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cluttonfred
 Post subject: T-18L pros and cons?
PostPosted: Thu Mar 24, 2016 12:47 am 
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The T18.net breakdown of T-18 models says:

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T-18L - This model uses both the standard fuselage and the standard wing plan form, but incorporates the LDS airfoil designed by Lu Sunderland and used on the S-18. There are no actual drawings for the "L" version, as builders have taken it upon themselves to make the airfoil change.


So the T-18L is really more of a builder's mod. Does anyone have any experience to share with this model and/or thoughts on the original vs. LDS airfoils in practice?

Cheers,

Matthew

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Bill Williams
 Post subject: Re: T-18L pros and cons?
PostPosted: Thu Mar 24, 2016 4:56 am 
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It is a quite simple change...both wings are the same from the spar to trailing edge, Builders use the leading edge Sunderlund airfoil and now you have a LDS wing


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James Grahn
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 12:42 am 
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If you are referring to an S wing versus a T wing, there are more changes than a few nose ribs. For instance, the spar on the S is significantly stronger than the T, which results in the increased load capacity.
If you are referring to the newest airfoil, there are TONS of changes.
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cluttonfred
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 2:48 am 
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Cubes, I thought we were discussing the original T-18 wing except using Sunderland's LDS airfoil. No?

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James Grahn
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 9:34 am 
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I've never heard of the T18L, so I don't know. But there are plenty of folks who fly with the T fuselage and S wing. You are correct that the builder will have to make a few changes. The 522 spar fittings will have to be modified to move inboard 1 inch on each side. The inboard wing skin of the S airfoil will have to be extended to be able to close the gap with the narrower fuselage. The aft spar fitting will have to be changed. It's not hard, but needs a bit of attention to detail.
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James Grahn
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 9:36 am 
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I'm a lucky man. I have an underworked machinist who loves my "little projects". He mills down to the 10,000th of an inch. Therefore, I'm able to get parts machined for modifications like this and for my own bird. You should see the hydraulic switch we designed and he made. Incredible.
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cluttonfred
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 9:56 am 
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Hmm...the T-18L designation is not my invention but from the T18.net site. My understanding is that the T-18L is a stock T-18 wing but built from scratch using the slightly more rounded Sunderland LDS-2 airfoil profile instead of the stock NACA-63-412, already tweaked by Thorp to eliminate the cusp in the rear section. It sounds like you are talking about adapting the S-18 wing to the T-18 fuselage. which is something different. Here are images of the two airfoils. If someone has an image of the actual T-18 airfoil as modified by Thorp, that would be helpful.


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lds2.gif
lds2.gif [ 11.72 KiB | Viewed 4622 times ]
n63412.gif
n63412.gif [ 12.26 KiB | Viewed 4622 times ]

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Hagle347
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 4:26 pm 
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James Grahn wrote:
I've never heard of the T18L, so I don't know. But there are plenty of folks who fly with the T fuselage and S wing. You are correct that the builder will have to make a few changes. The 522 spar fittings will have to be modified to move inboard 1 inch on each side. The inboard wing skin of the S airfoil will have to be extended to be able to close the gap with the narrower fuselage. The aft spar fitting will have to be changed. It's not hard, but needs a bit of attention to detail.
Cubes

;)


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leewwalton
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 4:37 pm 
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FYI there is a very nice Wide body T-18 with a standard wing w long range tanks on the market right now.

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James Grahn
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 5:12 pm 
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Well that's a new one on me. I can't help you. And, yes, Terry did what I was talking about.
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KWK
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 7:32 pm 
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cluttonfred wrote:
If someone has an image of the actual T-18 airfoil as modified by Thorp, that would be helpful.

Matthew, to get the T-18 airfoil, just take the drawing of the NACA 63-412 and draw a line starting at the trailing edge, going forward to a tangent to the NACA profile's lower surface, which comes at about 46% of chord. That's what Thorp did (drawing 547).

If you're using Xfoil, just take the NACA coordinates and delete the lower surface ones from 50% aft to the trailing edge; Xfoil's spline will fill in the details close enough. You'll find the profile is the same as the LDS-2 aft of the spar. Here are the coordinates:

T-18
1 0
.95023 .00881
.90049 .01739
.85070 .02618
.80084 .03492
.75089 .04344
.70087 .05153
.65076 .05899
.60057 .06562
.55031 .07125
.50000 .07576
.44964 .07894
.39924 .08062
.34882 .08059
.29840 .07872
.24800 .07499
.19765 .06929
.14735 .06138
.09718 .05063
.07218 .04379
.04727 .03544
.02257 .02460
.01041 .01719
.00567 .01320
.00336 .01071
0 0
.00664 -.00871
.00933 -.01040
.01459 -.01291
.02743 -.01716
.05273 -.02280
.07782 -.02685
.10282 -.02995
.15265 -.03446
.20235 -.03745
.25200 -.03919
.30160 -.03984
.35118 -.03939
.40076 -.03778
.45036 -.03514
1 0

You can plot these using AirfoilTools.com, where you will also find the LDS-2 coordinates already copied from the UIUC database, ready to plot there.

Xfoil shows the LDS airfoil to be an improvement to the original, and it should give a somewhat higher maximum lift coefficient. The larger leading edge radius should also be (slightly) more tolerant of flaws, less likely to trip to turbulent as soon.

I believe David Reed, who posts here, used this S-18 airfoil on his T-18. It flies beautifully.

Karl


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david read
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 9:04 pm 
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Karl is correct. N718DR is a standard T18 with the LDS airfoil. I plotted the points for the nose ribs but otherwise the rest of the wing is per plans. I was told this tames the stall with no significant high speed penalty. I flew N27DW with the original airfoil for 250 hours or so and can be completely happy with either. More important than the airfoil is that the plane be built straight. Both of my planes had benign stalls straight ahead with plenty of warning.


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jrevens
 Post subject: Re: T-18L pros and cons?
PostPosted: Sun Mar 27, 2016 10:15 pm 
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FWIW, I built my standard body/standard wing T-18 with the LDS airfoil also, and I built those wings over 30 years ago. In doing test flights in formation with my buddy Dean Cochran in his T-18 with the original airfoil, my airplane would consistently still be flying when his wing would stall. Both airplanes were very close in empty weight and many other aspects. Both have 160 hp but I was faster also, although I believe that was mostly due to different props. Ditto on what David said about being built straight.


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Bill Williams
 Post subject: Re: T-18L pros and cons?
PostPosted: Mon Mar 28, 2016 5:07 am 
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Huh! so the difference is the leading edge, where did I see that before?


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KWK
 Post subject: Re: T-18L pros and cons?
PostPosted: Mon Mar 28, 2016 9:47 am 
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jrevens wrote:
Both have 160 hp but I was faster also, although I believe that was mostly due to different props.

From Xfoil's calculations, there is no significant difference in cruise drag between the two wing airfoils. For that matter, any decent airfoil would have the same cruise drag, even the ancient M12. The airfoil doesn't have a big effect on drag, unless you can make the surface perfect and keep it that way. Parasitic drag of the airframe is the thing to concentrate on.


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