Re: [Phys-l] Bad physics in National park

["LaMontagne, Bob" <RLAMONT@providence.edu>]

One of my hobbies is radio controlled gliders. One day at the field where I fly them the usual discussion of Bernouli vs. Newton and the whole thing about  flat winged balsa hand gliders came up. I turned the wing on one of my competition gliders - one with little dihedral - over and launched it. It flew, but came back down faster than normal (no thermals). The only big difference was that I couldn't fly it with rudder only (probably because of the inverted dihedral), but fortunately that glider also had ailerons. I'm still not sure that it came down faster because the airfoil was inverted - I had to fight the roll instability so much that I was not flying it efficiently.

I also have an aerobatic electric powered plane. The airfoil is symmetric and it has no dihedral - it flies just as well upside down as right side up. (Wouldn't do that in my full sized Cessna 150!)

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Richard  Tarara [rbtarara@sprynet.com]
Sent: Friday, May 22, 2009 5:02 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Bad physics in National park

I don't want to get into the wing thing for the umpteenth time, but flying
upside down involves different physics than Bernoulli lift.  However it
works, a wing (such as the Wright Brothers or various recent human-powered
craft) can _probably_  be modeled using Bernoulli at least in part, but
upside down flight pretty much requires a conservation of momentum
argument--like sticking your hand out the window of a moving car and placing
the fingers slightly upwards so that the the palm hits the air--forcing it
down and the hand up.  The only point here is that upside-down flight
doesn't, in itself, disprove any Bernoulli explanation.  ;-)

Rick (who would guess that the Wright flyer can't fly upside down, but also
recalls a Bernoulli like explanation for wings presented at the Air Force
museum in Dayton!)




----- Original Message -----
From: "Donald Smith" <dsmith4@guilford.edu>


> Hi,
>
> No, that explanation makes more sense than what they said at the national
> park.
> What they said at the park was more like "the air that goes over the top
> of
> the
> wing has to travel further than the air going under the bottom of the
> wing.
> The
> two flows must travel across the wing in the same amount of time.
> Therefore,
> the top one must travel faster, and since it is travelling faster, the
> pressure is lower
> above the wing than below.  This difference in pressure results in an
> upward
> force
> we call "lift", which is what holds up the airplane."  I'm paraphrasing
> from
> memory,
> but that is as close as I can remember to an exact quote.
>
> Never mind that this "explanation" would imply that planes cannot fly
> upside
> down,
> which obviously they can.

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