Scratch Building, Aircraft Design, 3D/CAD If you are starting/building a project from scratch or want to discuss design, CAD or even share 3D design images this is the place. Q&A's.

Datum Line

Reply
Old 05-13-2003, 03:49 PM
  #1
triangle
Thread Starter
 
Join Date: Sep 2002
Location: Lake Worth, FL
Posts: 204
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

CAN SOMEONE TELL ME HOW TO CALCULATE THIS????? THE IMAGINARY LINE ON THE FUSELAGE WHERE FOWARD OF IT ALL WEIGHT ADDED IS POSITIVE AND BEHIND IT NEGATIVE..... AND ITS RELATION TO THE CG???????
triangle is offline  
Reply With Quote
Old 05-13-2003, 04:03 PM
  #2
Highflight
 
Join Date: Feb 2003
Location: Jackson, MS
Posts: 831
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

The Datum Line is a horizontal line that extends from the nose to the tail and has nothing to do with CG.

Are you thinking of something else?

Highflight
Highflight is offline  
Reply With Quote
Old 05-13-2003, 04:17 PM
  #3
Tall Paul
 
Join Date: Jun 2002
Location: Palmdale, CA
Posts: 5,211
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

There's no "datum line" in the vertical plane..
The c.g. is positioned relative to the airplane configuration using the wing and tail areas and moments.
For a conventional tailed airplane the c.g. is placed between 25% and 35% of the mean aerodynamic chord.
Tall Paul is offline  
Reply With Quote
Old 05-13-2003, 04:24 PM
  #4
Mike James
 
Mike James's Avatar
 
Join Date: Jan 2002
Location: Anchorage, AK
Posts: 2,562
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Measurements

Triangle,

The reference line you're referring to is arbitrary. Just put it where you like, and make a note for any who need to read your plans, explaining where your "zero reference line" is.

Most companies tend to put this "zero reference" near or at the nose, and measure from there rearward, but it's entirely up to you. (For a pusher aircraft, for example, you might find it convenient for the rear-mounted firewall to be "F1", rather than "F-10".)
Mike James is offline  
Reply With Quote
Old 05-13-2003, 09:05 PM
  #5
triangle
Thread Starter
 
Join Date: Sep 2002
Location: Lake Worth, FL
Posts: 204
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

THANKS MIKE, I KNEW MUST HAVE HEARD OF IT...... NOT TOO MANY PEOPLE KNOW AERODINAMICS....... AND PLANE DESIGN
triangle is offline  
Reply With Quote
Old 05-14-2003, 09:42 PM
  #6
starcad
 
starcad's Avatar
 
Join Date: Dec 2001
Location: Tucson, AZ
Posts: 257
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Re: Datum Line

Quote:
Originally posted by triangle
CAN SOMEONE TELL ME HOW TO CALCULATE THIS????? THE IMAGINARY LINE ON THE FUSELAGE WHERE FORWARD OF IT ALL WEIGHT ADDED IS POSITIVE AND BEHIND IT NEGATIVE..... AND ITS RELATION TO THE CG???????
The Datum is just a reference line used in the calculation of the C/G limits for a weight and balance calculation. This really isn't necessary on model aircraft unless your planing on something really big. ( over 100 lbs.) Then you might want to consider doing a weight and balance. There is an excellent publication you can get from the FAA "Weight and Balance" that will explain all the formulas involved and how to setup the plane and take the weight measurements.

For models just use the rule of thumb 25% to 33% of the MAC and you will be fine. I tend to go on the nose heavy side with a new design at say 28% for the first several flight and then work forward and backward just a little at a time.
starcad is offline  
Reply With Quote
Old 05-15-2003, 11:12 AM
  #7
Mike Denest
 
Mike Denest's Avatar
 
Join Date: May 2002
Location: Newark, DE
Posts: 724
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Lines

As Starcad said, the datum is an arbitrary vertical line used for c/g calculations. The most common locations can be anywhere from the nose to the wing leading edge. The basic calculation is W/A=M or Weight times Arm equals Moment. You would need three decently calibrated scales, a plumb bob and tape measure. Level the aircraft and suspend the plumb bob at your datum point. Mark this with a line perpendicular to the longitudinal axis of the airplane. Measure from this line to the center line of each gear leg. These are your Arm measurements. It dosen't matter whether the airplane has a nose or tail wheel. Record the weight on each scale referencing the distance you measured. When you have recorded each moment, total them up. Divide the moment by the total weight and you will then have the location of the c/g. The FAA website (faa.gov) has the Advisory Circular available for free. Search for AC91-23A, Weight and Balance for Pilots. It give a complete description of the weight and balance process. Again, this is not necessary for small models but when you have something cracking the 55# limit, it's not a bad idea.
Mike Denest is offline  
Reply With Quote
Old 05-15-2003, 03:15 PM
  #8
ProfLooney
 
Join Date: Jan 2002
Location: Moline, IL
Posts: 3,255
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

Ok U guys cornfusing me on the Datum Line. From what I always was told is the datum line was basically a Horizontal reference line that followed the thrustline on the plane. And it always seems to follow in a line along the widest part of the fuse formers which is very helpful when Lofting your fuse formers and has nothing to do with CG or its location. The Datum line you guys are talking about isnt called a datum line its some other reference line that I would agree with what Mike explained it as though I personally didnt know what it was called but knew it wasnt a datum line.

Joe
ProfLooney is offline  
Reply With Quote
Old 05-15-2003, 03:35 PM
  #9
Mike Denest
 
Mike Denest's Avatar
 
Join Date: May 2002
Location: Newark, DE
Posts: 724
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Re: Datum Line

Quote:
Originally posted by ProfLooney
Ok U guys cornfusing me on the Datum Line. From what I always was told is the datum line was basically a Horizontal reference line that followed the thrustline on the plane. And it always seems to follow in a line along the widest part of the fuse formers which is very helpful when Lofting your fuse formers and has nothing to do with CG or its location. The Datum line you guys are talking about isnt called a datum line its some other reference line that I would agree with what Mike explained it as though I personally didnt know what it was called but knew it wasnt a datum line.

Joe
Fellas,
Here is the official FAA definition of the Datum Line.

"Datum (reference datum)--is an imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of permissible c.g. range must be taken with reference to that point."

To answer the original question at the beginning of the thread, the datum line is not a calculation but an arbitrary reference line in the vertical plane. It has nothing to do with aircraft design whatsoever. The datum line is only a starting point from which to measure the moment arms (distance) from the line.

So, in reference to the datum line, all weight forward is indicated as a negative while anything aft is referenced as positive. I have never seen the negative and positive weights reversed but it is possible.
Mike Denest is offline  
Reply With Quote
Old 05-15-2003, 07:38 PM
  #10
Flypaper 2
 
Join Date: Mar 2002
Location: Kingston, ON, CANADA
Posts: 4,921
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

Key words in FAA reference are ; from which all measurements OF ARM are taken. How about the FAAs reference to the horizontal datum line. I'm with Prof Loony's line of thought.
Flypaper 2 is offline  
Reply With Quote
Old 05-15-2003, 09:19 PM
  #11
starcad
 
starcad's Avatar
 
Join Date: Dec 2001
Location: Tucson, AZ
Posts: 257
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

Quote:
Originally posted by Flypaper
Key words in FAA reference are ; from which all measurements OF ARM are taken. How about the FAAs reference to the horizontal datum line. I'm with Prof Loony's line of thought.
Gord, I think what your referring to is the CENTERLINE. When designing, the first line I draw is a centerline and then all measurements for formers, height, width are referenced from there. In all my years of designing I've never heard it revered as a horizontal datam line, however I guess someone could call it that.

When I start the line is usually two to three inches longer than my planned subject. I start with my fuse length and divide it by 2 to get the center of the fuse length and plot a reference on the centerline. From there it is just a matter of picking the points and drawing the lines or lofting as it is referred to. If your doing scale then just loft from the centerline to the points reference.

It's like plotting an airfoil, so many inches from the ( God I hate to confuse you here ) front end of the chord line and so many inches above and below the chord line.

I use AutoCad but any CAD program will have the different line types or if your on a board then draw out a centerline and start from there.

Guy
starcad is offline  
Reply With Quote
Old 05-15-2003, 09:38 PM
  #12
Tall Paul
 
Join Date: Jun 2002
Location: Palmdale, CA
Posts: 5,211
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

In full-scale practice a point will be chosen that will be in front of the furthest forward point on the airplane, and at or below the bottom of the wheels, maximum landing gear strut extension.
Then all calculations regarding the positions of structure and equipment will always have a positive displacement when figuring the moment arms..
.
For example, this "Station Point Reference Guide" for the L-1011..
The front of the radome is at fuselage station 80, and waterline 200..
this is WAY too complicated a procedure for model airplanes.
Attached Thumbnails
Click image for larger version

Name:	78820_25376.jpg
Views:	1672
Size:	67.7 KB
ID:	46589  
Tall Paul is offline  
Reply With Quote
Old 05-15-2003, 09:40 PM
  #13
Turbulence
 
Turbulence's Avatar
 
Join Date: Jan 2002
Location: Sahuarita , AZ
Posts: 1,252
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default HEY that looks familiar

That looks a lot like my MD-11 documentation. Been there, built 400 of them.


Turbulence
MDC-K2J
Turbulence is offline  
Reply With Quote
Old 05-16-2003, 01:12 AM
  #14
Flypaper 2
 
Join Date: Mar 2002
Location: Kingston, ON, CANADA
Posts: 4,921
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

I have blueprint drawings of the Avro Arrow. Has a fuselage datum line with all the stations marked, makes for an easy way to accurately scale up. The engines are cranked up at the back about 5 degrees with its own line called engine datum line from which the upper and lower engine measurements are taken. Maybe us Canucks do things funny.
Flypaper 2 is offline  
Reply With Quote
Old 05-16-2003, 01:17 AM
  #15
ProfLooney
 
Join Date: Jan 2002
Location: Moline, IL
Posts: 3,255
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default datum

I agree all the plans i have the datum line is marked as the horizontal thrust line. I also have the entire Paul Matt series of drawings and the horizontal thrust line is also labeled the datum line so thats where i get wondering whats everyone talking abt.

Joe
ProfLooney is offline  
Reply With Quote
Old 05-16-2003, 01:41 AM
  #16
starcad
 
starcad's Avatar
 
Join Date: Dec 2001
Location: Tucson, AZ
Posts: 257
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

Basically what everyone is referring to is just a reference line from which all the standard measurements are taken. I guess different designers call it different things such as thrust line, datam line and ME centerline. We're all in agreement I think as it is just a reference line.
starcad is offline  
Reply With Quote
Old 05-16-2003, 11:27 AM
  #17
SDR-Hammer
 
Join Date: Sep 2002
Location: , MI
Posts: 683
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

I too have seen plans refer to the centerline as the "datum line" but to me the datum line is what weight a balance is calculated from.

Guess a Webster’s definition of datum would be something that could encompass both. But what it means to each individual depends on your background and if it's full-scale datum means weight and balance. Just as a stall is something that happens to a wing and not the engine.
SDR-Hammer is offline  
Reply With Quote
Old 05-16-2003, 04:09 PM
  #18
ProfLooney
 
Join Date: Jan 2002
Location: Moline, IL
Posts: 3,255
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

I been doing a little research since this thread started hitting the library etc and I found several drawings showing an "X" and a "Y" datum line where one is the horizontal and the others on the verticle. So whats this mean? It means that any line on a drawing can be a datum line. like someone said earlier the airline industry used a verticle line meybe they do but mose 3 views and RC plans decide to use a horizontal line as a datum line. now I found some drawings for the automotive industry and they have both a horizontal and verticle where, though i dont know for sure, but they have the verticle on what appears to be the CG of the car or truck. So what I gather from this is we are all correct and most of all there is no incorrect answer they are all datum lines it is just which line a particular designer is using for the drawing at the time and next drawing he may use a different datum line.

Joe
ProfLooney is offline  
Reply With Quote
Old 05-16-2003, 05:59 PM
  #19
Mike Denest
 
Mike Denest's Avatar
 
Join Date: May 2002
Location: Newark, DE
Posts: 724
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

As I learned from my early start in aviation, the datum line is a weight and balance reference. As I got involved in engineering these reference lines, sometimes called datum lines by some manufacturers can be best identified as Water Line, Butt Line, Station Line. As shown by the picture of the MD-11 in the earlier post, Water Line 0.00 can be the horizontal line (thrust line" from which all +- (above or below) measurements are made. The same is true for the Butt Line which will be shown as a minus for measurements to the left and + for measurements to the right. The Station Line is the reference dimension either aft or forward of the datum line established by the manufacturer. The Station Line can also be a - or + measurement depending on the established datum. Now, is everyone thoroughly confused?
To make matters worse, most all aircraft designed today are done in 3D rendering. A master model is created, establishing the XYZ relationship to the part. For example, I can go into my design software I use in my job, indicate a point in space and show the XYZ measurement distance from STA. 0.00, BL 0.00, WL 0.00.

Hey guys, this is an excellent thread. I think we have found that despite the method and terms used to measure an aircraft structure, they all accomplish the same thing. For our models, the process is much simpler but when designing a new model we still have to start the design layout and some type of reference point. How you like to call it is totally arbitrary but accomplishes the same goal: a model that performs according to the specs we have selected for the type of performance we desire in that model.
Mike Denest is offline  
Reply With Quote
Old 05-16-2003, 07:53 PM
  #20
triangle
Thread Starter
 
Join Date: Sep 2002
Location: Lake Worth, FL
Posts: 204
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

GREAT!!!!!! THANKS GUYS, VERY USEFULL, AFTER READING THIS, I UNDERSTAND WHY I SHOULD GET THIS WORD OUT OF MY DESIGN VOCABULARY UNLESS I'M DESIGNING AT LEAST AN ULTRALIGHT.......HAHAHAHA
THANKS AGAIN....
triangle is offline  
Reply With Quote
Old 05-16-2003, 08:57 PM
  #21
Tall Paul
 
Join Date: Jun 2002
Location: Palmdale, CA
Posts: 5,211
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

Doing it the fuselage station/waterline method can be useful, IF you know the precise weights of all the materials in the structure.
As modellers usually sand a lot of the wood away, the true locations of the centers of mass of the parts varies from the unworked stock.
Basically it's possible, after building a lot of similar airplanes to be reasonably accurate in guessing where the center of mass for say a fuselage will be. Or the wing. The small parts; motors, servos, batteries are easy to size, weigh and position, but it's the unknowns in the assembly process that make this much precision less than a fruitful pursuit. Wood with density that varies along the length, or denser/lighter than expected, amounts of and types of glue used... covering, paint..
.
Experience teaches how much structure to have and to sand and where and where to place the parts to get a plane that requires a minimal amount of ballast to get the c.g. in the proper location relative to the wing.
Tall Paul is offline  
Reply With Quote
Old 07-15-2003, 11:35 AM
  #22
Gwyll0650
 
Join Date: May 2003
Location: Alvin, Texas
Posts: 1
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

"Triangle"........as a liscensed A/P Mechanic, there is a "Datum Line" in the vertical plane. Although it is completely arbitrary, and set by the air craft manufacturer, it is most commonly depicted as the fire wall, and used primarily as a reference point for W&B(weights and balance) calculations. As I said, this imaginary line is completely arbitrary, and can be placed anywhere convienent to you. Just make sure it is well established(referenced), and always used in any mathimatical computations. I prefer to balance my air craft using this full scale method generally used for light/general aviation air craft (three weight scales>>one for each ground-contact-point / read landing gear). This method tells you exactly what your weight paramiters are, how much needs to be added/subtracted, and exactly where. When using this method you get immediate results in both the longtitudinal and lateral planes. Good luck with your project(s), and P/M me back about your results.
Gwyll0650 is offline  
Reply With Quote
Old 07-15-2003, 05:35 PM
  #23
triangle
Thread Starter
 
Join Date: Sep 2002
Location: Lake Worth, FL
Posts: 204
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Datum Line

THANKS..........
triangle is offline  
Reply With Quote
Reply


Thread Tools
Search this Thread
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -8. The time now is 03:30 PM.