OT - Would it take off?

[Check Six 2]

I have been involved in this discussion before, and have even seen the Mythbusters try to solve the conundrum. Here is the scenario...

 

Would an aircraft taxiing on a giant treadmill take off?

 

The treadmill is stationary. When the AM calls "Contact...Clear" and fires her up, and she starts moving, the treadmill is adjusted to the aircraft's forward speed.

 

At this stage, the aircraft moves forward, but there is no airflow (theoretically) over the wing. The aircraft's wheels aren't driven (like in a car for instance), so its forward motion normally whilst taxiing is merely thrust and airflow from the spinning prop causing it to move forward.

 

This forward motion, when sufficient speed is built up, causes airflow over the wing, causing lift, and away we go. But on a treadmill, there will be no such airflow (assuming there is no wind or it is negligible).

 

The Mythbusters used a model airplane, and it took off, but I don't think the experiment was carried out in ideal conditions. They then towed a large canvas sheet behind a SUV and a pilot in a light airplane taxiied in the opposite direction, and he became airborne.

 

I am still sceptical. In ideal test conditions...would she fly?

[Duce Lewis 3]

Doesn't seem logical

Could the prop wash across the wings created the lift?

[Macklroy 2]

Lift is created by airflow over the surface of the wings. Prop-wash alone would not create enough airflow sufficient enough to lift the plane into the air. Simply, any aircraft has a speed at which it can lift off (while within ground effect) and this speed may be slower than the actual airspeed needed (out of ground effect) to maintain flight. This speed (airspeed) will not be reached if the aircraft is unable to make forward progress (eg. on a tredmill). In short. NO! a plane would NOT become airborn if forward momentum and thus the necessary airflow is not achieved. This ofcourse is simply "in theory".

 

 

The mythbusters project was so flawed it can't be used as a viable source on the subject. (yes i saw the episode and was unimpressed with their tests.)

 

S!

[UK_Widowmaker 571]

My question has to be... "Would Mythbuster's viewing ratings go up, if they didn't do silly test's like this one?"

[Mr. Lucky 0]

The airplane would roll off the treadmill, as the wheels would only be affected by the treadmill and not the airplane itself. As you pointed out, the propulsion of the aircraft is by the propeller into the air, so the wheels are the only component affected by the treadmill. They might squeal when they hit the pavement as they would be turning faster on the treadmill than they would be based on the aircraft's forward motion relative to the ground. Or, if the plane is light enough, could cause it to nose over. As has been pointed out, the lift is strictly generated by airflow over the wing, which can only be achieved by relative motion of the wing to the air, not the wheels to a surface.

 

Using a model airplane was spurious at best as I've seen many with enough of a power to weight ration take off almost instantaneously just by thrust and prop blast on the elevators. I'm sure many have seen the radio controlled version of Snoopy's doghouse flying. The large canvas sheet had a different effect as unlike the treadmill, the ground surface and therefore the aircraft was moved, relative to the wind as well. This had the effect of taking off in a tailwind, which would've increased the takeoff distance. In short, as UK said, it was silly.

[Olham 164]

The only almost vertical take-off with a pison engine propellor craft I ever saw, was on the

ILA Internationale Luftfahrtausstellung Berlin 2006.

They announced, that the wind conditions where right now such, that the pilot of a Fieseler Storch

would try to perform a vertical take-off. And he almost did - the craft did not move more than 2 meters

forward, then it was hanging in the wind like a kite, only 2 meters above the ground, and remaining

over the same point more or less, like a helicopter. But it had airflow through the wind.

If the craft in a giant treadmill would take off, then it would be from the airflow of the propellor.

[Gous 0]

I have seen this episode of Myhtbusters too and I must say that they..uh...screwed up.

.

This video though is very informative. When Adam turns off the car's motor, the car is dragged behind. This has to do with the moment that the friction creates at the wheels. So...my guess is that the plane would take off from the treadmill with no engine power, and with the nose looking at the opposite direction of the car's. Also, a hairdryer blowing towards the wings would help. In my opinion, the rest of the experiment from Mythbusters is wrong. And that's because your goal is to take the plane off the ground with no engine power.

[RAF_Louvert 101]

Interesting discussion, and I agree that the only way this works is due to the airflow over and under the wing and tail surfaces created by the prop wash. I tried something very similar to this years ago with an RC plane I had. My brother and I tethered it to the ground by it's wingtips with very long lengths of monofilament fishing line so that it could not move forward but could lift up. With the size and pitch of the prop and the airflow it generated the plane did indeed climb, despite having no forward movement. However, due to the turbulent nature of the prop wash it was a very uneven and bumpy climb.

 

And Myth Busters can be fun, but I take most of what they do with a very large grain of salt.

 

Cheers!

 

Lou

[HomeBoy 1]

The assertion that an aircraft cannot take off if it is on a treadmill is absolutely a myth! A propeller (jet, rocket engine, or any other thrust producing device) pulls its payload through the air (i.e. an "airscrew") and has no relation to the ground at all really. If this were not true, a helicopter would NEVER get off the ground as that is the extreme example of the "treadmill effect." Now, if the engine is not powerful enough to pull the plane through the air without the benefit of rolling across the ground (e.g. the need to roll down a steep hill to gather speed) then yes, the treadmill effect appears to be legitimate but what's really going on there is the engine is not powerful enough to pull the plane through the air sufficient enough to produce lift and the "rolling across the ground" is needed to assist the engine. In fact, you could secure that airplane with chains into concrete and if the engine is POWERFUL ENOUGH, it would rip itself free or pull the engine off the front of the plane and move through the air! Extreme example obviously but true non-the-less.

 

I saw the episode of Myth Busters as well and thought they could have done a better job with that particular myth. Their conclusion was correct however.

Edited July 8, 2009 by HomeBoy

[RAF_Louvert 101]

Sorry HomeBoy, but I must point out that you are in error Sir. Any aircraft that uses a wing requires lift in order to fly. A helicopter flys because its blades are the airfoils, (i.e. wings), and create their own air flow, and thereby lift, by spinning. Without air flow over and under the wings an airplane, or helicopter, cannot fly.

 

Cheers!

 

Lou

[HomeBoy 1]

Sorry HomeBoy, but I must point out that you are in error Sir. Any aircraft that uses a wing requires lift in order to fly. A helicopter flys because its blades are the airfoils, (i.e. wings), and create their own air flow, and thereby lift, by spinning. Without air flow over and under the wings an airplane, or helicopter, cannot fly.

 

Cheers!

 

Lou

 

Hi Lou,

Ok, I accept that. The helicopter is, based on your point, not the best example. However, you have, with your explanation of the helicopter "wing", proven that an airplane's contact with the ground is irrelevant. You say "A helicopter flys because its blades are the airfoils, (i.e. wings), and create their own air flow." Now, take that airfoil and mount it to the front of a fixed wing aircraft and it will "create it's own air flow" thus pulling the plane forward and create lift on the fixed wings which cause the plane to go airborne. NOWHERE in all of this is the ground EVER a factor. That is of course unless the engine does not have enough power to accomplish this which I previously mentioned.

 

None of this has anything to do with propwash really. It's the "airscrew" nature of a propeller that is the primary issue here. The prop pulls the airplane through the AIR and when enough speed is generated, the wings moving through that air generated from the forward movement will experience lift quite independently from mother earth.

 

Great discussion BTW. This really makes you come to terms with what makes an airplane fly. One of the hardest things when I first started learning to fly was how little of what goes on on the ground matters to the airplane flying through the air.

Edited July 8, 2009 by HomeBoy

[RAF_Louvert 101]

Agreed HomeBoy, it has to move through the air. But in the treadmill example, it is not moving through the air at all and so has no lift, apart from what little it will get from the airflow created by the prop wash across its wings.

 

Cheers!

 

Lou

[HomeBoy 1]

You know, this "treadmill theory" beautifully illustrates the mind-twisting problems student pilots have to overcome when thinking about ground and air. A student really struggles with the idea that an airplane flying at a ground speed of 100 knots flying into a headwind of 110 knots does not fall out of the sky. Well, this is an obvious example but is really exactly the same principle at work here as with the "treadmill theory." Ground speed ONLY matters to us humans when we care about how long it takes to get somewhere. The airplane has no concern at all about the ground. It is flying through the AIR not the GROUND.

 

These concepts get tricky sometimes but principles override what "feels right" every time! It's like that old saying "Speed of light, not just a good idea, it's the law!"

 

Have a great day Lou! Fun discussion.

 

-mark

[Check Six 2]

There's a wonderful film "The Gods Must Be Crazy" (if you haven't seen it...do yourself a favour...seriously funny). In the sequel (The Gods Must Be Crazy II) there is a microaircraft that a zoologist uses to get around in. Tiny thing, as the female lead remarks. The zoologist replies "We have a smaller plane, but we can't find it".

 

Anyway, he takes this female doctor up for a quick aerial safari, and they encounter a wild storm. There is great footage of them actaully flying backwards. The headwind was that strong that they weren't going anywhere (except backwards). They landed in a tree. Very funny to see.

[RAF_Louvert 101]

Always fun to have "what if" discussions such as these HomeBoy. And just so we have it sorted out, my final point is that the airplane will NOT fly as it tootles along on the treadmill. All that is happening in this scenario is that the power of the prop is being used to compensate for the rearward drag that is being created by the wheels on the treadmill, which without the prop spinning would result in the aircraft going backwards off the treadmill. Because their is no forward motion IN THE AIR, (nothing to do here with ground speed at all), the plane cannot lift up off the treadmill.

 

 

Cheers!

 

Lou

[Cameljockey 3]

It would roll off the end of the treadmill and take off, or crash.

[Launchbury 1]

It could NEVER fly on Prop Wash Alone, if it could, there'd be no need of runways, you'd just raise the throttle and Fly, only for Bugs Bunny is such a thing possible.

 

Mmm...Not too sure about that one. It would, of course, take one hell of an engine powering one hell of a prop that has to be wide enough to pass prop wash over the entire length of the wings, WITHOUT running so fast that the tips reach supersonic speeds. I've seen a 172 damn near blown over by the jetblast from a Gulfstream IV (lovely recovery by the Cessna pilot, btw) and that's more or less the same deal. As long as the prop could propel the air over the airfoil at a speed required to generate sufficient lift for flight, well there you have it.

 

Think of it this way, when you're on the run-up pad in your single, and you push her up to 1800 revs for your checks, if you manipulate the elevators you can feel the tail of your aircraft go up and down.

 

Of couse, there's the troubled nature of the air coming from prop wash. It's not nice and smooth like the air you usually slice through, it's curling and buffeting and all around angry. God what a brain-bender.

 

As far as the question of the treadmill goes, it's nigh-on irrelevant. Of course, if the treadmill was moving in the direction opposite to the aircraft's path of travel it would slightly lengthen the takeoff roll because of the slight increase of drag on the wheels (remember, there's no such thing as frictionless movement) and vice-versa. Those people who try and present the treadmill as a way to shrink airports and lower noise pollution are just a pack of fools with no grasp of aeronautics.

[HomeBoy 1]

Always fun to have "what if" discussions such as these HomeBoy. And just so we have it sorted out, my final point is that the airplane will NOT fly as it tootles along on the treadmill. All that is happening in this scenario is that the power of the prop is being used to compensate for the rearward drag that is being created by the wheels on the treadmill, which without the prop spinning would result in the aircraft going backwards off the treadmill. Because their is no forward motion IN THE AIR, (nothing to do here with ground speed at all), the plane cannot lift up off the treadmill.

 

 

Cheers!

 

Lou

 

Perhaps I've misunderstood the original assertion.

 

I agree with your comment here ONLY if we are talking about a PARTICULAR aircraft with a PARTICULAR engine. Maybe that's the point you and others are making. Sorry if I missed that part of the assertion along the way.

 

If you take a 65hp J3 Cub and put it on a giant treadmill and spin the treadmill such that the Cub cannot get off the ground then, yes [mabe, not even sure], I agree with you. BUT, if I put a large enough motor on that Cub, it will overtake the "treadmill effect" and will take off every time! That's the argument I thought we were having. Perhaps I was wrong and I apologize if so.

 

To give credence to this "treadmill theory" would be to say it would be impossible for any airplane to take off UP the side of a mountain, "with" a strong wind instead of into it, off an aircraft carrier that is turned "with the wind" instead of "into it", etc. All of which would actually be valid arguments in the event that this discussion was about a PARTICULAR airplane (example, the J3 Cub may not have enough power to do any one of those things). That was not however the myth that the Myth Busters were investigating and I was arguing along those lines and not this PARTICULAR airplane case.

 

I'm with you. The fun is waning on this discussion so I'm going to bow out too.

 

Nice energetic discussion. Makes us all pull out our aeronautic books and review the basics. Not a bad exercise.

 

<S>!

-mark

[Condor44 0]

Of course it would take off. That it would not is based on the assumption that the treadmill would prevent the plane from moving forward and accelerating to takeoff speed. What are the forces to stop it from doing so?; only the rolling resistance of the tires and wheel bearings (part of which would be a function of the weight of the airplane.) Those resistances would increase on a rolling treadmill since the wheels would need to spin much faster than normal but I doubt that there is an airplane that does not generate sufficient thrust to overcome those forces. Especially since most generate enough thrust to fly much faster than takeoff speed and the drag from the airframe moving through the air as speed increases is much greater than the rolling resistance of the wheels. Probably the best example of an airplane overcoming tremendous resistance to forward motion when taking off is a flying boat or float plane (Think Pan Am clipper). Anyone who has driven a boat appreciates the resistance of a hull through water. Yet the Pan Am clipper and spruce goose took off. I know the resistance to forward motion of a hull or float through water is much greater than the rolling resistance of wheels. If you don't beleive me take a boat up to 30mph and cut the throttle and see how long it coasts as compared to a wheeled vehicle.

[RAF_Louvert 101]

Alright, we are clearly arguing two different ideas here. Of course the plane can take-off from a treadmill, given a long enough treadmill and sufficient forward movement of the plane in relationship to the ambient air, (and the ground under the treadmill), to produce the required lift for it to climb. What I was stating was, that if what the treadmill is suppose to accomplish is to shorten the actual take-off distance the plane needs, as it moves through the ambient air, (and over the ground that is under the treadmill), then THAT is a load of rubbish.

 

All the Myth Busters experiment did was ultimately require that the plane's wheels spin twice as fast as normal before the plane lifted off, which it did in the same amount of distance along the ground that it would have needed anyway.

 

Cheers!

 

Lou

[Olham 164]

Geeze - I'm glad we only have to take off from grass fields!

Carrier landings and take-offs from treadmills came only later...

[Hellshade 110]

Well I don't know anything about if the plane would take off or not if it spent a little time on a treadmill. But I do know that if I spent a little time on a treadmill, the plane I flew in later would probably be able to fly a little faster!

 

Hellshade

[Launchbury 1]

Treadmills are good for the planes. Stops them from getting fat. :P

[Macklroy 2]

Yes, I will amend my previous statement. I was "assuming" that the 'tredmill argument' was that the planes forward motion was canceled out by the tredmill. (aka. NO forward motion) In THAT case then the aircraft would NOT take off. But like many have pointed out, a tredmill would not prevent the aircrafts forward momentum.

 

S!

[Duce Lewis 3]

The treadmill and propeller can can only create horizontal forces on the aircraft

They are in opposite directions, and since the plane is not moving, they must be equal

 

In order to get into the air there must be a vertical force

Propwash over the wing airfoil creates a high pressure zone below & a low pressure zone above - lift

If the lift is greter than the weight on the plane (downward force), the plane will rise vertically

 

I don't think an ordinary Cessina's propwash can generate enough lift to do this

The Mythbusters must have used a very light/high lift plane to accomplish this

 

Who would want such a crate

It'd probably be impossible to land and fly off in a light breeze