Math question

[Huntindog]

So after installing my new upgraded axles and springs, my WD needed a little tweak as the TT sits a tad higher.
While making the adjustment, I got to thinking...
It is often advised to error on the side of tounge low... My thought that I have never seen discussed here is that,, If it is tounge low, then the roof of the TT will act like a large spoiler creating downforce... I know it's not much of an angle (usually) but it is a very lage surface, and wind speeds can easily exceed 100MPH when towing into the wind.

So how much downforce would this add? Would it be enough to exceed the weight ratings of axles and tires? Especially since the industry tends to skimp on these components?

This could perhaps explain some failures that were unexplainable in the past.

Thoughts?

[christopherglen]

The roofs are so "dirty" with AC's, vents, shower domes, antennas, etc. The slight angle of attack you are getting, and the fairly low airspeed don't amount to much. You are looking at about 0.0807 lbs. per cubic foot of air at standard temperature and pressure. Assuming the trailer is 8.5 feet wide, 4 inches higher at the rear then front, and you have 100 mph wind over the top. That's 528,000 linear feet per hour (3600 seconds), or 146.667 linear feet per second. 8.5 feet across times 4 inches tall is 2.831 feet of displacement * 146.667 linear feet per second. So you are vertically displacing 145.141 cubic feet per second total, or about 33.5 lbs of air per second. The axles are over 50% of the way back, so over 50% of that occurs in front of the axles, and in theory is added to the TV.

The reality is there is a huge low pressure at the front, far larger then the slope of the trailer, random high and low pressure spots above and below the trailer down it's length, and an 8.5 by 10 foot box (tv and trailer)is shoving 12466 cubic feet of air out of the way, and dragging it back behind it - per second (at 100 mph air speed). The 1000+ lbs of air you are pumping at each end per second has far more to do with anything then the 30 something the roof is moving.

[TomG2]

No anger here. I just thought it was funny to think of a grown man riding down the highway holding a box out the window trying to prove a point.

[Huntindog]

TomG2 wrote:

Huntindog wrote:
....snip........ :B

I haven't tried this yet, But if I were to take a rectangular box, and hold it out the window on the highway in different positions, then that would be a pretty good test of my thoughts. I think I know what will happen

When you start playing around with your boxes, don't forget to place one in front of your simulated travel trailer to take the place of a tow vehicle that is punching a hole through the air. Also, include a roadway that will affect the airflow around the two vehicles. There about a hundred other factors to consider if you want a really scientific evaluation. In fact, they have such a thing and it is called a wind tunnel.

Why the anger? Drink your coffee and lighten up.
I don't happen to have a wind tunnel handy, do you?

Actually I have already moved on beyond holding a box out the window... A simple test that can prove or disprove weather a box will act the same as an airplane wing,, or not. I am having a hard time with the lift theory of the box... Yes I am very familier with how an airplane wing generates lift... But most of us are not towing airplane wings.

A scaled down home test can easily prove or disprove this without a lot of expense.

What is needed is a matchbox car, or a few different bodied ones of your choice. A scaled to size TT shaped wooden block. Some wire to simulated the hitch between the two, attached by a means that will work (I can think of several), some thumbtacks stuck into the block for wheels, a fan, a protractor and a digital grain scale. A wind speed meter would be a nice to have item.

Set the hitched up model TT on the scale, with the TV on some books etc by it to simulate how it would be on the road. First perfectly level, turn the fan on for wind, than take a reading from the scale.
Then repeat the scenario with different angles on the TT. From there it is just a matter of doing simple math to scale the results up to full size. If the actual windspeed is known, then it can be a very good experiment. I don't happen to have the scale needed (mine is an old beam style), but many reloaders do have this. It is very likely that someone reading this has one.

[SprinklerMan]

GrandpaKip wrote:

lbrjet wrote:
Nothing wrong with thinking.

Especially around a campfire.

With a 6 pack

[JIMNLIN]

Most TVs nowdays have spoilers or air dams to keep the air passing under them to a minimum.

Just a point here.... front air dams on our trucks were added to create a low air pressure under the engine area. Low pressure under the engine bay allowed a much better air flow through the radiator and on out the bottom of the engine bay.

[bguy]

GrandpaKip wrote:

lbrjet wrote:
Nothing wrong with thinking.

Especially around a campfire.

I thought he said drinking.
Cheers!

[TomG2]

Huntindog wrote:
....snip........ :B

I haven't tried this yet, But if I were to take a rectangular box, and hold it out the window on the highway in different positions, then that would be a pretty good test of my thoughts. I think I know what will happen

When you start playing around with your boxes, don't forget to place one in front of your simulated travel trailer to take the place of a tow vehicle that is punching a hole through the air. Also, include a roadway that will affect the airflow around the two vehicles. There about a hundred other factors to consider if you want a really scientific evaluation. In fact, they have such a thing and it is called a wind tunnel.

[GrandpaKip]

lbrjet wrote:
Nothing wrong with thinking.

Especially around a campfire.

[lbrjet]

Nothing wrong with thinking.

[dalenoel]

Definitely believe that more people on this forum need to spend more time around a campfire and less time thinking.

[Ron_Gratz]

Huntindog wrote:
---If it is tounge low, then the roof of the TT will act like a large spoiler creating downforce... I know it's not much of an angle (usually) but it is a very lage surface, and wind speeds can easily exceed 100MPH when towing into the wind.

So how much downforce would this add? Would it be enough to exceed the weight ratings of axles and tires? Especially since the industry tends to skimp on these components?

For virtually all TT front-end shapes, there will be a negative pressure resulting from air flow over the front of the roof.
If the roof is sloping downward at the front, the magnitude of the negative pressure will be decreased by about three percent per degree of slope.

If a TT is "nose down" by two inches and the distance from the coupler to the midpoint between axles is 200", the downward slope would be about 0.5 degrees.
The resulting reduction in the magnitude of the negative roof pressure would be about 1.5%.

IMO, the effect on axle and tire loading, due to roof pressure changes, would be negligible.

Ron

[frizzen]

TomG2 wrote:
The effect you are asking about is totally about aerodynamics and the application of Bernoulli's principle. I am thinking that there would be a slight amount of lift involved with a lowered front face, just as there is in an airplane wing. The kicker is that the drag associated with towing a travel trailer makes looking for other changes tiny in comparison.

What he said.

I'm not sure how much effective lift you would get since the airflow under the trailer would have to be considered as well.

[GrandpaKip]

Look up the Bernoulli Principle or Bernoulli Effect. Unless the angle presented to the wind is very severe, there should be a certain amount of lift, though because an RV is not smooth top or bottom, it is probably negligible.
Lots of fun and interesting experiments can be found to demonstrate this.
Huntingdog, my guess is the box would fly out of your hands at highway speeds.;-) that big sail will make most other forces insignificant.

[TomG2]

The effect you are asking about is totally about aerodynamics and the application of Bernoulli's principle. I am thinking that there would be a slight amount of lift involved with a lowered front face, just as there is in an airplane wing. The kicker is that the drag associated with towing a travel trailer makes looking for other changes tiny in comparison.