Space and Science BS Thread

It is important to compare the amount of power yielded from the motor vs. the power transmitted through the bearings in the wheels. This difference in power transmission is the key distinction. Of course there is some power transfer through the bearings, but it is much more insignificant than that from say pontoons in water.

In my world, the way to think about this sort of system is via energy and power transmission, which are convenient sets of units to evaluate relationships without having to work out the individual interactions. Wheels on bearings simply don't transmit much power, so the overall effect on the system is minimal.

That's all I have to say on it. I cede the floor back to you gentlemen.

 

The wheels and bearings don’t even need to spin, it’s no different than the pontoons in this theoretical case. Both are simply the means for the movement of the surface underneath the plane to impart motion to the plane and as a result create an airflow over the lifting surfaces.

 

One last thought. A similar thought experiment would be to stand on a skateboard and propel yourself with a high powered yard blower. How much would your acceleration be affected by locating this system on a moving conveyor belt at an airport, say SFO or Pheonix where the conveyor belts are long and presumably open for testing. Your maximum speed is governed by balancing the force imparted in the system by the blower vs the air resistance generated by your forward motion. Since the air resistance goes with the square of your forward speed, the balance readily at some speed. The frictional losses in the skateboard wheels also contribute to slowing you down in this setup. But because the bearings are designed to be low friction, the power from the blower and your air resistance govern the system and the frictional energy losses in the bearings contribute only minimally.

This is a pretty good analogy to what is happening with the airplane.

 

If I’m on a moving conveyer belt on a skateboard I’m now moving at the speed of the conveyer plus the speed imparted by my sweet jet blower motor.

 

The thing to do here is compare energy losses in the bearings vs energy required to accelerate the mass of the system. Bearings obviously have a static and dynamic component and if you don't exceed the static component the wheels will not spin. This relates the the comparison of the energy required to move the body vs. the energy relationships in the bearings.

If what you are saying is the plane will accelerate up to some speed even with wheels if you accelerate it fast enough, this is true. But the coupling of power (energy/time) via this mechanism is minimal vs. the power of the motor. Upon motor startup the momentum and KE of the moving plane would have to react to the engine power, but after it does, the moving surface has nearly no effect because the power transfer is so small vs the engine output.

/done

 

I totally disagree. You as a moving system have mass and the change in speed of that mass is affected by the blower, air resistance, and power transfer through the bearings. The bearings have minimal contribution.

 

I think we’re looking at this two different ways. In my thought game the conveyor is moving with the direction of the airplane, not against it like a treadmill. That might be where your confusion comes from.

 

How? Consider the moving sidewalk style conveyors at airports - standing still on one you are moving across the ground at a set speed. You can increase that speed across the ground by walking in the same direction as the moving sidewalk. If you were on a skateboard you wouldn’t just sit still with the wheels spinning between the board and the conveyor, you’d move down the conveyor at whatever speed it is moving. Add additional propulsion from the leaf blower and your speed increases.

 

Direction doesn't matter. Energy transmission has to balance. Power transfer through the bearings, external propulsion, and air resistance have to balance. Add in the momentum and KE effects, write the equations out and solve based on some initial condition. Whatever the case, air resistance and propulsion devices dominate power transfer through the wheels as long as the propulsion device is not insignificant. Write out the free body diagram and this is how you solve such a problem.

 

Direction absolutely matters when discussing how an aircraft would takeoff. Wings generate lift from airflow moving over them in one direction. Moving backwards at 50kts would generate 50kts of ground speed and apparent wind but do nothing to provide lift. Change the direction of movement (and therefore apparent wind) and now the aircraft will lift off the ground.

I still think there’s some misunderstanding here. At no point is energy transferring from the conveyor to the aircraft, the energy to lift off is created by the apparent wind caused by the aircraft being moved across the ground by that conveyor.

It’s not much different than when I land on a ship. My aircraft is now stationary relative to the surface it’s sitting on but still moving forward at the speed of the ship. Sitting still I have a 20kt airspeed.

 

Sorry I was referring to the direction of the moving belt. It doesn't matter in comparison to air resistance and propulsion devices if you interface it via wheels with bearings. Sure you can drag a plane on wheels with a moving runway if you wait a long time for it to speed up via power transfer through the bearings. But this effect is dominated by air resistance and propulsion.

Sounds like you might be a pilot. I'm not a pilot but I have flown a 172 for around 30 mins total and landed once. More importantly I'm a pretty experienced engineer in such things and have objects on other planets, which is why I read through here sometimes. I think we think about things very differently, and I'm prepared to accept that.

 

This really is the kind of conversation best over beers and with a white board or pen and paper to sketch stuff out.

 

In reality it would be absurdly difficult to keep the plane motionless with respect to the tarmac. It’s going to takeoff regardless of the conveyor system. The “trick” of saying the conveyor moves in reverse at the same speed but opposite the plane is a verbal ploy that has no real meaning. It could travel in the same direction as the plane and have hardly any different effect.

 

" At no point is energy transferring from the conveyor to the aircraft"
This is not true (bearings transfer some energy/time, i.e. Power), but what you are claiming here is the moving runway absolutely has no effect.

 

Go back to the example of the airport moving sidewalk but make it huge, fast and put a plane on it. The plane’s wheels are not moving yet the plane is moving across the ground and therefore creating an apparent wind flow over the wings. The conveyor moving the plane is transferring no lifting or accelerative force to the plane but the wings are creating lift due to air flowing over them.

 

Yup, it is confirmed, y'all making it way harder than it needs to be.

 

That’s the only way to do it.

 

Agreed. @crazysccrmd, happy to continue in PMs.

 

OK no doubt everybody wants to see calcs so here is the quickest back-of-the-envelope calc I could think of (and yeah I am out after posting this):

A typical wheel bearing has frictional coefficient of approx. 0.001. A loaded Cessna 172 weighs 2000 lbs. Wheel bearings divide load and not accounting for friction vs. load effects, the number of bearings cancels. Given these generalities, the force imparted by spinning the wheels with a moving tarmac is 0.001*2000 lbs = 2 lbs. That is, by spinning the wheels, you can push the plane with a force of two pounds. You can take this further to include rotational inertia effects of the wheels but this disappears at steady state.

A 172 engine produces around 470 lbs thrust. Thus the engine dominates the wheels by a factor of 235 which is why I was saying the wheels are negligible.

At what speed do you think the drag on the airplane reaches two pounds? This is the maximum speed the wheels on the moving tarmac can drag the plane. We can calculate this sometime, but the speed is low since drag scales with the square of speed. I maintain you cannot get a 172 off the ground with 2 lbs force, and I rest my case.

Edit: for a Cessna Cardinal (see: https://ntrs.nasa.gov/api/citations/19740006639/downloads/19740006639.pdf, page 16), approx. 55 HP is required to move the plane at 60 MPH. This is easy to calc so here it is:

(55 HP x (550 ft-lbs/sec)/HP)/(5280 ft/mi * 60 MPH * 1 Hr/3600 sec) = 344 lbs .

It takes 344 lbs force for a Cessna Cardinal to reach 60 MPH which is approx. min takeoff speed. The force imparted by the wheels is 2 lbs, which is 0.6% of the required force to take off. It doesn't take off, the speed it attains is nowhere near takeoff speed.

A fast way to find the speed it attains is fit a 2nd order equation through the points 0,0 and 344 lbs (y-axis) vs 60 MPH (x-axis). Then follow this curve down from 60 MPH at 344 lbs to whatever speed corresponds to 2 lbs. The speed is much closer to 0 MPH vs. takeoff speed.


Edit2: Here's the plot, the moving runway drags the plane less than 5 MPH.

 

Shuttle Enterprise:
https://twitter.com/nasahistory/status/1690370054485921793?s=46