Physics Question for you.....

[leehljp]

Going with dovetails.

Just how are these put together or taken apart?

[Alex Franke]

As an example. Below is a picture of a box (not mine). How do you open it?

I could see that working if the joint formed an arc of a circle...

An that's the craziest dovetail I've ever seen. It must go together at 45 degrees, right? Did you cut that?

[Russianwolf]

Here's the opened box. (Again, this isn't mine) It was cut using an Incra Jig from what I understand.


Although this isn't the case in this one, I've seen others that have two parrallel dovetails so that each of the two hidden faces of the box would also have a dovetail in them. All of them are great work IMO.

[gfunderb]

The Plane Will Take Off!!

A recent episode of MythBusters proved that the plane will take off.

They put a Piper Cub on a very large conveyor belt. The motion of the conveyor belt pulled enough air under the plane to give it lift.

Gene

[Russianwolf]

A recent episode of MythBusters proved that the plane will take off.

They put a Piper Cub on a very large conveyor belt. The motion of the conveyor belt pulled enough air under the plane to give it lift.

Gene

Hi Gene,
The conveyor has little to no effect on the air. It's just that the conveyor also has little effect on the plane's ability to propel itself throught the air to achieve lift.

[Kristofor]

P.S. I have to add: All the arguments about the plane's thrust acting on the air were also misleading. It doesn't matter where the plane is getting the force from. You can tie a rope on the plane's nose with the plane's engine off, apply enough force, whether by several guys pulling or with a truck or whatever, and the plane will still move. Move fast enough, and the plane will take off. Of course, with the engines off, bets are off on how long it stays afloat. But it will take off.

Well, it does matter in some ways... How do you develop acceleration if your thrust is being delivered through a drivetrain to the wheels? In that case you need to have a high degree of friction between the wheels and the conveyor (so the engine can turn the wheels which in turn push *relative* to the conveyor not the ground).

This follows the same 2x wheel speed issue as before, but now the engine is coupled to the wheels instead of letting them spin freely. Unlike the propeller engine which only needs to overcome a little extra wheel speed friction the drivetrain engine needs to drive the wheels twice as fast. If you can normally go 60mph in your car at 4000 RPM you would now need to go 8000 RPM to maintain a 60mph ground speed (120mph relative to the conveyor).

I don't know if average plane engines have enough beef to provide double the needed RPM for takeoff without blowing up, if not they could never takeoff if they were driven by their wheels. Of course, if they did take off that way I know they wouldn't get very far since there's not a lot of thrust generated by spinning your tires in the air.


Kristofor.

[Wood_workur]

even if the airspeed were precisely matched to conveyor it would take off. thats the speed in the cockpit. if it was the ground speed then it would not take off.

and I love those dovetails. I might just have to use those in the blanket chest I'm working on.... thankfully I have a lot of scraps to practice.

[leehljp]

Well, it does matter in some ways... How do you develop acceleration if your thrust is being delivered through a drivetrain to the wheels? In that case you need to have a high degree of friction between the wheels and the conveyor (so the engine can turn the wheels which in turn push *relative* to the conveyor not the ground).

This follows the same 2x wheel speed issue as before, but now the engine is coupled to the wheels instead of letting them spin freely. Unlike the propeller engine which only needs to overcome a little extra wheel speed friction the drivetrain engine needs to drive the wheels twice as fast. If you can normally go 60mph in your car at 4000 RPM you would now need to go 8000 RPM to maintain a 60mph ground speed (120mph relative to the conveyor).
Kristofor.

I FINALLY SEE CLEARLY what "Take Off" people are talking about. Yes it will.

In my profession and with a few knot headed people, I have to split hairs so much that I am just used to it. My NEXT illustration was to place an elevated "stop" a few inches over the rolling runway in front of the wheels to guarantee that the speed of the wheels matched the speed of the rolling runway WITHOUT FAIL, in which case - NO it will not.

THANKS KRIS, your explanation triggered that "Ah Ha"!

[JimD]

While corrected (correctly) once already (ideal gas law is not very applicable, Bernoulli's principle is the most typical explanation of lift but is only a partial explanation) I will still attempt another input.

It seems to me that the key confusing thing about the question is whether there is motion of air over the wings of the plane. If the air motion is higher than the take off velocity for the airplane, it will fly. If it is slower, it will not.

With respect to the conveyor belt questions, I think the key thing is to note is that the conveyor belt cannot be moving at a constant velocity, as they normally do, and keep the plane stationary. The engines supply thrust which results in acceleration - increasing velocity. A constant velocity conveyor belt would initially move the airplance backward and if the engines were powerful enough and/or the frictional forces low enough, the airplane would ultimately overpower the constant speed conveyor belt. For the plane to remain stationary and lift to not be generated, the conveyor needs to accelerate at the same rate in the opposite direction as the airplane. Moving at the same rate could mean it accelerates at the same rate but I think it is more clear to say it will accelerate at the same rate but opposite direction than to say it moves at the same rate - constant rate movement is what we assume for a conveyor which would allow the airplane to accelerate and overcome the conveyor (possibly what happened on mythbusters).

Jim

[Alex Franke]

I thought that Bernulli's principle (that's the air pressure above/below, right?) was only a minor part of the equation -- that the primary factor was in fact the air bouncing off the bottom of the wing.

It doesn't matter if the conveyor is at a constant speed or accelerating. It's impossible for the conveyor to accelerate enough to cancel out the forward movement of the airplane unless the wheels stop functioning properly.

Hmmm... Stop the plane, release the brakes, turn off the engines, throw the conveyor into reverse and crank it up as fast as it will go. Once the wheels start rolling backwards, it'll accelerate very slowly at a rate related to the friction in the wheels, right? So then at some point it'll reach a terminal velocity and you still won't be able to fling the plane into the air, so long as the friction in the wheels remains the same -- even if the conveyor is going a zillion miles a hour in a seemingly favorable direction. Is that right?

[tjmac44]

While corrected (correctly) once already (ideal gas law is not very applicable, Bernoulli's principle is the most typical explanation of lift but is only a partial explanation) I will still attempt another input.

It seems to me that the key confusing thing about the question is whether there is motion of air over the wings of the plane. If the air motion is higher than the take off velocity for the airplane, it will fly. If it is slower, it will not.

With respect to the conveyor belt questions, I think the key thing is to note is that the conveyor belt cannot be moving at a constant velocity, as they normally do, and keep the plane stationary. The engines supply thrust which results in acceleration - increasing velocity. A constant velocity conveyor belt would initially move the airplance backward and if the engines were powerful enough and/or the frictional forces low enough, the airplane would ultimately overpower the constant speed conveyor belt. For the plane to remain stationary and lift to not be generated, the conveyor needs to accelerate at the same rate in the opposite direction as the airplane. Moving at the same rate could mean it accelerates at the same rate but I think it is more clear to say it will accelerate at the same rate but opposite direction than to say it moves at the same rate - constant rate movement is what we assume for a conveyor which would allow the airplane to accelerate and overcome the conveyor (possibly what happened on mythbusters).

Jim

Best explaination yet to why the plane will not take off with regards to the original question. Thanks.

[niki]

Let's say that the conveyor has a fence from each side...the fences are connected to the ground...

Install a rocket on your back and roller skates on your feet.

Put your self on the conveyor, hold the fences and turn the conveyor at 500MPH...the rollers of your roller skates will turn at 500MPH. The force that you need to hold yourself from moving back is exactly equal to the roller friction/drag that is very small.

Now, turn ON the rocket on your back....you will fly like a "rocket" on the conveyor...yes, of course the rollers will turn at very high speed but you will be pushed forward like a....rocket...

Now, if you had some wings on your shoulders....

[JimD]

To anybody wanting to understand lift, I recomment NASA's website. They have an explanation that is not real complicated. Bernoulli's equation is correct but not the main source of lift. Bernoulli's tells us that if we increase the velocity of a fluid it's pressure must decrease (assuming the temperature is constant). The main force that generates lift is described by the F=ma. Because the airplane, mainly but not totally the wings, causes the air to accelerate, a Force is generated. Because of the direction of a good bit of the acceleration is downward, an upward force on the airplane is created - thus lift. All of lift is due to the change in air motion as it moves over the airplane. No movement of the air relative to the airplane, no lift.

I haven't been thinking much about the wheels. If the wheels have adequate friction with the conveyor the plane will initially move at the velocity of the conveyor. If the velocity of the conveyor is near constant, little friction is required. To accelerate the plane to the conveyor speed would require much more friction and plane wheels are so tiny there would probably not be enough at first and there would be slippage. When the thrust from the engines accelerates the airplane to a higher velocity that the conveyor (assuming a constant speed conveyor), lift will be generated decreasing the friction. The plane velocity in the forward direction does not have much to do with the wheels turning, however. If the wheels slip, the plane will move quicker forward if the conveyor is trying to drag it in the other direction. The wheels are drug forward by the plane and it doesn't matter a whole lot whether they turn or not. If they do not turn, there is friction drag but I think it is grossly insufficient to stop the plane at full thrust. As lift increases, the wheels will not have enough downward force and will stop turning. This will happen very close to the point at which takeoff occurs.

For the situation where the conveyor accelerates at the same rate as the thrust from the plane engines, the wheels will not turn. The plane will be stationary. This assumes that the friction of the wheels against the conveyor is enough to transmit that force to the airplane to balance the thrust from the engines. Given the size of airplane wheels and engines, it is not reasonable to assume that the conveyor can transmit enough force through the wheels IMHO. That would mean the wheels will slip when the thrust exceeds the friction force and the airplane will move forward.

So if you accept my judgement that the wheels of an airplane (unlike a car) are inadequate to stop forward motion from thrust then it does not matter a lot whether the conveyor is constant velocity or matches the acceleration of the thrust. At the point where the conveyor force cannot be transmitted through friction to the airplane, the plane moves and begins to lift. A way to test this would be to stand on the brakes on the conveyor and try to lift off. I think you could. Cars cannot overpower their brakes but I think airplanes can. Just to be clear, I don't think it's the brakes exactly that get overpowered. I assume that the brake friction is greater than the tire friction with the conveyor or pavement and the tire friction is the limitation.

Whether the plane flies would really seem to be a function of how much thrust it has. If it can take off with the brakes locked then it can overcome the conveyor too.

Jim

[niki]

Hi Jim

I could not understand the last part with the brakes applied. If you are applying the brakes, you are "locking" the airplane to the conveyor and it will never move, even with takeoff thrust.

No body limited the wheel speed (actually the tire speed) because it is a theoretical question. There is no "conveyor runway"...
In the real world the tire speed is limited to 225 MPH and a fully loaded 747 will lift of before this speed is achieved.

The bearing friction and tire drag of an airplane wheel is almost the same like the wheel on your car (a little bit higher because of the size of the tire)

Please read my post above (with the rocket) again and tell me what do you think...but without assuming that the wheels speed is limited or any other assumption...it's a theoretical question and situation.

Normally, the airplane cannot overpower the brakes but this kind of takeoff roll is almost never used because of other reasons (engine surge and slippery runway)...normally, a "rolling takeoff" is used, which means, applying engine thrust (or power) gradually till the max takeoff thrust.

Regards
niki

[OpaDC]

New here and got caught up in this post. Lots of the answers way over my head and finally quit about 3rd page, but have one simple question. If takeoff airspeed is 150 mph (no idea what it really is so picked a number), what happens if the plane lifts off the ground and starts flying? Does it jump to 150+ or crash back to Earth? No physics here, just thinking! I don't think it's going anywhere, but I'm a computer geek, kinda out of my league.