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More ways to get Braingle...

Pete the Pilot

Science brain teasers require understanding of the physical or biological world and the laws that govern it.

 

Puzzle ID:#16770
Fun:*** (2.83)
Difficulty:** (1.32)
Category:Science
Submitted By:airhead3***
Corrected By:BobbyKat

 

 

 



Adrenaline was coursing through his body. Our hero, Pete the Pilot, has so far survived a harrowing flight in his small, single-engine airplane, but he isn't home yet. After a long flight fighting his way through unexpectedly wicked weather, his destination is finally in sight.
"Almost there," he assures himself as he looks at the ground moving past a mile below him. He wipes the beaded sweat off of his brow as he lines his plane up to the runway that just emerged in the distance. Five miles to go now. Just as he thought, "Maybe I'll make it through this yet," the sobering sound of his sputtering engine filled Pete's ears, followed by silence. The comforting hum of the engine was now replaced by the sound of his pounding heart. His engine failed five miles from the airport! After a quick check of his fuel gauges, he knows he won't be able to restart the engine. Our hero doesn't have a parachute, yet he was able to survive the ordeal without even a scratch on him. How did he do it?


Answer

Pete the Pilot simply glided the aircraft the remaining five miles and landed on the runway without incident. Contrary to what many may think, planes won't fall like rocks without their engines. In fact, most can glide very well. Most typical airplanes have a glide ratio around 10:1, meaning the plane will fly 10 feet forward for every one foot it descends, without the help of an engine. (For example, the Cessna 172, perhaps the most common small single-engine plane, has a glide ratio ranging from 8:1 to as high as 13:1, while the 747 'Jumbo Jet' is around 15:1.) In this scenario, Pete only needed roughly a 5:1 glide ratio, one his small plane should easily be able to achieve.
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Comments

bluetwo*
Mar 03, 2004

are you sure he wasn't in a flight simulator?
toast9897Aus*
Mar 03, 2004

lol. flight simulator... kids these days. what is this world coming to? jk blue. good alternate answer
airhead3*
Mar 03, 2004

bluetwo - that simulator of yours has pretty impressive wrap-around visuals for Pete to be able to look *down* and see the ground below him. Most single-engine sims I've been in only have two monitors on the dash (if they have any visuals at all)! Your alternate answer is a good one, though - one that casts new light on the teaser.
bbngk
Mar 03, 2004

He must be a good pilot coz most planes need their engines to stop after landing
airhead3*
Mar 03, 2004

bbngk -
Actually, what nearly all airplanes use as their primary mechanism for stopping are brakes - just like cars. Larger, heavier, jet-powered airplanes (like the 747) can sometimes need more help to stop, though, so they utilize what is called reverse thrust. That means they redirect the jet engine's thrust so it's 'blowing' forward, to help the airplane slow down in a shorter distance. Small airplanes (like the one mentioned in the teaser) don't really have (or need) any way of stopping other than their brakes - the engine doesn't play a role. Let me know if that makes sense, or if I can explain anything further - I appreciate the comments!
BulletthrudahedAgb
Mar 04, 2004

If planes couldn't glide like that would they even be able to fly?
airhead3*
Mar 04, 2004

Bullet -
Great way to look at it. Airplanes the way we know them would have to be able to glide to a certain extent in order to be aerodynamically sound enough for flight to be possible. This holds true in even the most extreme example: The stub-wing X-15 rocket plane that broke all the speed records still has a 4:1 glide ratio! And that was probably the best contrary example I could come up with. (Heck, it's a different beast, but even helicopters can auto-rotate in a controlled descent if they lose their engine.) I know the concept of aircraft being able to glide is certainly not new or groundbreaking, and as such this isn't exactly the most challenging of teasers, but it never fails: When I take someone flying, I'm coming in for landing so I pull the throttle all the way back to idle, and they always freak out a bit at the lack of engine noise, because somewhere deep down, they expect the plane to somehow fall out of the sky without that comforting noise of the engine. Just one of those things...
electronjohn*us*
Mar 11, 2004

How much is the glide ratio affected by the altitude? I mean I would expect a plane aproaching Denver CO, say 1000 feet above the airport, to glide less than same plane flying the same speed and distance above Logan airport in Boston MA, since Boston is basically at sea level with denser air and Denver is about 1 mile up with thinner air.
airhead3*
Mar 12, 2004

electronjohn -
Yours is a very interesting and perceptive thought, and on the surface one would think that the glide ratio would be affected by altitude. Fascinatingly enough, though, the glide ratio is not affected, and here’s why: the glide ratio (distance/altitude) is equivalent to the ratio of the aircraft’s lift to drag (lift/drag). This can be explained easiest through vector triangles, and if you want further clarification let me know, and I can provide an explanation. For now, though, we’ll just take that statement as fact. So, let’s look at the equations for lift and drag: Lift=Cl x r x 0.5(V^2) x A and Drag = Cd x r x 0.5(V^2) x A, where r = air density, V = aircraft velocity, A = area (of the wing for lift, of the reference area for drag), and C = the coefficient of lift or drag, respectively (the coefficients account for many more complex aspects of aerodynamics, and are typically found through experimentation with a given aircraft). Note that the two formulas are identical in construction. From this we can plainly see that air density (which is affected by altitude) *does* affect lift, as you suggested, but it also affects drag the same amount. To phrase it differently, when looking at lift/drag, air density cancels itself out of the equation. Now, because the glide ratio is equivalent to lift/drag, we can see that the glide ratio is unaffected by altitude. I hope that was an intelligible explanation…let me know if I can clear anything else up. Thanks for the comment! (I love it when I'm forced to think...which is why I'm here.
)
hersheykiss8908*
Jun 27, 2004

i actually got that one! all i had to do was think of a paper airplane.
I_am_the_Omega**
Oct 12, 2004

Bah. I thought it was a flight sim.
silly_goose*us*
May 31, 2005

I love airplanes!
bookworm91**
Nov 10, 2005

very good one. i like the not-too-impossible to wrap your brain around teasers.
vbguy101*us*
Aug 20, 2006

I should've thought of that!
ElectronJohn*us*
Nov 27, 2007

Airhead, thanks for the answer about the lift/drag stuff. That does make sense now that you explain it.
mosca*us*
Dec 09, 2007

Do small planes still have to fly a traffic pattern before landing? they did when I was taking flying lessons years ago.
airhead3*
Dec 09, 2007

mosca, traffic patterns are not a requirement - more of a standard practice. At uncontrolled airports (without air traffic control), you can choose to do a straight-in approach, and you can request a straight-in approach at controlled airports, as well. The traffic pattern certainly makes things easier (many pilots would argue a straight-in is more difficult), in fact, in many emergency landings it is recommended to do a leg or two of a traffic pattern to aide in judging the approach. That brings us to the primary point: this would certainly be an emergency situation, and pilots are given a great amount of latitude in emergencies - regardless of standard practice, or even regulations (yes, in an emergency, pilots can break most rules). Thanks for the interest!



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