X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from ispmxmta05-srv.alltel.net ([166.102.165.166] verified) by logan.com (CommuniGate Pro SMTP 5.0.4) with ESMTP id 891475 for flyrotary@lancaironline.net; Wed, 21 Dec 2005 14:18:26 -0500 Received-SPF: pass receiver=logan.com; client-ip=166.102.165.166; envelope-from=montyr2157@alltel.net Received: from Thorstwin ([4.227.98.15]) by ispmxmta05-srv.alltel.net with SMTP id <20051221191738.VZOW8731.ispmxmta05-srv.alltel.net@Thorstwin> for ; Wed, 21 Dec 2005 13:17:38 -0600 Message-ID: <000001c60663$36d0a450$0f62e304@Thorstwin> From: "Monty Roberts" To: Subject: TAS vs IAS Date: Wed, 21 Dec 2005 13:17:24 -0600 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0005_01C60630.E1B266C0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2180 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 This is a multi-part message in MIME format. ------=_NextPart_000_0005_01C60630.E1B266C0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Interesting, Monty. My read was since IAS is a reflection of "q" or dynamic pressure which = is the pressure factor responsible for shoving air molecules through the = cores - that you would want to compare cooling at the same IAS.=20 From our own Al Gietzen http://www.tvbf.org/archives/velocity/msg02818.html The pressure we have to work with is limited to the dynamic head. And = if air isn't treated right in the ducting it will form back eddies and = pressure waves, and find lots of ways to give you less flow than you calculate = from your intake area. Core thickness can be traded for x-sectional area = only if the ducting is designed to get the air slowed and through it. =20 http://www.vansaircraft.com/pdf/hp_limts.pdf Because the airspeed indicator is The Gauge That Lies. Despite its name, an airspeed indicator does not measure speed. It measures "q" - dynamic pressure caused by packing air molecules into a tube. http://duxford.iwm.org.uk/upload/pdf/Instrumentation.pdf=20 Airspeed The airspeed is directly related to dynamic pressure. To find out what = the Dynamic Pressure is, Static Pressure (the pressure of the air surrounding the = aircraft) is subtracted from the Total Pressure, which is the force of the air = impacting with the aircraft (this is measured using a pitot tube which protrudes from the = aircraft to meet the oncoming airflow directly). So it would seem to me for apples and apples you would want to compare = your cooling at different altitudes at the same dynamic pressure (or = IAS) in order to isolate the effects of ambient temperature on cooling. = But, then I've been wrong before {:>) Ed A Ed, Q is indeed Q at any altitude and speed, which makes it very handy for = structural calculations. It involves density, which involves Temp and = pressure. So using Q alows you to eleminate these variables from your = calculation. That is fine for structural loads. It is not fine for thermodynamic calculations. What is important here is = how fast you are actually traveling through the fluid. In the jet world = all calculations are bassed on To which is the stagnation temperature = that you get if you decellerate a molecule adiabaticly to rest from free = stream velocity. We don't have to worry so much with compressibility and = stagnation temp at our speeds, but we still have to have the TAS to = calculate inlet efficiencies and sizes. We cannot just throw out density = (pressure and temperature) since they are intimitely related to what we = need to do here. TAS is free stream or Vo. What goes through your inlet gives you Vi/Vo = or Vinlet/Vfree stream. typically .6-1 depending on what you are doing = on the other side of the inlet. If the inlet is perfectly sized to the task, the capture area will = ingest the proper amount of mass flow at Vo (TAS) for CpDt to carry away = our cooling load WITHOUT DIFFUSION. Now that you have the air in the = airplane you use a diffuser to slow it down and raise the pressure in a = controlled manner to the point that it will cancel the pressure drop in = your heat exchanger (or better yet overcome your exit nozzle). As the = air flows through it picks up heat and expands a little. If your cooler = is the proper size and relatively efficient your pressure on the back = side of the cooler will still be greater than ambient and you can = accellerate the cooling air in a nozzle to Po (ambient pressure) If you = did good you will get close to Vo. If you did real good you will get Vo+ = a little. This is where the thin vs thick argument comes in. = Theoretically if you slow the air way down and recover all the dynamic = pressure and pass it ever so slowly through a Mac truck sized radiator = that is 1/4 inch thick you will get very little pressure drop in the = cooler and all the stagnation pressure will all be available (plus a = little from heat addition) to squirt out through the exhaust nozzle. =20 ------=_NextPart_000_0005_01C60630.E1B266C0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Interesting, Monty.
 
My read was since IAS is a = reflection of "q"=20 or dynamic pressure which is the pressure factor responsible for shoving = air=20 molecules through the cores - that you would want to compare cooling at = the same=20 IAS.
 
From our own  = Al Gietzen

http://www.tvbf.org/archives/velocity/msg02818.html

The pressure we have to = work with is=20 limited to the dynamic head.  And if
air isn't treated right in = the=20 ducting it will form back eddies and pressure
waves, and find lots of = ways to=20 give you less flow than you calculate from
your intake area.  = Core=20 thickness can be traded for x-sectional area only if
the ducting is = designed=20 to get the air slowed and through it.

 

http://www.vansaircraft.com/pdf/hp_limts.pdf

Because the airspeed indicator is = The Gauge=20 That

Lies. Despite its name, an = airspeed indicator=20 does not

measure speed. It measures "q" =96 = dynamic=20 pressure

caused by packing air molecules = into a=20 tube.

 

http://duxford.iwm.org.uk/upload/pdf/Instrumentation.pdf<= /A> 

Airspeed

The airspeed is directly related = to dynamic=20 pressure. To find out what the Dynamic

Pressure is, Static Pressure (the = pressure of=20 the air surrounding the aircraft) is

subtracted from the Total = Pressure, which is=20 the force of the air impacting with the

aircraft (this is measured using a = pitot tube=20 which protrudes from the aircraft to meet

the oncoming airflow = directly).

So it would = seem to me for=20 apples and apples you would want to compare your cooling at different = altitudes=20 at the same dynamic pressure (or IAS) in order to isolate the effects of = ambient=20 temperature on cooling.  But, then I've been wrong before=20 {:>)

 

Ed = A

 

Ed,

Q is indeed Q at any altitude and = speed, which=20 makes it very handy for structural calculations. It involves density, = which=20 involves Temp and pressure. So using Q alows you to eleminate these = variables=20 from your calculation. That is fine for structural loads.

It is not fine for thermodynamic = calculations.=20 What is important here is how fast you are actually traveling through = the fluid.=20 In the jet world all calculations are bassed on To which is the = stagnation=20 temperature that you get if you decellerate a molecule adiabaticly to = rest from=20 free stream velocity. We don't have to worry so much with = compressibility and=20 stagnation temp at our speeds, but we still have to have the TAS to = calculate=20 inlet efficiencies and sizes. We cannot just throw out density (pressure = and=20 temperature) since they are intimitely related to what we need to do=20 here.

TAS is free stream or Vo. What goes = through your=20 inlet gives you Vi/Vo or Vinlet/Vfree stream. typically .6-1 depending = on what=20 you are doing on the other side of the inlet.

If the inlet is perfectly sized to = the task, the=20 capture area will ingest the proper amount of mass flow at Vo (TAS) for = CpDt to=20 carry away our cooling load WITHOUT DIFFUSION. = Now=20 that you have the air in the airplane you use a diffuser to slow it down = and=20 raise the pressure in a controlled manner to the point that it will = cancel the=20 pressure drop in your heat exchanger (or better yet overcome your exit = nozzle).=20 As the air flows through it picks up heat and expands a little. If = your=20 cooler is the proper size and relatively efficient your pressure on the = back=20 side of the cooler will still be greater than ambient and you=20 can accellerate the cooling air in a nozzle to Po = (ambient=20 pressure) If you did good you will get close to Vo. If you did real = good=20 you will get Vo+ a little. This is where the thin vs thick argument = comes=20 in. Theoretically if you slow the air way down and recover all the = dynamic=20 pressure and pass it ever so slowly through a Mac truck sized radiator = that=20 is 1/4 inch thick you will get very little pressure drop in = the cooler=20 and all the stagnation pressure will all be = available (plus=20 a little from heat addition) to squirt out through = the exhaust=20 nozzle.  

 

 

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