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<P>Yes, you deleted many post without reading. I erased most but kept one because I need more time to read it and digest it. This is by Jerry Budd. </P>
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<P>Here is a copy. Happy reading:</P>
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<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">>Angle of attack, alpha or AOA, for the<BR>> aircraft is the difference<BR>> between the flight path angle, gamma, and the aircraft<BR>> attitude, theta<BR>> (assuming the airfoil zero lift angle is essentially<BR>> aligned with the aircraft<BR>> reference datum, which for all practical purposes on our<BR>> designs, it is).<BR>> Reference: </SPAN><SPAN id=OBJ_PREFIX_DWT544><SPAN class=object2><SPAN style="FONT-FAMILY: 'Arial','sans-serif'"><A href="http://www.aerospaceweb.org/question/aerodynamics/q0165.shtml" target=_blank>http://www.aerospaceweb.org/question/aerodynamics/q0165.shtml</A></SPAN></SPAN></SPAN><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">.<BR>> Flight path angle is the aircraft trajectory, or in<BR>> pattern speak,<BR>> "track" but along the pitch axis, i.e. climbing<BR>> or descending.<BR>> In level, steady state flight (~1-g), the flight path<BR>> angle is zero.<BR>> Which means that the angle of attack is equal to the<BR>> aircraft pitch<BR>> attitude. If you run the numbers using a reasonable<BR>> airfoil lift curve<BR>> slope at a representative level flight speed for our planes<BR>> you'll find that<BR>> our planes trim out around ~ 0.5 degrees alpha (Lift =<BR>> Weight = CL * Qbar * S<BR>> where CL is the lift coefficient, Qbar is the dynamic<BR>> pressure, and S is the<BR>> reference wing area). This is because of our<BR>> extremely low wing loading,<BR>> it simply doesn't take a lot of angle of attack to<BR>> generate 1-g of lift when<BR>> your airplane only weighs 10 or 11 lbs. Here's<BR>> the other part: with the<BR>> exception of velocity (or airspeed), the equation is<BR>> linear, which means that<BR>> if you double the aircraft weight, for the same flight<BR>> speed, you get twice the<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">alpha (again, for 1-g trim). Or if you kept the<BR>> weight at say 10 lbs, but<BR>> cut the wing area in half, the angle of attack would then<BR>> double to ~ 1 degree<BR>> (again, for level, steady, 1-g flight). Velocity is a<BR>> little trickier to<BR>> account for because it's a non-linear second order<BR>> function in the lift<BR>> equation (remember Qbar? Reference: </SPAN><SPAN id=OBJ_PREFIX_DWT545><SPAN class=object2><SPAN style="FONT-FAMILY: 'Arial','sans-serif'"><A href="http://en.wikipedia.org/wiki/Dynamic_pressure" target=_blank><FONT color=#00008b>http://en.wikipedia.org/wiki/Dynamic_pressure</FONT></A></SPAN></SPAN></SPAN><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">,<BR>> Qbar = 1/2 * rho * V**2) where rho is air density and V is<BR>> the velocity).<BR>> Essentially, the lift doubles every time we increase<BR>> Qbar by 2, or<BR>> velocity by the square root of 2 (or 1.414). So if<BR>> you're flying along in<BR>> level flight at 60 mph (88 feet per second), and you speed<BR>> up to 85 mph mph<BR>> (124.45 feet per second), you've doubled your dynamic<BR>> pressure (Qbar) and to<BR>> stay at level 1-g flight, you'd have to retrim your<BR>> plane in pitch to 1/2 of<BR>> what your AOA was before (or you'll start climbing).<BR>> In this case the AOA<BR>> would be ~ 0.25 degrees (as would the pitch attitude).<BR>> One last bit of<BR>> info for the point I'm about to make is that the lift<BR>> curve slope for our<BR>> airfoils at the Reynolds Numbers we are operating at is<BR>> linear out to around<BR>> ~15-16 degrees alpha, with separation of lift occurring<BR>> above that, closer to<BR>> 18-20 degrees alpha (Reference: </SPAN><SPAN id=OBJ_PREFIX_DWT546><SPAN class=object2><SPAN style="FONT-FAMILY: 'Arial','sans-serif'"><A href="http://www.aerospaceweb.org/question/aerodynamics/q0136.shtml" target=_blank><FONT color=#00008b>http://www.aerospaceweb.org/question/aerodynamics/q0136.shtml</FONT></A></SPAN></SPAN></SPAN><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">).<BR>> Note that the onset of separation is independent of<BR>> airspeed, it's purely<BR>> a flow angle phenomenon, i.e. you can stall an airfoil at<BR>> any airspeed, or<o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">attitude (if you can get to a high enough angle of<BR>> attack). That's<BR>> essentially what Chris is saying below. <BR>>So what does this all mean? <BR>> <BR>> If you look at the numbers, for our<BR>> planes, you can't get to<BR>> stall from steady level flight (at any reasonable cruise<BR>> speed) without<BR>> inducing a significant G-loading on the aircraft (which<BR>> will cause a<BR>> noticeable, and very observable, change in the flight path<BR>> angle), BEFORE the<BR>> airfoil stalls. <BR>> > Here's some more numbers to help<BR>> you connect the dots:<BR>> from level 1-g trim flight at 1/2 degree AOA,<o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">for our models), you<BR>> have to generate nearly 5-6 g's to reach stall (think<BR>> you'd notice that?).<BR>> Don't believe me? OK answer this: Have you<BR>> ever quickly but<BR>> smoothly from level upright 1-g flight at a normal cruise<BR>> speed input full aft<BR>> stick for a second or two and then release it but no<BR>> lateral input? What<BR>> happens? Unless something is grossly wrong with your<BR>> airplane you're<BR>> likely to see a rapid pitch up and a corresponding change<BR>> in flight path angle,<BR>> probably to something approaching a near vertical attitude,<BR>> but not much else.<BR>> Why? Our planes are so lightly loaded that only<BR>> at spin entry and<BR>> landing speeds can we induce enough angle of attack to<BR>> approach stall on the<BR>> airfoil on the plane without inducing significant g's<BR>> and grossly altering the<BR>> flight path angle (and flight path angle is what we really<BR>> see when we're<BR>> flying at cruise speeds BTW, not so much the pitch attitude<BR>> until we're at much<BR>> lower speeds. That's because we mentally<BR>> integrate the velocity vector in<BR>> our minds but that's a topic for another time/day).<BR>> Full scale aerobatic<BR>> planes (and to a lessor degree IMAC planes) don't<BR>> suffer this problem nearly so<BR>> badly since their wing loadings and inertia's are much<BR>> higher (dynamic stability<o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">is somewhat more complex than static stability so I'm<BR>> not going to go much<BR>> further than this on this topic). The bottom line is<BR>> this: we're<BR>> not stalling the wing when we do our snaps, not even a<BR>> portion of it (unless<BR>> you're VERY low on airspeed at entry such as a spin).<BR>> We're too lightly<BR>> loaded to get to stall at any reasonable airspeed, the<BR>> airplane will respond<BR>> too quickly in the pitch axis resulting in a rapid change<BR>> in flight path<BR>> angle, effectively unloading the AOA during the<BR>> response. <BR>> So what are we doing to make the plane<BR>> present what appears<BR>> to be a snap roll when we can't actually be stalling<BR>> the wing asymmetrically to<BR>> induce autorotation like many claim? Lots of control<BR>> power in pitch and<BR>> roll coupled with additional rolling moment induced by<BR>> dihedral effect<BR>> (sideslip driven by rudder input). Pretty much<BR>> everyone knows that at<BR>> higher AOA you can command/control roll with rudder, well<o:p></o:p></SPAN></P>
<P class=MsoNormal style="MARGIN: 0in 0in 0pt"><SPAN style="COLOR: black; FONT-FAMILY: 'Arial','sans-serif'">that's due to<BR>> dihedral effect (roll with rudder), it gets more powerful<BR>> with a little AOA.<BR>> That's where you get the part of the dynamic that<BR>> visually emulates a<BR>> full scale snap roll but physically is quite different (you<BR>> can make it look<BR>> like a full-scale snap, but it really isn't). <BR>> <BR>> So the bigger question is should<BR>> emulating a full-scale snap<BR>> roll be a pattern judging criterion or do we even care? (we<BR>> know what the<BR>> answer is for full scale aerobatics and probably IMAC too<BR>> but we are neither of<BR>> these). Until we decide the answer to THAT question,<BR>> we're really just<BR>> debating "how many angels can dance on the head of a<BR>> pin"... <BR>></SPAN></P>
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<P><BR><BR>Vicente "Vince" Bortone<BR><BR>----- Original Message -----<BR>From: "J Shu" <jshulman@cfl.rr.com><BR>To: "General pattern discussion" <nsrca-discussion@lists.nsrca.org><BR>Sent: Thursday, October 15, 2009 1:46:52 PM GMT -06:00 US/Canada Central<BR>Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)<BR><BR>
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<DIV><FONT face=Arial color=#000080>But they left NO doubt about pitch break... isn't that what everyone needs to see, or did I delete too many posts without reading them?</FONT></DIV>
<DIV><BR>Regards,<BR>Jason<BR><A href="http://www.shulmanaviation.com/" target=_blank>www.shulmanaviation.com</A><BR><A href="http://www.composite-arf.com/" target=_blank>www.composite-arf.com</A><BR></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B> <A title=vicenterc@comcast.net href="mailto:vicenterc@comcast.net" target=_blank>Vicente "Vince" Bortone</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=nsrca-discussion@lists.nsrca.org href="mailto:nsrca-discussion@lists.nsrca.org" target=_blank>General pattern discussion</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, October 15, 2009 2:42 PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)</DIV>
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<DIV style="FONT-SIZE: 12pt; COLOR: #000000; FONT-FAMILY: Arial">and SEBANDREW SNAP for the not normal one.<BR><BR>Vicente "Vince" Bortone<BR><BR>----- Original Message -----<BR>From: "Vicente \"Vince\" Bortone" <<A href="mailto:vicenterc@comcast.net" target=_blank>vicenterc@comcast.net</A>><BR>To: "General pattern discussion" <<A href="mailto:nsrca-discussion@lists.nsrca.org" target=_blank>nsrca-discussion@lists.nsrca.org</A>><BR>Sent: Thursday, October 15, 2009 1:40:03 PM GMT -06:00 US/Canada Central<BR>Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)<BR><BR>
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<DIV style="FONT-SIZE: 12pt; COLOR: #000000; FONT-FAMILY: Arial">Let's called NORMAL SNAP. <BR><BR>Vicente "Vince" Bortone<BR><BR>----- Original Message -----<BR>From: "J Shu" <jshulman@cfl.rr.com><BR>To: "General pattern discussion" <nsrca-discussion@lists.nsrca.org><BR>Sent: Thursday, October 15, 2009 11:45:13 AM GMT -06:00 US/Canada Central<BR>Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)<BR><BR>
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<DIV><FONT face=Arial color=#000080>I don't remember anyone saying they received a 0 at the Worlds. Andrew and Seba had the 'IMAC' pitch break, most of the the rest of us had the 'normal' snap.</FONT></DIV>
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<DIV><FONT face=Arial color=#000080>One thing I found funny was some of the other pilots that would 'show pitch break' would actually do mostly a roll afterwards. I guess if you 'show pitch break' the plane 'must be stalled' right...lol.</FONT></DIV>
<DIV><BR>Regards,<BR>Jason<BR><A href="http://www.shulmanaviation.com/" target=_blank>www.shulmanaviation.com</A><BR><A href="http://www.composite-arf.com/" target=_blank>www.composite-arf.com</A><BR></DIV>
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<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV style="BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: black"><B>From:</B> <A title=vicenterc@comcast.net href="mailto:vicenterc@comcast.net" target=_blank>Vicente "Vince" Bortone</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=nsrca-discussion@lists.nsrca.org href="mailto:nsrca-discussion@lists.nsrca.org" target=_blank>General pattern discussion</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, October 15, 2009 12:35 PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)</DIV>
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<P>Jason,</P>
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<P>It will be interesting to know at the WC in Portugal. I never got one in local contests this year. I don't think at local contest we are zeroing even when there is not a clear snap. At the Nats I zeroed some years ago. I never got a complain. I have been downgrading 3 points for not presenting the break in pitch trying to use the current snap description. <BR><BR>Vicente "Vince" Bortone<BR><BR>----- Original Message -----<BR>From: "J Shu" <jshulman@cfl.rr.com><BR>To: "General pattern discussion" <nsrca-discussion@lists.nsrca.org><BR>Sent: Thursday, October 15, 2009 11:17:46 AM GMT -06:00 US/Canada Central<BR>Subject: Re: [NSRCA-discussion] How I became an expert Snap Judge (TIC)<BR><BR>
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<DIV><FONT face=Arial color=#000080>Just curious... how many are getting zero'd for doing snaps?</FONT></DIV>
<DIV><BR>Regards,<BR>Jason<BR><A href="http://www.shulmanaviation.com/" target=_blank>www.shulmanaviation.com</A><BR><A href="http://www.composite-arf.com/" target=_blank>www.composite-arf.com</A></DIV><BR>_______________________________________________ NSRCA-discussion mailing list NSRCA-discussion@lists.nsrca.org http://lists.nsrca.org/mailman/listinfo/nsrca-discussion</DIV></DIV>
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