My biggest problem with quad-launching was that I found it hard to visualise. I've never seen anything get airborne like that. Given that birds are obligate bipeds and their legs are not connected to their wings by a continuous flight surface, they are free to either jump into the air, as with pigeons, or propel the animal along the ground with an energetic run-up, like swans and geese. Many palaeontologists agree that pterosaurs were obligate quadrupeds and that their fore-limbs and hind-limbs were, in life, connected by the wing membrane. Birds are, therefore, a poor analogue for launching pterosaurs, and it is for these, and other anatomical reasons, that palaeontologists believe that pterosaurs' primary launch method probably involved a highly-energetic 'push up'.
A recent post at Pterosaur Heresies again demonstrates its author's frustrations with the problems he sees with the forelimb launch mechanism. The article points out that vampire bats achieve a considerable height from an initial leap before they perform a single flap, and that pterosaurs would be unlikely to achieve such a feat. In a bid to attempt to understand bats taking off from the ground (only a few species can do this) I looked at video footage of a fringed myotis taking off. Adams et al, in their 2012 paper, looked at how bats use their uropatagium to facilitate launch, and made available the following video:
There are four video links in the online paper, showing launches from various angles. In order to get a better idea of what's going on, I rendered the bat as a very-basic stick figure, traced from screenshots of the first online video. The wings' tracings show the stroke, and the head shows the positions of the animal relative to the ground.
Sequence showing a bat (Myotis thysanodes) taking off from the ground, mapped from screen-shots of film footage. This section of the sequence totals around two-and-a-half seconds. (Sequence drawn by author, traced from footage available with Admas, Snode & Shaw 2012.)
|The same bat's take-off sequence, overlaid in order to better show the small area required for a successful launch. Black numbers denote head positions during launch; red numbers denote left wingtip positions. (Sequence drawn by author, traced from footage available with Admas, Snode & Shaw 2012.)|
|Overlaid launch sequence for a male Nyctosaurus gracilis. (Copyright © 2014 Gareth Monger)|
|Launch sequence for Nyctosaurus. Nicked from my deviantART profile, hence the whole lo-res thing. Copyright © 2014 Gareth Monger)|
Adams RA, Snode ER, Shaw JB (2012) Flapping Tail Membrane in Bats Produces Potentially Important Thrust during Horizontal Takeoffs and Very Slow Flight. PLoS ONE 7(2): e32074. doi:10.1371/journal.pone.0032074
Elgin, R.A., Hone, D.W.E., and Frey, E. 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica 56 (1): 99–111.