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Alex, I agree. Those are very good pictures. I just changed all my forward baffling from "seal on the cowl" to "seal on the ramps". I remember asking Van's which way to go (wayyyy back when...) and they said either way will work. I also extended the lower cowl seal way up the sides. They didn't even go out past the "curve". Now everything looks like yours. I don't have a lot of time on the mod but it looks like CHT's 10-12 degrees lower and oil temp 5-10.
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I understand from Zavatson that 60% is pretty good, but "very nearly full" would seem to be greater. Next, where can I view photos of your efforts? TIA mjb |
Big Curves
Hi Guys,
I have a question. I'm reworking my seals and need some advice. What size material should I use when working the curvy sections around the spinner and the back of baffled area. Should I just bend the **** out of 3" wide material or should I be using something wider? |
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Next time I would use a sheet of cheap vinyl or similar to mimic the baffle seal as I cut/trimmed and pieced together a pattern to make a 1 piece seal. The resulting pattern will tell you what size material you need. In the end I think it's faster and easier to make a 1 piece pattern to cut a finished piece of baffling than to fit lots of little pieces, not to mention a better seal. Just a theory but an easy one to try..... |
From reviewing this thread it would appear my front/middle baffle seal is pointing the wrong way. It's edge is tucked under(behind) the wrap around portion of the top cowl. I thought maybe the pressure inside would be higher than the pressure behind the gear and keep the seal tight?
Or, does this seal need to be angled backwards for better seal? If so, does the seal where it curves backward need to be redone? PS This RV-9A is new to me and runs hot. Hope the picture compensates for my poor description. Looks like I haven't figure out picture posting, maybe the following will help. http://tinypic.com/m/jike8x/3 |
Definitely should be the other way around. I'm guessing if you reverse them it will help with your cooling issues, but would also guess that there may be other baffling problems too. Worth going over and making sure every possible place is sealed.
Chris |
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Yup!! Should be pointing inward so high pressure entering inlet will force the rubber flaps up against the cowl.
Roberta |
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The curved inlet ramps are installed just aft of the inlet on the top cowling? The curved ramps set inside of the rubber baffle material that seals to the cowling top? |
Baffle photos at inlets
These are good. But would appreciate more shots around inlets. I'm working on my baffles now. Would appreciate seeing more photos around inlets, both sides?
Questions that arise when trimming the aluminum include: to what point do you trim against the cowl inlet? How much space between cowl and baffle? How does snorkel for horizontal induction look? Thanks |
This is an awesome thread.
I'm going to reexamine all my baffle seals ASAP. I had a really not-fun flight just now with the oil temps going way past my comfortable margin. I'm almost certain that the sealing around the inlets is no good and that there are some small leaks around the top where the baffle material overlaps. Before I do a lot of extra work to accommodate a bigger cooler, I'm going to try the continuous strip of baffle seal and rework the inlet areas to keep as much as possible from spilling out into the lower cowl. CHT's are fine, but if I can get an extra 15-20 degrees of oil cooling by sealing up the cowl, that would be great. |
inlet heII
yep, baffling is what these are to me!
....not a builder, but inherited some pretty effective, well done baffles. BUT!!! the better they are, the more they become an absolute BBEEYYYATCH! to get the lower cowling on! anyone addressed this with smaller, overlapping baffle elements attached to the cowling, that slip past some nice short ones on the engine/inlet ramp side? love to see a solution that will save time, vocabulary etc. (P.S. this is on my Glastar with with a LOT of Van's FWF components!) |
I do not understand why everyone does not use a plenum. They are much easier to fabricate, and they work better. They don't wear out, and are quickly removable. To me, baffle seals are for planes with hinged cowl doors.
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Plenum
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I've seen some beautiful plenums (Dave, Dan) but just didn't have the courage to do it. The part that intimidated me was the connection from the plenum to the air intakes. Considering how much time I spent on the "standard" baffles, I probably should have done a plenum to justify how long it took. |
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I'm completely revamping the baffles on the Rocket, but I am still not going to a plenum yet. We'll see how the flight test program works out before I throw in the towel on that one. |
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My CHTs are 320 +/- 30 degrees depending on OAT and power selection. Spread is usually 10F. I have to run a 1/2 blocked oil cooler in the summer to get temps up to 170F. No vent grills or cowl flaps. Did not trim the cowl exit either. It still sticks rearward of the firewall slightly. Why do i need a stinkin' plenum???????:) |
Various plenum musings..
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NASA ran some tests, and concluded that a plenum is 30% more efficient than the seals. Your MMV, but that says a lot.Danged if I can find the paper - I will keep looking. * found it - Nasa contractor report 3405 from 1981. Adding a plenum can be as easy as laying up a top cover inside of your top cowl. Of course, you might want to use plastic film to keep the new part from simply adding weight to your top cowl...:-0 One thing I have found that some don't get right is that the plenum attach flange should be on the top cover, so the flanges are not in the way when you work on the plugs. Auto trunk seal can be used to seal the inlet area...round inlets might be another trouble spot for some. Changing the top cowl so it does not include the inlets is one way around the inlet seal mystery... You might need to get creative when sealing the oil filler tube on the 6 cyl setups... Those who say that the long leaky seals are good enough for their setup forget that 30% leakage is still happening (OK - maybe it's 20% in your case), but that leaking air is still creating drag... I am not saying it is easy! Best, Mark |
Mark, Dan, or anyone who has instrumented their plenum...
Any idea if a well executed plenum delivers similar pressure as a pitot style induction system? In other words, if you have an excellent plenum, does it make sense to ditch the pitot style induction scoop and tap into this source of pressure? If the resultin MP is the same, then the loss of the induction scoop and resulting reduction in wetted area should net a gain. Of course, looking at the F1 guys seems to indicate this is not the case - the induction scoop is very prominent in most cases. |
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Mike, the plenum lid itself is just a sealing device. There are effectively two throttles in the flow, (1) the passages between the cylinder and oil cooler fins, and (2) the lower cowl exit. Leakage bypasses (1), so you should see increased upper deck pressure due to better sealing if (2) is relatively open. However, if the cowl exit is highly throttled, as it should be for least cooling drag, closing the leak path around the engine would have less effect on upper deck pressure. Big picture? You want to close the leak path primarily to boost heat transfer efficiency, i.e make sure every single molecule of air that passes through the system is required to contact hot parts. Doing so allows the same cooling with less total mass flow, i.e. drag reduction. To increase upper deck pressure (or more accurately, the ratio of upper deck static to available freestream static), concentrate on inlet configuration. In the case of a plenum, that means connecting the cowl inlet to the plenum so as to not promote flow separation in the inlet flow. A low Vi/Vo inlet configuration is relatively insensitive. A high Vi/Vo inlet is very sensitive, but if done right can work equally well. BTW, I was fooling around one day and measured the change in upper deck pressure with change in exit area. It was a valuable increase, but not huge even with a large change (about 35%) in cowl exit area. As a thought experiment, imagine a cowl with a very large exit (no throttling). A 35% reduction in upper deck leakage (replacing very leaky rubber flap seals with a good plenum lid) should net about the same pressure increase. http://www.vansairforce.com/communit...&postcount=175 |
My current plan is to baffle the engine in the usual manner (no plenum lid), but I'm going with the Glassair style "great wall" chin baffle and open inlets (low Vi/Vo). I think I can work a single piece of material from the back wall, along the side, down and around the chin, then back up the other side to the rear wall. If I'm successful, I'll have only 2 leak paths (one at each rear corner). That said, I'll also have about 12 feet of seal surface to maintain. This might actually hurt me in the end as a net loss. Then there is the significant fabrication of this fwd baffle... I might actually be labor and weight ahead going with a plenum after all. I still have a ways to go before I need to make that decision, but its certainly on the table.
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Yes. That configuration is what sparked the thought about harvesting the cooling air for induction. As an aside, I do intend to seal the prop shaft (pretty easy with the Rocket extended hub) to reduce the effect of the low pressure area right behind the blade roots. Seems a waste to spend a bunch of time packing the upper deck full of pressure with outboard inlets only to have the spinner gap suck a bunch of it away.
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Seperate of combined induction inlet
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Increasing capture area in the plenum increases pressure but also flow rate. You run into diminishing returns because flow rate keeps going up as you try to add induction pressure. For a pure induction inlet, volume flow rate is fixed by the engine. Increasing capture area increases only pressure. And the gain is easily calculated ahead of time. http://n91cz.com/InductionSizing/Ind...izing_rev0.pdf |
Thanks for that Chris. Your experimentation has been an inspiration to me for many years now.
As an update, I have decided to go with a full plenum for cooling (my current weekend project, in fact), and I am really going to town with integrated cylinder shrouding and sealing. My attempts to seal everything other than cooling fins is bordering on fanatical. I hope it pays dividends. |
A different thought:
Might it be easier to set up a well sealed cowl if the cowl is not trying to seal against a ever-moving engine? In other words, attach the cowl to the engine - not the airframe. The annular outlet at the firewall would be a detail that would need to be addressed, but that might be easier than fussing with 12? of sealing!
Might be that this type of system would find benefits in an augmentor system - I noted some out-of-the-box thinking on Race 30 two years ago, which has been improved on since. Their goal was reducing exh temps, but the cooling was increased too. |
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BTW, returning to the conventional, one of the best inlet-to-plenum connections I've seen on a conventional Vans rectangular inlet is on Ron Moring's RV-8. Those of you who have met Ron know he is too bashful to post a photo ;) |
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Mike, the photo doesn't come through.
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I've posted this before but this seems like a good place to post it again.
Remove your existing seals. Place paper clips all along the perimeter of your baffling so they stand tall and will hit the cowling. Put your top cowl on. Remove it. There's the perfect outline for your seals. Get the red/blue silicone seal material and cut it about 1 inch longer than the paper clips. Carefully work the corners to avoid puckers or gaps and bingo, you are ready to go. |
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I have some RTV between the corner folds to keep it all air tight. If you look closely you can see just a bit of orange on that lower left corner in the picture.
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I am just about ready to cut my baffle seals and drill the holes for the rivets to hold them on. Since there are no dimensions in the plans I am planning on starting with seals 3-1/4" high and setting them 1" down on the baffles. This will give 2-1/4" to seal against the cowl. The rivets will be 1/2" down from the top of the baffle. Does this sound like a reasonable starting point?
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So, you might start with your proposed length on a section, cleco the seals in place, and check fit looking in through the air inlets and oil door. Trim to length based on what you observe and cut the others to match. |
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We did a lot of work on cowl design and cooling sealing on the CAFE Foundation test Mooney back in the 1990?s. That bird actually ended up with a huge spinner from a turboprop and a 360? annular cooling inlet. I'm not suggesting that path for RV?s but the baffle seals were something completely different. We used thin, 10 mil, flexible, reinforced silicone fabric to create a baffle seal at the engine?s horizontal mid-plane. The seal extended around the circumference of the engine cylinder mid-point line with the outer edge of the fabric attached to the lower cowl inner wall and to the firewall itself below the oil filter. A bit of a hassle accessing the lower plugs, but when you removed the upper cowl you had full access to the Entire accessory section and the top half of the engine, with no aluminum flanges need to support heavy baffling engaging the upper cowl. The beauty of this installation is that it provided for a much larger volume decelleration plenum, and the entire upper accessory section was was in the ice cold pressure section. It was essentially a hermetically sealed partition. If I were building new I?d seriously consider such an approach. Pressure recovery of that system was in a class by itself. I?ll try to dig up some photos.-Otis |
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