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pchap(at)primus.ca
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 Posted: Thu Jul 15, 2010 8:40 am Post subject: Sabrina's a/c & drag of unfaired struts |
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At 09:59 15-07-10, you wrote:
| Quote: | --> Zenith-List message posted by: <s_thatcher(at)bellsouth.net>
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 [quote][b]
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craig(at)craigandjean.com
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| Posted: Thu Jul 15, 2010 8:48 am Post subject: Sabrina's a/c & drag of unfaired struts |
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Interesting. Fairing the round struts on a 701 reportedly only gains you 3-5 knots.
-- Craig
From: owner-zenith-list-server(at)matronics.com [mailto:owner-zenith-list-server(at)matronics.com] On Behalf Of Peter Chapman
Sent: Thursday, July 15, 2010 9:40 AM
To: zenith-list(at)matronics.com
Subject: Sabrina's a/c & drag of unfaired struts
At 09:59 15-07-10, you wrote:
| Quote: | --> Zenith-List message posted by: <s_thatcher(at)bellsouth.net>
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 [quote]
href="http://www.matronics.com/Navigator?Zenith-List">http://www.matronhref="http://forums.matronics.com">http://forums.matronics.com
href="http://www.matronics.com/contribution">http://www.matronics.com/c
[b]
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