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Old Jul 19, 2011, 02:03 AM
Go ahead. Meme me!
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,627 Posts
That's true, the ideal shape would be bent in a sort of phallic manner to match the curving free stream. But the point is you never want to try to create lift with such a thing, it should just blend in with the free stream as much as possible and have rounded sides that let any lift potential spill around. Even the symmetrical nose of the red plane in post #1 will produce a great deal of drag since it is wing shaped (square tips) instead of rounded into a torpedo shape. I know it seems logical to shape everything like an airfoil but the trouble is that stubby shapes like that are absolutely terrible at creating lift. It's always better to simply reduce drag, and let the wing handle all the lift.

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Old Jul 19, 2011, 10:09 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Design criteria

We have discussed elaborate fairings on this forum before. In the above case the pod could be rounded and faired into the wing and fuselage, but that would have added weight, cross section area and construction complexity. A wide faired mount would have been complicated by the wing dihedral. As a percent of overall performance, I don't see the flat pod sides as much of an issue. The pusher prop and high thrust line caused more problems.

Any design is a local minimum. The evaluation criterion is "how well does the constructed result satisfy its' design goals?":

1. Frontal camera view.

2. 20 minute flight duration.

3. Sufficient payload capacity and access.

4. Reliable hand launch and recovery.

5. Construction requirements within the skill set of the builder (me).

As Kevin said above, 'all it has to do is fly level and take pictures'.

Tom
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Last edited by Tom Harper; Jul 19, 2011 at 11:46 AM.
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Old Jul 19, 2011, 01:15 PM
Go ahead. Meme me!
vespa's Avatar
Thousand Oaks, CA
Joined Mar 2004
2,627 Posts
Quote:
Any design is a local minimum.
Absolutely, and that's why some aircraft are designed with lifting fuselages and such. Sometimes trying to avoid lift ends up adding more drag in the form of weight or frontal area. I'm just letting you know that in general, anything you can do to make the pod less cambered, less wing-shaped, and less likely to create lift will help.
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Old Jul 19, 2011, 01:31 PM
Herk
HerkS's Avatar
Virginia USA
Joined Jun 2007
1,649 Posts
Tom - did you also mention an autopilot???
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Old Jul 19, 2011, 02:02 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Herk,

Yes. I've been using Attopilot v1.8 and 2.0. This one will probably get an IMU.

Tom
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Old Jul 23, 2011, 12:30 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Got Started

I moved the building board into the den - understanding spouse.

Tom
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Old Jul 23, 2011, 03:23 PM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
13,613 Posts
I picked up an interesting book this morning while in town.

'Unmanned Aviation'. A brief History of Unmanned Aerial Vehicles' by Laurence R. 'Nuke' Newcome.

So far I have only had a quick scan through it, but it's amazing how long UAV's, RPV's, Drones etc have been around. Some interesting pictures as well, even a twin boom UAV, looking very much like some current models, flying back in 1974.

Couldn't find an asymmetrical one though.
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Old Jul 23, 2011, 04:54 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Thanks, eflight, sounds like a neat book.

Once I finish the fuselage, I'll slim down the section thickness a bit on the pod, then build it.

Should be interesting.

Tom
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Old Jul 25, 2011, 01:05 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Some Progress

I cut and glue a couple of pieces when I have a few minutes.

The spouse hasn't tossed me out of the den yet.

Tom
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Old Jul 26, 2011, 12:20 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Fuselage frame finished

I can set this aside and work on the pod.

Tom
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Old Aug 09, 2011, 10:55 AM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
POD sides

Completed the thickened NM57 section for the pod sides. Need to do a little more sanding to outline then I can assemble the pod.

Tom
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Old Aug 13, 2011, 01:36 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Pod

I had some time to rough out the pod structure. There's plenty of room for a variety of cameras. The thin wall aluminum tube will be the main spar. The outer wing panels slip onto it.

Pod construction and center wing panel next.

Tom
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Old Aug 17, 2011, 06:33 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
First Assembly

First try at assembly looks good. The center panel will be sheet covered after I add the circuit conduits and spar web.

Now the sections are held together by 1/4:20 nylon bolts. On final, the center section will be glued. The outer wing panels will slip onto the aluminum spar and the inner ribs will be held to the fuselage, and pod, by 1/4:20 nylon bolts and nylon wing nuts.

Tom
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Old Aug 19, 2011, 12:54 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
Joined Jul 2004
3,661 Posts
Center Panel

The center panel is sheet covered. The outer panels will be sheet covered to the 40% point with cap strips beyond that. The scheme I have for the TE looks OK but the bottom will have to be sheet covered through the cusp. The section is very thin near the TE and there's no way I could get Monokote to stick to that concave curve. There are small peaks where the sheets join, but those will sand out.

The LE is formed by rolling 1/16th balsa around the nose of the section. I was concerned about that, but it turned out to be the easy part - thanks Norm!

The wing nuts protect the nylon bolts from being dislodged by the sanding block.

Tom
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Old Aug 19, 2011, 03:33 PM
internet gadfly
nmasters's Avatar
Colorado
Joined Aug 2006
2,186 Posts
Quote:
Originally Posted by Tom Harper View Post
The center panel is sheet covered. The outer panels will be sheet covered to the 40% point with cap strips beyond that. The scheme I have for the TE looks OK but the bottom will have to be sheet covered through the cusp. The section is very thin near the TE and there's no way I could get Monokote to stick to that concave curve. There are small peaks where the sheets join, but those will sand out.
This is the the way I made my trailing edges back when I was still building:

Back then I tried to get my TEs sharp enough to slice tomatoes. All I succeeded in was making them so thin that they got dented during transport and that just looks bad. A small bluntness is okay. As long as the TE thickness is less than 5% of the airfoil thickness the extra drag is insignificant to a powered plane.

Quote:
The LE is formed by rolling 1/16th balsa around the nose of the section. I was concerned about that, but it turned out to be the easy part - thanks Norm!
You're welcome I don't remember recommending that but it is the way they did it on plywood wings back in the day when big planes were made of wood. I do have one concern though. Before you skinned that pod it didn't look like there was very much structure. It should be able to support 4 times the weight of the payload at the same CG location as the real payload. This load factor is to make sure that your camera doesn't stop somewhere before the plane rolls to a stop after a moderately hard landing. If you anticipate very hard landings or sever turbulence the load factor should be higher. If it looks like the pod may fail a couple strips of strapping tape, under the Monokote, to reinforce the D-tube will go a long way toward strengthening it.

--Norm
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