AERO DYNAMICS ANYONE?

[Night Train]

I was once told that a Defender was more aerodynamic in reverse than forwards.

Chris

If it had spare wheel on the back door then it would be.

Much of the areodynamic drag from a Defender is due to the blunt back end causing a low pressure area that literally sucks the vehicle backwards against its forward motion. Length compensates for this somewhat so a 130 would be better then a 110 which would be better then a 90.

[Scotian]

not sure if many people know what the front flaps are for? They are there so when you are driving a soft top with the rear flap rolled up, Air passes through the vehicle and out the back stopping the eddie forming dragging dust in from the rear causing everyone to get plastered in carp when driving through the desert/africa/dirt tracks or any other places that cause small particals to be sucked in by the flat shape of the rear.

SO with that in mind I would think that you would get better economy with the back open and the flaps open.

[Yostumpy]

If it had spare wheel on the back door then it would be.

Much of the areodynamic drag from a Defender is due to the blunt back end causing a low pressure area that literally sucks the vehicle backwards against its forward motion. Length compensates for this somewhat so a 130 would be better then a 110 which would be better then a 90.

so theoretically a RIBBED 110 roof c1985 vintage, would be better as it will break up the air flow over the roof! Hoe do you stop the suction at the rear then? does this come from the roof, or the sides, BUT the sides having a barrell roll in them this should ease the problem. So what would you do to creat a swirl at the back to brake up the suction.

[Night Train]

so theoretically a RIBBED 110 roof c1985 vintage, would be better as it will break up the air flow over the roof! Hoe do you stop the suction at the rear then? does this come from the roof, or the sides, BUT the sides having a barrell roll in them this should ease the problem. So what would you do to creat a swirl at the back to brake up the suction.

The ribs wouldn't have that much effect in reality. To reduce the suction at the rear you would need to taper the whole body to produce a smaller back surface area for the turbulance to suck on. You don't want to break up the flow until it reaches the rear end. You wnat smooth flow over the body but at the back, if you have a small area, you then want the flow to detach as quickly as possible. That is what the little lip spoiler is for on sports cars.

[Night Train]

From a top view you can see where the green areas of low pressure are, at the back, at each side window and the sides of the front wings.

The windows are worse then the front wings as the corners are sharper.

[Yostumpy]

Very interesting. SO with a spare wheel it would smooth out the plan view, but witha roof rack you have the resistance of the rack, but this breaks up the roof flow and leads to less vacuum at back. My old 110 had a cut down flat full length rack, but open bars ( no boarding) and on a run I got 33 mpg, but this was with an Isuzu lump, was queit though. I see the foward motion water theory now, wat effect would those small loory deflectors have at windscreen posts. Also theoretically then a 110 with an external front cage or 2 snorkels, or both, would be better as it rounds off the corners............phew!!!

[landroversforever]

[WritingsOfaMadMan]

how about making a long vent in the bonnet (hinge end) in the high pressure area. and then join it to the air intake hole + second on opposite side( intake moved!). For high speed, could even pump it.

[/WritingsOfaMadMan]

[Night Train]

[WritingsOfaMadMan]

how about making a long vent in the bonnet (hinge end) in the high pressure area. and then join it to the air intake hole + second on opposite side( intake moved!). For high speed, could even pump it.

[/WritingsOfaMadMan]

Ever seen the old hotrods of the 60's and 70's? They had bonnet scoops to force air into the carbs. The forward facing ones hoped to catch the ram air effect of the forward motion of the car and the rear facing ones took advantage of the eddie currents at the base of the windscreen. The ones facing backwards worked well when there was a windscreen and a normal height roof line relative to the bonnet. The forward facing ones worked when the windscreen and roof were low or non existent and there was a lot of engine relative to the body work.

This is also why on normal road cars the fresh air vents are located here and not at the nose of the car.

[Bull Bar Cowboy]

The CD of a Def is reckoned to be somewhere in the area of 0.9 ……………..but I guess nobody has actually measured it.

I am not particularly into aerodynamics, but I do know that aerodynamic drag increases with the square of speed and it is generally assumed in the motor industry that 60% of the power required to cruise at 56mph is used by overcoming air drag………….

Going back to the statement that aerodynamic drag increases with the square of speed idss the bit I usually quote when folk come on hear telling us that their 90 with a standard 300 tdi engine will top 120mph ! …………. To get a defender over a true 100 mph on a flat road requires an awful lot of power …………. 110mph requires significantly more power, and 120mph requires serious amounts of power……… all to overcome the aerodynamic drag……………

When I had some time on my hands, I remember doing some fag packet calcs with the assumption that the CD was 0.9………………IIRC 100mph required sub 250 bhp …………. 120mph required +400bhp………….

Although the 'van' has been looked at in this thread .............. what about the truck cab .................. IIRC they are worse as the open load bed becomes the low pressure area ?

Ian

[Night Train]

The truck cab presents an addition 'frontal area' by way of the inside of the tailgate. So not only do you get the grille, wings and cab pushing air out of the way but you get the tailgate doing it as well. You then get drag at the back of the cab and the back of the tailgate.

This is why curtainsider trucks keep their curtains closed even when running empty.

[Scotian]

Those figures for speed / power seem a bit extreem. Brand new wolf landrovers in the Army (in 1998) really were requlary doing up to and over 100mph (up to 120 on some occations, I was in the back of it and pooing my self) on a long flat road run. I know they are tuned up compared to the civvie 300tdi but I'm sure they are not anywhere near the bhp you stated..

There is a certain member on here who is planning on making a Army Hummer style slope back on his 110 van. I'll direct him to this thread when I see him online next. It will be interesting to get his before and after mpg figures with this mod.

[Night Train]

Thing is, there are so many factors involved with aerodynamics and with fuel consumption that some rough drawings and generalisations would, in reality, be meaningless.

My car does 65+mpg on a 120 mile run if I ignore the speed and drive for economy. If I drive steadily at 60mph I only get 50mpg.

My 6x6 weighed over 3 ton, had loads of transmission drag and only 96bhp and it was happy at 80mph at 2500rpm. The torque there was around 190lb-ft.

[mmgemini]

Look up two things.

Professor KAMM

Chaperall [sp]racing car. The G or H. The H I think. Like a brick

Makes interesting reading.

With the spare on the bonnet. I used to get the washer jet water squirting forwards.

I would think any of the model Defenders would make a wind tunnel model.

All we used was a Dinky Car Imp in the wind tunnel. I still have it somewhere with all the pipes attatched.

mike

[Daan]

I find the aero quite interesting myself. I spent one year designing truckfairings on trucks for M&S, and we took a Daf 75truck to the windtunnel. There are a few things that might be interesting for us: the a pillar deflectors work well for keeping airflow attached to the side of the vehicle, which reduces drag (and keep windows kleen). a flat floor helps reduces drag as well. Interestingly, the drag caused at the back of a truck is about the same as the drag caused at the front. You can reduce the drag at the back by having the rr wall recessed into the sidewalls. So the sidewalls are longer than the vehicle if you like.

I saw a defender 110 in Paris dakar that had the back end shortened for this reason, but the sidewalls were kept standard length and sloped inboard. It also had the roofline lowered and the area were normally the flaps are was rounded off.

To come back to the original posters question: I think if you measure the consumption penalty due to the roofrack, you will remove it if you dont need it!

daan

[FridgeFreezer]

How do the 109 Safari (double-skinned) roofs compare then? I'm always amazed how well they work at keeping the vehicle cool, but it strikes me that they are also acting to keep a layer of air flowing along the roof. Or, missing the boat totally and trying to pick air up in an eddy due to poor design.

And I don't care if it does save fuel, I'm not sticking a 37x1250 on my bonnet

[Astro_Al]

...I'm not sticking a 37x1250 on my bonnet

Would you be able to John...?! (Not sure I would either).

I wonder if a set of vortex inducers along the top of the windscreen would help keep flow attached over the roof at higher speeds. Might be a cheap way to help a bit, but I still think its a lost cause!

Parasitic drag of the roofrack will defo suck (pun intended) as Daan implies. Lots of little bits really add drag - like the cables between the wings of biplanes - added a surprising amount of drag for their size.

Al

[nicks90]

i suppose the sun visor wotsit from a series safari would make an effective difference to the vortex created above the windscreen if suitably altered.