will it take off?

[patrick_g]

daTeacha, No! Let me repeat for emphasis N O contest if you want to take the "will fly" assumptions (and be bored stiff.) As has been repeated MANY times the plane will take off almost normally with the biggest noticible difference being the wheels will be rotating at twice the plane's lift off speed. (This assumes as has also been said, the wheels don't come apart at 2x takeoff speed)

If you take the "extended" assumptions of the "Won't Fly" argument and nothing comes apart and there is sufficient traction on the wheel-conveyor interface and such then the plane's CG doesn't move more than an arbitrarily small distance forward and back (averaging zero) as the MCB may not be perfect. For our purposes, since any motion is of short duration, extent, and velocity that it is of no more importance that the molecular motion of the atoms comprising the plane's structure which are above absolute zero in temp. IF you take the will fly assumptioins the plane will fly. I don't want to work on the design team responsible for the MCB unless I get paid up front not on completion and successful testing.

Pat

 

[Egon]

Use rollers!

 

[NorthwestBlue]

Pat,

I too enjoy the mental stimulation, although I'm obviously outmatched. My degree in Aerospace Engineering has gone unused as I took the unenviable course of a career of directing high speed aluminum tubes. A once glorious occupation regarded highly by many and rewarded with wonderful perks and compensation. Alas, it has now been relegated to the bowels of the mass transportation system. So you might call me a frustrated something or other, or not.

I liked your question about the barometer and immediately thought of most of the methods you described with the exception of the trade and the staircase (I seem to always overlook the obvious). Another way would involve turning it into a fluid dynamics problem. Place two interconnected buckets filled with water next to the building. On one place a float just the right size to fit inside (kind of like a plunger). Drop the barometer on the float. The displacement of water will allow you to determine the kinetic energy at impact, etc. Or an optical method… place a mark on the ground and move the barometer until the mark is barely visible behind the barometer when looking down from the top of the building. Now use that angle and distance to calculate the height. Or another audio method… drop the barometer on the ground and measure the time it takes for the sound to reach the top of the building. Of course, you could also melt the barometer down and make a nice long wire out of the metal.

I don’t share your feelings on “real world” being un-fun or boring. Anytime you through people into the mix it often times makes fantasy or fiction seem dull in comparison.

Da Teacha,

Your idea about using the giant rubber band is intriguing. Start out with a large force that decreases with distance. That wouldn’t be unlike a rubber band powered balsa airplane. I wonder how many turns it would take to wind up enough to generate 70 kN of thrust. Of course if we discount the fact that the rubber band would eventually break, then we could get an unlimited force to deal with that pesky MCB.

 

[SPYDERLK]

No. If the tires have enuf traction to overcome engine thrust the magical conveyor could hold the plane stationary by applying a force, equal&opposite to engine thrust, to the tires. This thrust would rotationally accelerate the mass of the wheels. As long as the wheels, tires and bearings could withstand the increasing rpm the plane would not move.

NorthwestBlue, thank you for the good physics. I will review it in some depth later to refresh my education.

Above is my first post on this thread. Id like to point out that your numbers have shown the truth of the post.

-- Traction coefficient 0.8 x weight ~7.8KN = ~62.7KN
Engine thrust reduced by this number [70KN - 62.7 = 7.3KN] is quite close to the 7.2KN force you calculated, based on traction limited max possible rotational acceleration of wheel mass, to be left over to push the plane forward. Since the numbers dont match exactly there are probably some small errors of approximation that crept in.
Larry

 

[Egon]

small errors of approximation that crept in.
Larry

That's sorta like a fudge factor to adjust to whatever you need?

 

[NorthwestBlue]

Larry,

Good catch. Thanks for keeping me honest. At least someone was looking at the numbers. Correct values were 70000 N - 62784 N = 7216 N.

Egon,

If you don't like the numbers I first give you, I got a lot more where those came from. Just tell me to stop when you like them.

 

[daTeacha]

Northwestblue -- the rubber band I had in mind would be more akin to a slingshot. When I was a kid you could spend a quarter and buy a plane that had a prop spun by a rubber band, or a bit less and get one that you launched with a rubber band attached to a stick. Of course of all you had was a dime, you had to settle for the one you through, supposedly a model of the Sabre Jet, an F-86 if memory serves.

 

[NorthwestBlue]

daTeacha,

I guess I wasn't clear, but I did understand that you meant a slingshot. I was just making the comparison. Force is force whether you get it from pulling, pushing, thrust, gravity, etc. How many rubber bands would it take to make the non-believers believe?

Its funny you mention the planes you bought when you were a kid. I loved those little planes as a kid. They played a big part in fostering my fascination with flight. I wanted to buy some recently to play with my boys, however they are not easy to find and cost a lot more than a quarter.

 

[patrick_g]

I had several of the throw it yourself balsa gliders profiled and printed to be a Saber jet F-86 and later super saber F-100. May have had some F-80 as well (single seat fighter version of the T-33 two place trainer Straight vs the saber's swep[t back wings.)

Oh yeah... I am builty of working out from under the umbrella of a precise degree field. Although physics required more math then a math minor (a couple three classes short of a major) I never worked as a mathemetician. I never held a job title of physicist either but used physics a lot. I once had a job title with a small startup of Wizard, receiving mail so addressed. Actually I worked more realistically as an EE with my physics background and then after going back to school to be retread as a computer scientist and softwarre engineer I actually had job titles to match for a while. My last trip back to grad school was for Instructional Technology where you engineer training solutions. I needed that paper to be a manager of a team of courseware developers for Naval warfighting systems. The lab where my wife and I worked, NCCOSC, RDT&E (Naval Command Control and Ocean Surveilance Center, Research Development Test and Evaluation) abbreviated NRAD, we were the NRadicals. This place had very broad bins for labeling folks. My job title was Scientist (not rocket scientist.) I had friends who were labeled Engineer. Pretty non descriptive. What we actually did was whatever needed to be done. Besides, if I told you too much about what I did I'd have to shoot you.

Pat

 

[daTeacha]

Maybe, but since your plane won't take off, you could never get here from there.


I just had a thought regarding the rapidly spinning wheels -- air friction, friction in the bearings, etc. would bring about some serious amount of thermal expansion as the rpm of the wheels approached near infinity. The result would be that the wheels expand to a size large enough to contact the wings and tear them off, hence it won't fly.