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
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| Subject: | 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
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| Subject: | Re: Zenith-List Digest: 2 Msgs - 07/14/10 |
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
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| Subject: | 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
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| Subject: | 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
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