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#61
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
From the Aircraft Spruce website, the drag spec on a rod and ball transponder antenna is listed as 0.41 lbs. at 250 kts. The blade type lists as 0.09 lbs. at 250 knots. Drag increases by the square of the speed, so at half the speed (125 knots), the drag is around 1/4 the numbers listed. If you are concerned about the .0225 lbs. in extra drag vs. the .01025 lbs., buy the blade type and keep the yaw string straight. You will probably increase your final glide numbers by at least a couple of yards.
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#62
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
On Wed, 01 Jan 2020 06:32:46 -0800, India November wrote:
I am thinking of the theoretical advantage in drag coefficient of a blade vs rod antenna which holds for any speed. I found this: https://www.grc.nasa.gov/www/k-12/airplane/shaped.html which may help sort things out for you, though these numbers are for 3D shapes while what you really want is drag comparisons for 2D shapes with the shape extending right across the wind tunnel or (better) with one end attached to a side wall that's in the airstream. Other, less well written web pages, suggest that a teardrop shape with a blunt LE and max thickness at around 30% chord and slightly convex surfaces behind max thickness that terminate at a sharp TE should have about half the drag of a cylinder with the same frontal area. As others have said, drag will rise sharply if the AOA of the streamlined shape isn't zero and this will get worse as the max thickness moves back toward 50% and/or the LE becomes sharper. If you're really worried, find a University with an aerodynamics dept and try to persuade a student to write a term paper based on wind tunnel tests on a rod antenna and the best shaped blade antenna you can find. Now, you can calculate your glider's drag at, say, best glide and max cruise (easy to do from its flying weight and glide polar) and then calculate how much each adding each antenna would affect the glide angle at those speeds. We'll all be interested to see just how much effect the antenna has. -- Martin | martin at Gregorie | gregorie dot org |
#63
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
Martin Gregorie wrote on 1/1/2020 1:56 PM:
On Wed, 01 Jan 2020 06:32:46 -0800, India November wrote: I am thinking of the theoretical advantage in drag coefficient of a blade vs rod antenna which holds for any speed. I found this: https://www.grc.nasa.gov/www/k-12/airplane/shaped.html which may help sort things out for you, though these numbers are for 3D shapes while what you really want is drag comparisons for 2D shapes with the shape extending right across the wind tunnel or (better) with one end attached to a side wall that's in the airstream. Other, less well written web pages, suggest that a teardrop shape with a blunt LE and max thickness at around 30% chord and slightly convex surfaces behind max thickness that terminate at a sharp TE should have about half the drag of a cylinder with the same frontal area. As others have said, drag will rise sharply if the AOA of the streamlined shape isn't zero and this will get worse as the max thickness moves back toward 50% and/or the LE becomes sharper. If you're really worried, find a University with an aerodynamics dept and try to persuade a student to write a term paper based on wind tunnel tests on a rod antenna and the best shaped blade antenna you can find. Now, you can calculate your glider's drag at, say, best glide and max cruise (easy to do from its flying weight and glide polar) and then calculate how much each adding each antenna would affect the glide angle at those speeds. We'll all be interested to see just how much effect the antenna has. A 50:1 900 lb glider at best L/D has 18 lb of drag; the rod antenna at 62 knots has a drag of 0.41lb/16 = 0.025, or 0.14%. That's about 0.07 L/D loss. The rod drag quadruples at 125 knots, but the drag of the glider would increase substantially, also, so maybe the percentage doubles, which is still only 0.3% of total drag. -- Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me) - "A Guide to Self-Launching Sailplane Operation" https://sites.google.com/site/motorg...ad-the-guide-1 |
#64
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
On Wed, 01 Jan 2020 14:33:36 -0800, Eric Greenwell wrote:
Martin Gregorie wrote on 1/1/2020 1:56 PM: On Wed, 01 Jan 2020 06:32:46 -0800, India November wrote: I am thinking of the theoretical advantage in drag coefficient of a blade vs rod antenna which holds for any speed. I found this: https://www.grc.nasa.gov/www/k-12/airplane/shaped.html which may help sort things out for you, though these numbers are for 3D shapes while what you really want is drag comparisons for 2D shapes with the shape extending right across the wind tunnel or (better) with one end attached to a side wall that's in the airstream. Other, less well written web pages, suggest that a teardrop shape with a blunt LE and max thickness at around 30% chord and slightly convex surfaces behind max thickness that terminate at a sharp TE should have about half the drag of a cylinder with the same frontal area. As others have said, drag will rise sharply if the AOA of the streamlined shape isn't zero and this will get worse as the max thickness moves back toward 50% and/or the LE becomes sharper. If you're really worried, find a University with an aerodynamics dept and try to persuade a student to write a term paper based on wind tunnel tests on a rod antenna and the best shaped blade antenna you can find. Now, you can calculate your glider's drag at, say, best glide and max cruise (easy to do from its flying weight and glide polar) and then calculate how much each adding each antenna would affect the glide angle at those speeds. We'll all be interested to see just how much effect the antenna has. A 50:1 900 lb glider at best L/D has 18 lb of drag; the rod antenna at 62 knots has a drag of 0.41lb/16 = 0.025, or 0.14%. That's about 0.07 L/D loss. The rod drag quadruples at 125 knots, but the drag of the glider would increase substantially, also, so maybe the percentage doubles, which is still only 0.3% of total drag. I thought it was likely to be a pretty small effect. Thanks for confirming. -- Martin | martin at Gregorie | gregorie dot org |
#65
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
Thank you Eric!
Putting it in perspective like that was very helpful. Lou |
#66
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
Mark Mocho wrote:
"If you are concerned about the .0225 lbs. in extra drag vs. the .01025 lbs.., buy the blade type and keep the yaw string straight. You will probably increase your final glide numbers by at least a couple of yards." But Mark, the blade type just looks cool! Especially when mounted on top! 😛😂😵 |
#67
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
But I have flown with you, Cliff, and the drag from the antenna is only low when aligned with the airflow. Your yaw string looks like a windshield wiper! You don't go faster by using the rudder pedals like bicycle pedals.
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#68
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
Thanks. I needed a laugh.
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#69
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
Just mount the blade antenna by the front lug only, leave it loose a bit. Cut a slight smile slot for the coax, use a bit of grease, and,voila!
Now your blade style antenna can feather with the wind, netting you the least amount of drag possible. ;-) |
#70
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Locating Transponder Antenna on top of the fuselage instead ofunder it.
On Thursday, January 2, 2020 at 3:44:53 PM UTC-8, Chris Behm wrote:
Just mount the blade antenna by the front lug only, leave it loose a bit. Cut a slight smile slot for the coax, use a bit of grease, and,voila! Now your blade style antenna can feather with the wind, netting you the least amount of drag possible. ;-) Why not mount it on the side of the fuselage and use it as an angle of attack sensor as well? |
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