The effects of airflow on the weights and balance of a Caravan.

iana · Guru

The effects of airflow on the weights and balance of a Caravan.

KJB · Guru

Slowing intake air speed was one of the major problems they had to overcome to build the Concorde and when they were successful it was looked on as a major achievement (mainly because of the way they were able to use "computer" control of the air flow). Up until then, the only computer available was too large and heavy for aircraft use but the Concorde team were able to drastically reduce its size and "make it work" very successfully......

Greg 1 · Guru

A lot of years ago, my BIL was involved in the design and manufacture of aerodynamic aids for cab over trucks. As a licensed aircraft engineer he gets asked to do a lot of this sort of thing particularly on race cars. I spent quite some time helping test these aids. Aerodynamics is a very complex subject, and in answer to the original question about the effect on weights, on an aerodynamic brick like a caravan the effect would be negligible. Yes, pressure waves generated by trucks will push a van around as will strong cross winds, but it is more to do with the sail effect on a slab sided structure than anything. The amount of lift generated by a van at normal road speeds would not effect it's weight to any degree. With regards to the aerofoil on the roof of the LC200 as per the photo, it is better than nothing but has some major inherent issues. The main one being the distance between the rear lip of the foil and the caravan. The air spilling off the aerofoil falls between the two causing vortices in the gap. This is compounded by the fact that the foil doesn't reach high enough to fully direct the flow onto the van roof. The fact that it slopes away at the sides looks pretty but is inefficient. They would have been better to increase the width a little and fit it with side fences. The angle of attack is also fairly steep which causes separation off the rear lip and there is a significant gap between the roof and the leading edge of the foil which will cause more inefficiencies. Like I said, better than nothing but questionable whether the return is worth the outlay.

iana · Guru

Ok Magnarc, straight from the "Big Gorillas"" message.

Vr is defined as the speed at which the rotation of the aircraft should be initiated to takeoff attitude

Pilots say rotate because it is a verbal queue that an airplane has reached its predetermined rotation speed . This is the speed at which control column is pulled back to lift the nose off the runway.

V1 is the speed by which time the decision to continue flight if an engine fails has been made. It can be said that V1 is the "commit to fly" speed. V2 is the speed at which the airplane will climb in the event of an engine failure after lift off. It is known as the takeoff safety speed

In sequence V1 then Vr then V2. The Pilot NOT flying is the one to call the readouts. So they can be actioned by the Pilot Flying. Captains and First Officers take turns in control of the aircraft on different legs of the flight.

Big Gorilla ex 747 pilot.

Eaglemax · Guru

Be good if you stayed on topic Yobarr.

Let's be hypothetical, if a trailer is a single axle enclosed box trailer like those you rent, and let's say single axle, at say 100kph (which many here seem to be their cruising speed) I'd have a guess that the flat front on the trailer would have a tendency to lift the hitch upwards placing a negative effect on the towball weight. If that loss of towball weight is say 20kg on any trailer/caravan, it could be relevant to the rigs stability.

Many vintage caravans had flat fronts.

Factors that would lessen this would be - lower speed, twin/triple axles, etc

I dont need to "know" as you put it Yobarr, I can give an opinion, fancy that!

Tony

Noelpolar · Veteran Member

Fwiw.... when I travel with a small tinnie on the roof.... mounted on a special rack I built so that it could be loaded from the rear.... the rack kicks up at the rear to clear the 200 series open tail gate.... and the boat sits very rearwards on the car.... so it's transom is well over the hitch point.... I noticed I had built an almost pergect caravan wind deflector.... unitentionally.... top of the van (tall 3.1m van) remain bug free.

-- Edited by Noelpolar on Friday 27th of November 2020 06:26:45 PM

iana · Guru

I have to say Tony, that was the jist of my thread. No mention of airfoils etc., but only the turning moments about the axle, caused by pressure and drag forces trying to rotate the caravan rearwards. Reading about how persons carefully calculate their vehicles weights, but then the whole lot possibly goes out the window once moving.
Bearing in mind that drag increases at the square of the speed, for example a van like yours, compared with say a long pop top van that is in the stowed position very low. The moments compared when moving must be very different.
So as I said, could this effect at speed be a contributor to the unexplained van accidents.

Eaglemax · Guru

Thanks Iana

To expand on this "wind effect", we can imagine various scenarios, like...those wind reflectors that are attached to the rear roof of a 4x4. Imagine one was mounted at the rear roof of a caravan purely for the sake of effect. The faster you tow the less hitch down force you'd have. It might not be a lot however it would be a variable effect dependant on speed and that could make the handling unpredictable.

Based on that extreme example any surface of any object be it tow vehicle or caravan would be effected by wind. A good example is when towing our rig and we get caught following too close to the rear of a large vehicle say a semi. The turbulence tosses our rig around sometimes causing sway. Taking that one step further, we don't even need to be following that truck to get the same effect, crossing some bridges north of Sydney side winds can give you ulcers.

I'm suggesting that I think your query that wind could be a factor in some caravan rig accidents has merit. In that aspect every rig would be different and all other conventional factors like weights, centre of gravity etc would be part of the equation.

One could only determine how much effect speed has on the wind effect of each rig by wind tunnel measurements. But some of us know when our rigs are becoming unsafe. For me above 95kph I know my rig has more risk of instability and I also believe wind is a factor be it on all surfaces. With a towball weight of only 85kg if wind reduced the hitch downforce by 20kg it can become dangerous.

I'd speculate that speed has a greater factor than anything else regarding wind effect. If all caravans were restricted to 90kph (just for the discussion relax guys) and restricted to 20kph less than advisory speed signs on bends the varying towball weight and sway caused by wind would be greatly reduced.

Perhaps others have thoughts on this also.

Rob Driver · Guru

Hi guys,

I wrote this in a post above
......
I am probably guilty of going away from your original topic contents and enquiry but with your original observatIons of the instability varying with speed and movement is a fact and with a caravan and tow vehicle combination this situation is probably for the worse due to the simple fact that most of these combinations are a balancing act using near maximum allowances on both vehicle and caravan suspensions.
.....
I should have added this but I didnt consider it relevant to your original subject.
.....
The above balancing act combined with the lack of experience, training and skill with towing a combination as big as a car and caravan is in my opinion the main cause of caravan roll overs.
......
This link does really explain the instability in a car / van combination.

From a post above
youtu.be/PUHk6lqSKQI

Greg 1 explains this very well in his post.
The Sail Effect created by the size of the side of the caravan is most affected by wind or other vehicles.

With aerodynamics of big square trailers, I think we would rarely see a large rental furniture trailer or tradie trailer roll over but it is common with caravans, hence my comment re the balancing act with weights having a major cause in these accidents.

Then we could start on road surface conditions and design.
The drop off at the bitumen edge is very dangerous for causing instability of a caravan but when considering this, it is still the Balancing Act of near maximum (or sometimes over) weights on wheels and will be magnified by under inflated tyres.

The stability of a car caravan combination comes back at the end, to the operator.
Get everything right and we will all be a lot safer.

Regards

Rob

hufnpuf · Senior Member

bentaxlebabe wrote:
with the lack of experience, training and skill with towing a combination as big as a car and caravan

I just got a caravan, I was surprised that you don't have to do any training, that it's optional. I think in Britain you need to do a course, I've seen mention on-line of having to have your licence endorsed for towing van.

There are basic things I think everyone should know before hitting the road. One thing they told us at the short course I went to was if your van is starting to wobble, not to brake the car, also when a big truck/semi passes you. That would seem to be a natural tendency to put the car brakes on, but it's wrong. People can't be aerospace engineers to understand the forces involved, but they can absorb such basic instruction. Something like that could help avoid a roll-over/collision from somebody doing the wrong thing because of ignorance and inexperience. I think a requirement to at least learn some basic things would be a good idea. Also understanding how to load things, and why you need to do it that way.

Although people can't do anything about external forces (except not to be out there when there are strong winds, I suppose) like airflow from other bigger vehicles passing, they can do something about learning more to be able to safely handle what they are driving.

As for better engineering vehicles against external forces, the costs of doing it would be high, and given the other variables at play, probably not realistic.

iana · Guru

Eaglemax wrote:
I'd speculate that speed has a greater factor than anything else regarding wind effect. If all caravans were restricted to 90kph (just for the discussion relax guys) and restricted to 20kph less than advisory speed signs on bends the varying towball weight and sway caused by wind would be greatly reduced.

Perhaps others have thoughts on this also.

Just remember that when you are moving forward at 80kph, in no wind conditions, you then have a wind over your rig of 80kph. If you have a 20kph head wind, that wind (relative air flow), is now 100kph. Any wind from the side, is hitting your van as a resultant of the forward speed over the side wind speed.

Eaglemax · Guru

This thread is educational.

Jaahn · Guru

iana wrote:
Here's an other bit of trivia to make you think. Because of the effect of airflow when the speed of sound is reached, all the air, and gas flow through gas turbine motors must remain below the speed of sound. So our SR1 spy plane doing Mark 3, the gases in the motors must remain below mark 1.
All achieved by adjustable convex and concave ducting.

Hi smile

Gees it makes me think that you should not exceed the speed of sound towing your caravan. biggrin I will keep that in the front of my mind travelling ! aww Thanks for that information !

Jaahn

iana · Guru

I would not recommend anyone breaking the sound barrier with a caravan, the shock wave will knock the semi's off the road, and break all the house windows. smile