neuroza

Hi!
Now I'm building my first delta wing, so please, how to find where must be the center of gravity???
Any help is welcome!
Thanks!

mjfrederick

This might help : [link=http://www.nasascale.org/howtos/cg-calculator.htm]CG Calculator[/link]

neuroza

Thanks!
But there ask me to enter the CG, I just don't know how to use it...

Can anybody help, this is my measurement:

Root Chord (A): 350
Tip Chord (B): 241
Sweep Distance (S): 150
Half Span (Y): 500
%MAC Balance Point: ???

mjfrederick

I'm not positive on delta wings, but most aircraft use 25-33% MAC for the CG. Try using these and it may give you a decent range as to where the CG should end up.

neuroza

OK, thanks, I will try with 30% and that we will see...

Thanks!

Ron S

My Feedback: (2)

Here's an extra data point for you. I've flown an F-106 Delta Dart model for several years now, and the CG is at approx 22% of the Mean Aerodynamic Chord. The planform of the model is not a pure delta shape, though, because of the (small) contribution of the fuselage.

It is packed pretty tight, so it isn't really easy for me to test fly with different CG positions (without adding dead weight...) If I could, I'd like to go aft a bit more. Also, there might be some minor contribution in CG shift due to the fuselage, but I've never checked to see if it would drive the Aerodynamic Center fwd or aft - I'd assume fwd...

pimmnz

For any 'usual' delta of around 60 degrees L/E sweep you will find that the balance will be at about 50% of the centre chord.
Evan, WB#12.

Ron S

My Feedback: (2)

Ah, I've never heard that before! Lets see. For a simple delta wing (tip chord = or approx zero), mac length will be 2/3 the length of the root chord. 75% of the mac (measured from TE) is normally a reasonable spot for aero center, and correspondingly, 75% X 2/3 X Root Chord = 50% Root Chord! [8D]

So 50% Root Chord (measured at centerline of model) corresponds to a 25% mac or aero center.

pimmnz

Yes, a good safe place to start, adjust to taste as usual. It's all really simple, you see.
Evan, WB#12.

Mustang Fever

My Feedback: (2)

Tail less deltas should balance at 20-25% of mean aerodynamic chord (dry, with a tank forward of the CG), as long as the nose isn't way out there in front affecting the center of pressure. Ron S' F-106 is a perfect example. Right in the middle at 22%. It has a very skinny nose, even tho it's way out there, that doesn't affect the CP much. Some deltas, like Laddie M's Arrow, have a monster nose (5" wide) with a curved upper surface that generates lift. That puppy won't fly at 20-25%. It needs to be at about 15 or even a bit less.

The hard part is finding the Mean Aerodynamic Chord. You either have to draw out one wing panel and fuse, full size, on a big piece of paper, or lay measuring sticks on the floor parallel to one wing panel.

1. Find the root chord (RC) on the CENTERLINE OF THE FUSE (C/L)(or wing). Extend the LE to the centerline to do this. Measure along the Centerline (C/L) to obtain RC.
2. Measure the tip chord. (TC)
3. Draw a line (or place a stick) extending forward along the centerline (C/L) from the LE for the distance of the tip chord (TC), and aft along the centerline from the TE.
4. Draw a line (or place a stick) extending forward, parallel to the tip, from the LE for the distance of the root chord (RC), and aft from the TE.
5. Connect the four corners obtained diagonally as shown. The two lines will cross somewhere near the center of the wing. The location of the MAC is where they cross. Measure from the LE to the TE at that point, and you'll have the dimension of the MAC. Multiply by 0.80, and that is the CG location as measured from the TE, which is usually much easier than measuring from the LE. That should be good for a first flight. Have PLENTY of elevon up travel, and use lots of expo so they won't be too sensitive, and you'll have no trouble flaring the beast even if it is a bit nose heavy. You can always creep the CG aft as you gain experience.

It's a huge pain in the ***, but it works. My drawing isn't so good, but it works, too.

Mike Connor

My Feedback: (5)

%MAC Balance Point: 18
Balance Point @ Root Chord (CG) =124.18

Your measurments are as much a flying wing as a delta.
15%-20% MAC is a good place to start.
Delta's fly by their own set of rules.

neuroza

How can I know the best "%MAC Balance Point"???
Is it ok to start with 15 - 20 %

Thanks!

Mike Connor

My Feedback: (5)

I designed a delta using proven concepts from other delta designs and it flies excellent. Mine flies best balanced at 18% MAC. Most delta's I researched like the Diamond Dust, SlipStream and others balanced between 15%-20% MAC.

neuroza

Thanks!
I'll try with 18%...

longtalltexankwg

I am building a profile r/c electric convair pogo vtol with a length of 20 inches and a 24 inch wing span.I am starting with a small one to save on cash. When I can get the small one to work then I will make a bigger and full fuselage version.I already have a counter rotating prop design that I have been messing around with to make it work for a bigger version. Thanks for your time, Kerry

donnyman

Neuroza

I sent you a pm I trust you received it.

the information you seek is a subject often answered in our magazines. and example.

Flypaper 2

This old Pogo is at the 50% root chord mark.

beyeriii

Hi DonnyMan,

Please post the name of the magazine and issue (April 2009) that has this diagram. I want to order a back copy to read the entire article.

Thanks,
Keller

donnyman

THE MAGAZINE ARTICLE IS MODEL AVIATION THE A.M.A. MAGAZINE. APRIL 2009 ISSUE. AS I SAID IN A EARLIER POST, A HOST OF INFORMATION IS AVAILABLE, I THINK I HAVE SEVERAL ARTICLES ON THE SUBJECT. JUST HAVE TO FIND THEM.

Mustang Fever

My Feedback: (2)

I wouldn't put too much faith in that "25%" figure in the article. That may work for almost all conventional wing and tail section designs, but it won't work for any delta that I know of. Maybe the F-106 wing plan, because it has so much of the wing area aft, rather than forward. (After all, it's the wing area aft of the CG that's stabilizing the delta in pitch. Any kind of long or large projection in front of the wing makes that job harder, and requires a more conservative CG to compensate.)

I've built several of the Laddie Mikulasko "Arrows", which is a pure delta design, and the CG that gets you a controllable and stable airplane is between 0 and 5% of MAC. It does not have a severe sweep to the leading edge, and so has much of the wing area located forward, plus it has a rather long and wide nose with a square cross section, as it's a flying boat. It also has an aft mounted tank, which makes things even goofier.

The Lanier Shrike is a "sorta" delta with a short tail section tacked on just aft of the wing, and the factory CG works out at a little over 16% of MAC.

The CG tape is on both the pictured birds, so you can get an idea.

I wish there was a formula or a rule of thumb that used wing area opposite each 10% section of the root chord, plus a fiddle factor for the effects of the nose, but there's not. When you start getting into "tailed" deltas like the F-4 Phantom II or the Mig 21, then everything seems to go out the window.

The Pogo looks like it has a lot of wing aft. I'd start with 10-15% and go from there.

beyeriii

Hi Donny,

Thanks for posting the magazine name and issue, I hope I can someday return the favor

I have sent Academy of Model Aeronautics a $7 check for an April 2009 back issue.

Keller

beyeriii

Hi Bob (Mustang Fever),

Thanks foryourearlier (6/14/2008) explaination and drawing,it is very similar to the MODEL AVIATION April 2009 article.
Thanks also for the new posting with advise regarding thevarianciesof delta wings.
I am very new to RC Planes but have some expierence with electric cars.
I will be rereading your explainations as well as theModel Aviationarticle until I get a firm grasp on these topics.

Thanks again for your helpfulness
Keller

CloudyIFR

Go to www.TailwindGliders.com and download "Flying Wing Calc". There is a tutorial on how to use on the "Article/Files" page of the website. The article is for "Sailplane Calc" but it'll work for you. Deltas are a little different than most but they are very similar to flying wings except for the very low aspect ratios.

Just a little background:
All airfoils have a pitching moment around the 23-25% of the Mean Aerodynamic Chord (MAC). This means if you balanced your model here it would be neutrally stable, if balance further aft it would become unstable and if balanced forward it would be stable, i.e. it would try to self correct any wind shear changes, pitch changes etc....
The distance of the MAC from the neutral point (NP) is called the static margin (SM). The SM I recommend for first flights would be about 8% forward of the NP.
Wings are funny and you'll notice that you may be able to really backup the balance point with a small SM but and it'll fly fine but then be very squirrelly on landing. This is caused by the higher AoA on landingand movement of the pressure points. Thus wings normally fly with a little higher SM than conventional planes.

Curtis