X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 1 [X] Return-Path: Received: from ispmxmta05-srv.windstream.net ([166.102.165.166] verified) by logan.com (CommuniGate Pro SMTP 5.1.4) with ESMTP id 1735770 for flyrotary@lancaironline.net; Thu, 04 Jan 2007 13:59:47 -0500 Received-SPF: pass receiver=logan.com; client-ip=166.102.165.166; envelope-from=montyr2157@alltel.net Received: from ispmxaamta04-gx.windstream.net ([166.102.185.139]) by ispmxmta05-srv.windstream.net with ESMTP id <20070104185843.CUZW10623.ispmxmta05-srv.windstream.net@ispmxaamta04-gx.windstream.net> for ; Thu, 4 Jan 2007 12:58:43 -0600 Received: from Thorstwin ([166.102.185.139]) by ispmxaamta04-gx.windstream.net with SMTP id <20070104185842.DLNY7955.ispmxaamta04-gx.windstream.net@Thorstwin> for ; Thu, 4 Jan 2007 12:58:42 -0600 Message-ID: <003601c73032$6c11ea80$01fea8c0@Thorstwin> From: "M Roberts" To: "Rotary motors in aircraft" Subject: Ideal Cooling Date: Thu, 4 Jan 2007 12:59:12 -0600 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0033_01C73000.2153C5E0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.3028 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.3028 This is a multi-part message in MIME format. ------=_NextPart_000_0033_01C73000.2153C5E0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Lot's of true statements. But there's one vital component that's = overlooked. If you look at crash history, you may notice that one of the = main causes is marginal design. Designing systems on the edge of = failure.=20 We have excellent example from forced landing just a few months ago. The = guy that experienced overheating after changing to Evans coolant. If he = had robust cooling system, he would not have considered changing coolant = mid journey. Marginal cooling contributed to this decision. Forced = landing resulted.=20 I require myself to have compelling reasons to make a design change to = any engine system. So if I minimize the hose diameter, minimize the = radiator surface area. What do I gain? Likely I save a half lb. So for = that trivial weight advantage, I add risk every flight. I lug along a = spray bar to compensate. I would have to watch climb rate, make sure I = don't fly on hot days. This is the perfect setup for failure.=20 Strongly encourage robust cooling design. Don't let perceived value of = weight reduction lead to risky decision. The cause of the problem you mention is the change to Evans coolant = without a proper test program to see if it was adequate. Being on a = journey with a marginal cooling system in the first place. And not = fixing the cooling system during the flight test program.=20 The key word here is "marginal". I am not suggesting a marginal design, = I am suggesting a properly engineered and tested one. The thing I did = not talk about was the aerodynamic design of the cooling system, only = the heat exchanger sizing. Even if you use 3in hoses and a great big = thin radiator but a poor aerodynamic design, you will still have an = inadequate cooling system. This is where most of the installations drop = the ball. There is no magic size of this or that to give optimum = performance. It must be properly engineered, for each specific situation = with all components designed to work together. The math is not that = hard. Even the aerodynamic design can be done with little more than: P =3D 1/2 rho x Vo^2 Where: =20 P is the available dynamic pressure=20 rho is the density of air at your altitude Vo is the TAS of the aircraft You also need: Mdot =3D rho x Vi x Ai Where: Mdot =3D mass flow of air rho =3D density of air Vi =3D Velocity into the inlet which is usually not equal to Vo Ai =3D area of the inlet This is all you need. To find the heat rejected by the engine look up the drag curve for the = airplane and see what Hp you need at a given speed. Multiply this by 1.2 = to allow for 80% prop efficiency. For the rotary you will need to reject = about 2/3 of this value in the water and 1/3 in the oil. The Renesis may = need a little more on the water side, so add 20% on the water just to be = safe. Thanks to Tracy, we now have some idea of the delta T for air using the = thick radiators. The one remaining assumption is the ratio Vi/Vo. It = would be great if Tracy could get some data on this. Even without the = data you can look at a conservative range...say .5 to .8 so you at least = know where you will start to have trouble. Adjust accordingly. It is simple math. How many people do this? How many people actually = look at what the pressure drop in a given length of hose is at a certain = mass flow of water? This is engineering design. Everything else is just guessing. If you do this at various flight conditions and size for the worst = (reasonable) case. Then properly test and evaluate the system, You will = not have a "marginal" design. Using some simple math will get you in the ball park, to see if what you = are considering is even reasonable. Ground testing can then be used to = quantify things like pressure drop across the radiator for a given flow = of air. Part of your go no go list on test flight take-off is: have I = got the required pressure ahead of the radiator? No....then abort the = take off and regroup. Don't just take off and pray. Do your flight test = on a cold day. Have a spray bar just in case. Expand your flight = envelope into higher power, higher OAT and lower speed. Fix problems as = necessary. Don't conclude flight test until you have explored the whole = flight/operational envelope and are satisfied with the results.=20 What is the worst reasonable case? Some would say it is full power climb at VX on a hot day.=20 I don't feel that way. I prefer a spray bar and a properly sized = radiator for where the airplane spends it's time. The spray bar is only used in full throttle climb at Vx on a very hot = day with an already hot engine (a safety feature to get you out of = trouble which should be avoided operationally). Or while idling on the = ground for extended periods of time at 100F OAT. How often do you do = these things? What percentage of flying is this? If you design a = properly sized system for these conditions it will be 3X larger than = necessary for normal operations. The type of heat exchanger needed is = entirely different than the dense high pressure drop type needed for = cruise. It is a large area thin radiator with low pressure drop. You = will need an electric fan to make even this work for extended periods on = the ground, especially downwind taxi. During cruise you will have to accept a massive drag penalty for = perceived safety. I say perceived, because a properly sized and ducted = thick radiator will work just fine once you are over 100 mph or so. You = just can't climb at full throttle at 60 mph all day and expect it to = work. The large thin radiator greatly complicates the aerodynamic = ducting, and packaging design. You can't escape the Q=3DmdotCpDT = equation. You just went from DT=3D125 for the thick rad to DT=3D50 or = less for the thin one. Now you need a much greater Mdot air for the same = Q. So you need a huge inlet, an even larger exit. If you use the thin = giant radiator and an inlet that is too small, it will not cool as well = as the thick radiator with the same inlet. Assuming you do get the inlet = and outlet sized properly, you are flying around with a barn door = attached to your airplane all so you can amaze the crowds by flying = around in slow flight at full throttle on a 100 deg day without the use = of a spray bar. No real gain in safety. We are all design engineers now. At some point we must decide what we = are designing, an airplane or a tow truck. Do whatever makes you happy. Monty ------=_NextPart_000_0033_01C73000.2153C5E0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Lot's of true = statements. But=20 there's one vital component that's overlooked. If you look at crash = history, you=20 may notice that one of the main causes is marginal design. Designing = systems on=20 the edge of failure.
We have excellent = example from=20 forced landing just a few months ago. The guy that experienced = overheating after=20 changing to Evans coolant. If he had robust cooling system, he would not = have=20 considered changing coolant mid journey. Marginal cooling = contributed to=20 this decision. Forced landing resulted.
I require myself to = have=20 compelling reasons to make a design change to any engine system. So = if I=20 minimize the hose diameter, minimize the radiator surface area. What = do I=20 gain? Likely I save a half lb. So for that trivial weight = advantage, I=20 add risk every flight. I lug along a spray bar to = compensate. I=20 would have to watch climb rate, make sure I don't fly on = hot=20 days. This is the perfect setup for failure.
 
Strongly encourage = robust cooling=20 design. Don't let perceived value of weight reduction lead to risky=20 decision.
 
The cause of the problem you mention is = the change=20 to Evans coolant without a proper test program to see if it was = adequate. Being=20 on a journey with a marginal cooling system in the first place. And not = fixing=20 the cooling system during the flight test program.
 
The key word here is "marginal". = I am not=20 suggesting a marginal design, I am suggesting a properly engineered = and=20 tested one. The thing I did not talk about was the aerodynamic = design=20 of the cooling system, only the heat exchanger sizing. Even if you=20 use 3in hoses and a great big thin radiator but a=20 poor aerodynamic design, you will still have an inadequate cooling = system.=20 This is where most of the installations drop the ball. There is no = magic=20 size of this or that to give optimum performance. It must = be properly engineered, for each specific situation with all = components=20 designed to work together. The math is not that hard. Even the = aerodynamic=20 design can be done with little more than:
 
P =3D 1/2 rho x Vo^2
 
Where:
 
P is the available dynamic pressure =
rho is the density of air at your=20 altitude
Vo is the TAS of the = aircraft
 
You also need:
 
Mdot =3D rho x Vi x = Ai
 
Where:
 
Mdot =3D mass flow of air
rho =3D density of air
Vi =3D Velocity into the inlet which is = usually not=20 equal to Vo
Ai =3D area of the inlet
 
This is all you need.
 
To find the heat rejected by the engine = look up the=20 drag curve for the airplane and see what Hp you need at a=20 given speed. Multiply this by 1.2 to allow for 80% prop efficiency. = For the=20 rotary you will need to reject about 2/3 of this value in the water and = 1/3 in=20 the oil. The Renesis may need a little more on the water side, so add = 20% on the=20 water just to be safe.
 
Thanks to Tracy, we now have some idea = of the delta=20 T for air using the thick radiators. The one remaining assumption is the = ratio=20 Vi/Vo. It would be great if Tracy could get some data on this. Even = without the=20 data you can look at a conservative range...say .5 to .8 so you at = least=20 know where you will start to have trouble. Adjust=20 accordingly.
 
It is simple math. How many people do = this? How=20 many people actually look at what the pressure drop in a given length of = hose is=20 at a certain mass flow of water? This is engineering = design.
 
Everything else is just = guessing.
 
If you do this at various flight = conditions and=20 size for the worst (reasonable) case. Then properly test and evaluate = the=20 system, You will not have a "marginal" design.
 
Using some simple math will = get you in=20 the ball park, to see if what you are considering is even reasonable. = Ground=20 testing can then be used to quantify things like pressure drop across = the=20 radiator for a given flow of air. Part of your go no go list on test=20 flight take-off is: have I got the required pressure ahead of the = radiator?=20 No....then abort the take off and regroup. Don't just take off and pray. = Do your=20 flight test on a cold day. Have a spray bar just in case. Expand your = flight=20 envelope into higher power, higher OAT and lower speed. Fix = problems as=20 necessary. Don't conclude flight test until you = have explored the=20 whole flight/operational envelope and are satisfied with the=20 results. 
 
What is the worst reasonable = case?
 
Some would say it is full power climb = at VX on a=20 hot day.
 
I don't feel that way. I prefer a spray = bar and a=20 properly sized radiator for where the airplane spends it's = time.
 
The spray bar is only used in full = throttle climb=20 at Vx on a very hot day with an already hot engine (a safety feature to = get you=20 out of trouble which should be avoided operationally). Or while idling = on the=20 ground for extended periods of time at 100F OAT. How often do you = do these=20 things? What percentage of flying is this? If you design a properly = sized system=20 for these conditions it will be 3X larger than necessary for normal = operations. The type of heat exchanger needed is entirely different = than=20 the dense high pressure drop type needed for cruise. It is a large area = thin=20 radiator with low pressure drop. You will need an electric fan to make = even this=20 work for extended periods on the ground, especially downwind = taxi.
 
During cruise you will have to accept a = massive=20 drag penalty for perceived safety. I say perceived, because a properly = sized and=20 ducted thick radiator will work just fine once you are over 100 mph or = so. You=20 just can't climb at full throttle at 60 mph all day and expect it = to work.=20 The large thin radiator greatly complicates the aerodynamic = ducting, and=20 packaging design. You can't escape the Q=3DmdotCpDT equation. You = just went=20 from DT=3D125 for the thick rad to DT=3D50 or less for the = thin one. Now=20 you need a much greater Mdot air for the same Q. So you need a huge = inlet, an=20 even larger exit. If you use the thin giant radiator and an = inlet that=20 is too small, it will not cool as well as the thick radiator with the = same=20 inlet. Assuming you do get the inlet and outlet sized = properly, =20 you are flying around with a barn door attached to your airplane = all so=20 you  can amaze the crowds by flying around in slow flight at = full=20 throttle on a 100 deg day without the use of a spray bar. No real = gain in=20 safety.
 
We are all design engineers now. At = some=20 point we must decide what we are designing, an airplane or a = tow=20 truck.
 
Do whatever makes you = happy.
 
Monty 
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