Aerodynamics questions related to recumbant HPVs

[Otto]

Originally Posted by Bicycle Bob

There are a lot of trade-offs involved. A two-lump shape has more drag than a continuous curve, all else being equal, but by so little that the skin drag difference may matter more if you don't need all the volume. As for HPVs, You are dealing with problems such as solar gain, demisting access, vision through imperfect plastic, which is terrible at low angles with bad lighting and/or rain and so on, as well as doing trade-offs between frontal area, rider position, wheel placement and size, vision, cooling, surface area and so on.
A pure laminar shape tempted the HPV pioneers, but surface flaws and vibration generally defeated them, while adding considerably to frontal area in some examples. Lately, gains have been found by using small cranks to reduce frontal area, raising RPM 30% or so, until a 1 or 2% power loss wipes out further gains- until someone who has learned to spin a Hot Wheel at 300 RPM shows up.

We can change anything,
but we can never change just one thing.

What's your take on front wheel drive?

Also, since high gear is what apparently matters at top speed, why mess with derailer, rear sprocket cassett, etc., as opposed to in-the-hub transmission and 8 speeds with one crank sprocket? This way, shorter chain, properly aligned in the plane of rotation with crank and hub sprockets for presumably less friction loss.

And, do front and rear wheels need to be the same size? Might be better at the bottom line to have smaller front wheel for smaller nose, overall more efficient despite presumably somewhat greater rolling resistance in a smaller wheel.

[Bicycle Bob]

For a supine recumbent, FWD can reduce weight and driveline flex. Twisting the chain to steer is fine except for parking type maneuvers. It is better to add idlers than to let the pedals move with the steering or put the pedals above the wheel, unlesss the wheel is exposed below the main body. Internally geared hubs have more friction than derailleur setups except in direct drive, and in a top speed run, most of the time you are accelerating. It is tempting to use an intermediate block and two chains, with two or three derailleurs, but this costs maybe 3% more power than idlers. Idlers can float sideways as the chain shifts. Clearance is tight - try using the narrowest chain, but not many ratios, with a triple crank. Use an extra-big chainring, and try to stay away from tiny cogs - efficiency drops off quickly under 15 teeth. ( With a small wheel, it may be more tempting to use compound gearing.) An oil-bath chaincase might be worthwhile, too.

Wheel sizes probably should not vary any more than they did on the penny farthings, which were also done small-in-front. That should give you some wiggle room.

[aerohead]

Originally Posted by snowluck2345

So, what would be the most aerodynamic shape possible for one of these? A reverse teardrop with a reverse teardrop head pod?

First off,I'm way out of my element here so take this with a grain of salt.
HPVs are 'special' and aerodynamics is only one of many criteria which makes up the 'solution' to a successful bike.
I was fortunate to have attended the 2005 competition at Battle Mountain,Nevada.There were a wide variety of vehicle designs represented at the meet.
One entry,which some might think aerodynamically superior to Varna required the rider to lay on his back,head forward, facing skyward,looking through a periscope to see his advance 'behind' him.
He never approached the performance of Sam Whittingham and Varna.Nobody did.
Over the 5-mile course it was as much rider as vehicle which determined the victor.
Personnally,I'm attracted to trikes and I remain very impressed with VECTOR,which tunneled out at Cd0.10 or .11.
I would like to pull a trailer with PVs and electric assist,and this makes the trike appealing.
If you're thinking of competing in HPV,consider hiring a floor sweeper for your design chief.History reveals that they have an uncanny knack for accomplishing the impossible.