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Author Topic: Tip weight formula  (Read 8648 times)

Offline sleepy gomez

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Tip weight formula
« on: August 26, 2010, 07:53:31 PM »
Tip weight formula?  Can a formula be developed perhaps using half the weight of the lines and then adding an equal amount of tip weight?  Then could a percentage of the line weight added to the tip weight?  Would the plane then need to balance at something outside the center line of the plane?  Granted the 'formula' would not be exact but it might provide a good and safe starting point.  It seems this might be better than "add x amount of pennies".   So a penny for your thoughts.

Offline Jim Thomerson

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Re: Tip weight formula
« Reply #1 on: August 27, 2010, 12:43:34 PM »
I use steel BB's. I start out with a number of BB's based on engine displacement, in multiples of five.  35 for a 35, 40 for a 40, 15 for a 15, etc.  Then add or subtract five at a time until the airpane flies right. D>K

Offline Tim Wescott

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Re: Tip weight formula
« Reply #2 on: August 27, 2010, 12:47:06 PM »
I don't have enough experience to say on my own, but most of the advice that I read say balance the plane side-side then add half the weight of the lines.

An equivalent is to hang the plane so that you can easily see level, put your line reel (no handle) halfway out the inboard wing, then put an empty & identical line reel half way out the outboard wing, then balance.  The empty reel cancels out the effect of the reel on which the lines are wound, so you end up with balance + half the line weight.

Then go flying and see how it does, because any static balance procedure is just a starting point.
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Offline RandySmith

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Re: Tip weight formula
« Reply #3 on: August 27, 2010, 02:14:19 PM »
Tip weight formula?  Can a formula be developed perhaps using half the weight of the lines and then adding an equal amount of tip weight?  Then could a percentage of the line weight added to the tip weight?  Would the plane then need to balance at something outside the center line of the plane?  Granted the 'formula' would not be exact but it might provide a good and safe starting point.  It seems this might be better than "add x amount of pennies".   So a penny for your thoughts.

not really, too many things will kill any formula....it takes what it takes...  If you build the in or outboards a little heavy or light , this will affect your calculations
if you build the tips differant weights this will affect the calculations
if you put the heavy wood on the outboard, as you should this will affect it also
if you leave the leadouts longer or shorter this will affect the calculation
if you have more paint on one panel this will affect it
the asymmetry of the wing panels will greatly affect the calculations
if  blah blah  blah....  it can go on forever.

You can though get close, bench trimming by just adding weight to the tip until it drops slowly when held level then let go, or you can add clay to the outboard tip until it balances then add 1\3 ounce to start with a 1/2 inch shorter outboard and about 3\4 ounce with equal panels.....unless it is a profile..then that will change the weights


Regards
Randy
« Last Edit: September 06, 2010, 06:39:29 PM by RandySmith »

Offline Mark Scarborough

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Re: Tip weight formula
« Reply #4 on: August 27, 2010, 10:43:52 PM »

The way pat Johnston taught me to bench trim tip weight was to balance the airplane assembled ready to fly on the glow plug and tail wheel ( assuing that its accurately built and the tail wheel is pretty much centered. Then put a scale under the outboard tip, add weight until the tip weighs one ounce, or an ounce and a quarter . This is a good starting point, enough to insure you ahve enough, but not so much that it will fly like a carrier plane,,, y1
For years the rat race had me going around in circles, Now I do it for fun!
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Offline Bob Reeves

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Re: Tip weight formula
« Reply #5 on: September 05, 2010, 03:16:02 PM »
Tip weight formula?  Can a formula be developed perhaps using half the weight of the lines and then adding an equal amount of tip weight?  Then could a percentage of the line weight added to the tip weight?  Would the plane then need to balance at something outside the center line of the plane?  Granted the 'formula' would not be exact but it might provide a good and safe starting point.  It seems this might be better than "add x amount of pennies".   So a penny for your thoughts.

I believe it's possible, I even asked the same question some time ago. Biggest road block is trying to get 10 or 15 top trimmers/flyers to give you the test data needed. If we could get the NATS top 20 to weigh the outboard wing tip as explained by Pat Johnston and tell us what lines and maybe a few other details I am totally convinced a starting formula could be developed that would be pretty durn close.  I wanted to add it to Line-III but received the same kind of responses you have and gave up...

Offline sleepy gomez

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Re: Tip weight formula
« Reply #6 on: September 05, 2010, 10:11:03 PM »
Thanks Bob, I think you understand.  May I refer you and others to the main forum/ monkeys and stunt.  SLEEPY

Offline RandySmith

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Re: Tip weight formula
« Reply #7 on: September 06, 2010, 12:00:51 AM »
I believe it's possible, I even asked the same question some time ago. Biggest road block is trying to get 10 or 15 top trimmers/flyers to give you the test data needed. If we could get the NATS top 20 to weigh the outboard wing tip as explained by Pat Johnston and tell us what lines and maybe a few other details I am totally convinced a starting formula could be developed that would be pretty durn close.  I wanted to add it to Line-III but received the same kind of responses you have and gave up...

And it would still be useless in getting an "formula" for accurately figuring out tip weight for all the reasons I listed above plus dozens more.
All you could ever do is develop one that would get close on some or most planes, and not so close on others, You can do that just by adding weight to the ourboard tip until it just drops slightly...even this will vary , there are many factors that will change every trick or formula. It will in all circumstances normally come down to flying and adjusting the weight to get the proper amount.

Regards
Randy

Online Igor Burger

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Re: Tip weight formula
« Reply #8 on: September 06, 2010, 02:13:39 AM »
Tip weight formula?  Can a formula be developed perhaps using half the weight of the lines and then adding an equal amount of tip weight?  Then could a percentage of the line weight added to the tip weight?  Would the plane then need to balance at something outside the center line of the plane?  Granted the 'formula' would not be exact but it might provide a good and safe starting point.  It seems this might be better than "add x amount of pennies".   So a penny for your thoughts.
Can be :-)

Question is how exact answer you expect. It will give you number little under 1oz and for the security and with respect of material selection you will come to real weight little over 1oz. And it will be good and safe starting point. :- ))))))

Exactly like LINE-xx program will tell you line rake. It is also “some” useful number. Both methods are very scientific, but both miss reality.

I have formula which works to some extent, but since tip weight needs also subjective tuning, then no “computerized” method gives good answer.

Here are some thoughts:

Tip weight must counterbalance the weight of the fuselage. It is mounted inboard of center of lift, because center of lift is approximately an inch right of the geometrical center of wing because of circular path of the model (outer wing is quicker).

The same is necessary to do for wing and tail.

The problem is, that we use asymmetric flaps, and therefore position of center of wing depends of amount of extended flaps. But the tip weight is important in corners, so we have to calculate it with flaps extended to corner – and that is point which depends on personal preferences (or abilities).

Position of center of lift also depends on yaw.

We have also lines, but lines in corner does not follow model so quick, so they does not change the result so dramatically. For example when I tried thinner lines on my trimmed model, I had to do many changes, but tip weight was the same after retrim. I switched from 0.40 mm line to 0.36 what makes theoretical difference 5g/2. 

So for me … The best way I found is to calculate center of lift in corner with respect of “my” corner radius and flaps chord/are asymmetry. Then I add weight to put the CG to that point. It is still too little, but model is yawed little and it is enough to balance lines also in level flight. For example – my fuselage including rudder needs 21g … my wing asymmetry needs 9g … and my real tip weight measured by scale in center of symmetric wing is 29g

However it is good to start trimming with little heavier tip and subtract during trimming. So I would say, make it short and use 1oz   VD~

Offline sleepy gomez

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Re: Tip weight formula
« Reply #9 on: September 06, 2010, 10:48:55 AM »
From Sleepy - Thank you very much Igor.

Offline RandySmith

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Re: Tip weight formula
« Reply #10 on: September 06, 2010, 03:21:10 PM »


Hi Igor

As I wrote earlier these things, and many many more will kill that formula from being accurate,
It is just impossible to come up with a formula that will be accurate across the board, all you will ever get is something that will get you started in the right direction  ;D      see below:

"not really, too many things will kill any formula....it takes what it takes...  If you build the in or outboards a little heavy or light , this will affect your calculations
if you build the tips differant weights this will affect the calculations
if you put the heavy wood on the outboard, as you should this will affect it also
if you leave the leadouts longer or shorter this will affect the calculation
if you have more paint on one panel this will affect it
the asymmetry of the wing panels will greatly affect the calculations
if  blah blah  blah....  it can go on forever."

Regards
Randy

Offline Jim Kraft

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Re: Tip weight formula
« Reply #11 on: September 06, 2010, 06:14:49 PM »
Here is my formula; If you have your tip weight pretty close flying in Kansas moderate wind (10-15), and you are going to a contest where it will be blowing Kanas normal wind (20-30), add two pennies. You won't be able to tell if the wing is banging from to much tip weight or the wind, and it will stay out on the lines most of the time. y1 Works for me.
Jim Kraft

Offline RandySmith

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Re: Tip weight formula
« Reply #12 on: September 06, 2010, 06:40:35 PM »
You can get close, bench trimming by just adding weight to the tip until it drops slowly when held level then let go,
 or you can add clay to the outboard tip until it balances then add 1\3 ounce to start with a 1/2 inch shorter outboard and about 3\4 ounce with equal panels..
...unless it is a profile..then that will change the weights


Regards
Randy

Offline Dean Pappas

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Re: Tip weight formula
« Reply #13 on: September 07, 2010, 11:10:15 AM »
Oh! You're approximating the Log with a square root term ... How clever! n~
Dean
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Offline L0U CRANE

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Re: Tip weight formula
« Reply #14 on: October 08, 2010, 03:46:51 PM »
...and yet another unscientific, but reality-related approach...?

The weight of the lines is supported at both ends. One end is at the handle; the other - in effect - is at the tip guides.

...while the model is flying level, that is... In maneuvers, where additional forces vary a lot all but don't last long, things go screwball, but just about equally to each side of stable level flight.

So, here's a crude "do-it-until-it's-done" method:

Best done with the model complete and ready to fuel up, but adequately close while the finishing is incomplete.

Rest the model on its back, on a towel or something to protect it from scuffing. It rests on fin and spinner or forward fuse, right? (Ad lib for multiple fins, or where a single fin is not on the centerline. Say, rest the model on the front and aft fuse...)

Get out the reel with the lines you intend to use, and position it on the inboard wing panel, halfway from the fuselage centerline to the leadout guides.

Find another - identical - empty reel (if you prefer a bit more "precision" include the same number and type of line clips.) Put the empty on the outboard panel at the same distance from fuselage centerline as the full reel on t'other side.

Surprise! the inboard panel drops to the tabletop, right? Add "ballast" at your weight box location until the model balances with no significant tendency for either tip to drop from horizontal.

You've static-balanced the model on the fuselage centerline for the half of line weight supported at the leadout guides in level flight. How does this work?

Well, full weight at half the distance is the same as half the weight at full distance. The empty reel at equal distance from centerline cancels the weight of everything that isn't the flying lines.

You haven't measured anything; weighed anything; calculated anything. You just "did-it-until-it-was-done."  You have a decent starting point from which to final-tweak what the particular model needs - and they all ARE different... - and the more demanding the flier is, the finer and fussier the final adjustments get as the ideal is approached.

The other effects mentioned, e.g., Igor's mention of line inertia, and many others still appear. Forunately, line pull is a great stabilizing factor by itself. It is one of the strongest continuing forces on the model. Maneuver lift periods are all brief, and only the consecutive round loops last longer before forces unload or reverse. (Kirk Mullinix mentioned that at the recent Tucson meet. Makes great sense!)
\BEST\LOU

Offline Tim Wescott

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Re: Tip weight formula
« Reply #15 on: October 08, 2010, 03:55:14 PM »
As a newbie I don't know much, but one statement that I see stated over and over again for tip weight, fore/aft CG, tank position, needle valve setting, leadout position, and darn near everything else is: "that'll get you to a good starting point, then you'll need to tune it from there".

Which my lazy brain reads as "don't sweat the details too much up front: just make a SWAG, get it flying, and then sweat the details".
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Offline L0U CRANE

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Re: Tip weight formula
« Reply #16 on: October 08, 2010, 04:15:48 PM »
Tim,

I agree by and large, but I'd prefer to be in the same ballpark, in the same city, as the model... (That's a joke, son... ;) ) Some guys have enough experience and hands-on 'feel' that they can SWAG very close to what they want for a final result. Not everyone has that. So, for me a simple, sensible procedure makes a certain amount of sense.

And, the other important phrase is: "...they are all different..."  You will almost certainly need to make further adjustments in any case. A model that is waayy off at the time of first flight just might not be around for a second flight...
\BEST\LOU

Offline Tim Wescott

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Re: Tip weight formula
« Reply #17 on: October 08, 2010, 06:08:11 PM »
Lou:

I wasn't saying don't do your procedure -- in fact, yours sounds like an excellent way to get in that "safe first flight" ballpark.  It was more addressed to folks that may find themselves hesitating to fly because they're not sure if their starting tip weight is correct to the nearest milligram.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline L0U CRANE

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Re: Tip weight formula
« Reply #18 on: October 10, 2010, 03:09:34 PM »
Dean, to your Post #14:-

I've never seen a log sawed from a root. Roots aren't square, either. They're usually gnarly and mud covered...

Math! Pi are square? (That's rotten spelling, btw.) Naah, every pie I've ever seen was round. We called the "square" pizzas pizzaiola when I was a kid in Brooklyn. Since pizza means pie, why do we order 'pie-pies?'
\BEST\LOU

Offline Bill Little

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Re: Tip weight formula
« Reply #19 on: October 11, 2010, 12:42:57 AM »
Back in 1990 (maybe 1991??) Paul Walker put a trimming procedure table in his Impact article (Flying Models).  He also described a basic bench trimming procedure.  His suggestions were to balance the plane on the fin and nose and add weight to the outboard wing so that the wing will slowly (not *bang*) drop to the table.[/i]  I figured Paul probably knew what he was talking about. ;D  Sorta exactly like Randy Smith has posted a bazillion times (he pretty well knows what HE is talking about, too).  Super simple, and works on every plane to get a starting point.  All we can hope for is a starting point, and trim from there to suit your style. ;D  A tiny bit too much and even a tiny bit too little doesn't hurt for the first flight.  I would suggest a bit too much if you do not have much experience, I guess.  It has always worked for me...........

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Online Igor Burger

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Re: Tip weight formula
« Reply #20 on: October 11, 2010, 01:25:42 AM »
...and yet another unscientific, but reality-related approach...?

 ;D

I tried your unscientific approach to check my indoor model, but I failed. My (proper) tip weight is 20x heavier than one line, it somehow miss the reality. What now?  VD~

Offline L0U CRANE

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Re: Tip weight formula
« Reply #21 on: November 01, 2010, 09:40:25 PM »
Igor, late, but to the point...

The situation with your indoor F2B Pattern model is so far out of usual practice that it may serve as the exception which proves a rule.

I spoke of a simple 'rule of thumb' that is useful for most simple, advanced and - maybe - ultimate CLPA models. A few grams of model, with a propeller that reflects shaft torque loads very well, is something different.

My best wishes for  a successful 'demonstration model!'

\BEST\LOU
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Online Igor Burger

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Re: Tip weight formula
« Reply #22 on: November 02, 2010, 01:49:04 AM »
>>>so far out of usual practice that it may serve as the exception which proves a rule<<<

  ;D ;D ;D ... you are right, but such exception is always eye opening ... I recommend everyone do such model, I learned LOT

I just wanted to say that line weight is not the only thing we have to counter ballance. Such a model is on very short lines and all troubles of tethered flight are extremaly visible. Lines are plastic (Spiderwire fishing lines) and so does not weight anything and I need 10g of lead anyway. It is because of shifter center of lift, not because of lines weight. (motor torque must be counterballanced aerodynamically so it work also in reversed flight)

Funny is, that we have more rules which are logical for first look, give good results but does not reflect full reality. Another such rule is result of program LINEXX which is developed for speed and team racing models, does not reflect reality of stunt models at all, but also giving "some" good number  VD~


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