What would happen if you took a 747 and shrank it to handheld size?

Factual Questions
1

This should be a fitting topic for the 100 year anniversary of powered, heavier-than-air flight.I am fascinated with all things aviation related. I am working on my pilot’s license and love to play with model airplanes. However, I am still amazed when I see a large jet airliner fly overhead. I know how it works but it still seems like a miracle that it flys at all. I would like to get a better intuitive sense of how that aircraft would feel and behave if I could hold it in my hand.

Here’s the excerise: Take a 747 or any other large airliner and shrink it so that it can be held by me (say 2 feet long or so). How much would it weigh? Could it fly on its miniature jet engines? Would it still need to go 150 mph or so to fly the same way its big brother does?

2

A total shrinkage–same everything? Not just building something that looks similar but has a thicker wing and is made of balsa and tissue?

3

That’s it. Total shrinkage.

4

The problem is, if you shrunk the 747 that much the metal skin of the plane would be so thin you could probably see through it. A breath would tear it apart.

5

You can’t scale physics. The ratio of lift to weight will change (I don’t know if it’s for the better or the worse in this case). Not clear to me that miniature jet engines are feasible. That’s why an elephant is shaped the way it is and an ant like it is. An ant the size of the creatures in Them would collapse under its own weight. Similar problems in reverse for the Incredible Shrinking Man. It’s very difficult to make a scale model of a boat for a movie and have the surrounding water look realistic.

Let’s see, the weight will reduce as the cube of the scale factor and the area of the wing will reduce as the square. But I don’t know what happens to aerodynamics as you scale a wing surface down but something sure as heck changes. You aren’t scaling down the air itself.

6

The real question is where are you gonna find tiny, tiny little women to be the stewerdesses!

7

I should think scaling down would enable flight to still be possible. Unless (a) it’s the same mass, only much denser, and not merely the same materials or (b) the electronics, etc., shrink to the point where the size of electons, etc., become an issue (reminds me of the whole Fantastic Voyage issue of shrinking oxygen molecules for your trip in the submarine, since refualr oxygen molecules will be too large for your lungs…).

It’s scaling up that’s the problem. The wings have to grow much larger in propotion to the plane’s weight.

8

The leg room would be about the same though.

9

Ignoring viscous effects (the Reynolds number will be a lot lower on a wing gemotrically scaled like that, changing the laminar/turbulent characteristics of the wing), you’d find that your wings would be grossly oversized.

Lift is a function of density, velocity, lift coefficient, and wing surface area. All the other parameters could remain constant. But the surface area is reduced by only the square of the scale factor, while the volume (and hence the mass) will go down by the cube of the scale factor. For a plane that is geometrically scaled down by 400 times—making a Boeing 747-400 about 7 inches long and 0.013 lbs—your wing area would be 400 times larger than it needs to be, so your lift would be 400 times too large. It would look right, because it properly geometrically scaled. But from a dynamic scaling standpoint, it’s absurdly wrong.

Skin friction drag would be relatively increased by these large wings, and all that extra lift makes induced drag (drag due to lift) way too high, as well. In short, that plane isn’t flying at all.

In reality, however, a buildable scale model will weigh many times more than this perfectly geometrically scaled aircraft. That’s the only way such a small aircraft could hold itself together. The aluminum thickness won’t go down by 400 times, but more like a factor of 2 or 3 at most. In the end, the plane will actually be so overweight for its scale that the wings will be undersized, incapable of lifting the dense hunk of metal (say, 1 lb) off the ground at realistic speeds.

Most flying scale replicas of large planes have oversized wings. The jet-powered, composite F-22 replica I built in college, for example, had wings that were about 20% larger than simple scaling would dictate. Also keep in mind that model was too large to fit in the palm of your hand, and was a replica of a plane that is considerably smaller than a 747. While our hypothetical 747 model is 400 times smaller than the real thing, my F-22 was only about 1/24 scale.

So in practice, to make your palm-sized 747 fly—assuming it’s made of scaled down metal—the wings would have to dwarf the fuselage, and then you’ve got terrible stability and control issues. Make it out of balsa or foam, and it gets (somewhat) more practical.

10

It wouldn´t work; to begin with, the fuel wouldn´t flow through the fuel lines, it´s too thick for that; second, as was mentioned, the metal skin (to put an example) would be too thin to withstand any external force.
A 747, shrunk to a 2 feet wingspan (a bit less than 100th scale) would weight about a pound or so; at that size IMO it would have to travel much faster than it´s scale cruise speed of 5.something knots.

11

An intuitive sense of how an airplane would behave gained by holding a miniature one in your hand?

Have you already done this with your HotWheelsTM cars to get an intuitive idea of how a Masserati would handle?
…Vrooom… …Vrrrrooom!.. Vrrroooommm!!!

Have you talked about this with your flying instructor?

12

It seems appropriate to amend this :

13

In response to the amendment by panamajack:

I chose my words carefully already. I have yet to see a bird or a bat with an auxillary engine strapped to its back (although that would be pretty cool. Runs off to experiment…)

14

Pigeons and D sized Estes engines might be fun!

15

Actually, I was talking about model aircraft. (Not toy planes … a toy plane is something you wind up and it rolls along on the ground …) I suppose ‘human’ is an ambiguous term – the way I used it I meant the first time a human was in the plane. A better added term would have been controlled.

I wouldn’t have pointed it out as it’s obvious what event you were referring to, but since the thread’s about tiny planes, that’s why it seemed appropriate.

Further information on early powered flight can be found here.

16

Like TheLoadedDog says - I think the airline industry would finally be forced to remove a row of seats. :stuck_out_tongue:

17

Yep, they are. There’s a local fellow in my neighborhood who flies RC models powered by jet.

I should add that his jet engine occupies most of the fuselage, and he only uses one. There may be a lower limit below which jet engines aren’t feasible, and it may be that a scale model 747 at 2’ would not be feasible with four authentically scaled jet engines. But the flying model in question is under 2’ in any dimension.

18

WAG
sounds like a squared/cubed problem.
Reduce each dimension by a factor of 100, the volume or weight goes down 1,000,000 but the surface area, ie lift will only go down 10,000.
100 times the lifting power.
Bog alone knows what happens to the engines power though.

Have forwarded this question to a friend at Airbus Industries and am awaiting reply. They probably have this problem in reverse from wind tunnel tests on scale models.

CH

19

I’ll guarantee you wouldn’t be allowed to take it onto another plane though, sharp objects and all.

20

Regarding the engines; all the small turbines I´ve seen have an amazing RPM rate, the smaller the turbine, the faster it spins; the turbine Broomstick mentions probably tops at 100.000 RPMs, and that´s a 2´ diameter one. At 2" winspan the shrunk 747 turbines would have a diameter of about an inch, and they´d have to spin (if we accept the empirical observation I made) incredibly fast well over it´s design tolerances; it would probably self destruct