Scales for a flying glider fleet - Page 3

**ReynoldsSlumber** · #21 08-10-2023, 01:01 AM

The wing-body join, whew. As discussed with Bruno here, it turns out that the space between the two upper wing surfaces on these old A-4 plans is too narrow for the fuselage. Rather than split the bottom surface to space the wings further apart and thus reduce wing strength... instead on the top surfaces I cut and folded back the root edges, a little further than the pencil line shows.

Then it was a multi-round wrestling match to get the wings into shape, glued together, further into shape, and finally (after a big breath) glued to the fuselage in one big join. I did manage to shape some aft undercamber into inboard portions of the wings (not shown here); dunno whether it'll be worth it aerodynamically.
The part of the tail that straddles the tailpipe is cylindrical on the real thing, seemingly a bit different from the plan, so the way I made it, the little swoopy piece that's supposed to close it off doesn't quite fit. Might close it off later.

**ReynoldsSlumber** · #22 08-10-2023, 01:21 AM

Before all that I made a profile glider with paperclip nose weight to test out what the CG, dihedral, and tail incidence should approximately be. Fun! Again Bruno's bonus side view was helpful. Luckily the slight dihedral on a real A-4—there is some!—might just work out. Too much dihedral was actually bad, made it roll off to the side.

Also, after the profile glider's many crashes, it's now quite the gut-check to contemplate taking the soon-to-be-finished model and flinging it across the room on purpose. Gonna do it tho. Think I need to upgrade to a badminton crash net.

**mbauer** · #23 08-12-2023, 11:44 AM

Quote:

Originally Posted by ReynoldsSlumber

[*]Cut a circular hole on each side of the center body for intake air to pass through to the tailpipe, for a little less drag hopefully. I shaped each hole into a scoop.

I tried this technique on the SR71.

Actually adding more parasitic drag. Now the relative airflow travels on two different surfaces. This adds to the drag, not reducing.

The option of closing the nacelle, acts like an air brake. Might try experimenting with both options to see which one flies best.

When I used to fly paragliders, learned that the square openings on the leading edge actually create an unseen leading edge by the air getting stuffed into the double sail. The material is tested for porosity and once it looses the ability to trap air, letting it seep through you need a new parafoil.

Since air is not compressible, it plugs the openings, the air now splits to top and bottom flow around a bubble of air trapped at the square openings. On this build it might be less drag due to the shape of the nacelle opening. Both halves do not make a complete circle, so the bubble would be rather small on the flat surface, spilling the air over the sides.

Yes, I know you can compress air into a tank using mechanical means. It now is a form of hydraulics. Just like hydraulic fluid in a brake system. Most big trucks use air as the hydraulic since it is less dense than oil.

Air is fluid and is a really interesting thing to learn about!

Build is looking really good!

Always wondered about Origami and if it could help with some of the construction. Thank you for mentioning how you used it in this model.

Mike

**mbauer** · #24 08-12-2023, 12:17 PM

Quote:

Originally Posted by ReynoldsSlumber

Before all that I made a profile glider with paperclip nose weight to test out what the CG, dihedral, and tail incidence should approximately be. Fun! Again Bruno's bonus side view was helpful. Luckily the slight dihedral on a real A-4—there is some!—might just work out. Too much dihedral was actually bad, made it roll off to the side.

Also, after the profile glider's many crashes, it's now quite the gut-check to contemplate taking the soon-to-be-finished model and flinging it across the room on purpose. Gonna do it tho. Think I need to upgrade to a badminton crash net.

Paper models are really quite strong. Here is a photo of a Vertical landing pattern for one of my early SR71 models:

Scales for a flying glider fleet-vertical-landing-pattern.jpg

Here is the next flight photo clearly showing the bent fuselage:

Scales for a flying glider fleet-next-flight.jpg

Instead of a badminton net, found this deer netting on clearance for $3, not quite square openings but at 84inches wide by 100ft long, more than cheap enough to use as your catch it net:

Scales for a flying glider fleet-pict0258.jpg

Hoping you have some successful flights and patiently waiting for the flight test report.

Mike

**mbauer** · #25 08-12-2023, 01:22 PM

Yeti's link to the videos show something I don't see in your A-4 model. At :20 of this video you will see how the wing leading edge is angled on this A-4.
Paper A-4 Skyhawk and F-100 Super Sabre flights - YouTube

I took a screenshot of your photo above: A-4-35:

Scales for a flying glider fleet-pm-thread.jpg

It does not look like you have added any Incidence to the wing leading edge, following the flat shape of the fuselage bottom instead.

Did a quick paint 3D jpeg to show what Incidence is, since you don't have a way of providing flight controls other than bending the flying surfaces for control, you will need to provide some Incidence Angle to the wing:

Scales for a flying glider fleet-angle-incedence-wing.jpg

Wing Washout also helps to recover from a stall, but on these small models it could be hard to get the angle right on both sides of the wingtip. Washout and Angle of Incidence are measured from the angle of the relative wind as the model travels through the air during flight. Wing Washout provides a Twist to the actual wing length: the wingtip is twisted slightly from the root edge on production aircraft. If the aircraft stalls, the different angle of the wingtip will bring the aircraft automatically back to stable flight helping to recover from the stall.

The actual angle of incidence needed for your models might be as much as a half of a degree or greater than shown.

The model will fly slightly nose down, while the wings fly more or less horizontal. These two actions help the model recover from a stall. Instead of diving down towards the ground, accelerating till it hits; the wings will try to bring it back to horizontal flight due to the Angle of Incidence.

Dihedral adds stability to left/right or Roll axis. Longitudinal (Pitch) Axis needs Incidence as well as weight/balance. Yaw axis can be corrected with a bend to the rudder countering the nose trying to go left or right.

Mike

**ReynoldsSlumber** · #26 08-15-2023, 12:59 AM

Further modifications:

Trimmed the edges of the pylons and main landing gear fairings with a little curvature to match the wing surface undercamber before attaching them.

On the dorsal hump, trimmed the side edges as well for a better fit and cut a couple more slits in the front to make it rounder.

Gluing all of these pieces edge-on to the body and wings added a lot of weight, so next time I'll try adding little tabs to the edges for face-to-face attachment with way less glue.
Chucked up a toothpick in a drill and used and a needle file to turn it into a gun barrel. Then cut into the wing root to install it, one on each side, as seen above. Each gun barrel nestles along the fuselage wall, which is nice for strength for when one snags on something on landing. The Sharpie's black is too dark; I shoulda used pencil to color it silver-gray.
After rolling the refueling probe paper around 20 gauge steel wire (on the third try, when I learned to wax the wire so that it can slide out!) and before attaching the refueling probe to the body, cut the tube behind the nose so that the forward portion and the wire are removable. If the forward portion were in place for flight, no way it would hold up to even gentle landings. The cut is at an angle so that I know how to orient it when I re-insert it.
As seen above, made a little display stand out of a square of double-thick cardboard, like the paper stand on Ojimak's recent planes. The clear plastic finger grip on the belly of the plane slides into the slot.
Temporarily stuck a roll of steel wire in the nose for balance, straightened all the flying surfaces, took a biiiiiiig breath... and test flew it into the crash net.

The dang thing worked! I started from close range and stepped back a little further each time. I tried sticking a little cone of foam rubber to the nose for crumple protection, though with the crash net it wasn't necessary. The badminton net upgrade to the volleyball net-like coarse mesh that I used before was essential for this small of a plane. Very appropriate that my inaugural flight in this series is with a Navy carrier plane.
Then cut pieces of steel wire and glued them at the very front end of the nose, so that the longest possible lever arm from the CG would mean the least weight to balance, and also so that the pieces could be glued in solidly inside the nose cone.

**ReynoldsSlumber** · #27 08-15-2023, 01:40 AM

Hi Mike, excellent point about the ram air drag of a tube closed at the back potentially being less than the drag of an open tube. I was so burned by how much drag the nacelles on my earlier F7F Tigercat seemed to produce that I wanted to try something. Someday I'll have to homebuild a wind tunnel with force balances and do a comparison test! Plus incense smoke flow visualization.

Nice idea to use a deer net as your crash net! Best bang for your... buck.

I wasn't so worried about the incidence of the wing on the body, since that only has to do with whether the body is level at cruise. For me the priority was scale as well as not needing to modify things from the plans too much.
The incidence I was really worried about was the angle of the tail relative to the wing, since if the plane ends up needing a lot of nose-up trim, with that tiny horizontal stabilizer I was risking running out of elevator authority. The model, being a lot heavier than the profile test plane, does require more up elevator so that it flies at a higher angle of attack. But it's manageable, so the horizontal stab incidence worked out.

About the wing airfoil, again I was trying to keep the look scale, so rather than bend in a lot of camber at the front, I gave it just a little and also gave the back some undercamber. In between I had to let the bottom surface of the wing bulge out a little, to accommodate the shape of the fuselage. So the airfoil isn't quite ideal but at least better than if I had just gone with the all-convex airfoil that was in the printed plan.

Photos of the finished plane coming soon.