Topic: Can this airplane lift off?

[Markcb750]

new symbol to an old fart, thanks!  A qwerty effect I am sure.


I worked on the radar navigation set for the C130, how the system dealt with low or zero ground speed was a key test of correct calibration.

[cb650]

I dont know what "!=" means.   I gather he was just using the ! .

[Inigo Montoya]

Holy crap! How is this Still AN ISSUE? The plane WILL take off, it really IS that simple. OK picture this. Take an rc airplane. The operate on EXACTLY the same principles just on a smaller scale. Start the engine and hold the plane in your hand. When you give it gas you WILL feel it pull because the prop is PULLING air NOT pavement.

Here is another one. Say you have a hot wheels or matchbox car. Put it on a treadmill and hold it in place. See the wheels spinning? Of course you do. Now use your hand to push the car forward. It WILL move forward no matter how much you increase the treadmill speed. This is essentially how the prop or jet engine applies its force to the plane.

I know this sounds simple but it IS sound if you try not to over complicate things.

[Markcb750]

I dont know what "!=" means.   I gather he was just using the ! .

So this leaves me wondering what he intended, one is correct, one is incorrect...

I Goggled !=. "Google does this.

Your search - != - did not match any documents.

Interesting, is he just emphasizing that he believes airspeed and ground speed are always equal? 

[Bob Wessner]

[Markcb750]

The inertial navigation systems I worked on also included a correction for altitude to derive ground speed...

[Bob Wessner]

The inertial navigation systems I worked on also included a correction for altitude to derive ground speed...

Just posted the formula for clarification of the difference between the two. Just curious, how does altitude come into play or make a difference? My gray matter isn't functioning yet this morning.

Edit: Maybe because you are flying in a slight arch as apposed to a straight horizontal line? Just trying to prime the gray matter.

[Inigo Montoya]

Higher altitude, air is less dense. A prop has to spin faster OR have greater pitch to move the same amount of air that it does at lower elevations.

[Bob Wessner]

What about jets?

[mattcb350f]

Increase in speed of belt will increase friction on tires (more drag) which will heat up the bearings which will create more friction and higher belt speed will push the plane back with greater force (translation force will push on bearings which will in turn push the plane back). Don't forget drag from the wings.

The question is do you have more thrust than those forces working against it combined?

Please tell me where this logic is flawed?

Your logic is not flawed, the fact is that the drag on only the wheels will increase with an increase in wheel speed. The problem is we don't know enough about the aircraft in question to make an accurate assessment of either drag or thrust.

Take a Cessna 172 for example. If the lift off or rotation speed is say, 70 mph, then if the belt were to spin the wheels at twice that speed then the wheels would be spinning at 140 mph at the moment of lift off. Without knowing how much drag the three little wheels would be making at this moment, or the total thrust being created by the engine, than we can't mathematically answer this question.

What we do know is that the actual forward movement through the air will have to be 70 mph, and the total thrust created by the engine would have to be enough to spin the wheels at twice that speed, and still allow the aircraft to accelerate to takeoff speed.

For this question we don't know the tire inflation pressure, diameter, air temperature, air density, engine horsepower/thrust horsepower, wind velocity/direction, conveyor belt material, wheel bearing grease density/properties, ......the list goes on and on, and therefore can not make a mathematical assessment of whether the aircraft would be able to overcome the drag created by the wheels spinning at twice their normal speed.

It is my professional opinion that the aircraft I said (C-172) would be able to take off with the wheels at twice the takeoff speed. This aircraft would likely have enough excess horsepower on a good day to make it happen. On a hot day in high altitudes, aircraft will often need to achieve a higher speed than normal rotation speed to get airborne and they do it without a problem. I've heard of 172's hitting 90-100 mph before lifting off...but they did lift off.


Someone asked about a float plane earlier? Drag on floats increase exponentially with an increase in speed, much like a boat. There is a finite velocity that a float plane will be able to maintain while on the water given its available horsepower. Designers make the aircraft so the lift off speed is below this terminal velocity so the aircraft will become airborne. If the terminal velocity of the aircraft on the water is 100 mph, than the designers will make the lift off speed somewhere below that in order to allow it to fly.

 Matt.

[Inigo Montoya]

A jet is unaffected as most are made to operate at elevations well over 10000 feet. I doubt you would notice much change.

[Bob Wessner]

A jet is unaffected as most are made to operate at elevations well over 10000 feet. I doubt you would notice much change.

I guess that is what I was trying to point out. It seems to me, the calculation of airspeed vs groundspeed is independent of the type of aircraft..no?

[Inigo Montoya]

Type of propulsion does not really matter other than a jet produced more power so it can overcome greater friction. Ultimately both move air and that in turn is what moves the plane.

[MacM2010]

This thread has made me very happy. 

[Soos]

This thread has made me very happy. 

It made me want to beat my head against the wall......I'm no genius.... but come on.
(no offense. really! It just frustrates me when something that seems so clear to me is completely missed by others.)

The first few pages of people actually denouncing the ability of a plane to take off with a conveyor belt running the opposite(same? does direction matter?) direction..... was ?frustrating?

I can understand how it could fool some people the way the Q. was worded....
But all you have to remember a plane does NOT need forward momentum to gain elevation.


You could tie a plane to a immovable object with a unbreakable rope(or whatever) and if the prop(s)/jet(s) got the airspeed up fast enough over the wings IT WILL STILL TAKE OFF.

A plane does not have to move forward to take off, it simply needs enough velocity to the air over the wings to gain altitude.




BTW, I am by no means a college grad.
Dropped out of  high school 2 times. The only paper I have to my name is a machinist certification really.
I am very mechanically inclined, and spatially oriented in my thinking.
Trig is as far as my math skills go and thats pushing it.
My writing ability is atrocious(thank god someone put5 the spell checker on this board!), and socially relationship challenged.....
If it weren't for machining, I would probably be digging ditches, or flipping burgers.
So I am definitely not "above" the average intelligence IMO.


Odd... put5 didn't set off the spell checker....





l8r

[Soos]

A jet is unaffected as most are made to operate at elevations well over 10000 feet. I doubt you would notice much change.

I guess that is what I was trying to point out. It seems to me, the calculation of airspeed vs groundspeed is independent of the type of aircraft..no?

Only for vehicles that stay within the atmosphere.

Technically a object/space vehicle in a  "stationary" orbit is in fact doing several thousand MPH.
And if you wanna get picky, the earth and all things on it are doing what 18,000 mph around the sun?
Sorry, it traqvels at 65,000 mph (105,000 kph) along it's orbit around the sun.
If it weren't for internet searches, i'd look like the idiot I am.
With them, i just look like the arse I can be
 

The earth rotates on its axis once each day. Since the circumference of the earth at the equator is 24,901.55 miles, a spot on the equator rotates at approximately 1037.5646 miles per hour (1037.5646 times 24 equals 24,901.55) (1669.8 km/h).

At the North Pole (90 degrees north) and South Pole (90 degrees south), the speed is effectively zero since that spot rotates once in 24 hours, a very, very slow speed.

So it depends on where on the earth you are as to your actual speed as compared to a fixed point in the universe...



l8r

[markb]

I think some of you are over thinking the problem.  Friction in the wheels has nothing to do with it.  There are 4 forces acting on the plane: thrust, drag, lift & gravity.  Thrust has to be greater than drag to move forward and lift has to be greater than gravity to lift off.  If the conveyor is going in reverse at the same speed as the plane (initial problem) then there is no motion and the thrust is zero.  Also, if the plane is stationary there is no air moving past the wings (which causes lift) so the lift is zero.  It doesn't need forward momentum to fly, it needs air moving past the wings.  Again the flaw in the Mythbusters test is that the "conveyor" is not going fast enough in the opposite direction.  The plane is going faster forward than the conveyor is going backwards so of course there is thrust and because air is moving over the wings there is lift.  My opinion, it can't lift off.  

[Markcb750]

This thread has made me very happy. 

It made me want to beat my head against the wall......I'm no genius.... but come on.
(no offense. really! It just frustrates me when something that seems so clear to me is completely missed by others.)

The first few pages of people actually denouncing the ability of a plane to take off with a conveyor belt running the opposite(same? does direction matter?) direction..... was ?frustrating?

I can understand how it could fool some people the way the Q. was worded....
But all you have to remember a plane does NOT need forward momentum to gain elevation.


You could tie a plane to a immovable object with a unbreakable rope(or whatever) and if the prop(s)/jet(s) got the airspeed up fast enough over the wings IT WILL STILL TAKE OFF.

A plane does not have to move forward to take off, it simply needs enough velocity to the air over the wings to gain altitude.




BTW, I am by no means a college grad.
Dropped out of  high school 2 times. The only paper I have to my name is a machinist certification really.
I am very mechanically inclined, and spatially oriented in my thinking.
Trig is as far as my math skills go and thats pushing it.
My writing ability is atrocious(thank god someone put5 the spell checker on this board!), and socially relationship challenged.....
If it weren't for machining, I would probably be digging ditches, or flipping burgers.
So I am definitely not "above" the average intelligence IMO.


Odd... put5 didn't set off the spell checker....





l8r

Just not right; the correct conclusion but if you tie the plane to the ground, same thing as sitting there with the motor running at full power and the brakes on...



This is a fun thread, much like oil threads, except more insight into how people solve thought experiments.


Einstein would be proud, this is how he solved relativity, thought experiments.

[Inigo Montoya]

Markb, it will take off. Stop thinking groundbased. The wheels will spin as fast as they need to and the plane will still move forward as its thrust is NOT ground based but air based meaning NOTHING goes thru the tires for power from the plane. The tires for lack of a better word are free wheeling. So all that will happen is the tires will spin faster while that plane takes off like it always does.

[Soos]

Sorry I meant more like a tether to a immovable object...

A motor could not cause enough air to go over a wing to pull it and the ground up into the air.

l8r

[Soos]

Thw whole concept of a plane flying is the lower air pressure created by faster air going over the curved surface of the top of the wing VS the air under the wing.






l8r

[bunghole]

This entire community needs group therapy.

We have issues.

[Inigo Montoya]

Given 10 pages of discussion on a simple question, I am inclined to agree with you!

[my78k]

Are some people suggesting that the lift gained by an airplane is actually wind coming back from the prop? Ofcourse the plane has to move forward! Although the conveyor moving in this instance is not enough to be able to prevent forward momentum.

I realized I was the one who posted the you tube link but right now I am at work and they block youtube for some reason so I can't verify how they measured the speed of the wheels. IIRC, they actually moved the conveyor to match the forward speed of the aircraft (windspeed even though it is on the ground) and not actually the same speed as the wheels...

Wow, I was convinced a little while ago and now I am starting to doubt it again! Wow, I may just have to change my mind and run full synth in the bike now too!!!

Dennis

Oh I know why they banned youtube....have you seen what some girls will do for attention on there??  

[Bob Wessner]

This entire community needs group therapy.

We have issues.

Surely you knew that long before this thread popped up.