X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [24.25.9.103] (HELO ms-smtp-04-eri0.southeast.rr.com) by logan.com (CommuniGate Pro SMTP 5.0.4) with ESMTP id 891526 for flyrotary@lancaironline.net; Wed, 21 Dec 2005 15:17:02 -0500 Received-SPF: pass receiver=logan.com; client-ip=24.25.9.103; envelope-from=eanderson@carolina.rr.com Received: from edward2 (cpe-024-074-025-165.carolina.res.rr.com [24.74.25.165]) by ms-smtp-04-eri0.southeast.rr.com (8.12.10/8.12.7) with SMTP id jBLKGF1u005020 for ; Wed, 21 Dec 2005 15:16:15 -0500 (EST) Message-ID: <000801c6066b$68034570$2402a8c0@edward2> From: "Ed Anderson" To: "Rotary motors in aircraft" References: Subject: Re: [FlyRotary] TAS vs IAS Date: Wed, 21 Dec 2005 15:16:19 -0500 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0005_01C60641.7EE5D430" 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 X-Virus-Scanned: Symantec AntiVirus Scan Engine This is a multi-part message in MIME format. ------=_NextPart_000_0005_01C60641.7EE5D430 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Let me see if I've got this right. "q" is dynamic pressure, correct? = As best I recall the equation for dynamic pressure is 1/2pV^2 with p = being air density and V velocity.=20 So if I understand you correctly, it appears that even for IAS - the = case is we are using TAS (actual velocity of air molecules - or = aircraft) to generate the dynamic pressure in the pitot tube that gives = us an indication - we know as IAS? So I guess its correct to say IAS is = a reflection of "q" but the indication is actually a result of air = density and TAS, Correct?? I certainly understand your argument for TAS as being the speed of = the molecules and therefore the rate at which they carry away heat. = Its just I would have bet money that if you wanted to test the effects = of different ambient tempts on a core, you would ensured that they both = saw the same dynamic pressure. In order to do that, of course, at = higher altitude you would need a greater true airspeed (due to the lower = density) in order to get the same IAS (dynamic pressure) as you would = have at a lower altitude (denser air with less TAS). =20 But, you live and learn. Getting older and none the swifter {:>) Ed A ----- Original Message -----=20 From: Monty Roberts=20 To: Rotary motors in aircraft=20 Sent: Wednesday, December 21, 2005 2:17 PM Subject: [FlyRotary] TAS vs IAS 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_01C60641.7EE5D430 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
Let me see if I've got this right.  "q" is = dynamic=20 pressure, correct?  As best I recall the equation for dynamic = pressure=20 is  1/2pV^2 with p being air density and V = velocity. 
 
 So if I understand you correctly, it = appears that=20 even for IAS - the case is we are using TAS = (actual velocity=20 of air molecules - or aircraft) to generate the = dynamic pressure in=20 the pitot tube  that gives us an indication - we know as IAS?  = So I=20 guess its correct to say IAS is a reflection of "q" but the indication = is=20 actually a result of air density and TAS, Correct??
 
  I   certainly understand = your=20 argument for TAS as being the speed of the molecules and therefore the = rate at=20 which they carry away heat.    Its just I would have bet = money=20 that if you wanted to test the effects of different ambient tempts on a = core,=20 you would ensured that they both saw the same dynamic = pressure.   In=20 order to do that, of course, at higher altitude you would need a=20 greater true airspeed (due to the lower density) in order to = get the=20 same IAS (dynamic pressure) as you would have at a lower altitude = (denser air=20 with less TAS).
 
 
 But, you live and learn.
 
Getting older and none the swifter = {:>)
 
Ed A
 
----- Original Message -----
From:=20 Monty=20 Roberts
Sent: Wednesday, December 21, = 2005 2:17=20 PM
Subject: [FlyRotary] TAS vs = IAS

Interesting, = Monty.
 
My read was since IAS is a = reflection of=20 "q" or dynamic pressure which is the pressure factor responsible for = shoving=20 air molecules through the cores - that you would want to compare = cooling at=20 the same 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=20 to 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 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=20 indicator 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=20 of 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=20 tube 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=20 altitudes at the same dynamic pressure (or IAS) in order to isolate = the=20 effects of ambient temperature on cooling.  But, then I've been = wrong=20 before {:>)

 

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=20 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=20 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=20 the TAS to calculate inlet efficiencies and sizes. We cannot just = throw out=20 density (pressure and temperature) since they are intimitely related = to what=20 we need to do here.

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

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

 

 

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