Vehicle & Technical > Defender
Aerodynamics
drmike:
I did do that with my series. I had a steel lid fabricated for the tub and it was very effective - cheap too. £25 all in made up by local agricultural engineer. They won't do that again I'll bet! The series had a top speed of 65 or so as I wasn't so cruel as to ask it to go faster.
I'd guess the most telling statistic would be the top speed of a truck cab against a hard top. The guy who sold me the 90 said it would cruise at 70+ and more all day but of course didn't say how much of a run up it needed. I can certainly get it to 70 as a truck cab and being legal etc. I have no idea how much faster it would go but the acceleration at that end of the scale isn't sparkling.
Mike
extreme90:
i agree with drmike the rear tub wil act like a break...... it will be like sticking a parachute behind you, but as i said i have not noticed any change at all from when i changed mine for a hardtop with rollcage to truck cab with full rollcage.. and its now gr8 as i can see the rear corners unlike before
V8MoneyPit:
--- Quote from: "drmike" ---I have thought about this some more. I reckon that the tub will create a lot of turbulance and also act as an air brake with a truck cab. I also believe that it would be mnost noticeable at higher speeds.
--- End quote ---
An air brake is only effective in 'clean' air. The tailgate, as you correctly point out, is in turbulent air. Frontal area is by far the greatest factor in passing any object through the air.
To address the parachute comparison, once again, the parachute needs clean air to operate. That's why they are always well behind drag racers for example. If you hung a shute 6 feet behind a Land Rover (like the tailgate) it would not even open because of the turbulence.
--- Quote from: "drmike" ---I'm not convinced that the front profile is the only factor that counts. I'd have said that teh tub would have quite a daramtic effect but that's guess work!
--- End quote ---
Other factors do come into the equation, but they are very insignificant compared to frontal area.
--- Quote from: "drmike" ---I wouldn't have thought that larger tyres would affect the aerodynamics adversely although they lower the gearing.
--- End quote ---
Again, it is the frontal area that is the main problem. The wider tyres will present a greater area to the air.
--- Quote from: "drmike" ---What about rag tops? They flap about a fair bit - won't they affect the aerodynamics?
--- End quote ---
Anything that interupts the clean airflow around the moving object creates drag. However, the main consequence of this is noise and stability, not how fast the object can travel given a particular force.
If anyone wants their Land Rover to go faster, the best thing can do is lower the suspension to reduce tyre frontal area and give it a 6" roof chop! :lol:
drmike:
Well that's very interesting. It had never occurred to me that a parachute was ineffective in turbulent air but I think I can understand the logic.
However, (and there's always one of those!) won't the air shove its way past the cab, swirl over and round it and then swirl into and eventually out of the tub? Won't this cause drag simply beacuse the air is getting al het up in the back?
Thinking of water. I suppose if I had a big enough hose pipe I'd expect the water to hit the front of the LR, splash all over the place and in fact the tub to fill with water with some sloshing about but perhaps not actually changing the water in there very much which might be what you're telling me.
I think I may well forget about this as my head is beginning to hurt. In ye goode olde days I did some fluid mechanics at University but gave it up in favour of pure maths which always seemed to make more sense and avoided integration most of the time. I remember being very envious of the chemical engineers who had PDP11 computers to play with and I used to get bored and help them pinch a PDP from one experiment to another. All you needed was a sack trolley. Happy days.
Mike
V8MoneyPit:
--- Quote from: "drmike" ---However, (and there's always one of those!) won't the air shove its way past the cab, swirl over and round it and then swirl into and eventually out of the tub? Won't this cause drag simply beacuse the air is getting al het up in the back?
--- End quote ---
'All het up at the back'..... :lol: I like that description. Maybe I should pass that on to my ex-colleagues at Lotus.... I always thought they were using the wrong terms :lol: :wink:
The issue is that you are having to displace a particular amount of air around the object. Whether it is a cube or a sphere, if the frontal area is the same, you still have to displace the same amount of air. Once the air has been displaced, all the hard work has been done. What happens after that is purely relating to minor effects, mainly on the stability of the object.
--- Quote from: "drmike" ---I think I may well forget about this as my head is beginning to hurt.
--- End quote ---
Tell me about it! I'm having to think back several years too!
Just to expand on the roof chop comment I made earlier...... I was joking!
If you wanted to increase the top speed by 10 percent, it would take a lot more that 10 percent increase in power. However, if you reduced the frontal area by 10 percent, it would be far closer to achieving it without any power change.
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