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Balsa USA 1/4 scale Sopwith Pup balance point.

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Old 06-18-2003, 07:38 AM
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Default Balsa USA 1/4 scale Sopwith Pup balance point.

I just finished building the Balsa USA 1/4 scale Sopwith Pup.
I have a question re the balance point. I did some research here and elsewere and according to the standard formulas for this type of biplane, the balance point comes out to 5 1/4" from the front of the top wing, or right at the front spar of the bottom wing with the plane held upside down. This is 25% of MAC. The plans show about 4" from the front of the top wing.

My Plane balances right at 5 1/4' with no added lead. It would take a lot of lead to balance it at 4".

Can anybody shed some light on this discrepancy?
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Old 06-18-2003, 06:29 PM
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Default Balsa USA 1/4 scale Sopwith Pup balance point.

I balanced mine at the 4" mark with a g23 up front no added lead flys perfect
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Old 06-18-2003, 11:51 PM
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Default pup cg mystory solved

Well I dug a little deeper on this cg issue with the Balsa USA Sopwith Pup and this is what I found: 5 1/4" from the leading edge of the top wing is the standard optimal cg for this model. The assembly manual, page 30, photo 164 shows the construction of the machine gun , this photo also shows the location of the original CG at 5 1/4". If you compare the photo to the plans that come with the kit the cg on the plan is not in the same place and is well forward of the cg in this photo.

I used the full size plans to find the proper cg by drawing the center lines on the wing cross sections, put a point 25% aft of the leading edge on both wings on these center lines, connected these two points, marked the bisection of this line, and finally brought this bisection point up perpendicular to the datum line and to the bottom of the top wing. The two wings are the same size, they have no sweep , and the incidence appear to equal. This point is at 5 1/4", and matches the photo on page 30 of the assembly manual.

My pup has a Magnum 91fs, 16/6 prop, and a heavy prop hub. I mounted the engine, battery and servos as far forward as possible. i used the full length of the cowling and the engine is mounted upright. I had to use a 120fs motor mount to mount the engine as far out as possible. I used no lead for balance.
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Old 06-19-2003, 12:08 AM
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Default Balsa USA 1/4 scale Sopwith Pup balance point.

what can I say good luck
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Old 06-19-2003, 11:00 AM
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Default Balsa USA 1/4 scale Sopwith Pup balance point.

Why worry, look at this way the plane has enough lift as it is. I started off flying mine at the 4" CG (I just added weight to the nose to balance) No biggie. then if you think it flies too nose heavy, start removing the weight to get your 5 1/4 CG. I would rather start flying a nose heavy airplane than a tail heavy airplane. Especially this one.

You're under powering this airplane as it is with a 91 FS (It will fly very Scale like.) It will fly rather well with this engine as I had started out with a TT91 FS (wheels off the ground in 5 feet with this combo) myself before switching to a ST2000, this gave me a get-out-of-trouble power if i needed it and the only thing more power did was make it climb faster.

I agree with lazyace that the G23(g26) is a perfect match for this airplane.

Good luck and hey you can always call Balsa USA and see what they say. Good Luck....
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Old 06-25-2003, 02:26 AM
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Default maiden flight

Got to fly my pup today and after some trimming she flys just fine. it takes some down trim to keep it level at half throttle but it was well within the trim range. The magnum 91 flys it around just fine but then I didn't add a load of lead to balance the plane at the plan cg so My pup is on the light side.
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Old 06-25-2003, 10:48 AM
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Default Balsa USA 1/4 scale Sopwith Pup balance point.

WELL, where are the Pics? HuH Huh, Please please....

Glad to hear if flies well......
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Old 06-25-2003, 10:09 PM
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Default pup flight 2 3 & 4

I'll get some pics tomorrow at the field .got to try some touch and goes today. It takes about 1/4 throttle for the approach and about 2 or 3 clicks up from idle for the glide path. I had to stay on top of the rudder for the whole roll out because the tail takes forever to drop. I did some small loops and rolls. the rolls need a fair amount of down elevator during inverted flight, and the loops were small and needed to be proceeded by a dive, I guess thats where the bigger engine would come in handy ,but it flies and lands very scale.

I used the cross bar with the rubber bands at the ends on the landing gear and it works better than the science project I tried out at first. i also pinned the cowling stays, they needed to be beefed up due to the 12oz of lead I added to the front of the cowling.
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Old 06-26-2003, 09:54 PM
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Default pics

pup
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Old 06-26-2003, 09:55 PM
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Default pup

pup pic 2
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Old 06-27-2003, 10:24 AM
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Default Balsa USA 1/4 scale Sopwith Pup balance point.

VERY NICE! I like the way you did you wheels, too. What did you use to cover the airplane with? Here's a pic of my first one, I used monokote.
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Old 06-27-2003, 04:52 PM
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Default pup

I used mono on my pup also. I did some touch and goes with the pup today and I found that with my pup I needed to keep the landing speed up a little to get an accurate glide path and accurate rudder control, but for the final landing roll out as soon as the tail wheel touches the ground the steering gets real sensitive, and takes a gentle touch. My guess is that the landing would be easier on grass with just a tail skid, but the tail wheel makes taxiing so easy.
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