marbleturtle Posted January 8, 2004 Share Posted January 8, 2004 This probably should be in the egine area, or could be in another area related to aerodynamics... but here goes. http://home.earthlink.net/~rotaryeng/how-to-cool12.txt This may be old... but its news to me! It seems to say that an NACA duct with an obstruction inside (like the radiator) won't work well because of external diffusion (air in the duct gets blocked, air coming from behind goes around). I goes on to state that the P-51 type scoop off of the boundary air layer ala Bulent (sp?) works better. John... do you have any thoughts on this since is specifically discusses cooling a rotary? I was curious to see what arguments dispute this thoery. I know about the vortex generators ahead of the NACA but that seems to be more of a crutch than a solution. But I'm not a big fan of creating more drag with a scoop either... decisions decisions. Quote This ain't rocket surgery! Link to comment Share on other sites More sharing options...
John Slade Posted January 8, 2004 Share Posted January 8, 2004 I was curious to see what arguments dispute this thoery. Interesting way to put it. I've read that stuff, and its just that - theory. There are too many poarameters for this stuff to be an exact science. Ed Anderson is probably closest to understanding all the issues involved, but unfortunately he's of the backwards persuasion. As in the case of the Electric Water Pump and various other theories "proven" on the ACRE site, I'm hoping to disprove this one with facts. Incredibly, Paul still argues that the EWP can't work, even though Todd Bartrim and a few others I think, are flying around with them cooling happily. To be honest, part of the reason I'm persuing the NACA is that Paul says it can't work The other is that I want minimum drag / maximum cruise speed. I'm hedging my bets a bit by positioning stuff so I can easily cut armpit scoops, and I'm installing the vortex generators. Anything that can reduce operating temps by 40 degrees without impacting performance is more than a crutch, it's a damn good idea. I'm also using a big ferrari fan for ground cooling, and exhaust augmentation. It it doesnt work I'll admit defeat and add scoops. By the way, that Lamar paper is old and out of date. He now says the best way to cool a pusher is to dispense with the P51 scoop and hang the rad out in the wind. Sounds attractive, safe and efficient at 200+mph, don't you think? When paul puts down his wornout calculator, builds and flys a pusher (or any airplane for that matter) that runs well and cools well, then I'll listen to him. Join the ACRE list if you want 2MB pictures of RX8s, unproven theory and partial science. Join the flyrotary list if you want real world information based on actual experience. Well, you did ask. Quote I can be reached on the "other" forum http://canardaviationforum.dmt.net Link to comment Share on other sites More sharing options...
marbleturtle Posted January 9, 2004 Author Share Posted January 9, 2004 YES I DID! Thanks for the response. I'm trying to sort out the details of a few last decisions while I get the building area in the shop (ie: the corner of the garage the GIB's stuff does not occupy) ready. Whether to use the NACA duct or the P-51 duct seems important now, but may be less important later when I get my hands dirty and see if it is easy to make the change. I hope you do prove him wrong! The less drag the better. After I made this thread I surfed around for "NACA aerodynamics". Not much about aircraft, but alot of information and pictures of 200 MPH racecars. Can you believe that all of those rear-engine cars use NACA ducts feading directly into small radiators? Millions of dollars in aerodynamic testing flushed down the toilet! Wait, I see a forced air scoop... no, that feeds the airbox. The pursuit for knowledge continues! I'm still toying with the idea of 2 rotaries powering twin duct fans blended into the body... Quote This ain't rocket surgery! Link to comment Share on other sites More sharing options...
LargePrime Posted January 9, 2004 Share Posted January 9, 2004 If I understand NACA theory correctly (unlikely) the problem is with "back pressure", not with the duct. Per unit of drag you cant beat a naca scoop. But for it to work you need to feed it into a lower pressure area. Like all scoops, if you feed it into a high pressure area it will back up and create a wave at the entrance. An armpit scoop can add more pressure for sure, but per unit of drag it cant touch a NACA. If you look at the scoops on Rutan's pond racer you see an extreame example of a NACA type scoop. Quote We know who you are... Link to comment Share on other sites More sharing options...
JakeC Posted January 9, 2004 Share Posted January 9, 2004 If I understand NACA theory correctly (unlikely) the problem is with "back pressure", not with the duct.This is my understanding as well. It kind of makes sense considering how easy any back pressure would allow the air stream to slip right past the inward bend and continue to follow the contour of the fuselage. I suspect that the vortex generators give the air stream better adhesion to he inward curve and make it more affective at counteracting the back pressure. The method that seems to me to have the best shot at solving the problem is the one that John is experimenting with; a combination of Vortex Generators and exhaust augmentation. The exhaust augmentation SHOULD create a negative pressure within the cooling ducting effectively lowering the pressure at the scoop. In my "concept" of things, a scoop with only vortex generators and a scoop with both the generators and augmentation should loose it's effectiveness at the same back pressure level. However, the scoop with both should be moving a greater volume of air before this critical pressure threshold is reached because it is starting out with a lower pressure because of the suction effect of the augmentor. Here is where it gets even more interesting for me. Assuming what I just wrote above is correct, it would seem to me that a NACA scoop that uses an exhaust augmentor system to suck air through the system may act as a variable geometry scoop of sorts. When the throttle is firewalled the augmentor is sucking the greatest amount of air into the inlet. As the throttle is reduced, the pressure at the inlet is increased and the inlet begins to lose its effectiveness. When this happens, the air that cannot make it into the scoop just slides across the bubble of compressed air as if it's a continuation of the fuselage and continues on its way. Thus the area of the inlet is effectivly reduced. Since you don't need as much cooling air when throttled back at cruise as you do when climbing out on take off, the fact that this system would lose some of it's effectiveness at cruise may prove to lower the drag at cruise as well. Now, this is all highly theoretical but I would like to hear your comments on this. Jake Quote www.homebuiltairplanes.com Link to comment Share on other sites More sharing options...
LargePrime Posted January 9, 2004 Share Posted January 9, 2004 When a bunch of people who don’t really know what they are talking about start agreeing I always get a queasy feeling... Exhaust augmentation may work as a variable geometry scoop. But... Why not just vary the geometry directly? A moveable floor for a NACA is easy light and effective. Quote We know who you are... Link to comment Share on other sites More sharing options...
marbleturtle Posted January 9, 2004 Author Share Posted January 9, 2004 I wonder if squaring off the back of the cowl at the air exit would help cooling... creating a vacume to suck air out of the cowling helping more go through the NACA duct. Oh... and a really cool spoiler! Quote This ain't rocket surgery! Link to comment Share on other sites More sharing options...
marbleturtle Posted January 9, 2004 Author Share Posted January 9, 2004 ... on a more serious note. John, are you going to use the EWP to replace or augment the mechanical pump? It appears that 1 primary EWP and 1 backup EWP could be used and still save 6 lbs over the mechanical. It also appears that you could have control to manage the flow for desired cooling results, not just RPM based. Is Todd Bartrim flying now with this arrangement? If so, how long? Thanks! Quote This ain't rocket surgery! Link to comment Share on other sites More sharing options...
LargePrime Posted January 9, 2004 Share Posted January 9, 2004 I belive the exit is a key issue with NACA's. I have seen photos of people trying to get the cowling as close as possible and having cooling issues. If you open the rear cowling gap up you increase cooling. You also add drag. I would rather have a cool engine and work towards reducing drag then have low drag and work towards resolving cooling issues. Quote We know who you are... Link to comment Share on other sites More sharing options...
dust Posted January 9, 2004 Share Posted January 9, 2004 how about creating a venturi at the exhaust to pull more air out of the cowling enjoy the build Mike Quote maker wood dust and shavings - foam and fiberglass dust and one day a cozy will pop out, enjoying the build i can be reached at http://www.canardcommunity.com/ Link to comment Share on other sites More sharing options...
JakeC Posted January 9, 2004 Share Posted January 9, 2004 Exhaust augmentation may work as a variable geometry scoop. But... Why not just vary the geometry directly? A moveable floor for a NACA is easy light and effective. I was really thinking about the variable geometry inlet phenomenon that may occur as a side benefit of using the augmentor with an NACA scoop and not a reason to use the augmentor. Besides, you can mechanically vary the inlet area all you want but if the pressure is too great to allow the air in, you probably won't get any benefit from it. Jake Quote www.homebuiltairplanes.com Link to comment Share on other sites More sharing options...
John Slade Posted January 9, 2004 Share Posted January 9, 2004 Sorry Guys, I'm out of town. You'll have to chat among yourselves for a few days.... >but if the pressure is too great to allow the air in, you probably >won't get any benefit from it. Exactly. I'm hoping the cowl flap at the back will allow me to control the pressure in the plenum. WHen it's open the air can go straight through. On the EWP - no. I'm using a stock pump because it's easier to do at this stage. I'm watching Todd & Rusty closely. Maybe a pair of EWP's will be in my second generation setup, but I've got enough complexity in there for now. Gotta go.... Quote I can be reached on the "other" forum http://canardaviationforum.dmt.net Link to comment Share on other sites More sharing options...
No4 Posted January 9, 2004 Share Posted January 9, 2004 The link below takes you to the photo page of Thielert, for their water cooled Centurion 1.7 diesel. The photo's are too big to attach. http://info.thielert.com/centurion/main/photoarchive.php This link below shows the radiator and ducting of the 1.7 unit that has replaced an 0-320/360 in a Cessna 172. http://info.thielert.com/fotoarchive/45.jpg I imagine a Rotary would require as much or even more cooling than a 140 hp diesel. There are lots more photos including the intercooler set up, for three different aircraft types. cheers Quote The Coconut King Link to comment Share on other sites More sharing options...
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