Hensley Arrow: How does it REALLY work?

Willald wrote:
I did read through all your numbers, Ron, and most of it did sound accurate. However, I think that given all the wind resistance a trailer has, tire friction, weight, etc., the 'pulling' affect we've described, MUST account for considerably more than a .07% or 1# increase/improvement. I don't pretend to be able to prove such, though, except to say the following:

Think about when you're pulling your trailer down the road, all the pulling/resistance it puts on the tow vehicle. Enough, to typically drop your mileage in half! Imagine how much force it'd take, for you to pull the trailer forward .5" closer to the truck (assuming there was enough 'slack' in your hitch to allow such). That would take a LOT of force, seems like a good bit more than any 1# or .07%.

I'm sure I'm missing some technical detail, though, which Ron will bring up. Soooo, have at it, Ron

Will, I don't need to imagine how much force it would take -- I can calculate how much.

The technical detail you are missing is the fact that the additional force require to move the TT closer to the TV has nothing to do with the drag forces on the TT. The additional force only depends on the mass of the TT and the magnitude of the acceleration needed to get it to move closer to the TV. The key word is "additional". The baseline drag force depends on speed and remains constant. The additional force is equal to mass (of the TT) times its acceleration.

Lets assume a swing of 1 degree and a TT weighing 6440#. The mass (mass = weight/gravity) of the TT is 6440# / 32.2ft/sec/sec = 200 slugs. Now assume this mass is moved at constant acceleration so that it moves forward 0.1 foot in 1 second. The required acceleration is 0.2 ft/sec/sec. The required force (force = mass * acceleration) is 200 * 0.2 = 40#.

Now, remember, the 40# is the increase in the longitudinal component of tongue load. The corresponding increase in the lateral component of force is 40 * tangent(1 degree) = 0.7# (I was generous when I rounded it to 1#.)

Will, if the increase in longitudinal force due to the forward movement of the TT relative to the TV is anywhere near as large as you seem to think it is, it would cause the TV to decelerate and you could feel it (even if you are driving an Excursion).

You can check out what I'm saying by getting something heavy (concrete block, bucket of water, etc.) and standing on a scale with it. Lets assume the added weight is 32.2# (1 slug). If you stand on the scale without raising the weight, the scale will read your weight plus 32.2#. Assume you raise the weight 1 foot in 1 second at constant acceleration. The acceleration is 2 ft/sec/sec. The added force is 1 slug * 2 ft/sec/sec = 2#. The scale, if accurate enough, would read your weight + 32.2 + 2 lbs. Please note, this acceleration is ten times greater than the acceleration of the TT which moves ahead 1.2" in 1 second.

And, I don't know what else to say.

Ron