X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sat, 16 Feb 2008 19:51:51 -0500 Message-ID: X-Original-Return-Path: Received: from [64.12.143.99] (HELO imo-m11.mail.aol.com) by logan.com (CommuniGate Pro SMTP 5.2.0) with ESMTP id 2731651 for lml@lancaironline.net; Fri, 15 Feb 2008 17:40:04 -0500 Received-SPF: pass receiver=logan.com; client-ip=64.12.143.99; envelope-from=Sky2high@aol.com Received: from Sky2high@aol.com by imo-m11.mx.aol.com (mail_out_v38_r9.3.) id q.c26.2bb6daac (42809) for ; Fri, 15 Feb 2008 17:39:18 -0500 (EST) From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Fri, 15 Feb 2008 17:39:18 EST Subject: Re: [LML] Cold Induction, Power, and Speed X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1203115158" X-Mailer: Unknown sub 34 X-Spam-Flag: NO -------------------------------1203115158 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en Fred, =20 There is a free lunch - or at least a cheap one. Drag is an interesting=20 component. Those that fly rivet-bumped, strut-braced, fixed gear airplanes= with=20 poorly designed cowls and bad plenum seals are forever stuck in a world tha= t=20 just doesn't understand relative drag and its significance. In a laminar=20 flow airplane with very low form and induced drag, even something that used= to=20 be thought of as a small drag component soon looms as a large component. =20 Fixing the cooling drag in an already draggy airplane may buy little perfor= mance=20 but in an airplane where the only drag is from cooling, any improvement wil= l=20 have great payback. =20 So, let's take just one example. I saw a 6-8 knot indicated speed increase= =20 at cruise speeds of about 175 KIAS (depends on atmospheric conditions) by=20 sealing all control surface and flap gaps. Since 6/175 =3D 3.4%, the resul= t was a=20 conservative 3.4% speed improvement. From your argument, that would mean=20= a=20 10% reduction in drag. Pretty impressive for just $110. =20 =20 Gap seals on a spam can would not result in anywhere near such a performanc= e=20 improvement because of all the other drag. In other words, you are right=20 that a small % change in drag would yield an even smaller % change in=20 performance. You are wrong to claim there is no free lunch. The gap seal=20 modification was my most expensive drag reducer. There are so many others=20= that are free=20 and cooling drag is one of them. =20 Here's another on the power side. Suppose ram air adds 2" to the MAP (its=20 free also and this discussion will keep the speed around 200 Knots). If pow= er=20 is approximated by RPM x MAP, holding RPM constant and increasing MAP from =20 say 25" to 27" (2/25 =3D 8%), then an 8% improvement in power is almost a 3%= =20 increase in speed by your estimation. The 200 knot airplane now goes about= 205.5=20 Knots, certainly enough to win a race against a similarly equipped =20 challenger with some flow restricting filter on a leaky induction system pi= ckup. =20 Combining power gains with drag reductions results in significantly greater= =20 performance. =20 No Free Lunch indeed. Balderdash! =20 Grayhawk =20 In a message dated 2/15/2008 10:17:40 A.M. Central Standard Time, =20 fredmoreno@optusnet.com.au writes: =20 =20 =E2=80=9CIncrease the power, reduce the drag and the limit is raised but it= is in no=20 way linear past 200 knots . It is very exponential.=E2=80=9D=20 Not linear at all, in fact. At these speeds in the thick air we customaril= y=20 fly in with aspirated airplanes and with the weight and aspect ratio of the= =20 wings, the total drag is almost entirely parasitic drag. Only a =20 vanishingly small amount of induced drag (drag arising from lift) occurs at=20= the speeds=20 discussed. =20 This means that to a very good approximation, with a propeller airplane,=20 power (shaft horsepower) goes as the cube of speed. =20 So it is cubic curve. =20 However=E2=80=A6. One can linearize the curve for small deviations without=20= incurring=20 much of an error (mathematically throw away the third order and higher=20 terms) to arrive at a simplification for constant conditions (same drag=20 coefficient, same flight conditions) =E2=80=93 =20 Which is: =20 For a 1% increase in speed, you need 3% more power.=20 For a 2% increase in speed, you need 6% more power=20 For n% more speed, you need 3n% more power for small n (say less than 10%)= =20 Test - for 10% more speed, the simplification yields 30% more power. =20 Compare this to 1.1 cubed which is 1.331, or 33.1% more power. So the simp= le =20 approximation is not bad. =20 It is MUCH BETTER to reduce drag than increase power. Unfortunately, for=20 constant power, to get a 1% speed increase requires a 3% drag coefficient=20 decrease, and 2% speed increase requires 6% drag coefficient decrease etc.=20= etc. =20 (another linear simplification for small changes).=20 No free lunch. =20 Fred Moreno=20 AKA Captain Tuna, Chicken of the Skies=20 **************The year's hottest artists on the red carpet at the Grammy=20 Awards. Go to AOL Music. =20 (http://music.aol.com/grammys?NCID=3Daolcmp00300000002565) -------------------------------1203115158 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Content-Language: en
Fred,
 
There is a free lunch - or at least a cheap one.  Drag is an=20 interesting component.  Those that fly rivet-bumped, strut-braced, fixe= d=20 gear airplanes with poorly designed cowls and bad plenum seals are forever s= tuck=20 in a world that just doesn't understand relative drag and its=20 significance.  In a laminar flow airplane with very low = form=20 and induced drag, even something that used to be thought of as a small drag=20 component soon looms as a large component.  Fixing the cooling dra= g in=20 an already draggy airplane may buy little performance but in an airplane whe= re=20 the only drag is from cooling, any improvement will have great payback.
 
So, let's take just one example.  I saw a 6-8 knot indic= ated=20 speed increase at cruise speeds of about 175 KIAS (depends=20 on atmospheric conditions) by sealing all control surface and flap= =20 gaps.  Since 6/175 =3D 3.4%, the result was a=20 conservative 3.4% speed improvement.  From your argument,=20 that would mean a 10% reduction in drag.  Pretty impressive f= or=20 just $110. 
 
Gap seals on a spam can would not result in anywhere near such a=20 performance improvement because of all the other drag.  In other words,= you=20 are right that a small % change in drag would yield an even smaller %=20 change in performance.  You are wrong to claim there is no free=20 lunch.  The gap seal modification was my most expensive drag reducer.&n= bsp;=20 There are so many others that are free and cooling drag is one of them.
 
Here's another on the power side.  Suppose ram air adds 2" to= the=20 MAP (its free also and this discussion will keep the speed around 200 Knots)= . If=20 power is approximated by RPM x MAP, holding RPM constant and increasing MAP=20= from=20 say 25" to 27" (2/25 =3D 8%), then an 8% improvement in power is almost a 3%= =20 increase in speed by your estimation.  The 200 knot airplane now g= oes=20 about 205.5 Knots, certainly enough to win a race against a similarly equipp= ed=20 challenger with some flow restricting filter on a leaky induction system=20 pickup.
 
Combining power gains with drag reductions results in=20 significantly greater performance.
 
No Free Lunch indeed.  Balderdash!
 
Grayhawk
 
In a message dated 2/15/2008 10:17:40 A.M. Central Standard Time,=20 fredmoreno@optusnet.com.au writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>

Not line= ar at=20 all, in fact.  At these speeds in the thick air we customarily fly in= =20 with aspirated airplanes and with the weight and aspect ratio of the wings= ,=20 the total drag is almost entirely parasitic drag.   Only a=20 vanishingly small amount of induced drag (drag arising from lift) occurs a= t=20 the speeds discussed. 

This mea= ns=20 that to a very good approximation, with a propeller airplane, power (shaft= =20 horsepower) goes as the cube of speed.

So it is= cubic=20 curve.

However= =E2=80=A6. One=20 can linearize the curve for small deviations without incurring much of an=20 error (mathematically throw away the third order and higher terms) to arri= ve=20 at a simplification for constant conditions (same drag coefficient, same=20 flight conditions) =E2=80=93

Which is= :=20

For a 1%= =20 increase in speed, you need 3% more power.

For a 2%= =20 increase in speed, you need 6% more power

For n% m= ore=20 speed, you need 3n% more power for small n (say less than=20 10%)

Test - f= or 10%=20 more speed, the simplification yields 30% more power.  Compare this t= o=20 1.1 cubed which is 1.331, or 33.1% more power.  So the simple=20 approximation is not bad.

It is MU= CH=20 BETTER to reduce drag than increase power.  Unfortunately, for consta= nt=20 power, to get a 1% speed increase requires a 3% drag coefficient decrease,= and=20 2% speed increase requires 6% drag coefficient decrease etc. etc. =20 (another linear simplification for small changes).<= /P>

No free=20= lunch.=20

Fred=20 Moreno

AKA Capt= ain=20 Tuna, Chicken of the Skies

<= /FONT> 




The= year's hottest artists on the red carpet at the Grammy Awards. AOL Mu= sic takes you there.
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