Strength of that spar... Assumptions: _____________________ Basswood (failure stress 4.7 ksi - Spruce will take 6.0 ksi so would be about 25% stronger.) 10" Root Chord 10% thickness 99" Span H11 tool steel rod (stress to yield 239 ksi) Ignoring any added strength from D-box construction ======================================== Disclaimer: 1) These calculations are VERY sensitive to the height of the spar. Taller is better and shorter gets worse, fast. 2) I have yet to get these these calculations checked by a Mechanical Engineer ========================================
So... The spar will fail at about 14 lbs of pull on the towhook (with the wing pulling 14 lbs of lift) The wing rod will take a permanent set at about 20 lbs of pull on the towhook (with the wing generating that much lift as well) Width is better than height on the spar caps. 3/8"w x 1/4"h will increase the wing failure stress to about 20.8 lbs whereas 1/4"w x 3/8"h gets you to 15.6 lbs. If you cap the spars with 0.007 x 0.25" carbon the failure stress goes to 37.6 lbs. (for the carbon only - the carbon is much more stiff than the wood and the wood will not do much besides hold the carbon straight). Half inch wide will take the pull-to-failure to twice that above. You would then be advised to wrap the spar 6-8 times per inch with strong thread or Kevlar tow, etc. A carbon wing rod of the same size takes you to 22 lbs, but that is to failure, not yield. Any increase in diameter gets you a tremendous increase in allowable stress. Doubling the diameter gets you an eight fold increase in strength. A 3/8" tool steel rod will take the allowable stress to 100 lbs. Half inch to 237 lbs! Darn, I love spreadsheets. Materials will usually fail in compression and/or buckling of the top spar cap so the covering is of no real use for strength in spar calculation. A D-tube of balsa will add some if you make the spar of wood, but almost nothing if you use carbon. It still helps in torsional stiffness (preventing flutter) and in preserving the airfoil. Check out the wonderful stuff that Dr. Drela has put together about spars http://www.charlesriverrc.org/articles/allegro2m/markdrela_allegro2m.htm There are tips on building "Winch Proof" Spars and some basic calcs that are very close to the numbers that I have come up with, except I use 270ksi for carbon failure from data I have seen in different sources. I add a suitable safety factor, anyway. Anybody have data on music wire? . Tom Koszuta Clarence Sailplane Society (Buffalo) NY ----- Original Message ----- From: "Ed Jett" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]>; "RCSE" <[EMAIL PROTECTED]> Sent: Saturday, March 06, 2004 9:41 PM Subject: [RCSE] Aquila > Ok guys. I got an uncontrollable urge to build a 'new' sailplane today. I > wrecked my BOT ARF about 3 weeks ago and have decided instead of just buying > a new one to build something instead. > > I have tenatively decided to build an Aquila (standard class). I was mildly > suprised to find that the kit (yeah, I have an original Airtronics kit) uses > 1/4" spars. > > I know some of you fellas have some experience with these. Do I need to > upgrade spars to 3/8"? or go with some CF? What about the 0.218" dia. stock > wingrod? > > I fly my sailplanes off of a highstart. I feel like it will be strong enough > for that, but I'm a little concerned with the strength. > > Any advice is appreciated. > > Ed Jett > > > 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. 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.
