Expert: Paul Soderman - 12/22/2008

Question
QUESTION: Hello Paul,
Back at the book again and this chapter is on flying machines so hope you are up for a few questions. :>)
First one: what is your take on the debate about whether giant pterosaurs can fly? Recent studies on the albatross (see here for summary http://www.newscientist.com/article/mg20026763.800-were-pterosaurs-too-big-to-fl) suggest 40kg is the upper limit for wingpowered flight, while paleontologists figure the largest pterosaurs were four times that size. And yet - they probably did fly - they had the flying body plan afterall. So how can these facts be reconciled?  Also, it is weight, and not overall size that is limiting, right?  And, just suppose someone came along and invented an artificial muscle that could produce more power per unit weight - could that make way for an artificial flying machine based on a "Wing" design? Thanks in advance

ANSWER: Hi Judy
Sato makes a good point - heavy wings are hard to flap. But I have to side with the pterosaurs. They didn't evolve wings for nothing.  And maybe all they had to do was soar.  If they lived in mountain areas where they could launch easily and glide they wouldn't have to flap much.  Course, sooner or later they would have to gain altitude, but maybe they were good at using thermals.  We know that gliders, which are much heavier than 40 kg, can gain tremendous altitude by judicious use of thermals or mountain updrafts.  And the earlier Earth may have been a more dynamic place, weatherwise, than it is today.

A human-made flying machine that flaps is certainly possible.  And I think it has been done for simulations of insects or small birds.  To carry a person is more difficult because the strength and weight requirements go up so the power plant gets bigger and heavier.  It soon becomes an inefficient way to fly I imagine, but I can't say its not possible.  Creativity is a remarkable thing.
Paul

---------- FOLLOW-UP ----------

QUESTION: Nicely put, as always. So - Robert Hooke, my subject, attempted to make himself a winged flying machine. He had it figured that the problem was a lack of muscle power. His solution involved gunpowder and springs. That is, I'm afraid, all the information we have about it. I'm hoping you might be willing to conject whether powder and springs could get a winged man airborne, or keep him there?



ANSWER: I studied Hooke, he knew a lot about springs.  If the spring was rigged to pull the wings upward, the pilot would only have to pull them down, which is a high strength direction for humans.  But the gunpowder  ?? You've got me there. I can think about jokes with the gunpowder in a gun aimed at the pilot, but in reality the only advantage of gunpowder is rocket assisted launch I think. Unless in those days they could burn gunpowder, create steam, and create a crude engine.  But....  how exactly.  We need more information.
Paul

---------- FOLLOW-UP ----------

QUESTION: It sounds as though it might work...so just how big to wings need to be to hold up humans?

RE gunpowder: not enough information is so aggravatingly Hookeish. in his diary he says, "discoursd much of theat of the Long one of the ways of the strength of the Spring &c. of the way of bending springs by gunpowder for flying".  I suspect this won't help, but if you can think of a way that gunpowder could be used to bend springs, please let me know!

Answer
I believe Hooke meant stretch the spring in modern English.  If he was thinking of designing some kind of a piston device, then one could envision a gunpowder fueled single stroke engine that pushed the spring and wing and allowed the spring to return for the next firing.  I've seen old tractors that huffed and puffed along using single piston engines.  But things like that get heavy because it takes real iron to contain an explosive charge and push a piston.  I bet Hooke's idea never got off the drawing board.

How big a wing ?  I did a rough calculation assuming a 140 lb man (runner type) with 30 lb wing weight flying at 10 ft/sec.  Because of that low speed he would need roughly a 1300 ft^2 total wing area.  So if he had two wings they would have to be around 30 ft by 20 ft each.  Pretty big.  If the speed could be doubled, and I have no idea what speed could be achieved, the wing area goes down by 1/4.  That is, the wing area is inversely proportional to velocity squared. This is why gliders are as big as they are.  A bird man would have the same problem as a glider; even more because I didn't take into account the upstroke which doesn't produce lift without some fancy wing twisting like a hummingbird.

Someone may have better numbers on this. Man powered aircraft have crossed the English Channel, but they used a propeller. The wing was pretty big.  I haven't heard of any bird man since Icarus, but he may be out there.
Paul