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Old Feb 19, 2008, 08:59 PM
K4UAV
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Miami, FL
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Another twin boom pusher - spud

After wanting a twin boom pusher for some time, and reading someone say "Just cut some wood, dammit!" I decided to do just that. This design is heavily inspired from KD7OST's (Dan) twin boom pusher. Basically, it is his general dimensions but only 60% of the size and shortened a bit more as well. Since he photographs potato farms (amoung other more interesting things) I decided to name it 'spud'.

Since I have no design experience whatsoever, but I think I am a pretty good kit builder, I ventured into the world of scratch building. I took a rib from a herr cloud ranger and built a 60" wing with mostly carbon spars. The control surfaces have tyvek hinges sandwiched between 1/16 balsa.

I shortened the design considerably as I want my digital camera to sit in the nose so it needs to be closer to the wing, which then makes the tail need to come a little closer to get the CG where is should be. Will this make her overly pitch sensitve, I dunno, I'll have to wait and see. Will she even fly, I hope so. An Axi 2814/12 will be doing the pushin' with a 10-12 x 5-6 prop. 3 cell lipo somewhere between 2100 and 4200mah, a digital camera, video tx, and autopilot will be the payload. Here is where I am so far...

Any thoughts, suggestions, words of advice are appreciated. If neccessary, I can increase the length of the booms and/or make a new fuse (it is after all, just a box).

-dave
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Old Feb 19, 2008, 09:07 PM
K4UAV
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Miami, FL
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I just put in the camera (Nikon 3700) and she balances right on the main spar. Granted I need to add the motor, esc, batteries etc., but since the fuse is so big, battery placement should be able to compendate for a large number of configurations.


I'd also like to make the wingips curve downwards, like this pic of a Hermes 450. Any thoughts on that?


-dave
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Old Feb 20, 2008, 04:47 PM
Professor of Wood
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I think the tip drop, (Horner tips) are a good idea. They help separate the upper and lower airflow at the tips and reduce the vortices that rob lift at the tips of the wings. The tips used in commercial aviation (The ones that point upward) do the same thing but we can get away with pointing downward in high wing planes. Nice touch IMO.

Dan
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Old Feb 20, 2008, 08:07 PM
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Sydney, Australia
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Try Aerosonde for ideas

Down here in Australia there is a company which specializes in commercial UAVs of about 4m wingspan. Have a browse at
http://www.aerosonde.com/drawarticle/42
Which reminds me: I have a similar exercise almost completed which I must get back to...one of these days...
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Old Feb 20, 2008, 08:30 PM
Professor of Wood
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It does look quite short coupled. These mid motor/engine designs are pretty responsive to elevator as you stated. I would go pretty easy on the throw. I try to put the center of my stab rearward of the wing by about 50 percent of the span.

Dan
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Old Feb 20, 2008, 11:39 PM
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Quote:
Originally Posted by dalbert02
I'd also like to make the wingips curve downwards, like this pic of a Hermes 450. Any thoughts on that?


-dave
You could achieve the same effect with tip-plates. Simple flat things attached to the end of the airfoil. From the side view they can be in many shapes, make them look like missiles. Much easier to make.
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Old Feb 21, 2008, 02:41 AM
W4UAV
Gainesville, FL
Joined Nov 2005
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Dave Im concerned about your landing gear location, too.
Im forecasting rotation problems on takeoff.
Mike
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Old Feb 21, 2008, 09:49 AM
Professor of Wood
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Quote:
Originally Posted by RaptorAP
Dave Im concerned about your landing gear location, too.
Im forecasting rotation problems on takeoff.
Mike
I hadn't noticed that but I agree with Mike on that one too. It will sure stick on landing but rotation will be high speed with massive up and a leap off the ground.

I would draw a line from the center of lift of the wing. From the main spar. Have this be a straight line heading down and back at 15 degrees from the spar. It will work very well to have your main LG axle intersect that line.

Dan
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Old Feb 21, 2008, 10:04 AM
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Yes, now that you mention the gear positioning, I see it too.

I also tend to think the wing should be mounted as far back on that fuselage as possible, for cg purposes. A plane is supposed to be slightly nose heavy when the bird rests on something at approximately the airfoil hump. Put the wing mounting sticks as far back as they can go. Otherwise, the weight and distance of things behind the airfoil hump (motor, main gear, tail, tail-servos, covering) may win over what's ahead of the airfoil hump (nosegear, camera, cam servo, battery?).


And then back to the tip plates. They have an aerodynamic advantage over the droop tips. Except for smooth styling, they provide what droop tips provide but also add a "discontinuous leading edge". There's a thick NASA paper which talks about it's advantages in stability at slow speeds. I've been involved with others in a Rutan-type airplane forum on this.
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Old Feb 21, 2008, 10:19 AM
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Quote:
Originally Posted by RaptorAP
Dave Im concerned about your landing gear location, too.
Im forecasting rotation problems on takeoff.
The rotation on a smooth surface will be very long. On bumpy surfaces it'll cruise along until it's bumped off. The gear placement will still work. Just expect a VERY long takeoff. On the plus side you don't have to worry about prop strikes

Moving it back say about 3" behind the CG should work out. Basically in a nutshell. You want the plane to 'rock' back enough to allow an upward angle of attack during takeoff.. However you don't want it to close to the CG that it won't sit on its landing gear w/o falling on its tail feathers. Don't forget prop strike. Allow it to rock back, but don't let it rock back and hit the prop.
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Old Feb 22, 2008, 07:22 AM
K4UAV
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Miami, FL
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Thanks for all the tips. I think I will just lengthen the booms and remake the fuse.

Thanks
-dave
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Old Feb 22, 2008, 10:50 AM
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Just to muck up the waters, about the boom length. I'm not so sure I would change that if >I< were building that plane. It looks short-coupled relative to other aircraft that have tails, but not as extremely short as a flying wing. I tend to think it can be made to work well by how you rig your servo-controlsurface throws. I'm sure you'de get used to how it flies.

The longer the tail is the longer the nose needs to be and the more overall weight you'll have. More weight results in higher wing-loading and higher landing speeds. But you probably know all this.

It does look nice.
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Old Feb 22, 2008, 11:24 AM
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MAC*1.5 or MAC*2 = distance from Wing TE to Tail LE is usually the general rule.

I'd say have fun with this plane. You can always make another one with the lessons from this one.
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Old Feb 23, 2008, 07:25 AM
K4UAV
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Miami, FL
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Thanks again. Someone once told me, "one carefully controlled experiment is often times worth more than a thousand expert opinions". Not that I discredit any of you (believe me, you all know more then I), but since there is some conflict about will it or won't it fly, maybe I should just throw her up and see what happens. Maybe some lead rather then a camera but the rest is rather cheap balsa. The price of education I guess.

If I modify the existing fuse to put the wing further back, would that be safe? Also, the reason I have the fuse short in the first place is that I am using a digital camera in the nose for FPV flying and also to take pictures. If I make the nose too long, then I need to balance that weight out somehow.

Also, what is MAC? Sorry

-dave
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Old Feb 23, 2008, 08:59 AM
Professor of Wood
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I think it could fly Dave. The rotation will be an issue just due to the far rearward placement of the main gear. It is short coupled so may be pretty pitch sensitive. Just knowing that though you should be OK by making sure the elevator throw is pretty minimal. Or at least have the ability to go to low rates with maybe 1/4 inch or so throw in the upward direction. You can get by with less than that for down throw.

MAC is Mean Average Chord. I'm not sure what Scott, (Typicalaimster) is getting at there. I do know he has designed successful planes. In your case you have a constant chord, non swept wing so the average chord for you is the chord at anyplace along the wing. Lets say it's 12 inches for an example. 1.5 to 2 times the MAC then is a range between 18 to 24 inches. Scott is therefore suggesting that the distance between your wing trailing edge to your stab leading edge fall in that 18 to 24 inch range. (edited to include...Now that I think about it, Scott's numbers should align well in his process with my numbers using my process. They seem pretty close.)

I've never done it like that. In my designs I measure the half span of a conventional wing. For me this means a wing that has an aspect ratio of between 1:6 or 1:8 range. I'm sure you're in that range. (1 foot chord by 6 foot span for example is 1:6) In such a design I would measure from center of lift of the wing, (At the spar in a constant chord wing) and put the center of the stab 3 feet back.

These are numbers that are known to be stable and you can work them with a pretty good chance of success if your other lines are at good angles. Angle of attack (AOA) of the high lift wing, AOA of the stab etc. And motor thrust. You look to be good on those because everything lines up well along the flight path. You can get away with zero's pretty well across the board to start.

Anyway, these numbers aren't a hard fast rule. They are simply a good starting point. A glider wing for example may have a 20:1 aspect ratio. Neither Scott's or my process hold up. Very small chord and huge span. Some planes have stubby wings and longer moments and all variations in between.

The big thing perhaps is to start in a design with an idea of what kind of flight do you want to have. Radical, Gentle, trainer (self righting) etc. Then sticking with lines similar to what other designs follow that achieve your flight envelope is an easy way to go to give you a better chance of success. But it might fly sweet right off the board anyway.

Dan
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