How does a scale up affect airfoil center of lift?

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pjphilli

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Hi all,

Planning retirement dreams, next project after cub restoration complete. I've tried the search function last few weeks, just not seeing what I'm after.

Specific to George Spratt's use of the 23112 airfoil in his control wing flying boats and his terrestrial plane endeavors, he located the hinge point at 22.83% of the cord, 10.96" inches from the leading edge on a 48 inch chord, 12 ft wing panel (left and right mirror image, ~ 24 ft total span). Assuming the same ~70-80 MPH flight speed, would the center of lift remain at 22.83% were the wing scaled up in all dimensions to have a cord of 5 feet, and 15 ft wing panels span? Maybe my question simplified is "do airfoil characteristics scale" or are changes in say Reynolds number significant even at these slow speeds and "small change" from a 4 to a 5 foot cord... (You can tell I'm not an aeronautical engineer) :)

PS: Control wing plans are available in the public domain for download. Page 23 of the 79 page part-1 construction manual shows details, center of lift vs. speed, etc. in drawings. This is a fascinating read and a lot of history.

Thanks! - Phil
 

Norman

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What you're asking about is called the pitching moment (identified as Cm (for moment coefficient) on polar diagrams) and, no, scale does not affect it. As long as you plot your airfoil from an accurate set of coordinates and use the same percentage for the hinge the control forces will be comparable.
 

pjphilli

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Thank you both very much! My question was answered that I can scale up to a ~30 foot span and 5 ft cord and the pitching moment (a new phrase!) will scale.

Re: terrestrial planes, Mr. Spratt designed in addition to flying boats, at least three land based planes in control wing style (his words… ever seen a bird with a rudder? With an elevator?) In mid 30s there’s pix of what looks like a single seat rag and tube control wing with what looks like a 2 cylinder opposed air cooled engine, both naked and after covering pictures used for flight testing. Very cute….! Northrop took another land based 2 seat version of a Spratt designed and built airplane with 4 wheels that looked a lot like an air car. Reports ( I’ve read…) were it flew well as built. After Northrop “re-engineered” it, added about 240 lbs to the wings and weight to fuselage, it flew but reports were underpowered. Finally, his model 107 or 108 (phone type, I’m old…laptop not up… google Spratt control wing and images… the yellow one hanging in a museum) flew well per what I can find. This is the one I’m most interested in. Two rear wheels, a nose wheel carrying about 70% of the weight but designed as a 3 seat like my beloved j5A… two at center of gravity, PIC center- front.
 

pjphilli

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Edit addition… think the museum plane is a 2 place, left pilot seat ahead for CG, but right seat near center of gravity like the 2 seat flying boats. Have not made it there to crawl over it (yet), but I want to build a 3 seat, liquid cooled auto conversion powered version, if for no other reason than other than Spratt Sr. and his son, this idea being non-traditional from the Wright brothers never caught on. No stall, no spin, no stunts… just for learning and fun :)
 

pjphilli

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Some period INFO:
Wow, can't wait to dig into this... thank you!
View attachment 114939

Thank you for bringing this to our attention, most informative!
You are very welcome. The construction manual and plan drawing are in the public domain, 4 .pdf files. Google "spratt control wing plans" and "speleotrove dot com" pops up if interested.

Mid Atlantic Air Museum (MAAM) has the model 108 hanging from the ceiling, that's more the direction of my dream versus the flying boat. That comes up quick on a google search as well. Be Safe and Blessed! - Phil
 

Norman

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The NACA-23112 has come up on HBA several times before, usually associated either with the Pou-du-Ciel or Spratt Control Wing. The coordinates in the NACA report that Wingitis posted are pretty coarse. They're good enough for hand drafting but if you're going to draw your rib templates in CAD you need to make sure that the spline passes through all of the points (especially around the leading edge). We talked about that problem here. We've also talked about pitching moment and center of pressure because this is a very old airfoil and data for it is usually in the form of the old NACA standardized graphs which were made before the switch from c.p. to Cm. They're just different ways to display the same data and you can easily transform one into the other with a bit of math shown in Post #24 of this thread. A lot of people still have trouble with the fact that there's still a pitching moment at zero lift so l drew a picture showing that the center of pressure for both surfaces act at different locations. What happens when two unequal, and sometimes opposite forces act on a body at different points?.. It rotates.
 
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WINGITIS

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Airfoils tools can generate it to 200 points, assuming their version of the NACA formula is correct!?

I have attached the DAT file as a TXT of the original as they list above as well as the DEROTATED version with the Mean Camber line at 0.0

AIRFOIL TOOLS 23112.png
 

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WINGITIS

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Wow, can't wait to dig into this... thank you!
You are very welcome. The construction manual and plan drawing are in the public domain, 4 .pdf files. Google "spratt control wing plans" and "speleotrove dot com" pops up if interested.

Mid Atlantic Air Museum (MAAM) has the model 108 hanging from the ceiling, that's more the direction of my dream versus the flying boat. That comes up quick on a google search as well. Be Safe and Blessed! - Phil
The 108 is impressive and a 103 is super easy, BUT the flying boat DOES HAVE CHARACTER!

The plans are cool.

Thanks.
 

WINGITIS

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I have extracted the AIRFOIL image from the SPRATT Seaplane plans and compared it to Normans original one(NACAG 231) mentioned in a link above and they are both the same.

The 23112 generated from the Airfoil Tools site formula is different and is similar to Normans BTB one is the same link.

(SEE PDF FOR BETTER IMAGES)

So Norman where did you find your original version(NACAG 231) that is the same as Mr Spratts?

Because your BTB one from the same thread is close to the Airfoil Tools one!?

Cheers

Kevin
23112 VARIATION PROFILES.png23112 comparisons.png
 

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pjphilli

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I’ll have to catch attachments from PC tomorrow, but I really super really appreciate them!

I have read much stuff on the internet, always with a grain of salt being a computer versus a book, fearing that something changes with either algorithm or flat errors with this specific airfoil. The handwritten Spratt airfoil coordinates does not have quite that distinct concave dip in bottom leading edge side of cord the way I think the calculator generates it. Wingitus, I believe we agree. I was thinking to use the vintage hand drawn coordinates from the plans for real-go RC prototyping for two reasons… I can handle 8th grade math, but more importantly, I trust Mr. Spratt with a #2 wood pencil and a slide rule more than most stuff on the general internet.
I super-super appreciate everyone’s comments. No way can I learn unless I can shelve preconceived notions. Thank you all so much. Thread drift… N30312 is J5 referenced previously as family heirloom beloved.- Phil
 

WINGITIS

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I’ll have to catch attachments from PC tomorrow, but I really super really appreciate them!

I have read much stuff on the internet, always with a grain of salt being a computer versus a book, fearing that something changes with either algorithm or flat errors with this specific airfoil. The handwritten Spratt airfoil coordinates does not have quite that distinct concave dip in bottom leading edge side of cord the way I think the calculator generates it. Wingitus, I believe we agree. I was thinking to use the vintage hand drawn coordinates from the plans for real-go RC prototyping for two reasons… I can handle 8th grade math, but more importantly, I trust Mr. Spratt with a #2 wood pencil and a slide rule more than most stuff on the general internet.
I super-super appreciate everyone’s comments. No way can I learn unless I can shelve preconceived notions. Thank you all so much. Thread drift… N30312 is J5 referenced previously as family heirloom beloved.- Phil
There is always an issue getting things from plans because they may have been drawn incorrectly or the paper may have shrunk or the scanner may have been on an angle etc etc.

That is why I have asked Norman for his source so we can maybe sort it out.

BUT THAT WAS BEFORE YOU HAVE POINTED OUT THOSE DIMENSIONS IN PDF #2.....which I missed.

HAS SOMEONE ACTUALLY TYPED THOSE IN SOMEWHERE!?

If not and you type them into a Spreadsheet I will check them for you.

Either way there is not a GREAT DEAL of difference between the two in terms of analytical results, but one has less drag at low angle of attack cruising and the other has a better Lift/Drag at higher angles of attack.

K
 

WINGITIS

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I got excited and did the first phase for you, someone can please check my numbers, before I go on to create the DAT file.

A few issues I found:

1: The LE is not on the reference line, this can be rectified in XFLR5 with the Normalize and De-rotate features later in the process.

2: Where the tape is at the TE the station lengths are not the same, so it has been taped incorrectly.

3: The tape covers some of the measurements, I used a bit of math to solve for those missing points, using the slopes and intercept method(Overkill for this example, its more useful when the X axis values are to 6 decimal places and not equidistant, but I use it for all straight line interpolations as a rule)

Most of the other numbers are fairly readable, but it does not mean I have misread or mistyped 1 or 2, SO PLEASE CHECK!

Kevin
 

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Norman

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I have extracted the AIRFOIL image from the SPRATT Seaplane plans and compared it to Normans original one(NACAG 231) mentioned in a link above and they are both the same.

The 23112 generated from the Airfoil Tools site formula is different and is similar to Normans BTB one is the same link.

(SEE PDF FOR BETTER IMAGES)

So Norman where did you find your original version(NACAG 231) that is the same as Mr Spratts?

Because your BTB one from the same thread is close to the Airfoil Tools one!?

Cheers

Kevin
Man, that was 10 years ago, I can hardly remember breakfast. I do remember that I didn't change the file name so searched with that. "NACAG" stood for something but I don't remember what. Something to do with hang gliders I think. Anyway that group appears to be gone. The BTB version should be identical to what would be produced by the ordinates in the original NACA tech report. Splining those coords might yield a slightly different shape than The NACA version because the spline doesn't reproduce the slope and circle leading edge hand drafting method. If you did lay it out in CAD with the slope and circle method the resulting airfoil would require both normalization and de-rotation because the slope and circle method pushes the leading edge forward and up. Normalizing and de-rotating would make it look different in the XFLR5 direct design window. Simply splining those coords could also produce several leading edge shapes depending on the spline algorithm. B-splines often pass close to the control points but pull away depending on the radius of the bend so would produce a larger leading edge radius than intended (this would also shorten the chord, thus also increasing the thickness after normalization). The XFoil "refine" command produces a smooth spline that hits all the points in the original ordinate file but doesn't reproduce the slope and circle error at the leading edge. As I've said before the slope and circle method is a hand drafting expediency used because it's really hard to draw a smooth parabolic spline around the leading edge by hand. The XFOIL refin command can do it so why don't we just use it and stop fussing about how the spline doesn't exactly replicate the hand drawn shape?

BTW
JavaFoil has an airfoil generating application that can make many more airfoils than any other program I've seen. All of the NACA 4, 5, and 6 digit sections INCLUDING the 231 mean line family. I can't do it myself right now because my real computer is having a brain transplant but it would be interesting to see how the 23112 that it produces compares to the original NACA coords.
 

WINGITIS

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Thanks Norman,

Just about to go and get breakfast here because we are now out of the Level 1 lock down where I am, as of yesterday. I cannot remember breakfast from 10 years ago either, but can manage yesterday!☺

Once Phil has checked the numbers I entered in the spreadsheet I will create the DAT file of the PLANS and compare that to the Airfoil Tools one De-rotated as well, as it comes from the Airfoil Tools formula nose high like the plan shows.

Cheers
Kevin
 

WINGITIS

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Ok I have finished the conversion from the PLAN dimensions to a SELIG DAT, then De-rotated it and here it is compared to Normans NACAG 231 version, they are almost the same.

There is a little work for me to do on the nose yet, theres a point missing, and then that may bring it even closer during the De-rotate procedure.

Then I will do the Polar's analysis and re-post.

SPRATT CNTRL WING-DEROTATED COMPARED TO NACAG 231.png
 

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