Return-Path: Received: from smtp817.mail.sc5.yahoo.com ([66.163.170.3] verified) by logan.com (CommuniGate Pro SMTP 4.3c3) with SMTP id 853635 for flyrotary@lancaironline.net; Sun, 03 Apr 2005 17:43:20 -0400 Received-SPF: none receiver=logan.com; client-ip=66.163.170.3; envelope-from=dcarter@datarecall.net Received: from unknown (HELO Davidscmptr) (dcarter11@sbcglobal.net@64.219.115.122 with login) by smtp817.mail.sc5.yahoo.com with SMTP; 3 Apr 2005 21:42:34 -0000 Message-ID: <01bc01c53896$09f49180$6401a8c0@Davidscmptr> Reply-To: "David Carter" From: "David Carter" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] Re: Cooling -Learned a lot Date: Sun, 3 Apr 2005 16:42:30 -0500 MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2800.1437 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1441 I wonder - is it not more correct to say: Behind the diffuser, the velocity will be slower, density higher, therefore there will be "nearly the same mass flow of air" thru the radiator, with same cooling, BUT "at slower speed, higher air density, and therefore less cooling drag"? - Drag is a function of velocity squared. The air density factor is not squared, thus we seek a reduction of drag by cutting velocity thru the rad. Bernie, you are the SR-71 PW engine air duct man - am I even close to expressing any useful and true info above? David Carter ----- Original Message ----- From: "Ed Anderson" To: "Rotary motors in aircraft" Sent: Sunday, April 03, 2005 4:15 PM Subject: [FlyRotary] Re: Cooling -Learned a lot You are absolutely correct, Tracy. I did not make it clear but the diffuser does the velocity reduction and increases the pressure in front of the core by recovery of (some) dynamic pressure component of the air flow. This higher pressure in front of the core then results in an increased pressure differential across the core. This increase in pressure differential across the core, as you stated, actually speeds up the air flow through the core itself. My apologies for being less than careful on that point., Ed A. ----- Original Message ----- From: Tracy Crook To: Rotary motors in aircraft Sent: Sunday, April 03, 2005 12:24 PM Subject: [FlyRotary] Re: Cooling -Learned a lot Excellent summary Ed, correlates with my experience as well. Only exception I would take is in the following excerpt: "A good diffuser will reduce airflow velocity through the core which will reduces cooling drag. Pressure across the core is increased which further enhances cooling." A good diffuser will reduce velocity but the reduction occurs IN the diffuser, not through the core. As counter-intuitive as it may sound, the velocity through the core is HIGHER than it would have been without the diffuser's velocity decrease (and pressure increase). Think about it this way, How could velocity through the core be reduced by a pressure increase? It isn't. The velocity at this point (through the core) is increased. This is the single most misunderstood detail in liquid cooled engine systems. Tracy Subject: [FlyRotary] Cooling -Learned a lot Too right, Jerry My first 40 hours or so were in the marginal cooling zone. {:>). As other things in this hobby, there are so many variables that interact, that what may appear simply at first, is almost always a bit more complex. I say(Cooling Axiom 1) if you have enough cooling surface area and air mass flow then it WILL cool. However, you may incur a high penalty in cooling drag - which may not be as important for draggy airframes (such as biplanes) as it is to sleeker airframes. Also a system that adequately cools an engine producing 150 HP may not cool an engine producing 180 HP. Picking your cooling design point is important. Optimizing for cruise and your will be less than optimum for take and climb. Optimize for climb and you will probably have more cooling drag than required at cruise. Compromise, compromise - cowl flaps are sometimes used to try to have the best of both worlds. Some folks advocate a thinner, larger surface area core -which is great for slow moving automobiles stuck in traffic with low dynamic pressure potential, but I think is not the optimum for most aircraft. Once you trip the airflow and turn it turbulent you have incurred most of the drag penalty. Larger surface area cores disrupt a larger airstream and incur more drag. Yes, thicker cores produce a bit more drag than the SAME frontal area thinner cores. But, with a thicker core you can use a core with smaller frontal area. The NASCAR radiator's average 3" thick and on the long tracks where speeds are higher some even go up to 7" thick. My contention is their operating environment is more akin to ours than regular automobiles moving at slower speeds. You know that the NASCAR folks will spend $$ for just a tiny advantage - so clearly they don't use thick cores because it is a disadvantage. But, some folks will continue to point to the large thin radiators designed for environments with much lower dynamic pressure as being the way to go. Will it cool? sure it will (Cooling axiom 1 above). Is it the lowest drag option for an aircraft of the RV/TailWind type, I am convinced it is not. The diffuser makes a considerable amount of difference and can made the difference between a system that cools adequately and one which does not. The biggest culprit that lessens cooling effectiveness is turbulent eddies that form inside the duct due to flow detachment from the walls. These eddies in effect act to block effective airflow through part of the core. So keeping the airflow attached to the sides of the diffusers is crucial for good cooling from two standpoints. A good diffuser will reduce airflow velocity through the core which will reduces cooling drag. Pressure across the core is increased which further enhances cooling. I have gone from a total of 48 sq inches opening (total) for my two GM cores and that provided marginal cooling - down to 28 sq inches (total) with adequate cooling with an engine now producing more HP. Experimenting with the diffuser shape made the difference. The K&W book (Chapter 12) really provided the insight to how and which diffuser shapes provided the better dynamic recovery. The Streamline duct was shown to be able to provide up to 82% recovery of the dynamic pressure. Some folks reading the chapter misinterpreted the chart to show only 42% recovery where there chart was actually only showing the pressure recovery contribution due to the duct walls and did not include the contribution due to the core. On the same chart, an equation (which apparently gets ignored) clearly shows that the TOTAL pressure recovery is 82%. I have taken the Streamline duct as a starting point, but since I do not have the space to provide the 12-14" for a proper Streamline duct, I did some "creative" things to try to insure that there was no separation even though my walls diverge more rapidly than the Streamline duct. Won't claim mine are as good as a Streamline, but they clearly are much better than the previous design which basically just captured the air and forced it through the cores. FWIW Ed Anderson RV-6A N494BW 275 Rotary Hours (Plugs Up) Matthews, NC eanderson@carolina.rr.com ----- Original Message ----- From: "Jerry Hey" To: "Rotary motors in aircraft" Sent: Sunday, April 03, 2005 9:27 AM Subject: [FlyRotary] Re: phase I flight restrictions was:N19VX flys > It was not long ago that "cooling" was the major issue. Now it seems > that we have learned enough to make several different configurations > work. I can't lay my finger on what it is we have learned but my > recommendation is to use smaller radiators and EWPs. Jerry > > > >> Homepage: http://www.flyrotary.com/ >> Archive: http://lancaironline.net/lists/flyrotary/List.html