Physics Question for you.....

[Stytooner]

I knew I felt wrong about going against Loring. I should have known better.
All you would need is a flight plan.
I was thinking literally, but it is a physics question.

Sorry to have drug this on so long. I was wrong.
This plane will fly. I suddenly feel so enlightened.

The wording in the original statement is the culprit though.
The conveyor could be running ten times faster than takeoff speed in reverse and the plane would indeed move forward. It's free wheeling casters are what got me.

I actually figured this out on my belt sander.

Thanks for the mind work out guys.

[jerrye]

Is this making sense?

The conveyor moves only in relation to the plane's speed, so how fast will the plane be able to move? Once it moves 1 MPH, the conveyor will counter, moving at 1 MPH opposite, bringing the plane's speed down to 0 MPH, which will then make the conveyor 0 MPH. In this instance I don't see how the plane could fly unless it had sufficient acceleration to overcome the time lag between the conveyor sensor reading the plane's speed and the conveyor coming up to that speed. Nonetheless, it would be a very jerky ride!

[niki]

Hi jerry

Lets make an experiment.
Take your car, remove the engine and the gear so the wheels can rotate free forward or back.

Put your car on the conveyer. Put some barrier behind the car just not to let it go backward, the barrier will be anchored to the ground, not to the conveyer.

Turn the conveyer to 500 MPH.
Because the wheels can move free, they will rotate at 500 MPH but the force backward will be only the tire drag + bearing friction, I think that you can stop the car from moving backward only with your hand (lets say that you need 100 lbs to stop the car moving back)

Now, on the roof of you car, install an engine with propeller or jet engine or a rocket than can produce 10,000 lbs of thrust (it will not have any connection to the car wheels that still can move free).

Go back on the conveyer, turn it to 500 MPH, get into the car and start the engine/jet/rocket to maximum power.

To overcome the tire and bearing friction you need only 100 lbs, the rest 9,900 lbs are pushing the car forward off the barrier. Of course the wheels speed will increase but you are pushed and accelerated forward.

niki

[Stytooner]

The basic way I had the AHAH moment was to think about landing on a moving coveyor belt. Actually my belt sander. You can land on one, so you can do a touch and go or you can take off from a dead start on one.

[LCHIEN]

The conveyor moves only in relation to the plane's speed, so how fast will the plane be able to move? Once it moves 1 MPH, the conveyor will counter, moving at 1 MPH opposite, bringing the plane's speed down to 0 MPH, which will then make the conveyor 0 MPH. In this instance I don't see how the plane could fly unless it had sufficient acceleration to overcome the time lag between the conveyor sensor reading the plane's speed and the conveyor coming up to that speed. Nonetheless, it would be a very jerky ride!

Once it moves 1 MPH, the conveyor will counter, moving at 1 MPH opposite, bringing the plane's speed down to 0 MPH,

That's where you are wrong, the conveyor will move at 1 MPH but the airplane won't slow, just the wheels will spin faster.

[leehljp]

The basic way I had the AHAH moment was to think about landing on a moving coveyor belt. Actually my belt sander. You can land on one, so you can do a touch and go or you can take off from a dead start on one.

Hey, you can't do that! it has to have airspeed over the wings or it will fall or stall. Good thing the Wright Brothers didn't know about this on their first plane that had skids instead of wheels. It still took airspeed over the wings for lift.

Once the wheels on a 747 reaches 200 MPH on a conveyor going 200 MPH in the opposite direction, there still is NO lift from the 0 mph wind going over the wings.

Boats, river current and wheel speed are irrelevent to wind speed lift over the wings.

Now as to your belt sander - you can take off and land on it provided the belt sander is also moving (not just the belt) at the speed at which lift takes place.

On the conveyor, everyone is looking at ground speed. They need to be looking a WIND speed. Wind speed is what makes a plane or glider fly, not wheel speed.

[Stytooner]

Actually I was studying speed too much. I should have been thinking about the free wheeling castors on the plane. The guys that said it would fly are correct.

The coveyor belt could be moving forward or backwards and still have no effect on the jet itself because the wheels are freewheeling.

Consider when a plane lands, it goes from the wheels not turning at all to intantly rotating over a couple hundred MPH depending on the plane.
No problem. The plane and its engines operate copletely separate from the landing gear no matter how fast or what direction the gear is traveling. The conveyor is only acting on the gear and not the plane itself.

[niki]

I scanned this page from the book.
It shows the forces acting on the airplane during the takeoff roll.

As you can see the only force acting against (or opposing) the engine thrust is the "rolling friction" (axcepr the aerodinamic drag) and it's decreasing as the takeoff speed increases.

[mpc]

In an aircraft, air speed and ground speed ARE different most of the time, differing by any winds present. For example, a 10 mph headwind will make airspeed 10 knots HIGHER than ground speed. Which is why airplanes generally pick runways aligned with the wind (or have the aircraft carrier aim into the wind) - the headwind is "free" energy. Takeoff distances need to accumulate enough air speed to get enough airflow over the wings to make enough lift to counter the aircraft weight. If the headwind is really strong the ground roll distance can be very low - and the ground speed will be low too. Thus F=ma doesn't need to integrate up long to get sufficent ground speed (velocity). For landing the situation is similar: headwinds combine with ground speed to create airspeed and wing lift. When the brakes are applied they have to overcome only the ground speed of the aircraft... so landing INTO a headwind lowers the ground speed, so stopping distances are shorter. If you watch small private airplanes, that land at 50 to 60mph typically, strong headwinds make them appear to be flying really slowly or almost hovering.

Now on any multi-engined commercial airliner the FAA regulations mandate that it be able to complete a takeoff and climb upwards (at some minimum rate) with an engine completely failed. A failed engine not only doesn't produce thrust... it actually makes a lot of extra drag as it "windmills" in the airflow. Somebody posted a page back wondering if there would be enough engine thrust to overcome all the drag added by the conveyor belt spinning the wheels. Yes, plenty. A two-engined airplane has more than 2 times the thrust it needs to takeoff actually (since it has to fly on just 1 engine); a 3-engined airplane has 1.5 times what it needs. A 4 engined plane can survive TWO engines crapping out and still takeoff. Only private airplanes and some biz jets have wussy engines; commercial airplanes (jet or turboprops) have lots of excess power when all engines are running. Military aircraft are another matter completely; most can barely stay airborn if an engine dies - and they won't be able to take off with a dead engine. Military requirements are totally different than commercial airliner requirements... after all, military pilots can eject and parachute to safety. United Airlines passengers can't.

No matter what that conveyer belt operator does the aircraft will barely notice the effect. The wheels will spin like gangbusters but the aerodynamics don't care a whit about that. The engines will easily overcome the extra rolling friction and the whole aircraft will accelerate relative to some fixed point (or that pole)... accumulating real air speed and it will be able to take off. Even if the belt was moving 100mph backwards (taking the airplane with it due to that rolling drag) BEFORE the pilot advanced the throttles the aircraft would still be able to overcome that and eventually lift off. The REAL ground speed (measured relative to the pole, not to the stupid conveyor belt) and the air speed will be the same; the ground speed experienced by the wheels will be totally different.

Niki - that belt sander & toy car analogy was terrific.

mpc
Level 5 Engineer - Senior Principal Engineer/Scientist
Aerodynamics, Stability & Control, Flying Qualities, and Simulation
Boeing (formerly McDonnell-Douglas)

[leehljp]

MPC, I am glad you are here.

But you missed some points and added things that were not in the original statement.

Original statement: This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).


No matter what that conveyer belt operator does the aircraft will barely notice the effect. The wheels will spin like gangbusters but the aerodynamics don't care a whit about that.

This is correct.


The engines will easily overcome the extra rolling friction and the whole aircraft will accelerate relative to some fixed point (or that pole)... accumulating real air speed and it will be able to take off.

This is totally incorrect. According to the original statement, if the plane were to accellerate . . .say to 500 MPH on a conveyor going 500 MPH in opposite direction, its resulting airspeed (and relative ground speed would still be ZERO.
Original statement in different format:Conveyor matches the speed of the Airplane. The plane cannot accellerate faster than the conveyor according to the statement. The logical conclusion is that relative to the ground it will not accumiliate any speed, air or ground, only to the conveyor.

I do not understand people's statement that at some point it will accellerate beyond the conveyor when it is stated in a way that it cannot. Without forward air speed (or a typhoon hitting it head on) it will not fly.


IF, per chance, it did accellerate faster than the conveyor to get the Bernouillis rolling over the wing, of course it would, but the original statement is contrary to this.


Even if the belt was moving 100mph backwards (taking the airplane with it due to that rolling drag) BEFORE the pilot advanced the throttles the aircraft would still be able to overcome that and eventually lift off.

AHHH you are going against the original statement here. You and others are assuming that you can get some extra speed up relative to the air or ground but his statement says othewise. Mentally, I think some of you are trying to "help" it into the air. Wish that that could be so.



The REAL ground speed (measured relative to the pole, not to the stupid conveyor belt) and the air speed will be the same; the ground speed experienced by the wheels will be totally different.

Yes, the REAL ground speed and airspeed will be "0" according to the way the original statement is made.

mpc
Level 5 Engineer - Senior Principal Engineer/Scientist
Aerodynamics, Stability & Control, Flying Qualities, and Si

[Tom Slick]

assuming air speed is 0, the plane doesn't fly, period. relative ground speed can be mach 1 and it still won't fly. it may bounce around but with no airflow over the wings it's not going to be in controlled flight.

It would be the same as (previously mentioned) it sitting still with it's brakes on.

[JTimmons]

The question is:

Will the plane take off or not? Will it be able to run up and take off?

No!

71 replies now counting mine, surely someone has gotten the right answer. Russianwolf, care to end the madness?

Edit: Missed reply #44 completely, just read it, thanks to Ray!

[leehljp]

If IF those who think that it will fly by virture of enough thrust to break it free, then it would have to be a thrust factor to the extent it would not need wings. At that point, all flight would be as a result of thurst force and thrust direction, not wings.
IN this case,
1. we are not talking about an airplane (except maybe a Harrier) and
2. if it had that kind of thrust it would not need to "roll" to begin with.

If rolling forward were needed for flight speed, then it would never happen if the conveyor matched the planes wheel speed.

[mpc]

No air speed = no flight, everybody agrees on that. But I don't agree with the conclusion that if the belt is moving opposite what the airplane thinks it's doing that the actual air speed is zero.

Original statement: This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Okay, the belt is going gangbusters backwards. So why does that make the airplane NOT move when the engines push it forwards? Niki's little car tugged by a string while it rides on a belt sander is the same thing. The string wins. The car's wheels, like the airplane's wheels, are spinning way faster than the rest of the car is traveling but that does not mean car speed relative to the fixed pole is zero.

The belt can do anything it (or it's controller) darn well pleases; because there's no rigid/strong connection between the belt and aircraft body center of gravity it can't cancel the body speed. There will be ground speed relative to a fixed pole, and there will be air speed. If the engines are off, and the airplane's parking brake is set (yes, they really have such a thing) then whatever the belt does the plane will do - the wheels are now "locked" to the belt. If the engines are off, and the brakes are off, for the most part the airplane will ignore what the belt does... only that 15% rolling friction/drag term will affect it... so the airplane will move, slowly accelerating until it's moving at the same speed as the belt.. but it will not accelerate at the same pace as the belt. Just like yanking the tablecloth out from under the dishes... do it slowly, the dishes move with the tablecloth. Tug it a bit more quickly and they sort-of move, sort-of stay put - they do not move as fast as the tablecloth because you've broken the "static friction" and turned it into the much smaller "dynamic friction." The "rigid connection" of static friction is gone... only a smaller term remains. Yank the tablecloth really fast and there isn't enough time for the dinky dynamic friction to do anything to the dishes - they barely move. The rolling wheels of the airplane are dinky friction relative to the mass of the airplane. The drag force = 15% of the weight (approximately, and it's fairly independent of speed too) so the aircraft could accelerate backwards at 0.15g's maximum; if the belt accelerated at 0.3g's backwards the airplane would only do 0.15g's backwards still. That's all the force that can be transmitted from the belt, through the wheels, to the center of gravity of the airplane.

Now turn the engines on... that 15% drag is small compared to engine forces acting on the airplane - the engines will win, accelerating the aircraft forwards. A 130,000 pound MD90 aircraft has 2 engines with 28,000 pounds of thrust each. That's 130,000 pounds responding to 56,000 pounds of thrust... acceleration is 0.43 g's; well over the 0.15g's the belt can pull it backwards. (yes, I know I'm using "pounds weight" instead of "pounds mass" in the F=ma equation, but fortunately both F and m need the same correction factor so it cancels anyway). As the aircraft builds airspeed, rolling drag will decrease (as the wings lift some of the weight off the gear) and aeroydnamic drag will increase: the basic "profile drag" (drag of shoving something through air) and the "induced drag" (drag due to making lift). Engines will still win though. Why? Because another aerodynamic/performance equation basically boils down to Climb Rate = (Thrust - Drag)/Weight. We know the aircraft can climb with one engine conked out (cutting Thrust a lot)... so Thrust has to be greater than drag (with a konked out engine) to keep Climb Rate positive. With all engines operating, Thrust is waaayy bigger than drag. More than 0.15g's worth.

I doubt the original intent of the question included the 15% drag anyway; like most folks I'm sure it intended zero rolling drag. I'm putting too much "reality" into the answer. Without it though the aircraft has even more reason to overcome the belt and take off.

Niki's string on his toy car represents the airplane engine thrust. All it takes is enough thrust to overcome the rolling friction... which the airplane has in spades. So do you when you tug on the string. You can make the car move forwards regardless of the sanding belt.

Oh, one other clarification based on a prior post: for most commercial jetliners, the job of the engines is not to draw/blow air over the wings; in fact the designers try to lessen this effect. The engines just create forward thrust to overcome drag to make the aircraft mass accelerate to some velocity... enough velocity relative to the air to generate enough lift over the wings to fly. Unless it's done very carefully, engine airflow actually screws up wing airflow more than it helps.

Some airplanes do use "augmented lift" techniques - military "STOL" aircraft (Short Takeoff or Landing), "blown flaps" airplanes like the C-17 heavy lifter, etc. This type of technology though is risky for commercial airliners - if an engine dies, the augmented lift on that wing would drop to nothing... but the other engine+wing would still be making loads of augmented lift. The aircraft would roll over - rolling towards the dead engine - which is a challenge for pilots to control. The FAA and NTSB don't like the phrase "challenge to control" when passengers might be on the airplane. Prop airplanes do get some augmented lift from the propwash but it's not huge and there are other things added to help the pilot if the engine does fail. The FAA rules for performance, on commercial airplanes, do not allow engine thrust induced lift to be used in performance computations either - all aircraft capabilities are computed based on the "bare wing" lift only... so the prop airplane doesn't get credit for the propwash induced lift anyway. Many turboprop airplanes "cross shaft" the engines too - the engines are connected so if one dies, the other engine still turns both props to eliminate the asymmetric lift and roll problem. Jet engines induce so little flow over the wings (by design - it's a lot of effort to position jet engines actually, to design the pods and pylons, etc.) that the asymmetric roll is not a challenge.

mpc

man I wish I'd kept my fingers shut. I'm just going to annoy somebody and that's not my desire. I'm going to (try to) refrain from any more posts in this thread.

[niki]

Thank you Mpc
Everything that you said is totally correct except that the last time (that I know about) that a B-747 lost two engines during takeoff, it could not hold itself in the air and they crashed (EL-AL, cargo at Amsterdam) but maybe there were also other factors that we don't know.

The problem to understand the question and the answer is because most of the people thinks about airplanes in comparison to cars.

They are two totally different animals.

The car is using its engine to power (rotate) the wheels and by that advancing the car forward (or back).

Airplanes are using the engine(s) just to push it forward (in some cases also back). It's not an "engine" in normal terms but it is a "Propulsion Unit" meaning, it produces work by "Action and Re-action".
The "Action" is accelerating mass of air. The "Reaction" is, push force created in the opposite direction to the "Action".

I gave an example in reply #57 (Page 6) and reply #63 (page 7), I will try different approach:

Put the airplane on the conveyer. Hang the airplane with hot air balloon so the wheels are lifted to 1" above the conveyer. To emphasize, the airplane tires does not touch the conveyer but are 1" above it.

Now, start the airplane "Propulsion unit" or "Power Plant" (if you want, call it an engine).
Push the "Thrust lever" (jet) or "Throttle" (piston) to maximum power and see what will happen.

The airplane will accelerate forward gaining airspeed, and at some point will liftoff. The tires did not rotate and the airplane liftoff.

Let's imagine, just for the experiment, that at some point during the forward acceleration, you are going under the airplane and start to rotate the tires. Will it stop or reduce the forward acceleration of the plane? of course not, because the wheels are separate unit from the plane and are connected to the plane through bearings and if you rotate them or not, it does not have any influence on the airplane or the Propulsion unit(s).

Now lets make the same experiment like above, but without the hot air balloon, the airplane will sit with the wheels on the conveyer.
What dragging or friction or stopping force is added now? only the tires friction on the conveyer and the tire bearing friction of the wheel. That's the ONLY parameter that changed.

And as Mpc stated, we shall need only small additional Propulsion (or thrust) to overcome this additional drag of the tires and bearing friction.

Just to visualize how much extra thrust we need to overcome the tires and bearings drag, go to your car, gear Neutral and brake Off and push it...
Call a few gays more and you can push also the 747...

niki