewhere. We've all looked out the windshield and seen how the cowl edge blows open between fasteners in flight. That's what happens with 0.25 psi. Consider what the edge of a poorly designed or fastened plenum lid is doing at twice or three times the pressure. Moral of the story? The mere presence of a plenum lid does not guarantee great sealing.

Yeah, I'm not surprised I was off as I was measuring with a mental picture in my head and tried to be conservative :) Thanks for the accuracy check!!!

I'm in the process of fabricating 4 additional cowl attach points. Two along the upper engine mount brace to the top cowl and two from the lower engine mount brace to the lower cowl. I've reinforced the cowl already but still see scalloping between each skybolt fastener.
 
Pulling air all the way around the fins on the head seems like the way to go based on my limited experience. My next engine installation will look a lot more like the air cooled VW and Porsche baffling than the typical "throw some air in the general direction" Lycoming installation. There's a lot of volume moving through an RV cowl, but not as much as should be going through the fins.
 
If I could offer my experience/advice.
#1 Be happy you have as much room in your RV cowl to build your plenum. I am jealous.
#2 Sure baffle seals work, but inefficient is an understatement.
#3 Cooler than normal CHT's is a bad thing. As said before, you're paying for that with speed.
I have 400 cubic inches cooled by less than 20 square inches of inlet area. Two 3.5" inlets. At race power (3,000 rpm) during the air races, I barely get over 400 CHT. Standard baffles could never do this.

Here is my current baffle/plenum I am running.

baffle_30.jpg


baffle_32.jpg


Read about it being built here:
http://www.n54sg.com/baffle-kit-diffusers-and-plenum/

This is bang on the money - we pay for cubic feet of air in the inlet (via drag) and then a lot of gets wasted by not putting it where it needs to be. Take every cubic foot you have and put it where it should be - and don't buy any cubic feet you don't need.
 
Has anyone done any tests to determine how deep into the inlets it takes for the relative velocity to reach minimum? In other words, if the relative velocity is near cruise speed right at the inlet lip, how far down the pipe does it take to reach max pressure/min velocity?

If it is only an inch or two before it stuffs up, then the only critical part of the duct is that first part which manages the aerodynamic part - after that, you are dealing with a pressure vessel and the shape of the divergent duct should become less critical. I'm thinking specifically about the "6 degree maximum divergence rule" so often used when discussing divergent ducts.
 
Has anyone done any tests to determine how deep into the inlets it takes for the relative velocity to reach minimum? In other words, if the relative velocity is near cruise speed right at the inlet lip, how far down the pipe does it take to reach max pressure/min velocity?

If it is only an inch or two before it stuffs up, then the only critical part of the duct is that first part which manages the aerodynamic part - after that, you are dealing with a pressure vessel and the shape of the divergent duct should become less critical. I'm thinking specifically about the "6 degree maximum divergence rule" so often used when discussing divergent ducts.

The short answer is that the velocity reduction can take place before reaching the inlet plane (low Vi/Vo, large diameter), or well back inside the inlet, in the divergent duct (high Vi/Vo, small diameter). The shape of a duct (if any) inside a low Vi/Vo inlet isn't critical. The shape of the diffuser inside a high Vi/Vo inlet is apparently quite critical. Either can result in good inlet performance. In my own (so far unproven) opinion, the low Vi/Vo inlet probably works better at low speed and high AOA, while the high Vi/Vo inlet probably results in less total drag (including exterior form drag) at high speed.

Note the physical space requirement. Good high Vi/Vo inlets require space (length) aft of the opening for good diffusers, which pretty much dictates the need for a prop extension. See the Lancair photos posted above, or look at the nice shapes on Chris Z's diffusers:

http://www.n91cz.net/cooling/Cowl_report_4-12-2005.pdf

For my own airplane I wanted a 390 with a Hartzell BA, a vibration-surveyed combination with no restrictions. The big motor and heavy prop made a prop extension undesirable, thus the low Vi/Vo inlets, for which the ductwork (if any) can be quite stubby.

Consider the case of a midsize inlet with ducts connecting to the plenum....ducts with no particular diffuser shape, maybe some misalignment, maybe a sudden turn upward and volume expansion right at the cylinder face. Think it might not ring the bell for inlet performance, i.e. low conversion of Q to static?
 
Can anyone comment on the differences between inlet styles and how angle of attack may affect their ability to cool.
 
The short answer is that the velocity reduction can take place before reaching the inlet plane (low Vi/Vo, large diameter), or well back inside the inlet, in the divergent duct (high Vi/Vo, small diameter)...

Makes sense.

Therefore, the most effective cooling in a standard length RV inlet would likely be achieved with tightly shrouded fins on the cylinders and head - with small openings only at the top and bottom. In other words, closely duplicate the typical "bottom" shrouding on the top as well. If there is little relative velocity and the upper cowl is essentially a pressure vessel, then the real cooling path is vertical, just like a VW bug. Shrouding would direct air down and around much more of the fin area than is typically seen. Should remove a lot more heat.
 
The inlets I made for my cowling and plenum (posted above) are also divergent.

3.5" at the inlet, expanding to 4" over a 2.5" depth. Then 4" SCEET tube to connect to the main diffusers. A lower divergent angle would probably be better, but the space (or length in this situation) is limited, hence the prop extension.

inlet.jpg
 
?.....but to be fair, you are operating at a different MO than the normal cruise. It would be interesting to see what you get operating at a DA of say 8-9K and WOT and 10-20LOP.
By the way your % power is wrong, some engine settings are not entered right. It should be showing about 79% power......

Ok David, I had a few minutes before work today and gathered some new numbers. This is a normal cruise set up for me, actually if I'm going somewhere I will be higher to clear the mountains. But for 8000DA this is about right. I don't know how LOP I am, as I just lean the ol fashioned way. Pull until rough and then smooth it out. My engine gets rough, then just seems to quit soon after.

WOT, 2450rpm, 9gph (when I'm at 11500 I'm <8gph)
1360344308545_zps0718a807.jpg


I haven't looked into my Dynon %power settings yet, but think it is fairly close in the above pic, based on the formula you gave me.
 
??

Does the D-180 not have a "Lean" mode? With my D-100, push the "menu" button (second from right) and then "more" and you'll see a "Lean" button. Pushing this allows you to see your EGT climb to peak and then the big numbers disappear and a -15 or so. will appear, indicating degrees lean-of-peak.

To exit this mode, return to the "Lean" button and simply press it again.

Best,
 
Yes the D180 has the Lean mode. I'm just not in the habit to use it. I set my fuel flow and tweak it to get the smoothness and speed I want for that day.
 
Funny... I find the "lean" function to be one of the most valuable aspects of modern glass. I would not even consider buying a new engine monitor without it.

I have even used it to diagnose an ignition problem... There I was, pounding along at 50 LOP for an hour or so and I noticed that I was suddenly at about 10 LOP. Engine was perfectly smooth. So I did a quick mag check and sure enough, one side was dead.

Edit: The above is not meant to be a sales pitch or anything... Just pointing out a difference in perspective
 
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About a month ago, I change from a plenum to the Vans baffle kit.
Results, after 2 days work fitting them, only 8-9 deg F cooler. I still can get over 400F on CHT #3 on a normal to hot day.
My last run with an OAT of 11C, max climb out temp was 398 F. ( its winter here now )
For a lot of work, very poor result. I wonder if my 3 blade prop has anything to do with this ?
Im also going to try an exit air ramp on the lower cowl.
Will post results later.
:(
 
Take measurements

Jamie

Your prop has nothing to do with your cooling. You say you converted to stock baffles, is that complete stock, cowl and all? Or is that just baffles inside a modified cowl?

The cooling system is just that, a complete system from entry to exit. There is no one "silver bullet" that solves all and each part is interrelated.
For example, the inlet pressure is determined by the inlet total area, their shape, the quality of the pressure recovery, and how tight the seal is.

You can start troubleshooting by asking these questions.

What is the max amount of air mass I have available to cool with?
With stock inlets you have much more than enough air mass available. And the shape of the inlets are good, so no issues with flow disturbance. If not stock, then how much area and what is the shape of the inlets. To work well the should be nicely rounded at least 1/4 inch radius.

What is the pressure differential across my system? And is all of the air mass going where it is supposed to?
The pressure differential is the force that allows that volume to move through the system. With stock cowl and good baffle seals, the inlet pressure is going to be adequate. What is the outlet pressure? Where is the air going? Remember it is under pressure, so it will want to go anywhere the pressure is lower.


If you want to actually know what is going on you have to measure the pressures in your system (the upper plenum, below the engine, the outlet, ect) . You can use an old pitot tube, water manometer, magnahelic gauge.
 
No Plenum independent of the cowl for me

I see no advantage in reducing the capacity of this area. There is speed and efficiency to be gained in the engine cooling system. I gained approximately 5.7 knots over the original system with good cooling following years of experimenting and many failures. It is tiresome at this point to repeat all the stuff and boring to you I'm sure - especially if you have already committed to one course of action that works.

To me there are 5 areas of the system from the inlet to the outlet and I have worked with all of them except #3. My testing and experimentation has been extensive but not exhaustive. First a question or two when you go to the smaller plenum independent of the cowl (the baffles and the cowl form a plenum in the standard system), did you eliminate all ports in this part (#2) of the system? If you did , did you add any new external openings for the heater and blast tube functions?


Part 1 (inlets: made many small changes on the inboard side of the inlet to reduce area - I gained no speed with mods in this area)

- Reducing the inlet size directly effects the CHT - smaller is hotter
- The way you block the air by reducing the inlet size can increase drag
Part 2 (plenum: covered the heater and blast tube ports for racing only and seal joints with Dow 736 RTV - slight speed gain wit the cover plate mods in this area)

- Blocking the plenum ports for heater and blast tubes increases the speed of the airplane slightly and reduces the CHT of cylinder #3 (in front of the large heater port).
Part 3 (Cylinder passage - no modifications tried)

Part 4 Lower cowl (Too many modifications tried to mention - all had to do with perceived flow paths but later realized I was also changing the volume and thus pressure - I gained 4 knots with mods in this area)

- There may be a speed gain possible from making special baffles to control the orderly flow of cooling air from the engine to the outlet.
- There is a point in a given system where you reduce the lower cowl cooling air flow path volume the aircraft speed is reduced.
- It appears that improving the flow path can increase the air mass flow and increase the drag.
- It appears that if you improve the flow path and reduce the volume, to a limit, you can reduce the cooling drag and increase the airplane speed and retain good engine cooling.​

Part 5 outlet (Many combinations of outlet fairings with many opening shapes, coanda bumps with many shapes, positions and sizes, center flow separator and none - increased speed 1.7 kts)

- There appear to be interactions between the outside air, the cooling air and the exhaust that can be effected by the surfaces between and around them in a way that effects the way the three air sources recombine to reduce drag to a value less than the standard truncated cowl cooling air outlet.​

Outlet mod test table:

IMG-29.jpg


Bob Axsom
 
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Outlet area?

My experience is that outlet area is one of the critical parameters for good cooling with a standard cowl and baffles. For my 6A with an O-320 and FP the CHTs would regularly get to 380 to 400F in the cruise (at any altitude), despite great care with all aspects of the baffling. Since I installed louvres in the lower cowl the CHTs have plummeted. When throttled back (130kt) I'm now struggling to get the above 320F. Clearly the louvres are too large (flowing too much air = too much drag), and need to be blocked off some, but at least I have the temperatures under control. I have a friend who added a louvre to his SJ cowl with no effect whatsoever - so that cowl is not outlet area limited.

Pete
 
I have just seen you all reply to my report. Im off to the airfield just now, I will do a better report, with photos later today. :)
 
Here is the home made plenum I used.


p6052443custom.jpg


p6052444custom.jpg


p6052445custom.jpg


p6072449custom.jpg


p6072448custom.jpg


I removed top ( white ) plenum, note that the ramps at the front are part of the plenum.
I then fitted the standard black baffle seal around the edges as per the Vans FWF install plans.
I then had to fiber glass new ramps to the top cowl, and blocked off the sides of the new ramps, so air cant go around the sides.

At 4500ft


CHT 314 , 370 , 338 , 375
EGT 1418 , 1427 , 1394 , 1423
65% power TAS 145kt 29lt/ph LOP

CHT 302 , 343 , 339 , 356
EGT 1392 , 1385 , 1383 , 1396
55% power TAS 130kt 25lt/ph LOP

Oil temp was 184F now 174F

Things that I have changed.

rv7a6custom.jpg


In the above picture,
1) the air inlet for the cab had to be made 1.5 inches lower so the FAB would fit.
2) R/H side exit area I had to make a valley so the exhaust pipe would clear the lower cowl ( gray painted area )
3) This is a very old type RV6 cowl and engine mount, Vans told me this is the same as the new type RV7 cowl, I dont have another to measure.
4) I have also removed the two small dam walls in front of #1 and #2 cylinder.
 
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I removed top ( white ) plenum, note that the ramps at the front are part of the plenum. I then fitted the standard black baffle seal around the edges as per the Vans FWF install plans.

So what sealing measures were in place around the plenum inlet?
 
So what sealing measures were in place around the plenum inlet?

p6052445custom.jpg


you can see in this photo, the black seal for the bottom, the same was used for the top cowl.

Also, how is the air being controlled around the cylinders?
No other control around the cylinders fitted or removed.
 
One Data Point

I considered to plenum or not to plenum when starting the RV-10. I had some experience by helping a friend finish a -10 with a homebuilt plenum and also converting a -7a from stock to plenum that resulted in a 30 deg drop in CHT's at the same conditions and retained the same TAS.

The goal with the RV-10 was to spend as much time as necessary to produce maximum airspeed at cruise without overheating in the climb.

The louvers on the bottom of the -10 that most people are using looked less than optimal to me so I set out to eliminate them and still cool the engine.

The engine is a IO-540 C4B5 that was fitted with 9.1:1 CR pistons and new flow matched tapered barrel ECI Cyl's.

Here is some of the baffle and plenum pics.
IMAG0102.png

IMAG0109.png

IMAG0228.png

IMAG0229.png


here is a pic of the cowling to inlet intersection. The gap that you can see is sealed completely around the inlet with a rubber seal.
IMAG0311.jpg


below the engine we tried to help smooth the air flow as it went out the exit.

IMAG0716.jpg


I flew the first 25 hrs or so and determined that about the only way to keep the CHT below 400 F in the climb was to reduce climb power to 25" MAP and 2500 RPM and keep forward speed up around 120 KIAS.

I still was against putting louver on the bottom but felt I needed a slight increase in outlet area so I opened it up 3/4" like this.

IMAG0447.jpg


After these modifications the cooling system will keep the cyls below 400 F when the OAT is 100 and plane is at gross weight taking off with a heat saturated engine with the Air Conditioner eating up 70 amps on the 24 volt alternator and climbing at 110 KIAS.

As far as efficient cooling flow goes. I have no way to quantify the amount of cooling drag reduction, if any, that was gained because this was not a modification but rather incorporated into the initial build. However, It will consistently fly 179-181 KTAS on 12.0-12.4 GPH so I have to believe we gained something for the effort.

I think there are gains to be had by utilizing a plenum but its not gonna work to just smack a lid on the stock baffles and expect good results......it takes work and work and lots of it!!!!!!!!!!!
 
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Jamie

Hard to have a good opinion from the web. You are using a parallel valve engine and a stock cowl, which should not be an issue. There are ways to improve the exit, see Vetterman's fairing and Bob Axom's work. I would say that using stock size inlets that you should have way more than enough air available. I would suspicious of how well the upper is sealed. Once pressurized there will be high pressure at the leading edge of the inlet all the way back. The seal needs to be pressure tight, like a balloon. And it needs to make the air go through the cylinder fins to get out.

Your rear cylinders are hottest, so they are not seeing the same amount of mass air flow as the front. Therefore, leaks are robbing them, the pressure below them is higher, and or the flow is being diverted elsewhere.

Once you have everything sealed well you want to make sure you have the air going where you want it. To increase your efficiency and get the maximum amount of heat transfer, the air should stay inside the fins all the way around. Borrowing the NACA radial engine research I made cylinder baffles like those suggested for radial engine cylinders. The barrel shrouds were made with high temp epoxy and they are band clamped to the cylinder. The plenum hugs against the head fins and does not let it out until it gets to the bottom. Air wants to follow a curved surface so that is why the entry edges are curved.

Sorry, no really detailed pictures yet. I made a "half engine" mock up with red tagged cylinders. Here is a simple quick sketch and the pictures I have handy.



Early first try. Ended up extending to the base to be able to clamp them.




 
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Wade,

Thanks for the info and pictures.
On the weekend we changed from a plenum to STD Vans Baffle system, I had some help from a friend that has made many baffles on Rvs and all of them have very low temps.
We did everything that we could to seal all gaps, knowing that a small leak can cause problems, and yet I still have problems.
I might look at re-fitting the dam wall in front of #1 to see if that pushes air back to #3.
All the other local Rvs are around 20 - 30 F lower than mine, I guess I just have to keep looking for the problem.
 
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