Something to think about

[mgh 89]

I explained that the thrust will accelerate the aircraft and so it will be moving relative to the air, including its wings. What generates the the equal and opposite force to the thrust that would prevent the aircraft from moving?

[Guest oyvindhansen]

>It is not a short circuit at all, as wheel speed could be>used to define forward speed while the aircraft is on the>ground.The short circuit is actually believing that the ground going backwards can do anythink to stop the aircraft moving forward. The speed of the wheels therefore become totally irrelevant for determining whether the aircraft can take off or not. The threadmill can move with an arbitrary spedd, forwards or backwards or stand still. The aircraft will need the same takeoff distance in all cases. -

[victorwest2 0]

>I explained that the thrust will accelerate the aircraft and>so it will be moving relative to the air, including its wings.>What generates the the equal and opposite force to the thrust>that would prevent the aircraft from moving? But what if the treadmill matches the speed produced by the thrust of the engines - as it should do to comply with the original question ?

[Guest]

>I think it is a complete short circuit of thought to focus on>the wheels at all. Aircraft can take of with floats or skis or>whatever, the only thing that matters is its velocity relative>to the air.>>-

[Guest]

Correct. In the theoretical example the treadmill will cause the aircraft to stay stationary relative to the earth on which the treadmill is mounted stationary itself, therefore the aircraft will never gain forward speed however much power the engines put out (which will go towards accellerating the wheels or skis which normally would create speed over ground but now will create nothing because the treadmill counteracts that speed).

[Guest oyvindhansen]

>Which will be zero if something counteracts their attempts to>gain such speed...>Something like a treadmill, a strong enough tailwind, or a>very strong current trying to take off upstream in a>floatplane.Tailwind yes, threadmill no. Based on your logic an aircraft shouldn't be able to take off, since it involves rolling over ground fast, the equivalent of rolling over a threadmill going backwards. As is proved in thousands of takeoffs every day, wheel friction is negligible compared to engine thrust. -

[Guest Charlie]

Before going further, I want to point out that is not necessary to use inflammatory terms like "short circuit of thought" in this debate. It is an interesting discussion with many thoughtful comments from both points of view. We don't need it dragged down with that kind of comment.You, sir, continue to present factually incorrect information. The treadmill moving backward CAN and DOES do SOMETHING to impede the aircraft

[mgh 89]

You didn't answer my question. Newton's 2nd Law is quite clear; Acceleration = Force divided by Mass. If there there is a thrust, T, then the aircraft must accelerate and thus increase speed unless there is an equal and opposite force equivalent to minus T. In the scenario envisaged where does this force come from?

[Guest Adverse Yawn]

R1200T,I hope you're happy. Look at the kefuffle you've caused :)

[Guest oyvindhansen]

"Short circuit of thought" is actually a polite term for my opinion of some of the statements made in this thread. Maybe you find the discussion informed, I do not.Please read my reply to jwenting about friction and thrust. For all practical purposes the friction force is small. Rolling friction (like all dynamic friction) is fairly constant with speed, it is air resistance, together with high temperature in rubber and bearings that eventually limits maximum speed.>"The aircraft will need the same takeoff distance in all cases."Please do not nitpick! An aircraft catapult is an additional propulsive force that increases the aircraft's acceleration. It has nothing to do with the discussion at hand, where the aircraft accelerates by means of its own engines.-

[victorwest2 0]

>You didn't answer my question.> >Newton's 2nd Law is quite clear; Acceleration = Force divided>by Mass. If there there is a thrust, T, then the aircraft>must accelerate and thus increase speed unless there is>an equal and opposite force equivalent to minus T. In the>scenario envisaged where does this force come from?Well I'm not disputing that the aircraft accelerates. However, if the treadmill accelerates at an equal amount to the aircraft then surely the position of the aircraft on the tradmill cannot change. If the position were to change then there is not an equal level of acceleration between the treadmill and the aircraft. For take off does the aircraft not have to move faster forward then the treadmill is moving back?

[Guest Adverse Yawn]

>>For take off does the aircraft not have to move faster forward then the >>treadmill is moving back? That's exactly what Mgh and I have been writing :) But think carefully by what you mean as faster. There are four relevant speeds: 1) Wheel rotation 2) Treadmill speed 3) Air speed 4) Ground speedThe airspeed is solely dictated by the amount of forward force (thust) minus the total backward force (drag and friction) realtive to the air. You already know that thrust is more than drag and friction or else airplanes couldn't accelerate down the runway and take off in normal conditions. Treadmill speed is tied to aircraft speed, but aircraft speed is realtive to the air, so there is no speed unless the aircraft is moving forwards through the air. If the aircraft if travelling at 65kts, then the treadmill is moving at 65kts backwards.Wheel rotation speed is the sum of the aircraft's Ground Speed (speed relative to the treadmill as a whole) + Treadmill Speed. As we should sensibly assume mean seal level and no wind then TAS = IAS = Ground Speed. So the wheel rotation speed is The aircraft's speed + treadmill speed, which is the same as saying IAS * 2.Not forgetting of course, that is there is no airflow, there is no airspeed, if there is no airspeed, the treadmill remains stationary!

[victorwest2 0]

>>>For take off does the aircraft not have to move faster>forward then the >>>treadmill is moving back? >>That's exactly what Mgh and I have been writing :) But think>carefully by what you mean as faster. There are four relevant>speeds: 1) Wheel rotation 2) Treadmill speed 3) Air speed 4)>Ground speed>>The airspeed is solely dictated by the amount of forward force>(thust) minus the total backward force (drag and friction)>realtive to the air. You already know that thrust is more than>drag and friction or else airplanes couldn't accelerate down>the runway and take off in normal conditions. >>Treadmill speed is tied to aircraft speed, but aircraft speed>is realtive to the air, so there is no speed unless the>aircraft is moving forwards through the air. If the aircraft>if travelling at 65kts, then the treadmill is moving at 65kts>backwards.>>Wheel rotation speed is the sum of the aircraft's Ground Speed>(speed relative to the treadmill as a whole) + Treadmill>Speed. As we should sensibly assume mean seal level and no>wind then TAS = IAS = Ground Speed. So the wheel rotation>speed is The aircraft's speed + treadmill speed, which is the>same as saying IAS * 2.>>Not forgetting of course, that is there is no airflow, there>is no airspeed, if there is no airspeed, the treadmill remains>stationary!>>>The original question does not state airspeed,treadmill speed or wheel rotation speed. It purely states "this treadmill could match the forward speed of your aircraft".

[kerosene31 349]

>I explained that the thrust will accelerate the aircraft and>so it will be moving relative to the air, including its wings.>What generates the the equal and opposite force to the thrust>that would prevent the aircraft from moving? The treadmill. You guys are making this too complicated. Airspeed is the only important variable.

[Guest Adverse Yawn]

And the forward speed of the aircraft is its airspeed. Aircraft don't measure speed in any other way. The other speeds come into play as a result.