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Author Topic: Lead out position at tip  (Read 5638 times)

Offline Bill Little

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Lead out position at tip
« on: July 12, 2015, 08:59:06 PM »
Hi All,

I have a pressing question before I start building a Classic design.  It is the swept back wing F3H Demon from Jack Sheets.

How do I calculate the lead out position at the tip with a swept back wing??  I will do all I can to install an adj. guide, but I have no way of finding a mid point for it.  I am sure some of you can give me an answer.

Thanks!
Bill
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Offline Tim Wescott

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Re: Lead out position at tip
« Reply #1 on: July 12, 2015, 09:57:07 PM »
You want to locate the leadout guide in the correct position relative to the CG, regardless of where the wingtip may be.  As to where the CG should be -- I dunno.  I haven't wholly sussed the whole problem of where to put a CG on a stunt ship (other than by fussing with it until it flies right), but I'm guessing that you want the CG to be at about the point where the bellcrank pivot is located on the plans, or maybe a bit back.

I suspect that where the plans show the leadouts is actually a good starting point.  Figuring out how to get much adjustment in the leadout position without too sharp a bend where the front leadout goes into the leading edge should be a nice challenge, particularly because it's probably not a good idea to have a honkin' big slot in the leading edge.
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Offline Bill Little

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Re: Lead out position at tip
« Reply #2 on: July 12, 2015, 10:42:43 PM »
Hi Tim,

Thanks for the comments!  I would have no problem with a "standard" wing, with constant chord or a swept LE/TE.  But with the entire wing panel swept back about 7*, I have no clue.  I forgot to mention that often the pictures of the model in the article show the lead outs in one fixed position while the plans show them somewhere else altogether.  And like you say, I would not really want to run a long slot in the LE and make the lead out guide look like a huge pitot (sp?) tube! LOL!!  Need to hear from Ted or Brett, I guess, they may know a formula to use for a swept wing panel ;D

Thanks, again!
Bill
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Offline Tim Wescott

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Re: Lead out position at tip
« Reply #3 on: July 12, 2015, 11:58:41 PM »
Need to hear from Ted or Brett, I guess, they may know a formula to use for a swept wing panel.

Well, as to that, use this http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/formulas/mean_aerodynamic_chord.htm, and treat the resulting "MAC" as the chord of a rectangular wing.

Before Howard takes a break from preparing to win the Nats and points this out, the results will be approximate -- but they should at least put you enough in the ballpark that you won't be cutting up your plane after the first test flight.
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Offline Bill Little

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Re: Lead out position at tip
« Reply #4 on: July 13, 2015, 12:05:55 AM »
Thanks, Tim!

Bill
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Offline Chris Wilson

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Re: Lead out position at tip
« Reply #5 on: July 13, 2015, 09:02:34 PM »
Well, as to that, use this http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/formulas/mean_aerodynamic_chord.htm, and treat the resulting "MAC" as the chord of a rectangular wing.

Before Howard takes a break from preparing to win the Nats and points this out, the results will be approximate -- but they should at least put you enough in the ballpark that you won't be cutting up your plane after the first test flight.
Hey Tim, does that assume a constant section along the span?

Thanks.
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Offline Tim Wescott

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Re: Lead out position at tip
« Reply #6 on: July 13, 2015, 10:12:54 PM »
Hey Tim, does that assume a constant section along the span?

No, but it does require a straight taper.
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Offline Serge_Krauss

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Re: Lead out position at tip
« Reply #7 on: July 14, 2015, 02:47:18 AM »
I would have no problem with a "standard" wing, with constant chord or a swept LE/TE.  But with the entire wing panel swept back about 7*, I have no clue. I Need to hear from Ted or Brett, I guess, they may know a formula to use for a swept wing panel ;D

Bill-

I haven't posted much recently, but if you do a search on either SSW or SH forums, you'll find several of my posts on this. The main point I'd make is that you do NOT even have to calculate these things yourself. Some RC and other sites have calculators on line, where you just enter your model's dimensions and they feed you the answer. I see that the best RC site says that they have a virus and will not display their page now, but the Palos sites are still up:

http://www.palosrc.com/index.php?option=com_content&view=article&id=51:mac&catid=41:ic&Itemid=50

http://www.palosrc.com/index.php?option=com_content&view=article&id=50:cg&catid=41:ic&Itemid=50

They've taken down their A.C. calculator, but you can use either of these to find your answer. For the aero center (AC), just keep varying the CG position until the % MAC reads 24% or 25%. That root position will give you your longitudinal AC position. Then you vary it to get the % MAC you desire for your CG or just use the CG calculator for that.

If you really want to calculate the AC position yourself, I'll send you a PDF version of my derived formulas (illustrated) for several combinations, including straight or swept (fore or aft) tapered wings, elliptical wings of varied types and sweep, and some others. I offered them on the forums when I derived them years ago, but no one was interested. Regarding the elliptical, my derivations are correct, while some given as "gospel" on the internet are not. If you'd like these formula sheets, just contact me off-forum with your current e-mail address, and I'll e-mail them to you. I think the Palos site is what you want though, unless you have more complicated shapes. Approximate the tips; the answers will fall within the errors of the understood simplifications inherent in the assumptions used.

SK


Offline Bill Little

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Re: Lead out position at tip
« Reply #8 on: July 14, 2015, 03:29:08 PM »
Hi Serge,

Thanks for the help!  It is good to see you again, I hope all is well with you!

Bill
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Offline Trostle

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Re: Lead out position at tip
« Reply #9 on: July 14, 2015, 07:18:40 PM »
Well, as to that, use this http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/formulas/mean_aerodynamic_chord.htm, and treat the resulting "MAC" as the chord of a rectangular wing.


Tim,

I am not trying to be critical of the information in your reference above regarding the calculation of the MAC.

There is another formula that is useful in locating the position of that MAC relative to the root chord.  In the following:

m is the distance from the root chord leading edge to the leading edge of the MAC.  
s is the sweep distance from the tip LE to the root LE.
a is the root chord
b is the tip chord

m = s(a+2b)/3(a+b)

Or another way to express this if I do not have the correct usage of the computer here:

m = (s/3)((a+2b)/(a+b))

Works for any swept wing with any amount of taper.

This works with  the calculation for the MAC as

MAC = (2/3)((a+b - (ab/(a+b))

Showing my age a bit, but this is from my Perkins and Hage text book, Airplane Performance, Stability and Control, 1958.
« Last Edit: July 15, 2015, 12:06:54 AM by Trostle »

Offline Matt Spencer

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Re: Lead out position at tip
« Reply #10 on: July 15, 2015, 10:19:20 PM »
Thats fearfully Modern . Its from after the war . My aerdynamic manual predeceeds the war .  ^-^

Offline SH Stunter

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Re: Lead out position at tip
« Reply #11 on: August 11, 2015, 02:45:33 PM »
The first thing to do is to ask the designer, Jack Sheeks, about the lead out placement.  He should know if anyone would.  Try him.
S-H Modeler.

Offline Mark Scarborough

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Re: Lead out position at tip
« Reply #12 on: August 11, 2015, 04:23:51 PM »
Mike Haverly built something similar,, in fact he is such a glutton he built two,, he may have some helpfull input as I know he dealt with that problem on his
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Offline Bill Little

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Re: Lead out position at tip
« Reply #13 on: August 11, 2015, 04:41:04 PM »
Mike Haverly built something similar,, in fact he is such a glutton he built two,, he may have some helpfull input as I know he dealt with that problem on his

Hi Mark,

I totally forgot that Mike built the Freedom 45 from Jack two times!  I will contact him.  Jack had at least three models with fully swept wings before the Freedom 45, maybe four.

Thanks!
Bill
Big Bear <><

Aberdeen, NC

James Hylton Motorsports/NASCAR/ARCA

AMA 95351 (got one of my old numbers back! ;D )

Trying to get by


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