CG of Gieseke BEAR

I just got the plan of the Gieseke Nobler "BEAR". The plan does not show the CG. It does show the pivot postion of of the bell crank.
Does anyone have a idea where the design CG should be?

>I just got the plan of the Gieseke Nobler "BEAR".
>The plan does not show the CG. It does show the pivot postion
>of of the bell crank.
>Does anyone have a idea where the design CG should be?

Gerban,

I was hoping Doug Moon would jump on this since he's the second most experienced Bear flyer I know of.

Since he hasn't let me give you a quick way to get the CG almost spot on.

First figure out the wing area. I'm guessing it's about 700 square inches or so ... but measure it and find out. Simple measure the span tip to tip, find a spot halfway out one wing from the fuse centerline and measure the chord at that point (yes, including the flap). It'll probably be around 11 or 11.5 inches or so. Multiply the span by that "average chord" and you'll get the wing area close enough for Gov'm'nt work or fixing the proper location of the CG.

Do the same thing with the stab/elevator. I'm guessing the area will be in the vicinity of 175 square inches or so.

Divide the wing area into the tail area and you'll come up with a number right around 25% or so ... smaller if the tail is smaller and larger if it's larger. It will almost certainly be somewhere between 20 and 26 or 7 percent.

Whatever that number is, go back to that average chord you located and multiply the chord length by the tail percentage of the wing. Let's continue with our "supposed numbers" and divide the 175 square inch tail by the 700 square inch wing. Son of a gun, that's exactly 25%.

Now multiply the chord length (we're using that 11 inch number I made up) by .25. 25% of 11" = 2.75".

At that average chord location measure back 2.75" on each wing and make a removable mark on the bottom of each wing. Adjust the nose and tail weight until the ship balances at that 25% of the average chord. I guarantee the CG will be perfectly safe for first flights and will most likely end up within a fraction of an inch or so after you've completely flight trimmed the airplane.

The last important part of that "bench trimming process" is to properly locate the leadouts in relation to that computed CG.

Pick the airplane up by the wingtips with your index fingers as close as you can to the same fore and aft location of each wingtip. The ship will, of course, balance in the same fore and aft "plane" as it did when you set the CG at the Average chord location. Make another mark on the inboard wingtip exactly where that point is on the tip. Measure 1.25" aft of that point and make that point the middle point between the up and down lines where they exit the tip. Keep the lines close together for best flight trim.

Make sure you've got an appropriate amount of wingtip weight based on how much longer the inboard wing is than the outboard. Make sure the wings and tail and rudder are straight and the hingelines sealed and go fly.

Don't make CG changes initially based on how quickly the ship turns. Your best clue to the proper CG is how the ship glides after the engine quits. If it's correct approaches and landings should be reasonably controllable and gliding a lap or so should only require a modest amount of whipping.

If the airplane slows quickly and/or balloons up into a headwind, you need to move the CG forward. If you can easily whip the airplane more or less as far as you want, the CG should probably be moved back a bit to optimize maneuvering performance. Most likely, the glide will be just about as described in the first instance, controllable with no tendency to balloon into a headwind and 'whippable" with some effort on the part of the pilot.

If the plane is too responsive or not responsive enough with the CG set to provide the proper glide, use your adjustable handle spacing to make you happy with the rate of response.

You will almost certainly NOT have to add or subtract large amounts of weight based on your bench set CG. It will be "that" close. There are reasons for making very small changes but they would be determined only after significant numbers of patterns have been flown and would be the result of observed tendencies to do things less than perfectly on a repeating basis.

A subject for another time.

Ted

Now, If Doug comes on and tells you his CG is totally different from that, you should probably listen to him because his Bears flat fly! The above will "not" get you in trouble.

You got this plan from UHP correct? Steve Moon drew the plan? I need to make sure this is the right plane. The Bear has many designs and I dont know if anyone else has inked plans besides UHP. I call it the 97 Bear because it was 1997 when Bob finalized the design and the plan came from those numbers. He had a host of planes and said that was the one. There have been no changes to it since then either. I think it was inked in 1999. If so the following should work for you.

You can do what Ted says about the CG and it will be pretty darn close. I can take out some of the work for you.

The tail section on that plane should be right at 20%. It has stablets on there as well. The plane will NOT turn without them. You can not skip them. The Bear builds and designs for ultimate light weight. The idea was to use stablets and get an efficient tail with plenty of power without having to build it bigger. Less size means lighter. The theory worked as his planes, ranging 650-675 sqrs, were routinely in the 52-54 oz range. This is a must when building that model. You will notice it has a pretty sharp leading edge on the wing. It will not support heavy building weights, it will stall. And the tail section will give up as well. You bring it in at 56 or lower and you will have no troubles. I would suggest following Ted's advice and start with the CG at 20% of the wing at MAC and go.

On the first Bear I built I asked him where the CG should be. He said, "As far back as it will go and not crash then forward a bit." Then he pointed to a spot and said somewhere around here would be good. I know it isnt much but that is what he said.

On my last 97 Bear that I built I checked it once and it was around 20%. The tail was from the plan but I had extended the wing and it the tail was around 18% at that point. But it still flew fine at 20% and I won many a contests with that old bird.

As far as LO position. I think you will end up a little more forward than what Ted is saying. Reason is that it uses an airfoiled rudder and is inducing more yaw than a standard straight rudder. You will need a little more forward LO position to keep it straight. I always end up much more forward than the standard numbers you see on the net. But then again using an airfoiled rudder isn't to common these days. Bob's theory on that was, it is the way I have always done it. But after some attempts later the foiled rudder seemed to work better with how he liked to trim a plane. I myself have used both and tend toward a foiled rudder as well. But the latest model has a Rabe rudder as it will have a monster prop. My own thought on it is induced yaw with the foiled rudder will demand a more forward LO position therefore getting the model more tangent to the circle path it flys in. I have been told on several occasions that the foiled rudder will need a new LO position if the lap time speed is changed. Or as the model speedn changes during maneuvers it will go in and out of optimum trim. Maybe so but I have not found this to be true and I have not changed LO position in 6 years on my current Geo-Bear. But lap speed has varied from 5.65-5.95. And typical LO issues have not shown their ugly head at the different speeds. We all like a certain speed and I like it slow as possible on my piped models and the LOs as forward as possible. Seems to work out pretty good.

If you have any questions about this model please do not hesitate to ask. Ask here or PM me any time. I will be more then happy to tell you what I know. I have built 4 97 Bears myself. My brother inked the plans for it and built one himself. I have several flying partners who have built them and flown them over the years. Not to mention I have had the pleasure of flying with the Bear for the past 15 or so years. I am confident I can find an answer for you as you build and fly that model.

PS. there is another Bear based on a Geo-Bolt wing. It kills! If you want more on that let me know. No plans for it though as of now.

OK Doug! I got the UHP version inked by Steve Moon(10/98). Sorry I already mixed up your names.

Thanks for the hint of 20%.

I already mailed you,through your website.

>PS. there is another Bear based on a Geo-Bolt wing. It kills!
> If you want more on that let me know. No plans for it though
>as of now.

I am trying to decide on my next 60-sized stunter project. Can you tell me a little more about this "Killer Bear".
What exactly is a Geo-Bolt wing?
I think I have a set of plans and short kit for the 97'Bear up in the attic. How much of that kit could be used for the Killer version?


Allan Perret

I call it the Geo Bear. It is a design from Bob Gieseke using the Billy Werwage Geo-bolt wing. That of course is the wing you will find in Billy’s T-bolt. The rest of the design is all Bob. It is very evident in the photo below.

Here are some note about the model and the construction. There is no plan at this time. But these numbers and notes should get you enough to build by or draw something up.

It is kind of random. Sorry about that.

It uses a pretty standard motor crutch assembly. 3/8x1/2 motor mounts that travel from the nose ring to the top of the wing. They should taper to go around and glue onto the top of the wing. Then the nose ring ties them together coupled with F1 to create a box around engine.

The tank floor is glued in place as well. This will need to be as low as possible to let the pipe go through but high enough to get an 8oz Sullivan tank inside the model. You will need to relieve the engine beams some, .25", in the tank area so the tank can get high enough in the plane to create and equal run. Then fill the middle area between the bearers with cross grain balsa. The nose moment on this model is short. You will need room to get the rank in there and not have the header butt up against it. I usually cut out the front of the wing, about 1" back, once it is installed in the plane. This gives the tank the extra room needed.

The width of the fuse is wide enough to get the motor and tank in. I would go with at least 2.5 inches on the outside. Use a 2" spinner.

The top and bottom blocks are molded 3/32 balsa shells, from the nose ring all the way to the rudder. If you dont want to make male molds to mold on just sand and carve from blocks. That large deep bottom block will be a big beating to hollow out. You can mold that one around little league baseball bat and slice to fit. The turtle deck can be done this way as well. I feel molding is the only way to go. Once you have it down, or you may already have it down, you can setup a top and bottom in one evening after the molds are dry.

Here are the numbers.

Nose 9"
Tail 18"
The height of the fuse sides is 3.5", the over all height of the fuse at the high point of the wing is 5 5/8"

Thrustline numbers,
The wing centerline is 7/8" below the thrustline.
The stab centerline is 3/8" above the thrustline.


Stab
1/2" thick, flat. Must be light.
Span 29"
Root is 4.5"
Tip is 2.5"
Elevator
Span of one elevator is 13.75"
root is 3.5"
tip is 2.5"
Elevators are tapered.
Stablets-1.75" tall, 5.5" long along the bottom and
4.75" along the top. The front of the stablet is
vertical 90degrees and the rear of the stablet follows
the angle of the rudder TE.

Rudder
From the stab it is 4 5/8" to the highest point of the rudder. The rudder extends back 2.5" from the TE of the elevators. Bob and I both use an airfoiled rudder. Looking at Bear 97 plans the shape is very similar if not the same.

Tips at the trailing edge are 2 3/8" out from the tip rib and at the spar they are 1.75" out from the tip rib. They are swept back styled tip.

Flap span OB 27.25
root 3 5/16
tip 1 9/16
Flap span IB 28"
root 3 5/16
tip 1 9/16
(I know the inboard flap has more area than the outboard flap and that will make some people crazy but the plane flat out flys good. Everyone built from these numbers so far has been darn good! I have new model using this same wing and I have built the OB flap larger at the tip to accommodate the offset and it flys very good as well. Either way seems to work very good.)

I used built up flaps but Plank 1/4" will work as
well. I do NOT taper my flaps.

My gear is wing mounted. Bob G uses fuse mounted carbon gear from Bolly. He gets them through Randy Smith.

The BC is 4" and a Tom Morris setup will run this rig perfectly. I use 1:1 flap to elevator controls. No need to get fancy.

And of course it has the normal stuff...TW box at the outer most rib on the CG line. It has adjustable LOs as well.

The wing is 63" span. It is the Geo Bolt wing. It uses a 5/8" offset already built into the jig. The Lost Foam jig comes from Bob Hunt. I don't know any other way to build it. It is worth every single cent!

If you need some more detailed looking photos to help you get a good visual please let me know and I can take some from any angle you need. Like cowling pipe tunnel and stuff and stuff like that. Please it is no problem for me to do so.

I think this is enough to get you started. If you need more let me know.




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The next photo is my plane hanging in the CG machine. It is a good side view of it to give you a good shot of the profile top and bottom.

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The following photos are of Bob's last full blown stunter. It is a very large model. The tail moment is 21" (no rudder overhang, wouldn't fit in the car if it did have it!) the nose is only 9"!! It uses the Geo Bolt wing as well but the tips are round. The fuse is 3" wide and round. You cant see it but it is not the typical box fuse. The craftsmanship on this one is amazing. I think he received 19 points and was on the front row all along with this one.

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Are the ribs from my '97Bear short kit close enought to the airfoil of GeoBolt wing to build the killer Geo-Bear ?

Allan Perret

Nope. The ribs are not the same at all. Conatact Bob Hunt he may be able to get you root and tip outlines so you can see the difference. Or you can contact AMA plans. The T-bolt plane was published in MA.

What do you consider a good weight on the bigger GeoBear, 60~62 ?

Looks like it has a more typical tail volume around 25%. Are the stablets still necessary, or just keeping with the original design cosmetics?

I think you mentioned an 8oz Sullivan.
Which one, round,slant, or rectangle?

Mine originally weighed 58oz. One repair and 6 years later it tips the scales at 62. Still flys awesome and is my top stunt plane.

I would think 66 would still be a good weight.

26% tail section.

8 oz rectangle.

Or better yet I'm sure Bob Hunt can provide a lost foam jig for this wing. Terriffic way to build. Especially a geodetic wing.

Randy C