Mat That was a great explanation for a layman like myself, thanks very much for taking the trouble. As a matter of interest I have found out that our C.A.A has set the rear limit for the RAF48 at 28% chord. regards Dene Collett KR2S-RT builder Port Elizabeth South Africa mailto: [email protected] P.S: checkout www.whisperaircraft.com
----- Original Message ----- From: "Matthew Elder" <[email protected]> To: "KRnet" <[email protected]> Sent: Friday, January 21, 2005 12:46 AM Subject: Re: KR> AS 504X CofG > Actually it's the other way around. > > The designer of the airfoil comes up with (emperically usually) > a point known as the center of lift. The aircraft designer uses > that information based on the mission of the aircraft. > > Assume that we are talking about conventional aircraft here (big > wing in front, little wing in back) > > For most GA aircraft, the idea is to keep the CG of the aicraft > ahead of the center of lift (nose heavy). This creates a > pitching moment that pushes the nose of the aircraft towards the > ground. The designer then sizes the tail accordingly to have > enough force to overcome that pitching moment. IF he gets it > right, the aircraft will be stable. > > If the center of lift and the center of gravity are coincident, > the tail plane has to apply zero force to keep the aircraft > level... however, this is akin to balancing a ball bearing on an > up-side-down bowl or other convex surface.... > > The nose heavy condition will make the aircraft, in general, > more stable, slower in cruise (due to more induced drag because > the wing is carring more effective weight) and make landing > distances longer. Stall speed will also usually increase, but > with more of a tendancy to mush (again, in general). Controls > will feel heavier as well. Yes, there is such as thing as too > nose heavy. This is where the tail plane simply cannot apply > the needed down force required to overcome the pitching moment > created by the wing. Very bad when you are 3 feet off the > runway and slow... > > The more towards the tail heavy end you go, up to the > coincindence point, the more touchy (sometimes that can be fun) > the aircraft becomes, and the faster it will fly (less induced > drag). Stall speed will likely decrease, but it may make > recovery longer. Control feel gets lighter as well. If you go > too far aft, well lets just say that's not a good idea. Anyone > that has flown a tail heavy aircraft, real, model or otherwise, > knows what I'm talking about. > > I know that I have grossly generalized some information, but > there are many good books on this subject alone if you want/need > to get into the nitty-gritty. > > Hope this helps, > Matt > > > > > > --- "Dene Collett (SA)" <[email protected]> wrote: > > > Hi guys > > Thanks to everyone that tried to stear me in the right > > direction regarding > > CG location. I have really very limited knowledge of > > aerodynamics and have a > > few questions of how the CG is determined. The way I see it is > > this : After > > the "designer" has finished designing his aerofoil, he does > > some more > > calculations or some testing (wind tunnel?) to determine the > > CG range that > > the aerofoil is happy at regardless of what plane it will > > ultimately be used > > on. Then again I suppose that would depend on the amoumt of > > load the wing is > > expected to carry and the speed it is expected to carry it at. > > So if I stick to an ultimate aft limit of 30% MAC I should be > > OK??? By the > > way, what are the published figures that everyone talks about. > > I only have > > the remains of a very old manual and I guess that bit of info > > is missing. if > > it wasn't for two friends who had built three KRs between > > them, I might > > never have got this far. > > Mark, the tube is for the mains and yes it is a retract I'm > > afraid (all > > three wheels). I am basing the system on a design by Stelio > > Frati called the > > "Pinguino". I figured it took me nine years to get this far, I > > might as well > > try to produce something really unusual as well as not make it > > any heavier > > than the fixed gear. > > Thanks again guys. > > Cheers > > Dene Collett > > KR2S-RT builder > > Port Elizabeth > > South Africa > > mailto: [email protected] > > P.S: checkout www.whisperaircraft.com > > > > > > > > _______________________________________ > > Search the KRnet Archives at > > http://www.maddyhome.com/krsrch/index.jsp > > to UNsubscribe from KRnet, send a message to > > [email protected] > > please see other KRnet info at http://www.krnet.org/info.html > > > > ===== > ------------------------------------------------- > Matthew Elder > Orangeburg, SC > http://www.infinigral.com/melder > > _______________________________________ > Search the KRnet Archives at http://www.maddyhome.com/krsrch/index.jsp > to UNsubscribe from KRnet, send a message to [email protected] > please see other KRnet info at http://www.krnet.org/info.html >
