---------------------------------------------------------- Zenith-List Digest Archive --- Total Messages Posted Thu 07/15/10: 5 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 07:03 AM - Re: Zenith-List Digest: 2 Msgs - 07/14/10 () 2. 08:47 AM - Re: Zenith-List Digest: 2 Msgs - 07/14/10 (Sabrina) 3. 09:40 AM - Sabrina's a/c & drag of unfaired struts (Peter Chapman) 4. 09:48 AM - Re: Sabrina's a/c & drag of unfaired struts (Craig Payne) 5. 10:29 AM - (4rcsimmons@comcast.net) ________________________________ Message 1 _____________________________________ Time: 07:03:37 AM PST US From: Subject: Zenith-List: Re: Zenith-List Digest: 2 Msgs - 07/14/10 Nice work Sabrina. I wouldn't have thought that the 180 sq inches of exposed tubing would have slowed down the aircraft so severely! Isn't that only about 5% increase in the exposed area? I'm not wanting to take the time to calculate the wetted area (and increased drag) but the speed reduction seems high to me. Oh well, guess that's why I became a structural rather than aeronautical engineer. Are you using any strain gauges to capture any movement of the wing or other structures? Once more though your photo shows real professionalism in your design. And congratulations on the IATA Industry Achievement Award and a CABAA scholarship to MIT. Scott _________________________________________ Scott Thatcher, Palm Beach Gardens, FL 601XL with WW Corvair, Registered as E-LSA N601EL, http://placestofly.com, http://eaa203.com, http://mykitlog.com/sdthatcher > Hey Guys! > > The Sabrina Mark 1 took to the air again in calm skies above Sugar Grove, > Illinois > this morning! ... ________________________________ Message 2 _____________________________________ Time: 08:47:46 AM PST US Subject: Zenith-List: Re: Zenith-List Digest: 2 Msgs - 07/14/10 From: "Sabrina" Thank you Scott... I have provisions for strain gauges, both a simple mechanical device that could be mounted between two internal portions of the struts as well as the types which would be mounted externally either as a layer or bridge. The later are fairly expensive and I will see if I can borrow some from MIT for my first weekend home in early October. I waited 324 days between the last two flights, I am in no hurry. I have five more weeks here in Chicago, so I will concentrate on the fairing design and build. NACA 0025 strut fairings are commercially available from Florida and Canada, but I am told motorcycle enthusiasts and float plane owners have had better success with NACA 67-025 fairings. Is there any first hand knowledge out there on this list? Does the NACA 0025 fairing have twice the drag compared to the 67-025 in real world settings as some people claim? Read this topic online here: http://forums.matronics.com/viewtopic.php?p=304911#304911 Attachments: http://forums.matronics.com//files/naca67_025_135.jpg ________________________________ Message 3 _____________________________________ Time: 09:40:56 AM PST US From: Peter Chapman Subject: Zenith-List: Sabrina's a/c & drag of unfaired struts At 09:59 15-07-10, you wrote: > >Nice work Sabrina. I wouldn't have thought that the 180 sq inches >of exposed tubing would have slowed down the aircraft so >severely! Isn't that only about 5% increase in the exposed >area? I'm not wanting to take the time to calculate the wetted area >(and increased drag) but the speed reduction seems high to me. The key thing is that it is round tubing. The wetted area isn't the problem, it is the drag of round tubing. The tube will have a drag coefficient of around 1.0-1.2, given its size and typical light aircraft speed. (For a much higher speed aircraft, it would be lower.) A faired tube's drag will be a lot lower. I see numbers like 0.05, although at the low Reynolds number, it might be a little more. And if the fairing is stubby it may also rise. I think a value of 0.1 is reasonable for the maximum possible. So the unfaired tube may be easily 10 to 20 times more draggy than a faired tube would be. (You hear "10 times" being bandied about on the web for faired vs. rounded struts, but I wanted to check some of the aerodynamic data to make sure I wasn't just repeating something not accurate in this case.) But so what, what's the overall effect on the whole aircraft? As for the overall aircraft effect, 180 sq in of frontal area at say 1.1 drag coefficient is about 1.4 square feet of equivalent flat plate drag area. Light plane drag area varies by plane, but for a side by side airplane with moderate streamlining, 5 square feet is plausible. Very roughly then, adding 1.4 is a 28% increase in overall drag. Well, that ignores the lift induced drag, and I haven't calculated that, but it is relatively small at high cruise, and might be only 5-10% more. Leaving out that drag source, with drag proportional to speed squared, that works out to a 12% decrease in speed. If one started at say 120 mph is about a 15 mph reduction. Voila! Although there are some approximations involved, that works out to the range of speed loss Sabrina talked about. Plus with the way the bracing tube on her aircraft only slowly "descends" to the wing, being close to and nearly parallel to it for quite a distance, I'm guessing it will cause a some interference drag and kill some of the wings lift, requiring more angle of attack and drag to make up for it. Appendix with details: A 100 mph, 1.5" dia, Reynolds number will be around 150,000 so the drag is still up at that level, at the higher 'subcritical' level, rather than down at 0.3 coefficient if it were at 400k+ Reynolds number. For the faired tube, one sees 0.05 numbers but I didn't see much on how it increases with lower Re numbers. Some wind tunnel test data of faired tubing at similar Re numbers showed .05 is reasonable for good fineness ratios of 3, or 0.1 for a stubby fineness ratio of 2. So I think I can trust "0.05 if good but up to 0.1 in the worst case". The flat plate area I guessed based on tables of flat plate areas calcuated for a bunch of other light planes, so it wasn't entirely pulled out of the air. Peter Chapman Toronto, ON 601 HDS / 912 / C-GZDC ________________________________ Message 4 _____________________________________ Time: 09:48:15 AM PST US From: "Craig Payne" Subject: RE: Zenith-List: Sabrina's a/c & drag of unfaired struts Interesting. Fairing the round struts on a 701 reportedly only gains you 3-5 knots. -- Craig _____ From: owner-zenith-list-server@matronics.com [mailto:owner-zenith-list-server@matronics.com] On Behalf Of Peter Chapman Sent: Thursday, July 15, 2010 9:40 AM Subject: Zenith-List: Sabrina's a/c & drag of unfaired struts At 09:59 15-07-10, you wrote: Nice work Sabrina. I wouldn't have thought that the 180 sq inches of exposed tubing would have slowed down the aircraft so severely! Isn't that only about 5% increase in the exposed area? I'm not wanting to take the time to calculate the wetted area (and increased drag) but the speed reduction seems high to me. The key thing is that it is round tubing. The wetted area isn't the problem, it is the drag of round tubing. The tube will have a drag coefficient of around 1.0-1.2, given its size and typical light aircraft speed. (For a much higher speed aircraft, it would be lower.) A faired tube's drag will be a lot lower. I see numbers like 0.05, although at the low Reynolds number, it might be a little more. And if the fairing is stubby it may also rise. I think a value of 0.1 is reasonable for the maximum possible. So the unfaired tube may be easily 10 to 20 times more draggy than a faired tube would be. (You hear "10 times" being bandied about on the web for faired vs. rounded struts, but I wanted to check some of the aerodynamic data to make sure I wasn't just repeating something not accurate in this case.) But so what, what's the overall effect on the whole aircraft? As for the overall aircraft effect, 180 sq in of frontal area at say 1.1 drag coefficient is about 1.4 square feet of equivalent flat plate drag area. Light plane drag area varies by plane, but for a side by side airplane with moderate streamlining, 5 square feet is plausible. Very roughly then, adding 1.4 is a 28% increase in overall drag. Well, that ignores the lift induced drag, and I haven't calculated that, but it is relatively small at high cruise, and might be only 5-10% more. Leaving out that drag source, with drag proportional to speed squared, that works out to a 12% decrease in speed. If one started at say 120 mph is about a 15 mph reduction. Voila! Although there are some approximations involved, that works out to the range of speed loss Sabrina talked about. Plus with the way the bracing tube on her aircraft only slowly "descends" to the wing, being close to and nearly parallel to it for quite a distance, I'm guessing it will cause a some interference drag and kill some of the wings lift, requiring more angle of attack and drag to make up for it. Appendix with details: A 100 mph, 1.5" dia, Reynolds number will be around 150,000 so the drag is still up at that level, at the higher 'subcritical' level, rather than down at 0.3 coefficient if it were at 400k+ Reynolds number. For the faired tube, one sees 0.05 numbers but I didn't see much on how it increases with lower Re numbers. Some wind tunnel test data of faired tubing at similar Re numbers showed .05 is reasonable for good fineness ratios of 3, or 0.1 for a stubby fineness ratio of 2. So I think I can trust "0.05 if good but up to 0.1 in the worst case". The flat plate area I guessed based on tables of flat plate areas calcuated for a bunch of other light planes, so it wasn't entirely pulled out of the air. Peter Chapman Toronto, ON 601 HDS / 912 / C-GZDC ________________________________ Message 5 _____________________________________ Time: 10:29:33 AM PST US From: 4rcsimmons@comcast.net Subject: Zenith-List: http://ecutrip.the-best-web-sites.com/web/about.php ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message zenith-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/Zenith-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/zenith-list Browse Digests http://www.matronics.com/digest/zenith-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.