Newbie to ornithopters, carbon rod build

[anestho]

Hi, I am new to ornithopters. I built up a carbon rod version (17" wingspan)of the Freebird by Nathan. I built my own gearbox with 28:1 gear reduction using a 7 mm superslick pager motor. It will come in about 15-16g using the 0.9g plantraco reciever, 80 mah Lipo, and 1.1g actuator.

How do I determine the CG - I glide RC planes to figure out where to place CG, but how to do it for ornithopters?

What can I use for covering? Mylar? plastic garbage bag? Or do I have to use tissue?

[Chronister]

You can use a plastic film covering, but it should be a hard plastic like cellophane, not soft like mylar or plastic bag. Softer plastic will require more bracing.

The CG is normally about 1/3 chord, but may differ depending on the particular design. The only way is to test with wings flapping.

[anestho]

Is the tissue stiff? Where can I get cellophane? Is it that stuff that clear present wrap is made of?

[Tail Spin1]

I think that’s the same stuff. I’m no Martha Stewart (thank goodness) but I think that’s the stuff they use to cover gift baskets too. Check with your local flower shop, they may have some scraps, or some they will sell. I wonder if cellophane comes in different thicknesses, or if one size fits all? I obviously have never used the stuff before.

Zach

[Sean Kinkade]

What's wrong with Mylar polyester film? I have two rolls of clear Mylar, 1mil and .5mil. I sell it by the foot or by the yard. The rolls are pretty wide. I'll have to check their exact width but they are at least a yard wide, maybe 54".

For an example, the standard Tim Bird toy ornithopter uses 1mil Mylar for its wingsail. They also made a "robotic" design Tim Bird with a prismatic metallic finish and its sail was thinner, .5 mil Mylar. They both work fine for a small ornithopter.

[anestho]

It is a custom 28:1 gearbox made with timer gears and basswood. The motor is interchangable. the orange or red 7 mm motor can be used. The gear ratio can also be varied by changing the spur gear 8 T or 9 T.

I have not covered it yet.

Everyhting can be adjusted including the upward and downward wing movement. The linkages are carbon rod with 22G wire for attachment.

[anestho]

The frame is carbon rod. The linkages are adjustable so that the upward and downward wing swing can be adjusted.

[anestho]

I have a flapping video but its over 1 MB. Don;t know how to make it smaller.

[Mikey C.]

I have a flapping video but its over 1 MB. Don;t know how to make it smaller.

Try to host it on http://www.putfile.com

You have to register first but they will host 10mb for free I think.

[anestho]

On low throttle, the frame is stable, but on higher flapping rate, the frame shakes. What could be the cause? Is it a out of balance issue or is this normal for orni's?

[Chronister]

You can't expect it to run properly with no tissue on the wings. The inertia loads will be too high and that causes all sorts of problems.

I didn't know Tim Bird had mylar wings. Are there different types of mylar or is it just the added thickness that makes it so crisp?

To clarify what I meant by "stiff": A material that can stretch easily will not work as good as something that's non-stretchy. Many plastic films are too stretchy. Why is this important? A non-stretchy material when curved acts as a rigid element. This allows the portion of the wing membrane extending beyond the frame to support air pressure and produce lift. A stretchy film cannot support any load outside the frame boundaries and therefore requires more bracing, which adds weight and interferes with the airflow.

For more info on bracing, see The Ornithopter Design Manual (http://www.flyabird.com/pub.html).