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Prandtl Glauert Singularity

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Considering the phrases "Normal Shock", "Boundary Layer", and "Ground effect" have been not been mentioned once in this conversation, I would suggest the following.

Read: http://www.amazon.com/Fundamentals-Fluid-Mechanics-Bruce-Munson/dp/0470262842/ref=sr_1_1?ie=UTF8&s=books&qid=1273678454&sr=8-1

then Read: http://www.amazon.com/Mechanics-Flight-Warren-F-Phillips/dp/0470539755/ref=sr_1_1?ie=UTF8&s=books&qid=1273678490&sr=8-1-spell

Then come back and talk until you're blue in the face. I've read them both cover to cover and would vouch for their quality of content.
 
Another good book is Aerodynamics for Naval Aviators.

I have learned quite a bit from that book.

Cheers,

Ken
 
Well, if an automobile was to be dropped and travel "through" the earth and come out the other side...... we probably wouldn't live long enough to even know about it, much less see it.
 
An automobile achieves thrust by the friction between the tires and the road as the tires achieve rotational velocity through a drive system.

An airplane achieves thrust by a propeller or jet engine propelling the aircraft forward on free turning wheels.

The basic difference is summed up by a straightforward examination:

1. Take a car and put it on a treadmill and tie a chain to the frame and the other side of the chain to a steel pyling. Turn the treadmill on and something on the car will break, or the chain will break, or the treadmill's surface will tear. This is because the drive wheels on the car will resist the velocity of the treadmill surface.

2. Take an airplane and similarly chain it to a steel pyling. Turn the treadmill on and the aircraft at rest will initially move aft until the force on the chain counters the friction of the tires at rest on the treadmill surface. At that point, the airplane's wheels will match the velocity of the treadmill's surface. There will be no additional force on the chain.

Now, with regard to the airplane, leave the chain attached. Fire up the engine and add power. How many vote that the aircraft will not move forward and relax pressure on the chain?

The point is that all the thrust that is needed is what is required to break the initial at rest inertia of the wheels on the surface.

One final point. When aircraft are engine run, you have to put chocks down to keep the aircraft from rolling forward.

When cars are engine run, you simply place them on a dynamometer, which is a free rotating cylinder on the drive wheels. The faster you go, the faster the cylinders are rotated to match the velocity. This how race cars are engine tested.

The critical difference is that cars again get thrust from the frictional effect of tires on the road as the wheels they are attached to rotate. The car only moves if it encounters resistance on a stationary surface. Another example of the point.

I am sure everyone has sat in a car at idle on sandy soil and tried to leave quickly. The wheels spin, right? You go nowhere while the wheels spin and they keep spinning until the tires make contact with ground surface firm enough to provide resistance to the spinning drive wheels.

An aircraft doesn't have this problem. Every time you apply full and immediate power, whether on grass, dirt, or asphalt, the aircraft never spins its wheels, does it? Of course not! Instead, the wheels don't need any surface resistance for the aircraft to move forward. Instead, for a prop plane it merely needs the prop to cut through the air and produce a lift force in the horizontal direction. A jet engine simply produces more thrust out the back and relies upon Newtonial physics to provide the matching thrust in the opposite direction.

And this is why, folks, a treadmill cannot prevent an airplane from taking off.

Cheers,

Ken
 
Well, you've left out stuff that will make the anal ones among us squirm a bit...and of course I include myself in that grouping ;)
But of course the aircraft will take off, we've established this already.

I'm waiting to hear about what conditions you could theoretically apply to make this a hard problem!
Chris H said:
...In real life the aircraft will take off, the aircraft will be able to accelerate quicker than the conveyor belt
...But on paper it can't, due to these little physics assumptions that we make, and the correct answer is obviously no, it wouldn't take off...
Click to expand...
 
Wing_Z said:
Well, you've left out stuff that will make the anal ones among us squirm a bit...and of course I include myself in that grouping ;)
But of course the aircraft will take off, we've established this already.

I'm waiting to hear about what conditions you could theoretically apply to make this a hard problem!
Click to expand...

I agree. This doesn't appear to me to be a "hard" physics problem. The additional component is that if you allow the treadmill a full head start, then in addition to the inertial force, the thrust of the aircraft would have to also overcome the horizontal momentum in the aft direction. But, momentum is not thrust, and therein lies the essential point.

Yes, it will take more time for the aircraft's forward thrust to counter the aft velocity the treadmill was allowed to generate -- again due entirely to the frictional force and inertia Newtonian laws.

But, that thrust will overcome that aft momentum and soon as those aft forces are broken, the aircraft will experience forward velocity relative the stationary ground and provided the treadmill is long enough, that velocity will increase and once rotate velocity is achieved, it becomes a normal takeoff with the free rolling tires going twice as fast in rotational velocity as they normally would be. The only practical difference once airborne is that the wheels on the aircraft will take longer to spool down.

Ken
 
Actually, as JohnC has pointed out, ground effect might be quite significant.
It's the reason cars are put in wind tunnels with rolling roads to tweak aerodynamics.
 
Wing_Z said:
Actually, as JohnC has pointed out, ground effect might be quite significant.
It's the reason cars are put in wind tunnels with rolling roads to tweak aerodynamics.
Click to expand...

The effect of ground effect is simply to reduce the airspeed at which an aircraft can remain aloft. It has the effect of lowering the stall speed. So, if anything, same as for a soft field takeoff, it should assist the takeoff, not harm it.

For those who do not know, a soft field takeoff has the pilot rotate and lift off at a lower airspeed and then remain in ground effect while the aircraft builds up speed to climb out at Vx (best angle of climb).

Cheers,

Ken
 
Ken Stallings said:
And this is why, folks, a treadmill cannot prevent an airplane from taking off.

Cheers,

Ken
Click to expand...

If that said treadmill were thrown through the windscreen, striking the pilot across the bridge of the nose...I bet that air plane will not take off until said pilot gets out of the plane and whoops the hind end of the joker who threw the treadmill through his wind screen.
 
OBIO said:
If that said treadmill were thrown through the windscreen, striking the pilot across the bridge of the nose...I bet that air plane will not take off until said pilot gets out of the plane and whoops the hind end of the joker who threw the treadmill through his wind screen.
Click to expand...

Not to mention I'd kick his butt fierce for breaking my airplane! :icon_lol:

Ken
 
Ken Stallings said:
... it should assist the takeoff, not harm it...
Click to expand...
Correct from a pilot's perspective, yes, on a stationary runway.
Because the treadmill moves however, there will be more rotation of air (tumbling anticlockwise seen from the port side) under the aircraft, due to boundary layer drag.
Now I'm trying to imagine whether this would drop the pressure under the plane.
I think at higher speeds: yes
So that would suck the plane back down!
The faster plane and treadmill go, the more it would suck!
Is there a moral here??
 
tigisfat said:
Anyone with any scientific background knows that there must be a terminal velocity for the penny and the car.
Click to expand...

It's a lot more than just terminal verlocity.

Energy dissipation on impact, surface properties, etc...

A car is rather weak compared to your average ground.
 
Bjoern said:
It's a lot more than just terminal verlocity.

Energy dissipation on impact, surface properties, etc...

A car is rather weak compared to your average ground.
Click to expand...

My comment was only to say that gravity has it's limits. Whatever you drop from any height will not continue to accelerate to the speed of light. Eventually, and in most cases, there will largely be an equilibrium.

Of course gravity and atmosphere changes with distance, but we're talking about stuff being dropped 1,300 feet, or however high the empire state building is.
 
Wing_Z said:
Correct from a pilot's perspective, yes, on a stationary runway.
Because the treadmill moves however, there will be more rotation of air (tumbling anticlockwise seen from the port side) under the aircraft, due to boundary layer drag.
Now I'm trying to imagine whether this would drop the pressure under the plane.
I think at higher speeds: yes
So that would suck the plane back down!
The faster plane and treadmill go, the more it would suck!
Is there a moral here??
Click to expand...

OK, I see your point. It would be an interesting measurement to take. I suspect it wouldn't suck enough.

Although I confess on that point, I have no empirical data to back up my assertion. It's just a hunch based upon my own flying.

Cheers,

Ken
 
tigisfat said:
I wouldn't even bring that up around Boxcar, he believes in them, and very strongly to boot.
Click to expand...

Wow... I come here to check in for the first time in weeks to find you speaking for me. Never being be so audacious & reckless to do so again. You've done this elsewhere & I'll speak for myself. I do not speak for you, Memphis man, so stop speaking for me.

If anyone cares to read the full-meal-deal on how this assertion of "tigis" came about then you are most welcome to read the thread: http://forums.flightsim.com/vbfs/showthread.php?t=209947 He subsequently accused me in another forum of saying that "we were being gassed" by our government which is completely untrue.

Like I said, read the whole thread. You'll see me speaking for myself as well as standing up to a trouble maker.
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boxcar said:
Wow... I come here to check in for the first time in weeks to find you speaking for me. Never being be so audacious & reckless to do so again. You've done this elsewhere & I'll speak for myself. I do not speak for you, Memphis man, so stop speaking for me.

If anyone cares to read the full-meal-deal on how this assertion of "tigis" came about then you are most welcome to read the thread: http://forums.flightsim.com/vbfs/showthread.php?t=209947 He subsequently accused me in another forum of saying that "we were being gassed" by our government which is completely untrue.

Like I said, read the whole thread. You'll see me speaking for myself as well as standing up to a trouble maker.
..
Click to expand...

How on earth do you consider the comments of mine you quoted offensive OR speaking for you? It's clearly neither. I shouldn't even be concerning myself with this....


Mods, not only is a lock called for but a delete is as well. Please do me a favor. No good can come of this (way) off topic public display. This can only go REALLY downhill from here.
 
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You .just ... don't ... get it. Someone mentions "chemtrails" & you bring up my name in connection, saying not to bring them up around me, how greatly I believe in them, how strongly. That certainly is speaking for me. Was going to edit out your city of residence though, but you've already quoted me.

Am seeing no need for any lock on this interesting thread, as I'm done, though what will be will be.
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