In a message dated 2/18/2005 12:47:33 PM Central Standard Time, [EMAIL PROTECTED] writes: D(N) = [ W(N)/L(m) ]^2 / [Pi*q(kg/m*s^2)],
(note, that W = m*g (N = kg*m/s^2)), therefore for the units: N = [N^2/m^2]/[kg/m*s^2] = N^2*s^2/kg*m = N^2/N = N So nothing is missing, there is no average chord there. ****** Oleg, OK, we've got a nomenclature difference (it's always the communication issues, isn't it?) My standard usage is W/L as "Wing Loading" which is Weight / Area = Weight / (Span * Cavg) I believe your W/L refers to the Weight (W) divided by the Span (L) (- which I usually call "S"). Hopefully this clarifies things. If you multiply the W/L term by Cavg/Cavg you get Cavg * Weight / (L*Cavg) = Cavg * Weight / Area and things are consistent. Works the same in english, cgs, mks, etc. Sorry for the digression but with this sorted out, the result is mathematically the same. The reason I prefer to keep it in the "wing loading" form is that the scaling is a bit more apparent (to me). The lift force equation relates these two as: Weight/Area (wing loading) = q * Cl (since Lift has to equal Weight for steady flight) Since q contains the V^2 term, Wing loading is proportional to V^2. - assuming Cl stays about the same. The derived Drag formula then scales as W^2 / L^2 / V^2 ~ Cavg^2 * Wing Loading)^2 / V^2 or Drag (induced) scales as Cavg^2 * (Wing Loading) For a fixed span, as aspect ratio increases, the wing loading does not go up as fast as Cavg goes down so Cavg wins. For a fixed Cavg, as aspect ratio increases, wing loading will go up so drag should increase (??) ***** I don't see how increasing AR changes W/L. It is constant unless you want to account for mass changes due to AR changes, which is true in reality, but again for a fixed span the weight of the wing is usually proportional to the wing area, therefore W/L will actually decrease for higher AR (smaller wing area). ***** Again, this really depends on the assumptions one makes when doing an analysis. This was discussed most recently in RCSD, I believe the Jan 2004 or Feb 2004 issues for a 2M design (server seems to be down right now but available as PDF files form the general site: _http://www.b2streamlines.com/RCSD.html_ (http://www.b2streamlines.com/RCSD.html) .) My assumption is that you have to look at the entire package. That means keeping a constant weight for the fuselage and radio equipment, and scaling the wing and tail surfaces for aspect ratio (constant volume coefficients are good enough for the stabs). If you weigh all of those components, you'll usually find that the wing is less than half of the total aircraft weight (take out the servos and it's even less). So as you scale down the wing area, the weight of the whole aircraft doesn't come down linearly with the wing area. Since those analyses were for a fixed class (2M, for instance), as you change Aspect Ratio, you reduce the average chord at a fixed wingspan. Although the wing weight scales down with area, the weight of the fuselage and ancillary equipment stays relatively constant. Thus the wing loading will go up. If you let the span be unconstrained then the options are much more flexible. But when you get to the open class design, the improved efficiency from the higher achievable aspect ratios (at a reasonable wing loading) set them quite a bit apart from 2M. Here it's the aspect ratio and loading effects that dominate, not Re. So bigger flies better, but not strictly due to Re effects. **** Meanwhile, I hope the science fair project is going OK! Thanks - and sorry for the confusion on the terminology. - Dave R RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
