CG help, please
 

I just got a short kit of the old (1941) Cleveland "Super Condor"FF glider, which I intend to make r/c electric assist.

The plans do not show any CG information. Here is what I have measured from the plans:

Wingspan 84", chord 5", except the tips, which taper.
Wing Area - 324 Sq in
Airfoil is undercambered

Horizontal stab:
66 sq in area, 3" avg chord (4 at the root, 2 at the tip)
Lifting stab, clark-Y airfoil

Without measuring, it looks like the wing and stab are set a about 2 deg positive.

The wing LE is 19" away from the stab LE


My gut feeling is that the CG should be around the middle of the wing flat plan area, but I've seen ff rubber with CGs way further back.

RE: CG help, please
 

If that helps:

static stability margin is
30% with C/G at 50% (2.5in)
18% with C/G at 60% (3.0in)
12.5% with C/G at 65% (3.25in)
6.5% with C/G at 70% (3.5in)

Neutral point is 3.8" behind LE.

I'd shoot for 62% to 66% balance point.

BUT, if incidence angles are given and no advice is given to change them (by shimming), I'd just test glide the model and modify C/G position until a straight glide is achieved. That's what I was told to do in the 1960s.

RE: CG help, please
 

The wing and tail incidence angles sound fine.

One thing you WILL need to do is replace the spars with spruce for RC. Thanks to having an elevator control you'll easily be able to generate too much G load to get away with balsa spars. In fact I'd make the center section spars from spruce that is about 50% thicker than shown for just this reason. For the outer panels you'll want to start them with the same 50% thicker and taper down to the regular size at the tips.

Over at RCGroups a fellow did a Cleveland Albatross in the Vintage and Old Timer section. It's very similar to the Super Condor in planform and the fact that it uses gull wing dihedral. He tried it at first without ailerons but the dihedral in a gull wing is in the wrong place what with being in next to the fuselage where it has too little leverage to roll the model. He ended up with ailerons in his which I'm sure you'll need as well.

Here's a link to that thread that discusses all the issues related to gull wings on these models....

http://www.rcgroups.com/forums/showthread.php?t=1181136

RE: CG help, please
 

<thinking>...and glide it over tall grass....(so it doesn't get broken)...

RE: CG help, please
 

Wedj,

Run the information through "Sailplane Calc" and you'll be very happy with the first flight.

See www.TailwindGliders.com for the file and a tutorial article.

Curtis
Montana

RE: CG help, please
 

Wow, all fantastic advise. Thanks.

RE: CG help, please
 

The albotross is much larger than the Condor, but a great build thread. Thanks for all the good input, I will strengthen the center panel spars with spar cap strips and shear webbing, making a good solid I-beam construction. I will post all further comments in the golden/Vintage section as a build thread. Thanks to all of you for your input, especially Ustick!

PS - might take a few days to start the build thread.

RE: CG help, please
 

Is that chord an average chord or what? 5 x 84 is 420, so I assume the airfoil is tapered as per another post.

Lifting stab is same as down elevator on normal airplane. The faster it goes, the more nose-down force due to the lifting stab. That is the way FF and gliders maintain some stability.

That means that there is no difference between stab and wing. In normal sub-sonic converging airflow the stab chord-line sets the zero incidence. Those models that have the longitudinal axis - a straight line the designer draws from with no other aeronautical value - and a bunch of + and - numbers can be averaged out using the stab as zero and find the true incidence angles.
EXAMPLE: using the long. axis, plans show engine at -3°, the wing at +2° and the stab at +1°, then the airflow sees the wing at +1° with 4° downthrust and of course the stab is the zero line. Airflow doesn't see the "designer's" drawings, just the real-world differences.

Most FF with your statistics would balance around 50% as the stab area is relatively small, at around 18% of wing area. When I flew competition FF, 60-70% worked but 30-35% stabs were the normal. I would start at 40-50% but if tests show touchy stability, maybe more forward CG. The touchy factor is the lifting stab which will change trim with any change in airspeed.

Now you are going to power up with electrics. I predict you will need significant trim change between climb and glide.

RE: CG help, please
 

There is a number of online CG calculators out there. Links are in the sticky resource threads at the top of this forum post titles. Input the numbers for the design to get a good idea of where to start. The oddball wing shape of the Condor will require a bit of fudging to come up with a good "straight wing" equivalent root and tip chord and equivalent sweep angle. But even if done roughly it'll still give you a darn good idea of where to start for your initial CG location.

RE: CG help, please
 

From my own experience you could hang the plane from the cord line.It should hang level when it's all together? Good Luck Charlie

RE: CG help, please
 

Yes, but the question is WHERE on the chord line. The CG calculators are designed to do a bunch of calculations to determine where the aircraft's neutral point is located and then tells you how much ahead of that point to balance the model. The actual balance point is quite flexible depending on the model's design and the degree of pitch stability that the flyer desires. And a lot of the old time plans didn't tell you where the balance point was to be located.

RE: CG help, please
 

Thanks for all your inputs. Build thread here:<a href="http://www.rcuniverse.com/forum/m_10077015/tm.htm">

http://www.rcuniverse.com/forum/m_10077015/tm.htm</a>