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Author Topic: C of G  (Read 987 times)
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Johnny Formica
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« on: August 25, 2017, 03:31:26 PM »

I made a new glider today by taking the 8" sweepette plan and making it a little bigger but using the wing from another plan called the 'hang 10'- I know this is probably a recipe for disaster but anyway.. After a few hand launches and adding a little weight I got it gliding nicely but when I later checked the C of G it is about 1/3 of the wings width back from the leading edge. The 8" sweepette plan is more like 2/3 back. Is 1/3 bad.

Thanks
John
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skyraider
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« Reply #1 on: August 25, 2017, 06:06:08 PM »

Seems that 1/3 ( 25 - 30% back ) from the LE is about right. Depends on
the model and wing layout. You must have hit that lucky combination. If it
were me, I'd still be hunting for it.

Skyraider
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Hepcat
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« Reply #2 on: August 25, 2017, 08:41:35 PM »

Formica John,
That is about as bad as it can get.  If you are only going to throw the glider it may not cause too much damage, just a few bruises on the back of your neck but as it is only 8" span I guess you are intending to launch with a catapult in which case it could draw blood.  Seriously though FJ  if a trimming hand launch looks OK with a CG that far forward than a fast hand launch, and certainly a catapult launch is going to whip the glider round in a very small loop.  It is difficult to suggest a correct CG without seeing any plans but I think it is safe to say that you will seldom find a HLG or CLG with a CG further forward than 65%.
John.
 
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Mike Rolls
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« Reply #3 on: August 26, 2017, 03:23:05 AM »

Back in the 50s many folk - including me - rigged chuck gliders with wing and tail at zero and the CG at or only very slightly in front of, the CG. Gave a good climb with no looping tendencies, but the transition to the glide could be a bit fraught - sometimes the 'transition' would be a vertical dive to terra firma! Later days (haven't flown a chuckie in decades) folk seem to have moved the cg a little further forward and trimmed for a spiral, sacrificing a bit of height for a safer transition. The very big difference in speed between the model's normal glide speed and a vigorous launch, whether by bicep or catapult, means that 1/3 back is not a good idea!
Mike
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OZPAF
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« Reply #4 on: August 26, 2017, 06:07:56 AM »

FJ I think it is accepted practice today with catapult gliders particularly, trim the launch first and then adjust the glide if necessary - with out changing the launch settings - ie angular difference between the wing and tail.

I agree with John(Hepcat) that the Cg at 1/3 (of the root chord?) on a wing plan form that is swept back, is too far forward for the reasons John mentions.

I would suggest you move the Cg back to at least 50% of the root and remove the up elevator warps/gurney that worked for your 33% CG. Check the glide and if necessary remove more up elevator to stop a stall - but don't move the CG forward. If it is diving on the glide - and it has reasonable tail area and moment( the Sweepette should be ok in this area) - ensure the tail is definitely flat and then move the CG back slightly.

When you have a reasonable glide - then start launching on your catapult. If you are lucky- when launched with a 30 deg or so right bank and around 60degs up - it will climb in a shallow turn to the right and then possible turn and dive vertically to the ground.

Great - this means that you will only need a small amount of up elevator to transition to the glide - providing your CG is not too far aft.

Once it transitions from a steep climb to a glide - whilst being launched with a consistent stretch on the catapult - then you can trim the glide.

I do this by removing very small amounts(size of a pin head) of nose weight(easy if you are using modelling clay) insufficient to make any real difference to the stability, but enough to trim the glider to fly at a higher angle of attack.

Do not change the stab settings to trim the glide if you are getting a good transitioning launch.

This is very abbreviated and the road to success requires more study and thought.

I would suggest that you search HPA for the pearls of wisdom form Lee Hines(Sweepette), Tony Mathews,  Stan Buddenbohm, Ralph Ray.

Also it would be worth trying to find anything from some of the leading English glider flyers such as Mick Paige who has an enviable contest record.

There are no real shortcuts just lots of study and practice.

Good luck.

John
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Crabby
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« Reply #5 on: August 26, 2017, 09:14:14 AM »

Great pearls of wisdom and knowledge going on here. As a kid I seemed to be able to crank these chuckies and cat gliders out en masse, and seem to remember doing quite well with them. Of course Thee Olde Man and my buddies old man were experts in aerodynamics and were not afraid to go to chalk and blackboard with complicated concoctions of numbers and letters that put me asleep. But I owe my early successes to their efforts. So I can't seem to get back to anything that works like it used to, till I built the Tick, and even it is touchy and moody. So I am going to look at the Sweepette, and whatever guidance I can get back to the trail of promise! I see guys out at our field shoot these things way the "H" up there, and they softly and slowly return to earth with glide ratios like dandelion seeds.
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Johnny Formica
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« Reply #6 on: August 26, 2017, 09:47:44 AM »

I really appreciate all your input, I think I have made this more difficult for myself by not following a tried and tested plan but I love making these little gliders and I learn more after every one I make. The original wing was quite wide with the high point being about 15mm back from the leading edge which has enabled me to use my trusty razor saw and cut the leading edge back 10mm. The aerofoil shape has been re sanded back and all looks better with an approximate C of G of about 60%. When the wind hopefully dies down later I shall endeavour to take to the skies. I have put a couple of pictures on showing before and after. Thanks again.
John
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skyraider
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« Reply #7 on: August 26, 2017, 10:50:12 AM »

FJ,
  I stand corrected. I should have read your post several times before replying.
Somehow enlargement from an 8" w/s to a larger one missed being stuck in my
old head. Sorry about that. I agree with Hepcat and the others here with their
recommendations. Also a pic really helps.

Skyraider
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OZPAF
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« Reply #8 on: August 26, 2017, 08:08:16 PM »

Thanks for the pics JF - that helps. It looks ok to me although you won't know for sure until you try a launch.

I noticed that your tail boom is tapered and it appears that it is all on the bottom, where the stabilizer is attached. This will give a small amount of 'Up Elevator" or more accurately a small amount of positive longitudinal dihedral. I find it easier to start with as close to a zero - zero setting as possible(ie no angular difference between the wing/tail). Thus I taper the top of the boom and place the tail underneath on the flat bottom of the boom, parallel to the underside of the wing. I also usually glue the fin to the right side of the boom after sanding in a couple of degrees left rudder. It is a much stronger joint as well.

Adding a small amount of "washin" on the centre panel on the inside of the turn and a small amount of up elevator will provide sufficient positive angular difference in most cases. The launch trimming will then determine whether you need more of less.

I would also consider adding a couple of light triangular gussets under the wing between the TE and the fuselage. These only need to be 1.5 light with of course the grain parallel to the hypotenuse of the triangle - crossing the grain on the wing. These  will help to keep the wing on hard arrivals.

I apologise if I have gone overboard here but hope it helps.

Incidentally another source of good info on small Cat gliders is Peter Becker (Pit) here on HPA.

Happy flying.

John
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Johnny Formica
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« Reply #9 on: August 27, 2017, 03:59:28 PM »

OZPAF- Many thanks for your detailed reply- I know I will be refering back to it in the future. You were correct that you don’t know until you try. (I will stick to a plan in the future). Not knowing how much nose weight required to achieve a nice glide makes it difficult to know where the wing needs to be for the correct CG. I removed the wing again and stuck it temporarily with office stick glue as recommended on here somewhere- and messed about until I thought it was right and tried again. The plane glides quite well but flies right instead of left. I think I will put it in the hangar for another day.

On a sad note my 8“sweepette was lost today- I changed the stab tilt this morning to achieve a larger glide radius and it worked really well- my downfall was flying in the gusty conditions- I couldn’t  wait for the evening quieter air and the plane stayed up for ever getting slowly blown downwind until it flew over a hedge into a garden. I may get lucky but it has no waterproof jacket on and after several rescue attempts I think the chances of recovery are slim.

OZPAF- One more thing- Do you build with stab tilt or rely on rudder? I have no experience of trimming.

Thanks
John
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OZPAF
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« Reply #10 on: August 28, 2017, 12:40:30 AM »

I use both actually. A small amount of built in rudder ( left) is built in and is used mainly to roll the glider out of it's right bank into it's left glide.

I then have tail tilt to provide most of the glide turn. How much depends on the model and the % of tail area(of the wing). It shouldn't be too much - 10 deg max at a guess.

That sounds like good progress despite losing your sweepette 8" glider.

I'll try to dig up some words on trimming for you and if you personal message me with your email I could send you the docs.

happy flying

John
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