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Old Oct 26, 2013, 07:24 PM
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wing design question

I am looking at building a large wing. I recently started a thread about it but I deleted it. I built one a few years back and it flew pretty well but it was a fluke accident. I somehow managed to design in enough room to allow for a decent COG. So, here I am with a fully taped wing complete with motor mount, control horns, and hinged ailerons. I did a dry run with all the equipment sitting on the wing annnnnnnd it just flips backwards. There is no way to add enough weight to the nose to make it work. If I move the motor up to just behind the COG and move the COG back an inch it balances.... mostly. CLose enough for me to use a little weight to bring it in anyway. But thats only IF the original COG is off from the COG calculator ( http://fwcg.3dzone.dk/ ) .
Current wing design already built:
Name: current wing.PNG
Views: 41
Size: 25.6 KB
Description:

After looking around I noticed that all of the successful wings I see buzzing on youtube share a common characteristic: a much thicker chord than I chose to use. I went back to the COG calculator and played with some numbers and I came up with this:
Name: deeper chord.PNG
Views: 42
Size: 36.5 KB
Description:

This design places the COG past the halfway point of the root chord and gives me way more area to play with for component placement and ultimately more leverage for the battery to offset the motor weight. Is this what I need to make my wings balance better or is there a factor Im ignoring?
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Old Oct 26, 2013, 08:07 PM
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center of pressure or pressure distrubutions...That's why the center of pivot is different from the center of mass.
That why delta has a better chance of success because most people just comprehend it as just moving the CG
around ...by chance...which isn't the same from using the CG calculator.lol
The wing being swap back more...it acts sort of like a rear stabilizer. Hope that make sense.

Try to balance a pencil around 6 " on your finger tip.
Then a 12" ruler...
It's easier to balance the 12" ruler.
The swap back wing being wider or taller. It's easier to keep in balance.
I can balance a jet model such as an f-18 on it's nose with my finger tip easily because it's long.
Hope that makes sense.

Pressure Distribution on a Wing (5 min 54 sec)


Mass & Balance: The relationship between CG and CP (1 min 28 sec)
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Old Oct 27, 2013, 05:51 AM
An itch?. Scratch build.
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Also if you have that pencil and a 12" rule, (or preferable something a bit stronger), you can use them to make a simple seesaw balance.

Then place your motor/adapter/prop at one end of the seesaw, and your battery at the other end, and see where the balance point is.

That will give you a rough idea of where the battery will need to go in relation to the motor either side of the models calculated CG.

The ESC, Rx and servos can almost be ignored if nearer to the CG line. Just make sure the battery is movable slightly backwards and forwards for CG adjustment.
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Old Oct 27, 2013, 06:50 AM
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Neat videos. A wee bit over my head but mostly understandable. The first video is alittle misleading. that model works well for a straight wing or maybe even a warbird type wing that is more or less swept equally on leading and trailing edge. but with a delta and especially with a swept delta, calculating lift is much more difficult. you end up with varying amounts of lift at different points along the length of the wing. Not only that but the area of lift extends from in front of the COG to well behind the COG. That portion of design boggles my brain. In its static condition there will seem to be a constant backwards tilt. But in motion the lift coming from behind the COG banaces out. How do you plan for that?

I lke the idea of the sea saw balance. Have you used it at all? Im curious to see how well that works. Last night after I tried to test balance my plane and it failed I stripped off the control surfaces and prepared to file 13 it. Then I tested the COG again. It still back flipped. Should the wing be able to balance on its own when empty? Additionally, if I balance out my motor/battery/flight gear and add that to a plane that already tips backwards, wont it still just tip backward?
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Old Oct 27, 2013, 08:43 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
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The seesaw balance is just to give an idea or the weight difference between the motor and the battery about a balance point.

Assume the 12" rule - if the balance point turned out to be in the middle, (6" mark), you then you know whatever distance from the CG the motor goes, the battery needs to go the same distance the other side.
- if the balance point was say 3" from the battery, (heavy battery), and 9" from the motor, (ratio of 3:1), then you know whatever distance from the CG the motor goes, the battery needs to go 1/3rd that distance the other side.

It's just to get an idea of where the battery is likely to be to balance the motor weight. Other things like were you put the ESC, Rx, and servos etc, still need to be considered.
But say you wanted the motor in the nose, (that's probably further away from the CG than a pusher), then the balace gives you some idea of how far the battery needs to go back behind the CG. For a swept back wing that may be a major problem with a short 'fuselage', and shows why many wings have the motor as a pusher.

Whether the wing has all the gear or nothing, the wing still needs to balance at the CG, where the calculation recommends.
So even with no motor radio gear and battery, you still need to add some weight to get the CG correct for it to glide. The calculated CG is the flying balance point whatever the weight of the wing.
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Old Oct 27, 2013, 09:43 AM
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It's just to help you understand better the different forces at play.
The motor's location and thrust angle will also effect it.

Example...when you did the dry run...the angle of your hand and the point where you held
the model might had cuased the wing to flipped backwards.

I do know....If I have sufficient vertical stabilizer. it'll prevent the wing from falling out backwards.
slight forward swept.The CG to that thing is probably around the most forward tip of the wing or more forward.lol
Alula sloping (1 min 10 sec)



I prefer to distinguish between Center of Pivot and Center of Mass.
The reason is...I prefer to get the mass as close as I can to the Center of Pivot.
Rather than moving weight to the nose to balance the model. Id add dead wieght
close the center of pivot...or relocate other electronics...such as the servos.
It'll help the control surfaces to be more effective at a slower air speed because less
force are required to counter whatever force...ect
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Old Oct 27, 2013, 08:27 PM
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Here is another question/design issue. I have at my disposal 3 motors. 1: The small parkjet motors from Grayson hobby that rcpowers recommends for his designs. It's a small 40 gram 2200kv motor that draws 20 amps or so. With a 6x4 prop this thing makes an incredible amount of thrust. It's also loud. Lol. 2: a turnigy G10 out runner. It turns a prop anywhere from 9-11" at about 30 amps. 3: a ridiculous G25 that can crank a 12" prop and probably bigger. My last 60" wing I made the parkjet seemed to push just fine.
Flying Wing Dollar -Tree Foam Board- (4 min 26 sec)


Should I keep it or should I move up to the G10? The G25 is not a real option lol. I think the G10 will be overkill personally but I don't know. A 9" prop on a 60" wing doesn't sound too bad but wings require less HP than fuse planes. I use dollar tree foam for my builds. I remove the paper on the sides that touch to reduce weight so my planes usually come in light. I use colored packing tape for color, 9 gram servos, orange ex radio gear, and right now I'm limited to 3000 mAh batteries.
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Old Oct 27, 2013, 08:50 PM
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[QUOTE=DEBAY777;26475514
After looking around I noticed that all of the successful wings I see buzzing on youtube share a common characteristic: a much thicker chord than I chose to use. [/QUOTE]

Erm.. Look Harder.. Mate.
Thinner Moderne foils of moderate chord are the preference amongst those who know what they are doing with ..'wing' ..designs.
Youtube isn't actually the Font of all knowledge :-)
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Old Oct 27, 2013, 10:34 PM
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well, I guess I can only point to the origin of this thread where I am asking for assistance. No offense but I obviously have been looking around but perhaps I have not seen what you see.

The issue I found with my initial design was that the weight behind the COG could not be countered by the flight gear. There was just no way to make it work. As seen in my earlier post the COG was very close to the front of the LE. The second model's COG is much further back and gives me more room to maneuver the flight gear to get the proper COG. Many many many designs follow a similar pattern. Not all of them, but many of them. Im reasonably sure I could have made it puller and not a pusher and salvaged the wing, but I like pushers. No real reason, I just do.
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Old Oct 28, 2013, 03:09 AM
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Id keep it , if you're not wot all the time. That motor is lite.
if you put the g-10 on, you don't have to use all the power.

Since you're building longer. You're not looking for top speed or super roll rates.lol
DTF is only 3mm...with paper removed it's lite.
You're model flies nice and lands nice. I like it.
It has plenty of lift...you don't need anymore.lol

How did you built the wing? 3 layers? If you did it that way...you can build it different
to save weight. So you can install a bigger pack.haha
Just fold the front part of the wing instead of laminating sheets on top and bottom.
or you can just cut little strips for the KFs. You're taping over anyway...you don't need all that foam on there.lol
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Old Oct 28, 2013, 08:54 AM
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Quote:
Originally Posted by smcx66 View Post
Id keep it , if you're not wot all the time. That motor is lite.
if you put the g-10 on, you don't have to use all the power.

Since you're building longer. You're not looking for top speed or super roll rates.lol
DTF is only 3mm...with paper removed it's lite.
You're model flies nice and lands nice. I like it.
It has plenty of lift...you don't need anymore.lol

How did you built the wing? 3 layers? If you did it that way...you can build it different
to save weight. So you can install a bigger pack.haha
Just fold the front part of the wing instead of laminating sheets on top and bottom.
or you can just cut little strips for the KFs. You're taping over anyway...you don't need all that foam on there.lol
The wing in that video is long dead. I built that in the early stages of my DTF building. It was just a whim. I cant even find my original plans with the dimensions I used. I got lucky and managed to land the COG far enough back to make it balance out in the air. Unfortunately I did not make the center strong and it got wet on a frosty landing. A few flights later I must have gotten the battery in the wrong place because it fluttered quite a bit at WOT. It did this weird up/down oscillation and then transitioned into a very bird like flapping motion. And then it folded in half and dropped to the ground from 100' or so. That particular flying V was not very effective at flying.

On this build Im thinking of doing the KF airfloil that uses 2 steps instead of 1. It may be a little too much foam but it also is stronger than just 2 layers. I plan to mount my GoPro on this once it gets to flying good so I want that strength. Im also thinking of switching to spray glue versus hot glue. I use the large hot glue gun and it gets too hot. I have considered the dimmer switch for it but never bought it. The glue comes out in a boil sometimes. Cant they put a thermostat in these things? Anyway, I will use the smaller motor. If need be I can cannibalize my quad and use a motor. It turns a bigger prop but is still fairly efficient. I may get more out of it with a smaller prop. But thats all later on.

Last night I sat down and played with the COG calculator some more. Im beginning to see the mechanics of the wing a little clearer. I can reduce the length of my chord and still maintain the COG at just past the halfway mark. I may keep the longer chord just for room though. I dont want to short change myself when I mount my camera.
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Old Oct 28, 2013, 10:02 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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Quote:
Originally Posted by DEBAY777 View Post
I am looking at building a large wing. I recently started a thread about it but I deleted it. I built one a few years back and it flew pretty well but it was a fluke accident. I somehow managed to design in enough room to allow for a decent COG. So, here I am with a fully taped wing complete with motor mount, control horns, and hinged ailerons. I did a dry run with all the equipment sitting on the wing annnnnnnd it just flips backwards. There is no way to add enough weight to the nose to make it work. If I move the motor up to just behind the COG and move the COG back an inch it balances.... mostly. CLose enough for me to use a little weight to bring it in anyway. But thats only IF the original COG is off from the COG calculator ( http://fwcg.3dzone.dk/ ) .
Current wing design already built:
Attachment 6210208

After looking around I noticed that all of the successful wings I see buzzing on youtube share a common characteristic: a much thicker chord than I chose to use. I went back to the COG calculator and played with some numbers and I came up with this:
Attachment 6210209

This design places the COG past the halfway point of the root chord and gives me way more area to play with for component placement and ultimately more leverage for the battery to offset the motor weight. Is this what I need to make my wings balance better or is there a factor Im ignoring?
The recommended CG location from the Flying Wing CG Calculator you used should work fine with that wing. I have used it several times for Zagnutz and Divinity KFm4 flying wings and it was right on the money as far as the CG location.

To set up the planes I taped a larger round dowel to the bottom of the wing at the recommend distance back from the nose. Then I moved the battery and electronics around until the nose just tipped down. I used Velcro for the receiver and ESC and taped the battery down for a gentle and brief maiden flight. Then I made the batter mounting permanent.

If you put a strip of Velcro loops on the battery and a matching strip of adhesive backed hooks on the top of the wing you can move the battery forward and back a little to adjust the CG later. I cut narrow slots down through the wing and used a strap made from 1/2" wide sew on Velcro to hold the battery to the top of the wing.

You will usually find that you want to move the battery back a little after you do the maiden flight. That will give you a more balanced plane and make it more responsive. But that location won't be very far from what the calculator recommends.

What style wing are you building? The KFm4 makes for a very nice flying wing, that has been proven in a number of designs like the Divinity builds:

48" Divinity II - http://www.rcgroups.com/forums/showthread.php?t=1048425

Jack
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Old Oct 28, 2013, 01:28 PM
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Have you thought about doing an armin wing..if you're going to do muilty steps KFs.

I know for a fact...if I laminate 3-4 strips of dtf with paper.1/2 tall or greater.
It's seriously strong in one direction. Vertical instead of flat. Much liter and stronger
than dowels as a spar. I seriously wouldnt have any problems adding stacks of those
at the center section of the wing with some tape wrap around it. On top of packing tape on the inside and outside of the wing.
Plus an armin wing allow me to recess the servos inside of the wing. Minimizing parasite
drag.
Add bottom section to the vertical stab as a sort of skid to protect the ailerons
during landings...that's when servos get damage.
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Old Oct 28, 2013, 02:35 PM
Jack
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The Armin is slower to build with the same materials and does not fly as well as the KFm4 in my experience. If you haven't tried both an Armin and a KFm wing build it is really hard to describe how much better the KF can be.

I won't argue the point on parasitic drag, the KF might have more drag but, all things considered, I like it better. This "my airfoil is better than yours" thing is pretty subjective anyway. I prefer not to argue the point with anyone that has not tried both...

Jack
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Old Oct 28, 2013, 02:44 PM
I don't do Normal
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I'm new to this myself but I do a CG / equipment location mathematically sort of. It's not exact because of extra stuff added on but its close enough if your battery can move a bit

1. Get a weight on each piece of equipment you need in the same unit of measurement

2. In the middle of a piece of paper mark an X or something labeled CG.

3. ruefully get a measurement for how far away each piece of equipment will be from the CG

4. sketch each piece of equipment on the paper on the corresponding side of the CG and the measurement

5. at the top of the page mark an equal sine then start multiplying the weights by the measurement and labeling them on each side of the = sine at the top

6. when you have them all done and on the right side of the = sine add each side together and mark it down

7. the total of each side should be the same for the CG to be located properly

8. you can't change the weights of equipment but you can move it farther away or closer to the CG to make the two sides equal. simply play with it a bit to get it to work

See pic for a bit better understanding hope this helps a bit
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