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Old 06-18-2020, 01:45 PM
sreinmann sreinmann is offline
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Extracting the cross section of a wing

Still plugging away at my RQ170 recolor/resize. On the advice of better designers than I, I'm trying to increase the interior structure and allow the model to keep its shape better, and make it easier to assemble. However, not knowing the true wing cross-section, I'm left putting my best guess down. But this has left me with a fiddley gap at my seam.

Is there a better way of constructing a body's cross-section rather than hunting and pecking with shapes? And no, I don't have a 3D model.
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Extracting the cross section of a wing-img_1073-1-.jpg   Extracting the cross section of a wing-img_1076-1-.jpg   Extracting the cross section of a wing-img_1077-1-.jpg   Extracting the cross section of a wing-capture.jpg  
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  #2  
Old 06-18-2020, 09:14 PM
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mbauer mbauer is offline
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Had the same issue when I built the last version of my jet powered paper airplane project.

In other models have to modify the "root" edge to have extra curves in it to fit a round fuselage.

When trying to fit both wing roots together with curve wing skin, you have to ensure all edges are straight.

At that point I would suggest using the wing rib and glue it to the skin at the root on both sides. Once they set up, just glue the two root ribs together to join the two wing halves.

On the final version of my jet project, plan is to glue the wing skin to the aluminum ribs I've created for my power skid system. This will leave a gap in the center that will be open for running the leads needed for the RC controls. Once everything inside has been completed, plan is to glue the center section to the end of wing skins/ribs.

The easiest option for you: Create a joining strip, like AirDave uses to join the wings. Basically a glue strip/tab that is glued inside one of the wings, then the other wing is now glued to the tab created.

Make sense?

I've got a photo somewhere on how I used the joining strip. Will try to find it and post.

So far all the designs I do have been 2D, working on 3D training at this time.

Mike
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Last edited by mbauer; 06-18-2020 at 09:16 PM. Reason: 2d
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Old 06-18-2020, 09:29 PM
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mbauer mbauer is offline
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Photo of Wing Center Section

Found a photo of what happened on my build trying to join wing to wing.

Notice I printed a "center section" gap filler joining strip:
Click image for larger version

Name:	PICT0252.JPG
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ID:	420289

Notice also that I really messed this whole thing up, my gap was about 1/4". This is the top, it was covered with a metal heat shield, effectively hiding the fix.

One of our members years ago, Jim Nunn, came up with a novel idea. In fact it was so good he posted on April 1. His version of a paper stretcher.

Too bad there isn't a paper stretcher, could of used one several times during my model builds!

Mike
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Last edited by mbauer; 06-18-2020 at 09:39 PM.
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Old 06-19-2020, 07:20 AM
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The RQ170 seems likely to use a symetric airfoil (same upper and lower curves) on the outer panels but by the time it gets to the centreline the upper surface curves a great deal more than the lower.
At least one person seems to agree with this - see picture 21/23 at
https://www.pond5.com/3d-models/item...q-170-sentinel

Off now to the somewhat ancient NACA 4 digit symetrics at
NACA 4 digit airfoil generator (NACA 0012 AIRFOIL)
which should be good enough for the RQ170.
(that site also gives links to many other airfoils)

How about 6% thickness at the tip with 12% at the panel joint, and at the centreline combine the lower curve of the 12% with the the upper curve of perhaps the 18%.

Cut out enough of these sections to make up 2 handed inner and 2 outer panels from card, and decide the spar shapes and dihedral required so that the sections can be set up vertically relative to each other and also brace them to be parallel horizontally

Card Modeling FAQ
is still a worthwhile place and in this case because it includes
Card Modeling FAQ Appendix: Designing Paper Models
and down that page is a section on "Rollation"


As a bit of a slightly off topic aside
The Incomplete Guide to Airfoil Usage
https://m-selig.ae.illinois.edu/ads/aircraft.html
may tell you which airfoil was used on an a particular aircraft and I reckon model designers ought to check whether it does. A web search may then turn up the actual shape - or not
- but certainly not in this case.
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Old 06-19-2020, 08:42 AM
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mbauer mbauer is offline
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For what's its worth:

I have several symmetrical airfoils plotted. I can print them to a PDF and post in the download section, if others would like.

Mike
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Old 06-19-2020, 08:49 AM
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Quote:
Originally Posted by maurice View Post
The RQ170 seems likely to use a symetric airfoil (same upper and lower curves) on the outer panels but by the time it gets to the centreline the upper surface curves a great deal more than the lower.
At least one person seems to agree with this - see picture 21/23 at
https://www.pond5.com/3d-models/item...q-170-sentinel

Off now to the somewhat ancient NACA 4 digit symetrics at
NACA 4 digit airfoil generator (NACA 0012 AIRFOIL)
which should be good enough for the RQ170.
(that site also gives links to many other airfoils)

How about 6% thickness at the tip with 12% at the panel joint, and at the centreline combine the lower curve of the 12% with the the upper curve of perhaps the 18%.

Cut out enough of these sections to make up 2 handed inner and 2 outer panels from card, and decide the spar shapes and dihedral required so that the sections can be set up vertically relative to each other and also brace them to be parallel horizontally

Card Modeling FAQ
is still a worthwhile place and in this case because it includes
Card Modeling FAQ Appendix: Designing Paper Models
and down that page is a section on "Rollation"


As a bit of a slightly off topic aside
The Incomplete Guide to Airfoil Usage
https://m-selig.ae.illinois.edu/ads/aircraft.html
may tell you which airfoil was used on an a particular aircraft and I reckon model designers ought to check whether it does. A web search may then turn up the actual shape - or not
- but certainly not in this case.
Wow, what a great link for the NACA 4. Have all the ones up to the 0024 already plotted. Just looked at a few of the others. Thank you once again. Now have more airfoils to mess with.

Mike
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Old 07-31-2020, 08:47 AM
sreinmann sreinmann is offline
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Thanks again for the links and descriptions. It was very helpful in getting my design to its first real prototype. But I now come with a new question.

The attached photos show the dorsal engine "bubble" from the intake back to the exhaust. This is a significant design change from the original version which was broken into 7 segments. That design allowed for a more stable structure ... but still had a surface attachment using glue tabs. My new design is a "single piece" and will most likely use glue tabs but is challenging to afix to the fuselage since its fairly ... elastic (I hope you understand).

I've marked using circles and squares the points where alignment points exist. I am thinking that I might need to add some bulkheads inside the engine bubble and maybe stop blocks on the fuselage (paper squares on either side) to help with the alignment. This will still be rather fiddly, so I'm curious if anyone has come across this kind of issue and can say, "Don't worry about it Scot, modelers of planes expect this." or "OMG, I can't believe you didn't design it such-and-such way!"

Please accept my apologies as aircraft design is completely new to me and my lack of experience is certainly a hurtle. I really appreciate your guidance!
Attached Thumbnails
Extracting the cross section of a wing-inkedimg_1180_li.jpg   Extracting the cross section of a wing-img_1182.jpg   Extracting the cross section of a wing-img_1181.jpg   Extracting the cross section of a wing-capture.jpg  
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Old 08-01-2020, 07:26 AM
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maurice maurice is offline
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Perhaps, angular or curved, the formers can only make predictable point contact with the wing surface (where you've put spots) but angled flanges
should be able to keep them in place. A backbone notched into the tops of the formers might then give a steady enough base for skinning.
One piece skining - probably not without shallow V notches in the sides to allow the top part to change direction.
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Old 08-01-2020, 10:30 AM
sreinmann sreinmann is offline
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Thatís a great idea Maurice. Iíll give it a try but in my mind I can see the idea has promise.
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Old 08-02-2020, 02:50 PM
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I would recommend trial and error alleviate any fitting issues. Since you design without 3D, one need plenty of testing before settling on a developed shape. You can try tracing an untrimmed piece that is fitted onto your bulkheads rather than testfitting different trimmed pieces.

As to aerofoils, I *highly* doubt a truly symmetric aerofoil will be used in this day and age on a swept flying wing. You will normally find symmetric aerofoils on tail control surfaces and canards, but there are plenty of reasons to not do it on a primary lifting surface. Without knowing the exact aerofoil, I would suggest something like NACA2415 to get started. Don't be afraid to elaborate on your own, nobody is going to complain about not getting this correct.

The following image seem to confirm an asymmetric aerofoil with a much flatter lower surface. (also note the washout at the wingtip)
https://www.nytimes.com/2011/12/08/w...lance-bid.html
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