1/12 Scale Reiher Sailplane foam

[lincoln]

If this flies slowly enough, that rudder can be added by the left thumb*. Another possibility is aileron differential. I'm assuming mode 2.


*I try to use thumb and forefinger, actually. Not sure I actually do that on the left stick, though.

[Piecost]

That model is looking very nicely finished. Would you mind sharing how you did such a neat job cutting out the wings?

The replies with suggestions of others. materials and techniques is appreciated.

[bcarter1234]

Let me know if I'm not answering the question you are asking. ;-)

The wings are cut using a gravity feed hotwire cutter. I purchased a used commercial unit ages ago but there are several sites with instructions to make your own.

I cut a new set of blanks for a fuselage and wings last night. I'll cut cores and shape the fuselage today. The wings will be glassed, equipped with ailerons and the front half of the fuse will be glassed.

lincoln,

Knowing this glider won't see the kind of speed range that a DLG would is the AG04 still a good choice? I considering something slightly thicker like the AG24 for the root and the AG26 for the tip.   

[OZPAF]

I think that with such a light model - the original suggestion by lincoln of AG04 would still be the best selection.

A quick look(on XFOIL) at Ago4,AG24 and AG26 for a low operating Reynold's Number of around 50,000 shows AG04 having an advantage - lower drag over most of it's speed range.

Generally as the flying speed and thus Re No reduces - thinner airfoils will be best.

John

[bcarter1234]

Here are some shots of the new cores and fuselage cut this afternoon. The cores are Ag24 root, Ag26 tip, they look so thin compared to the E392. I'll need some time to adjust before going even thinner with AG04. ;-)

I'll weigh the everything before and after glassing for reference.

[Piecost]

Can ypu please post a photo of your gravity wing hotwire setup?

[SP250]

I built and have used for years the drop arm device described in Radio Control Soaring Digest back in 2008. I works well, if you read, remember and apply all the little tips and tricks therein.
https://redirect.viglink.com/?format=go&jsonp=vglnk_177192117058111&key=5e5e6e04e939b9674c384ca8b03117f4&libId=mm0bvjv4010248ul000ULbyxn75vl&loc=https%3A%2F%2Fwww.rcgroups.com%2Fforums%2Fshowthread.php%3F1779705-Gravity-Drop-Arm-Foam-Cutter&gdprConsent=CQgE_UAQgE_UAAKA1AENCTFsAP_gAEPgAAQAMBtR_G__bWlr-bb3aftkeYxP9_hr7sQxBgbJk24FzLvW7JwXx2E5NAzatqIKmRIAu3TBIQNlHIDURUCgKIgFryDMaE2U4TNKJ6BkiFMZA2tYCFxvm4tjWQCY4vr_9lc1mB-t7dr82dzyy6hHn3a5_2S1UJCdIYetDfv8ZBOT-9IEd_x8v4v4_EbpEm-eS1n_pGtp4jd6YnM_dBmxt-Tyff7Pn__rl_e7X_ve_n3zv84XH77v____fv-7___2b_-___aDAQAJhoVAEZZEAAQKBhBAgAUFYQAUCAIAAEgaICAEwYEOQMAF1hIgBACgAGCAEAAIMAAQAACQAIRABAAQCAACAQKAAMACAICABgYAAwAWAgEAAIDoGKYEEAgWACRGRQaYEoACQQEtlQgkAQIK4QhFngEACImCgAABAAKAAAAeCwEJJASsSCALiCaAAAgAACCAAgRSFmAIIAzRaCsCTgMjTAMHzBMkpgGQBEEJGQZEJvwiAAAAAAAIEAQAAAAAAAQAABAA.YAAAAAAAAAAA&ccpaConsent=1NNY&gdprApplies=true&v=1&opt=true&out=http%3A%2F%2Fwww.rcsoaringdigest.com%2Fpdfs%2FRCSD-2008%2FRCSD-2008-11.pdf&ref=https%3A%2F%2Fwww.google.com%2F&title=Gravity%20Drop%20Arm%20Foam%20Cutter%20-%20RC%20Groups&txt=http%3A%2F%2Fwww.rcsoaringdigest.com%2Fpdfs%2F...SD-2008-11.pdf
Sorry its a large link but article starts on page 103
John M

[bcarter1234]

Here is a picture of the same unit I have. The concept is brilliant in its simplicity. Many people have built their own from wood or other items. A commercial unit is not required.

[lincoln]

Let me know if I'm not answering the question you are asking. ;-)

The wings are cut using a gravity feed hotwire cutter. I purchased a used commercial unit ages ago but there are several sites with instructions to make your own.

I cut a new set of blanks for a fuselage and wings last night. I'll cut cores and shape the fuselage today. The wings will be glassed, equipped with ailerons and the front half of the fuse will be glassed.

lincoln,

Knowing this glider won't see the kind of speed range that a DLG would is the AG04 still a good choice? I considering something slightly thicker like the AG24 for the root and the AG26 for the tip. 

First of all, keep in mind that I'm not an expert. I've read quite a bit about this stuff, and I have some engineering background, and I've messed around with model airplanes. That's it. Anyway, if at one point I figured out the Reynolds numbers for your model, I've forgotten. As I mentioned before, a little thinner than the AG04, and maybe a bit more camber. Thinner, percentage wise, as the chord gets narrower.

Speaking of gravity feed hotwires, I wonder if anyone has used a gearmotor, rotary damper,clockwork, etc. to give them a constant speed. I know a guy with an NC cutter he concocted. He controls the speed so that it cuts without actually touching the foam. That way, the wire remains straight.

[bcarter1234]

lincoln,

Do you have any idea if he is cutting with a higher wire temp to achieve the cut without the wire touching? Typically my wire moves through the foam at about 2 inches per minute or so but it is in contact with the foam.

[Piecost]

Thanks gents for the photo, explanation and links. I might have to build one when I next cut a wing.

[OZPAF]

As I mentioned before, a little thinner than the AG04, and maybe a bit more camber. Thinner, percentage wise, as the chord gets narrower.

I believe Lincoln is correct. I'm also not an expert but model aerodynamics is a large part of my hobby and my experience supports this comment by Lincoln.

At the tip I would suggest reducing the thickness below that of the main section , and reducing the camber.I also try to improve the airfoil near the tip by moving the thickness high point forward, but this needs access to XFOIl to see the benefits or lack of.

Typically my wire moves through the foam at about 2 inches per minute or so but it is in contact with the foam.

This is around the speed that I have seen used successfully. The wire melts the foam and then moves through the liquid melt. The slower you can cut the more accurate the wing aerofoil will be as the drag of the wire moving through the liquid increases with the square of the speed. Thus the lower the speed the lower the drag and thus for a given tension - the lower the bow in the wire.

I saved that same link John M years ago - it's about time I built it!

John

[SP250]

OZPAF, John yes it makes a good manual set up.
Regarding speed of cut to reduce wire drag - that's all well and good, but you do have to play with the wire temperatures as well.  Because if you cut too slowly to avoid drag, the hot wire melts more of the foam to a larger radius around the wire diameter, so you have to account for the bigger kerf when making the root and tip templates to end up with the correct aerofoil section and thickness once cut.
Ends up being an iterative process where many trials on offcuts are made before the final process is arrived at. (Certainly for me it was).  Then write it down so you can set up the next cut 6 months later correctly without wasting foam or time practicing!
John M

[OZPAF]

Because if you cut too slowly to avoid drag, the hot wire melts more of the foam to a larger radius around the wire diameter, so you have to account for the bigger kerf when making the root and tip templates to end up with the correct aerofoil section and thickness once cut.

That's true unless you have a variable DC supply , and this is a major help. There were many approaches to achieve this - from Variac's to using light dimmers.


John

[bcarter1234]

Finally an opportunity to use some rock hard balsa I've had for decades. The grooves are sanded in the top of the wing. The spars are 1/16" deep, 1/8" wide. I'll  glue the top spars in and then sand the grooves for the lower.