News:



  • March 28, 2024, 01:55:19 PM

Login with username, password and session length

Author Topic: Today's Quiz  (Read 11146 times)

Offline t michael jennings

  • AMA 83322
  • 2016 supporter
  • Trade Count: (0)
  • Commander
  • *
  • Posts: 221
Today's Quiz
« on: November 02, 2015, 05:54:18 PM »
Gentlemen,

Attached is a sketch of the Inboard section of the wing that I am building.

Need to know two things:

1)  What is the dimension for the CG (Center of Gravity)?

2)  What is the dimension for the LO (Lead Out) Centerline?

The given dimensions are in inches.

Thanks for your assistance.


T Michael Jennings    :-\
Knoxville, TN.


Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Today's Quiz
« Reply #1 on: November 02, 2015, 05:59:09 PM »
My calibrated eyeball says that your "CG" dotted line is pretty close, so slap a ruler on your drawing and start scaling it out.

That same calibrated eyeball suspects that your leadout location is too far back; I'd deal with that by building in plenty of adjustment and then finding out.

Don't expect either of them to be dead nuts on when you get the plane in the air.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Today's Quiz
« Reply #2 on: November 02, 2015, 08:14:14 PM »
I oughta make a spreadsheet to do that.  It still depends on some other stuff.  However, it's easier to figure where the CG should be than it is to figure out how to make it come out there.

Use Line III for leadout position: http://www.tulsacl.com/Linelll.html .  As Igor will tell you, that's only for one flight condition.  Make them adjustable about that point. 

 
The Jive Combat Team
Making combat and stunt great again

Offline Steve Helmick

  • AMA Member and supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 9920
Re: Today's Quiz
« Reply #3 on: November 02, 2015, 09:24:24 PM »
Wouldn't having the position of max thickness on the airfoil move aft to something silly like 60% back then require the CG and LO's be shifted similarly? Do I have to provide the math?  VD~ Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Today's Quiz
« Reply #4 on: November 02, 2015, 09:35:25 PM »
Wouldn't having the position of max thickness on the airfoil move aft to something silly like 60% back then require the CG and LO's be shifted similarly? Do I have to provide the math?  VD~ Steve

CG is mostly a function of the wing and stab planform, and the distance twixt the two.  Wacky airfoils primarily affect elevator trim and when and how the poor thing stalls.

But if you want to build a stunter to prove differently go right ahead.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Steve Helmick

  • AMA Member and supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 9920
Re: Today's Quiz
« Reply #5 on: November 02, 2015, 09:37:33 PM »
Well, then why does most everybody suggest starting out by putting the CG just forward of the high-point of the wing airfoil and/or spars?  VD~ Steve
"The United States has become a place where professional athletes and entertainers are mistaken for people of importance." - Robert Heinlein

In 1944 18-20 year old's stormed beaches, and parachuted behind enemy lines to almost certain death.  In 2015 18-20 year old's need safe zones so people don't hurt their feelings.

Offline Serge_Krauss

  • Moderator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 1331
Re: Today's Quiz
« Reply #6 on: November 02, 2015, 10:29:43 PM »
Go to one of the online MAC (Mean Aerodynamic Chord) calculators (a search here will find at least a couple of my posts on that, giving links), type your wing dimensions in, and read out the wing's (half span's) aerodynamic center. Mark it on your scale wing drawing. The program will give you the MAC and it's position. Then use the rules of thumb that for a flapless model, you want the c.g. at maybe 17% to 20% of the MAC, and for flapped, you want the c.g. between 20% and 25% of the MAC, depending on the ratio of horizontal tail area to wing area. Make the % MAC equal to the percent the tail's area is of the wing's. Those will get you close for a modern proportioned model. The online calculators usually will do that computation for you and therefore position the c.g. for you.

These computations are made according to theory with simplified assumptions that nets useful answers. On line, you don't have to do any of the work. If you want the MAC equations, they're mostly on the internet, or you can get them and their derivations from me. My elliptical wing stuff is better than what I found out there. Anyway, if your model is conventional in configuration, most of the things people tell you will suffice for a start. If not, they will not, especially for higher than average wing sweep. Wing high point of any useful section will not have any significant affect on c.g. position, but as said above, wii affect stalling and thus max lift and drag.

For the leadout position, use the "Line II" or "Line III" freeware. Leadout position can be approximated as 3/4 - 1" behind the c.g. of a "normal" sized model, but it's proportional to size, and depends on line length, model weight, line diameter, speed, air density, etc. Those programs seem to provide a reasonably accurate answer, when used with information from from a site like Palos RC or RC Proving Ground. Do the search or just google these two.

I'm supposed to be doing something else pretty important now, or I'd check out and give you all the answers you need, but I really can't. A search should give you several links. This is important enough that I think you should spend the 5 minutes to 1/2 hour getting a good answer from  these sites. 'wish I could help more now, but I'm sorta "outta here."

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Today's Quiz
« Reply #7 on: November 03, 2015, 03:15:02 AM »

Use Line III for leadout position: http://www.tulsacl.com/Linelll.html .  As Igor will tell you, that's only for one flight condition.  Make them adjustable about that point. 

 

Igor will especially ask you where is the rudder size and deflection setting, or motor offset in those programs :- ))))))))))

Offline t michael jennings

  • AMA 83322
  • 2016 supporter
  • Trade Count: (0)
  • Commander
  • *
  • Posts: 221
Re: Today's Quiz
« Reply #8 on: November 03, 2015, 05:38:16 AM »
Gentlemen,

Thanks for your response.
I will follow up on your suggestions.

There was an error in the sketch.  Attached is the corrected sketch.

T Michael Jennings
Knoxville, TN.




Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Today's Quiz
« Reply #9 on: November 03, 2015, 10:42:22 AM »
Well, then why does most everybody suggest starting out by putting the CG just forward of the high-point of the wing airfoil and/or spars?

Because most everybody whose stuff you read doesn't understand the theory.  As Serge said, "...if your model is conventional in configuration, most of the things people tell you will suffice for a start. If not, they will not..."
The Jive Combat Team
Making combat and stunt great again

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Today's Quiz
« Reply #10 on: November 03, 2015, 10:52:54 AM »
Gentlemen,

Thanks for your response.
I will follow up on your suggestions.

There was an error in the sketch.  Attached is the corrected sketch.

I stand by what I said.  "By the book" CG location and especially leadout location is subject to tons-o-factors, not least of which is personal preference.

For that matter, the correct CG location is strongly affected by the tail configuration, and you haven't shown us that at all.  I wouldn't stress too much on getting it exactly on some calculated spot -- I'd build the plane, fly it, and adjust it to suit me.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Today's Quiz
« Reply #11 on: November 03, 2015, 12:55:49 PM »
I also noted the aspect ratio of the wing is very low...in the vicinity of 4.3 (span squared divided by the wing area or, for straight tapered wings like this, span divided by the average chord.)  Your flaps constitute a significant percentage of the wing area so you'll want to be aware that you'll want to use modest flap deflection as the induced drag (drag due to lift) will already be quite large while maneuvering due to the low AR and deflecting flaps excessively will exacerbate the situation.  Low aspect ratios on unflapped designs aren't quite as handicapped but any time you go much below a classic (Nobleresque) five to one (span/average chord) flap deflection will be worthy of adjustability to maximize trim.

Also, as--I believe--Tim has pointed out the CG location is hugely impacted by the size and location of the stab/elevator.  A useful rule of thumb I've employed over the years (on reasonably conventional planforms) is to determine the area of the tail as a percentage of the area of the wing (500WA/100TA= 20% for example) and plan that percentage aft of the leading edge at the MAC as the proximate ideal location (MAC is the Mean Aerodynamic Chord; chord length at the half span for practical purposes). In the example if the half span is 10 inches long 20% of that in the example is two inches aft of the leading edge at the half span and the optimum CG is "likely to be" at that point.  Initial flights for "not yet expert designer/fliers" should probably start with the CG about five percent forward of that predicted location which will allow "certain" controllable flyability and allow subsequent fine tuning without an intervening period of time back at the repair shop.

Other important variables that will affect the "optimum" are more esoteric and/or a matter of personal taste.  Aspect Ratio of the wing and tail as well as control system sensitivity are of import in the first case and what feels right to the pilot in the latter.  I would caution in either case that excessive quickness in response with an airplane that glides and lands with positive control should almost certainly be resolved by desensitizing the control system rather than simply adding nose weight.  Having the CG at a close to optimum location has a huge effect on the the plane's willingness to be flown well in all weather conditions.  Adding a lot of weight to the nose to slow down the rate of turn on an aerodynamically stable plane will make it harder and harder to fly well in bad air...especially strong winds.

Howard will probably poo-poo some of this and be technically correct but I think, as a lay person such as myself, you'll find the above very valuable information to consider when fine tuning a basically sound ship.  Sometimes Howard's correct explanations make me look like this:  n~ n~ !  My brain looks worse but, fortunately, it is shrouded in bone and fat and is, thus, not illustrate-able as an emoticon display.

Ted Fancher

Offline Serge_Krauss

  • Moderator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 1331
Re: Today's Quiz
« Reply #12 on: November 03, 2015, 10:09:45 PM »
(MAC is the Mean Aerodynamic Chord; chord length at the half span for practical purposes). In the example if the half span is 10 inches long 20% of that in the example is two inches aft of the leading edge at the half span and the optimum CG is "likely to be" at that point. 

I think Ted meant to type, "In the example if the Chord at the halfspan is 10 inches long, 20% of that in the example is two inches aft of the leading edge at the half span, and the optimum CG is 'likely to be' at that point."

I should point out that usual guesses get less and less accurate as taper increases. For the extreme taper of a pointed wing (taper ratio = zero), the MAC is only 1/3 of the half-span out from the root, and it's length is significantly greater than for a rectangular wing. For instance, if you change a 50"-span rectangular wing of aspect ratio 4.0 into a pointed wing of the same span and area (AR still 4.0), you'd miss your c.g. spot by about 3 1/3 " with this approximation. It works fine for "normal" wings, but breaks down progressively with increased taper.

As said earlier, the tail area and distance from the c.g.  are all parts of the total aircraft's aerodynamic center and thus of the c.g. placement. These considerations (including 'tail volume coefficient') are much more difficult to analyze. That's why Ted's ratio of Tail area to wing area is such a valuable rule of thumb. That rule of thumb and and the other one I mentioned above are pretty good starting points to trimming the model to its optimum (for the particular pilot) c.g.  Again though, if any dimensions (like chords) are stretched out of proportion to what constitutes a "conventional" plane, these too break down.

Finally (?), the flaps are always considered part of the wing area. So when using on-line calculators or conventional short-cuts, your measurements must include the flaps.

Offline Brett Buck

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 13716
Re: Today's Quiz
« Reply #13 on: November 03, 2015, 10:56:42 PM »
As above - you can tell very little about the characteristics of the airplane by looking at *just* the wing by itself. To make even a good guess at the CG, you need to show us the tail volume coefficient, or the dimensions of the tail and fuselage.

    Brett

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Today's Quiz
« Reply #14 on: November 04, 2015, 01:33:28 AM »
Howard will probably poo-poo some of this...

 "(500WA/100TA= 20% for example)"  looks potentially poo-pooable, but I'd have to give it more study.
The Jive Combat Team
Making combat and stunt great again

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Today's Quiz
« Reply #15 on: November 04, 2015, 03:26:05 PM »
"(500WA/100TA= 20% for example)"  looks potentially poo-pooable, but I'd have to give it more study.

OMG!  Who could have altered my original eminently correct post...oh, wait...

See?  I told you he'd do dat.  Where is the blushing face emoticon when you need it.  That's what I guess I get for gargling with Knob Creek just prior to pre beddy-bye posting.

signed
Anon

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Today's Quiz
« Reply #16 on: November 04, 2015, 03:36:29 PM »
I think Ted meant to type, "In the example if the Chord at the halfspan is 10 inches long, 20% of that in the example is two inches aft of the leading edge at the half span, and the optimum CG is 'likely to be' at that point."

I should point out that usual guesses get less and less accurate as taper increases. For the extreme taper of a pointed wing (taper ratio = zero), the MAC is only 1/3 of the half-span out from the root, and it's length is significantly greater than for a rectangular wing. For instance, if you change a 50"-span rectangular wing of aspect ratio 4.0 into a pointed wing of the same span and area (AR still 4.0), you'd miss your c.g. spot by about 3 1/3 " with this approximation. It works fine for "normal" wings, but breaks down progressively with increased taper.

As said earlier, the tail area and distance from the c.g.  are all parts of the total aircraft's aerodynamic center and thus of the c.g. placement. These considerations (including 'tail volume coefficient') are much more difficult to analyze. That's why Ted's ratio of Tail area to wing area is such a valuable rule of thumb. That rule of thumb and and the other one I mentioned above are pretty good starting points to trimming the model to its optimum (for the particular pilot) c.g.  Again though, if any dimensions (like chords) are stretched out of proportion to what constitutes a "conventional" plane, these too break down.

Finally (?), the flaps are always considered part of the wing area. So when using on-line calculators or conventional short-cuts, your measurements must include the flaps.

Darn, Serge!  You got me, too.  Can I hire you as an editor?  I'd ask Howard but he'd be way too eager to do so and might even be willing to pay me to allow him to do it.  Hmmmm.  Let me think about that... 

Your comments on the effects of taper ratio on MAC are on the money as well, of course.  That's why I generally use the qualifier "remotely conventional planform" or similar; in which case the half span "substitute" is within the "noise range" for a "rule of thumb."  How's that for equivocation overkill?   #^ #^

Ted

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Today's Quiz
« Reply #17 on: November 04, 2015, 03:45:26 PM »
Well, then why does most everybody suggest starting out by putting the CG just forward of the high-point of the wing airfoil and/or spars?  VD~ Steve

Steve,

You might well ask such suggesters in return whether they locate the CG just forward of the airfoil's High point at the root of the wing or the tip--or anywhere in between--of their super stunter's tapered wing?    Re locating the CG at the spar, ask them if that is where they'd balance their Ringmaster.  >:D >:D

Ted


Offline t michael jennings

  • AMA 83322
  • 2016 supporter
  • Trade Count: (0)
  • Commander
  • *
  • Posts: 221
Re: Today's Quiz
« Reply #18 on: November 04, 2015, 04:09:29 PM »
Gentlemen,

Spent a couple of hours reading and studying the recommended reading that you posted. 
Also, reread my Bill Netzeband info that he gave me several years ago.  His article "Control-Line Aerodynamics Painless" provided help.

Came up the following dimensions.

   Mean Aerodynamic Cord:    8.92 inches,
   @ 20% MAC   CG is 2.2 inches behind the Leading Edge,
   @ 30% MAC   CG is 2.9 inches behind the Leading Edge,
   Line Rake behind the CG is 0.628 inch.

Do they seem reasonable?

Also, attached is my Excel DATA SHEET (P-39 Airacobra) that I stole from Mr. Fancher several years ago.  It has some or the remaining data you suggested.

Thanks Igor and Ted for the Helping Hand.


T Michael Jennings    HB~>
Knoxville, TN.




 



Offline t michael jennings

  • AMA 83322
  • 2016 supporter
  • Trade Count: (0)
  • Commander
  • *
  • Posts: 221
Re: Today's Quiz
« Reply #19 on: November 05, 2015, 08:12:31 AM »
Serge,
Others that responded,

Left you our for thanks.

Thanks again.

T Michael Jennings
Knoxville, TN.

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12804
Re: Today's Quiz
« Reply #20 on: November 05, 2015, 10:53:02 AM »
Darn, Serge!  You got me, too.  Can I hire you as an editor?  I'd ask Howard but he'd be way too eager to do so and might even be willing to pay me to allow him to do it.  Hmmmm.  Let me think about that... 

Your comments on the effects of taper ratio on MAC are on the money as well, of course.  That's why I generally use the qualifier "remotely conventional planform" or similar; in which case the half span "substitute" is within the "noise range" for a "rule of thumb."  How's that for equivocation overkill?   #^ #^

Ted

I have found that when I try to do this toy airplane stuff by calculation I get it all wrong.  So I try to dampen my enthusiasm for doing it in an eggheaded way, and I just eyeball the thing.  If I'm doing some really unconventional thing for fun (saucer, canard, whatever) then I'll make some chuck gliders with balsa or foam, and adjust them for a fast, flat glide -- that usually puts the CG at the right spot for RC, and I know about how much to move it forward for CL, so I'm good.  Also, if I'm going to do something really oddball I plan on building more than one plane, and I build the first one rough and fast, so that I won't feel bad about hacking it up to make it work.  Once it's working, the second plane will be a cleaned-up version of whatever ends up working in the first plane.
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Serge_Krauss

  • Moderator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 1331
Re: Today's Quiz
« Reply #21 on: November 05, 2015, 12:29:09 PM »
Tim-

Those sound like pretty good approaches. I have found, however, that my calculations, using the "rules of thumb" and TVC have come out pretty well for trimming. My present long-standing but almost finished project is the first where I've used a slightly modified original wing, but intentionally deviated from the designer's intentions, leaving  some room to modify for comparison. So I'll learn a bit how far afield I can go, when I fly it.

I designed a radical model 5+ years ago intending to proportion, size, and cg it entirely by theory. I wanted to see if I could make it "fly right off the board." I spent several pages on weight of materials and weight distribution of surface finish and structural parts, re-aquainting myself with freshman calculus. I finally ran out of steam. I'm just finally going to build the thing with a guesstimate for size. What I did find out though was that my a.c. computation, using everything I knew about MAC's, TVC's, and approximate wing efficiencies, came out very close to my experimental results. To find the aero center of the whole plane, I glided a 24"-span, non-airfoiled, foam-board model with no elevator deflection in windless conditions. The resulting c.g. range is not really where the casual modeler would have guessed. So I think that the simplified theory worked out pretty well on an extremely unorthodox plane.

Incidentally, the latter plane got quite a rise out of Bill Netzeband during his last year. He was very kind and enthusiastic in talking about it with me.

SK


Offline Serge_Krauss

  • Moderator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 1331
Re: Today's Quiz
« Reply #22 on: November 05, 2015, 12:42:35 PM »
Ted-

'love your posts. I think the "gargling" is often needed these days (and ages!). Mine needs an olive though.

SK

Offline Serge_Krauss

  • Moderator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 1331
Re: Today's Quiz
« Reply #23 on: November 05, 2015, 09:28:00 PM »
OK, these links will give you what you want - within reason:

http://adamone.rchomepage.com/cg_calc.htm

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

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

http://www.southernsoaringclub.org.za/a-AC-calculator.html

The first one purports to find the CG and a.c. taking the whole plane into consideration. It looks like the old RC Aircraft Proving Grounds calculator (missing now from the internet), which used a curious adjustment for tail moment and introduced static margin to the computations done by the other sites listed below it.  If you figure out some static margins for known CL stunters or compute your own, you'll find that CL planes use c.g.'s that are noticeably forward of those used by RC and full-sized aircraft and therefore ahead of their recommendations.

If your plane is of a usual configuration, iI suggest that you just use one of the two Palos calculators in conjunction with the "rules of thumb" for flapless (in my post) or flapped (Ted's - in both posts). All you have to do is type in your wing measurements, and out will come a usable c.g. location in terms of distance behind the root leading edge. It takes a minute.

SK



Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Today's Quiz
« Reply #24 on: November 06, 2015, 01:50:33 AM »
 If you figure out some static margins for known CL stunters or compute your own, you'll find that CL planes use c.g.'s that are noticeably forward of those used by RC and full-sized aircraft and therefore ahead of their recommendations.

because we must use little bit more rules then only calculating static margin (remember, we have flaps), and also Ted's rule "CG % = tail volume %" works in "usual range" ... I have for example 33% tail volume, and believe me I cannot put CG at 33% of MAC :- ))) I have it even more forward then I used to have on airplanes with < 25% tail ... just because I "can" with still sharp corners :- )))))))) ... opposite extreme - small tail will probably lead again to CG close to 25% of MAC because tail uthority will not handle nose heavy model

Offline Serge_Krauss

  • Moderator
  • Trade Count: (0)
  • Admiral
  • *****
  • Posts: 1331
Re: Today's Quiz
« Reply #25 on: November 06, 2015, 03:02:00 PM »
Igor, is this far-forward c.g. in some way related to your exponential bellcrank/flap combination? That maight maske your plane less sensitive around neutral, ....?

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Today's Quiz
« Reply #26 on: November 07, 2015, 07:18:45 AM »
Well hard to answer by few words :- )))

1/ I think that CG of usual flapped airplane (not canard, tandem or any other tricky exceptions) should not be back of 25% MAC. The reason for that is that tail will become “lifting” in level flight. If it is lifting then any gentle maneuver like we do to maintain level flight will mean deflection of flaps which will cause the tail will change from lifting to pushing regime due to its moment, what will cause change of transfer points, separation points (if any) and that can cause nonlinearity leading to hunting and hard co control behavior. It means that even if you exceed tail area 25% (like I have 33%) you will still can come to troubles if you move CG back of 25% MAC.

2/ The point of Ted’s rule “CG % = Tail %” is based on tail auritority to keep stable flight. It means that stabilizing effect of CG front of AC of wing (at 25% of MAC) is replaced by tail stabilizing effect. So that is what allows to move CG back with larger tail. The advantage is that at the same stability, model can do easy corners if I move CG back. BUT if I go over the 25% of tail volume, and I do not move CG, the tail has still enough authority to make corner – even more then 25% large tail. So if I have 33% tail, I can even move CG back front :- ))) … and to get even better stability at the same sensitivity. I would say that 22 or maximally 24% is aft most position for CG. I do not say that CG at 33% will not fly, but it is far from whot I call as good stunter.

3/ My nonlinear system will make stronger flaps in level, so it really asks to keep CG front of AC even more then with classic linkage (explained in point 1/) … it also limits maximal deflection of flaps in corners because it slows down its flap to elevator ratio, what makes deflecting controls to corner easier – so it goes to corners very easy also if you move CG little front – result is stable model, which goes to corners very easy also with CG even more front then those 22%. I found that CG on model with logarithmic linkage should be approximately 10mm forward of CG of the same model with classic linkage. I did two identical models differing only with linkage just to see the difference.

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Today's Quiz
« Reply #27 on: November 07, 2015, 05:02:24 PM »
Gentlemen,

Spent a couple of hours reading and studying the recommended reading that you posted. 
Also, reread my Bill Netzeband info that he gave me several years ago.  His article "Control-Line Aerodynamics Painless" provided help.

Came up the following dimensions.

   Mean Aerodynamic Cord:    8.92 inches,
   @ 20% MAC   CG is 2.2 inches behind the Leading Edge,
   @ 30% MAC   CG is 2.9 inches behind the Leading Edge,
   Line Rake behind the CG is 0.628 inch.

Do they seem reasonable?

Also, attached is my Excel DATA SHEET (P-39 Airacobra) that I stole from Mr. Fancher several years ago.  It has some or the remaining data you suggested.

Thanks Igor and Ted for the Helping Hand.


T Michael Jennings    HB~>
Knoxville, TN.

TMJ,

Unless you are measuring those percentages of MAC somewhere other than "at" the MAC your math is suspect.  20% of (roughly) 9" is only 1.8", not 2.2"  and 30% 2.7" vice your 2.9".  Did you pass the CG at the MAC "through" the fuse and make those measurements aft of the leading edge "at" the root of the airfoil rather than at the MAC?  That's fine if that is how you arrived at them but the general convention is to make the measurement at the MAC for consistency.

Ted




 




Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Today's Quiz
« Reply #28 on: November 07, 2015, 05:04:09 PM »
Ted-

'love your posts. I think the "gargling" is often needed these days (and ages!). Mine needs an olive though.

SK


Serge, I may have been known to marinate olives in clear cold liquid on occasion my own self!

Ted

Offline Ted Fancher

  • 23 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2323
Re: Today's Quiz
« Reply #29 on: November 07, 2015, 05:29:45 PM »
Well hard to answer by few words :- )))

1/ I think that CG of usual flapped airplane (not canard, tandem or any other tricky exceptions) should not be back of 25% MAC. The reason for that is that tail will become “lifting” in level flight. If it is lifting then any gentle maneuver like we do to maintain level flight will mean deflection of flaps which will cause the tail will change from lifting to pushing regime due to its moment, what will cause change of transfer points, separation points (if any) and that can cause nonlinearity leading to hunting and hard co control behavior. It means that even if you exceed tail area 25% (like I have 33%) you will still can come to troubles if you move CG back of 25% MAC.

2/ The point of Ted’s rule “CG % = Tail %” is based on tail auritority to keep stable flight. It means that stabilizing effect of CG front of AC of wing (at 25% of MAC) is replaced by tail stabilizing effect. So that is what allows to move CG back with larger tail. The advantage is that at the same stability, model can do easy corners if I move CG back. BUT if I go over the 25% of tail volume, and I do not move CG, the tail has still enough authority to make corner – even more then 25% large tail. So if I have 33% tail, I can even move CG back front :- ))) … and to get even better stability at the same sensitivity. I would say that 22 or maximally 24% is aft most position for CG. I do not say that CG at 33% will not fly, but it is far from whot I call as good stunter.

3/ My nonlinear system will make stronger flaps in level, so it really asks to keep CG front of AC even more then with classic linkage (explained in point 1/) … it also limits maximal deflection of flaps in corners because it slows down its flap to elevator ratio, what makes deflecting controls to corner easier – so it goes to corners very easy also if you move CG little front – result is stable model, which goes to corners very easy also with CG even more front then those 22%. I found that CG on model with logarithmic linkage should be approximately 10mm forward of CG of the same model with classic linkage. I did two identical models differing only with linkage just to see the difference.


Hi Igor,

In my way too wordy articles from decades past when I developed the tail area percentage rule of thumb I also advocated a CG no further aft than 25% for much the same reasons but expressed slightly differently.  The lift of a  more or less symmetrical surface is centered more or less longitudinally right about 25% MAC.  To the extent that the lift remains reasonably co-located with the CG location during pitching maneuvers any moment between lift and mass is minimized and no longer tries to open up the maneuver as G forces increase(as happens with the CG more forward)...especially as airspeed increases/decreases during maneuvers especially in the wind.  As the CG is moved  aft of the location the lift is centered that lift will tend to accelerate the model "into" the maneuver  making tracking and concentricity more difficult.  Ergo; don't do that.

Way too wordy yet again and probably not clear.  Let's just say that we agree that there is no pitch maneuver benefit to a CG aft of 25% MAC even if the aircraft could remain stable due to a large tail volume co-efficient  (See Canards to utterly confuse the relationship)

Offline phil c

  • 21 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2480
Re: Today's Quiz
« Reply #30 on: November 07, 2015, 08:42:03 PM »
As above - you can tell very little about the characteristics of the airplane by looking at *just* the wing by itself. To make even a good guess at the CG, you need to show us the tail volume coefficient, or the dimensions of the tail and fuselage.

    Brett

Add that you can't position the leadouts without knowing the weight of the plane!
phil Cartier

Offline Howard Rush

  • 22 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 7805
Re: Today's Quiz
« Reply #31 on: November 07, 2015, 10:54:55 PM »
Let's just say that we agree that there is no pitch maneuver benefit to a CG aft of 25% MAC even if the aircraft could remain stable due to a large tail volume co-efficient

I think you agree based on valid experimental observations, but the explanations sorta diverge.

To the extent that the lift remains reasonably co-located with the CG location during pitching maneuvers any moment between lift and mass is minimized and no longer tries to open up the maneuver as G forces increase(as happens with the CG more forward)...

Then you go on to say that the instability exists even if the airplane is stable because it has a stabilizer.  It doesn't.  Both the wing and tail have positive rates of change of lift with angle of attack (assuming dε/dα from the wing < 1 at the stabilizer).  The propensity for the loop to tighten up is overcome by the stabilizer opening it up.  

In his item 1/ above, Igor describes what happens when the CG is just behind the quarter mean aerodynamic chord of the wing, to wit that stab lift passes through zero, where there are likely to be anomalies in the stab lift curve slope due to flow separation and the transition point wandering around, as he and Frank Williams have described here or on SSW.  I did the ciphering for an example: For a 64-oz. Impact on a sea level standard day, flying at 88 ft./second with the CG at 30% m.a.c., and assuming the wing lift acts at 25% m.a.c. (which it might not-- it's a function of flap position), the stab operates at a lift coefficient of .01.  That's an angle of attack of about .16 degree.  A vertical gust of 3 inches per second could put the stab at zero angle of attack.  So tiny control movements or light turbulence--not to mention "hunting" itself--could cause the stab angle of attack to continually pass through zero.   The same thing could happen with the CG a little forward of the quarter m.a.c.  Another problem might come from flap or elevator hinge moment passing through zero.  It there's any slop in the controls, you could find it difficult to fly level despite having a statically stable airplane.  

I operated Impacts with CGs close to or behind the wing quarter m.a.c. and got away with it.  I think that may be because the Impact's elevator downrig causes any stabilizer squirreliness to happen at some innocuous loop radius, rather than near zero lift coefficient.  I think the transition pretty much stays put at the back of the roundy stab LE, particularly if you help it with a trip strip.  

In his item 2/ above, Igor describes what I learned trying to find a CG for a T-Square-type combat plane and learned again sizing tails for transport airplanes: the bigger the tail and farther back it is, the wider the range of CG location you can have.  The Impact that I operated at 30% had stiff flaps and a tail that was too small.  I kinda got painted in a corner and had to put the CG there.  
The Jive Combat Team
Making combat and stunt great again

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2165
Re: Today's Quiz
« Reply #32 on: November 08, 2015, 01:29:55 AM »
Then you go on to say that the instability exists even if the airplane is stable because it has a stabilizer.  It doesn't.  Both the wing and tail have positive rates of change of lift with angle of attack (assuming dε/dα from the wing < 1 at the stabilizer).  The propensity for the loop to tighten up is overcome by the stabilizer opening it up.    

Yes, that "stability" and that "force opening loops" is the same thing and it is related to distnace between CG and neutral point (includes wing and tail), not CG and AC ow wing in 25% MAC (wing only). So larger tail will cause opening loops also if CG is at 25% MAC. Fortunately larger tail has also larger elevator, so we do not see it as it is more effective. But in any case, the same airplane will go easier to loops if we move CG from 20% to 25% and also easier if we move from 25% to 30% but it will be still stable and still will go out of loop if we release handle :- ))


Advertise Here
Tags:
 


Advertise Here