Mike Connor

My Feedback: (5)

The only CG calculator I could find that would do complex shapes would not work for a delta. Any one have a link?

combatpigg

My Feedback: (3)

Mike, I would turn your drawing into an uncomplex shape. Project a line along the LE from the wingtip into the root, then just do a simple calculation for tapered and swept wings. The area forward of the imaginary line will contribute a little lift, so base your CG on 17-18% of MAC [disregarding those strakes] and it should be flyable.

Mike Connor

My Feedback: (5)

cp,
I did it your way and marked my copy (8.75" back). It is probably close but if someone comes up with an actual calculator it will be interesting to see where the CG ends up.

I always use the point where the leading edge would intersect in front of the plane to measure my CG and not the leading edge of the motor mount (yes, electric motor mount). Is that the correct point to calculate from?

CloudyIFR

Mike,

How about Flying Wing Calc at www.TailwindGliders.com

Here's a few screenshots of your model in the Excel Spreadsheet.

I estimated the weight so the wing loading may be incorrect.

Curtis
Montana

Mike Connor

My Feedback: (5)

Curtis,
Your #'s look right except for the sweep angle. The inner panel is 45* and thee outer panel is 30*. The trailing edge is a constant 12*. Your NP and CG points look about right but wonder if the sweep angle being off would change the #'s?

That Excel spreadsheet looks like what I want but it wont open in my OLD version of Excel. I may download the spreadsheet software from Java and see if it will open it.
Thanks

CloudyIFR

\Mike,

The dimension I need is the chord length of the inner panels tip or the outer panels root. This is the same dimension, I had to guess a little with the use of a CAD program.

Note that the dimensions in the spreadsheet are 1/4 chord points NOT the leading edge angle. Also, If you give me the leading edge angles I can work it out easily as there is a Sweepback converter in the spreadsheet too!

If you still have issues opening the file send me your version of Excel and a Private message with your email and I'll save it in that version and we'll see if that works. You may lose some eye candy in the graphs but that's all it is.

Curtis

Mike Connor

My Feedback: (5)

Curtis,
You were real close on the outer panel cord. It is 11". I am embarrassed to say my Excel is 97 SR-2. If you can save it to that version and it still works I would appreciate it. My email address is [email protected] . I am downloading "Open Office" as I type to see it it works. I would rather use Excel if your save works.

I noticed you used the front of the motor mount as a datum point rather then where the leading edge lines would have meet in front on the wing. Is that the correct way to calculate the CG point?
Thanks,
Mike

CloudyIFR

Well I guess I couldn't tell in the photo where the leading edge of the wing is.

How's this look?

Mike Connor

My Feedback: (5)

Great!! Thank you.
I am asking because I am not sure. It appears the datum point should be where the imaginary lines intersect from the leading edge being extended and not the front of the motor mount. I think I understand you to be saying that from what you have done in your last post. The 9.82" woud be the 15% MAC point measured from the datum line in front of the airplane using the 18.5" cord line. Right? Just want to be sure I understand.
Thanks.

EDIT - These questions were answered via email and it seems my assumptions are correct. I knew this not long ago, I think. Getting old I guess.

DanSavage

My Feedback: (5)

Hi Mike,

It looks like you drew the wing in CAD.

A lot of CAD programs have a centroid command, which finds the geographic center of a plane. (geometric, not aeroplane)

The geographic center just so happens to lie on the Mean Aerodynamic Chord.

Drawing a line from this point through the leading and trailing edges gives you the MAC.

From there, it's a simple matter to calculate the percentage along the MAC on which to balance your model.

I've been using this method for years for both my models and a lot of others' models and it works very well.

The only wing-shape that's given me problems is a FSW design like the X-29 or Su-47.

Here's your wing. the forward CG is 20% MAC and the aft one is 25% MAC.

Dan

Mike Connor

My Feedback: (5)

I drew the wing in an old version of TurboCAD Learning. I am self taught and could not find the "centroid" (or similar) command after looking around and searching help. Your drawing shows the CG well aft of what the Excel spreadsheet says and another wing calculator after converting to a simple shape. I typically fly deltas at about 18% MAC but your #'s are still about an inch behind the other CG location considering that. That does concern me a little as the wrong CG may mean one flight. Maybe an unpowered glide test is in order before it's maiden. Thanks for taking the time to look at it.

I did finish the airframe but am waiting for a motor that is out of stock. Pictures attached. I like the way it turned out.

DanSavage

My Feedback: (5)

Ah, that explains it. I don't think TurboCAD has the centroid command.

I understand completely your concerns about being tail-heavy. I'm the same way. That's why I never trust what the instructions say and always calculate my own CG.

If you look at my drawing, the forward mark on my drawing is at 20% of the MAC. The aft line, which is at 25% of the MAC is only about 1/2" aft of the 20% mark. So that makes each percentage point along the MAC just under 1/8" apart. Using this scale, 18% is only 1/4" ahead of the forward mark on the drawing. Given my experience using this method to find the CG, I'd say the calculators are off if they're saying the CG, at 18% of the MAC, is an inch ahead of where I put it.

Another way to test the results besides an un-powered flight is to find the CG the old-fashioned way. Make a cardboard cut-out of one half of the wing planform, like what I traced. Then find the centroid, or geographic center by getting the cardboard planform to balance on the tip of a pencil. That's where the MAC lies. Then, follow the instructions I posted.

FWIW, I've built two F-106 Delta Darts. I flew the first one at 23% of the MAC and I'm flying my current one at 25% of the MAC. The double-delta arrangement of your wing planform is similar to my Su-27 design. Here's where it balances at 23% MAC.

Here's a link to the maiden voyage. (See: [link=http://SavageLight.com/F-106/video/F-106-V2_Maiden-Lo.wmv]F-106 Maiden Flight[/link]) The rough landing happened because I copied the programming from my FlyFly F-22, which needed down elevator mixing at low-throttle settings, and I forgot to delete it before I flew the F-106 for the first time.

If you didn't use a reflexed airfoil, then you'll need to add some up-trim to the elevons before the maiden flight to keep it from wanting to pitch down.

Good luck!

Dan

Mike Connor

My Feedback: (5)

Dan,
I went back and checked all my #'s again and see where some of my measurements were calculated from the front of the motor mount and some were from the true datum position on the cord line. Making these corrections in the Excel spreadsheet brought the CG location much closer to what you came up with. Thanks for taking a look.

Your maiden flight Looked like you had good pitch control and a little sensitive on the roll axis. I had to watch the landing a few times. My expression probably matched the sounds coming from your camera man's mouth.

DanSavage

My Feedback: (5)

Hi Mike,

That's good that you found the error.

Yes, you're right on the money. With such a short wingspan, the roll axis of this model is extremely sensitive. I reduced it before the next flight and it's much nicer to fly now.

Ha ha! That's funny. I try to look on the bright side and say that now I know that the wing spars and landing gear mounts are really strong. I've got the video to prove it!

Dan

nbawannab

can any one please explain the difference between Delta wing mixing and Elavon mixing...i thinking about buying a jet and have narrowed my choices down to 2 models. the F-22 Raptor and the MiG-29,both from scaleflying.com...with the Raptor calling for delta mixing and the MiG calling for elavon mixing. any and all help would b greatly appreciated. thanks guys

Mike Connor

My Feedback: (5)

Elevon - A control surface on an airplane that combines the functions of an elevator and an aileron.
Delta wing require elevon mixing so pretty much the same thing.

combatpigg

My Feedback: (3)

Plot the cg for this........

rctech2k7

My Feedback: (1)

The cg for aero center is quite complex due to curvature of the wings. You can try graphical or you might ended up using integral calculation...

Mike Connor

My Feedback: (5)

My bet is that CP probably used the "that looks about right" method

combatpigg

My Feedback: (3)

Howdy Mike..!

Pretty much nailed it. I built a model ahead of this one that was much simpler, but the same basic shape. The first flight was tail heavy, but I bring plumber's solder and sheet lead to the field for fine tuning. If the plane is massively tail heavy, I just do consecutive loops till it runs out of fuel.
I did a semi-scale SR71 years ago and found that the forward body contributes more than a trifle amount of lift, so that maiden ended up needing not just some lead in the nose, but 5 minute epoxy, too.

BMatthews

The two terms mean the same thing.

By the way, it's not really proper to haul back up a 3 year old thread to ask about a new question. Your topic deserved its own NEW thread.

rctech2k7

My Feedback: (1)

That’s too intense to fly… Good thing TLAR works. Depending on how critical it is but without excellent pilot skills the aircraft requires additional cg consideration to minimize error prior to maiden flight. Believe me, even wing loading for this kind of aircraft is not that easy... Actually it’s not that but some part of aerodynamics is not easily accessible without testing even if we have data. In electrical although it's invisible there are basic law that's applicable without predicting just to double check that the aircraft exceeds minimum safety requirements before flying.

Mike Connor

My Feedback: (5)

Can someone explain the purpose of these words to me. I know my hospice drugs are strong but.... [&o]

nbawannab

thanks

combatpigg

My Feedback: (3)

Just think that many years ago weird models like these were flown either with 0 or 1 channel control.