RE: Can It Take Off??

RE: Can It Take Off??
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RE: Can It Take Off?? - 8/11/2006 8:33:13 PM

mesae

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Oryx, about low-speed aerodynamics being incompressible: As I understand compressibility, pressure variations due to the production of lift and thrust are not precluded (lower pressure on top, etc). Its just that the air isn't compressed (and heated) significantly due to the speed of the aircraft. Correct?

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RE: Can It Take Off?? - 8/11/2006 8:40:06 PM

dick Hanson

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kinda fun - like examining an elephant in the dark - without walking around it
try this:
visualize a closed tube -- with a piston in the centre held by a latch.
evacuate the tube at one end (reduce pressure ) and or increase pressure in the other end.
Now
release the piston - obviously it will move to the low pressure end - as the forces try to rebalance

In my mind --this is all that is happening with a airscrew - pressure is unbalanced and momma nature tries to make it all right again.
Propeller theory and wing theory are explained thru various charts and books, ad infinitum -but in the end - this is what is happening.
Not making fun -just telling it like I see it

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RE: Can It Take Off?? - 8/11/2006 9:17:32 PM

Oryx

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quote:

ORIGINAL: mesae

Oryx, about low-speed aerodynamics being incompressible: As I understand compressibility, pressure variations due to the production of lift and thrust are not precluded (lower pressure on top, etc). Its just that the air isn't compressed (and heated) significantly due to the speed of the aircraft. Correct?

Indeed, there are local pressure variations, but the density varies very little through the fluid. It is in fact quite different to what you would get when you compressed the fluid in a piston - in that case the ratio of compression is usually enough that you increase the density significantly. In "incompressible flow" the pressure variation is small enough that the density changes are insignificant, and that is in principal what defines incompressible flow. You can for instance put pressure taps on a blade or wing producing lift under water (lets say the keel of a sailing ship), and you will see pressure variations over this blade similar to what you would see on an airfoil in air. Yet, the density change around the blade is insignificant and for all practical purposes the water is still incompressible.

I was once involved with one of the America's cup teams - airfoils were designed using the same software and methods used for airplane airfoils. We tested the rudder airfoils in a wind tunnel rather than a water tank, because we could get a closer Reynolds number match in the tunnel than we would have got in a tow tank. There are some differences when you work with sea water due to precipitates in water, but these are secondary effects and there are methods to compensate for them. Essentially, as long as the real thing won't have low enough pressures to encounter cavitation in water, and the tunnel version runs at low enough speed (lets say less than about Mach 0.3), then there is no difference between testing in air or water - both are incompressible for all practical purposes. The answers are nondimensional and you just calculate the actual forces using the water density... Worked very well - tunnel and actual testing on an instrumented keel and rudder gave very good agreement.

On the other hand, place an airfoil in supersonic flow and you have significant density variations through the flowfield - enough that you can actually see these variations (shock and mach waves) under the correct lighting conditions such as with Schlieren or Shadowgraph photogrophy. The density variations correspond with very large pressure changes. In this case, what you see resembles much better what would happen in a piston and the "incompressibility" assumption is obviously not valid anymore.

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RE: Can It Take Off?? - 8/11/2006 9:22:05 PM

mesae

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Fascinating.

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RE: Can It Take Off?? - 8/11/2006 10:19:20 PM

da Rock

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OK, this discussion has gone way off the original path. That, by itself is of little import as long as there is an interchange of ideas.

However, when the ideas being interchanged shift to a discussion of each other, that's too far off.

Discuss the issues. Leave out anything personal.

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RE: Can It Take Off?? - 8/12/2006 6:05:32 PM

Tim Green

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quote:

ORIGINAL: ptulmer

Tim, what are you babbling about? How does my getting blown about move an airplane forward? Look, I said something that was out of place and was going to let mesae have the last word rather than take this thread down that road. That doesn't extend to you being nasty. (and wrong) Obviously, when the air is past the prop, it's not imparting a force to the prop.

No - but all that moving air should give you some idea of the force the prop imparts ("move an airplane forward" in your words) to the airplane as the propeller moves all that air.

I didn't mean to be nasty - sorry.

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RE: Can It Take Off?? - 12/13/2006 3:31:13 AM

bear cub

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Always remember:AIRSPEED ---true air speed (TAS) indicated air speed (IAS) not ground speed, is what causes an airfoil to produce lift.

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RE: Can It Take Off?? - 12/13/2006 1:44:18 PM

mesae

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quote:

ORIGINAL: bear cub

Always remember:AIRSPEED ---true air speed (TAS) indicated air speed (IAS) not ground speed, is what causes an airfoil to produce lift.

Not TAS; only IAS (dynamic pressure). TAS is used with density to compute dynamic pressure and this in turn is used to compute other aerodynamic forces, including lift. An airspeed indicator effectively measures dynamic pressure, but the indicating scale is calibrated in knots, rather than PSI or some other pressure units, because pilots relate better to knots as a measure of speed rather than PSI, though PSI could cave worked just as well theoretically. True airspeed is not measured directly but has to be computed using a density correction (pressure altitude and temperature) against indicated airspeed either by the pilot or automatically by an Air Data Computer.

edit <spelling error>

< Message edited by mesae -- 12/13/2006 1:45:37 PM >

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RE: Can It Take Off?? - 12/14/2006 2:02:38 AM

bear cub

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Your'e right, indicated air speed is the force that works on the airfoil to produce lift. I believe that this would hold true for the Custer Channel Wing also.

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RE: Can It Take Off?? - 12/15/2006 7:10:58 PM

mesae

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I had never heard of the Custer Channel wing concept before your post but I just read up on it a little bit. I can see why it never became popular. During a very low speed multi-engine climb-out, an engine failure would mean certain death. There are safer and lighter ways to get STOL performance with reasonable cruise speed.

And yes, pressure differences, not TAS would govern that wing's lift too. Any object at all in fact, even a sofa or a 2 by 4. I bet if you put a big enough engine on a sofa and arranged it just the right way, it would fly...

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RE: Can It Take Off?? - 12/15/2006 9:07:33 PM

manks7477

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Easy answer, the speed of the wheels is not an issue, as long as air is flowing over the wings it will take off.

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Post #: 536

RE: Can It Take Off?? - 12/18/2006 1:21:33 AM

cyclops2

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Wrong again.

Fuel tank ran dry.

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Post #: 537

RE: Can It Take Off?? - 12/21/2006 4:03:23 AM

F2G-1

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quote:

ORIGINAL: bentwings

Here is one to use up band width. The diesel truck guys made 302 posts in 5 days. only 2 personal flames recorded
can the experts beat this??? Let's have at it. Can the "wing guys" beat the "wheel guys"

*** Brain Teaser ***

Imagine an airplane is on the beginning of a massive conveyor belt, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off?

OK, I think I solved the problem.

The plane will fly.

"But Ted, the mathematics do not suport this result." you say?

Well, the mathematical explinations of the 'nay-sayers' have been flawed from the start, just as the parameters of the question.

#1) the parameter/stipulation of belt speed matching the wheel speed is unobtainable in this scenario, the equivalent of '1+1=3'. It is imposible for the belt to match wheel speed because inevitabley wheel speed is obtained by adding the speed of the belt+forward speed of the a/c, (because this vector is independant of the belt, it is additive)

#2) the belt will accelerate infinitly, with the wheel speed accelerating infinitly + X (x= net velocity from outside propulsion, resulting in forward motion due to the unequal opposing vectors.

I spent an hour getting 1/2 way throught this thread, thinking to my self "self, I honestly believe my IQ has droped 2 points for subjecting my grey matter to this thoughtless banter" But after a few more pages, some of your theories started making scense mathimatically, but not in practical applications. So after reading the blah blah about lack of math skills evident within this thread, I thought about it some more. I'm kinda pissed i had to read it all to see nobody else picked up on this.

Ted

OK, i'm ready,.......flame on.

< Message edited by F2G-1 -- 12/21/2006 4:31:04 AM >

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RE: Can It Take Off?? - 12/21/2006 4:21:35 AM

bear cub

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I guess you told us

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RE: Can It Take Off?? - 12/21/2006 4:33:56 AM

F2G-1

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quote:

ORIGINAL: bear cub

I guess you told us

What?
so......
tell me?
sound right?
No, really,.....be honest, dont hold back....icantakeit.

LOL!

Ted

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Post #: 540

RE: Can It Take Off?? - 12/23/2006 11:58:48 AM

Woodmandan2

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From: NanjingJiangsu, CHINA
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quote:

ORIGINAL: DLSmith2

This is a job for the MYTHBUSTERS!!!

do do do do do who yo gonna call, MYTHBUSTERS!!!! do do do do do do do do do do do WHo yo gonna call do do do do do do do do do do MYTHBUSTERS!!!!!
Lol yea they should do somthing like this.
Dan

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Post #: 541

RE: Can It Take Off?? - 12/23/2006 10:09:51 PM

Edge 540