Van's Air Force
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Is a plenum a good idea?
- Thread starter szicree
- Start date
KirkGrovesRV8
Well Known Member
For what its worth
Hi Steve!
For what its worth I have a carbon fiber plenum from Jon Johanson, and it seems to work well I have never had any cooling issues and I have a rather quick airplane.
Go for it
1 more day and I am off for OSHKOSH!!!!!!!!!!!!!!!
Hi Steve!
For what its worth I have a carbon fiber plenum from Jon Johanson, and it seems to work well I have never had any cooling issues and I have a rather quick airplane.
Go for it
1 more day and I am off for OSHKOSH!!!!!!!!!!!!!!!
Having a plenum is not an option
The question is whether it is going to be independent of the upper cowl or not. All my "friends" tell me I will eventually go the hard box route but I haven't given in yet. If you can get one that someone has taken thought the development and test process for your engine and cowl combination the risk should not be too much more than the standard baffle and cowl closed plenum. If you plan to go racing and you pick the best one available for your engine and cowl you probably will get more speed out of it if you don't poke it full of holes for heat air source, oil cooler, mag blast tubes, vacuum pump cooling air, etc. If you start down this road it can be very interesting and educational if you do your testing carefully and keep good data. Myself, I kind of like to see the top of the engine when I take the cowl off ... but no less that John Huft tells me I will come around eventually.
Bob Axsom
The question is whether it is going to be independent of the upper cowl or not. All my "friends" tell me I will eventually go the hard box route but I haven't given in yet. If you can get one that someone has taken thought the development and test process for your engine and cowl combination the risk should not be too much more than the standard baffle and cowl closed plenum. If you plan to go racing and you pick the best one available for your engine and cowl you probably will get more speed out of it if you don't poke it full of holes for heat air source, oil cooler, mag blast tubes, vacuum pump cooling air, etc. If you start down this road it can be very interesting and educational if you do your testing carefully and keep good data. Myself, I kind of like to see the top of the engine when I take the cowl off ... but no less that John Huft tells me I will come around eventually.
Bob Axsom
I just started on my baffling today. I am modifying the Van's cowl into a James shorty cowl. I will be using the hard plenum from James with the round inlets. He claims that I will see at least 30 degrees cooler CHT and Oil temps over a stock cowl. The inlets are 40% smaller than the Vans cowl and all this will translate into airspeed. I also have the Ram air intake for the FI that will get an inch or so more manifold pressure at cruise. Alan Judy's airplane has even smaller inlets and he doesn't have any cooling issues. Most of the RVs around here running the Van's cowl have cooling issues. Don
szicree said:
Steve,
A hard plenum is the way to go if you seek efficiency. It does add complexity, however. You will need to consider how you will transition to your inlets, for example.
Composites offer much flexibility in creating complex shapes. They are perfect for this sort of application. It doesn't necessarily have to be carbon.
Chris
Look no further - here is your man !
http://www.jeffsrv-7a.com/KEVLAR PLENUM.htm
Jeff's plenum fits - it is simple, he provides the fixings, I haven't flown it yet, but it looks really neat and it was easy to fit - no snags on a stock Van's baffle kit. He has put a lot of thought and work into it.
It sits about 1/2" below the cowl line and sufficiently above the engine. The air intakes fit perfectly.
Email me for my shots or contact Jeff.
(Hey Jeff - normal commission - yea )
http://www.jeffsrv-7a.com/KEVLAR PLENUM.htm
Jeff's plenum fits - it is simple, he provides the fixings, I haven't flown it yet, but it looks really neat and it was easy to fit - no snags on a stock Van's baffle kit. He has put a lot of thought and work into it.
It sits about 1/2" below the cowl line and sufficiently above the engine. The air intakes fit perfectly.
Email me for my shots or contact Jeff.
(Hey Jeff - normal commission - yea
)
Woah there. Heed the comment that just any plenum isn't necessarily a magic bullet. I have, and AM FLYING Jeff's plenum on my O-320. I am really fighting through some high CHTs. I am certain that my lower baffles are as good as most and better than many. I'm not saying the plenum top has flaws, but I am preparing to make changes to it as I learn more about my setup via real pressure instrumentation.
I must admit, I thought a plenum top, (any plenum top), would be a magic bullet with my little 160hp O320. Not the case.
I must admit, I thought a plenum top, (any plenum top), would be a magic bullet with my little 160hp O320. Not the case.
Careful how you evaluate some of these systems
There is quite a bit to these systems and you need to take in the entire configuration when you make decisions for your own airplane. Alan Judy has a system with an extra inlet that can be opened and closed in flight for the oil cooler so his inlets around the spinner do not need to let in as much air mass as the normal configuration.
Bob Axsom
There is quite a bit to these systems and you need to take in the entire configuration when you make decisions for your own airplane. Alan Judy has a system with an extra inlet that can be opened and closed in flight for the oil cooler so his inlets around the spinner do not need to let in as much air mass as the normal configuration.
Bob Axsom
yakdriver said:
Norman CYYJ
Well Known Member
I wonder why everyone thinks that they need a plenum?
I have built 6 RV type aircraft now and have never used a plenum and never had any temperature problems. If the baffles and cowling are built properly there is no need for a plenum. The plenum will only give less cooling drag and I doubt very much that there are many out there that will see a speed difference. There are a lot of draggy areas that could be cleaned up with more attention to detail and a lot less work.
I have built 6 RV type aircraft now and have never used a plenum and never had any temperature problems. If the baffles and cowling are built properly there is no need for a plenum. The plenum will only give less cooling drag and I doubt very much that there are many out there that will see a speed difference. There are a lot of draggy areas that could be cleaned up with more attention to detail and a lot less work.
plenum for sale
I have a Sam James Plenum forsale never been used brand new, for 0-320 or 0-360 good deal $275
send me a E-mail [email protected]
I have a Sam James Plenum forsale never been used brand new, for 0-320 or 0-360 good deal $275
send me a E-mail [email protected]
scard said:
WHOA
Well until you are flying with 2 working ignitions you might want to rethink your cooling issues for now. As you reported, the only numbers you have were most likely from flying with only one P-mag. Not a perfect scenario for posting CHTs IMO. After you jam 2 Bendix Mags in that Mattituck the world may never know....
-Jeff
mark manda
Well Known Member
thanks chris
nice site chris.
Easy to understand reading and before and after data makes it reliable and believable.
I have a plenum, intake K and N chamber and Louvers on my Harmon Rocket. As of 20hrs., from all appearances the engine is running CHT's considerably lower than other HR-2s without the above.
And this is a newly reblt engine with 10:1 pistons which would usually run hotter.
Your site almost makes me want to build a plenum for my RV-7a; but not quite, I'll let the next owner do it.
thanks for the informative homepage.
nice site chris.
Easy to understand reading and before and after data makes it reliable and believable.
I have a plenum, intake K and N chamber and Louvers on my Harmon Rocket. As of 20hrs., from all appearances the engine is running CHT's considerably lower than other HR-2s without the above.
And this is a newly reblt engine with 10:1 pistons which would usually run hotter.
Your site almost makes me want to build a plenum for my RV-7a; but not quite, I'll let the next owner do it.
thanks for the informative homepage.
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Norman CYYJ
Well Known Member
One of the problems that I have seen in a few RV's around here with high cht's is that the engine is not running rich enough. The carb should have the jets changed to get more fuel running through it. After this has been done the temps have dropped into the normal range.
mark manda
Well Known Member
yes, that's a good point. Lycon was talking about changing the injector size (?) on my IO-360 180hp but I haven't followed up on it.
scard said:
The plenum is only one piece of the cooling system. Unfortunately there are many ways to end up with high CHTs despite a plenum. In fact, the hottest running aircraft engine I have ever come across (pushing 500 deg F on take off in the summer) has a plenum. This particular installation had a fatal flaw in the transition from the inlet to the plenum. The plenum itself merely provides the opportunity to create a leak proof chamber thereby improving one aspect of the system. You still have to get the air to the plenum efficiently, seal all the sheet metal baffling and so on. I was able to lower the CHTs 30 deg F on one installation, also with a plenum, using nothing more than two tubes of RTV. Cooling Systems are a package deal.
Norman CYYJ said:
Not everyone needs a plenum. In fact, if one is already flying and CHTs are good, the motivation level for a plenum can be rather low. The only real benefit is then speed. How much speed? The increase will be approximately proportional to the cube root of the gain in power. In my case that was about 7.6% (1.076) engine power and 2.5% air speed. (1.076)^(1/3)=1.025. Of course results will vary, but this is one data point that was actually measured.
Here is a plot of before and after cooling drag as a function of altitude.
http://www.n91cz.com/Pressure/N91CZ_Cooling_Drag.htm
N91CZ said:
I whole heartedly agree. I think the inlets are precisely where I might be able to make some improvements in my SYSTEM. I sure did use one tube of RTV in my setup. Maybe I should find a place to put a second tube
. I have a digital manometer and am rigging to be able to measure and quantify any changes that I decide to try.
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Possible Improvement
In your case Scott I would try to attach the rubber seal to the upper and lower cowling and have it lay inside the plenum/baffling inlet as opposed to attaching it to the plenum and having it surround the cowling inlet. You could easily be loosing gobs of air right at the high pressure inlet/seal area.
Shown here is how it is currently configured.
-Jeff
In your case Scott I would try to attach the rubber seal to the upper and lower cowling and have it lay inside the plenum/baffling inlet as opposed to attaching it to the plenum and having it surround the cowling inlet. You could easily be loosing gobs of air right at the high pressure inlet/seal area.
Shown here is how it is currently configured.
-Jeff
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gmcjetpilot
Well Known Member
Go for it
Does this mean Vans set up does not work? No Van's set up works fine for cooling but you can improve or reduce cooling drag. My normal recommendation is stick to the plans & keep it simple.
As far as cooling problems, soft seals may contribute, but I don't think a solid plenum is to blame in my opinion or can fix the main reasons for high CHT, which are many. I list them below; what I think the top three reasons are for high CHT, in my opinion.
Besides better sealing, not having to deal with the soft seals and less cowl wear, I don't think a "dog-house" plenum will gain speed or much better cooling. To gain speed significantly you need to change other things in concert with the solid plenum, which I mention below. If you want to gain some speed you should consider a smaller inlet area or even different shape (round v. rectangle).
Common sense, zero leaks, all air going into the upper plenum (regardless of how you seal it) and down through cylinders means more cooling and efficiency. Any leak, where air goes direct into the lower plenum with out cooling the cylinders, pressurizes the lower plenum and is bad for efficiency. The solution for the stock set-up is make those seals as leak free as you can. Clearly a solid "dog house" plenum can help but the seal between the cowl and plenum is a problem. I have an idea I show below on how to keep the cowl stock but change the inlet.
Leaks do reduce the differential pressure across the cylinders. Differential pressure from above to below the cylinders is what drives the air to do cooling work. A sealed solid plenum is KEY to no leaks.
The question is whether to stick with the stock rectangular inlets or round widely spaced inlets (ala James Aircraft). If you stick with rectangle you will have some challenge sealing the plenum to the cowl. Round obviously with a soft flex hose and clamps is easier to seal 100%. Round is just more practical for duct work. However what jeff did "RV_7A did is awesome, I'm impressed, nice work. You can seal rectangles and ducts but its a challenge.
There are other key reasons for using a round inlet. With a round inlet it's easier to make a more aerodynamic transition, which causes less turbulence in the first part of the inlet. Note smooth transition and 100% sealing between cowl inlet and plenum:
You will have turbulence with any design, but the design of a round "scoop" with an aerodynamic lip and inner profile means the air will not become turbulent, stall or stagnate as soon, which determines the efficiency. Also the air the farther from the spinner is cleaner due to the prop blades. The prop blade shank is blunt. Note reverse flow in the inlet near the spinner:
With a round inlet, you can put all your required area as far outboard as possible, verses a narrow letter box rectangle shape. Note how round inlet give better pressure. (Cp=coefficient of pressure but don't worry what it is, just compare the two inlet styles and a 10% is not insignificant.) Also not the even inlet flow and more pressure outboard, near the hot cylinder valve area. (In the top left corner, prop air flow is shown. Further towards the tip the better.):
More efficiency means you can make the inlet smaller and still maintain equivalent cooling with less cooling drag, which means more speed. The idea is NOT better cooling just the same cooling with less air or less drag. If I did not want to go the full James Aircraft style I would make a plenum and stock inlet smaller. Reduced height and width filling in from inboard edge. You can seal a rectangle duct with clamps. You make a plug deal like this, which nests inside the existing cowl inlet. The beauty is you can change is. This is some what like what Al and Bob Axom have done.
Hot CHT is NOT the plenums fault:
In my opinion its a red herring. Van's set up is fine from a cooling stand point if you pay attention to details and seal as best you can. The inlet size of Vans cowl are actually large enough, even if there are leaks. Now if you go to smaller round or rectangular inlets, the inlet area goes down by a large amount, so you have to have more efficient cooling with less air, to stay on par. The reward is less drag.
HIGH CHT is cause by several things like:
-Casting flash (lycs known for cast-flash left in fins during manufacture)
-Engine break-in (believe it, carbon build-up makes a thermal shield, lack of it on new engines allows engine to absorb more heat)
-Lower baffling TOO tight on cylinder #3-right rear or #2-front left shallow fins (see pic below, trick increase lower baffle/fin gap)
- At 100F OAT, gross wt climb to 8.5 might mean you need to increase speed to keep CHT in check. It's a trade off of cruise efficiency and climb CHT.
Yes, in my opinion a plenum is value add. This is also the opinion of the NASA cooling report, which made the recommendation to avoid soft seals because they are hard to seal up against the cowl. Also a secondary consideration (but not trivial) is air pressure on the upper inside of the cowl (plus out side air pressures) tends to wear the rear side/lower hinges; Hinges may last a good long time, but in my experience, 1000 hours they are shot. Also with soft seals engine vibrations/movements tend to load the cowl, even transmit more vibration (I would guess).szicree said:
Does this mean Vans set up does not work? No Van's set up works fine for cooling but you can improve or reduce cooling drag. My normal recommendation is stick to the plans & keep it simple.
As far as cooling problems, soft seals may contribute, but I don't think a solid plenum is to blame in my opinion or can fix the main reasons for high CHT, which are many. I list them below; what I think the top three reasons are for high CHT, in my opinion.
Besides better sealing, not having to deal with the soft seals and less cowl wear, I don't think a "dog-house" plenum will gain speed or much better cooling. To gain speed significantly you need to change other things in concert with the solid plenum, which I mention below. If you want to gain some speed you should consider a smaller inlet area or even different shape (round v. rectangle).
Common sense, zero leaks, all air going into the upper plenum (regardless of how you seal it) and down through cylinders means more cooling and efficiency. Any leak, where air goes direct into the lower plenum with out cooling the cylinders, pressurizes the lower plenum and is bad for efficiency. The solution for the stock set-up is make those seals as leak free as you can. Clearly a solid "dog house" plenum can help but the seal between the cowl and plenum is a problem. I have an idea I show below on how to keep the cowl stock but change the inlet.
Leaks do reduce the differential pressure across the cylinders. Differential pressure from above to below the cylinders is what drives the air to do cooling work. A sealed solid plenum is KEY to no leaks.
The question is whether to stick with the stock rectangular inlets or round widely spaced inlets (ala James Aircraft). If you stick with rectangle you will have some challenge sealing the plenum to the cowl. Round obviously with a soft flex hose and clamps is easier to seal 100%. Round is just more practical for duct work. However what jeff did "RV_7A did is awesome, I'm impressed, nice work. You can seal rectangles and ducts but its a challenge.
There are other key reasons for using a round inlet. With a round inlet it's easier to make a more aerodynamic transition, which causes less turbulence in the first part of the inlet. Note smooth transition and 100% sealing between cowl inlet and plenum:
You will have turbulence with any design, but the design of a round "scoop" with an aerodynamic lip and inner profile means the air will not become turbulent, stall or stagnate as soon, which determines the efficiency. Also the air the farther from the spinner is cleaner due to the prop blades. The prop blade shank is blunt. Note reverse flow in the inlet near the spinner:
With a round inlet, you can put all your required area as far outboard as possible, verses a narrow letter box rectangle shape. Note how round inlet give better pressure. (Cp=coefficient of pressure but don't worry what it is, just compare the two inlet styles and a 10% is not insignificant.) Also not the even inlet flow and more pressure outboard, near the hot cylinder valve area. (In the top left corner, prop air flow is shown. Further towards the tip the better.):
More efficiency means you can make the inlet smaller and still maintain equivalent cooling with less cooling drag, which means more speed. The idea is NOT better cooling just the same cooling with less air or less drag. If I did not want to go the full James Aircraft style I would make a plenum and stock inlet smaller. Reduced height and width filling in from inboard edge. You can seal a rectangle duct with clamps. You make a plug deal like this, which nests inside the existing cowl inlet. The beauty is you can change is. This is some what like what Al and Bob Axom have done.
Hot CHT is NOT the plenums fault:
In my opinion its a red herring. Van's set up is fine from a cooling stand point if you pay attention to details and seal as best you can. The inlet size of Vans cowl are actually large enough, even if there are leaks. Now if you go to smaller round or rectangular inlets, the inlet area goes down by a large amount, so you have to have more efficient cooling with less air, to stay on par. The reward is less drag.
HIGH CHT is cause by several things like:
-Casting flash (lycs known for cast-flash left in fins during manufacture)
-Engine break-in (believe it, carbon build-up makes a thermal shield, lack of it on new engines allows engine to absorb more heat)
-Lower baffling TOO tight on cylinder #3-right rear or #2-front left shallow fins (see pic below, trick increase lower baffle/fin gap)
- At 100F OAT, gross wt climb to 8.5 might mean you need to increase speed to keep CHT in check. It's a trade off of cruise efficiency and climb CHT.
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tonyjohnson
Well Known Member
inlet issues
I have been following this thread in order to get smart on these issues since this part of the project is just over my horizon.
If I understand correctly, the plenum helps with cooling issues, and reduces drag which contributes to more speed. I am getting the impression that the inlets are a significant issue here.
I like the fact that with Jeffs design you can use the standard vans cowl and baffelling. I am wondering, particularly in light of George's very informative input, how much value the plenum would have if you did not also maximize the efficiency of the intakes, Ie going to the round ones.
I visited Jeffs website and I am interested in his plenum design. The easiest setup would seem to be the standard vans cowl with the standard inlets, vans baffeling, and Jeffs plenum.
I see that Bob Axom, who is always looking for speed mods, has opted to use the original rectangular inlets and modified them with a "nestled plug". Bob, if you read this I would appreciate your input on that method. Would you do it that way again?
Has anyone modified a standard vans cowling to accept round inlets?
Jeff, could you please provide some thoughts and or observations on that issue. What are your thoughts on using round inlets with your plenum? Are you flying now? Any data on how your plenum is working out?
Does anyone who has used the Sam James plenum have any feedback on the difficulty of making the baffeling for that setup, as opposed to the standard Vans baffeling that Jeff uses?
I have been following this thread in order to get smart on these issues since this part of the project is just over my horizon.
If I understand correctly, the plenum helps with cooling issues, and reduces drag which contributes to more speed. I am getting the impression that the inlets are a significant issue here.
I like the fact that with Jeffs design you can use the standard vans cowl and baffelling. I am wondering, particularly in light of George's very informative input, how much value the plenum would have if you did not also maximize the efficiency of the intakes, Ie going to the round ones.
I visited Jeffs website and I am interested in his plenum design. The easiest setup would seem to be the standard vans cowl with the standard inlets, vans baffeling, and Jeffs plenum.
I see that Bob Axom, who is always looking for speed mods, has opted to use the original rectangular inlets and modified them with a "nestled plug". Bob, if you read this I would appreciate your input on that method. Would you do it that way again?
Has anyone modified a standard vans cowling to accept round inlets?
Jeff, could you please provide some thoughts and or observations on that issue. What are your thoughts on using round inlets with your plenum? Are you flying now? Any data on how your plenum is working out?
Does anyone who has used the Sam James plenum have any feedback on the difficulty of making the baffeling for that setup, as opposed to the standard Vans baffeling that Jeff uses?
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Tony,
My plenum top provides a positive seal up top and a fairly maintenance free setup as compared to the conventional seals like Vans and others suggest. It creates a rigid structure with the baffling and also helps eliminate vibration cracks from forming. I designed my plenum top to work with the stock cowling and baffling with no extra glass work needed. Opting to convert and obtain the look and performance of the James cowling design would certainly take a bit of glass work which most of us hate. If you merely want a clean easy to install trouble free setup and are happy with Vans speed numbers then my plenum top is the ticket. I?m not selling low cooling numbers here with my plenum, but if everything is set up properly (i.e. ignition timing, fuel mixture, baffling sealed without unnecessary cooling ports to accessories) you will have acceptable cooling levels. Every airplane is different and will most cetainly take some tinkering to balance the numbers between cylinders.
-Jeff
My plenum top provides a positive seal up top and a fairly maintenance free setup as compared to the conventional seals like Vans and others suggest. It creates a rigid structure with the baffling and also helps eliminate vibration cracks from forming. I designed my plenum top to work with the stock cowling and baffling with no extra glass work needed. Opting to convert and obtain the look and performance of the James cowling design would certainly take a bit of glass work which most of us hate. If you merely want a clean easy to install trouble free setup and are happy with Vans speed numbers then my plenum top is the ticket. I?m not selling low cooling numbers here with my plenum, but if everything is set up properly (i.e. ignition timing, fuel mixture, baffling sealed without unnecessary cooling ports to accessories) you will have acceptable cooling levels. Every airplane is different and will most cetainly take some tinkering to balance the numbers between cylinders.
-Jeff
tonyjohnson
Well Known Member
thanks
Jeff,
Thanks for that information.
Tony
Jeff,
Thanks for that information.
Tony
Christopher Murphy
Well Known Member
no plenum
If you are looking for alot of work for not much gain, go for the plenum. Vans cowlings and baffle set up is pretty efficient and will cool your engine properly. Cooling problems are more likely the result of other factors if the baffles and cowling are installed correctly. If you want all out speed keep your airplane light and make good fairings.
Most guys that I have raced with, that have plenums, say they would forgo the trouble and expense.
Race 34 RV-4 207.52mph A-V Cup
If you are looking for alot of work for not much gain, go for the plenum. Vans cowlings and baffle set up is pretty efficient and will cool your engine properly. Cooling problems are more likely the result of other factors if the baffles and cowling are installed correctly. If you want all out speed keep your airplane light and make good fairings.
Most guys that I have raced with, that have plenums, say they would forgo the trouble and expense.
Race 34 RV-4 207.52mph A-V Cup
tonyjohnson said:Has anyone modified a standard vans cowling to accept round inlets?
QUOTE]
Here is one spotted at the Corning, CA airport during an EAA chapter meeting.
Moving the inlets away from the spinner and the blunt blade roots is essential to making an efficient inlet.
David-aviator
Well Known Member
I went to 3 seminars on cooling and/or drag reduction at OSH -
Sonja Englert (German aeronautical engineer)
Curt LoPresti (of the LoPresti family)
Klaus Savier (efficiency expert king)
Everything George said is confirmed except the benefit of a plenum and round inlets, there is no agreement on that.
For sure there is a dead area next to the prop hub, Sonja Englert had tufted it and showed a prop pressure graph depicting it. She also tufted and photographed the inside of the cowling showing that air came in the left side, made a u-turn at the aft baffle and returned to the front of the right side rather than go down through the cylinders. She designed a simple fence of sorts and reduced the CHT's 50 degrees - this was work on the Columbia. Someone asked if round inlets were better and she simply replied "no" the air doesn't care. A lip about the size of a finger was necessary to prevent separation, the shape of the hole did not matter.
The LoPresti's do not use plenums, the reason being it reduces area above the engine which reduces the amount of air available to cool the engine at any given moment. They believe a good baffle seal works just as well. They have done some remarkable stuff with production airplanes, many of which are not at all optimized for speed, efficiency or cooling.
Klaus Savier is incredibly intelligent with regard to props, engines, cooling, drag, ignition and now fuel injection. His 0200 runs at 2900 rpm with 10:1 pistons with an experimental fuel injection system based on pulse time and manual mixture control. He has achieved 100 mpg with the VEZ. Unfortunately, I did not catch whether he uses a plenum or baffles. Suffice it to say, he does not have a cooling problem.
Just hearing these people talk is enough to light anyone's fire on improving the performance of what we have. So far as plenums are concerned, even the experts do not agree so it probably is a matter of personal preference. It may not matter performance wise.
Sonja Englert (German aeronautical engineer)
Curt LoPresti (of the LoPresti family)
Klaus Savier (efficiency expert king)
Everything George said is confirmed except the benefit of a plenum and round inlets, there is no agreement on that.
For sure there is a dead area next to the prop hub, Sonja Englert had tufted it and showed a prop pressure graph depicting it. She also tufted and photographed the inside of the cowling showing that air came in the left side, made a u-turn at the aft baffle and returned to the front of the right side rather than go down through the cylinders. She designed a simple fence of sorts and reduced the CHT's 50 degrees - this was work on the Columbia. Someone asked if round inlets were better and she simply replied "no" the air doesn't care. A lip about the size of a finger was necessary to prevent separation, the shape of the hole did not matter.
The LoPresti's do not use plenums, the reason being it reduces area above the engine which reduces the amount of air available to cool the engine at any given moment. They believe a good baffle seal works just as well. They have done some remarkable stuff with production airplanes, many of which are not at all optimized for speed, efficiency or cooling.
Klaus Savier is incredibly intelligent with regard to props, engines, cooling, drag, ignition and now fuel injection. His 0200 runs at 2900 rpm with 10:1 pistons with an experimental fuel injection system based on pulse time and manual mixture control. He has achieved 100 mpg with the VEZ. Unfortunately, I did not catch whether he uses a plenum or baffles. Suffice it to say, he does not have a cooling problem.
Just hearing these people talk is enough to light anyone's fire on improving the performance of what we have. So far as plenums are concerned, even the experts do not agree so it probably is a matter of personal preference. It may not matter performance wise.
David-aviator said:
I wish I could have been there. It would have been interesting to listen in and speak with the presenters.
One thing I have found in the area of fluids and cooling is that the knowledge base of some that are widely regarded as experts in the field can be overly constrained, in particular with those without a formal education in fluid dynamics. For example, in a recent discussion with one of the other LoPrestri's I was being informed that the pressure recovery I was obtaining was not possible; that a quoted NASA report was in error; and that my oil temperature must be going through the roof. After some back and forth it occurred to me that their experience in cooling systems had been limited to very conventional low inlet velocity systems and that they were having trouble with some fluid dynamic concepts with which they simply had no prior experience. A concept not fitting their conventional thinking? It must be in error.
The pressure that I could not possibly be obtaining was a direct measurement with calibrated equipment; I tend to believe the accuracy NASA reports, and my oil temperatures are fabulous. After addressing each of the points in detail, I could see their gears turning. It was an eye opening experience to say the least.
Consider also that there are many non-technical reasons not to use a plenum on a certified plane: Cost, complexity, maintenance, certification issues etc.
"...the reason being it reduces area above the engine which reduces the amount of air available to cool the engine at any given moment."
Now that is amusing. The first order effect of reducing area does not affect the amount of air available, it simply make the air move faster. Granted, faster air can generate higher losses, but let?s put this into perspective. With a good plenum you loose 5-10% of your volume. So the air on average moves 5-10% faster. But wait, by creating a better seal you actually reduced volume flow and velocity. So in the end, you really are better off with the plenum.
<<She also tufted and photographed the inside of the cowling showing that air came in the left side, made a u-turn at the aft baffle and returned to the front of the right side rather than go down through the cylinders. She designed a simple fence of sorts and reduced the CHT's 50 degrees - this was work on the Columbia. Someone asked if round inlets were better and she simply replied "no" the air doesn't care. A lip about the size of a finger was necessary to prevent separation, the shape of the hole did not matter.>>
Dave, Chris, could you comment on the above two items?
What sort of fence was it, a divider along the center of the case?
As for standard inlets, what sort of "lip" are we talking about?
Dave, Chris, could you comment on the above two items?
What sort of fence was it, a divider along the center of the case?
As for standard inlets, what sort of "lip" are we talking about?
DanH said:
Fences are often used to keep sinks from competing for the same local air. Without seeing the whole system and what was driving the air from one side to the other its tough to comment in detail. Using fences or diverters is a common approach to solving local flow issues.
I agree with the shape comment. The air doesn't care whether the inlet is round or not. There are some non-aero reasons for round inlets, however. First it is immensely easier to seal to a round inlet than non-round; an issue for plenum users. One also has more manufacturing options with a round inlet. Contours can be done on a lathe for example.
The extent to which a radiused lip is required is a function of the inlet velocity ratio and thus where your stagnation point occurs. Large inlets with low inlet veloctity ratios do not need much internal radius to keep flow attached. At the opposite extreme, I use an inlet that is two inches deep, the cross section of which looks more like an airfoil than a lip.
When a solution to a problem is presented, one must be very aware of the context to know when that particular solution applies. Taking a solution out of the proper context has caused many builders a lot of grief and frustration.
David-aviator
Well Known Member
DanH said:
Dan,
Chris knows much more about this subject than do I. I think he answered your questions. I am very much on the front side of a learning curve here.
I liked the presentation by Sonja Englert so much I bought her book, Efficient Powerplant Installation. (she's written at least a half dozen books) She is no theorist. Just about everything in the book is backed up with experience. In addition to the Columbia project, she also did the liquid cooling design work on the experiment Continental Deisel installed in a Skymaster which is of particular interest to me with the H6 Subaru. I really need to clean up the cowl, get better cooling and most important, more speed.
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Dave,
Sonja Englert has an interesting resume and seems very practical:
http://www.caro-engineering.com/
Thanks for the tip. I think I'll buy a copy of her powerplant book too. Yeah brother, I know what you mean about the learning curve <g>
Chris,
Ahh, now I know what "lip". A friend with a Phd in CFD once suggested that about 7 degrees was a reasonable "rule-o-thumb" max for the divergent angle in order to maintain attachment. Hard to do within the length constraints of a cowl...he was talking about a wind tunnel design at the time.
I previously mentioned the inlets on the new Skycatcher cowl. Compared to conventional cowls, they are higher up the cowl face in relation to the cylinders. Seems like that position would allow better flow control with a rectangular inlet. You could provide a nice radius on the lower lip rather than ramping up into the face of the forward cylinders. The upper lip wouldn't need much angle at all, as it is almost on the same plane as the upper cowl surface.
They also eliminated open area near the prop hub; the inlets are outboard.
Anyway, it struck me as the right way to do a clean sheet large rectangular inlet. Am I guessing right, or is this another case of CFD being non-intuitive?
Sonja Englert has an interesting resume and seems very practical:
http://www.caro-engineering.com/
Thanks for the tip. I think I'll buy a copy of her powerplant book too. Yeah brother, I know what you mean about the learning curve <g>
Chris,
Ahh, now I know what "lip". A friend with a Phd in CFD once suggested that about 7 degrees was a reasonable "rule-o-thumb" max for the divergent angle in order to maintain attachment. Hard to do within the length constraints of a cowl...he was talking about a wind tunnel design at the time.
I previously mentioned the inlets on the new Skycatcher cowl. Compared to conventional cowls, they are higher up the cowl face in relation to the cylinders. Seems like that position would allow better flow control with a rectangular inlet. You could provide a nice radius on the lower lip rather than ramping up into the face of the forward cylinders. The upper lip wouldn't need much angle at all, as it is almost on the same plane as the upper cowl surface.
They also eliminated open area near the prop hub; the inlets are outboard.
Anyway, it struck me as the right way to do a clean sheet large rectangular inlet. Am I guessing right, or is this another case of CFD being non-intuitive?
