A simple conversion, made terribly difficult.

[ziomatrixacs]

Hey guys.. I was working on an airplane and my chinese friend wanted to watch.. half way though we ended up in a mathergument :) I knew I was right because I have been doing this for years and it has worked, but I knew he had themz new maths that make 1=0

Here are some things he asked me and I didnt have a real answer for..

Converting square inches to square feet for use in wing loading calculation.

3x / 6x = 1/2
The x's cancel correct? I agree.

400square inches / 144 square inches = 2.77 square feet

(144 square inches comes from 1 foot = 12 inches, and 12 inches squared is 144 square inches)

My chinese friend gave me a dumbfounded look upon this statement. Sure 400 square inches = 2.77 square feet but he absolutely insisted that the square inches drop out and 2.77 is unitless. I want to agree with him but 2.77 square feet does equal 400 square inches. I think I might be missing a step in the conversion or something.. Could some one explain this so I can give a better answer than "Well it works because I can show it on paper."?

The second thing was probably a language thing..

Wingloading. WL= gross weight / wing area right? And the output (for typical imperial models) is Oz per square foot. He insisted that it was square feet per ounce. He said it is either WL=GR/WA = sqft per ounce or WL=WA/GR = oz PSF..

Im not sure what they taught him over there, but its hard to tell someone from china they are wrong when it comes to math.. :eek: I need solid evidence like an article on wikipedia to back my statements! :o

[Cory]

Don't need no stinkin' Wikipedia article to back you up on the first question, you just need the proof from this non college degreed math dummy from little ol' Idaho whose not afraid to tell someone from China they are wrong when it comes to math. ;)

You're using an incomplete equation. 144 sq. in. can also be written as 144 sq. in. / 1 sq. ft., so when you invert to divide you get 400 sq. in. * (1 sq. ft. /144 sq. in.) = 2.77 sq. ft. The sq. in. do cancel, but the unit "sq. ft." is left on the top.

You are right for the second one, it is usually expressed in weight per sqare area. You should be able to find that with a Google search.

[Brandano]

or just switch over to metric and avoid the whole mess....

[ziomatrixacs]

I thought I was missing a step. I remember doing the conversions from mili to pico and that stuff and the teacher said we couldnt skip those steps. Now I get why! We had the same class, so I am sure my friend will understand.

I knew I was right.. I think it was a language issue there with the wing loading. I have several books to prove it..

[Cory]

or just switch over to metric and avoid the whole mess....

Measurement system doesn't make any difference. If your starting out with wing dimensions in anything other than what you want your results in, you still need that step. Say for example your wing dimensions are in cm and you want your wing loading in grams/meter squared, you still need the conversion. It is easier to do the conversion in your head with the metric system, but you still do it.

[eflightray]

zio

Point you friend to some of the model adverts that quote area and loading or the Ivan' Plans website (http://www.geocities.com/ivansplans/) where it's quoted many times.

Could be worse, it could be someone in Europe using area in sq.mm and loading in dm2, (decimeters squared). Wing loading is in grams/dm2.

Me?, I still think in sacks of coal/square fortnight :D

[ziomatrixacs]

zio

Point you friend to some of the model adverts that quote area and loading or the Ivan' Plans website (http://www.geocities.com/ivansplans/) where it's quoted many times.

Could be worse, it could be someone in Europe using area in sq.mm and loading in dm2, (decimeters squared). Wing loading is in grams/dm2.

Me?, I still think in sacks of coal/square fortnight :D


I never got the decimeter thing. Im like 90% percent sure its a meter divided by 10 like a milimeter is a meter divided by 1000. I think of it as the centimeters red headed brother. Why dont they use just plain meters?

[Montag DP]

I never got the decimeter thing. Im like 90% percent sure its a meter divided by 10 like a milimeter is a meter divided by 1000. I think of it as the centimeters red headed brother. Why dont they use just plain meters?If you had a wing loading of 30 grams/dm^2 (about 10 oz/ft^2) it would be 3000 grams/m^2. They use decimeters just because it gives nicer numbers like 30 instead of 3000.

[Brandano]

Because in some situation it would be unpractical, like using meters to measure long distances. It's just a multiplication or division by 10, so it's not that hard to reconduce everything to a measure in meters. Btw, on full size planes, the wing loading is measured in kilograms per square meter (I think).

[vintage1]

yeah..we use millimters, cos they are nice, and centimeters, occasionally..but decimeters are rare things!

Better to do kg / square meter really!

Is a decameter 10 meters? Or just rhyming slang? :)

[biber]

Well, people use daN (Dekanewton) sometimes, because it is almost identical with the old kp (Kilopond).
Just like you often see hPa (Hektopascal) being the same as the common mbar (Millibar).
In technical issues stresses are given in N/mm², that are often called MPa (Megapascal), perfectly valid.
All those conversions are shifting commas at most, the digits stay the same, easy to do.

biber

[HugePanic]

I don't like french stuff, but the SI (http://en.wikipedia.org/wiki/SI) rocks.

Murphy is always right... Mars Climate Orbiter (http://solarsystem.nasa.gov/missions/profile.cfm?MCode=MCO)

[crashdaily]

yeah..we use millimters, cos they are nice, and centimeters, occasionally..but decimeters are rare things!

Better to do kg / square meter really!

Is a decameter 10 meters? Or just rhyming slang? :)

That's right, deca means 10. :)

[BMatthews]

Going back to the old units issue. You're idea is right but you missed a step. The right equation is not 400 sq inches/ 144 sq inches. Done that way your friend is right and you end up with a dimensionless number. Also your physics teacher would've rapped you a good'un upside the head for writing it that way.... :D

The correct and full equation is....

1 sq foot x 400 sq inches = 2.77 sq feet
144 sq in's

Done this way with a proper sq ft to sq inch relationship the units all work out fine. The inches cancel out and the sq foot is left to provide the correct final units. What you did in the first place is fine for short hand but it's actually wrong if you want to properly express the relationsip of the conversion from sq inches to sq feet.

[Cory]

Isn't that what I said in post #2?

[BMatthews]

Why yes, you did. I missed it in flying down through all the other posts looking for someone that realized the need for tracking the units other than me. I guess my incredably fallible powers of observation didn't cut in until around post #4 or so.... My bad. :D

[Cory]

;) :)