Mark C
Key Veteran
Location: Houston, TX - USA
| Feynman's Airplane
No, this plane is NOT on a friggin treadmill. But it is likely to generate some responses that are just as wacky.
I named this puzzle after another brain teaser made famous by a really cool Physicist twenty something years ago. After I got my head around the original brain teaser I had to throw everything I had been taught up to that time about airfoil lift away and re-think it. It was certainly the cause for several intense after hours discussions between me and my college physics professor which is likely still scratching his head.
The original puzzle was made popular by Richard Feynman and dubbed Feynman's Sprinkler It is my understanding that he did not originally come up with it but he published it somewhere and was able to explain the reasoning behind it and somewhere along the way his name got attached to it.
So if you followed those links and got your head around his sprinkler dilemma, I want you to consider a couple more scenarios before we fly his airplane:
1. Would it (the sprinkler) behave any different if you were using air instead of water?
2. If you attach the hose to the OUTPUT of your ShopVac (the side that blows air) and laid the hose on the floor and turned it on, it would certainly whip around on the floor like a loose firehose. But if you attached it to the INPUT side (the side that sucks air) instead, would it still whip around the floor like loose firehose?
So now for Feynman's Airplane.
We have a Cessna - Feynman's "magical" Cessna. Of it's many magical qualities it has invisible pressure sensors all over every inch of it's wings both above and below to record the pressure differential between the top and the bottom of the wings. The sensors make these measurements magically without disturbing the aerodynamics of the aircraft whatsoever.
We fly Feynman's magical Cessna and record the pressure differential above and below the wings on a magical computer - or maybe even a DELL XPS. Now we know the EXACT profile of the pressure differential over every inch of the airfoil during flight.
Did I mention that Feynman's magical Cessna was hollow and made of uncrushable unobtainum? Because it's skin is so strong and uncrushable it requires absolutely no ribbing or bracing of the wing or airframe. If you sit in the cockpit you can see right down the inside of the wings. It's bada$$.
Now, we take our special unobtainium drill and drill the TOP (and only the top) of the wing until it looks like Swiss cheese. Literally thousands of holes all over the top of the wing. We leave the bottom of the wing alone. And we didn't damage any of the magical sensors either.
We now take and attach our ShopVac model 747 to the Cessna using a large diameter ultra lightweight, ultra flexible hose made of uncrushable unobtainum II. The ShopVac 747 is sitting outside the Hangar and it's exhaust is in no way interacting with our aircraft. Only it's intake is attached to the airframe. We are going to create a massive vacuum inside the hollow Cessna.
Now with the Cessna anchored to a large scale (it's propeller engine not running), we power up the ShopVac 747 a massive vacuum develops inside the airframe and air begins getting sucked into all of the thousands of holes on the top of the wing of the Cessna.
We then adjust the ShopVac 747 until the computer reading the sensors tells us that we have the EXACT same pressure differential above and below the wing that was measured during our original data gathering flight. We even go around and drill extra holes on the top of the wing and patch up some holes if necessary until the computer shows us that every inch of the profile shows to have the EXACTLY the SAME pressure differential profile as originally measured above and below the wing during flight.
With it all adjusted to perfection, we now grab up our trusty scientific notebook, run over to the display on the large digital scale and record the weight of the aircraft on the scale.
So the real puzzle here is what are we going to see on the scale?
Are we going to see:
A. The Cessna is actually buoyant and would suck it's way into the air if we released its anchors.
B. The Cessna has sucked it's way to perfect neutral buoyancy and the scale reads nearly zero plus or minus some spooge.
C. The scale hardly shows any difference in the weight of the Cessna whether the ShopVac 747 is running or not. Even if we run the Shop Vac 747 wide open and get twice the pressure differential profile - we read no real difference on the scale.
What's the answer?
Mark C.
EDIT To Clarify the question:
If you have an airplane stationary on a scale without the engine running and you are able to "artificially" recreate the EXACT pressure differential conditions found on the wing during flight WITHOUT moving air across the wing surface, what would you observe? |
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