allanflowers

Graham wrote: "Any system of venting cooling air through the vehicle interior represents energy lost from the airstream so is to be avoided at all costs. For this reason, cooling is usually kept to the minimum required to maintain reliable operation of the equipment."
Not good advice, unless you want to be responsible for somebody's death. This race is LOOOOONNGGGG and, it ain't run at night. You need to protect the driver with some ventilation. Properly done, there won't be significant drag when the vents are open and they should be designed to be closable. A thermometer inside is important, and get some medical advice on what is an acceptable temperature. Just vent when necessary to keep the temperature where your doctor says. You can isolate the cooled area and place inlet and outlet vents for efficiency.
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Ben, I take it that the story about the P-51 oilcooler is true, then?
Allan

allanflowers

Ben,
How hard would it be to make a small model to test this idea? And what kind of efficiency would it have as a purely thermal powered vehicle, compared to using photovoltiac cells?
Allan

old git

I seem to remember that the Wright Bros managed to get some useful figures from their own wind tunnel and it was pretty simple.

The second factor about streamlining is vague in my memory but fastback cars lost aerodynamic efficiency due to an excessively steep taper. The upshot of the matter was to streamline and cut the excess length off sharp. Many of the hot GT models have the distinctive blunt rear. there was a name for that shape but I cannot for the life of me remember what it was.

There are a couple of cars that have been built for economy runs at our local academy (low tech kids). Would it be acceptable for non aircraft people to join in on this thread. I saw the cars during a visit, don't know the people involved but will contact the teaching staff if that would be acceptable. The academy is a high cost educational experiment for local youngsters in a seriously deprived area.

rmh

Git--The race car design is Kamm-
He used a severely truncated shape - simply whacked off flat.
The tapered back on some US cars lifted the wheels at speed s over 100 mph. The "streamliner" shape was used by Campbell and others and of course the look was simply carried over to make a car look "racy".
Nice look . pispoor engineering.
Low tech kids can really be a surprise - some of them are simply brilliant types -caught in poverty.
We do presentations (show how education can be a way out ) for kids in detention and the attention and questions they present show that they are not all bad - most are extremely inventive - they learn from necessity -not because someone carried them off to a fancy school.

I also worked years back with the mechanics who made the huge clothing manufacturing plants function - in the southern US low income areas.
That was a real education for me..
Some were incredibly talanted and instantly spotted the flaws in machinery designed by various companies - better yet - they sometimes came up with fixes which were excellent design improvements -

Ben Lanterman

Allan, beats me! Someone that remembers how to do it would need to calculate the pressures, energy input, temp rise, internal duct shapes and final velocities before something was made. I doubt it would ever be something other than a little bit of thrust. The question would be if it would be enough to overcome the rolling drag penality due to extra weight.

See Dick, the theory could be useful at times.

rmh

I would have to see the theory at work first

Doubting Thomas

Salto

"How hard would it be to make a small model to test this idea? And what kind of efficiency would it have as a purely thermal powered vehicle, compared to using photovoltiac cells?
Allan "

One way to look at the feasibility of this idea is to look at the thermal efficiency of your proposed heat engine. The thermal efficiency is a function of delta T, the difference between the high temperature, and the sink temperature of the surrounds. If you could get an absorber panel up to decent temperature then the idea might work. But this would require a lense or reflective concentrator of some sort. For just a flat 8sq.m. panel you haven't got a hope.

In fact, in 1987 one of the World Solar Challenge entrants used a parabolic mirror to heat up the "hot" end of a Stirling Engine. It didn't work, and as far as I know the idea has not been used since in a solar car event. In that same year there was also a steam engine powered entry - it didn't even start the race. Simple physics quickly shows that PV power is the way to go.

Graham.

allanflowers

Even given perfect efficiency in converting the solar energy to heat the air, the expansion thereof would still not produce much thrust. Most of the heat would just go out the back. I strongly suspect the story about the P-51 producing net thrust with its oil cooler has probably never really been verified although it is an interesting theory.
The extra complexity, rolling weight and internal drag for this application are hard to deny. Like a previous post stated, there is a lot of energy there but harnessing it is another matter.
Allan

rmh

there is theory and then there is applied theory
Sometimes they don't have same results-----------------