I was wondering if anyone could help me in how to determine the CG on my all star Biplane.
It is an older kit that Ace has now modified to a mini Ultimate.
It only has a 33" wing span (both wings being equal in length) x 6" cord.
Overall fuse length (including spinner) is about 31.5".
The top wing sets forward of the bottom wing by approx one-third of it's wing cord.
I had modified the kit by squaring off the wing surfaces to "mock" an Ultimate.
I have also added ailerons to the bottom wing.
Any help wood be appreciated.
TY

The approximate Mean Aerodynamic Chord (MAC) position on a swept wing is found just like it is on a tapered wing, or (tapered and swept wing).

You need to measure the wing root chort and tip chord. Ignore any wing fillets or tip shapes - they don't count. Tip chord is often best measured at the last wing rib, particularly with rounded wing tips.

You then measure aft from the leading edge at both the tip and root locations. For example if you want to find the 25% CG location and have an 8 inch root chord and a 6 inch tip chord, you would measure aft 2 inches at the root and 1.5 inches at the tip.

You then connect these points with a straight line. (A dry erase marker or piece of taunt strings works well). The point where this line intersects the midpoint of the wing panel (half way between the tip and root) is then the location of the 25% MAC of the wing. These two points can then be connected with a straight line to show the 25% chord location at the wing root.

If both wings are evenly tapered and staggered you can use the composite wing method indicated above combined with the root and tip measurments done above. Just add the stagger to the root and tip measurments assuming the same chord is used for upper and lower wings. Alternatively with unequal wing chords you'll need to estimate the measurements from the upper wing LE to the lower wing TE (with a positive staggered wing, the reverse would be used with a neagative staggered wing). You can also level the model and obtain the measurements with a square over a level surface as you mark the surface directly below the points on the wings and then measure the distance from leading and trailing edges there rather than trying to measure it accurately on the upper and lower wings on the model.

Where it gets tricky and complicated is on a Pitts type configuration with a swept upper wing and straight lower wings both of different areas. That gets hard to explain and involves more calculus from my ERAU days than I care to remember.

I would figure both the 33% MAC and the 25% MAC, mark them at the wing root and start the test flying with the CG at the 25% location and move aft toward the 33% location until you get the agility you want.

OK, I also have come across an All-Star Bipe and need to know the CG. After reading the last thesis on the way to figure it out left me with this question ......... Anyone have one and could you tell me the location of the CG. Do I balance it rightside or upside down??


Thanks Fred

As with most biplanes, balance upright with the BP at the leading edge of the lower wing.

My All-Star balanced here as does my 4-40 bipe.

With the modified All-Star (Ulti-Star?), I would still balance it at the leading edge of the lower wing since both wings are swept equally.

Dan

I don't have mine quickly available, but if you have access to RCM's illustrated plan guide, you might look up the original plan reproduction. It's kind of small, but the CG might be on it.
I loved the little guy when I had one. Bought it from an estate sale, all ready to go except for drilling the mounting holes in the engine mount and adding radio. Had a little trouble deciding to use either a Fox BB 15 or a new HB 15. So I drilled 6 holes so I could switch between them, and went with the HB. Figured if I needed more power, I could switch to the Fox. (14,200, 8-4 on the Fox, 12,800 for the HB, same prop, same fuel). Turned out the lower powered HB was perfect, especially 2 years later when a couple people asked me how I got the thing to fly so well off grass. They had several, (Father and son) but always tip stalled coming off the ground. Turned out they were using Enya 19s and ST 23s. They had trouble believing that you can overpower the All Stars. They were. I never tried the Fox after that. The higher RPM might have given me problems. Never checked the CG, but it flew great, till I was doing some low level aerobatics at our annual show, and our commentator called out "Don't CRASH!! So, of course, I did.
I had the original construction article, and the designer talked about take-offs from pavement with Cox Medallion 09. Some folks claim you can't overpower a plane, but this was the second plane I had that would prove that stement wrong. You can.

I used an OS .15FP/8x4 Master Airscrew in mine and flew it exclusively off pavement. My favorite trick was to do pirouettes on the ground.

First, I would feed in full-left rudder and a little forward stick pressure then start applying power until the tail wheel would come off the ground. Once that happened, I cobbed the power and kept the prop off the ground using the elevator. It would do about 60rpm in this configuraton. As expected, I sometimes broke a prop performing this 'manuever', but it sure was fun!

Dan

So, I balance it right on the leading edge of the lower wing? Right now it balances 2 1/2" from the LE of the top wing.

Also, how do you get it to and balance rightside up? It does not "hang" like my top and low wing one's do.

Yes, this is my first Bipe and if I balance it where you said, I need alot of lead in the tail. I am using 3 Hitec HS-322HD servos, Futaba Rx, and an Magnum .15 w/ stock muffler.

Thanks for the help,
Fred

Seems Ace might still be in business in Higginsville Mo. Might look them up on their website and give them a call. When the company was being run by Paul Runge and his sone, they were very into customer service. That may still be the case.

Draw an imaginary line straight up from the leading edge of the bottom wing to the underside of the top wing.

Balance upright at this point. It should 'hang' as expected.

Like 50+ wrote, these little bipes don't like to fly heavy, so I'd avoid the use of lead if at all possible. If it is tail-heavy when balanced at the point I recommend, then it would be better if you can move your receiver pack, or even re-locate the servos rather than adding lead.

Dan

Might even check the balance before you tap and drill the engine mounts. Slide the engine back and forth to try to get the correct balance, and then if you're not too experienced a pilot, add some removeable weight to the nose to get it SLIGHTLY nose heavy for the first couple of flights. It doesn't take much tail weight to rekit a plane, compared to nose weight.

Or, as I like to say, "A nose-heavy airplane doesn't fly too well. A tail-heavy one doesn't fly too long."

Dan