CG help on unknown plane
 

Last fall, I picked up a large pattern plane from a retired club member. The ony thing he knows about it is that it was built by another person. I haven't been suscessful in determining the model so I am unable to get recomended CG and throw info. I've just about got it air ready, but I'm at a loss as to where to place the CG.

I've been playing around with a couple of the CG caculators and I'm a bit confused as one shows a range between 6 and 7.75 inches from the root LE. This looks like a huge range from planes I've built. The wing span is 74", Root cord is 15.75, and tip cord is 11.5 with a 3.5" sweep on the LE. The horziontal Stab is 32" span, 10" root cord, and 6" tip cord with a 1.5" sweep on the LE. The Root LE of the wing to Root LE of the stab is 37"

Based on this and using a 10% static margin, I get the 7" CG from the Root LE. using this caculator. http://adamone.rchomepage.com/cg_calc.htm

I'm looking for someting that is going to let me get it up and flying without fighting a nose heavy, or tail heavy condition. Based you your experience, does the 7" sound OK???

Don

RE: CG help on unknown plane
 

http://www.geistware.com/rcmodeling/...calc/index.htm

RE: CG help on unknown plane
 

Here's the direct link. http://www.geistware.com/rcmodeling/cg_super_calc.htm


Dan.








edited to fix the link to show up as a link

RE: CG help on unknown plane
 

Guys, I ask for some practical advise, not directions to the library. I've been to that site and others. I'm still a bit confused, and that is why I posted the question.. Sorry to have bothered you.

Don

RE: CG help on unknown plane
 

Post a picture of the airplane, there a lot of pattern pilots trolling this forum, perhaps we could identify it. Most guys I know fly their pattern planes at 28 - 32% of the MAC.

RE: CG help on unknown plane
 

Interesting question. It really gets to the heart of designing airplanes.

In my experience, finding the Mean Aeodynamic Chord (MAC) and Aerodynamic Center (AC) for the wing and stab are straightforward for all but the most irregular planforms. Finding the Neutral Point (NP) gets involved with several incremental ratios and factors that are complicated to estimate without experiments so the on-line calculators assume built-in constants rather than ask for specifics. This is perhaps why you've encountered such disparity in answers among them.

One of the NP factors is relative lift coefficient ratio between wing and stab. If the calculator didn't ask for this it may have assumed a flat plate stabilizer which is probably incorrect for the plane you have. (Most pattern type planes have airfoil shaped stabilizers.) Another is stab "efficiency". It's another fudge factor that could be from 30% for an obstructed tail on a scale ship like a Gee Bee up to 90% for a T-Tail.

I calculated the points by hand and got the exact same MAC's and AC's as the one in the link you posted but my number for the NP came out at about 7 7/8" from root leading edge, not the 8.42" they suggest. (I assumed a airfoil stab with 60% efficiency.) Allowing a 10% static margin I would suggest a starting CG at 6 1/2" back from root LE.

This plane would likely still be controllable with CG from 5 to 8" from LE without too much drama. However, I always make sure I have more than adequate throws on all control surfaces prior to any maiden so the pilot has more than needed to wrestle a badly trimmed plane back under control on lift off. The throws can be returned to something more appropriate later.

RE: CG help on unknown plane
 

First, the stab is a flat slab type, no airfoil there. The wings look to be full symetrical, or nearly so. The stab sets high, maybe an inch or more above the wing. I was thinking this guy weighed a ton, but when I put it on the scales, it came in just a tad over 11 lbs, but that is without the battery.

The 6 1/2" sounds closeer than what the caculator suggested. I've attached some photos of the plane, I found the relation between the rudder and elevator to be somewhat unique, based on the searches of plans and kits of that size plane. It could well be that it was a home designed plane, I'm sure it was scratch built as there is no indication of die cut parts that I could see.

Thank you for your input, I'll install the battery and balance it out today and see if it may run, first problem, then fly tomorrow at the club meeting.

By the way, if anyone recognizes this bird, please let me know. When I posted some photos just after I got it, there was a lot of guesses that it was Bridi (Sp?) design, but looking over all of his kits and plans that I could find, none were the size of this plane, and none had the rudder/elevator relationship this one has.

Don

PS, I'm going to fill the 3/4" gap between the spinner and nose ring after I find out if it flys OK. The O.S. 1.20 pumper fit the motor mount that was in the plane but I had to mill clearance in the top of the mound for the regulator. The cowl must have been built around a two stroke, given the hole for a needle valve extention and the short nose compaired to the four stroke.

Don

RE: CG help on unknown plane
 

No bother.

The library is not a library. It's a tool that's dead simple to use. If you have a yardstick and 10 minutes, you can work out an absolutely reliable CG range for any airplane model or full scale. OK, the full scale would need more than one yardstick to measure.

RE: CG help on unknown plane
 

This tool asks for 9 measurements. It uses them to figure the important details of the wing and the horizontal tail, which are the important details for working out a CG. It also asks you to make a decision on the Static Margin and suggests how to do that. It figures MAC of both wing and h.tail and the neutral point etc etc etc. All you gotta do is measure what it shows you to measure.

No need to do any figuring, the program does all that.

It uses the formulas that the aeronautics industry has used ever since it figured them out.

http://www.geistware.com/rcmodeling/cg_super_calc.htm

RE: CG help on unknown plane
 

Da Rock, I did use that tool, and another one. I had questions about the results. Crate Cruncher has answered my question for now.

Don

RE: CG help on unknown plane
 

Excellent

Lots of people read these threads and some answers are written for all who wander in.

RE: CG help on unknown plane
 

I've written an Excel Spreadsheet that will calculate the MAC, NP, Static Margin and more with a Tutorial on how to use it written for Radio Controlled Soaring Digest (RCSD).

All I've done is to take formulas from Martin Simons, Mark Drela and DJ Aerotech and placed into an easy to use spreadsheet.

It's available FREE at www.TailwindGliders.com

Curtis
12" of Snow in Montana but I guess it's only April!

RE: CG help on unknown plane
 

You can overthink this using the calculators.

With an unknown I use the "That Looks About Right" rule to put the C.G. forward of the thickest forward part of the wing.

I fly the plane and see how I like it's behavior then adjust the C.G. accordingly.

While a tail heavy plane must be avoided, most planes will tolerate sizable C.G. ranges.