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I need help locating CG on straight flying wing

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Old 01-28-2004, 08:51 PM
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SLaPPiE
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Default I need help locating CG on straight flying wing

As the subject says, I need help locating CG on straight flying wing.

What I want to do is take a 30" span, 7" chord, 1.25" thick funfly wing with 2" wide aelirons (elevons), and top it with a fuse like the Kavan Projeti "Reno Edition" http://www.hobby-lobby.com/projeti_reno.htm

The 13" fuse is pink foam, and looks kinda like a ducks head, wing mounted at the very rear of the fuse, is designed for speed 300, geared for direct drive. It will have two depron vertical stabs.

Right now it is balanced at the spar. Motor is flush with wing trailing edge of wing, and battery is about an inch forward of leading edge. I want to use hs-50 servos and 6 cell 2/3 A Packs, knowing full well that can't be enough.

Can anyone suggest the cg with this info, and do you think it will fly?

Thanks in advance.
Rich
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Old 01-28-2004, 09:28 PM
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Tall Paul
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Default RE: I need help locating CG on straight flying wing

Should work fine, begin with the c.g. about 12%..
Here's one I changed to tailless.
http://home.earthlink.net/~pjburke1/Tailless.htm
Did it to a profile Somethin' Extra also..
The most important is to have the c.g. less than about 20% to start, 12% to 15% is a good place. Move it back as you feel the plane out.
I built the profile SE at the same time as the kit version, to see if I could lighten it, down to where my Fazers get... difference in weight
was only about 6 oz, not significant.
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Old 01-28-2004, 11:12 PM
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SLaPPiE
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Default RE: I need help locating CG on straight flying wing

Tall Paul, thanks for the reply.

That looks cool. How does it fly? Spinsation?
That looks pretty close to what I'm going for, except the vertical stabs won't or weren't going be as far back as yours.

I'm sort of noob to the terms, but do you mean to take the wing chord (7") and go 12% of 7 back from the leading edge when you say "begin with the c.g. about 12%.. ", or should I add the aeliron width to the wing chord then go 12% back.

I get the feeling you want the plane pretty nose heavy to start.
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Old 01-29-2004, 12:28 AM
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Default RE: I need help locating CG on straight flying wing

Slappie,
You got it right....
I've gone as far back as 20% on my wings..so, if you have poor elevator response you may be a little nose heavy.
I average about 16-18%...
which if calculated on your straight wing of 7 inches
7 x .16 = 1.12 inches from the leading edge
7 x .12 = .84 inches from the leading edge

So...nice estimate would be to balance it right at an inch from the leading edge and go....

Don't forget to have a couple degrees of reflex in your elevons (both slightly up).
Wings like to have a little reflex...

I've done the same thing as Tall Paul has before....funfly airfoils work great at flying wings....

Good Luck!
CrazyHerb
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Old 01-29-2004, 12:52 AM
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Tall Paul
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Default RE: I need help locating CG on straight flying wing

Be sure you DO include the ailerons in the chord measurement. They are part of the wing.
.
For a non-symmetrical airfoil, the reflex (trailing edge up) will be somewhat more than a symmetrical airfoil would need.
I used about an 1/8" up on the Krafty modification.
The SE needs a lot less, trim can do that once it's flying.
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Old 01-30-2004, 10:24 PM
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Default RE: I need help locating CG on straight flying wing

Hello All,
Wow! Thanks for the great info you all! Unfortunately, I have a similar dilema on one my wife and I are building. We want to build a flying (hopefully it will fly!) diamond. Essentially, it will be a perfect square with the engine mounted on one of the tips. It will be fully symetrical from the engine being the thickest point to the aft edge being the smallest. We are going to put a vertical fin right down the middle. My predicament is trying to figure out how to do it all. It works well on paper, but how do you do the CG of a wing that shape?! It will still have elevons and a rudder, but any ideas on the rest? Help with the CG? Anything is more than I have.

Thanks everybody!


Red
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Old 01-31-2004, 12:43 AM
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Tall Paul
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Default RE: I need help locating CG on straight flying wing

Put the c.g. about 25% of the diagonal.
It would fly better with ailerons and elevators instead of elevons.
The forward sweep screws up the elevator action if it's "full-span".
The inboard part of an elevon contributes little to the rolling ability.. a "real" aileron has a better mechanical advantage on the air, so cutting out the elevator part and have an elevator won't hurt the roll.
Start with a bit of reflex for the first flight. You can always hold it out if you don't need it, but if you need it at takeoff, you may not have enough time to put it before the plane goes down.
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Old 01-31-2004, 02:47 AM
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Default RE: I need help locating CG on straight flying wing

For the diamond you'll need to use the proper MAC to determine the CG. But on your diamond that's easy because it's actually just 1/2 the root chord located at 1/2 the way out the span of one wing. So if it's an 18 inch root chord and 18 inch span the MAC will be the 9 inch chord point 1/2 the way out the one wing. Taking 20% of that as your balance point puts it at 1.8 inches back at the MAC. Or 4.5 + 1.8 = 6.3 back at the root or fuselage. This will mean that you have 20% of the area in front of the balance point and 80% behind it just as it should be.

Sorry Paul but this one isn't that automatic. 25% back along the leading edge diagonal is only 25% of the first half of the wing or 9 inches. That would only put the balance back to the 2.25 inch mark at the root and make it totally too nose heavy.

I do agree with Paul's idea to separate the elevator and ailerons in this case. The sweep forward angle of the trailing edge means that only 1/2 the surface of the elevons would be doing what they are supposed to be doing anyway unlike with a plank style wing.
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Old 01-31-2004, 11:09 AM
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Default RE: I need help locating CG on straight flying wing

Thanks Y'all! I really needed that kind of help. I will be printing this out to use when I get it that far. You guys are the bomb! Thanks again.


Red
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Old 01-31-2004, 01:20 PM
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Default RE: I need help locating CG on straight flying wing

Oh, I dunno... 25% of the root isn't seriously nose heavy...
For a starter position it's close to the 25% mac point... in the "probably can make a 2nd flight" direction.
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Old 01-31-2004, 09:34 PM
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Default RE: I need help locating CG on straight flying wing

Well, while you all are being so helpful... what do you think about the fin? Any advice on size and dimension? I would like it to go from the front to the back. How much should I use for the rudder? Will it be a percent or a dimension? Any information is still helpful!

Thanks

Red
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Old 01-31-2004, 10:07 PM
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Tall Paul
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Default RE: I need help locating CG on straight flying wing

Vertical.. about 9 inches back.. root chord about 8, tip about 4
About 4" tall...
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Old 02-01-2004, 02:24 AM
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Default RE: I need help locating CG on straight flying wing

ORIGINAL: no1speshal

....the fin? .....I would like it to go from the front to the back......
A full length spine type fin is not going to work. The area forward of the center is going to act as a destabilizing force and cancel out the part to the rear. Paul has the right idea in his cute and perky little sketch. Follow that and you'll be fine.

Paul, when you said 25% of the diagonal I was thinking the diagonal leading edge rather than your centerline diagonal of the "square". My boo boo. With your's it would be nose heavy but safely so and at least it shouldn't lawn dart right off the bat like the other position would....

The sketch has me all enthused to try something like this.
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Old 02-01-2004, 01:24 PM
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Tall Paul
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Default RE: I need help locating CG on straight flying wing

1/4" foam board, and a slope would be a good combination for trying the idea, both for c.g. and control mix, forgetting the power for the present...
This one took about an hour to build..
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Old 02-01-2004, 01:30 PM
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Default RE: I need help locating CG on straight flying wing

It is refreshing to ask a question and get straight educated answers. I frequent another site related to Subaru performance, and replies range from guesses to smart arse comments.
Tall Paul, I had no idea it was PJB, You changed your handle. You sent me a 3D drawing of an FP-40 for my Profile design some time back. I went to your link, figured it out and LMAO.
Thanks for the info Herb, I knew nothing about needing reflex (except that mine are slow).
Do you guys think 3/32" reflex in the elevons?
Should I put some thrust in the motor? Right now it is parallel with the wing center line on the horizontal. Should I tilt the prop (rear) up a degree or two?

I like the Diamond Delta Rendering. Seems like a cross between a PBF and a FloppyDisk. What if you changed the hinge line at the center of the wings, so that the elevons were like 4" at the root, and 1" at the tip?> Would that be enough to offset the angle of the control?
Thanks
Rich
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Old 02-01-2004, 04:01 PM
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Default RE: I need help locating CG on straight flying wing

That motor look too high. There's a serious nose-down pitch lurking there on launch.
A couple degrees of tilt up at the back end wouldn't hurt, but lowering the motor would be better.
I'd suggest trying it off a slope first. Power-off launch, then motor on, to see how it flies, and you'll have some ground clearance
.
Then try a motor-on launch.
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Old 02-02-2004, 01:23 AM
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Default RE: I need help locating CG on straight flying wing

I agree, you should add about 4 or 5 degrees of upthrust. Also it looks like you have not turned the prop around yet to act as a pusher. The printing on the front face needs to face the.... uh,.... FRONT Sorry, sorry, I know, snarky comment.... LOL

The amount of reflex you need with your symetrical airfoil will depend on where the balance point is. The further back the less you need. Bearing in mind it does need to balance at least a little forward of the 25% point.

It would be good to have a friend do the first launch so you can be ready on the stick for more or less elevator in a hurry. Or launch out over tall grass.
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Old 04-03-2004, 08:52 AM
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Default RE: I need help locating CG on straight flying wing

Hello All,
Well, since you all helped me so much, I figured I would send you the finished results. I name all of my airplanes, and this one deserved its name. This one is called Organized Confusion. I have a Magnum .40xla powering the craft, with futaba 3004 servos all the way around. I listened to each of you and decided to split the elevons into elevators and ailerons for better control. It balanced out just where you all said it should, but it was by accident that it happened that way.
This was the second airplane I have built from scratch. Actually, the first was a J&C Funbat, so that wasn't really a scratch built. Anyway, my wife and were batting around ideas for a new plane, and this was what we came up with. It is all made from that foam board (paper covered foam from a store like Micheals) with balsa ribs. It weighs in at around four and a half pounds. Since this is my first endeavor from conception to airworthy, can you all give me some feedback on what you think?
Thanks again for all your help. It's guys like you that keep this hobby thriving!

Red
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