Why is towing something easier than carrying it?

Towing something IS easier than carrying it, right?

Trucks’ towing capacities are always higher than their carrying capacities. Or is this because of suspension issues?

I was surprised in a show about the Ansari X Prize, where one team meant to tow their spaceship to high altitude and mentioned that it was easier to tow than to carry it. I didn’t expect this to be true on the air.

Can I get a primer on the principles involved? (simple terms good)

For a car, it’s a matter of sprung weight. (Sprung weight is that that is supported by the suspension).

Picture using a wheelbarrow. You can load it up with amazing amounts of weight, and move it along nicely, as the wheel holds the weight, but you simply need to provide the motion. Try picking up the weight inside of the bucket, and you won’t be able to lift nearly what you can with the wheelbarrow.

For an aircraft, the lead plane provides the propulsion, but the trailing plane also provides it’s own lift, due to it’s wings. If it was ‘strapped on’ to the carrier plane, the carrier’s wings must provide the lift.

Won’t the carried plane’s wings also provide lift? (assuming there is a pilot in it making sure the control surfaces are not doing anything stupid)

And for the car, is it then a matter of suspension? Is it then not that it cannot MOVE the load, but that it cannot STAND the load?

ETA: Or in other words, suspension aside, if the engine can tow it, it can also carry it?

I can’t speak as to towing/carrying aircraft, but you more or less got it with trucks. Carrying something that weighs, say, a ton will put a ton of additional stress almost entirely on the rear axle, leaf springs, shock absorbers, dampers, wheel bearings and so on.

If the load was being borne in the middle of the truck it wouldn’t be that bad, since the load would be equally distributed between the front and rear.

Now, if you stick something in a trailer, the trailer suspension takes the majority of the load.

It’s not as though the truck can pull the load more easily if it’s in a trailer; quite the opposite. Adding the trailer wheels to the equation increases total rolling resistance and adds the weight of the trailer.

Carrying and moving are different things altogether. One is supporting the weight/mass of the item, the other is proving the power to get the thing in motion. For a thought experiment, imagine a block of something weighing a hundred tons but sitting on a perfectly frictionless surface. If you had a good foothold, you could push the thing and eventually get it moving. Picking it up, no way. The closest you can get in real life is probably something like a canal boat - the water is taking the weight of the barge, all the horse has to do is get it in motion. Put the boat on the horse, no motion (and no more horse).

Towing - something (usually the wheels/wings of whatever you are pulling) are supporting the weight, you are simply pushing it about.

We know F = ma*. At rest, the only forces are in the vertical plane,holding the object up (F = m * g). To move it, a force must be applied in the horizontal plane. That force must just be F = (m * g)sin(theta)* mu, where (m*g) is the normal force, and mu is the static friction coefficient. This number is usually less than one, hence pushing an object takes less force than holding it up, and even less force than holding it up and moving it. It is interesting to note that there is also a coefficient of kinetic (moving) friction, k. This number is always lower than the static friction. This, along with momentum, is why it’s easier to -keep- an object in motion than to start it moving.

On another note, Sapo’s back!!!

*All good physics problems start like this.

:wink: Yes I am.

So in summary, an engine can carry as much as it can tow, as long as the suspension holds. Right?

Now let’s go to the airplane. How is towing a plane easier than piggybacking it?

Besides the logistics of putting it up there before takeoff and the relative simplicity of a towing hook versus a stand (both serious concerns, I would guess), that is.

It seems possible there might be a draft effect where the towed plane encounters lower air resistance, which would undoubtedly help fuel economy if nothing else.

If you include the weight of the trailer, an engine can carry more than it can tow.

The answer is going to have a lot to do with how it’s towed. If you put it into a wheeled vehicle and tow it along a reasonably smooth road, the force needed to tow it will be a small fraction of the combined trailer-payload weight (probably no more than a few percent). But if you tie a rope around a 50-lb sack of concrete and try to tow it through a gravel pit, you’ll soon find that carrying it is easier.

As has been noted, up to some loading limit you can carry the load on the towing vehicle and get the same benefit conferred by wheels, without having to incur the penalty that the weight of the trailer represents. Beyong that limit, the trailer is the way to go. Of course, there are often volume as well as weight constraints that dictate the use of a trailer.

For an extreme case, look at a train. Here, the force required to move a towed vehicle (train car) is a tiny fraction of its weight (on a level grade, probably one part in several hundred). So an engine can tow a train loaded with freight massively larger and of enormously more volume than it could carry.

It will depend on the plane being towed. Much the most common form of this is when a glider (sailplane) is launched by a single-engine tug.

A typical towplane will have a lift-to-drag ratio around 12. If you could somehow stick a 720-lb glider inside, this would increse the drag in straight-and-level flight by 60 lbs (720/12). But that glider probably has an L/D well above 30. So in tow, it’s adding less than 24 lbs of drag. With careful design, “piggybacking” the glider could be more efficient than carrying it inside, but it would be very challenging to make this as efficient as towing.

Not really. Some benefit from the airflow around the towing plane has been shown to be possible, but very little practical use has ever been made of this.

Thanks, that was the most helpful answer to me. Two question:

  1. Would this apply to flying something through air into orbit, when (I think) the friction would exist in all dimensions?

  2. Why is mu usually less than one?

*All good physics problems start like this.

The train is a great example.

For the sake of evening out the field, let’s assume a flat level surface, and that carrying the load also implies carrying the trailer (or that the trailer doesn’t weigh anything)

Once we add enough wheels to distribute the weight of the load, then there is no difference between towing and carrying. Is an 18-wheeler towing or carrying? What if it is a fixed bed instead of a hinged trailer?

Once you factor out suspension, friction and the weight of the equipment, all is left is an engine moving a load and it makes no difference how they are linked. Or am I missing some other factor?